Caliornia Geology Magazine Mar-Apr 1992

8/7/2019 Caliornia Geology Magazine Mar-Apr 1992

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CALIFORNIAGEOLOGY

A PUBLICATION OF THEDEPARTMENT OF CONSERVATIONDIVISION OF MINES AND GEOLOGYSlate 01 California PETE WILSONGovernorThe Resources Agency DOUGLAS P. WHEELERSecretary for ResourcesDepartment of Conservallon EDWARD G. HEIDIGDirectorDiviSIOn of Mines & Geology JAMES F. DAVISState Geologist

CALIFORNIA GEOLOGY

In This IssueLOMA PRIETA DATA ARCHIVE PROJECT 30THE HONEYDEW EARTHQUAKE 31SOURCES OF NORTH COAST SEISMICITY 40EXCERPTS FROM THE SACRAMENTO BEE, 1892 54THE CAPE MENDOCINO EARTHQUAKES, 1992 56TEACHER FEATURE 58NEW MINING BOARD MEMBERS 60BOOK REVIEWS 61CORRECTIONS FOR DECEMBER ANDJANUARY/FEBRUARY ISSUES 62DMG OPENFILE REPORT RELEASE 63PUBLICATIONS REQUEST 63UNIVERSITY OF NEVADA SHORT COURSE SCHEDULE 63CALIFORNIA GEOLOGY SUBSCRIPTION FORM 64DIVISION OF MINES AND GEOLOGY MOVES 64

Assistant Technical Editor:Assistanl Edilor:Graphics and Design:

Elise MattisonLena Tabil ioPeggy Walker LOMA PRIETA DATA ARCHIVE PROJECT

Pnnted' Department ot General ServICeSOIltce ofSial Printing

D,vlSlon Headquarters. 801 K Srreel, MS 1230sacramenlo. CA 95814353t9164451825Publicanons and Informalion Office:801 f( Street. MS 1433Sacramento. CA 95814-3532Public Informat,on' 916-4455716Los Angeles Office' 107 South Broadway. Room 101>5lo s Angeles. CA 9001244022136203560Bay Area RegIOnal Office: 1145 Marl


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The rocks of the King Range Terrane. with peaks reachingover 4.000 feet (1,200 m), are within a few miles of the coa t.south of the Mendocino triple junction. Mclaughlin and olhers(1982) have uggested that the King Range Terrane wasabducted (thrust) onto northern California along a southwestdipping reverse fault as recently a 2 mi ll ion year ago. Sub equent folding and uplift are recorded by marine terraces alongthe coa t (laJoie and others, 1982: Mclaughlin and others.19 3: Merrits and Bull, 1989) and fluvial terraces to the eastnear Garberville (Bickner, F.. Humboldt State University.Arcata. California. oral communication. 992). tudies u ingf ission tracks (Dumitru. 1991) and stream profile (Merritsand Vincent. 19 9) corroborate the late Holocene history ofrapid uplift in the region.

on the outh by the Cooskie shear zone, and on the west bythe coastline (Figure 2). The epicenter of the Honeydew earthquake i within this region. and therefore must be considered atriple junction event.

128

Figure 1. Location map of large earthquakes in the north coastal Californiaarea during July and August 199 t . Times are Pacific Day1ight Time.Epicenter locations and magnitudes from the National Earthquake Information System (NElS), 1991.

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The Mendocino triple junction, because it can ists of poorlydelineated plate boundaries, i not a single point but rather aregion bounded on the northeast by the Petrolia hear zone.

GEOLOGIC SEITING

The boundary between the Pacific and North Americanplates is a broad zone of faults parallel to and east of the SanAndreas fault (Kelsey and Carver, 1988: Eaton. 1989). Thesurface rupture of the San Andreas faul during the 1906earthquake can be traced from south of San Jose northward toP . Arena (Law on. 1908). From PI. Arena. the San Andreasfault is thought to run offshore to just south of PI. Delgada(Griscom, 1973: McCulloch. 19 9). North of Pt. Delgada. thean Andreas fault is not well defined. but may run parallel andclose to the coastline. curving westward to join the Mendocinofault (McCulloch. 1989). or across land outheast of PI.Delgada, eventually joining the Mattole shear zone(Mclaughlin and others. 1988).

This report ummarizes surface phenomenaobserved during the 2 weeks following the earthquake (Photo 1). pre ents an i oseismal mapcon tructed from field data and individual surveyresponse and proposes a simple fault model forthe earthquake.

The Honeydew earthquake occurred about 15mile (25 km) southeast of Cape Mendocino in thevicinity of the Mendocino triple junction (Kel ey andCarver. 1988; Clarke, 1992). the region where theGorda. Pacific. and North American plates meet(Figure 2). orth of the triple junction. coa tal tectonics are dominated by the convergence of theGorda and orth American pia es along theCascadia subduction zone. To the south, tectonicsare controlled by the San Andreas fault system andhe right-lateral transform motion of the orthAmerican and Pacific plates (Kelsey and Carver.1988). The Mendocino fault, extending due west ofCape Mendocino. forms the boundary betweenthese two tectonic regimes.

orthwest of the Mendocino triple junction, thesouthern end of the Cascadia subduction zone bendseastward and comes onshore in the vicinity of thePetrolia Shear Zone (Clarke. 1992). The boundarybetween the Gorda and Pacific plates, theMendocino fault. can be followed eastward alongthe base of the Gorda Escarpment offshore to about1240 45' longitude. but landward its location becomes less certain. Seismicity trends of shallowearthquakes (McPherson. 1989; Oppenheimer andMagee, 991) suggest that he Mendocino fault follows the Mattole River Canyon and eventually joinsthe Cooskie hear Zone as proposed by Clarke(1992). Therefore, two of the three plate boundaries that form the triple junction come onshornear two previously recognized on-land shear zones.the Cooskie and Petrolia hear zones.

32 CALIFORNIA GEOLOGY MARCH/APRIL 1992


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During field tudies. it became clear that theamount of damage and the peak intensity werehigher than initial estimates. The Humboldt CountyOffice of Emergency Services (DES) has two volunteers (in Petrolia and Shelter Cove) to cover about965 square miles (2,500 km2) of very rugged terrainand poor road . The DES estimated county roadand bridge damage at 45,000 to 75,000, andprivate property damage at $50.000 (MikeMcGuire. Humboldt County DES. 1992. oral communication). We estimate the private sector damageto be several t ime higher than the DES estimatebecause many re ident did not report damage touninsured structure .

There are several ob tacles to the study of earthquake effects in remote areas. There i a tradition ofself reliance and many residents have strong feelingabout their privacy and tre passing. During the earlystages of the field study, one of u was rudelyalerted by gun hots fired overhead. We found itessential to respect the local population. to explainwhat we were doing and why. and to let the wordof-mouth network spread the information about ourtudy, A local radio station continually reported ourprogres , thu involving the residents and keepingtheir interest high. Mo t people were very cooperative and consequently we have a network to rely onafter future earthquakes2

In the 2 weeks following the earthquake. weconducted field investigation of the effects of trongmotion in the remote epicentral area. We wereinterested in studying thi earthquake because itsepicenter was on land and shallow. Beginning 2day after the earthquake. we mapped surface fea-tures and damaged tructures on 7-l/2-minutequadrangle . primarily along roads. Residents wereinterviewed and many invited us to look at featureson their property. The region covered by the mostdetailed field mapping i shown in Figure 3.

IS'

2Th,s nelwork enabled lIS 10 construCI a pr Ilmlnary ISOselsmalmap Within 2 weeks of thearea's damaging March 7. 1992 avanl

The ational Earthquake Information Service(NElS) a signed a peak inten ity of VII to the Honeydew area, ba d on postmaster survey re pon e ( EI .1991). However, effect ob erved during field work andinterview with residents convinced us that this intensity estimate was too low. To clarify the peak inten itie and regionalinten ity distribution, we found it necessary to change the Modified Mercalli scale to account for the lack of ma onry tructureand large tructures of any kind in the epicentral area. We puttogether an intensity questionnaire more suitable to a rural areaand surveyed more than 300 north coa t residents.DamageStructural damage was concentrated in a 3-mile (5-km)radiu zone centered near the town of Honeydew. Houses inadequately attached 0 foundations, and mobile homes on pos

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THE AUGUST 17thHONEYDEW EARTHQUAKE

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-10Figure 2. Simplified geologic sketch map of the epicentral region of theHoneydew earthquake (after McLaughlin and others. in press; Clarke.1992). Towns: P = Petrolia; H =Honeydew; SC =Shelter Cove. Geologicfeatures: KRT = King Range Terrane; PSZ = Petrolia shear zone;CKSZ =Cooskie shear zone; RF =Russ Fault; MF = Mendocino fault;CSZ = Cascadia subduction zone boundary. Geographic features:CM =Cape Mendocino; PG = Punta Gorda; PD =Point Delgada;CP = Cooskie Peak; KP = King Peak. Stars mark the epicenters of theJanuary 16, 1990, August 17, 1991. and March 7, 1992 earthquakes. Theregion of oriented surface cracks and the projected surface posItion of thefault plane are also shown. The brown rectangle is enlarged in Figure 3.

The Honeydew earthquake wa felt as far away as SanFrancisco, 185 miles (300 km) to the outh. It producedstrong ground-shaking (intensity IV or greater) over an area ofnearly 3. 60 square miles (10,000 km2). Residents of the epicentral area reported 6 to 10 seconds of violent ground motion that was trong enough to cau e ome people to fall tothe ground. Debris was shaken from both live and dead trees.Rockfalls were common and landslides were reactivated over alarge region. As the shaking ub ided. dense dust cloud rose.giving some observers the impression of a major forest fire.prompting several calls to the California Division of ore try(Photo 1).

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CALIFORNIA GEOLOGY MARCH/APRIL 1992 33


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Figure 3. Simplified field map of the epicentral region. Mapped surface effects: LS =reactivated landslides; DB =displaced boulders;SB = sandblows; OC = oriented cracks. Viewpoints of photos 2-7in this article are also marked.

Oriented Surface CracksA linear zone of surface cracks was observed along Wilder

Ridge Road about 4 miles (6 km) south of Honeydew. Individualcracks were typically 30 to 65 feet (10 to 20 m) long. Theyoccurred along the ridge and on the ridge flanks (Figure 3).

Honeydew and Petrolia areas have been affected by at least 14intensity VI or greater events and five intensity VII events in thepast 40 years. Chimneys have been reported damaged in thePetrolia area 16 times since the late 1800s (see Dengler andothers, this issue). As a precaution, many residents have replaced their masonry chimneys with metal stovepipe, whichwithstands ground shaking much more effectively.Roadbed slumping resulted in tensional cracks that cut

Mattole Road in several places on a 3-mile (5-km) stretch justwest of Honeydew. Similar failures occurred southeast ofHoneydew along 2 miles (3 km) of Wilder Ridge Road. The ettling caused 3-inch- (7- to 8-cm-) high escarpments to formacross the approaches to Bear Trap and Honeydew Creekbridges. A landslide on the north side of Honeydew CreekBridge covered Wilder Ridge Road with over 600 cubic yards(460 m ~ of debris, blocking traffic the day of the earthquake.South of the landslide, numerous tensional cracks and slumpingfeatures were obvious for 3.7 miles (6 km) along Wilder RidgeRoad.

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and cinder block foundations had the most damage. We interviewed, or received survey responses from, 53 residents in theHoneydew, Wilder Ridge Panther Gap, and Petrolia areas. Ofthese, 33 reported structural damage, some as far away asEttersburg, about 10 miles (16 km) to the southeast. At least adozen structures in the Panther Gap, Wilder Ridge nd Hon-eydew areas were jarred from their foundations, resulting inconsiderable structural damage. One mobile home, resting oncinder blocks, had been strapped to the ground with tautcables but was damaged when the cinder blocks disintegrated.Well constructed homes with shear bracing and proper boltingsuffered little or no structural damage. In the region of strongest shaking, nearly all unattached objects were thrown fromshelves and table tops.Al least 15 chimneys were down or damaged in the

Petrolia-Honeydew area. lncidence of chimney damage waslower, however, than one might have expected from observations of damage by previous earthquakes. Residents of the

Photo 2. Oriented tensional crack along Wilder Ridge Road, 4miles (6.4 km) south of Honeydew. Crack orientation is approxImately N 30Q W, with the downslope side having moved upforming an east-facing scarp. Photos by Robert C. McPhersonunless otherwise noled.

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LiquefactionLiquefaction of loosely consolidated water-laden sediments

can result in surface ejection of water and sand along tensionalcracks. This type of surface response to vibration occurred in

than smaller ones were dislodged. Several boulders werenipped, with some displaced 10 inches (25 em). Similarly, logsnear the cracks in the streambed were displaced and rotatedto the west. Disrupted boulders were also found near tensionalcracks in Bear Trap Creek. the next drainage to the northwest.The narrowness of the zone of disrupted stream-channel boulders suggested that strong ground-motions had been focused inthis region.

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Photo 4. Displaced stream cobble in Honeydew Creek, Thiscobble has been displaced 10 the west.

the Mattole River and Honeydew Creek channels where. moments after the shaking subsided, the water became turbid.This initial muddying was caused by liquefaction of the riverbedsediments. not from land sliding into the stream from the surrounding hills (although landslides provided a subsequent muddying of the flow). Sand and turbid water were ejected fromtensional cracks along the dry sections of the river channel.Some vents were marked by a group of cobbles surroundedby f ine sand. with a periphery of finer. brown-stained material(Photo 6).Intensity SurveyAn isoseismal map (Figure 5) of the Honeydew earthquake

was constructed from more than 300 written sUlVeys. telephone intelViews, and field reconnaissance. The map shows asomewhat elongated area of intensity VIll shaking. trendingnorth-northwest. The western boundary of the zone is uncertain because it coincides with the edge of the King RangeNational ConselVation Area. an unpopulated. roodless regionfor which we have no detailed information on ground shaking.The boundary between intensities V and VI is uncertain because of a similar lack of data.The VlIlzone encompasses the region of mapped surfacecracking. Within this zone. more than 50 percent of the surveyrespondents reported some structural damage, Our decision toassign an VI11 value was also supported by the statements ofseveral residents who described this event as stronger than anyother they had experienced in their 40 or more years in the

area. The size of the intensity Vlll zone is comparable to theone mapped for the magnitude 5.9 Whittier Narrows earthquake of October 2. 1987. The Whittier Narrows earthquakealso had a similar depth and focal mechanism and its intensityVIII zone is likewise located up-dip Irom the hypocenter(Hauksson and others. 1988).

Figure 5 also shows regions of mapped broken t r " " ' _ ~ . rockfall. landslides. and reported changes in water flow describedabove. The arrows on the map show the direction of groundmotion inferred from the motion of objects during the earthquake. The single-pointed arrows denote directions detenninedfrom hea\oY ob;ect displacement (Photo 7). For example. if awood stove was found west of its usual place. the arrow pointseast because the ground and foundation under the stationarystove moved east. The double-pointed arrows show the direction that hanging or shelved objects moved. The intensity Vlllzone shows a consistent pattern of ground-east motion. Else-where the pallern is less clear. Both north-south and east-westmotions were reported in the Petrolia and Point Delgada areas.Motion near Garberville was predominantly north-south,

RelATION OF FAULTINGAND SURFACE PHENOMENAThe most striking feature of the Honeydew earthquake wasthe concentration of structural damage. landslides. gravityslumping. rockfalls. and sandblows in a narrow zone 6 miles

(10 km) east of the epicenter (Photo 6), Within this zone. tensional cracks lonned along a narrow northlA'st trend. and werefollowed lor approximately 4 miles (6 km). The first motionsrecorded by seismographs suggest that slip occurred at deplhalong a northwest-trending reverse fault. having either a northeast or southwest dip of 45 degrees (Oppenheimer and Magee.1991). Since the area has soulhwest-dipping reverse faults(Mclaughlin and others. 1988). the southwest dip is the moreplausible.

The pattern of strong ground-motion effects can be ex-plained in two ways. Both explanations involve reverse slip ona fault that does not reach the surface. The concentration ofeffects near Honeydewmay be due to the focusing 01 strong

Photo 5. Displaced stream cobble in Honeydew Creek. The baseof this cobble had been imbedded in the streambed before beingdisplaced to the west.

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ACKNOWLEDGMENTS

theast of

Wheth r Ih bdu I focusing, n

ARYFission tracks: The palhs 0 radiation damage made by nuclear particles In a mineralor glass by the sponlaneous fission of uranaum 238 Impurities FiSSion tracks h vebeen used to determine ages from 20 years 10 14 X 109 years.Sandblow: A sand deposilleft by Ihe ejection of sand and waler during vibrationIndUCed compaction of salurated sedlm nts.MagnitUde: The measure ot the strength of anearthqu ke, or the strain en rgyreleased by il. usually express d by the RIChter magmtude seal Each whole numberstep of magnilude on Ihe scale represents a I n-Iold Increase In lh size of lh waveson a seismogram and about a 31 fold Increase In nergy rei ase Magnatudesdet rmlned within about 400 miles (600 km) of an eptcenter are local magnitudes.Surface-wave and bodywave magmtud s are measur d from seismograms recordedfarther away Energy m natudes are d t rmlned from f ull dimenSions, dl placem ntand rlgldlly These four magnitudes are usually Similarfor Ih same ev nt

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1 11.) Thl e planation. h \ ver, doesnot e i1y explain the narrown of thezon of disruption, nor does It pr vid amech n t m for the cracks which crotwo ridg and two tr m channel inthe vicinity of Bear Trap r k.A more pi u ibl scenario 1 that asth hp propag ted up-dip, it c used theblock f ind cont ining Co o ki P

to udd nly move up nd to the ea t.motion con !stent with Ihe region u lifthistory. Th no e of the h nging wallfail in a tension I mann r producing thenorthwest-tr nding tensional era (Figur ). In this mod I, the proj Ii n ofthe f It would in t cl the surfac I

Photo 7 Displaced concrete ~ In vlclnaty of Honeyd w Lid has movtan .



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Raben McPhe:non was the chief_,ooIogost ro. !heH_ BaySetsmic a 1 6 ~ arrayoperated by TERA Cofporaoon aspart 01 a set5ITUC safety study for thet-Unboktt Nuclear POWI!I" Plant Hecunently 'ectures at Hlmtddt StatelJnMrsIty. Arcata and \IoOfks


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Sources of North Coast SeismicityL. DENGLER, G. CARVER AND A. McPHERSON. GeologistsHumboldt State UOIverSlty. Arcata. California

INTRODUCTIOT he vicinity of pe Mendocino i -one of Califomi . most sei mlC lIyclive areas (Real nd th r , 19 2,T ppozada and oth 19 ) TheM ndocino triple jun i n I lh g ologi-cally dyn mic area wh r lh t end ofthe endocino fault m th uth endof th dia subduction lone nd thnorth m xt n i n of the n An rfaull. Th f uI d fine the bound riebetween th rda. Pacific. and orthAmerican tectOnl pI t (Figure 1).oastaJ Humboldt ounty has beenaffected by earthquak on the SanAndrea fault system. on th M ndocinofull. nd in both the rd nd orthAm ri n plates. At lea t f thearthqu k that have occurred mce themid-1800 have produced d magrrab! 1). Ren t tudies mdi t thallhCascadia ubduction zone puts north mcoo tal California t ri k also 0 leatonand Hartzell. 1 7). Although the subduction zone has not g n rated greathi tone earthquak . p a l o ' mic evidence suggests that it ' th urce ofI rg rthquakes in th r e n t prehi -tori pa (Clarke and Carver. I 2)

GEOLOGY A 0 TECTONIC SETTINGTh pr nt I lion of theendocino tripl junction i estimatedto be about 15 mile (25 kIn) southof Cape Mendocino ( I rke. 1992;Mclaughlin and oth . in pr ). Southof the triple junction, th n Andrea

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Figure 1 S,mplihed map of northwestern California regional tectOniCS To the soulh 01the Mendocino tnple JunctIon (MTJ). the San Andreas tault system (SAF) is the transform(stnke-shp) boundary between the Paclhc and North Amencan plates. North of CapeMendocino (CM) Ih Juan de Fuca and Gorda plates areconverging WIth the NorthAmerican pial along th Cascadia Subducllon zone West of Cape M ndocino. theMendocino fault (MF) IS the transform boundary between the Pacllic plate and the Gordaplate. White arrows d nole plate motion relallve to North Amenca; black arrows denoterelative plate mOllon at pi t boundaries. Th Inset IS a simplified cross seclton 01 thesouthern Gorda plate being subdueted beneath th North American plate In northernCallfomla

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TABLE. ,. NORTH COAST HISTORIC EARTHQUAKE.S OF MAGNITUDE 5.5 AND/OR I N T E N S I T Y ~ VI.,"",von Dale M MMI La!. eN Long. CW Reference

Eureka area (whar f sank 4 'eet. or 12 m) 1012311853 (5.7) VII 40.80 124.20 Toppazada and others, 1981Eureka (affected llow 01 streams, milk thrown Irom pans) 312011855 VI Townley and Allen, 1939Eu""'" 6/14/1657 VI Townley and Allen, 1939Humboldl Bay (damage to plaster and chimneys) 1111311860 (5.7) VII 40.80 124.20 Toppazada and oth8fS, 1981Eureka (damaged brick houses, fissure nearFort Humboldt) 1011/1865 (5.4-5.7) VIIVIII '"'.80 124.20 Toppazada and others, 1981Petrolia (chimneys damaged in RohnefVille and Petrolia) 31211871 (5.9) Vltl '"'.'"' 124.20 Toppazada and others, 1981Crescent City (damaged most buildings InCrescent City, landslides) 11/23/1873 (6.7) VIII 42.00 124.00 Toppazada and others, 1981Southeasl of Eureka 9/3011875 (5.8) VII 40.70 124.00 Toppazada and others, 1981West of Briceland (knocked down all chimneysIn Petrolia) 51911878 (5.8) VIII 40.10 124.00 Toppazada and others, 1981North 01 Hoopa 112811884 (5.7) V 41.10 123.60 Toppazada and others, 1981Petrol,a (chimneys down in Ferndale and Petrolia) 712611890 (6.0) VII 40.33 12425 Toppazada and others, 1981Near Miranda (chimneys down in southernHumbold1 County) 9130f1894 (5.&5.8) VII '"'.30 123.70 Toppazada and others , 1981OIIshore Arcata (damaged mif1ln Eureka) 4/1611899 (6.4) V, 41,00 124.40 Toppazada and others. 1981OIIshore Eureka 121911903 V-VI 40.80 124.20 Woodward Clyde, 1980'san Francisco (ruplure began near San Francisco 4f1811906 6.3 X, 37.70 122.50 Real and others, 1982and extended north possibly to Shelter Cove, VII-IX In southern Humboldt Co. and Humboldt Bay regiondamage throughout Humboldt Bay region'McKinleyvllle (chimneys lell in Ferndale) 4/2311906 VII 41.00 124.00 Real and others, 1982OIIshore Eureka 8111f1907 V, '"'.80 12420 Toppazada and othef}l, 1978'Eureka (chImneys lell) 8118/1908 5." VII 40.83 124.17 Simila,198O'Petrolia (damaged chimneys) 511811909 VIII 40.25 124.17 Simila,198O'West of Scotia (much damage In Rohnervitle&Upper Manole) 10/29/1909 '.4 VlIf '"'.58 124.17 Simila, 1980OIIshore Petrolia 311911910 6 V 40.83 124.17 Boll and Miller, 1975OIIshore Cape Mendocino 1213"'915 6.2-6.5 "' 41.00 126.00 Bolt and Mlller, 1975'West 0' Ferndale 71511916 '. 5 V, 40.58 124.25 Bolt and Miller, 1975OIIshore Arcata 7/1511918 6.06.5 V, 41.00 125.00 Bolt and Mitrer, 1975Eureka (chimneys 'eU) 9fl5/1919 V, 40.80 124.20 Simila, 1980Offshore Cape Mendocino lf26J1922 6." , 41.00 126.00 Real and others, 1982-Ollshore 37 miles (60 km) west 01 Arcata (felt InSan Jose, California, and Oregon and Nevada) 1131/1922 7.3-7.6 V, 40.87 125.35 Smith and Knapp. 1980'OI1sl1ore Cape Mendocino (buildings damagedin Petrolia area) 112211923 6.5-7.3 VIII '"'.30 124.50 Smith and Knapp. 1980OIIsl1ora. west of Orick 6J4f1925 6." , 41.50 125.00 Bolt and Miller, 197580 miles (13O km) wes! of Eureka 1211011926 6." ,.. 40_75 126.00 Boll. and Miller, 1975'OIIshore of Arcata (chimneys lell, landslides) 8/2011927 5." VIII 41.00 124.60 Bolt and Miller, 1975

'Eureka (chimneys leU) 912311930 5.0-5.5 VII '"'.80 124.20 Bolt and Millar, 1975OIIshore Cape Mendocino 12111/1930 5." V, 40.08 124.50 Bo lt and Miller, 1975OIIshore Cape Mendocino 3/10/1931 56 V 40.00 t25.00 Rea! and others. 1982OIIshore Cape Mendocino 8/2311931 5.3 VI ,",.20 125.60 Real and others, t982OIIshore Eureka (chimneys damaged) 91911931 5.8 V, '"'.80 125.00 Real and others, t982112 miles (1 80 km) west 01 Cape MendOCIno 31211932 5.6 ,.. 40.20 127.00 Woodward Clyde. t980'Near Arcata (one death, much damage 10 Eureka) 61611932 5.9-6.4 VUI 40.87 124.02 Smith and Knapp. 198056 miles (90 km) west 01 Trinidad 71611934 6.5 ,.. 41.25 125.42 Smith and Knapp, 1980Offshore Cape Mendocino 11211935 5.8 V ,",.25 125.25 Real and others. 198293 miles (150 km) west 01 Cape Mendocino 6J3f1936 5.8 V 40.16 126.45 Woodward Clyde. 1980OfIshore Cape Mendodno (slight damage) 21611937 5.75.8 V '"'... 125.25 Collman & von Hake. 1973'Southeast 01 Cape Mendocino (slight damagein Femdale) 911111938 5.5 V, '"'.00 124.00 Real and others, 1982'OIIshore Cape Mendocino (chimney damage) 11119/1940 5.5 V-VI 40.75 124.90 Bef1(eley Seism. Stn.Near Shaner Cove 12/2011940 5.5 V, '"'.00 124.00 Real and others. 1982'OfIshore nor1hwest 01 cape Mendocino 21911941 6.4-6.6 V, 40.70 125.40 Real and others, 1982OfIshore Cape Mendocino 511311941 6." V 40.30 126.40 Berkeley Seism. Stn.'OIIshore nor1hwesl 01 Cape Mendocino(cl1imneys damaged) 101311941 6.' VII '"'.54 125.00 Smith and Knapp, 1980 Ro::ttIer__ (1ftIgn\Ucles in ..__ .. . _l l lClJ Dal.l from _______ used . . makII F.gwe5 map~ Mo:6!iedMetcaI: Mo


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TABLE' (continued). NORTH COAST HISTORIC EARTHOUAKES OF MAGNITUDE 2 5.5 AND/OR INTENSITY 2 VI.location Dale M MMI La\. oN Ilong. oW Reference

Offshofe Cape Mendocino 5119/1945 62 V 40.60 126.40 Coffman & von Hake, 1973'Offshore Cape Mendocino (damage in Ferndaleand Capetown) 1211811946 4.7 VI 40.30 124.60 Woodward Clyde, 1980Offshore Cape Mendocino 313011947 46 VI 40.38 124.68 Woodward Clyde, 1980Offshore Cape Mendocino 5/2711947 5.2 VI 40.40 124.70 Woodward Clyde, 1980'Offshore Cape Mendocino 912311947 5.6 VII 40.40 125.20 Coffman & von Hake, 197393 miles (1SO km) west 01 Orick 3124/1949 5. ' II-III 41.30 126.00 Real and others, 1982'Otfshofe Cape Mendocino 1I14119SO 4.6 VI 40.22 124.42 Woodward Clyde. 1980'PetroUa 3/1011951 4.1 VI 40.30 124.30 Wooctwatd Clyde, 1980'Otlshore Cape Mendocino 10 miles (16 km) west ofPetrolia 101BJ1951 5.8-6.0 VII 40.35 124.60 Smith and Knapp, 1980'Near Scotia (minordamage) 11/1411951 4.' VI 40.43 124.05 Woodward Clyde, 198012 miles (20 km) southwest 01 Petrolia (chimneys lell,foundation damage) 912211952 5.25.4 VII 40.20 124.42 Real and others, 1982Otlshore Cape Mendocino 1112511954 6.1-6.3 V 40.48 125.46 Smith and Knapp, 198012miles (20 km) northeast 01 Arcala ($2,100,000damage, one dealh) 12121/1954 6.56.6 VtII 40.85 123.96 TERA. 1977Offshore nonhwest 01 Cape MendOCIno (slightdamage in Ferndale) 10/1111956 6.0 V 40.67 125.77 Real and others.. 1982'Near Petrolia 5/2411958 48-4.9 VI 40.30 124.02 Berkeley 5eism. Stn.Otlshore, 56 miles (90 kmJ west of Tnmdad 7( 2311959 5.8 IV 41.13 125.30 Real and others, 1982'Offshore Arcata (plaster leU at EurekaCity HaU) 61511960 5.7 VI 40.87 124.50 Smilh and Knapp, 1980Otlshore Cape Mendocino (telt in san Franciscoand southern Oregon) 81911960 6.06.2 V 40.32 127.07 Berkeley Seism. Stn.Offshore 11 miles (18 km) wes1 01 Cape Mendocino(triggered landslide) 4J6f1961 5.05.5 VI 40.49 124.81 Smith and Knapp, 1980'Oflshore Crescent City (slight damage) 8123/1962 5.6 VI 41.84 124.39 Nowroozi, 1973Offshore I'lOf1hweSt of Arcata (sllghl damaoe) 9(411962 4.95.0 VI 41.01 124.21 Nowroozi, 1973c:>tlshoi"e 12 miles (20 km) west 01 PetroNa(slight damage) 1211011967 5.65.8 VI 40.56 124.58 smith and Knapp, 1980'Offshore Cape Mendocino 612611968 5.5-5,9 VII 40.29 124.67 Nowroozi. 1973'Near Petrolia (slight damage in Pelrolla) ar9I1973 4.9-5.0 VI 4035 124.30 Smith and Knapp, 1980'Forluna 61711975 5.2-5.7 VII 40.51 124.27 Smith and Knapp, 1980Offshore 93 miles (150 kml not1hweSI of Eureka 11/2611976 6.' IV 41.30 125.70 Berkeley Selsm. Stn.'Offshore 16 miles (25 km) west ot Arcata (pipesand Windows broken In Eureka) 21311979 5.2 VIVlI 40.92 124.42 Berkeley Seism. Stn.'30 miles (48 km) west of Trinidad (bridge collapsed,Sill injured, $1,750,000 damage) 111811980 6.9-74 VII 41.12 124.67 Berkeley 5eism. SIn.Offshore Cape Mendocino 8124(1983 5.5 V 40.38 124.83 Berkeley 5eism. Stn.Oflshore 155 miles (250 km) west of Eureka (Ielt fromsan Francisco to Oregon) 911011984 6.6 V 40.50 126.83 NElSJust offshore Cape Mendocino (two evenlS in1minute, slight damage) 11121/1986 5.1.5.1 VI 40.37 124.44 Berkeley 5eism. Stn.'Just offshore Cape Mendocino (damage androckslides in Peltolia) 7131/1987 5.5 VII 40.42 124.41 NElSNear Honeydew (caused damage In Honeydew andPetrolia) 1/1611990 5 .' VII' . 40.23 124.14 Berkeley Seism. Stn,Offshore 50 miles (80 km) weslnorlhwest of CrescentCny 7/1311991 6.76.9 V 42.14 125.61 NElS

Oflshore 62 miles (100 km) west 01 Crescent City 811611991 5.96.3 V 41.73 125.39 NElSNear Honeydew (chimney, loundation damage.landslides, well changes) 811711991 6.06.2 VIII"' 40.21 124.28 NElSOffshore 62 miles (100 km) west of Crescent City 8117/1991 6.9-7.1 V 41.61 125.51 NElSSouth ol Petrolia (foundation damage, landslides) 317/1992 5.3-5.6 VW' 40.23 124.29 Berkeley Seism. Stn.3 miles (5 km) north of Petrolia (preliminary estimate- 4/2511992 7.0 ~ V I I t " 40.37 124.31 NElSS48 mUlion in damage, 35 6 injuries, lSunami, coastaluplif1, liquefaction, landslides)20 miles (33 km) westnorthwest 01 Petrolia (fire 412611992 6.0 VII" 40.44 124.58 NElSdestroyed SCotia Shopping Center)15 miles (25 km) west 01 Petrolia (additional damage) 412611992 6.5 VIIVIII" 40.40 124.56 NElS" ~ ~ ~ , " p w e n ~ a l e - . . . . . d l ' [MI;I ~ o __ _8 US


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Figure 2. Sketch map oltaults and folds in the viciOlly ot Humboldt Bay.MRFZ _ Mad River tault zone: MKF McKinleyville fault; MRF Mad River !aull:lSF _ lllt te Salmon !aull. Aher Clarke and Carver. 1992.

floor, and aftershock sequences of recentearthquakes showa pattern of left-lateralstrike-slip faulting within the offshoreportion of the Gorda plate (Silver. 1971:Smith and olhers. 1981; McPherson.1989a: Eaton, 1989). The faulls trendnortheast to e a s t ~ n o r t h e a s t and reflect aclockwise rotation of the southern porlion of the plate (McPherson, 1989a:Eaton. 1989). The pattern of deformation has been interpreted as a response10 north-south compression created bythe northward movement of the Pacificplate and the southeast motion of theJuan de Fuca plate (Spence. 1989:McPherson, 1989a).

,...

MRF

CI:.m Beao:.:h

,",,

N

t

C,'"McndlX;1l1O

'- thruslfaultX axis of synctine

axis 01 antICline

,I,

The Mendocino fault extends due westfrom Cape Mendocino. forming thetransfonn boundary between the Gordaand Pacific plates. This fault appears tobe nearly vertical and motion is primarilyTight-lateral strike-slip (McPherson.1989a; Eaton 1989: Wilson, 1986;Jachens and Griscom. 1983). However,reverse movement has been recorded inearthquakes in the compressional zoneclose to the triple junction (Nowroozi.1973: Simila. 1980; McPherson, 1989a).The occurrence of earthquakes withinthe Gorda plate indicates internal d e f o r ~

malion. Faulted sediments on the sea

fault syslem foons the lransform (strikeslip) boundary between lhe Pacific andNorth American plates. ln e Pacific plaleis moving north relative to the NorthAmerkan plate. resulting in strain accu'mulation and episodic ruptures such asthe 1906 San Francisco earthquake.Allhough the nalure of the northern termination of the San Andreas fault is unclear. there is agreement that it does notextend beyond Cape Mendocino (Smithand Knapp. 1980: UsowskJ andPrescott. 1989: Mclaughlin and others.in press: Clarke. 1992).

TIle Cascadia subduclion zone. northof lhe triple junction. is the convergentboundary between the Gorda and NorthAmerican plates (Atwater, 1970:Engebretson and others. 1985). A schemalic section across the southern portionof lhe Gorda plate and the subductionzone is shown in Agure 1. Subsurfaceearthquake locations trace the subdueledportion of the Gorda plate (Smith andKnapp. 1980: McPherson 1989a;Waller. 1986). The Gorda plate dips easlbeneath the North American plate at anangle of 10 to 15 degrees (Smith andKnapp. 1980: McPherson, 1989a).About 60 miles (100 km) east of thecoaslal margin. al a depth of 25 to 30miles (40 to 50 km). the dip steepens to30-45 degrees (McPherson and others,1981; Cockerham. 1984: Walter.1986). The deepest earthquakes (50miles or 80 km below lhe surface) associated vAth the subducted plate are east ofRedding (Walter. 1986). The active volcanos of the Cascade range are about 30miles (50 km) farther east where the sub-dueted plate has likely reached depths ofover 60 miles (100 km).

The collision betlAleen the NorthAmerican and Gorda plates has defonned the leading edge of the NorthAmerican plate (Carver, 1987). as seenby north-northwest trending folds andthrust faults onshore and offshore (Aek!and others. 1980: Clarke and Carver,1992). At least nine major thrusts and asmany folds defonn the young sedimentsalong California's north coast (Carver.1987) (Agure 2). Offsets of the FalorFormation (Manning and Ogle, 1950)along several of these faults indicate displacement of half a mile (a kilometer) ormore within the last mill ion years(Carver, 1987).

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HISTORIC SEISMICITYPattern 01 EarthquakesTabk 1 summarizes regional historic

e a ~ k e s . f lO( Including aftershocks.WIth R)(:hter magniludes" of 5.5 orlarger. and/or Modified Merca1li intensi1M5 (MMl) of Vl or IargeT MMI valuesare a measure of the degree of damageat a particular IocaIIOl1 and are based onpeT5OI'\aI ac:counlS and damage reports_1he Modlf.ed Mercalh IntenSity Scale(VJood and Neumann, 1931; CAUFOR-IA GEOLOGY, 1991) ranges from I(generally no t fehlto XII {total damagewilh viSibie ground surface wavesl. 1hethreshold of damage is typK::aIIy Vl characterized by faUing or displacedobjects and some plaster and chimneydamage. 1he intensity recorded at a par'ticular site is affected by the magnitudeof the earthquake. type of faulting. dis-lanCe of the site from the epicenter. andthe local geology. For example. northcoast communities wiD often sustainmore ground shaking and a higher MMIvalue from smaller earthquakes centerednear them than from large distant offshore earthquakes,Intensities listed in Table 1 are thepeak values reported and most havebeen transcribed from published earth

quake catalogs (Coffman and von Hake,1973; the University of California IU.C.JSeismographic Stations at Berkeley Bul-letins; the National Earthquake Information Service Detennination of Preliminary Epicenters; and WoodwardClyde.1980). Intensities for earthquakesmarked with a double asterisk ("") havebeen estimated using our examinatlon ofpersonal reports and the newspapersThe Humboldt Times. The HumboldtStandard. The Arcata Union. The DelNorte Triplicate. and the Susie BakerFoundaHon Papers (Fountain. 1%7)."Magn!lude .. thf.-. 01 rt. 01 .. .~ or th f ....., enerw'-'-' by II, . . , -.. e < by thf Roc:hMr [ a d ,"*'ole IU'I'lber*p 01 . . . . . . . . . on Ih f .epre'Ift1ls " wn-6oId _ lro Ih f .,plitude 01 th f_01"1" _.iO!I_ .. ..nd.toA ,,3t 'ok!

. . energy ~ d e l e r - - n w l l l d. . . 400 milD (600 ""'" 01 .. . . . . Iggol~ s.rirra_..m llollor-1Udn . . . meB.nlI from W&'IOgi_.1I recordod "'"Ilwr......, E n t o w ~ .. .dele"." &omI.Iull d""Olls... cftstMoe". . . ..nd r , . I Iy n" - ~ ~ " " , , , , " ' b l h f _-

For early events, it is likely that reportswere not received from all areas and theintensity value listed is smaller than thetrue maximum. Also, we have found thaiIt is not easy to differentiate amongMMls VII. VIII. and IX In o;parsely settledrural areas with Vt100dframe buiklingsBecause of the lack of large structures,masonry construction. buned pipe lines.and othet" MMI ~ i n d i c a t o r s . II IS po5SIbkthat peak mtenslty estimates forearthquakes are inaccurate by one ortwo intenSity Ievels_Epicenter Iocat)ons ~ s t e d in Table I

are from the r e f e n ~ n c e s in the last colWTIn. A range of magmtudes giYen for aSingle event reflects different reportmgsources and/or different methods ofmeasuring magnitude. Toppozada andothers (198 I) detertnllled epicenter IocalionS for pre--l900 events from the pattern of reports of earthquakes feit, andsuggest there may be misklcatlOf\s of 60miles (1 ()() km) or more, To cstlJl'l3lemagnitudes, they compared the pallemsof old felt evenlS to those of more recentones with known magnitudes. Althoughlocation errors of early events are likelylarge. the record of damaging events after 1850 is probably fairly comp&etc Thenorth coast area has had a widespreaddlstribuhon of small Jogging and fishingtOUlllS throughout the last century andtwo to five nev.rspapers have been inprint at aU times since 1850 (foppozadaand others, 1981). Offshore events that\l.'ere not widely fclt may not have beenreported. but it is unlikely that earthquakes causing significant damage havebeen missed.

In 1932 the U.c. Seismographic 5tations at Berkeley installed the first northcoast seismograph in Ferndale_ (TheBosch-Omori seismograph, althoughno k>nger used by U.C. Berkeley, is stilloperating and on display at the FerndaleMuseum,) Many of the e v e n ~ prior to1932 were recorded by the Berkeleyand/or Mt Hanulton stations of theearly Berkeley nelwork (established in1887). Because these stations are so farsouth. insttument-delermined ep;centerlocations priOr to the eswblishment ofthe FemdaIe station may have uocertamlIeS of 60 miles (1()() Ian) or more(Urhammer, RA, 1991, Uc. Berkeley.peT5OI'\aI communicatlOl1)_ Event locationimprc:llJetf gradually throughout the

middle part of the century as morc In struments were installed. U.c. Berkeleyestablished the Arcata station in 1948and the FkkJe Hill statoo (aboul 12miles or 20 km eaS! of Eurekal in 1968Because of the thin distnbuhon of statlOOS and the complexities of local geoIogy, It is likely that locatiOns of even on-shore or near-shore earthquakesoccumng as recently as the late 19505may be in error by as much as 30 miles(SO kmllKnapp. 1982)

In mid-1974. a IfHtation anay ofseismographs was inscaBed in the northc regJOn and operated by TERA CorporalJOn as part of a seismic safety srudyfor Pacific Gas and Electric Company'sHumboldt Bay nuclear p ( ) ' , ~ I ' e r planttTERA CorporatJon. 1975; WoodwardClyde, 1980; McPherson and others.1981. McPherson. 1989al_ Earthquakesrecorded dunng the arrayOs 12-year operatIOn are the best located of the hislone dald set After the network was dbbanded. two of the stationsweredonated to the Berkeley network and arepart of the U_c. Seismographic S1atlOOS,In response to the damaging November1980 earthquake. the U.S. GeologicalSurvey established a network oftelemetered instruments in the region.which conllnue to operate (Eaton.1989). Recent onshore and near-offshorc epicenters are likely to be accurately located, but locating evenls far offshore continues to be a problem duc tothe region's lack of oceanbotlom seismographs.Smith and Knapp (1980) published anumber of relocations of the older earthquakes. lhey used the Joinl EpkentcrDetennination technique. comparing the

seismk traces 01 a well-located recentevent (recorded by new and old seismicstations) 10 older events and slmultaneousty solving for a set ofepicenter lo-caliOns. origin times. and slation corrections of the older events relative 10 therecent event 1llese relocated epk:entersprobably represent an i m p r ~ estimate01 the true Iocalion of the earthquakesource.

Epicenters of the best-located histoneearthquakes (those after 1960. andSouth and Knapp's rekx:ations) areshown in PlgUTe 3. 1h e mapnty arescattered throughout the southeastern

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Figure 3. EJ)tCenlers and dales of best located north coast historic earthquakes of magnitude~ and/or intensity 2:VL

1906 san Francisco EanhquakeThe 1906 San Francisco earthquakedeserves special attention because itcaused the greatest damage in history ofthe north coast region and because thefault rupture was south of Cape

Mendocino. Rupture extended fromsouth of the San Francisco Bay areanorthward along the coast perhaps asfar as Point Delgada near Shelter Cove(Lawson. 1908; Prentice. 1989).Lawson (1908) noted surface rupture(interpreted as fault displacement) inShelter C o ~ . duration of strong groundmollon in excess of 40 seconds throughout the Humboldt Bay region. and ext e n s i ~ damage 10 communities in the

The April 25. 1992 Cape Mendocinoearlhquake is the only historic damagingevent which may h a resulted fromthrusting along the Cascadia subductionzone. Prior to this earthquake. researchers had found no c o n c 1 u s i ~ evidencewithin Ihe historic record to suggest thatsuch an event had occurred anywherealong the subduction zone boundaryfrom Cape Mendocino to Vancouver [s-land. British Columbia during the last150 years (Heaton and Kanamori.1984; Heaton and Hartzell. 1987).Preliminary data suggest that the CapeMendocino earthquake was the result ofr e ~ r s e slip along a shallow northeastdipping. northwest striking plane whichcoinckles with the estimated locationof the subduction zone boundary(Oppenheimer and others. 1992;Oppenheimer. 1992, U.5.G.5.. MenloPark. personal communication). Thisearthquake produced a tsunami recorded at tide stations in California, Oregon. and Washington (NOAA. 1992.personal communication) and preliminary investigations support coastal upliftfrom near Cape Mendocino 10 near themouth of the Mattole River. This earthquake lends strong support to theseismogenic nature of the subductionzone; however it was a much smallerearthquake than the main portion ofthe locked zone Is thought capable ofproducing (Clarke and Carver. 1992).

1974 and 1984 occurred at Gorda platedepths (McPherson. 1989a). stronglysuggesting Ihat the Gorda plate is themain source for both onshore and offshore historic seismicity.ORI,(;(I....-;r

124

II H' O K I ~

f O I ' f \ ~

1962125

immediate vicinity of the triple junction(see McPherson and Dengler this issue).The April 25. 1992 M,," 7.0 earthquake was located just north of the Au-gust 17th event and is discussed in moredetail below. Of the onshore earthquakes north of the triple junction. onlythe December 1954 earthquake isthought to be shallow enough to haveoccurred in the overriding North American plate (Tobin and Sykes. 1968;TERA Corporation. 1977; Smith andKnapp. 1980; Knapp. 1982). Hamilton(1975) suggested that this earthquakewas associaled with a fault in the MadR i ~ r fault zone. although lack of documented surface rupture and uncertaintyabout the type of faulting involved makeit impoSSible to confinn. The 14-mile(23-km) depth of the June 1975 earthquake (considered accurate to Within 0. 6mile [1 kml [TERA Corporation. 1977:Knapp. 19821) is clearly within the portion of the Gorda plate thaI has beensllbducted beneath the North Americanplate. Over 80 percent of well locatedmicroearthquakes recorded by lheHumboldt Bay seismic netVJOrk between

. , 9 3 41980

1922.,99,

.,99,

1991JZ6'

1976 N\o '0 20_I , ' , "o 10 20 3QK_

MAGNITUDE> 7 MAGNITUDE 6-7 MAGNITUDE < 6

12r

The epicenters of only eight earthquakes shown in Figure 3 fall within theonshore portion of the map. The January 1990. August 1991. and March 7.1992 earthquakes were centered in the

portion of the Gorda plate and along theMendocino fault. For earthquakes in thesouthern portion of Ihe Gorda plate(such as 1941. 1954. 1961. and 1984in F"lgure 3). it is difficult to establish theirsources as the Mendocino fault or theGorda plate. due to the imprecision inlocation methods. However. most off-shore earthquakes are clearly intraplateevents. Aftershock data and fault planesolutions suggest that the Gorda plateearthquakes result from movement alongnortheast trending left-lateral stnke-slipfaults (rERA Corporation. 1977: Smithand others. 1981. McPherson. 1989a).Mendocino fault earthquakes show predominantly right-lateral slip parallel tothe trend of the fault. but there havebeen a few reverse-slip events in thevicinity of Cape Mendocino (Bolt andothers. 1968: Simila and others. 1975;McPherson. 1989a).

,984 1954 . 1987~ - M ; ; ; ; o ; ; ; ; ; ; ; ; - ; ; ; ; ; ; ; ; - - - - ~ ~ ~ t - : : : : : s ; . ,19861-. Mendocino F3Uh1960-'0.5

41.5

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san Fr,lIll;:lsc;Q9 earlnquake7

~ 31 , ,"'SO '860 1870 '860 1890 '900 1910 1920 1930 19" 1950 1960 1970 1980 '990""..Figure 4. Frequency of hlstonc earthquakes prodUCing peak Modified MercaJIi IntenSitIes VI in Humboldl and/or Del Norte counlles.

Humboldt Bay area and in southernHumboldt County. The MMls were atleast VIII in Briceland. Eureka. andFortuna and may have reached IX inPetrolia and Ferndale rroppozada andParke. 1982). In Ferndale, not a chimney remained standing and brick buildings were badly damaged. Liquefactionfeatures were observed in the Eel RiverValley and around Humboldt Bay,Since the middle 180Ds. the northcoast region has experienced at least 60

earthquakes with peak MMls of VI. 28intensity VIis. 11 intensity Vilis (the epicentra! area intensity of the 1989 LornaPrieta earthquake) and possibly. one IX(Agure 4). Intensity VII or greater eventshave recurred on the average of every 5years. with 13 years being the longestintetval between these events. The recurrence of MMI VI or greater eventshas averaged 2-1/2 years, Most ofthese events likely originated within theGorda plate or along the Mendocinofault,Agure 5 is a composite intensity

map of the north coast area since1900. The map was constructed fromthe MMI data for the 34 earthquakesmarked by single asterisks in Table 1.For each olthese earthquakes. communities reporting VIs or greater weremarked on a map anclthe resulting p a t ~tern was contoured according to thenumber of times an area had experienced this level of intensity. Not surprisingly. communities in the coastal regionnear Cape Mendocino from Eureka tosouth of Petrolia have been damagedfar more frequently than the rest ofHumboldt or Del Norte County. Thefrequency decreases in roughly a radialpattern away from Cape Mendocino.

The northern part of Humboldt Countyand all of Del Norte County have rarelyben affected by earthquakes since1900.

PALEOSEISMICITYThe historic earthquake record in thenorth coast region covers fewer than

150 years. These recorded earthquakeshave occurred mainly offshore and few.if any. are associated with mappedfaults. However. the youthful characteristics of the faults mapped within theNorth American plate and recognition ofIhe potential of the Cascadia subductionrone require looking beyond the briefhistoric record and using paleoseismologic evidence to assess the seismicpotential of these sources.North AmerICan Plate FaultsSeveral studies have focused on a sys

tem of northwest trending thrust faultsthat intersect the north coast and extendoffshore (WoOOward-CIyde. 1980;Carver. 1987: Carver and Burke. 1988;Kelsey and Carver. 1988; Clarke andCarver. 1992; Pigure 2), Siudies of sediments exposed in trenches dug acrossfault traces suggest that these faults havegenerated many seismic events dUringthe paSt 10.000 years (Carver andBurke. 1988; Clarke and Carver. 1992).The three most studied faults are thelillie Salmon in the southern HumboldtBay area. and the McKinleyville and MadRiver of the Mad River fault zone northof Arcata (Figure 2),The Little Salmon fault has been themost active fault in the region during the

Holocene. INith a minimum of threeevents In the last 2.000 years. about300.800. and 1.700 years ago (Carver

and Burke. 1988), Each of these earthquakes resulted in more than 16 feetIS m) of slip along the fault. Based onlhe measured slip. the minimum size ofthese earthquakes is estimated to haveben between magnitudes 7.6 and 7.8.larger than any historic north coastearthquake (Clarke and Carver. 1992).The evidence of recent activity has ledto the designation of special studieszones for the Little Salmon fault andseveral segments in the Mad River faultzone. in accordance with the AlquistPriolo Geologic Hazard Zones Act of1972 (Hart, 1988).Cascadia SubductIon ZoneHeaton and Kanamori (1984) drew

national attention to the seismic potential of the Cascadia subduction zone bynoting similaritieswith other seismicallyactive subduction zones. Since then anumber of paleoselsmicity studies(Alwater, 1987: Grant and olhers.1989: Peterson and Darienzo. 1989;Adams. 1990: Clarke and Carver.1992) and recent geodetic data(Lisowski and Prescott. 1989: Savageand Lisowski. 1991) have strengthenedarguments that very large earthquakeshave occurred along this zone. and arelikely to recur_ The onshore system offolds and thrust faults in the HumboldtBay region clearly pins similar structures offshore (Clarke and Carver.1992) and is likely the southernmostmanifestation of the subduction zone_ Insimilar convergent boundary settingsthere is litde evidence that thrust faullslike these move independently of theirassociated subduction zones (Heatonand Hartzell. 1987). The emergingchronology of slip events on the LittleSalmon fault is similar to the timing of

.. CALIFORNIA GEOLOGY MARCH APRIL 1992


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125 00' 12430' \24 00' 12330',------L--------'----,,=-...L---T - , : - - ' - - - - - - , " 0 0 '

"-"- ' ,

SISKIYOUCOl.I:\IY

POINT ST G1':OH(;I,

Number ollJmes localJOn hase ~ p e r i e n c e d Modified Mercalhinlensity greater lhan or equalto VI since 1900.

15-2310143-9

Ga"'luct

U I ~ L 1\ORn:COL'NTY

/..\ ' . / Happy{ Camp((, ,\

\(, ,---_ ..41 30'

40'30'

".,

)(!"'"

Orlean ...Orick

,, . j' \,,/..r-.. .../:

\ Blue\ Lake ~ u " m O L J ) T ' ,ka \ \OUNTY:, \ I. TRINITYI \ 'I COUNTYI ,

n\it1e !a I i

I,I1I

Big Lagoon'triqidad

McKin1c.ly\ill

TNI.VIIMD 1I1.AI)

,',tTHleAS POINT

CAI'I::' MENDOc/.vO

0,3

20 MilesI I30 40 Kilometers

10

D

10 20, I

NIoIo

I

IIIGartJrvil le II :

POINT I)f.I.GADA IL --,- r-:====::....c....".......--''''-..L_----'....,.__ ---' ... . ,FIgure 5 Cumulative frequency 01 groond shakillg ill north coast oommUIli\leS. Commumties within the dar\(ast area have experienced alleast 15 earthquakes with ground shaking equlvalenllo or larger than VI on the Modified Mercalli scale. Dala used tocompile this map are from 34 earthquakes since 1900. denoted by smgle aSlefisks n ill Table 1.

CALIFORNIA GEOLOGY MARCH/APRIL 1992


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sudden submergence events in HumboldtBay and uplift events along Clam Beachsouth of Trinidad (Vick. 1988; Clarkeand Carver, 1992). This suggests thatslip measured on the thrust faults in theregion renects movement on theCascadia subduction zone.

There is no consensus as to whetherthe Cascadia subduction zone ruptures insegments or as a single giant event fromCape Mendocino to Vancouver Island(Rogers. 1988). A rupture of the southernportion 01 the zone could extend fromCape Mendocino to just nonh of the

Oregon border. corresponding to an earthquake of about magnitude 8.5 (Clarke andCarver. 1992). II a rupture of the entirezone occurred. the energy released couk!be comparable to that of the 1960 Chilean or 1964 Alaskan earthquake (energymagnitudes of 9.6 and 9.2 respectively).

TABLE 2. POTENTIAL SOURCE AREAS OF DAMAGING EARTHQUAKES.NAME LOCATION

AND STYLE OFFAULTING

MAGNITUDESOF DAMAGINGEARTHQUAKES

PEAKINTENSITIES

RECURRENCE

Gorda plateExample: 11/8/80(surface wavemagnitude 7.2)

primarily oflshore:strike-slip laulling5to 7,5(historic)

VII to VIII(historic) 5.5 years for mtensi\leS 2:VI1(combined Mendocino faultand Gorda plale sources)

SpaCial notes: accounts lo r mal0rlty Of historic seismIcity. most probabte source of damagam the near futule; pf lmary Impact to coas/at communities 01 Humboldt Coumy

MendOCinOfaultExample: June 25.1968 (localmagnitude 5.9)

oflshore west ofCape Mendocino:primarily strike-slipfaulting

5107.5?(historic) VII to VIII(historic)5.5 years. tor intensities;>:VII (combined Mendocinofaull and Gorda platesources)

Special notas: second most common soulce of historic ealthquakes. prlmarlty Impaclmgcommunities near Cape Mendocino and lhasa In tha Eel R,vel Valley and Humboldt Bay areas

San Andreastransform systemExample: Aprit 18.1906 (magnitude 8.3)

onshore and near-shore, south of triplejunction: primarily strike-slip5.0 \0 8.3(historic) VII to IX(historic) InsouthernHumboidtCounty

01 1906 San Andreas-typeearthquakes 200-400 yeals

Special notes: tess probable bur pOlentlally more damaging than Gorda ptate or Mendocmotault earthquakes: primary impact in southern Humboldt County and Humbotdt Bay aleas

North Americanplate onshore and shallow:primarily thrust-faulting 6.5 to 8+(expected) ;>:IX for thrustevents(expected)for thrust events. hundredsof years

no historic largethrust earthquakesSpec,at notes: tess probabta in near lurure than Gorda plate or Mendocmo fault earthquakes; much greater impact: pOSSible tsunam,

300 to 500 yearsin Humboldt andand Del Nortecounties: ;>:X(expected)

7.0? (historic)southern segment- 8.5whole zone - 9to 9.5(expected)

onshore am:! offshore:rupture length 125 miles(200 km) or more:thrustfaulting

Cascadia subductionzoneno historic great subduction zone earthquakes.April 25. 1992 M .. 7.0 mayhave involved slip at thesouthern end of the zone.Speaal notes: less probabie in near luture than Gorda plate or Menoocmo lautl eaf/hquakes. most damagmg 01all potentiat sources: hkely ro genelilta a tsunamI

" CALIFORNIA GEOLOGY MARCH/APRIL 1992


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'23'2'127 126 125r - - - ~ - - - _ - " ' : ~ - - , - - - ' : " " = " , " " " " , ~ - : - P T ~ - ' - ~ " "'2.

GOtda PtaleMendocino Fault

IIIII.' I

. : . ~ : ~ . : : : ' : : : : - San Andreas Transfonn Sysle' - INorth Amencan Ie I

~ , - - - ~ - = - - + - - - - l - - - f \ . - - + - - . . . , - j - - - i r - - - - l "cascadia SubductIOn ZOne 1III,,IOREGONY I ~ c - i . ; . ; : 7 ~ ~ ~ - j "CALIFORNIA

j Ir ~ c ~ n l Citr\)I,II

M - ~ . . . , I r - - - ; "

I0 100 200 300 '0 0 500 600 . . . . .I , , , ,I0 200


22/36

the t\lJO largest earthquakes of m


23/36

The 1991 HOfle!,.dew earthquakeinvolved reverse motion along a north\I.'e$! trending fault at the north end ofthiS system (Oppenheimer and Magee.1991: McPherson and others. 1991).The relationship among this fault. theSan Andreas fault and other strike-slipfaults in the transform system is notdear We believe this earthquake reflectscompression near the termination of theSan Andreas system and may be a partof this syslem. The Honeydew earthquake produced peak intensities of VlJI.a 3.6-mile-long (6-km-long) zone of surface cracking. widespread changes ingroundwater flow, and accelerationshigh enough to dislodge and flip slreamboulders (McPherson and others, 1992).Recurrence of this event, and the seismic potential of other faults in the transform system are not known.NorthAmencanPlate EarthquakesThe potential sources of these earthquakes are thrust faults WIthin the NorthAmerican plate north of the triple junc-tion. Epicenters IAIOUId be onshore andat depths of fewer than 12 miles (20

km). Primary evidence for this sourcearea is the zone of Holocene nonhwest-trending thrust faults extending fromsouth of Bridgeville to north of BigLagoon and offshore to north of theOregon border. There a re no verifiedhistoric examples of large thrust earthquakes along any of these faults.Evidence does not indicate whetheroffsets observed on these faults represent intraplate events restricted to theNorth American plate or ruptures ofthe Cascadia subduction zone. Only

one damaging event. the December 21 .1954. magnitude-6.5 earthquake,appears to have been located within theoverriding North American plate. However. the absence of documented surfacerupture and the lack of infonnationabout the earthquake source make itdiffk:ult to associate the 1954 event withany of the mapped faults in the epicen-tral region. This earthquake caused onedeath. much structural damage. andnumerous landslkies and rockfalls. and ittemporarily reversed water flow in theMad River tTERA Corporation, 1977).Recurrence of events like the 1954earthquake cannot be estimated withthe present data.

Paleoseismic CVJdence suggests thatseveral large thrust earthquakes occurredin the Holocene (Carver and Burke,1988) with recurrence intervals onthe order of hundreds of years. If theobserved offsets are the result of slipwithin the North American plate independent of the subduction zone. magnitudes of 7.5 to 8 or greater are sug-gested (Clarke and Carver. 1992). Thissize earthquake onshore could cause surface ground rupture and would producestronger ground shaking than any eventsin the ISO-year historic record. Movement along an offshore portion of one ofthese faults might trigger a local tsunami.Cascadia Subduclion Zone"Great" Earthquakes

These earthquakes 'NOU1d be causedby movement along all or part of theCascadia sulxluction zone between theGorda and/or Juan de Fuca plates andthe North American plate. Rupture ofthe southern portion of the zone mightextend from Cape Mendocino to northof the Oregon border; rupture of the"-hole zone coold extend to VancouverIsland. These earthquakes would havemagnitudes of at least 8.5. and couldproduce strong ground-motion lasting aminute or more. cause coastal uplift andsubsidence of several feet. and generatelarge local tsunamis affecting coastal areas of northern California. Oregon. andWashington. Distant tsunamis might alsobe generated, with the potential to affectother coastal regions of California andlarge areas of the Pacific basin. Eventsof this magnitude are rich in long-periodground motion (Heaton and Hartzell.1987) and have the potential to damagelarge structures hundreds of miles away.particularly when local soil and geologicoondiHons amplify the ground motion.PaJeoseismic ev\dence suggests thelast such earthquake occurred about

300 years ago and the recurrence interval of Cascadia subduction zone earthquakes is 300 to 500 years (Clarke andCarver, 19921. There is less probabilityof such an earthquake within the nearfuture than of ruptures within either theGorda plate or on the Mendocino fault.However. the increasing weight of scientific evidence supporting the recurrenceof such earthquakes. and the enormityof their effects make emergency planning and mitigation efforts essential atcounty, stale, and nalional levels.

ACKNOWLEDGMENTSThe predecessor of this paper was a

section of the Humboldt State UniversityEmergency Plan compiled by Kevin O'DeaSandra Potter compiled much of theintensity data for histolic earthquakesand constructed the composite isoseismalmap (F"tgure 5). She received supportfrom CALEEP (California EarthquakeEducation Project) and National SdenceFoundation Grant #8950 114 for undergraduate faculty teaching enhancement.Comments by Bud Burke and Tom Lisleon various stages of this manuscript werevery helpful.Preliminary information on the April

25. 1992 Cape Mendocino earthquakewas provided by the U.S.G.S. in MentoPark, the California Division of Mines andGeology's Strong Motion InstrumentationProgram (SMIP), the University of California at Berkeley Seismographic StatK>ns.NElS. and NOAA. Special thanks to DaveOppenheimer. Bob Urhammer. GregAnderson, Sam Clarke, Thomas Dunklin,Dave Valentine, and Mike McGuire.Lori Dengler is a geology professor at

Humboldt Slate Uniwr.;ity, Arcata For 7years she has been researching historic: setsmicity 01 California's north coost area Shehasworked WIth emergency planning personnel and educators in community andStale earthquake awareness and preparedness programs and now directs a regionalcenler for CALEEP (California EarthquakeEducation Project). Application of geophysical techniques to the study of near-surfaceprocesses is an ongoing research interest.Gary Carver is a geology professor atIlumboldt State University. Arcata. He has

been studying the fauhing. uplift. and deformation hislOfY of Quaternary deposits in~ t e m Califorma, southwestern


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REFERENCESAdams. J., 1990, Paleoselsmlclty of theCascadia subduction zone: Evfdence fromturbidites ot! the Oregon-Washington margin: Tectonics, v. 9. p. 569583.Atwater. B.F.. 1987, EVidence for great Holocene earthquakes along the outer coastof Washington state: Science. v. 236.p.942-944.

Atwater, T., 1970. Implications of plate tectonICS for the CenozoIc evolullon of westernNorth America: Geological Society ofAmerica Butletln. v. 18. p. 3513-3536.Bolt, B.A .. Lomnitz. C.. and McEVilly. T.V ..1968. SeIsmological eVidence of the lectonics of central and northern Californiaand the Mendocino escarpment: Bulletinof the Seismological Society of Amenca.v. 58. p.1725-1767.Bolt. B.A .. and Miller, R.. 1975. Catalogue ofearthquakes in northern California, Jan. 1,

1910 Dec. 31,1972: seismographic Stalions, University of California, Berkeley,567 p.CALIFORNIA GEOLOGY, 1991. ModifiedMercatli Intensity Scale: v. 44, p. 203.

Carver. G.A.. 1987. Late Cenozoic tectoniCSof the Eel River basin region. coastal northern California. in Herman SChymiczek andReinhard Sushsland. editors, Teclonics,sedimentation and evolutIOn of Ihe EelRiver and associated coastal basins ofnorthern California: San Joaquin Geologl'cal Society Miscellaneous Paper No. 37.p.61-72.Carver, G.A., and Burke, R.M . 1988.Trenching investigations of northwesternCalifornia faults, Humboldt Bay region,unpubfished final report. National Earthquake Hazards Reduction Program:U.S. Geological Survey, 53 p.

Clarke. S.H., Jr., 1992. Geology of the EelRiver basin and adjacenl region, Implica\lons for late Cenozoic tectonics of thesouthern Cascadia subduction zone andMendocino triple junction: AAPG Bulletin,in press.Clarke, S.H., Jr. and Carver, GA. 1992. LateHolocene tectonics and paleoseis- micityof the southern Cascadia subduction zone,northwestern California: Science, v. 255.p. \88-192.Cockerham, R. 1984, Evidence for a 180kmlong subducted slab beneath northernCalifornia: Bulletin of the SeismologicalSociety of America: v. 74, p. 569576.Collman, J.L., and von Hake. C.A., 1973,Earthquake History of the United States:U.S. Department of Commerce Publication411.

Eaton J .. 1989. Dense microearthquake nelwork study of northern California. in J.J.litehiser, editor, Observatory Seismology:An Anniversary Symposium on the OCCa-sion of the Centennial of the University ofCalifornia at Berkeley SeismographicStations: University of California Press.Berkeley and Los Angeles. p. 199-224.

Engebretson, D.C., C o ~ . A., and Gordon.R.G., t985, Relative mallons betweenoceanic and continental plates in thePaCIfic baSin: Geological Society ofAmenca Special Paper 206. 59 p.Field. M,E., Clarke, S., and White, M.. 1980.Geology and geologiC hazards of ollshoreEel River baSin, northern California contlnenfal margin: USGS Open-File Report80-1180,80 p.

Fountain, S.B., 1967, The Susie Baker Fountain Papers: Humboldt State UniverSity,Arcata, CalifofOla. v. 62, p. 1-32.Grant. W.C. Atwater, B.F., Carver, G A ,Darienzo, M., Nelson. A.R .. Pelerson.C.D.. and Vick, G.S., 1989. Radiocarbondating of late Holocene coastal subsidence above the CascadIa subducllOnzone - Compilation for Washington. Oregon and northern California. [abstract];EOS. Transacllons of the Amencan Geophysical Union, v. 70, p. t33t.Hamilton, D., t975, Geology of the HumboldtBay regiOn, unpublished report to thePacific Gas and Electric Co., San FranCISCO, California.

Hart, EW., 1988, Fault rupture hazard zonesIn California; California DiVISion of Minesand Geology Special Publication 42, 24 p.Heaton, T.H .. and Hartzell, S.H .. t987. Earthquake hazards on the Cascadia subduction zone: SCience, v. 236. p. 162-168.Heaton, T.H., and Kanamori, H., 1984, Seismic potenllal associated With subductionin the northwestern United States: BulieMof the Seismological Society of Amenca,v. 75. p. 933-942.Jachens, R.C.. and Gnscom. A., 1983,Three-dimensional geometry of the Gordaplate beneath northern California: Journalot Geophysical Research, v. 88. p, 93759392.

Kelsey, H.M., and Carver, G A , 1988. LateNeogene and Quaternary tectoniCS associated wllh the northward growth of theSan Andreas transform fault northernCalifornia: Journal Geophysical Research,v. 93, no. B5, p. 4797-4819.Kilbourne, R.T., and Saucedo. G.J.. 1981.Gorda basin earthquake northwesternCalifornia: CALIFORNIA GEOLOGY,v. 34, p. 53-57.Knapp, J.. 1982, Seismicity, Crustal Structureand Tectonics near the Northern Terminat ion of the sa n Andreas Fault. unpublished Ph.D. dissertation; University ofWashington, Seattle, 316 p.

Lawson, A.C. 1908, The California earthquake of April 18, 1906: Carnegie Institute, Washington, D.C.; v. 1. p. 5459and p. 165170.Usowski. M.. and Prescott, W.H., 1989,Slrain accumulation near the Mendocinotriple junction, California, [abstract]; EOSTransactions American GeophysicalUnion, v. 70. p. 1332.Manning, G A . and Ogle, B.A., 1950, TheGeology of the Blue Lake Quadrangle,

California: California Division of Minesand Geology Bulletin 148. 36 p.McLaughlin, R.J., Sliter, W.V., Frederiksen,N.O .. Harbet, W.P.. and McCulioch, D.S..Plate motion recorded by lectonostratigraphic terranes of the Franciscan Complex In the vicinity of Ihe Mendocino tripleJunction: USGS Butlelln, In press.McPherson, R.C .. 1989a. seismicity andlocal mechanisms near Cape MendoclOo,northern California, unpublished Master'stheSIS: Humboldt State Umversity, Arcata,California, 75 p.McPherson, R.C.. 1989b, Focal mechanismsand seismicity at the southern end of theCascadia subduction zone, [abstract]:Geological Society of America AbstractsWith Programs, v. 21, p. 116.McPherson, R.C.. Dengler, L.A. andOppenheimer, 0.,1991, Evidence ofcompressionallectonlcs in the KingsRange, California: the 1991 Honeydewearthquake [abstract]: EOS. Transactions

01 the Amencan Geophysical Union,v.72, p.315.

McPherson, R.C., Smith, S.W., and Severy,N.I., 1981. The Humboldt Bay seismiCnetwork: 1974-1980 [abstract): Earthquake Noles. v. 52. p. 41-42.National Earthquake Information Center.Preliminary determination of epicenters:U.S. Government Printing Office, Washington. D.C. 20402.Nowroozl, A. 1973, seismicity of theMendocino escarpment and the aftershock sequence of June 26. 1968 oceanbottom seismic measuremenls: Bulletinof the Seismological Society of America,v. 63, p. 441-456.Oppenheimer, D., Reasenberg, P., Walter,S., Macgregor-SCon, N.. Hirshorn, B.. andLindh, A.. 1992. Weekly seismicity report,April 2329. 1992; U.S. GeologicalSurvey, Menlo Park, California.Oppenheimer. D.H., and Magee, M.E. 199t,The 1991 M6.0 Honeydew, Californiaearthquake [abstract]: EOS, TransaC\lOnsof the American Geophysical Union,v. 72. p. 311312.Peterson, C.D., and Darienzo, M,. 1989.Potential eVidence of subduclion zonetectonics from stacked peal horizons inthe lale Pleistocene coastal terraces ofthe nonhern Cascadia margin labstract]:EOS, Transactions of the ArnencanGeophysical Union, v. 70, p. 1331,Prentice, C.S, 1989, Earthquake geology ofthe northern San Andreas fault near POintArena. California, unpublished Ph.D. dissertation: California Institute of Technology, Pasadena, California, 235 p_Real, C.R., Toppozada, T.R., and Parke,D.L., 1982, Earthquake epicenler map ofCalifornia: California Division of Minesand Geology Map Sheet 39.Rogers, G.C.. 1988. Seismic potential ofthe Cascadia subduction zone: Nature,

v.332,p.17.



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savage. J.C. and USOWSkl. . . . . 1991. Strainmeasurements and the potential lo r agreal sutxluCtJOn earthquake oN !hecoast 01Washll'lgton: Science. v 252.p.101103.

5Wer, EA, 1971, TectOBCS of !heMendocIno lnple IUnctJon Bullebn ofthe GeologIcal SooeIY of Amenca, v. 82.p. 2965-2978.Smllla. G W 1980, SeIsmologICal eondenceon the tector1lCS of the northern sectJOnof the San Andreas Fault RegIon. 1(1SlrQlIZ, R. and Shertlume. R.. editors,Studies of the sa n Andreas lau lt zone Innor1hem Calilornla; calitomla 0Msa0n ofMInes and Geology Speoal Report 140,P 131137

Slmlla. G W . Peppn. W and McEvity. TV1975. SetsmoIectona of !he CapeMelldoollO. CailomIa area; Bullebn 01the GeoIogtcaI SoCIety of Amenca. v 86.p 1399-1406Sm4h. S.W. and Knapp, J.S . 1980. Thenorthern temw'la1Jon of the San Andreas

fault. 1(1 Strertz. R. and Sherburne. R.. editors, Studies of the san Andreas faultzone in northern taIitomIa c a ~ f o m l a0rVtSl0f'l 01 MlIles and Geology SpeaalReport 140. p. 153164SmIth, Sw. , McPherson. R.C.. and severy.N.I., 1981. Breakup of the Gorda plate(abstract): Earthquake Notes. v 52. P 42.Spence. W., 1989, Stress OI'IgIl'lS and earth

quake polenbals II'l cascadia Journal ofGeophysical Research, v 94. P 3076,.,..TERA Corporation. 1975, Humboldt BayS8lsmic nerworx annual repof1. August1974 - August 1975: unpublished repooto the Pacif ic Gas and ElectricCo .. SanFrancisco, CalilOfl'lla.

TERA Corporallon, 1977, Tectonic s.gnlli.canee of large hislone earthquakes in

the Eureka reglOll. September 9. 1 9 7 7 ~unpublished reportlo the PaCIficGas andElectnc Co. San Frar1Clsco, C a ~ l o r r n a .TERA Corporatton. 1982, November 8.1980, Tnmdad offshore earthquakeand aftershocks: unpublished repot1 to

the Paafic Gas and EIectnc Co. SanFranosco. californiaTobm, OG. and SyKes, LA, 1968, se.srr.c-Ity and tectOBCS of the northeasl Paollc

O o e a n ~ Journal 01' Research.v. 73, p 38213845Toppozada. lR .. and PaN, 0 L. 1982,Areas damaged by C a earth19(10.1949 talitoma 0Msa0nof MAts and Geology Open-F. Report

82-17 SAC. 65 pToppozada. T R. PaN, 0 L. and Higgins.

C.T. 1978, 5eI:smoty of Califoma 19001931: calitorrwa OMSIOO of Mines andSpeoaJ Report 135,39 P

Toppozada TR, Real, CR , and PilrlUI.O.L, 1981. Preparabon 01' ISOSeISlTIafmaps and sunvnanes of reponed ettectslor pre-l900 caifomla earthquakescaJilomLa 0MsIJn of Mines and GeologyOpen-File Report 81-1 1SAC. 182 P

Toppozada, TR., Real. C.R.. and PatXe,DL, 1986. Earthquake tustory of eatilOfnI&: CALIFORNIA GEOLOGY, v. 39.p.27-33.

Townley. S.O.. and Allen. MW., 1939.Descnptwe catalog of earthquakes 01!he PaCIfic coast oflhe Unlled States.176910 1928: BullelJn of the 5elsmologicaISodetyoIAmenca,v.29.p 1-297.

University 01 Ca6lomia at Berkeley SeISmographic StabonS. Bulleon or Ihe seismographic statlOfls of the University of

Califorma: UnIVersIty or CaJi1omia.Berkeley, CaJifomla.Vick. G . 1988, Late Holocene paleos8lSmlClty and relatIVe sea level changes ofthe Mad RIVer Slough, northern HumboldtBay. Calilornla, unpublished Master'sthesis: Humbold1 State UnlVefSllY, Arcata.calilOfl'lla.Walter. S.A.. 1986. Intermediate locus earthquakes assoaatedWIth Gorda plate SlJb.ducbon in northern Caifomia: BuletJn of!he SeIsmologICal Sooety of Amenca,v 76, p. 583-588.Wilson, D.S., 1986, A kinemabCmodel lorthe Gorda plate delormatlOll as a diffusesouthern boundary of !he Juan de Fuca

p 1 a l e ~ Journal of GeophysIcal Research.v 91. p. 10259--10269Wilson. D.S., 1989, Deformation oflhe so-caIed Gorda plate: JoumaI of Ge0physIcal Research. v. 94, p. 3065-3075.Wood, H.O. and Neumann, F., 1931. Modlied Mercalli Intensrty ScaJe 01 1931:BullelJn of the SetsmotogICal Society 01Amenca, v 21, p. 2n-283.WoodwardCIyde Consultants. 1980. EvaJua

bon of the potenlJal for resolving the ge0logie and seISITIiC ISSUes at the HumbOldtBay Power Plant Unit No.3: WoodwardClyde Consultants. Walnut Creek. calilornia. Appendix 0, 145 p.: catalogue 1,71 p.; and Catalogoe 2.10 p.Woodward-Clyde Consultants. 1981. Reporton the Tnnldad olfshore earthquake of 8November 1980:Woodward-Clyde Consultants. Walnut Creek, California.Woodward-elyde Consultants, 1982, Evaluation ot the selsmlCrty data associaledwllh the November 1980 Trinidad offshoreearthquake lo r the Humbold1 Bay PowerPlan! Unit No.3: Woodward-ClydeConsultants, Walnut Creek., Califomia.'X

I-Vacaville-Wmters earthquakedamage, 1892.

CALIfORNIA GEOlOGY "


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CBlllfU..

..,n.",.. : : ~ ' .

n

neepip

' - - ~ -

h 0lIor

-

'" v r 1 Toy Ja.rre


27/36

ApriIlllO:2la!heoanlilrVAl of Vac:awtle-W...... ~ " . T h & M _lrom.. AflII 21 Ind 22, 1892 edllions of fieSa a . , . . . 8M . Allprn.d WI'fl01 . . S'O.'..... S..li!a2 eoun..,t11,. s.. lbwy. c.:mr.. s.e....


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The Cape Mendocino EarthquakesApril 25-26, 1992

During the preparation of this issue of CAUFORNlAGEOLOGY three large earthquakes rocked the CapeMendocino area. Humboldt County. California. 1lley werefelt throughout much of northern California and in southernOregon. Peak intensities \lJfC at least VIII (Lori Dengler.Humboldt State University. I4IIillen communication).

There were at least 144 aftershocks of magnitude 3 orgreater within a month of the magnitude 7 event (RickMcKenzie. UniveTSity of California. Berkeley. oral communica-tion).The earthquakes triggered numerous landslides. damagedroads and bridges. and caused lNidespread liquefaction in theEel River Valley. Structural damage was concentrated inFerndale. Fortuna. Petrolia. Rio Del. and Scotia. President

Bush declared Humboldt County a major disaster area.The National Oceanic and Atmospheric Administration(NOAA) reported a 3-looc- (l-m-) high tsunami. an earthquakegenerated sea wave. at Crescent Oty. The tsunami measuredabout 0.6 feel (18 em) 211 Point Arena. California and about

0.3 feel (9 em) in Hawaii.The April 25 event occurred along a northeast-dippingreverse fault very close to the postulated location of theCascadia sutxluction zone. Thus. this may be the first historicearthquake along lhe Cascadia subduction zone. However.the area is geologkally complex so the rupture could haveoccurred along anyone of the other thrust faults in the area.The April 26 earthquakes struck along a northwest-strikingright-lateral strike-slip fault in the Gorda plate (Lori Dengler,

Humboldt State University. written communication), No surface rupture has been discovered,This sequence of earthquakes occurred in one ofCalifornia's most geologically complex and seismically activeareas (see Sources 0/ North Coast Seismicity, this issue, for adescription of the tectonics and an overview of the seismic history). The region's largest historic earthquake (magnitude 7.2)

occurred in 1923. More recently. a magnitude 6.0 event struckabout 13 miles (21 km) southeast of Cape Mendocino (see TheHoneydew Earthquake: August 17, 1991. this issue). The front door was at the top at the slairs before this Ferndalehouse was shaken all lIs foundation. Photo by Kevin BaylISS.

Prelimmary /ocalJOns and magrnllJdes defermmed by the U.S. GeologIcal SOCIety and the UmverSity 01Gallforrua al Berkeley.Dale Time ON. Lal. W. Long. Depth M, M,

4125192 11 :06:04 PDT 40.37 124.31 15km 7.0 6.44126192 00:41 :40 PDT 40.44 124.58 18km 6.0 6.24126/92 04:18:26 PDT 40.40 124.56 2 ' km 6.5 6.4

.. CALIFORNIA GEOLOGY MARCH,APRlll992


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" '"

,20-.

- . - ' . , . . . . , . - - - - - - - - J . - - - - - - n - - - - ' - - - - - - . - - - - - - - , c ' - - - ~ " O O '

LocaliOl'l map of the Apnl 1992MenOoclno area earthquakes

Humboldt County roads. wt'llch serve rural communitIeS near the epcenters,Wefe damaged and blcx:ked at numerous IocalJons by debris slides However.mamtenance crews were able 10 clear one lane on mosl 01 theSe roads wllhln2 Of 3 hours 01 the earthquake. These roads cracked due to settlement 01roadway embankmenls. but none 01 the cracks were large enough to blocklrallic. PholO by Oscar Huber.

-.

Sandblows adjacent to !he Salt Rrver near PortKenyon prOVIde evidence olliquelacoon of saturatedsand layers beneath younger deposrts 01 Silt andday. dunng the ApnI2526, 1992 earthquakesequence. Water and sand squirted 10 the surfacethrough fissures ,n the slllicLay layer. and left smallcraters of f ine gray coheslonless sand on the surfaceThe craters In thiS photo are about 12 ,ncheS (30 em)In diameter. Port Kenyon 1$ about 1.5 miles (2.4 km)northwest 01 Ferndale. alld the Sail River is atnbutary to the Eel River. Similar sandblows wereobserved near the lower reaches of the Eel andMallole fivers. Photo by Oscar Huber.

CALIFORNIA GEOLOGY MARCHIAPRll1992 "


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c'---- )EARTH SCIENCE RESOURCES FOR TEACHERS 1992

This listing includes nonprofit sourcesof precollege earth science referenceand instructional materials. Whenrequesting free material. use schoolstationery.AMERICAN GEOLOGICAL

INSTITUTE. National eenler for EarthScience Education, 4220 King Street,Alexandria, VA 22302. (703) 3792480.Contact ACI Publications Cenler.P.O. Box 205. Annapolis Junction, MD20701. or telephone (301) 953-1744,

to delennine shipping and handlingcharges and volume discounts. Ordersmust be prepaid. Virginia residcnts add4.5% lax. Prices are subject to changewithout notice.Aduenlures in Geology. Teacherworkbook employs cooperative learningstrategies to help upper-elementary andmiddle-school students become activescience learners. Includes nine upbeatactivities. $6.95.AGI Doto Sheets. 3rd Ed. Ideal forquick reference on nearly 100 topicsfrom rocks, minerals, earthquakes, andvolcanoes to time scales and information sources. $34.95.AGl/NSTA Earth Science &omina-tlon. This multiple-choice exam. devel

oped by a ;Oint AGI/National ScienceTeachers Association (NSTA) task force.can be used by secondary school teachers to evaluate their science curriculumand their students' success. Order formsand additional information are availablefrom AGI and NSTA.

Careers in the GeoscIences. The fuJlcolor fokl-out describes what geoscientists do and where they work, future jobprospects, salary figures, and where tofind more information. Obtain a freecopy from the National Center for EarthScience Education, ACI. For bulk orders. contact the AGI Publications Center at (30 I) 953-1744.Diclfonary of GeologIcal Terms.

3r d Ed. 8.000 terms and definitionsthat cover the core vocabulary of theearth sciences without technical jargon.$12.00.

Earth Science Con/em GuidelinesGrades K-12. The report. developed byteachers and scientists. is a set of queslions to guide the inclusion of earth science information into the K-12 curriculum. The questions are organized byarea: Solid Earth. Water , Air, Ice, Ufe,and Earth in Space; and by grade level:K3, 36. 6-9 . and 9-12. Corresponding key ideas and teachers' suggestionsare given for each area and grade \evel.$15.00.

Earth Science Education for the21st Century: A Planning Guide.Developed by educators. administrators.and scientists, this report provides decision makers with a planning tool forexpanding ear1h science programs.$10.00.

The Making of a Continent.Companion Guide 10 the PopularPBS Series. Explains the events andforces that shaped the natural historyof North America. $16.95.

MInerals Foundations of Society.2nd Ed. Provides a nontechnical overview of nonfuel minerals: their significance and origin. and the complexitiesof making them available. $11.95.A Study in TIme, TVJO-Color beautifully illustrated poster depicts geologictime as a spiral leading from 4.5 billion

years ago. through each major period.10 the present. IT' x 22". $4.00.Why Scientists Believe in ElJOlution. Pamphlet that explains why scientisls believe lhat the complex organisms

of today evolved from simpler and moreprimitive ancestors. Single copies arefree from the National Center forEarth Sdence Education. AGI. For bulkorders. contact the AGI PublicationsCenter.

AMERICAN ASSOCIATIONFOR THE ADVANCEMENT OFSCIENCE. 1333 H Street, N.W. Washington, D.C. 20005, (202)3266620.Science Books for a Lifetime of Read-ing . An annotated reading list for preschool through adult. Free. ScienceBooks and Films. 9 issues/year.$35.oo/year. Provides critical reviewsof printed, audiovisual, and electronic

resources for use in science. technology.and mathematics education.

AMERICAN GROUND WATERTRUST. 6375 Riverside Drive. Dublin.OH 43017. (614) 761-2215. GroundWaler Education in America's Schools.A 4 I-page catalog of resource materialsfor elementary and secondary educationprofessionals describes 80 different re 'source packages for classroom instructionabout ground water and related environmental concepts. Single copies are free toeducational organizations. Requests mustinclude a 9" x 12" addressed envelopeand $2.00 postage.AMERICAN MINING CONGRESS.

1920 N Street. N'w. Washington, D.C.20036-1662. {2021 8617525. WholMining Means to Americans. A colorful16page booklet that conveys the impactmining has on civilization's growth andmining's importance in everyday lire.Other topics include preserving the environment and resource conservationthrough recycling. For 50 or more copies. $0.75 each. For fewer than SO.$1.00 each,

ASSOClATION FOR WOMENGEOSCIENTISTS. Geology Department. Macalester College, 1600 GrandAvenue, 51. Paul, MN 55105-1899.Career Profiles is a 22-page booklet thatshows the types of careers available inthe earth sciences. Profiles th e careers of20 women geoscientists. Free.

FEDERAL EMERGENCY MANAGEMENT AGENCY. Earthquake Program. 500 C Street, S.W. Washington,D.C. 20472. Attn: Marilyn MacCabe.(202) 646-2812. Earthquakes; ATeacher's Package for K-6. This six-unitbook, developed by the National ScienceTeachers Association with a grant fromFEMA. is a complete earthquakemultidiscipline curriculum containing activities. lesson plans. line masters. andbackground information. One free copype r school while supplies last. Additionalcopies may be purchased from NSTA at$15.00 a copy.

GEOLOGICAL SOCIETY OFAMERICA. 3300 Penrose Place, P.O.Box 9140, Boulder. CO 80301, (303)4472020. (BOO) 472-1988. Numerous



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Teacher Featurecontinued.

geological publications and other materials. Publications Catalog. Free. FutureEmployment Opportunities in the Geologica/ Sciences. Free. The Earth Has AHistOTY. Twenty-minute program introduces the concept of geologic time by"walking the viewer through t i m e ~ in theRocky Mountain Front Range near Boulder, Colorado. VHS cassette $25.00.15%discount for schools and institutionswhen purchasing the video. For purchases under $25.00, add $3.00 forshipping and handling.

N A T I ON A L A C A D EM Y PRESS.P.O. Box 285, Washington, D.C.20055. (202) 334-3313. (80016246242. Science for Children: Resourcesfor Teachers. A 176-page guide 100uI4standing hands-on, inquiry-based materials for elementary science leaching prepared by the National Science ResourcesCenter. $9.95.NATIONAL ASSOCIATION OF

GEOLOGY TEACHERS. P.O. Box5443. Bellingham, WA 98227-5443,(203) 676-3587. NAGr sponsorsregional meeting with field trips andworkshops: annual meetings held in conjunction with the Geological Society ofAmerica, membership: $25.00. Journalof Geological Educalion (five issuesyearly with membership). A source ofinfonnation. ideas. and reviews for highschool and college teachers.

NATIONAL EARTH SCIENCETEACHERS ASSOCIATION. ScienceDepartmentlJ14, Lansing CommunityCollege. 430 N. Capitol Avenue, lan-sing, Ml48901. NESTA membership:1 year. $10.00; 2 years,$19.00; and 3years, $27.00. lncludes a quarterly publication, The Earth Scientist. Slide setsare available for upper elementarythrough introductory college levels. Widevariety of subjects: modest prices. Writefor free brochure.NATIONAL ENERGY FOUNDATION_ 5160 Wiley Post Way, Suite 200Salt Lake City. VT 84116. (801) 539

1406. Resources lo r Education listsposters. teacher guides, glossaries. lessonplans. and kits to help teach all aspectsof energy usage. Free.

NATIONAL GEOGRAPHIC SOCIETY. Educational Services, P.O. Box98019, Washington. D.C. 20090-8019,(800) 368-2728. Teacher's guides forgrades 4- ] 2. Film and Video Catalogand Educational Services Catalog.Free.NATIONAL SCIENCE TEACHERS

ASSOCIATION. 1742 Conne


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MINING BOARD

THREE APPOINTED TO THE STATE MINING A

Documents

Caliornia Geology Magazine Mar-Apr 1992