EERI SPECIAL EARTHQUAKE REPORT -OCTOBER 1993 · EERI SPECIAL EARTHQUAKE REPORT -OCTOBER 1993 radius of 660 km (412 miles)! At various times in the days following the earthquake, the

EERI SPECIAL EARTHQUAKE REPORT -OCTOBER 1993

2


radius of 660 km (412 miles)! Atvarious times in the days followingthe earthquake, the epicenter wasplaced at all points of the compassaround the island, as different ormore recent subsets of data andinterpretation were considered.

SEISMOLOGY

'(0Background

Trench contains the greatestmeasured depth of the world'soceans, approximately 11 .3 km(37 ,000 feet), located to thesouthwest of Guam. The trench isarc-shaped like the island chain,and its minimum distance fromGuam is 112 km (70 miles) to thesoutheast, where it is alignedpredominantly at right angles tothis measurement (Figure 3). Tobe close enough to Guam to causesignificant structural damage, itwould seem that deep thrust earth-quakes on this fault system wouldhave to be located in the southeastquadrant measured from Guam.

Guam is the southernmost islandof the Mariana Islands in thewestern Pacific Ocean (Figure 2).The islands form a slight arc,convex to the east, and span from13°13' to 20030' N, and 145° to146° E. The island chain is 770km (480 miles) long. Guam is 46km (30 miles) long, averages 11km (7 miles) wide, and is alignedapproximately NE-SW. The islandarc outlines the Mariana Ridge atthe eastern edge of the PhilippinePlate, which is forced to rise inelevation as the Pacific Plate divesbelow it in a northwesterly direc-tion. At the same time, thedescending Pacific Plate, whiledragging the eastern edge of thePhilippine Plate with it, forms theMariana Trench where the twoplates converge. The Mariana

As of 18 days after the earth-quake, the USGS/National Earth-quake Information Center (NEIC)had located the magnitude 8.1earthquake at 12.97° N, 144.74° Eat a depth of 61 km (38 miles).This places the epicenter 60 km(37.5 miles) due south of Agana,the commercial center of the is-land, and a few kilometers furtheraway from Tumon, where most ofthe tourist hotels are located.Seismology

For this remote island in the west-ern Pacific Ocean, epicenter anddepth calculations cannot be madeas confidently as in California. Fora 90% confidence level in epicen-trallocation the error circle has a

The most likely fault plane strikesat N70E and dips steeply (77°) tothe NNW. The first motion has thePacific Plate moving down dip (tothe NNW) and along the strike (tothe WSW) in equal amounts.

~,b,

143° 144° 145° 146°""1""1"",.".1.,..,.,..1 ,.".1..,,,,,.,1 "...1,..., 1,.", 1 ,.

~

14°-

1975 M7.1.

GUAM

Tum-on ('Agana

13.~1993 M8.1

0,*

/'.~~/ "a/Ca~~~

,/~aa/ 00..

0 50 KM / " +~ ., , J / .,. .

c.~-<,.~c,~

~

'b-.'b-~

"tI.'b-~...

~

Figure 3 -Location of A ugust 8 main event ,and aftershocks; and epicenters of notable quakes since 1975.

3


GEOTECHNICAL

EFFECTS

Strong Ground Motionbuildings of one and two stories,and to very flexible buildings insome cases, suggests a lack ofhigh frequencies. The damage tothe four to twelve-story hotelsindicates the prominent groundmotion components had periodsbetween 0.5 and 2 seconds.

,The u.s. Navy, over a period ofyears ending in 1988, maintained anetwork of strong-motion accelero-graphs at some of their U.S. andoverseas bases, including threeinstruments in Guam. Althoughthe instruments had not been re-moved during the following years,there was no recording recovered.

Geologic effects of the earthquakewere restricted to areas generallyconsidered highly susceptible todamage from strong ground shak-ing. Primary surface fault rupturewas not observed. The most com-mon geologic effects in low-Iyingareas included ground fissures,horizontal ground displacements,differential settlement and sandboils. Common effects along cut-slopes and cliff faces includedsmall-scale sluffing and slumpingof loose surficial materials, andlarger rock-fall slope failures.

It is not likely that a magnitude 8event will ever occur closer to theisland than this one did. But lessermagnitude events can occur oncloser faults. These have the po-tential for causing much damage,particularly to stiff structures suchas one or two-story buildings.

An estimate can be made of thecharacter of the ground motionfrom observations on the island.The origin time was 08:34:25.0 onSunday, August 8. 1993 (GMT),6:34 pm local time, so mostpeople were awake and alert at thetime of the quake. Many peopleinterviewed agreed with the news-paper accounts of the shaking last-ing 60 seconds.

Ground Motion at DamageSites

The 61- km focal depth ensuredthat the drop in ground motionamplitudes was not rapid from thesouthern to northern ends of theisland. All firm foundation soils orrock in Agana and Tumon probablyreceived about the same shaking.Accelerations are unknown atpotentially liquefiable sand layersoverlain by fill that failed andspread during the earthquake.

The aftershock distribution indi-cates that the mainshock rupturemust have been for the most partin one direction, namely to thenortheast, and could well havelasted 40 seconds or more. Be-cause of the orientation of theisland to the rupture plane, wavemotion arriving from the rupture'sstart will have been only slightlystronger than the motion arrivingfrom the rupture's end. Travellingnortheast, the rupture would havereceded from Guam, increasingtravel distance and attenuation.The strong shaking likely lasted 30seconds, and then slowlylessened .

Liquefaction-induced ground fail-ures and settlement occurred inlimited areas. A firm limestonesubgrade, not susceptible to lique-faction, exists on most of theisland. Significantly, neitherliquefaction nor ground settlementwas observed in the Tumon Bayarea, where several hotels sus-tained major damage.

t

The 61-km depth and 60-km epi-central distance permits geometricthree-dimensional spreading tooccur for a greater distance andlonger duration than would beexpected for a shallow high-magni-tude California event. It is rea-sonable to conclude that peakacceleration amplitudes were low,(about 0.25 g), and that high fre-quencies, greater than 5 Hz, werenot significantly present. The lackof damage to small, solidly-built

Figure 4 -Liquefaction-induced lateral spreading damaged the wharf at i

Naval Station. The seawalls shifted laterally Up to 2.7 m (8 feet).Underground utl"lity lines were broken. ,

~III

4

Facilities that were damaged inlow-Iying areas include the Piti


deposits oc-curred duringthe earthquake.Local subsi-dence was alsoobserved at thefill abutmentsat many of theisland bridges.

Power Plant. the commercial port and some of thei(~aval port facilities. These areas are underlain by\~oose medium to coarse-grained coral fill. which

overlies fine-grained lagoonal and estuarine deposits.Evidence of lateral spreading included bent cranetracks and warped bulkheads at the port facilities. Inthese areas, cracks were observed extending 2 to 2.7m (6-8 feet) below ground surface where they wereobscured by the water table (Figure 4). The containerloading facility at the commercial port was closed dueto damage (Figure 5).

Up to three feet of ground subsidence was observedin the area around the Piti Power Plant. Evidence ofseveral inches of ground subsidence and differentialsettlement was observed adjacent to the new court-house in downtown Agana in an area where fill hasbeen placed over the soft marsh soils of the Aganaswamp. Post-earthquake ponding of surface water inparking lots in the downtown Agana area also indicatethat differential settlement of the underlying marsh

'!III!!

~

Only one majorlandslide wasobserved duringreconnaissanceof the island.This slideoccurred alonga remote sea-cliff on thenortheast sideof the islandnear lajunaPoint. Mostslope failures Figure 6 -A rock fall along Marineoccurred as Drive crushed two cars.slumping androck falls along the steep cut-slopes and cliffs that arecommon around the island. Although the majority ofthese slides were relatively small, many occurredimmediately adjacent to areas of dense development.In particular, several slope failures that occurred alongMarine Drive abutted commercial buildings, and in onecase two cars were crushed (Figure 6). Several slidesalong Marine Drive appear unstable and may be reacti-vated during future earthquakes.

~

Reconnaissance of the shoreline did not reveal evi-dence of either coseismic emergence or submergenceresulting from this magnitude 8.2 event. This may bedue to the relatively great distance ( -60 km) anddepth ( -60 km) of the earthquake, earthquake sourcemechanism (strong oblique component), geometry ofthe subduction zone, and/or architecture of the accre-

tionary prism complex.

Surficial faults previously mapped on the island wereassessed to evaluate whether sympathetic movementoccurred during the earthquake. No evidence ofsympathetic surficial displacement was observed oneither the Tamuning-Yigo, Adelup or Cocos faultzones.

~

Geomorphic features on the island suggest that upliftand tilting has occurred since late Quaternary time.Observations suggest that Guam is being graduallyuplifted and tilted westward due to compression anddeformation within the Mariana subbduction zone.

5- At the commercial port liquefaction caused

lateral spreading and ground distortion severe enough to

result in the shutdown of the container loading facl'lity.

5


BUILDINGS

~

Building Design Criteria and

Building Types

The island uses the Uniform Build-ing Code (UBC) as the basis ofdesign. By law, it automaticallyadopts the latest edition of theUBC as it is published by ICBO.The island is designated Zone 3 forseismic design. A design windspeed of 155 miles per hour isrequired, in recognition of the dam-aging typhoons that periodicallypass over or near the island. Forsome (relatively tall) buildings,wind will control the design lateralforces. However, because of theZone 3 designation, the specialdetailing requirements for seismicregions must be followed.

Figure 7 -The second story columns on the south side of the Royal Palm

Hotel disintegrated. The 10 stories above settled about 1.3 m (4 feet) onto

the remaining half-height walls and leaned about 2.6 m (8 feet) at the roof.

are founded on driven piles. Steelframe structures are rare. Thereare a number of wood frame struc-tures, mainly residential, in the lesstdeveloped portions of the island.Apparently more of these buildingsexisted in the past, but the recenttyphoons destroyed many of them.

The buildings on Guam, includingboth residential and commercialstructures, are constructed mainlyof reinforced concrete, reinforcedconcrete masonry or a combinationof the two. The commercial struc-tures, including the hotels whichare often up to twenty stories tall,are typically either pure concreteframes or concrete frames with

concrete or concrete block infillwalls. The residential buildings aregenerally one- and two-storymasonry structures with concreteroofs. The concrete structures areusually founded on concrete mator spread footing foundationsgiven the firm limestone subgradepresent throughout most of theisland. Some structures in Agana Building Performance

The most significant damage oc-curred in the tall hotels in theTumon Bay area with the mostspectacular failures occurring atthe Royal Palm Resort, the GrandHotel and the Guam Reef Hotel.

~~

The Royal Palm Resort consists oftwo adjacent 12-story reinforcedconcrete frame structures foundedon a concrete mat foundation.The resort had only been openabout one month at the time of theearthquake. A number of thesecond story columns on the southside of the structure nearest thebeach crushed vertically in theearthquake, causing the building to.drop approximately 1.3 m (4 feet) /

and lean about 2.6 m (8 feet) atthe roof. It appeared that the

Figure 8 -The masonry in fill walls surrounding the stair enclosure at the

Royal Palm Hotel crumbled in the first alld second stories.

6

EERI SPECIAL EARTHQU.4KE REPORT -OCTOBER 1993

to resist the typhoon winds orfrom the lack of strong high-frequency ground motion.

14

One of the most common types ofdamage was the spalling of con-crete from walls. frames and slabsof buildings. While the spallingoccurred as a result of the earth-quake. the root cause was clearlythe corrosion of the reinforcingsteel due to severe water intrusioninto the concrete elements. Theclimate of Guam and the quality ofsome of the concrete appear toresult in a corrosion problem. Thereinforcing steel rusts. expands,and cracks the concrete around it.If the concrete does not falloff onits own, an earthquake is theperfect vehicle to cause it to doso. In several cases observed inthe aftermath of this earthquake,the corrosion damage and spallinghas occurred at critical elements ofthe structure.

Figure 11 -The heavy concrete canopy in front of the entrance to the GuamReef Hotel came to rest on the crumbled remains of the four concretecolumns which originally supported it.

frames from drifting laterally. Thehospital is fortunate that the pan-els did not fallout into the ambu-lance receiving area.

The Guam Greyhound Race Trackgrandstand is a reinforced concreteframe structure with a heavy cano-py roof extending out over theseating area and back to the areabehind the seats, where bets areplaced. The roof is supported onseveral bays of deep beams andcolumns. To allow natural lightinto the betting area, the roof hasa clerestory infilled with decorativeconcrete block. The clerestorycauses a discontinuity in the dia-phragm. During the earthquake,the diaphragm loads were trans-ferred at the discontinuity directlyinto the beams along their weakaxis. This caused severe crackingand spalling of the beams, expos-ing only light vertical shear rein-forcing. The beams in each baywere similarly damaged, creating acollapse concern due to a lack ofredundancy in the vertical load

carrying system.

structure. Concrete block infillwalls created short column effects(Figure 13). At the second storythe columns experiencedsevere cracking and someshortening as loads weredelivered from the roof intothe infill walls through thecolumns. Ties were widelyspaced in the columns andthe quality of the concrete

appeared poor, Aggregatewas clumped together withvery little cement evidentin some places. The gymstructure consisted of anextremely heavy concreteroof spanning over basket-ball courtS" to perimeterconcrete columns. The col-umns experienced hingingat the bottom due to theheavy mass of the roof .

There was very little dam-age in low- and mid-risebuildings. The lack ofbroken glass was surpris-ing, especially in the low-rise commercial structures I : ! which have large storefront Figure 12 -The clerestory at the Guam

windows. This lack of Greyhound Race Track caused a discontinuity

damage may be due to the in the diaphragm, which resulted in damage

stout construction required to the supporting beams.~I

The Inarajan High School, on theeast side of the island, is a two-story reinforced concrete frame",

8


Liquefaction Effects on

Neither foundation settlement norliquefaction appear to have beensignificant along the hotel strip.The buildings were set on the nat-ural coral limestone soil, not on fill.

Structures located in liquefied soilzones performed fairly well al-though there was a spectacularcollapse of a building at the NavalStation. The buildings thatperformed best were supported onpiles founded below the liquefiedzone. General ground subsidencearound a number of pile-supportedNavy warehouses caused settling,shifting and warping of the groundfloor slabs which severely ham-pered forklift operations. TheNavy estimates that repairs will bequite costly.

Figure 14 -None of the approximately 100 people inside Andy's Hut, anenlisted persons' canteen on the Naval base, was seriously injured when thebuilding slid into the harbor and partially collapsed due to liquefaction.

The most spectacular failure was

of a one-story structure known as

,(8

(4 to 6 feet) and was below waterin some areas. The structure itselfwas constructed of unrein-forcedconcrete masonry blocks withrubble infill between wythes. Steelpipe columns embedded in thewalls supported steel roof trusses.The brittle wall con-structioncaused the walls to break up likeunattached building blocks. but theductile and redundant roof framingprevented total collapse.

Post-Earthquake BuildingAssessment

Andy's Hut (Figure 14) .The build-ing serves as an enlisted persons'canteen and was serving approxi-mately 100 patrons at the time of

~ the earthquake. The build-ing is located directly adja-cent to the harbor on twosides, with the ground re-tained along the shorelineby a deteriorating canti-lever steel sheet pile sea-wall. When liquefactionoccurs near a naturalshoreline or man-madeshoreline such as a wharf ,lateral spreading of theground toward the body ofwater can occur and resultin large cracking anddistortion of the groundsurface.

At Andy's Hut, the lique-faction caused groundspreading that carried thestructure and the seawallinto the harbor, partiallycollapsing the building.Miraculously, no one waskilled or severely injuredalthough the original

Figure 13- Concrete block infill walls created ground surface and theshort column effects in the concrete frame structure it supportedInarajan High School. dropped roughly 1.3-2 m

Guam did not utilize a formal post-

earthquake building inspection pro-gram such as ATC-20, Proceduresfor Post-Earthquake Safety Evalua-tion of Buildings, which is com-monly used throughout Californiaand which was proved effective inthe aftermath of the 1 989 LamaPrieta earthquake. On Guam,building inspections were typicallyperformed by structural engineershired directly by the individualbuilding owners. This approach isnot thorough and cannot guaranteeconsistency in identifying hazard-ous structures and therefore maynot adequately address public

safety.

9


BRIDGES

Nearly all of the bridges and majorculverts on Guam were visited bythe EERI team. At the time of theinvestigation, all the bridges wereopen to traffic. Speed and loadlimits were posted for a fewbridges due to the rough ride atthe approaches and uncertaintyabout structural adequacy of thebridges. Twenty-four of thetwenty-nine structures that wereinvestigated are owned and main-tained by the Guam Department ofPublic Works. The other five arelocated in the Leo Palace Resort,which is privately owned .

Figure 15 -More than one-half of the bridges observed by the EERI teamsuffered some damage, but all bridges were open to traffic by the time theteam arrived on the island. The abutment damage shown in this photographwas typical of damage to the concrete bridges on the island. The watermain carried below this bridge was broken in the quake.

The majority of bridges in Guam

are single spans of reinforced or

prestressed concrete girder con-

struction and were built from 1945

to 1987. The bridges in the Leo

Palace Resort were built in 1992

and are of steel girder composite

construction. Both integral abut-

ments and seat-type abutmentsare used. There are several three-

span bridges with reinforced con-

crete piers consisting of two or

more columns. The abutments are

typically supported on piles. The

bridge design specifications are

based on the American Association

of Highway and Transportation

Official Standards.

II

More than one-half of the bridgesinvestigated were damaged by theearthquake. The extent of damagevaried, but was generally small tomoderate (Figure 15). The pre-dominant types of damage were:1) approach fill settlement, 2)damage in abutment walls due toimpact from the superstructureslab, 3) damage to abutment wallsdue to girder movement in thelongitudinal direction, 4) damage

to connections between abutmentwalls and wing walls, and 5)damage to reinforced concretegirders at the seats due torestrained longitudinal move-ments. The damage was mainly inthe form of concrete cracking andspalling, although yielding of rein-forcement steel was apparent in afew cases.

. ",

'j

The two modern three-spanbridges in the Leo Palace Resortexperienced only minor impactdamage in the sidewalks at theabutments. These bridges are ontall, relatively flexible piers.

~ ~

II'

!l

I

'ir c

;1

!

r,fI

1IIIiii~l;

:'

il

r\nIi~li

~

The primary reasons for the goodoverall performance of the bridgesmay be the following: 1 ) theground motion seems to have beenof only moderate amplitudes; 2)the vibration frequencies for themajority of bridges were relativelyhigh while the ground motion isbelieved to have been of lowfrequency; and 3) the majority ofthe bridges were single spanstructures with relatively wide seatwidths and no "high" rocker

bearings.

10


j)~IFELINES

Power Generation

Almost all electric generationplants on the island suffered somedamage, but the hardest hit wasthe Piti Power Plant, site of seriousliquefaction. Differential settle-ment of Up to 1.3 m (4 feet) oc-curred as well as lateral spreading.

Approximately 50 unrestrainedbatteries, which power the protec-tion instrumentation, turbine lubepump and turning gear, fell overand were damaged. One of theunanchored motor control centersfell over, knocking out the supplyto several motors (Figure 17) .Unanchored station transformersslid on their foundations; onetipped over (Figure 18).

Figure 17- Very few cabinets at the Piti Power Plant were anchored. When

this unanchored motor control center tipped over, the 480-volt supply to

several motors was cut off. Asbestos contamination released by damage

to the plant limited access for repairs for about a week.

up the buried water transmissionsystem. The system suffered amoderate amount of damage. withnew significant leaks still beingdiscoverd one week after thequake. Some of the over 100underground breaks reported maybe the result of large inducedcyclic motion and not groundfailure.

Transmission SystemsThe loss of power caused loss ofpumping capacity. The island-widesystem has little reserve gravityfeed capacity (about one hour).While most of the system re-mained pressurized in the heavilypopulated areas surrounding Aga-na, some of the villages to thesouth of the island were still with-out water one week after the quake.

((Q)rhe transmission/substation sys-" tern experienced relatively little

damage. There was a moderateamount of porcelain breakage ontransmission lines. Line sagrequirements for high wind loadingmay have contributed to this.

Telephone System

Installation of a new buried fiber-

optic cable system for local phone

service was nearing completion at

the time of the earthquake. Ac-

cording to the supervisor of the

Agana Station, there was no dam-

age to batteries at any of the three

main switching stations. DC

power was retained. The loss of

AC power was made up by emer-

gency diesels at the stations.

Phone service on the island did not

even overload.

Water Systems

\\UAbOUt 800 km (500 miles) of clay,cast iron, ductile iron, asbestoscement, steel, and PVC pipe make

Figure 18- At the Piti Power Plant, many of the station transformers wereunanchored and slid on their foundations. This transformer slid so far ittipped over.

11

EERI SPECIAL EARTHQUA.KE REPORT -OCTOBER 1993

Figure 19- This aerial view shows evidence of liquefaction and lateral spreading, and movement of the bulkhead

at the Naval port.

4. A smaller I closer event with

higher frequency content could

have a much greater effect on the

large inventory of stiff masonry

residential construction. The po-

tential for this type of shaking and

the risk associated with this type

of construction on Guam should be

investigated.

unspectacular yet significantdamage occurred.WHA T CAN WE

LEARN FROM THIS

EVENT?2. The rapid development of theisland in the past ten years fortourism may have resulted ininadequate design and construc-tion quality assurance in someinstances, particularly in some ofthe damaged hotels.on TumonBay. Similar developmentscenarios could have led toundetected seismic vulnerabilitiesin other locations of the world.

by Craig Comartin, EERI Team

Leader

5. The lack of strong ground mo-tion records for this event signifi-cantly limits the usefulness of thedata on damage. Steps should betaken on Guam and elsewhere toavoid this situation in futureevents.

3. Seismic hazard mapping ofGuam, or other developing areas,would serve a very useful purposefor land use planning and riskmanagement. Guam has thepotential for future earthquakes onfaults on the island itself. Therecent event demonstrates vulnera-bility to liquefaction and seismical-IV induced sliding.

6. The economic and social impactof this event is likely to be verysignificant. The compact nature ofAlj)the island and its economy makes .,

Guam an interesting laboratory tostudy these factors which areoften neglected .

1. The earthquake was very largebut was located at a distancewhich appears to have led toground motions on Guam thattended to affect moderately longperiod structures more adverselythan others. The causes of theobserved structural damage aregenerally typical of those found inprevious earthquakes. contribut-ing factors include combinations ofpoor structural configuration,interaction of masonry in-fill withstructural elements. design detailswithout adequate ductility, andpoor construction Quality. Some

'2

Documents

EERI SPECIAL EARTHQUAKE REPORT -OCTOBER 1993 · EERI SPECIAL EARTHQUAKE REPORT -OCTOBER 1993 radius of 660 km (412 miles)! At various times in the days following the earthquake, the