VOL.

81, NO.

JOURNAL

5

OF

GEOPHYSICAL

RESEARCH

FEBRUARY

10, 1976

Microearthquakesin and Near Long Valley, California DON

W.

STEEPLES • AND A.M.

PITT

U.S. GeologicalSurvey,Menlo Park, California 94025

Sixteenportableseismograph stations weredeployed in thevicinityof theLongValleygeothermal area,

California, fromApril27to Ju, ne2, 1973.Onlyminormicroearthquake activitywasdetected in theLong Valleycaldera,but a highlevelof activitywasdetectedto the southandeastof the caldera.The abrupt spatialseismicity decre. aseat thesouthern boundaryof thecalderasuggests thatthecalderaiseitherstrucrurallylesscompetent thanthesurrounding crustor isat ajunctionof differentregionaltectonicdeformation trends.No significant attenuationor delaysoccurredfor eitherlocalP or S wavesthat traversedthe caldera.

works suffered from

INTRODUCTION

azimuthal

control

and distance with

respectto Long Valley, and eventssmallerthan magnitude1.5 Sixteenportable seismographstationswere deployedin the in Long Valley couldnot be detected.Pitt andSteeples[1975] area of Long Valley, California, from April 27 to June2, 1973, detected only two microearthquakesin the Long Valley calderawhile theyconfirmedthe seismicitypatternestablished aspartofa U.S.Geological Survey (USGS)multidisciplinary south and east of Long Valley. investigation for possiblegeothermal resourcesin the Long by the earlier investigators historicseismicity in theregion, Valley caldera. The principal objectiveof this study was to Therehasbeenconsiderable detect and locate microearthquakesin the Long Valley including the great 1872 earthquake in Owens Valley, about 100km southeastof the Long Valley caldera.Pitt andSteeples caldera, an area of present-dayhydrothermalactivity. Facca and Tonani [1964] suggestedthat the delineationof [1975] interpreted microearthquakefocal mechanismsjust specificfaults would be a valuable tool in the search for outsidethe southeasternedgeof the caldera in terms of right were locatednear permeablefracture zones in geothermalareas. Ward [1972] lateral strike slip. Those microearthquakes suggested'that microearthquakes mightbe usedto locateac- the epicentersof two magnitude6.0 earthquakesthat occurred within the last 50 yr [California Department of Water tive faults that may channel hot fluids toward the surface. Bruneand Allen [1967] and Thatcherand Brune [1971] have Resources,1964]. reportedmicroearthquakeactivityassociatedwith geothermal TUE NETWOlt}C areasin the Imperial Valley southof the SaltonSea. Wardand The instrumentsusedfor this studywerethe USGS portable Bjornsson [1971]located over2100earthquakes in Iceland, and they noted a differencebetweenthe continuousmicroearth- seismographsystemsthat have been describedin detail by quake activity in the geothermalareasand the larger shocks Eatonet al. [1970].Sixteen of thesesystems wereoperated at followedby aftershocksequences in other parts of the island. 20 different locationsduring the recordingperiod. The array Lange and Westphal[1969] detectedmicroearthquakeactivity was centered on the eastern half of the caldera, and six locanear The Geysers,California, during a recordingperiodof 5 tions from a previousstudy by Pitt and Steeples[1975] were days.In severalother geothermalareasof the westernUnited reoccupied(Table 1), States they detected such activity in recording periods of CRUSTAL MODEL similar duration. Hamilton and Muffler [1972] located 53 A shallowcrustalmodel for the immediateLong Valley area microearthquakeswithin 10 km of the then producingarea at was describedby Hill [1976].The microearthquakes recorded The Geysers,California, in a 3-weekperiod. Microearthquakes may have other usesin geothermalex- (discussedlater) were mostlyoutsidethe caldera,so the local ploration. Hamilton and Muffler [1972] suggestedthat the Hill model was bypassedin favor of a 'regionalmodel derived maximum focal depths of microearthquakes may be a from the reversedre&actionprofile betweenM OhOLake and temperatureindicator.BraceandByerlee[1970]showedthat China Lake, California, describedby Eaton [1966]. This hightemperature maypreventstickslip(i.e.,earthquakes) and profilepassesnearthe centerof the seismicarray of the present

study. The model consistsof the followingsequenceof

induce stable sliding.

The Long Valleycalderais an ellipticaldepression 19by 29 horizontal constant velocity layers, where velocity is in km (Figure 1). Rhyolitic extrusionhas occurredin the last kilometersper secondand depth is in kilometers: 1500yr, and hot springactivitypersiststo the presenttime. Baileyet al. [1976]havenoteda difference in the characterof P WaveVelocity DepthtoTop•of Layer faultinginsideand outsidethe caldera,and they suggestthat 3.0 0.0 th"ematerial insidethe calderawas decoupledfor a time from 6.0 1.7 the uplift of the Sierrasto the southand west. 6.4 15.0 6.9 28.0 Gumperand Scholz[1971] and Ryall et al. [1972] located

microearthquakes in ChalfantValleyandin a zonetrending northwestwardfrom Bishoptoward Mammoth. Their net-

7.9

54.0

Using this model, well-recordedblastswithin the network were locatedto within 1 km horizontallyand 2 km vertically. • Now at KansasGeologicalSurvey,Universityof Kansas,Law- It is assumedthat well-recordedearthquakesthat occurred rence, Kansas 66044. within the network were locatedto this sameorder of accuracy and that earthquakelocationsoutsidethe networkmay be in Copyright¸1976 by the AmericanGeophysical Union. 841

842

STEEPLES AND PITT: LONG VALLEY SYMPOSIUM

$7'45

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olley Coldero

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•e.•• •10 events

N37e30 '• ß ß EXPLANATION

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SeismogrophStotion

ß--,'""-,

Foult

• -- •

Long Volley

4p

coldero

boundory

Eorthquoke epicenter o I

5 i

i

KM

ill

Fig. 1. Microearthquakes in the vicinityof LongValleycalderafromApril 27 to June2, 1973.On thenodalplanesolution, solidcirclesrepresentinitial compression, and opencirclesinitial dilatation'smallercirclesindicatelessreliablefirst motions.Stationsmentionedin Figure 2 are designatedby three letterson the nodal plane solution.

error by 2 km or more. Relativelocationsof closelyspaced computerprogramHYPO71 [Lee and Lahr, 1972](Table2). determinedheremay differfrom theclassical eventswithin the network may be accurateto a few hundred The magnitudes meters. No elevation or station corrections were used. Richter local magnitudesby as much as one third of a magnitudeunit and shouldthereforebe regardedas local SELECTION OF DATA SET magnitudeapproximations. All local eventsthat appearedat three or more stationswere analyzed.P arrivalswere picked and timed to 0.01 s by using

MICROEARTHQUAKEDISTRIBUTION

The eventslistedin Table 2 are plotted in Figure 1 (except for two eventsoutsidethe map area). The activity is concentrated in a WNW trending band extendingfrom near Bishopalmostto Mammoth.Focaldepthsrangefrom 1 to 15 +0.02 s. km with no systematicpatternto the depthdistribution.The and S arrivals were only usedto locate eventswhen the P-only seismicitypatternis very similarto that found by Gumœer Scholz [1971] and Ryall et al. [1972]. It is also similar to the solution producedS residualssuggestingpoor quality locations. A Wadaft diagram was used to establishthe proper pattern found by Pitt and Steeples[1975], exceptthat the Vp/Vs ratio for the region(1.70). The uncertainties in picking swarm near 37ø36'N, 118ø57'W (Figure 1) is in an area in the S arrivalsmay be• s or more, but balanced$ residuals(i.e., which only two eventswere detectedin a 20-day recording somepositive,somenegative)are consideredto indicatestable period in 1970. The 1973data alone(Figure 1) are insufficient to presentthe (good quality) hypocentersolutions.Eventswith fewer than five P arrivalspickedto betterthan 0.25 s weredroppedfrom trendsthat emergewhenall our data are plottedtogether.In the data set unlessthe locationwaswithin 5 km of the edgeof Figure 3 all of the microearthquakedata from 1970and 1973 the Long Valley caldera,in whichcaseall eventswith threeor are displayed,alongwith stationlocationsfrom 1973.Approximately50 of theseevents(of over200 plotted)had fewerthan more stationswere kept. Hypocenters andmagnitudes fromthesignalduration[Lee five readings,and perhapsa half dozen(shalloweventswithin et al., 1972]weredetermined by Geiger'smethodusingthe 15 km of the D in SierraNevada(Figure 3)) of the 50 couldbe station WWVB or int.ernal chronometer corrected to WWVB.

S arrivalsweretimedon the fi.vethree-component stationsand wherepossibleon the 11 one-componentstations.Readingerrors (as distinguishedfrom picking errors) are no more than

STEEPLES ANDPITT;LONG VALLEY SYMPOSIUM

843

TABLE 1. SeismographStation Data

Location Station

NorthLatitude

WestLongitude Elevation, m

Instruments

I 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21

LVA LVB LVC LVD LVE LVF LVG LVH LVI LVJ LVK LVL LVM LVN LVO LVP LVR LVS LVT LVU DFOD

37ø36.16 ' 37ø36.13 ' 37034.97 ' 37ø45.14 ' 37o46.82 ' 37039.03' 37ø44.41 ' 37038.83 ' 37044.20 ' 37o38.50 ' 37042.73' 37038.05 ' 37035.22 ' 37035.87 ' 37ø29.21 ' 37023.03 ' 37038.82' 37o35.47 ' 37055.86 ' 37ø36.71 ' 37ø33.18 '

118ø59.68 ' 118ø51.15 ' 118ø33.17 ' 118046.03 ' 119004.96 ' 119ø03.01 ' 118ø57.81 ' 118057.33 ' 118050.36 ' 118048.37 ' 118041.28 ' 118039.38 ' 118040.42 ' 118020.25 ' 118ø38.19 ' 118ø33.14 ' 118055.67 ' 118050.07 ' 118056.30 ' 118023.92 ' 117ø45.16 '

2768 2310 2231 2195 2365 2755 2292 2365 2121 2140 2249 2316 2170 1792 1719 1451 2341 2426 2077 1378 2173

USGS Portable USGS Portable USGSPortable USGS Portable USGS Portable USGS Portable USGS Portable USGSPortable USGS Portable USGS Portable USGS Portable USGSPortable USGS Portable USGS Portable USGS Portable USGSPortable USGS Portable USGSPortable USGS Portable USGS Portable USGS Telemetered

22

TIN

37003.30'

118o13.70 '

1195

Cal Tech

NCER

explosions. The hypocenters of thesepoorlylocatedevents thesphere isa pointof intersection of thetwoP nodalplanes. couldbe in errorby morethan5 km horizontally and10km The northpoleis alsoa pointof intersecting S nodes.The vertically. Thegoodeventsfrom 1970areplottedin thework southpoleof the focalsphereis theonlyotherpointwhere by Pitt and Steeples[1975]. bothP nodes andS nodes coincide. Thisspecial case ofperfect A north-south trendisapparentin Chal.fant Valley.A larger vertical strikeslipcanbe generalized to thepresent (or any andmoreextensive trendfromnorthernOwensValleytoward other)casesimplyby rotatingthe focalsphere. LongValleycalderareinforces thetrendsuggested in Figure1, Figure2cshows thatstationLVEwasat orveryneara point includingthe abruptdecrease in activityat the edgeof Long ofcoincidence of P nodeandS node,sothatthediametrically Valley caldera.It is particularlynoteworthythat few events opposed nodalpointsarebothknownto withina veryfew weredetectedwithin the LongValleycaldera.Thisis discussed degrees. In addition,stationLVF showed P nodalcharacter, in detail later.

so that threeP nodalpointsare knownand thusone P nodal

planeisuniquely determined. Thesecond P nodalplaneisalso uniquelydetermined, sinceit mustpassthroughthe two The directions of first motion of P arrivals were noted when diametrically opposed P andS nodecoincidence pointsandat sufficiently strongandclassified onthreequalitylevels: (1) cer- thesame timebeperpendicular totheP nodalplanepreviously NODAL PLANE SOLUTION

tainlycorrectbecause of theamplitudeandcharacterof thearrival; (2) probably correctbecauseof the characterof the ar-

rival but of loweramplitudethaneventsin the firstclass;and

determined.

Thenodalplanesdrawnbythismethodarein totalagreement with the first motions for both eventsat the other 13 sta-

(3) lesscertainthanclasses (1) and(2) butstillworthusingin tionsasshownin Figure1.In thiscasethedataagreewellwith orderto increase the densityof coverage on thefocalsphere. the double-couple mechanism theory. ComputerprogramHYPO71 provides an equal-area projecThisparticularpairof eventsshows howimportantradiation of first motionson the lowerhemisphere. tionpatterncanbein determining theamplitude of S waves. Figure 1 shows a nodal plane solution with an arrow pointingto the epicenteron the map. The solution,a compositeof twonearlyidenticalevents, hasnodalplanesthatcan

Whenthissevere attenuation of S waves wasfirstobserved by theauthors, thetendency wasto thinkit mightberelatedto attenuation by magmaasin studies byKubotaandBerg[1967] be drawnby usingarrivalsfrombotheventsbut usingonly and'Matumoto [1971]at Mt. Katmai,Alaska,andPitt [1974] two fortuitously located stations.The two eventsused had at Yellowstone.This is an examplethat indicatesthat the hypocenterswithin a few hundred metersof each other. At 13 radiationpatterncan drastically affectamplitudes but that stations(of 15 operating)the seismograms couldalmostbe near total attentuation is limited to small areas of the focal tracedone eventuponthe other.Figure2a showstypicalex- sphere. amplesof this similarity.The remainingtwo stations(LVE As originallyconceived, part of thisinvestigation wasto inand LVF) possessednodal character (much smaller cludeattenuation datafromlocalearthquakes thatsentenergy amplitude) for the P arrival and showeda reversalof senseof

first motionfrom oneeventto the other(Figure2b). It is known[e.g.,Helmberger,1974]that P nodesandS nodescoincide at only two diametricallyoppositepointson a focal sphereandalsothattheP nodalplanesaremutuallyperpendicular,intersecting alonga straightlineconnecting the two points.This canbe visualized by imagining a perfectvertical strip-slipfocalsphereviewedfrom above.The northpoleof

through theLongValleycaldera in a studysimilarto thatat Yellowstone by Pitt [1974].This studyproduced negative results, astheray pathsinvolveddid notpenetrate to sufficient

depthin the calderato encounter significant low-velocity material,otherthan that in the upper3 or 4 km described in therefraction studyof Hill [1976].Theraypathsin thecaldera weretoo shallowto detectthelow-velocity materialdescribed by Steeplesand lyer [1976]and Steeples[1975].

844

STEEPLES ANDPITT' LONGVALLEYSYMPOSIUM TABLE

2.

Microearthquakes in the LongValley VicinityFrom April 27 to May 30, 1973 Depth

Epicenter

of

Origin

Date

April 27 April 30 April 30 May 1 May 1 May 1 May 1 May 1 May 1 May 1 May 1 May2 May 2 May2 May2 May 2 May 2 May 2 May 3 May 3 May 3 May 5 May5 May 8 May 8 May9 May9 May9 May9 May9 May9 May9 May9 May9 May 10 May 10 May 10 May 11 May 12 May 12 May 12 May 13 May 13 May 13 May 13 May 13 May 14 May 17 May 17 May May May May

17 18 19 19

May 20 May 20 May 20 May 20 May21 May21 May 21 May21 May21 May21 May 21 May 22 May 22 May 23 May 23 May 23 May 23 May 25

13h 05m 23h 05m 07h 28m 05h 58m 07h 35m 07h 40m 08h 59m 10h 37m 1 lh 27m 19h 14m 22h 43m

09.13s 04.48s 43.08s 15.53s 29.05s 38.65s 58.56s 34.68s 25.07s 18.85s 24.89s

031•14m 21.92s 09h 40m 58.78s 10h 37m 08.42s 1 lh 58m 33.73s 14h 21m 17.69s 17h 06m 59.16s 17h 53m 44.03s 09h 1 lm 58.34s 14h 36m 53.39s 15h 03m 24.08s 09h 21m 51.38s 09h 40m 06.85s 09h 09m 04.61s 09h 10m 28.41 s 06h 41 m 14.48s 08h 06m 43.82s 08h 58m 44.41s 16h 04m 56.41 s 16h 47m 59.12s 19h 17m 23.33s 20h 32m 59.19s 21h 23m 59.54s 22h 28m 40.76s 05h 28m 28.62s 16h 48m 24.17s 21h 08m 17.75s 08h 14m 03.58s 04h 48m 10.85s 12h 23m 44.34s 12h 28m 26.15s 02h 3 l m 50.50s 04h 35m 36.23s 05h 42m 36.22s 09h 50m 34.42s 17h 48m 45.60s 10h 22m 56.58s 08h 12m 14.19s 08h 43m 34.11 s 17h 44m 19.26s 14h 54m 39.74s 1 lh 53m 09.82s 1 lh 53m 40.73s 15h 18m 15.67s 15h 24m 05.75s 16h 14m 36.17s 21h 03m 54.33s 06h 29m 28.56s 08h 21m 20.18s 11 h 12m 41.04s llh 13m 39.21s 13h 03m 09.61s 20h 36m 56.25s 22h 58m 15.89s 06h 21m 16.33s 10h 11 m 22.46s 06h 04m 14.24s 07h 05m 48.23s 09h 00m 57.80s 17h 22m 35.34s .._

07h 48m 36.20s

37ø37.01 37027.72 37o33.35 37036.56 37o30.49 37ø30.61 37o28.37 37o35.40 37ø35.41 37ø35.17 37o35.22 37ø30.71 37o33.34

' ' ' ' ' ' ' ' ' ' ' ' '

37o35.75 ' 37o36.26 37o34.09 37o35.47 37o35.88 37o33.30 37o32.07 37o26.88 37o25.99 37030.66 37o35.59 37o35.86 37ø23.13 37o23.78 37o36.57 37o31.83 37o31.57 37o35.70 37035.46 37o35.47 37ø20.91 37ø21.16 37o34.79 37o35.57 37o35.54 37o35.56 37o34.05 37o34.24 37o35.35 37o36.50 37o35.90 37o35.36 37o31.30 37o32.65 37o22.27 37o21.99 37o21.77 37o34.45 37o32.20 37o32.20 37o23.55 37ø37.41 37o37.55 37o37.52 37o37.47 37o37.35 37o37.58 37o37.05 37o57.88 37o31.03 37o32.48 37ø38.17 37ø32.09 37o33.48 37o24.23 37ø37.10 37023.72 37o34.83

Longitude

km

M

N

Gap, deg

Dmin, km

rms, s

118046.90' 118038.63' 118o39.68' 118o57.06' 118o25.09' 118ø24.91' 118o36.80' 118057.43' 118o57.43' 118o57.65' 118o56.84' 118ø26.14' 118039.62 ' 118o25.63' 118ø57.14' 118056.27' 118o56.36' 118ø46.14' 118023.73' 118o43.74' 118o39.77' 118040.95' 118025.34' 118o56.85' 118ø57.19' 118026.74' 118o27.43'

5.00 5.36 7.99 8.35 9.87 13.01 5.00 8.06 7.70 5.00 5.00 9.53 9.56 7.88 7.40 5,00 2.00 9.61 12.62 13.69 7.20 6.49 9.57 4.89 6.86 9.04 9.09

1.29 1.79 1.34 2.07 2.44 2.59 1.59 1.89 1.42 1.44 1.66 1.67 1.07 2.08 1.15 1.18 1.35 1.51 1.21 0.95 1.23 1.01 1.71 0.86 1.10 1.78 1.36

3 7 5 23 16 10 3 11 10 6 7 10 8 12 16 4 4 8 6 14 4 9 13 7 9 25 15

187 163 143 172 269 270 163 187 197 201 181 264 115 265 152 234 193 135 285 217 174 189 181 171 165 247 235

6.5 2.8 3.6 4.2 18.2 18.5 2.6 6.3 6.3 10.6 4.5 13.0 7.9 11.2 3.7 6.3 5.1 8.5 14.2 7.6 4.9 7.2 12.2 4.3 3.7 9.4 8.5

0.00 0.08 0.06 0.14 0.15 0.12 0.03 0.10 0.12 0.13 0.11 0.12 0.23 0.15 0.13 0.04 0.08 0.14 0.13 0.37 0.00 0.05 0.14 0.04 0.06 0.24 0.17

11'8ø53.31 '

2.96

0.83

7

161

3.3

0.04

118o52.54' 118o52.63' 118o57.57' 118o57.22' 118o56.97' 118o35.43' 118035.53' 118o45.82' 118o56.94' 118056.85' 118ø49.10' 118o26.87' 118026.58' 118ø58.01' 118o45.24' 118o57.25' 118o56.99' 118o28.44' 118o27.72' 118o23.02' 118o23.36' 118o23.73' 118ø26.31' 118o42.43' 118042.45' 118o33.78' 118o49.46' 118049.50' 118o49.28' 118049.42' 118o49.36' 118 49.39 118o49.20' 118o42.58' 118o42.52' 118027.76' 118o28.96' 118o59.20' 118027.95' 118032.69' 118o26.06' 118o32.28' 118o42.68'

5.54 3.33 9.08 6.25 6.85 12.42 11.83 6.34 6.56 5.00 5.81 0.92 4.03 9.27 10.35 7.73 6.41 9.43 5.76 6.49 9.27 5.00 2.40 12.85 12.22 11.93 9.09 9.26 10.73 11.26 12.40 11.51 10.86 14.50 0.32 9.73 11.30 1.93 7.94 5.00 2.27 5.00 8.17

1.02 1.28 2.49 1.74 1.34 2.52 1.78 1.37 1.34 1.05 1.02 1.74 1.80 2.56 0.85 1.63 0.97 1.29 2.00 1.71 2.97 1.22 1.08 0.99 1.12 1.55 1.28 1.03 1.11 0.44 0.37 0.57 0.44 1.29 2.87 1.81 1.55 0.85 1.23 1.61 1.98 1.93 0.55

9 12 17 11 10 14 15 13 9 6 9 12 11 19 10 12 8 8 16 14 20 6 5 22 23 24 13 11 7 5 7 11 10 17 18 8 14 5 6 13 15 8 5

204 208 162 177 175 235 235 138 172 203 148 144 146 164 157 164 199 149 143 266 120 309 167 145 145 225 119 115 126 140 144 116 134 273 221 218 159 267 207 259 197 261 310

8.2 8.7 3.2 3.8 4.2 5.2 4.9 7.8 4.2 4.3 3.2 9.4 9.8 2.9 5.9 3.6 4.2 9.7 9.1 15.0 14.6 14.1 9.3 6.3 6.3 21.1 2.6 2.4 2.3 2.5 2.6 2.3 2.9 20.4 8.4 9.2 7.9 7.6 8.2 19.9 34.1 20.8 3.4

0.08 0.07 0.09 0.09 0.08 0.09 0.11 0.08 0.08 0.06 0.09 0.17 0.15 0.15 0.11 0.08 0.06 0.15 0.13 0.12 0.16 0.13 0.06 0.13 0.11 0.39 0.09 0.09 0.07 0.04 0.04 0.11 0.12 0.14 0.21 0.17 0.14 0.08 0.11 0.14 0.08 0.12 0.03

West

North Latitude

' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' '

' ' ' ' '

o

Focus,

,

,

ERH, km

ERZ,

1.1

1.4 2.0 0.8 3.0 3.7

1.0 0.5 1.6 3.4 0.8

1.2 2.1 1.3 2.7 2.1 3.3 0.7

1.4 5.5

2.3 0.5

1.4 0.6 0.5 1.3 1.1 0.4

0:8 0.6

0.5 0.8 0.8 1.0 0.9 0.4 0.9 1.0 0.8 1.5 1.7 0.8 1.3 0.7 0.8 1.7 0.8 2.6 0.8 5.5 1.4 0.5 0.4 3.3 0.6 0.7 0.8 2.3 0.6 0.9 1.0 1.1 1.2 3.6 1.4 2.3 4.4 2.9 0.8 3.4 1.0

km

1.3 1.9 12.0 2.6 2.7 4.6 2.9 0.9

4.3 3.9 2.7 0.9 3.0 1.4 0.8 1.6 1.3 1.2 2.1 3.8 0.6 1.2 1.2 0.6 0.6 1.1 1.3 1.8 1.2 142.1 6.0 0.8 2.1 1.0 1.1 3.3 2.5 4.5 1.4 5.1 21.9 0.7 0.6 6.5 0.9 1.2 1.1 1.6 1.0 1.9 2.1 1.8 0.9 3.8 2.2 20.8 3.3 14.6 94.7 19.6 0.4

AD BC BD AC BD

CD AD AD BD CD BD CD BB CD AC AD AD BC DD CD AD AD BD AC AC BD BD AC BD BD AC

AC AC AD AD AC AC BD AC CC CC BC BC AC AD BC BC CD BB DD CD AC AC CD AB AB AB BD AC AB BB BD BD CD BC CD CD CD CD CD AD

STEEPLES AND PITT: LONG VALLEY SYMPOSIUM

84:5

TABLE 2. (continued) Epicenter

Depth

'

Date

__

of

North West Focus, Latitude Longitudekm M

Origin

37ø23.78 ' 118ø35.20 ' 37ø28.09 ' 118ø32.47 '

9.71 8.58

N

Gap, deg

Drmn, rms, ERH, ERZ, km s km km

Q

May28 May28

08h 13m31.43s 14h43m 16.81s

1.58 9 2.00 13

268 239

20.9 12.8

0.07 0.21

3.2 3.0

6.3 5.2

CD CD

May28 May29 May30

15h24m19.72s 37ø25.13 ' 118ø37.55 ' 8.84 1.52 9 13h04m33.77s 37ø38.04 ' 118ø28.48 ' 8.20 1.99 14 12h25m19.55s 37ø33.65 ' 118ø25.40 ' 7.09 2.81 16

282 171 184

19.1 7.1 6.1

0.08 0.16 0.15

2.8 1.6 1.4

4.5 2.9 1.7

CD BC BD

M is themagnitude of theearthquake; N is thenumber of stations used in locating theearthquake; thegapisthelargest azimuthal separation between stations; Drmn istheepicentral distance to thenearest station; rmsistherootmeansquare errorof thetimeresiduals (rms= 2•,R,•'/N, where R, is theobserved seismic wavearrivaltimelessthecomputed timeat theith station); ERHis thestandard

errorof theepicenter (ERH= SDX:+ SDY:,where SDXandSDYarethestandard errors in latitude andlongitude, respectively, of theepicenter); ERZ is thestandard errorof thedepth(leftblankif it is _>t0). Q is thesolution qualityof thehypocenter (givenastwoletters) based on boththestatistical measure of thesolution andthenature

ofthestation distribution withrespect totheearthquake. Each ofthese twofactors israted independently according tothefollowing

scheme. Forthestatistical measure (firstletter), forA, rms< 0.15,ERH< 1.0,andERZ_