D D C - DTICAZTX Amarillo, Texas BLWV iveckley, West Virginia BMSO blue Mountains Seisinological Obs*ervatory bMIX Balmorhea, Texas BRPA Berlin, Pennsylvania bXUT blanding, Utah CK;%C

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CO

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UNCLASSIFIED TECHNICAL REPORT VÜ-65-4A

AIR FORCE TECHNICAL APPLICATIONS CENTER HEADQUARTERS UNITED STATES AIR FORCE

WASHINGTON DC 20333

MAGNITUDE DETERMINATION AT

REGIONAL AND NEAR-REGIONAL DISTANCES

IN THE UNITED STATES

UNCLASSIFIED VERSION

C AND TION

CLEARINGHOUSE FOR FSDEKAL SCIENTIFIC

TECHNICAL INFORT'*T

Miorofiohe

30 AUGUST 65

Bardcopf

PREPARED FOR

ARPA PROJECT VELA-UNIFORM

BY

AF TECHNICAL APPLICATIONS CENTER

VELA SE1SM0L0GICAL CENTER

D D C UJ JUN231966

iWLbü U Lb C

DISTRIBUTION OF TH'S DOCUMENT IS UNLIMITED.

Best Available

Copy

• .-I

AIR FORCE TECHNICAL APPLICATIONS CENTER HEADQUARTERS UNITED STATES AIR FORCE

WASHINGTON DC 20333

-

MAJOR GENERAL J. F. RODENHAUSER, CHIEF

.■ MAGNITUDE DETERMINATION AT REGIONAL AND NEAR-REGIONAL DISTANCES IN THE UNITED STATES

AFTAC/VELA SEISMOLOGICAL CENTER TECHNICAL REPORT VU-65-4A

PROJECT VELA-UNIFORM

30 August 1965

PREPARED BY: JACK F. EVERNDEN Research Associate

APPROVED BY CARL F. ROMNEYf Assistant Technical Director AF Technical Applications Center

''S DISTRIBUTION 0^ Tf- DOCUMENT IS U^Lli.VuE.U

i NOTICE

This report is Che unclassified version of Technical Report VU-65-4. All figures and discussions revealing classified data on weapons tests at the Nevada Test Site have been deleted. Such data in no way influences the seismological conclusions of this report.

l

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ABSTRACT

L. This rspoct deals with the problems of variation of Pn amplitudes in the regional and near*regional distance ranges (200-2100 Kilo- meters). The data used were recorded by Long Range Seismic Measurement (LRSM) vans cf the VELA SelsmologicsI Center as a result of earthquakes throughout the United States and numerous nuclear and chemical explosions in the same region.

2. It is shown that the patterns of Pn amplitudes versus L in Western and Eastern United States (VUS and EUS) are markedly different and that these differences are related to different velocity structures In the two regions. These differences extend to at Least 150 kilometers depth.

3. Neither the WUS nor EUS patterns conform even approximately to that predicted or suggested by Gutenberg and Richter in the 1000-2000 kilometer range. The hypothesized shadow-zone does not exist and overestimation of magnitude by as much as 1.3 magnitude units is frequently done because of failure to properly understand the patterns of radiation.

4. By proper calibration of WUS by use of numerous events. It is now possible to get consistent estimates of magnitude at all distance ranges for most explosions and earthquakes. This is very Important when body wave magnitudes are used as an essential element In an identification criterion.

5. Gotftining consistent estimates of amplitude as a function of distance for a particular event requires a knowledge of the energy distribution between the several refracted phases ufed between 200 and 2300 kilometers distance. Data on hand show that the energy partition function is reasonably uniform throughout the regions investigated but that locally It may vary radically» resulting in a ten-fold change in relative excitation of two refracted phases.

6. Patterns of energy radiation, such as those for EUS and WUS, are probably approximately determinable from knowledge of velocity structure and vice versa but are not determinable from earthquake investigations. They are only determinable from explosion data where origin time and focus are accurately known.

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CONTENTS

Page

INTRODUCTION 1

EASTERN UNITED STATES 3 1. Travel-Time Curves for EÜS 2. Magnitudes as Computed by the Shot Report for EUS Events 4 3. The Dependence of A/T on A and the Computation of

Magnitude for EUS Events

WESTERN UNITED STATES 9 1. Travel-Time Curves for WUS 2. Travel-Time Curves of FORE 10 3. Travel-Time Curves of MISSISSIPPI 4. Travel-Time Curves of BILBY and CLEARWATER II 5. Regional Variations of Travel-Time Curves in WUS 6. Conclusion

ESTABLISHMENT OF A/T VERSUS & CURVES FOR WESTERN UNITED STATES 12 I. P7.9 Arrivals 2« Pl0.5 Arrivals 14 3« P8.5 Arrivals 4. Pg.i Arrivals

DETAILED ANALYSES OF STATION MAGNITUDE VALUES FOR MISSISSIPPI, 15 BILBY, AND HARDHAT

1. Introduction 2. MISSISSIPPI 16 3. BILBY 17 4. HARDHAT 18

COMPARISON OF MAGNITUDES COMPUTED BY msr AND THE SEVERAL 20 CURVES OF THIS REPORT

RECALCULATION OF ALL SHOTS PREVIOUSLY ANALYZED IN SHOT REPORTS 20

RELATIVE MAGNITUDE BY USE OF DATA OF FIXED STATIONS 25

APPLICATION OF THE PHASE CURVES TO EARTHQUAKES OF WESTERN 26 UNITED STATES

1. General 2. Earthquakes Studied 27 3. Small Earthquakes 28 4. Libyan Earthquake of 21 February 1963

USE OF P PHASES AS DIAGNOSTIC AIDS 28

Page

VELOCITY STRUCTURE OF CRUST OR CRUST AND UPPER MAiNTLE IN 28 EASTERN UNITED STATES AND WESTERN UNITED STATES

1. EUS 2. WUS 29

1. 2. 3. 4. 5. 6. 7. 8. 9.

10. U.

12.

13. 14. 15.

APPLICATION OF IDEAS OF THIS PAPER TO REGIONS OUTSIDE THE 30 UNITED STATES

1. General 2. Europe 31 3. Asia

CONCLUSIONS 31

APPENDIX - ANALYSIS OF Pg DATA 34 1. General 2. Analysis of Pg Data from Shot Reports 3. Pg Data for WUS Earthquakes 35

FIGURES

Epicenters of Events Studied Reduced Travel-Time Data for SS VILLAGE Reduced Travel-Time Data for SALMON Reduced Travel-Time Data for GNOME - EASTERN PROFILE Reduced Travel-Time Data for West Virginia Earthquake Reduced Travel-Time Data for New Madrid Earthquake Reduced Travel-Time Data for Poplar Bluff Earthquake Magnitude versus Epicentral Distance for SS VILLAGE Magnitude versus Epicentral Distance for SALMON Magnitude versus Epicentral Distance for GNOME - EAST Magnitude versus Epicentral Distance for Texas-Louisiana Earthquake Magnitude versus Epicentral Distance for West Virginia Earthquake Magnitude versus Epicentral Distance for New Madrid Earthquake Magnitude versus Epicentral Distance for Poplar Bluff Earthquake Magnitude versus Epicentral Distance for South Dakota-Nebraska Earthquake

16. Magnitude versus Epicentral Distance for Western Vermont Earthquske

17. Reduced Travel-Time Data and Curves for FORE 18. Reduced Travel-Time data and Curves for MISSISSIPPI 19. Reduced Travel-Time Data and Curves for BILBY 20. Reduced Travel-Time Data and Curves for CLEARWATER 21. Reduced Travel-Time Data and Curves for CLIMAX

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22. Reduced Travel-Time Data and Curves for GNOME - WESTERN PROFILE 23. Reduced Travel-Time Curves for EUS, WUS, and Europe 24. Magnitude versus Epicentral Distance for SHREW 25. Deleted 26. Magnitude versus Epicentra 27. Magnitude versus Epicentra 28. Deleted 29. Magnitude versus Epicentra 30. Magnitude versus Epicentra 31. Magnitude versus Epicentra 32. Deleted 33. Magnitude versus Epicentra 34. Magnitude versus Epicentra 33. Magnitude versus Epicentra 36. Magnitude versus Epicentra 37. Magnitude versus Epicentra 38. Magnitude versus Epicentra 39. Magnitude versus Epicentra 40. Magnitude versus Epicentra 41. Magnitude versus Epicentra 42. Magnitude versus Epicentra 43. Magnitude versus Epicentra 44. Magnitude versus Epicentra 45. Magnitude versus Epicentra 46. Magnitude versus Epicentra 47. Magnitude versus Epicentra 48. Deleted 49. Magnitude versus Epicentra 50. Magnitude versus Epicentra 51. Magnitude versus Epicentra 52. Magnitude versus Epicentra 53. Magnitude versus Epicentra 54. Magnitude versus Epicentra 55. Magnitude versus Epicentra 56. Magnitude versus Epicentra 57. Magnitude versus Epicentra 58. Magnitude versus Epicentra 59. Magnitude versus Epicentra 60. Magnitude versus Epicentra 61. Magnitude versus Epicentra 62. Magnitude versus Epicentra 63. Magnitude versus Epicentra 64. Magnitude versus Epicentra 65. Magnitude versus Epicentra 66. Magnitude versus Epicentra 67. Magnitude versus Epicentra 68. Magnitude versus Epicentra

I Distance Eor MAD 1 Distance for MINK

1 Distance i Eor CHINCHILLA II 1 Distance for R0AN0KE 1 Distance l for CHINCHILLA

I Distance for ALLEGHENY 1 Distance : for KAWEAH 1 Distance i :or STILLWATER 1 Distance 1 for CODSAW 1 Distance 1 :or BOBAC 1 Distance for RINGTAIL 1 Distance 1 for SACRAMENTO 1 Distance for STOAT 1 Distance 1 for WICHITA 1 Distance i for AGOUTI 1 Distance i :or SANTEE 1 Distance i or ARMADILLO 1 Distance i :or PACKRAT 1 Distance 1 :or CASSELMAN 1 Distance 1 for HYRAX

1 Distance f 'or ACUSHI 1 Distance 1 or DORMOUSE PRIME 1 Distance f or DORMOUSE 1 Distance i :or CIMARRON 1 Distance i for YORK 1 Distance 1 or PEBA 1 Distance f or PASSAIC I Distance i .or FISHER 1 Distance i or MERRIMAC 1 Distance i or HAYMAKER 1 Distance i "or SEDAN 1 Distance f or AARDVARK 1 Distance i :or STONES 1 Distance i or FORE 1 Distance f or WAGTAIL J Distance f or MISSISSIPPI 1 Distance t or BILBY 1 Distance 1 or FEATHER 1 Distance f or CHENA 1 Distance f or PLATTE

vii

69. Magnitude versus Epicentral Distance 70. Magnitude versus Epicentral Distance 71. Magnitude versus Epicentral Distance 72. Magnitude versus Epicentral Distance 73. Magnitude versus Epicentral Distance 74. Magnitude versus Epicentral Distance 75. Magnitude versus Epicentral Distance 76. Magnitude versus Epicentral Distance 77. Magnitude versus Epicentral Distance 78. Magnitude versus Epicentral Distance 79. Magnitude versus Epicentral Distance 80. Magnitude versus Epicentral Distance 81. Magnitude versus Epicentral Distance 82. Magnitude versus Epicentral Distance 83. Magnitude versus Epicentral Distance 84. Magnitude versus Epicentral Distance 85. Magnitude versus Epicentral Distance 86. Magnitude versus Epicentral Distance 87. Magnitude versJS Epicentral Distance 88. Magnitude versus Epicentral Distance 89. Magnitude versus Epicentral Distance 90. Magnitude versus Epicentral Distance 91. Magnitude versus Epicentral Distance 92. Magnitude versus Epicentral Distance 93. Magnitude versus Epicentral Distance 94. Magnitude versus Epicentral Distance 95. Magnitude versus Epicentral Distance 96. Magnitude versus Epicentral Distance 97. Magnitude versus Epicentral Distance 98. Magnitude versus Epicentral Distance 99. Magnitude versus Epicentral Distance 100. A/T versus A for P Phases in the United States 101. Mean A/T versus Distance (10 KT - NTS Shots) 102. ints versus log (A/T)5oo (NTS Shots) 103. A/T versus A for Pg for RINGTAIL 104. A/T versus A for Pg for FISHER 105. A/T versus A for Pg for MISSISSIPPI 106. A/T versus A for PR for ANTLER 107. Deleted

108. log (A/Tsoo^g v«""» "V 109. Deleted HO. Deleted

for ANTLER for MADISON for DES MOINES for MARSHMALLOW for CLEARWATER for HANDCAR for HARDHAT for SHOAL for DANNY BOY for GNOME - WEST for CLIMAX for SWORDFISH for Earthquake of 2 Feb 1962 for Earthquake of 5 Feb 1962 for Earthquake of 15 Feb 1962 for Earthquake of 15 Feb 1962 for Earthquake of 25 Feb 1962 for Earthquake of 14 Apr 1962 for Earthquake of 27 May 1962 for Earthquake of 20 Jul 1962 for Earthquake of 21 Aug 1962 for Earthquake of 30 Aug 1962 for Earthquake of 5 Sep 1962 for Earthquake of 16 Sep 1962 for Earthquake of 6 Nov 1962 for Earthquake of 18 Nov 1963 for Earthquake of 22 Mar 1964 for Earthquake of 5 Jul 1964 for Earthquake of 24 Oct 1964 for Earthquake of 22 Dec 1964 for Libyan Earthquake

(2)

viii

TABUES

I West Virginia Earthquak«, 23 Nov 196A T«ixaa-Louisiana Earthquak«, 24 Apr 1964

II nib* versus Distance in EUS

III Lake Superior, Oct 1964

IV ng,} Magnitude Values

V MISSISSIPPI

VI BILBY

VII HARDHAT

VIII Magnitude Data on Selected NTS Shots

IX Comparison of B^' and Individual Station Relative Magnitudes

Page

5 6

8

8

15

16

17

18

21

25

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4.0NG RANGE 3EISMIC MEASUREMENT VAN LOCATION DESIGNATORS

Ar OS Aprfchc, Oklahoma ARWS Aurora, Wisconsin AVNv Aus.tin, Nevada AtSD Academy, South Dakota AZTX Amarillo, Texas BLWV iveckley, West Virginia BMSO blue Mountains Seisinological Obs* ervatory bMIX Balmorhea, Texas BRPA Berlin, Pennsylvania bXUT blanding, Utah CK;%C Cache Creek, British Columbia (PCL Campo, California CPSO Cumberland Plateau Seisnologics I Observatory CTOK Clayton, Oklahoma CVTN Centervllle, Tennessee DHNY Delhi, New York DRCO Durango, Colorado DICK Durant, Oklahoma EBMI East hraintree, Manitoba EFIX Eagle Flat, Texas EPTX El Paso, Texas El'AL Eutaw, Alabama I^Ml T nilmore, Utah IRMA iorsyth, Montana TSAZ flagstaff, Arizona C1MA v lendive, Montana CNNM Tnome, New Mexico i PMN v'r.ird Kap ids, Minnesota CVTX orapevine, Texas hbOK Hobart, Oklahoma HDPA Howard, Pennsylvania hETX Hempstead, Texas Hr:ND i'annah, North Dakota liLi:» Hailey, Idaho mw 100 Mile House, British Columbia IITMN Hastings, Minnesota 1; MA Itysham, Montana JELA Jena, Louisiana Jinx Juno, Texas KM'T Kansb, Utah LCNM Las Cruces, New Mexico LPTX La Pryor, Texas • LSNH Lisbon, New Hampshire

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MLNM »Mogollon, New Mexico MMTN McMinnville, Tennessee MNW Mina, Nevada Mi-AK Mouncain Pine, Arkansas MWA Mount Baker, Washington MVCL Marysville, California MJWS Niagara, Wisconsin PMWY Pole Mountain, Wyoming PTOR Pendleton, Oregon PJOK Perkins, Oklahoma KKON Red Lake, Ontario RINM Raton, New Mexico RVND Ryder, North Dakota SEMN Sleepy Eye, Minnesota SfAZ Snowflakc, Arizona S ilX San Jose, Texas SKTX Shamrock, Texas SS1X Sanderson, Texas SVAZ Sprmgervil le, Arizona TDNM Tres Piedras, New Mexico IftL lalt, California 11 SO Tor.to Forest Seismological Observatory 1KWA Tonasket, Washington I'USO . inta Basin Seismological Observatory VN'JT Vernal, L'tah VTOR v'cnator, Oregon WIN". Winnemucca, Nevada WMAZ Williams, Arizona WMSO Wichita Mountains Seismological Observatory WNSD Winner, South Dakota

XI

INTRODUCriON

1. Little advance has occurred In our understanding of so-called magnitude values computed at regional or near-regional distances bv use 01 h. dy w.ivoi ««intc the work of Richter and Gutenberg (VESIAC Report CulO-7l.X, Proceedings of the VESIAC Conference on Seismic Event Magnitude Determination, May 1964). Work done by Romney some time ago and described in detail In this report constitutes one of the few exceptions to this generalization. Seismologists have tended to simply adopt the curves given by Richter, extend them by arbitrary assumption», or to ignore the whole matter. The most cursory glance at available Amplitude/Period (A/T) versus Eplcentral Distance (A) data impresses one with the profound control exerted on such data by rtgional crustal characteristics. Imposition of a "standard" A/T v( rsus A riirvo on data lor tvonts east of the Rocky Mountains, here- aft»r riurfid lo .is Eastern United States (EDS), with the associated plotting of magnituoe (m) versus A rather than A/T versus A obscured the simple A/T versus A relationship of that region. Even in the Western United Slates (WUS) the standardized approach to magnitude computations as exemplified by VELA reports dealing with individual Nevada Test Site (NTS) events (so-called "shot reports") effectively obscured significant aspects of travel-time and amplitude data as well as evidence for the vertical velocity structure in the crust and upper mantle, llu- whole concept of the "crust" of the earth as defined by seismic velocity data may require revision based on data obtained by the VELA program. This report attempts to advance understanding ol lh< probhms associated with magnitude determinations at

rtö'^n^l and near•regional distances. Corollarv problems that became evident during the analysis will be discussed at the appropriate piacfs.

2 Roih short and l-^n^ period amplitude measurements of high quality lor both earihquakts and explosions are now available as a result of the ARPA/AFIAL program They show that the A/T versus A relationship in the rangi' 200-2100 kilomrters tor the first P phase as published In tabulated loim in the VELA shot reports represents a comparatively crude at»«>mp^ lo express the observed pattern of variation of P amplitudes with distance. That curve bears no relation to patterns of P energy resulting from events in the EUS. The supposed profound decrease and subsequent- Increase of amplitude in the neighborhood of 1000 kilometers epicentral distance (the "observational" basis for the "Shadow Zone") in the WUS has little basis in fact. The misconception

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arose by unknowingly .Utempting to analyze the A/T versus A data for three independently propap.at in«» refracted phases by one curve. It will In shown UMC magnitudes computed according 10 the lornvil.i appropriate to tach phase largely remove the apparently profound dependence of determined magnitude upon distance of observation. Systematic variations of 1.9 magnitude units occur when using the shot report A/T versus Ä table at regional and near-regional distances (for example, see Figures 8, 9, 10, 58, 60, 62, and all plots of data from earthquakes). Such overestimation of magnitude becomes a matter of concern to bomb detection and identification studies as some proposed identification criteria demand a valid and meaningful ordering of seismic events on a body wave magnitude basis. For example, miscalculation of the magnitude of events can effectively garble such a criterion as AR * versus body wave magnitude. Most of this report deals with the clarification of the problem of understanding the variation of P amplitudes as a function of distance in the regional and near-regional distance ranges (150-2100 kilometers).

3. The data of this study refute widely held ideas about the functional relationship between amplitudes of P phases and epicentral distance. In so doing, the data clearlv show that some current ideas about the proper placement of seismometers for optimum detection capability are grossly misleading for the WUS and are untrue for the EUS and, by extension, for much ol the world. The WUS represents only one of many possible areas in terms of velocity structure and, unfortunately, most studies of amplitude as a function of distance have treated WUS events. Misinterpretation of the data resulted in invalid conclusions for even that region. We will demonstrate the complete irrelevancy of the currently accepted A/T versus A assumptions to conditions in EUS though many seismologists assume these assumptions to be valid No shadow zone exists in EUS and amplitudes of tho ^85 phase increase continuously from tcleseismic distances to the epicenter (that is, to the inner limit of existence of the phase at a hundred or so kilometers). A 10-fold increase in signal-to-noise ratio, relative to that at an epicentral distance of 65°, can be obtained by

observinc Pio*) at 2o0 or p8.5 at ll0•

Brune, J. A Espinosa, J Oliver, 1963, Relative Excitation of Surface Waves by Earthquakes and Underground Explosions in the California-Nevada Region, J. G. R., Vol 68, pp. 3501-3513,

2 Mansfield, R. H., Measurements of Pg/Pn, Lg/Pg, and AR for Nuclear Explosions and Earthquakes, VSC Technical Report VU-64-1.

4. The observed tt a^c l-tioe compl icot ions in Wl'S nt distapcc. o: U-.-> than 2000 hilomctrr-i arc Inr^cly tiont<wiscent tor r.nny otlicr nrtar», the general belief in the universality of such corr.pl ical ions results i ru-n confusing the pro' lem of multiple- refractors with that of regional variations of refraction velocities. Possibly, highly seismic regions have, as a corollary foature, complex velocity structures.

t). Ibis rnpo-t purports to demonstrate the general statements made above and to present the quantitative relationships between amplitude, distancej a-id pha^c for Fi S and WUS. We will consider initially El'S because of its Tcmarkable simplicity and will then auompt to explain the major problems inherent in understanding amplitude relationships in the P phases arising from WUS events. Any careful attempt to develop a meaningful A/T versus A r».'lat lonship requires the ide^t it ication of phases., the dttermination of thei' distance dependence, and their av.iage energy content relative to othvr phases.

6 Magnitude has varied definitions with none universally accepted. Sinê the important points to consider include relative ordering of events and establishment of their relative magnitude, we can accept any reasonably meaningful poi^t of departure for oar definition. The definition of the maîtude we accept is as it is deti-ed by several obseivations at 20-30 , according to Richter's^ published curve for P body waves. Formulas dertved herein yield a uniform estimate of the magnitude by use of data obtained between 200 kilometers and teleseismic distances. We can then use those formulas to determine a meaningful estimate of relative magnitude of events over any distance range.

7. Ihe bubsmp*. nom,. ^clat ..r .■ used to characterize paiticular phases should not bo •aK-. too literally. A subscript of (7.9) mea^s that we attempt to treat under one phar.e and equation profil»s with observed velocities m the ra^g« C T-7.9,>. The (8 b) subscript should be considered as indication of velocities in the range (8.3-8 7) while (8.1) covers (8.0-8.1) in WUS. The P[0.i subscript identities the first segment of the travel-time curve observed beyond P^.S- These phases are distinct a-id arc- associated with distinct refractors.

EASTERN UNITED STATES

l- TiavoUIime Cuives fot EUS. The standard (T • A/8.1 versus L) figures of the shot reports show clearly the essential character of Pn travel- time curves for EUS (Figures 2 through 7). The Jeffreys-Bullen (J-B) tiavel -time cu'-'o is included in all figures. Figures 2 through 7 give

•^See, for example. Richter, C. P., i958. Elementary Seismology, Freeman and Company, San Francisco., p. 688.

K • the first .irrivnl data observed for the indicated events, numerous figures being presented in order to make it clear that this pattern of travel-time profile typifies epicenters throughout BUS. Figure 1 indicates the epicenters of these events and all others considered in this report. Note that the events ot CUS are distributed from Eastern New Mexico to the Western Atlantic Ocean and from the South Dakota-Nebraska Border to the Louisiana Gulf Coast. Clearly, the simple profile of a Pn leg of approximately 8.3 velocity extending from around 150 to about 2100 kilometers typifies the region. At that distance, a deeper refraction becomes first arrival, travel times then following the J-B curve with regional characteristics no longer playing a significant role. A comparison of this ttav«'1 tim«? curve pattern with the incomplete pattern of curves of WUS presumed in Figure 23 indicates the gross contrast of velocity scri'Cwurts ot the two areas. More discussion of this point will follow a uiscussion ot iht WUS profile. For the moment, we note only the existence ot t!u centrast with its implications of dlfferenr patterns of rtnpl i tuacs .-is a function of distance from epicenters in the two areas.

2. Irf-ni tudes as c ompuled by the Shot Report Table for EUS Events.

The iic If circles on Fißures 8 through 16 indicate magnitude values (msr) based upon the stdiidurd shot report A/T versus Zi table in the ranr.e of ' 2100 l.i loroct «rv. Clearly, the ma.nitude estimates are a function ct distance and thu« unsatisfactory in estimating the magnitude of EUS events. Not si'rpr isin.-ly, a Generalized A/T versus /^ curve attempting to cxnrcsn rlu- com;)!* xi t IFS of WUS velocity structures fails completely viu ii i.i'.jlud tr t'US, a simple region displayin;; essential phase con- tim.ity ov» T i !u i m i re interval, 150-2100 kilometers.

Lli. '1'.Tj^* m ' (1 A/' on M and the Cor.ipiTiation of lla^nitudc for EIS

a- It is .«vsiirud that t hr relationship between m, A/T, and A is of t h- torm

m - a ♦ loz (A/T) + n log A (1)

whore m is m.igni tud*-, a and n are constants to be determined, A/T is measured in mlil-m.crons per second, and A» the epicentral distance, is measured in l;i loiP'jt cr* The equation can be rewritten as C - m - a • Ic; (A/T) ♦ n log A . the correct value of n being that for which ( is independent of A Alter n is determined, a is evaluated by t'.se ot the rvr;ti'tüd' value determined at teleseismic distances (beyond th«- Ps#5 le- «f ira.d time curve or, in this case, beyond 2100 kilo- met f rs) .

b An estimate «. t n can be obtained by determining a least square fit ot lo^. A/T vtr-us n log t data for events occurring in EUS. The six events

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selected and the computed values of n with their standard deviations are: n

West Virginia Earthquakes (25 Nov 1964) -1.84 + 0.23

Texas-Louisiana Earthquake (24 Apr 1964) -2.26 +0.33

GNOME - Eastern Profile -2.07 ♦ 0.39

SALMON -2.37 ♦ 0.34

SS Village -1.56 ♦ 0.36

Poplar Bluff Earthquake (3 Mar 1963) -1.88 ♦ 0.54

Average -2.0

As examples of the influence of the value of n on data interpretation, a detailed presentation of the data for the first two events above is given in Table I. Note that, according to VELA custom, all standard deviations indicated are standard deviations of a single determination. Standard deviations of the mean are generally one-third to one-quarter of the standard deviation of a single determination.

TABLE I

West Virginia Earthquake 25 Nov 64 02:50:07. 1 m - 3.58 ♦ ts

0.20

Station & A/T log log A/T log A/T log A/T log A/T Äm8.5

tn (km) nvt/s t ♦ log A ♦ 2 log< ̂ ♦ 3 log

s r

BRPA 377 51.8 2.576 1.714 4.290 6.866 9.442 3.60 4.61 CPSO 403 55.6 2.605 1.745 4.350 6.955 9.560 3.69 4.75 HDP A 537 14.5 2.730 .161 3.891 6.621 9.351 3.35 4.64 DHNY 795 19.7 2.900 1.294 4.194 7.094 9.994 3.82 5.09 LSNH 1122 3.26 3.050 0.513 3.563 6.613 9.663 3.34 4.81 GPMN 1507 2.93 3.178 0.467 3.645 6.823 10.001 3.55 4.27 WMSO 1556 4.36 3.192 0.639 3.831 7.023 10.215 3.75 4.24 RKON 1784 2.87 3.251 0.458 3.719 6.970 10.221 3.70 3.36 RTNM 2022 2.79 3.306 0.433

C - 6.890

3.739

- 3.58 -

7.045 6.890

a, a - •

10.351

. 3.31

3.78 3.62

3.35

J

\

Texas-Louisiana Earthquake 24 Apr 1964 07:33:53.5 mts - 3.85 ♦ 0,34

Station A A/T log A log A/T log A/T log A/T log A/T in8#5 msr

(km) ♦ log A ♦ 2 logA ♦ 3 logA

JELA .'/() 4<J2.2 2230 '2 692 4,922 7.152 9.382 3.88 4.89 ^ GVTX 333 3/4,0 2.522 2.573 5.095 7.617 10.139 4.34 5.27

WMSO 365 14.1 2.752 1.149 3.901 6.653 9.405 3.38 4.55 EUAL 574 40.5 2.759 1.607 4.366 7.125 9.884 3.86 5.01 JUTX 715 52.0 2,854 1,716 4.570 7.424 10.278 4.15 5.41 CPS0 883 23.8 2.946 1.377 4.323 7.269 9.466 4.00 5.38 RTNM 1127 (2.04) 3.052 0.310 3,362 6.4U 9.980 (3.14) 4.61 LCNM 1212 5.35 3.084 0,728 3.812 • 6.896 10.249 3.63 4.93 BLWV UJ3 7.49 3.125 0.874 3,999 7.124 10.237 3,85 4.98 BPPA 1614 3-96 3.213 0.598 3.811 7.024 3.75 4.00

7.070 3.80

C - 7.070 - 3.85 - a, a - -3.22

c. Columns 6, 7, and 8 give values of C for n - 1, 2, and 3, tespectively. The values of Column 7 (n " 2) appear independent of A while those of Columns 6 and 8 (n - 1 and n - 3) show a dependence of C upon A. The values of "a'* follow immediately (-3031 and -3.22), their average value of -3.27 being adopted. Finally, the formula desired reads

"eus " «"g.^ " -3-27 ^ l0« A/T ♦ 2 log A . (2)

This equation is j^hown graphically in Figure 100 as the Pg. 5 curves, A/T being measured in millimicrons per second and A in kilometers Column 9 gives the magnitude compur^d according ro this formula as a function of distance. Compart these values with those of Column 10 which were computed according to the shot leport table. There is obvious disagreement of msr values with the teleselsmlc determination (mrs) while the mo ^ values yield the statistically Identical estimate of magnitude at both distance ranges. Note that magnitude estimates that differed bv 1.74 units (DHNY and RTNM) according t ■> insr differ by only 0.04 units when computed by mg 5.

d. The application of formula (2) to other events in EUS achieves clarification and greater Internal consistency of computed magnitudes. Figures 8 through 16 give the relevant data for all events studied. The events studied Include surface explosions of zero focal depth, underwater explosions, ard earthquakes of unknown but shallow focal depth The variation of ivpv of ev«;nl is such that dependence of the observed patterns upon a spec', fie source type cannot be seriously considered. Also, A/T appears little dependent on profile azimuth. The only GNOME data used in this part of the report comprise the profile laid out eastward from the test site. Tne only event showing marked discrepancy between mg^ and mtg is the Lake Superior 10-ton shot, mgj being 4.3 ♦ 0.35, whereas mts is 3.5 + 0.35. The reason for this discrepancy cannot be demonstrated but

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•

it will be shown under the discussion of the Nevada Test Site (NTS) events that variation in rock type at the source can lead to large variations in the pattern of energy distribution between generated P-wave signals.

e. Note the systematic discrepancy of the msr values, maximum predicted values by this formula being around 1000 kilometers. Figure 100 shows that msr overestimates the magnitude by 1.5 units at 1000 kilometers and underestimates it by 0.4 units at 1900 kilometers, a difference of 1.9 units introduced not by the tata but by the use of an inappropriate formula.

f. Note also that the A/T versus A data of these events indicate the total lack of any obvious shadow zone phenomenon. No shadow zone exists in EUS and the supposed problems of observing at distances of less than 2000 kilometers because of low signal levels do not exist for EUS events. The signal level at 1000 kilometers attains 30 times that predicted by the msr table and more than 10 times that predicted by msr at all distances between 400 and 1400 kilometers. The implications for ehe relative advantage of regional and near-regional stations versus teleseismic stations for regions of the crustal type of EUS appear obvious.

g. The Vermont Earthquake data depicted in Figure 16 provide an excellent example of the misconceptions that can be reached by use of Inappropriate formulas« Very few stations recorded this event, no station beyond 2017 kilometers receiving a detectable signal« How- ever« by use of mgr values, one obtains an estimated magnitude of A.7, an average of 5 values ranging from S.4 to 3«3« An estimate of 4.7 magnitude on the basis of observations at regional and near-regional distances is in pronounced conflict with the fact of total lack of recordings at teleselsmic distances. Such an event should yield an A/T value of 10 tt\j, at 50° and should be widely recorded at teleselsmic distances« The significance of this misevaluation will be pointed out in the forthcoming discussion of AR versus m. Here simply note that, by use of%«8.5 values, the spread of m values for signals clear in both period and amplitude is 0.1 magnitude units as compared with 2.1 for msr values. The estimated magnitude becomes 3.8 or 3.9, consistent with no recordings beyond 2000 kilometers.

WESTERN UNITED STATES

I. Travel-Time Curves for WUS. In the WUS, the situation becomes much more complicated than in EDS. Several events must be studied In order to appreciate the complexity even to the extent that It Is now under- stood. One could not find a suite of events which would establish the basic uniformity of the WUS as such uniformity does not exist. It seems only possible to appreciate the major patterns resulting from explosions at NTS and to then use the limited data available from other areas to suggest the similarities and dissimilarities with NTS data caused by varying epicentrpl positions and varying azimuths of profiles. Ue shall consider GNOME, CLIMAX (the 0.2 KT chemical explosion at Climax, Colorado), 50 shots at NTS, and 17 earthquakes distributed fron Central Mexico through WUS to Queen Charlotte Islands.

8 & 9

2. Travel-Time Curves ot FORE.

a. It is con enient to begin the discussion with FORE because of the excellent and significant profiles that were installed for this event. See Figure 17, The generally observed P79 refracted wave was well obseived from 200 kilometers out to 1000 kilometers to the southeast, reaching Las Cruces, New Mexico (LCNM). Remember that all arrivals toted on figures of the type of Figure 17 are the first observed arrivals at the several stations. No observed second arrivals are shown. A Pg 3 leg was received as first arrival at Uinta Basin Sciätnological Observatory (UBSO), Blue Mountains Seismological Observatory (BM90), Forsyth, Montana (FRMA), and at Glendive, Montana (GIMA), the positioning of Raton, New Mexico (RTNM), on this line being probably fortuitous. Note that the line through these points appears parallel LO the SS Village profile but with a time intercept on the zero distance axis of 14 seconds rather than the 9 seconds of the SS Village profile. The time arrivals along the profile to the ESE provide both complex and interesting data. Blanding, Utah (BXUT), and Durango, Colorado (DRCO), lie well below the two pteviously mentioned refractions and cannot be related to either of them. Mote discussion of these arnvnls will be presented later. RTNM probably did not actually receive a Pg 5 arrival consistent with the 8.5 profile through FRMA and GIMA but we cannot identity the phase measured at this time. The next stations on this profile appear to have recorded a signal with a slope of about 8.5 but with a time intercept on the zero distance axis of nearly 20 seconds. At least, they did not record the phase seen at BXUT and DRCO but something much later and with a higher refracted velocity.

b. Finally, the P phase generally received as first arrival beyond 2000 kilometers is well observed a\ EBMT and RK0N, 2148 and 2338 kilometers icspectively, as well as at Hempstead, Texas (HETX), Hannah, North Dakota (HHND), and Ryder, North Dakota (RYND), the latter being at an epicentral distance of 1700 kilometers. Thus RYND failed to observe earlici but less energetic rcf.actions, recording instead the strong P^Q 5 arrival a^ first arrival.

3. Travel-Tim*- Curves of MISSISSIPPI. MISSISSIPPI (Figure 18) shows similarity to FORE, P^ 9 being observed towards LCNM to 1200 kilometers and to shorter distances in other azimuths. Hai ley, Idaho (HLID), and DRCO received as first arrivals signals preceding the expected P7 9 event. The profile across Colorado and the Dakotas follows the previously mentioned 8.5 leg to WNSD but then jumps to Pio.5 ôr

Academy, South Dakota (AYSD), at 1601 kilomete:s. Other points are associated, in some cases, with unclarified phases.

10

4. Travel-Time Curves of BILBY and CLEARWATER. The interpretation of the BXUT, HLID, and DRCO arrivals becomes clear as soon as larger events such as BILBV and CLEARWATER are considered (Figures 19 and 20). Thess events show the existence of an 8.0 -8.1 refraction extending eastward from NTS through BXUT, DRCO, TDNM, and RTNM. Shamrock, Texas (SKTX), did not record this phase» a more energetic phase arriving at essentially the predicted time, while the figures show clearly that Apache, Oklahoma (APOK), Grapevine, Texas (GVTX), and Durant, Oklahoma (DÜOK), observed other phases as first arrivals. As will be noted below, much of the supposed difference between MISSISSIPPI and BILBY and the inability of a formula based upon MISSISSIPPI data to reasonably interpret BILBY data results from different patterns of observation in a heterogeneous region.

5. Regional Variations of Travel-Time Curves in WUS. Regional variations within the WUS appear when events from other than NTS sites are considered. For CLIMAX (Figure 21), located in Colorado, the first arrivals at most stations in WUS were an 8.1 refraction with no stations measuring a 7.9 event as first arrival. As noted above, the eastern profile from GNOME (Eastern New Mexico) recorded only Pg.S while Pg ^ and Pj^ were seen to the west and northwest (Figure 22), the Pg.l event being observed only very near GNOME. Note that the intersection of the Pg.S segment through Montana with PlO.5 occurs at approximately 2000 kilometers while for the SS Village profile these two refractions intersect at about 2200 kilometers. The suggested Pg.S segment through Texas and Okalhoma for NTS events intersects Pl0.5 at. about 1700 kilometers. Pg.S and P7.9 intersect at a distance of 1230 kilometers to the southeast and approximately 1000 kilometers to the north and northeast.

6. Conclusion. Thus, one may observe at least six different P phases from NTS shots and shots from different localities give rise to different patterns of travel-time curves. Compare this with EUS where essentially one travel-time curve for one phase satisfied all observed profiles. Not surprisingly, amplitude interpretation which does not depend upon phase identification, but which uses a single A/T versus A on such complex data as that recorded from NTS and other WUS foci has yielded nearly chaotic results.

11

■

I

ESTABLISHMENT OF A/T VERSUS A CURVES FOR WESTERN UNITED STATES

l^ P7 Q Airivals. Romney extensively employed Py^ arrivals (VSC Technical Note 10) to determine magnitudes of'the smaller NTS events. He observed this phase as first arrival fro« NTS to LCNM and beyond to a distance of approximately I0S0 kilometers. Northward from NTS, another excellent profile extends through Mlna, Nevada (MNNV), to Pendleton, Oregon (PTOR), Including HUD for events In which Pg.i la too small to be recorded. PTOR frequently misses P7.9 in favor of P8,5« Beyond PTOR In Washington, Montana, and the bakotas, either PQJ or P^Q.S records as first arrival measured. DRCO, to the east of NTS never records Py^ so that the data of that station should be Ignored when considering Py g. The dependence of A/T on A for the NTS-LCNM profile (data fxom FISHER, DORMOUSE, STOAT, etc.), tne LCNM-NTS profile (data from GNOME), and the NTS-PTOR pro* file (numerous events) Is Identical in all cases. This energy path should serve as an excellent source of Internally consistent estimates of magnitude of events within the region. The procedure adopted by Romney for calibrating this energy event against teleseismic data Is as follows (Romney, personal communication):

"a. (U) Since 1961 recordings from numerous underground nuclear explosions at the NTS have been made at mobile stations operated under the VELA LRSM program. These stations have been located at many distances and azimuths from the test site. Some of the stations have operated more or less continuously and have recorded data from many explosions, while others have moved from time to time, recording only a limited numbf cf explosions at a givtn location. The explosions have included yields smaller than ) KT and larger than 100 KT. Shot points have been somewhat dispersed within the test site, with the result that the propagation paths to fixed stations, and hence the amplitudes, have been affected somewhat. Variations In coupling which appear to depend upon the characteristics of the rock and other conditions near the explosions have also been noted. It Is, therefore, necessary to correct the measurements to common source conditions (explosive yield and coupling factors) before the data can be combined to give a good representation of the amplitude vsrtus distance relationship.

"b, (U) In the course of analysing recordings from the explosions it was found that, for any given station, the values of log A/T versus log Y (Y - yield) fit an approximate straight line if the measurements were restricted to explosions in Yucva Flats. This made It possible to find A/T at a standard yield for each station by fitting the best straight line to a plot of amplitude versus yield. The amplitude of 10 KT was read from this Line and used to represent the best value of A/T at the average distance of the station fron t^t recorded shots.

12

It was r«quired -.h^» at least lour shots which bracktl 10 KT De recotdrd before the datd w-e ufed This procedure tends to smooth out scatter in ih* data cau«od t^v small variations in coupling or in the path of propagation and by notse.

M (U) Ihi average valuos .1 log A/T at 10 KT for each station were plotted a* a function of distance (tiguxe 101). These points detin^ a line giv^n by

log A/T - 9,50 ♦ 0 72 - (304 ♦ 0.26) log A (3)

where A Is measured in kilome*^«-, T in seconds, and A in millimicrons. First, arrivals at stations between 200 and 1030 kilometers were used. The slope of this curve is almost identical with that fcund in LOGAN and BIANCA.

"d. (Uj Iho amplitudes ot P. 0 can he assoc»aved with telescismlc magnitudes by comparison with the signals xfcorded at long rangfs from the LRSM stations. The basic da a used came from the 13 explosions which were recorded at fire or mere stations at telesetsmic distance ( A> lb ) as wr11 as In the Pn range Thest shots v-re generally larger than 10 KT since only these larger shots can be safisfactorlly recorded at t*-leselsmic ranges. Unified magnitudes were computed for each such <xplcsi M from the- miasur-.d amplitudes and pnod of P. The average magnitude for »ach even«- was then compared with the value of log A/T at 500 kllcmerors, a? found by a least square fit of the P. _ amplitude versus distance data Results are shown in Figure 102 It may be seen that the Atagnirude 'netfast? more rapidly than log A/T at 500 kilometer*. The r6idHon<hip was tound by least squares *o be

in7 9 - 2.it*0/?.i *■ vl>2l ♦ 0.11) log (A/T>500 (4)

This implies that the *<Mesolsm'c amplitudes increase more rapidly with yield than does the amplitude at !>Ö0 ki lerntttrs. This probably results from the shift toward Iswe? It<quvrcles n the source as the yield inctcas*« together with th« reauced attenuation at low fre- quencies.

"•■> (u) Equations (3) and (4) may be combined to give the P. amplitudes at a function of magnitude The result is

m7 9 - -7.i5 *• 1.21 (log A/T ♦ 3,04 log A )," (5)

13

i

Note on Figure 100 that a discrepancy between msr and my g of 0.6 occurs at nj ^ equal to 5.0, 1117,9 ê^n8 smaller than msr.

^' ~10«5 Arrivals» We shall fit a smoothed curv« through the Gutenberg- Richter curve between 2100 and 3000 kilpmeters and, in addition, obtain the values of amplitude for Pio.3 observed at 1600 kilometers in North Dakota at RYND, etc. The equation obtained is

m10.5 " -10'35 * lo8 A/"1 + ^ log A (6)

where A/T is measured in millimicrons per second and A in kilometers. The corresponding curve is shown on Figure 100 as Pio.S* As far as is known, this dependence of A/T upon A occurs independently of azimuth.

^' £8.5 Arrivals. As noted above, PTOR, at approximately 1000 kilometers, gene-rally records Pg.ij as fust measured arrival. A phase with this velocity is observed as first arrival in Montana and the Dakotas, To the southeast, stations in Texas, Oklahoma, and beyond observe Pg.S as first arrival to 1800-2000 kilometers. The time intercepts for these three Pg 3 profiles appear quite different, both from each other and from that typical of this refraction in EUS. Even so, we proceed by attempting to employ the meus formula for these arrivals. The results agree satisfactorily with the magnitude computed for P7.9 and teleseismic distance. Therefore, we adopt the meus (i. e., mg^) curve for computing magnitudes for arrivals on Pg.S profiles in WUS.

^, £8.1 Arrivals. When establishing a curve for mg.p a most interesting result appears in this case, A well-observed profile for this refraction for NTS shots require« a large explosion, only DRCO recording this phase for most events. Either this phase possesses very little energy or the energy dissipates rapidly along the refractor. In this connection, rememb*1 that for the CLIMAX shot in Colorado, a small event generated a widely obser/ed Pg.j refraction. A/T values from CLIMAX measured toward Arizona decreased as about the inverse third power of the distance while along the profile from NTS through Southern Colorado into Northern New Mexico, A/I values for Pg.i arrivals decrease as the inverce seventh power of the distance. Note that DRCO occurs OP both the CLIMAX-Arizona and the NTS New Mexico profiles. High amplitudes typify Pg 1 arrivals at short distances from NTS. The signal decays so rapidly with increasing distance that it disappears into the noise beyond DRCO for small events. When one attempts to derive equations to use along the NTS-Ncw Mexico profile by use of CLEARWATER and BILBY, it becomes apparent that the data of the two events require a 0.9 difference in tne leading coefficient of the derived equations. Thus, for CLEARWATER, shot in mesa tuff, one obtains

mg ! (CLEARWATER) - -17.20 ♦ log A/T + 7 log A (7)

14

■

while for BILBY. shot at 2314 foot depth in valley tuff,

in0 . (BILBY) - -16.30 •»• log A/T ♦ 7 log A . (8)

Apparently, CLEARWATER coupled eight times as much energy (relative to that coupled Into the other measured refracted phases) into this phase as did BILBY. If one computes DRCO Pg . arrivals for events with varying shot locations according to Pa , ^CLEARWATER) and Ps , (BILBY), the following pattern emerges:

g ^ ,y~~^* -«, -g j

TABLE IV DRCO by mg 2. DRCO by mg#1

Depth . Magnitude (CLEARWATER^ (BILBY)

1191 ft 3. ,64 3. ,/2 1615 4. ,76 -

1000 3. ,98 3. ,71 625 3. ,46 3. ,27 696 3. ,40 3. ,03

630 2. ,72 3. 06 1351 A. ,52 -

3. ,72 3. 71 3. ,99 3, 63 3. ,71 3. 73

DORMOUSE Alluvium (3) MISSISSIPPI Valley Tuff (9) 1615 4.76 ' 4.89 CIMARRON Alluvium (9) STILLWATER Alluvium (9) C0DSAW Alluvium (9)

MINK Alluvium (3) HAYMAKER Alluvium (12) 1351 4.52 4.46 DES MOINES Mesa Tuff (12) MARSHMALLOW Mesa Tuff (12) MADISON Mesa Tuff (12)

Thus, all of the mesa tuff shots behaved similarly to CLEARWATER but so did all of the shallow valley shots. Only HAYMAKER and MISSISSIPPI coupled as little energy Into this refractor as did BILBY. Thus, the use of data obtained from this refraction throughout WUS requires extensive crustal calibration (CLIMAX profile versus NTS profile) as well as calibration of energy In the phase relative to source location. In other words, this phase does not appear very useful for routine Investigations. The very high rate of amplitude decrease eastward from NTS together with the markedly different behavior on another azimuth through the same area Imply the disappearance eastward of the velocity contrast requisite for the refraction. Such an azimuth dependent phenomnon requires lateral crustal inhomogeneitles.

DETAILED ANALYSES OF STATION MAGNITUDE VALUES FOR MISSISSIPPI, BILBY, AND HARDHAT

1. Introduction. Three events will be discussed In detail. Two of them, MISSISSIPPI and BILBY, Illustrate the problem of observing different profiles and thus possibly different phases for different events while Interpreting all data by an inadequate generalized formula. All three events illustrate a type of problem that even proper phase assignment

15

will not defeat. Obtaining the same estimated magnitude from several phases of the same event requires that the energy partition function for the event be Identical to that for the event or events used in establishing the function. Otherwise, magnitude becomes a function of phase.

2. MISSISSIPPI. MISSISSIPPI (Figures 18 and 64) probably represents the most significant event used in the establishment of the m_ _ formula. Not surprisingly then, m_ a and m values agree closely, use of the standard EUS m. . formula'obtained excellent agreement with m ..... w 8»5 ^ ^ * ,, ^ ts station-by-sta 8 ' it ion data are as follows:

The

• TABLE V

MISSISSIPPI

A Magnitude m sr A Magnitude »sr

MNNV 234 5.06 (7.9) 6.0 SSTX 1496 4,33 (8.5) 5.0 KNUT 278 4.68 (7.9) 5.5 WNSD 1507 U,Si (8.5) 5.6 FMUT 408 - ? 4.8 HBOK 1555 4.98 (8.5) 5.5 TFCL 415 3.70 (7.9) 4.5 CKBC 1578 4.52 (8.5) 5.0 WINV 483 4.83 (7.9) 5.8 WMS0 1596 4.55 (8.5) 4.9 FSAZ 485 4.86 (7.9) 5.5 AYSD 1601 4.95 (10.5) 6.0

CPCL 490 5.10 (7.9) 5.7 HMBC 1670 4.84 (8.5) 5.0 MVCL 517 - ? . PUOK 1714 7 TFSO 536 4,67 (7.9) 5.0 CT0K 1910 4.30 (10.5?) 4.3 DRCO 733 4.89 (8.1) 5.5 SJTX 1970 4.86 (10.5) 4.9 HUD 739 4.61 (8.1) 4.6 SEMN 1971 4.42 (10.5) 4.5

BMSO 863 4.47 (7.9) 5,2 MPAR 2083 4.49 (10.5) 4.6 PTOR 971 4.88 (8.5) 5.3 LR 17 2099 4,85 (10.5) 4.9 LCNM 1101 4.41 (7.9) 5.2 HTNM 2120 5.12 (10.5) 5.2 PMWY 1027 4.40 (7.9) 5.2 LR 16 2460 5.11 (10.5) 5.1 HKWY 1126 4.51 (8.5) 6.0 ARWS 2460 5.05 (10.5) 5.1 MUUA 1392 4.27 (8.5) 5.3 CVTN 2565 4.95 (10.5) 5.0

Average of all stations at less than 1900 kilometers - 4.76 (4.3-3.2) Average of 28 stations between 1900 and 4064 kilometers- 4.69 (4.3-5.1) Average of all m values at less than 1900 kilometers - 5.3 (4.3-6.0)

9 r

The figures in parentheses are the extreme values in each group of values.

16

Apparently, computotions based upon phase assignment and appropriate formulas have made the computed magnitude independent of cpicentral distance and have also reduced the range of variation of the data. Whereas the range In computed values was 4.3 to 6.0 using the shot report table, the use of phase curves reduced the range to 4.3 to 5*2, I. e., half the previous range. The pairs PMWY - HKWY and WMSO - AYSD clearly show the influence of proper phase identification. For the former pair, m values were 3.2 and 6.0, while properly interpreting the PMWY arrivÜT as P. g and the HKWY arrival as P 5 leads to magnitude estimates of m- • 4.40 and mft _ " 4.51, respectively. The m values for WMSO and AYSD are 4.9 and old, while, when making proper phase identifications, m . for WMSO is 4.55 and m.. 5 for AYSD is 4.95. Regarding the rejection of such stations as TrCL and MVCL, we argue that, for interpretation, we require a reasonably clear-cut relationship between data obtained at a given station and that recorded at teleseismic distances. If we have not established as yet such a relationship, the proper mode of interpretation of the data of such stations is unknown and we Ignore their recordings. Further investigations than reported here may render the data of such stations meaningful in a magnitude sense, and thus useable.

3. BILBY. For BILBY (Figures 19 and 65), the data are as follows:

TABLE VI

BILBY

A Magnitude m sr

A Magnitude m sr

MNNV 242 5.46 (7.9) 6.0 TDNM 880 5.59 (8.1) 5.3 KNUT 284 5.42 (7.9) 5.9 RTNM 1039 5.32 (8.1) 5.2 CPCL 483 ? early AZTX 1278 5.05 (8.5) 6.2 WINV 493 5.01 (7.9) 5,6 FRMA 1232 5.17 (8.5) 6.3

5.13 (8.1) TKWA 1337 7 6.0 MVCL 520 ? early SKTX 1426 5.26 (8.5) 6.2

TFSO 529 5.68 (7.9) 6.2 GIMA 1487 5.05 (8.5) 5.9 BXUT 586 5.68 (8.1) 5.9 WMSO 1593 5.18 (8.5) 5.5 UBSO 668 5.46 (8.5) 6.7 RYND 1705 5.40 (10.5) 6.0 DRCO 732 5.91 (8.1) 5.9 GVTX 1794 5.75 (8.5) 1 5.4 HLID 747 5.51 (8.1) 5.6 DUOK 1831 5.81 (8.5) ? 5.5 BMSO 371 ? 4.24 (8.5) ? 5.6 HHND 1926 5.83 (10.5) 6.0

Average of 18 stations at less than 1900 kilometers - 5.44 (5.1-5.9) Average of 18 stations beyond 1900 kilometers (1926-9100) - 5.6 (5.2-6.0)

17

N-»te that. •f\ ih- '„.; il 550-104Ü kilomoter?, the only profile observed lor BIL^V wa? th- MS - BXUT - DRCO - TDNM - RTNM path, and that P8.1 was cbsc^'ed as first atxival at all stations. When calculated according to mj 9 (r.aoitly ac«jpilpg that all arrivals in this distance range ate Py.^t nuigp.'.cudr ««tlnat-ra at these stations are 5.33, 4,64, 4.64, and 4.55, rc«pectlv-ly, val^-s below estimates made upon fj^g arrivals and values appartrtW decreasing with distance. The values entered in the tabulation above v< 'e found, as described previously, by calibration against the tele«eUmU data of this event. Note that the m-j tg and fflgj values appear sv5'-matIcally lev relative to m^Q ^ and mts values. We obtained, by use of th-. same fcrmulas, excellent agreement between magnitude estimates at aU ranges for MISSISSIPPI. This ImplUs that the energy partition for »hese two events, BILBY and MISSISSIPPI, differed markedly. The apparent agTc-iitn» of nig | and mts occurs because we calibrated the mg^ formula by use of this event. Probably, calibration against n-j g and mg 5 would have had more meaning, making all of the shallow refractions less energetic relative to m1Q 5 and deeper refractions for BILBY than for MISSISSIPPI. Such variation in the energy partition function requires either nonradlal symmetry in the source radiation function, varying patterns of vertical velocity structure In the two shot areas sufficient to give the observed differences of energy level In th« lunes of the focal sphere giving rise to the several refractions, or frequency and depth dependent attenuation factors.

4. HARDHAT. For HARDHAT (Figure 75), the data are as follows:

TABLE VII

HARDiiAT

MNNV 2?8 4.96 (7.9) 5.96 MLNM 781 3.56 C8.1CL) 4.29 ATNV 2r\ ^.30 (7.9) 6.11 TCNM 902 4.47 (8.1CL) 4.66 KNur 288 5.12 (7.9) 5,89 5,34 (8.IB*) 4.86 INCL 336 4,09 (7.9) 4.92 PTOR 961 4.44 (8,5) 4.86 WMAZ 399 4 90 (7.0) 5.49 LCNM 1017 3.79 (7,9) 4.64 FMUT 403 4,06 (7.9) 4.83 RTNM 1043 Very late WINV 474 5,19 (7.9) 5.78 TRWA 1070 4.27 (8,5) 5.74 FSAZ 490 5.18 (7.9) 5.80 EPTX 1095 3.46 (8.5) 4.94

CPCL 500 ? early EFTX 1209 2.89 (8.5*) 4.21 MVCL M? ? early GNNM 1244 3,30 (8.5*) 4.56 SFAZ 5h9 5.r3 (7 9) 6.13 BMTX 1324 4.05 (8.5') 5.18 VNur 669 5.19 (7„9) SSTX 1501 3.60 (Ö.5') 4.32 VTOR • 688 4,90 (7.9) WNSD 1304 4.24 (8.5») 4.96 SVAZ 711 4,47 (7.9) HBOK 1557 4.18 (8.5) HUD

DRCO

7?9

734

3 48 (8.1CL) 4.08 (8,IB) 4,32 (8.1CL) 4.92 (8.IB")

4,34

5.20

•

18

.

TABLE VII (Contd)

Magnitude in sr Magnitude msr

WMSO LPTX SEMN SJTX

1598 1765 1967 1975

? 4.41 (8.5) 4.02 4.09 (10,5) 5.37

MPAR 2084 3.90 (10.5) 3.00 NGWS 2500 4.13 (10.5) ^.13 hWTN 2728 4.34 (10.3) 4.37

The three symbols for P8.5 arrivals identify probable 8.5 refractions from different profiles with different intercept times. The Identification of the primed events as P8.5 is tentative. Uncertainty exists as to the appropriateness of using mg^ (CLEARWATER) ormgti (BILBY) (prime implying 0.3 adjustment of BILBY formula based upon discussion of BILBY). It the computed magnitudes are grouped according to phase, the following tabulation is obtained:

^7.9 m8#l(CL) m8#l(B') m8.5 m8.5* "'S.5» ml0.5

4.96 3.48 4,08 4.44 4.05 3.46 4.09 5.30 4.32 4.92 4.27 3.60 2.89 3.90 5.12 3.56 4.16 4.24 4.18 3.30 4.13 4.09 4.74 5.35 4.41 4.34 4.90 4.06

5.19 5.18 5.63 5.19 ^0 4.47 3.79

The two arrivals that were I se>ond early relative to P^ 9 and had very low computed m7>9 magnitudes (4.09 and 4.06), topethex with the unusually low estimate at LCNM (3.79) ir-itroduce an abnormal spread into the 1117 9 data. If one ignores these arrivals, the average m; 9 • 5.i7, if they are includtd, the average m7#9 - 4.87. The mean mg^ (B*) - 4.62> t He mean mg^ ■ 4.34, the mean m^Qj ■ 4.12. with the mg_5* and mg 51 values even lower. Obviously, the foTnulas used for MISSISSIPPI do not satisfy HARDllAT. No obvious distance dependence appears in any phase. The path length differences bttween P79, p8<c> and Pio.5 represent small frac- tions of the total path lengths so that one cannot explain a dependence of magnitude upon »ihase as being due to a higher frequency signal for HARDHAT than for MISSISSIPPI with a more rapid decay of signal with distance. The la^k of dependence of magnitude upon distance in each phase also suggests this conclusion. Another explanation seems required. The decrease In mô.5 values shown above from those in the shot report largely results from miscalculation in the shot report.

19

COMPARISON OF MAGNITUDES COMPUTED BY Bisr AND THE SEVERAL CURVES OF THIS REPORT

A comparison of the magnitudes computed by msr and the several curves .established here clarifies the significance of employing the proper curve (see Figure 100). At 1900-2000 kilometers, msr underestimates relative to mg.J by 0.4. At 1600 kilometers, msr underestimates the magnitude by 0.3 relative to me,5 and 1.0 relative to miO.S* Even if using the new curves, an error of 0.7 will be made if one mlsidentlfles the phase measured. At 1100 kilometers, msr overestimates with respect to both relevant curves, 0.5 relative to my.9 and 1.5 relative to mg(5. Again, if 1117,9 and mg.S are used but mlsidentlf 1 cation of a phase occurs, an error of 1.0 will be made in magnitude, PTOR providing a perfect example of this phenomenon. This station routinely sees Pß.S as first arrival for NTS events though being only 970 kilometers from NTS. Note the gross discrepancy between msr and ins.3 at all distances less than 1400 kilometers. Fortuitously, these curves cross around 1700 kilometers due to the attempt to get by some means from somet'iing approximating a mean between my.9 and mg.S at 1000 to mio.5 at around 1800 kilometers. Between 1000 and 2000 kilometers the msr and my.9 curves nearly parallel one another, the difference between the two curves being 0.63 at msr - 5 and 0.9S at msr - 3. We included mg.i (CLEARWATER) on Figure 100 to demonstrate the grossly different behavior of this phase with distance from that of all other curves. Inside 600 kilometers, Pg.i (CLEARWATER) appears larger than either P7#9 or Pg(5 for m - 4, while it decays to 1.5 v\k at 1000 kilometers for n - 4.

RECALCULATION OF ALL SHOTS PREVIOUSLY ANALYZED IN SHOT REPORTS

Subject to the limitations and uncertainties of the type just described, the A/T versus A versus magnitude data of all previously analyzed events were recalculated based upon the formulas given above. As none of them, other than SHOAL, were In granite, one does not expect problems of the type met In analyzing the data of HARDHAT. Figures 24 through 79 present the recalculated magnitude values, the figures for selected events (Figures 8, 9, 10, 58, 60, 62, 64, 65, 73, 76, and 78) including shot report values of magnitude for comparison and study purposes. The mag« nltude of these events was obtained by averaging all recomputed values, the symbol m^1 being adopted for this average value. The values of m^* nr6 those used throughout the following discussion and tables. It is clear from the indicated figures that the gross dependence of magnitude upon distance which was present in shot report values of magnitude has been largely eliminated by use of the phase formulas. Small events only recorded at less than 1500 kilometers are reduced in magnitude (m^*) from msr values by as much as a full magnitude. The following tabulation (TABLE VIII) gives all 1115* values computed by procedures of this, report for explosions, together with m8r values and values derived by Romney by use of A/T values at 500 kilometers for the smaller events.

20

«

TABLE VIII

Magnitude Data on Selected NTS Shots

SHOT •V «"R "ar

Alluvium

SHREW 1.2 2.3 MAD 2.37 3.7 MINK 2.72 2.93 3.7 CHINCHILLA II 2.73 2.84 3.8 ROANOKE 2.94 3.21 3.9 CHINCHILLA 3.16 3.33 4.2 ALLEGHENY 3.0^ 3.31 4.0 KAWEAH 3.15 3.36 4.1

STILLWATER 3.46 3.40 .

CODSAW 3.40 3.54 4.3 BOBAC 3.39 3.60 4.2 RINGTAIL 3.21 3.36 4.1 SACRAMENTO 3.32 3.40 4.1 STOAT 3.33 3.60 4.5 WICHITA 3.47 3.58 4.5 AGOUTI 3.55 3.83 4.5

SANTEE 3.30 3.56 4.2 ARMADILLO 3.51 3.78 ■i

PACKRAT 3.69 3.73 4.2 CASSELMAN 3.64 3.96 4.6 HYRAX 3.43 3.60 4.4 ACUSHI 3.51 3.75 4.4 DORMOUSE PRIME 3.73 3.91 4.6 DORMOUSE 3.64 3.83 4.5

CIMARRON 3.98 4.38 m

YORK 3.69 3.92 4.4 PEBA 3.54 3.78 4.4 PASSAIC 3.69 4.06 4.3 FISHER 3.45 3.68 4.3 MERRIMAC 3.80 3.87 4.3 HAYMAKER 4.52 4.5 4.9 SEDAN (crater) 4.12 . 4.7

21

TABLE VIII (Contd)

SHOT

Valley Tuff

AARDVARK STONES FORE MISSISSIPPI BILBY

inbi

4.55 4.50 4.85 4.76 5.61

nfc

4.7 4.6 4.9 4.7 5.6

tn sr

4.9 4.8 5.2 5.1 5.8

Mesa Tuff

FEATHER 2.13 CHENA 2.76 PLATTE 3.44 ANTLER 4.03 MADISON 3.71 DES MOINES 3.72 MARSHMALLOW 3.99 CLEARWATER 4.71

HANDCAR 4.19 (Dolomite)

HARDHAT 4.l5(mC8) (Granite)

SHOAL 4.62 (Granite)

DANNY BOY 3.06 (Basalt)

GNOME 4.27 (Salt)

CLIMAX 3.73 (Granite, Chemical)

CHASE III 4.7 (tnts) (Water)

4.0 4.9

4.6

4.8

3.2 3.9

4.7 4.5 4.5 4.5 5.3

4.7

4.9

4.9

3.6

4.6

4.4

22

RELATIVE MAGNITUC P . USE OF DATA OF FIXED STATIONS

Another approach to establishing relative magnitudes of NTS events is to compare A/T values of the events as recorded at fixed stations. Relative magnitude should be determinable to within several percent by this approach and one measure of the meaningfulness of the overall scheme here employed would be the agreement of relative magnitudes of NTS events as computed by the two methods of analysis. Several events are analysed by relative amplitudes at fixed stations in Table IX, all stations being restricted to ones receiving P79 in WUS from NTS «vents.

TABLE IX

Comparison of m^, and Individual Station Relative Magnitudes

Event "V Relative mî (CHIN II)

Relativ* m (CHIN II)* Av MNNV WINV KNUT FSAZ 6 m

CHINCHILLA II 2.73 2.73* 2.62* 2.35* 3.12* 3.0l* 2.78* *.05

KAWEAH 3.15 .42 .73 1.18 -.10 .03 .51 ♦ .09 STOAT 3.38 .65 .89 ■ .59 .62 .70 ♦ .05 CODSAW 3.40 .67 .84 1.10 .34 .19 .62 -.05 FISHER 3.45 .72 1.20 .80 .42 .59 .75 ♦ .03 ACUSHI 3.51 .78 1.11 .96 .35 .84 .82 ♦ .04 DORMOUSE 3.64 .91 1.30 . .53 .70 .83 -.08 CASSELMAN 3.64 .91 1.16 1.49 .61 .68 .99 ♦.08 PASSAIC 3.69 .96 1.23 1.34 .72 .75 1.01 ♦.05 MADISON 3.71 .98 1.08 1.32 1.07 1.09 1.14 ♦.16

*CHINCHILLA II data are basis of comparison for other events,

23, 24, & 25

In Table IX, the dar a of" CHINCHILLA II are the basis of comparison. Thus, in the row lor CHINCHJLLA II, values are nagnitudes as estimated by m;^ while in the rows fot other events, values are differences between magnitude for these events and CHINCHILLA II at the indicated stauon or stations, based on measured A/T values. Note that such an approach does not necessarily eliminate a fu\l magnitude spread in the data (KAUEAH, for example). Column 8 is an average of the relative magnitude values obtained at the four fixed stations (KNUT, MNNV, FSAZ, and WINV) while Column 9 is the difference between this relative magnitude estimate and that of the magnitude values of this report (mî). In only one case, MADISON, does a value in Column 9 exceed 0.1 magnitude unit. The conclusion is that the procedural scheme used of averaging values computed by various phase formulas has passed another significant test.

APPLICATION OF THE PHASE CURVES TO EARTHQUAKES OF WESTERN UNITED STATES

1. General. Problems of two ôrts fc«pertedly arise in the application of these formulas to data from eaxthquakes. First, the factor of relative excitation of phases, significant in such a small region as NTS, expectedly introduces inconsistencies in predicted magnitudes of earthquakes. Second, the certain definition of phase for ea^h arrival measured becomes more difficult than tor NTS shots. For shots, phase definition Is comparatively simple since we know accurately the time of origin and epicenter and 2-second differentials in travel-times are clear and meaningful. However, the programs used in locating earthquakes do not consider the presence of two dominant refracted phases in the distance range 300-1800 Kilometers. Ihe attempt to locate epicenters on the basis of a formula that obscures these two arrivals by treating them as one or as an invalid two can lead only to phase confusion and lack of definition on a T - A/8.1 plot, SHOAL and the Fallon Earthquake form an excellent demonstration paii of this phenomenon. These rwo events occurred very near each other and many of the same stations recorded them. The T - A/8.1 plot for SHOAL (see shot rspo^t) clearly shows the several refra>.ted phases. Time of origin and epicenter were very accurately known and were rot derived from the travel-time data. The Fallon Earthquake, however, of necessity was located by use of the travel-time data and by use of an earth model which did not represent the velocity structure of the region. Thus, appreciable uncertainties in the time of origin and epicenter resulted in a confused and virtually undecipherable T - A/8.1 plot. In such a situation, one must, almost of necessity, establish arbitrary ranges for the applicability of each phase magnitude formula. We adopt the convention that m^ ^ applies to all stations with < A ^ 1000 lilometeis, mg^ to all stations with 1000 < A ^ 1800; and mi0.5 and mts beyond 1800 kilometers. The breaking of this rule occurs when obvious reason to do so exists and we note such exceptions. Expectedly, the shadow-zone effects of the Sierra Nevada influence relative amplitudes of these phases. With these limitations in mind, we selected for study a set of WUS earthquakes. Several events

26

reported by the L'S (.oast, and Geodetic Survey (USC&CS) to he of map.nitudo 5 or greater augjncnr earthquakes previously reported by VELA in shot report style. As an interesting sidelight of this investigation, note that I'SC&GS or Pasadena often greatly ovtrcatimale nidp.nitude* of small earthquakes (5 or'less) relative to telcseismic magnitudrs. Much of iln-. is clearly duo to u^fr o{ invalid formulas r^laJ inJJ magnitude, distaiivt, and A/1.

2, tarthqiinkcs Studied. The earthquatcs investigated arc those oi:

2 Feb 1962 at 18 3 Feb 1962 38 IS Feh 1962 16 \'> Feb ]962 37 2.') Feh 1962 AS

:■> Apr 1962 38

12' U 51 04 12 24

104 107 1.12 11? Ill 119

54* 36 26 S8 12 18

W Colona Earthquake Report

hebgen La^thnja'xe Report Bridgeport Earthquake Report

U Apr 27 May 20 Jul 21 Aug 30 Aug 5 Sep 16 Sep

1962 1962 1962 1962 196? 1962 1962

41 31.7 39 30 42 41 40 36

54 62

123 115.6 118 18 127 111 112 119

24 00

Fallon Earthquake Report

Cache Creek Earthquake Report Cache Creek Earthquake Report

6 18 22 S

24 22

Nov Nov Mar Jul Get Dec

196^ 196) 196/*

1964 1964

4,;

^0 18 .'6 4A. 31.

123 114 119 I ti

129.1 116.9

Figuies 81 llvougn ^8 display i ht- msr values arid the phase magnitude values. The general phenomenon of pêdi« ting »oo high a magnitude by use of msr at less than 1700 kilom»;teTs becomeb as apparent as for NTS shots, Fallon, in particular, interests us as its pattern of computed magnitude values is so similar to that of SHOAL« implying essentially identical energy partition functions of theae tvo events as regards F-wave energy. The events ot 14 April 1962, 2 7 May 1962, 6 November 1962, and 24 October 1964 display e-x-.ellent adjustment of the data by use of the phase formulas in contrast to the poor adiustment when using mSr, Events of 2 tebruary 1962, 21 August 1962, 16 Sep^mbet 1962, 22 March 1964, 31 March 196V ri-'d S July : 96/4 mie lowd estimates of magnitude at less than approximately 1400 kilometers than at greater epicentral distances. Note that these events occurred either immediately west of the Sierra Nevada or in Mexico, or near Queen Charlotte Islands.

27

•

3. Small Earf'.q'.iak-^. The quakrs of 5, 15, and 25 February 1962, 5 April 1962.. and 6 August 1963, i. e., Colona, Kanab aftershock, Hebgen, Bridgeport, and o-t? other, were not recorded at teleseismic distances because of their r.mall si?e. The first four were used by Mansfield in his discussion of AR. Note that, of all of them, the use of msr values predicts a teleseismic magnitude completely inconsistent with the short

fdistanc»;« over which »-hese events were re'-orded. Adjustment by the phase formulae brmgi, the individual magnitude data into more reasonable intcraai ^^tcuiunt and, in addition-, yields reasonable magnitudes in light of the short rar.gc of recorded signals. A more detailed discussion ftorn a regional pattern point of view seems unwarranted until a large suaipie ul cxf'ts hem been analyzed. It seems obvious, however, that treatipent o( the A/T data of earthquakes of WUS by the WUS phase formulas yield« a more internally consistent body of data than that obtained by use ot m-r vdliits.

U, 1 ihy.r. Kar- * vi-^k^ oj A Feb.-vd? y 196}. As a 'i-^l "o^c on this point the two Libyan earthquakes (îg^re 99) studied by Goidon^ sugges'. the appropriateness of application of these formulas (or at least mg.s) to the Mediterranean area. For the two events xeporced in the above-

0. CO mentioned paper, ATU and 1ST, ^,8 and 10.6 , rtspec-ively, yielded excessive magnitude eatimates by use of msr values but yield values consistent with mt s hy mg ^ for IST and m^g tor ATU.

USE 0? P PHASES AS DIAGNOSTIC AIDS •

The agreement o» m^Q fs*« Appendix), ^79, mg 5V rn^Q 5V and mLs values for both explosions a-d carthquaK« s means that relative exaltation of these phases « annot b» ust.d as a diagnostic, criterion for distinguishing explosions and parth-juak^s. Li^^le dojbt otsts that patterns of variflblc relative e>:itation such as observed fox BILBY and HARDHAT will occur xn earthquakes also cut a pattern separating earthquakes and explosions dot? not exist.

VELOCITY STRUCIUHE 0^ CRUST OR CRUST AND UPPFR MANTLE IN EASTERN UNITED SFATFS AND WFSTERN UNITED STATES

I. EUS

a. limited Pg data indicate that in EUS this phase is never the clearly marked phase that it is in WUS, The events picked as Pg appear to have a /ero-dlstance time-intercept of several seconds and a velocity ot abo^t 6,4 V ilomete's pex second. However, the data are exceedingly

. poor.

b. Figur»» 2^ is the basis of the following discussion, being based upon numerous shot reports. The Pg.S event records as first arrival to 2100 kilometei j. The SS Village*prof lie (recorded by land otations,

5Gordon, David W., 1963, Libyan Earthquake of 21 February 1963, USC&GS Potll-atlon.

2P

the nearest being a tew hundred kilometers from the shot point) has an intercept of +9 seconds. This represents a path which includes one trip through the EL'S crust, one trip through an oceanic crust, and a horizontal leg of a few hundred kilometers to continental structures at about 8.1 kilometers per second rather than the continental velocity. GNOME-East shows an intercept of -«-ll seconds, this intercept being representative ot two trips through the EUS crust. We will assume that 5.5 seconds characterize crustal thickness in EUS, the 3.5 second difference of the SS Village intercept being explained by the two last path segments mentioned above.

c. Very simple crustal models suffice for the discussion which follows. The discussion illustrates the contrast between EUS and WUS, a contrast so gross that slight details of crustal model are of second- order significance. Therefore, we assume a velocity of 6,0 kilometers per second above the Pg.s refractor and obtain a thickness of 46 kilometers for the crust in EUS. This thickness varies as other investigations have shown, but an average figure suffices for this discussion.

2. WUS. A cursory glance at Figure 23 reveals immediately the gross contrast between travel-time curves for EUS and WUS in the range 150- 2100 kilometers. As we have already illustrated, marked differences occur within WUS itself.

a. Pg is well observed in WUS out to 1000 kilometers and has a velocity of very nearly 6.0 kilometers per second.

b. P7,9 is widely observed in WUS for NTS events. Its intercept does not depend upon azimuth irom NTS and is about ••■5 seconds. Use of a layer velocity of 6,0 kilometers pet second indicates a depth of 26 kilometers to this retractor.

c. The P8.5 - NTS/Dakotas profile has an intercept of -»-14 seconds, representing the sum of 1-way trips through WUS and EUS crustal types, all times having been measured in EUS for this phase on this profile. If 5^ seconds represent a 1-way trip through the EUS crust, 8^ seconds characterize a 1-way trip through the WUS crust,

d. For Pg.S - NTS/Washington, an observed intercept of about •♦■18 seconds should characterize a 2-way trip through the WUS crust. A value of 9 seconds for a 1-way journey agrees nicely with the value of 8^ found on the NTS/Dakotas profile,

e. If we assume an average value of 8.0 kilometers per second for the velocity in the layer between the P79 and Pg^ refractors in WUS, a thickness of approximately 130 kilometers is found for this.

29

layer. The Pg ^ reftattoi in WUS then oc.urs at a dcptn of about 150 kilometers while in EUS it occurs at only approximately 50 kilometers. The quest'on immediately arises as to what to call the crust in these areas. No logical basis exists for calling the P7 9 refractor base of crust in WUS and the Pgj refractor the equivalent level in EUS. This customary approach may obscure a fundamental and profoundly significant contrast between these two parts of the United States, No logical explanation of the supposed thinning of the crust from east co west exists. The general conclusion of compensation below the crust has been invoked in order to explain the conflict between gravity and seismic data. Real understanding of the relations between these two types of data may result from the realization that the same velocity surface is present in WUS as is termed base of crust in EUS but is approximately 100 kilometers deeper in WUS. Probably, celling the Pg s refractor in both areas base of crust would constitute a geophv>-icaliv meaningful definition.

f. PgBl - NTS/Northern New Mexico, On Urge evonts at NTS, a clear 8.0 - 8.1 arrival is recorded as fcr as ?.INM, the intercept being approximately 7 5 seconds Detailed inspection of records obtained along this profile indicates absence of a 7.7 7 9 refractor. Indefinite recordings of Po 5 have been measured at a tew hundred kilometers, really good recordings not being obtained until the profile extends into EUS. The Pg j refractor apparently replaced the P^ Q

refractor in this region and Is at a somewhat greater d^pth, the values given above yielding a depth of 36 kilometer». The Pg^ refraction leg observed in Texas and beyond has an intercept cf 18 seconds.. In light of the fact that the GNOME East profile traversed the sam»1 region with an intercept of only 11 seconds, the suggested explanation is that the Pg 5 energy observed along this profile for NTS shots is the energy that was following the Pg^ refractor m WUS

APPLICATION OF IDEAS OF THIS PAPER TO OTHER AREAS

1. General. The functional relation betw^n A/T and A for a given region quite obviously depends upon the velocity structure of the region, A simple velocity structure such as that of EUS will result in a simple dependence of A/T upon A- Conversely, the eKi5tence of multiple refractors will result in a complex dependence of A/T upon A. The general suggestion is that Shield or Cratonal areas of low mean elevation will dis-play a simple A/T versus A relationship while regions of high mean elevation on a regional basis will display complex velocity structures and a complicated dependence ->( A/1 upon A. Therefore, an investigation of the velocity structure of a region will lead to a basic understanding of the operative A/T versus A relationships. Such velocity data cannot be based upon an epicentral location and origin time based in turn upon an assumed velocity structure. Thus, only data for explosions are acveptable for such an analysis.

30

\

2. Europe. The Haslach travel-time curve for Europe allows an immediate prediction of the expected A/T versus A relationships for this region. The data on which the travel-time curve of Figure 23 1% based were obtained as the result of an explosion near Haslach in Southwest Germany (48016,N OS^T'E). Note that a single phase with uniform apparent velocity was recorded from 200 kilometers to 2000 kilometers, a pattern strikingly similar to that observed in EUS. It seems reasonable that A/T will be found proportional to very nearly the inverse second power of A in Europe. There seems to be little possibility of a pattern similar to that of WUS. No shadow sons for P phases will be found in this area.

3. Asia. The USSR has not released adequate explosion data to permit a prediction of the expected A/T versus A relationships. Obtaining such data is important as it would greatly advance our understanding of the problems to be expected from observatories at specific localities within the area of interest.,

CONCLUSIONS

1. Contrary to the curve presented by Richter , Pn amplitudes in WUS in the range 1000-1700 kilometers are not obeying a shadow zone interpretation as envisaged by Gutenberg. There is no evidence for a broad interval of several hundred kilometers in which P amplitudes are very small. There is a large jump in amplitude at approximately 1000 kilometers due to Pg^ becoming the first arrival. For some uncertain reason, the Pg 5 refractor and associated refracted phase were never recognized in WUS by Gutenberg and the high amplitudes of this phase were either missed or misinterpreted. Signal levels decrease continuously from approximately 1000 kilomctors outwards except for one or more comparatively small Jumps in amplitude associated with the beginning of the first arrival of deeper refractions. These small steps in amplitude (such as from P3 3 to P^Q,!)^ cannot be termed shadow zone phenomena in the Gutenberg sense or in any real sense. Measurement of P amplitudes in the range 1000-2000 kilometers are more desirable than in any more distant.

2. In EUS, amplitudes increase continuously from 1000 kilometers to the epicenter. There is total absence of any phenomenon which could be interpreted as a shadow zone for P>

3. In WUS, the amplitude discontinuity in the neighborhood of 1000 kilometers is abrupt and associated with the first arrival of a deeper refraction (Pg.S)» replacing the weaker P7,9 event. The P7.9 event can occasionally be followed as second arrival beyond

Richter, loc. cit. and Figure 100, this report.

31

the point where Pß.5 becomes first arrival. The Pg.S arrival In VIUS appears to obey essentially the some decay law with distance as does the P8.5 refraction In EDS.

4. The A/T versus A table used In the VELA shot reports Is grossly inaccurate at distances out to 15o-20o for virtually all events, even for UUS. Errors in magnitude estimates at single stations of as high as l.S magnitude units occur when this lav Is employed. Extensive confirmation of this failure of the shot report table or for any single curve for both explosions and earthquakes is contained In this report. All previously studied explosions and earthquakes plus several additional earthquakes have been recomputed according to the scheme proposed in this paper. Romney's procedure for computing m for small events by use of P7.9 data was calibrated against teleseismic data of large events and thus avoided the Inaccuracies of the shot report computations.

5. A/T data for the Pg phase In WUS are as useful and as meaningful for magnitude computations for both earthquakes and explosions as are the refracted P phases (see Appendix).

6. Magnitude computations based on refracted phases can be made essentially Independent of distance by proper calibration of the crust and by establishing the correct energy partition function for the several phases generated. Such energy partition functions appear quite variable and must be Influenced by focal and near-focus conditions.

7. Epicenter location of unknown events by use of regional and near- regional data will be markedly improved only when crustal models expressing the multiple refractors present in a region are used as the basis of analysis. Any effort to attach regional corrections must fail in general application.

8. Analysis of the amplitude/period data of any region In the range 200-2100 kilometers is only meaningful after elucidation of crustal structure and establishment of the general energy partition function and decay rates of each phase. It appears probable that velocity structure will characterize decay rates. Thus, the Haslach travel- time curve for Europe, based upon surface explosion data, is very similar to that of EUS. The implication is that mg.s may be appli- cable to A/T data of European seismic events in the regional and near-regional distances but only Investigation of A/T data of Europe will Indicate the actual relationships. The situation in the USSR is not certain and will not be clarified until data obtained by observing Russian explosions become available.

32

APPENDIX: ANALYSIS OF Pg DATA

1. General. P5 Q (pg) was observed widely and well throughout WUS to distances of lOOÖ kilometers with a velocity of 6.0 kilometers per second. Pg gives rise to higher A/T values than other P phases in WUS. In many cases it can be recognized by waveform character even though, at other than very near stations, it is a second arrival. In WUS it decays as the third power of the epicentral distance and its velocity is very near 6.0 kilometers per second. It does not occur beyond WUS and loses definition beyond about 1200 kilometers in any azimuth from NTS. One wishes to know whether one can interpret Pg A/T data in a magnitude sense to the extent that the Pn phases have been successfully intcrpreied. If so, two useful and significant results accrue. Firstly, one can obtain confirmation of our proposed A/T versus A versus mî interpretive scheme from an independent phase. Secondly, such a uniformity of behavior would imply one :ould use Pg for estimation of magnitude of earthquakes with as much or more reliability than the Pn phases as the scheme of identification of Pn phases for an unknown epicenter may prove difficult while one can generally identify Po by record character. If the beginning of P is measurable to within two or three seconds, better epicenters probably could be obtained by use of this phase than by use of the Pn phases in WUS. Pn phases as first arrivals are timed to a fraction of a second but the proper classification of the phase measured may be so uncertain and the crustal velocity structure assumed so inaccurate that one can introduce errors of several seconds into the calculations of epicenters. When using Pg in WUS an extremely simple but valid model can be used for computation purposes.

2. Analysis of Pg Data. The procedure followed here begins by estiii.ating the best value of A/T at 500 kilometers from the A/T versus A plots for Pg in the shot reports. See Figures 103 through 106 for examples of A/T versus & plots of Pg data. We plot these (A/T)500 values against radiochemical yield in Figure 107 and against m^t values of this report in Figure 108. Compare Figures 107 and 108 to see that the (A/T)500 values of Pg display a linear relationship to the magnitude values of this report , being independent of shot medium. These two figures confirm the usefulness of Pg amplitude data for estimating magnitude and support the relative magnitude determinations of this report. The resulting Vd.o equations are:

m6>0 - 1.28 ♦ 1.14 log (A/T)500 (13)

m6.0 " "8-00 * l-u l08 A/T * 3-42 loß Ä UM

34

The curves of Figure 100 for Py^q can be used for P6 Q if magnitude values are decreased by 1.0 units. In other words, P^.Q gives A/T values 10 lines those of P7.9 at the same distance.

3. P Data for WUS Earthquakes. A/T data for the Pg phase are available for several earthquakes of WUS. The computed mg.O vaûes

follow:

Earthquakes

Colona (5 Feb 1962) Hebgen (25 Feb 1962) Cache Creek (30 Aug 1962) Cache Creek A/S (5 Sep 1962) Bridgeport (5 Apr 1962) Fallen (20 Jul 1962)

*Based on WUS data.

The Pg data demonstrate their usefulness for the estimation of magnitude of WUS earthquakes. Remember that we derived this scheme for Pg by calibration against mt values of P phases and noting the linear relationship between log Y and log (A/T^QQ* Therefore, the agreement at magnitudes above 4 occurs independently of Pn values inside 2000 kilometers and the agreement at lower magnitudes depends upon control other than previously computed magnitude values.

(A/T)500 V 40 3.1 50 3.2

2500* 4.3 500* 4.1 42 3.03 240 4.4

V 0 3. 11 3. 22 5. 16 4. ,36 3. 29 4. 16

35

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Documents

D D C - DTICAZTX Amarillo, Texas BLWV iveckley, West Virginia BMSO blue Mountains Seisinological Obs*ervatory bMIX Balmorhea, Texas BRPA Berlin, Pennsylvania bXUT blanding, Utah CK;%C