A preliminary study of the Santa Barbara, California, earthquake of August 13, 1978, and its major aftershocks

CECIL D. ANDRUS, Secretary

H. William Menard, Director

Library of Congress catalog-card No. 78-600153 Abstract • Introduction Tectonic setting • • • • Data processing and analysis • Distribution of hypocenters Location of the main shock • • Focal nechanism of the nain shock Correlation • Conclusions • References cited •

Figure 1. Map showing locaticns of principal seisnograph stations am major e~ in Santa samara channel • 2 2. Diagram sharing crustal structure nodels 3. Map shCMing epicenters of Santa Ba:rbara earthquake and its major aftershocks • 4. Diagram shcMing fault-plane solution of Santa Barl>ara earthquake • 5. Cross section of area of figure 3 showing hypocenter distribution and faults .

Figure 1.

2. Diagram sharing crustal structure nodels
3. Diagram shcMing fault-plane solution of Santa Barl>ara
4. Cross section of area of figure 3 showing hypocenter distribution

graph.ic stations used in the present study . • 4

2. List of Santa Barbara earthquakes, August 13-18' 1978.

3 o Criteria for the four quality grades of Q o

ILLUSTRATIONS

showing locaticns of principal seisnograph

shCMing epicenters of Santa

CONTENTS

its major aftershocks •

faults .

TABLES

'Ihe M_r.5 .1 Santa Barbara earthquake of August 13, 1978 occurred at lat 34° 22.2'N., long 119° 43. 0' 4 km south of Santa Barbara, Calif. at a depth of 12. 5 km in the northeast Santa Barbara Channel, part of the western Transverse Ranges gecnn:rphic-structural province. 'Ihis part of the province is characterized by seismically active, east-trenQing reverse faults and rates of coastal uplift that have averaged up to about 10 m/1000 years over the last 45,000 years.

No surface rupture was detected onshore. Subsurface rupture propagated northwest from the main shock tavard Goleta, 15 km west of Santa Bamara, where a maxinrum acceleration of

0. 44 g was measured at ground level and exten-

sive minor damage occurred; only minor injuries were reported. A fairly well- constrained fault-plane solution of the main shock and distribution of the aftershocks indicate that leftreverse-oblique slip occurred on west-northwesttrending, north-dipping reverse faults ; inadequate dip control precludes good correlation with any one of several mapped faults. Had the earthquake been larger and rupture propagated to the southeast or a greater distance to the northwest, it could have posed a hazard to oilfield operations. The fault-plane solution and

regional defo::rmation and the solution closely resembles those of five previously mapped events located within a 15-km radius.

A m:xlerate-sized earthquake (Mr.. = 5 .1, an average fran five Wood-Anderson stations operated by the california Institute of Teclmology) occurred 4 k:m offshore of Santa Bal:bara, California at 3:54 p.m. local time (2254 GCT) on the 13th of August, 1978. Minor local damage occurred at the city of Santa Barbara; the carrpus of the University of California at Goleta

INTRODUCTION

15 k:m to the west suffered extensive minor damage. Hospitals treated scores of people for minor injuries; no major injuries were reported. 'Ihis report sumnarizes the preli.minaJ:y results of our investigation of the main shock and the major aftershocks that occurred in the follcwing five days.

'Ihe Santa Barbara Channel region i~ one of the nost active seismic areas of Ca1if01nia. 'Ihe earliest recorded destructive earthc:'Jake, on Decerrber 21, 1812, heavily damaged several missions along the coast and had an estimated magnitude of 7. Since then, numerous ffi>ents have been felt and several damaging earthquakes have occurred. In particular, alnost tr~ entire business section of Santa Barbara was destroyed or rendered unsafe by the J1.me 29, 1925 earthquake of magnitude 6. 3. Santa Barbara c.lso was damaged by the June 30, 1941 earthquake of magnitude 6. 'lhese two eart:hiuakes are poorly located but are inferred to have occurred very near to the August 13, 1978 event (fig. 1). A list of significant earthquakes in the fanta BaiDara Channel area was prepared by Hamilton and others (1969) and later revised by Iee and Ellsworth (1975).

With increasing population along tl',~ coast ani extensive petroleum developoont in the Santa Baxbara Channel, even m:xlerate-sized earthquakes may be hazardous. Lee and Ellsworth (1~75) argued that tectonic conditions in the C".hannel region are capable of generating an earthquake as large as magnitude 7 .. 5. In view of the continuing likelihood that a large earthquake will occur in the Santa Barbara Channel, a IliC'jor concern is the correlation of seismic data with recognized faults.

AaknowZedgments.--we thank Larry Porter and Tom Wootton of the california Division of Mines and Geology and Gerry Brady of the U.S. Geological Survey for providing stJ::ong-notion data. We are grateful to Mari Gunn and Al Walter for qssistance in data processing, to Bob Burford and Gary Fuis for their stimulating discussions, and to Peter Leary, John M::Raney and DeJ-:-ek Monov for their tireless efforts in the speedy proc-

Figure 1.--Locations of principal seisrrograph stations and major earthquakes in Santa Ba:t:bara Charmel.

essing of the USC data. The USC Santa Barbara network is supported by the Conservation Division, U.S. Geological Survey.

TECTONIC SETTING

Santa Bal:bara Charm.el occupies the southwest quarter of the western Transverse Ranges, a gearorphic-structural province of southern California. Relative to adjoining terrain, the Transverse Ranges are unique in several inportant respects: the distinct east-west orientation, the type, age, and history of exposed basenent rocks, and the spectacular rates of corrpressive defonna.tion as indicated by the inposing reverse-fault-controlled IIDUntain fronts and the extrenely deep basins filled with young, intensely deformed secli.Irents (the Santa Baroara Channel-Ventura basin axis is coincident with the steepest known gravity gradient in California) .

The western Transverse Ranges are botmded by major faults: the east-trending Santa Ynez

40 KILOMETERS

on the north, the southeast-trending San Gabriel on the east, and the east-trending Ar:acapa-Santa ~nica on the south. Onshore segrrent.s of each of these faults juxtapose dissimilar baserrent rocks: the Santa Ynez fonns the south boundaJ::y of the central coast Franciscan, the San Gabriel fo:rms the southwest boundaJ::y of expos~ed Precambrian anorthosites of the western Sar Gabriel M:>untains, and the Malibu Coast-Sante: ~nica fonns the north boundary of the weste:~ Los Angeles basin-continental borderland Franciscan terrane ..

'Ihe structure of the western Trc:nsverse Ranges is daninated by east-trending reverse faults; one of the best k:nown of these is the Red 1buntain fault. Well data, geolc :ric napping, and several congruent fault-plane solutions shew that the fault dips northward at about 60° , offsets strata as young as about 500 ,000 years, and has a :max:imurn stratigra:tfric separaticn of about 7,500 m (Yeats and others, in press).

A band of rroderate seismicity is associated with sarre of the east-trending revers~ faults within the western Transverse Ranges. '1hl.s result is based on a syste.ma.tic stu:ly of the 6- year (1970-1975) record of seisrrographic stations operated by the u.s. Geological Survey, California Institute of Technology, University of Southern California, and California Departtrent of Water Resources (Lee and others, in press} • One or nore of about 200 fault-plane solutions derived from this 6-year record can be associated gearetrically with segments of the Red M:>untain, Pitas Point-Ventura, and San cayentano faults, and perhaps the Mid-<llannel fault and fault X (see fig. 3) o '!he solutions shaN generally near-horizontal P axes oriented at an average of N. 24° E. The inferred c:anpressive stress is reflected in earthquakes of magnitude approximately 1 to 6.5 and reverse displacerrent on the east-trending faults; the average slip vector indicates approximately

(Yerkes and lee, in press).

All the evidence on the rate and sense of defonna.tion is :mutually consistent for individual segments of the faults in the Santa Bal::bara Channel area: geologic data on the sense of latest displacement and arrount and sense of stratigraphic separation, geodetic data on tilting of coastal areas underlain by the faults, uplift of dated marine terrace deposits in such areas, and associated fault-plane solutions. '!he average rates of uplift (up to 10 m/1000 years) , indicated by dating of deposits as young as 2, 500 years 1 show no slc:Mi.ng over the last 45,000 years (Yerkes and Lee, in press) • '!he east-trending reverse faults that dominate the structure of the western Transverse Ranges may be viewed as slip surfaces between a series of north- to northeast-di:pping shingles along which many kilareters of north-south shortening and east-west extension occurred in latest Quaterna:cy time. '!he Santa Barbara earthquake of 13 August 19 78 and its aftershock pattern fit well with this nodel, and its fault-plane solution neatly fits those of five previously mapped events within 15 km of it.

DATA PROCESSING AND ANALYSIS

'!he Santa Barbara earthquake and its aftershocks were well recorded by the California Institute of Teclmology (CIT) -u.s. Geological Survey (USffi) cooperative network in southern California and by seisnographic stations operated by University of Southern California (USC) , University of California at Santa Bal:bara (UC:SB), and California Depa.rt:nent of Water Resources It is very fortunate that the USC group established four stations (three in Santa Barbara Olannel) near the epicentral area one day before the earthquake o After the earthquake, additional stations were installed by usc, USGS, and others.

In order to nake a study in a short time, we selected about 100 earthquakes (out of several hundred well-recorded ones) and processed IIDStly data recorded at the critical stations (fig. 1). Initially, the data were processed ind£'1etldently at CIT, USGS, and USC. At CIT, the earthquakes were processed and analyzed in a routil1~ manner using a computer-assisted system desigiled by c. E .. Johnson. At USC, seismic data recorded on magnetic tapes were played back at a scue of 1 an = 1 second and arrival times were read manually. CIT's Develocorder film reccr.dings of SYP station Cabout 30 km fran the epicenter} were scanned at the USGS. P-arrival, S-arrj val, and signal duration were neasured for events of duration 20 seconds or :rrore. Fran the scar list, arrival times for the larger aftershocks were read from Develocorder fil.ms that recorded the Santa Barl>ara Channel region stations. OVerall errors in the arrival tine data are gerP.rally less than 0 .1 second. ·

nerged and analyzed. We located the ecrthquakes using the HYP071 conputer program (Lee and Lahr, 1975) . Initially, we used Healy's (19f 3) crustal structure nodel and station delays 'ill"nrked out for the Western Transverse Ranges by Le:~, Yerkes, and Simirenko (in press). This allCMeC' us to eliminate gross errors in arrival tin:es' quickly. We then selected 17 well-recorded eart:l1'1Uakes and derived a set of station oorrectiors using a crustal nodel (fig. 2) which approximates a tentative velocity profile in Santa Barbara Olannel obtained fran a geophysical survey usir') the seismic-reflection nethod.

Station ooordinates and station de:lays are given in table 1. Finally, we relocate:-1 all earthquakes using this crustal nodel ard the station delays. Earthquakes were located on the basis of P-wave arrival tines.. '!he HYI"171 cx:mlputer program erploys Geiger's (1912) ~thod to detennine hypocenters by minimizing the: residuals between observed and calculated c:rrivals. Travel tines fran a trial hypocenter tc the stations and their Partial derivatives are cx:mputed on the assunption of a horizontal :multilayer nodel by a tedmique introduced t:' Eaton

using the signal duration nethod (lee c:nd others, 1972}. Hc:Mever, the present earthquake: magnitude estimates are very cn:rle and should :be calibrated against the local magnittrle scale originally proposed by Richter in 1935 (Richter, 1958}. For exanple, magnitude estimated fran s-ignal duration for the main shock (table 2) is 4 .. 9, whereas the average Richter magnitude cf five 'WCXXl-Anderson stations is 5 .1.

DISTRIBUTION OF HYPOCENTERS

A total of 71 earthquakes that occurred from 2254 GCT August 13 to 0718 GCT A~JSt 18, 1978 are listed chronologically in table 2. Included are the origin time, location of hypocenter (epicenter and focal depth), magnitude, and nurrber of arrival tines used. In aildition, five parameters are listed as a neans of evaluating the quality of the hypocenter solution: Station

*These stations are located more than 80 km from the Santa Barbara earthquakes and were not used in the earthquake location.

A brief discussion of the accuracy of hypocenter solution of earthquakes was given by Lee, Eaton, and Brabb (1971). To obtain a reliable epicenter, the largest azimuthal separation between stations (a) should be less than 180° , so that the earthquake epicenter is surrounded by stations. To d:>tain a reliable focal depth, epicentral distance to the nearest station (B) should be less than the focal depth, so that there is a direct ray-path. In addition, systerratic errors arise fran uncertainties in the crustal velocity nodel. These errors cannot be detemd.ned without controlled experiments, such

Table 1.-- Coordinates and delays of prinaipaZ seismographia stations used in the present study

occ

SBLP SBSC SBSM SBSN*

KYP* PTD* SAD* SIP* SYP longitude (W)

34° 18.72' 34° 18.57' 34° 22.00' 34° 26.84' 34° 24.32' 34° 56.48' 34° 22.12' 34° 29.79' 34° 6.57' 34° 33.62 33° 59.68' 34° 2.25' 33° 14.70' 34° 15.27' 34° 27.48' 34° 6.10' 34° 0.25' 34° 4.88' 34° 12.26' 34° 31.60 I 119° 33.68' 119° 39.35' 119° 37.35' 119° 36.98' ugo 42.85' 120° 10.32' 119° 20.63' 119° 42.81' 119° 3.85 120° 24.03' 119° 37.99' 120° 20.99' 119° 30.40' 119° L99' 119° 5.44 t 118° 52.77' 118° 48.37' 118° 39.90' 118° 46.92' 119° 58.70'

as calibrated eJq?losions in the focal region. OWing to the irregular distriliution o.f stations and occasional loss of data fran critical stations, the quality of hypocenter solu'::ions in table 2 varies. Although standard er.urs of epicenters and focal depth are given, they nust be interpreted with caution, especially for quality C and D solutions. These standard errors are conputed with respect to the assured crustal velocity m:rlel, which is not necessarily a good approxination to the real earth ..

The epicenter distriliution (fig. 3) shaNS a linear trend of N. 60° W. with the main shock at the southeastern end. The d:i.rrension of the innediate aftershock area is approxim;..,tely 3 by 12 km. The main shock was preceded four hours by a small earthquake located at the lower righthand corner of the area of figure 3. It is not clear to us whether this earthquake is related to the Santa Barbara earthquake. Howt1ver, it occurred in an area where a swann of earthquakes

took place in March and April of 19 78. Inmedi-

ately after the main shock, seismic activity was

Delay (s)

concentrated 7 km nort:hwest of the main event. rater, a feN aftershocks occurred nearer to the main shock; only two aftershocks located southeast of the main shock occurred in the first five days. On August 16 a feN earthquakes occurred 10 km south of the epicenter; they appear to be associated with a different fault.

LOCATION OF THE MAIN SHOCK

We are fortunate that the Santa Bal:bara earthquakes were surrounded by seisnograph stations and especially that three stations were within 10 km of the earthquake epicenters (see fig. 1} • Hc:Mever, nost of the stations are located northeast of the earthquakes. To lessen the station-distribution bias, we enployed azimuthal weighting (Lee and Lahr, 1975}, ignored stations farther than 80 km fran the earthquake location, and also assigned greater weights to stations within 40 km of the earthquakes.

'lhe biggest uncertainty in eart:lquake location is due to our lack of knowledge of the crustal structure under the Santa Barbara Olannel region. Experinents with various crustal roodels indicate that the epicenter error may be as high as ±3 km and the focaldepth error ±5 krn. For a given crustal roodel, we also experinented with different subsets of arrival-ti..ne data for the main shock. '!he results showed that (1) epicentral locations do not differ nore than ±1 km if the earthquake is surrounded (naximJm azimuthal gap between stations less than 180°), and (2} focal depths do not differ nore than ±2 km if there is a station within 10 km of the earthquake. 'Ihe relative location errors between different earthquakes are srrall because we use station corrections derived fran a set of better recorded earthquakes. 0 In vieN of the above discussion, we suggest the follCMing main-shock paraneters:

Origin time= 22h 54m 52.4s (±0.1 s)

'!be location para:rreter:s are also supported by the strong-notion data obtained fran both the USGS and the California Division of Mines and Geology (a:M;) • 'Ihe tirre interval between s-wave arrival and the initiation of recording at the accelerareter (S-trigger tirre) is a mi.ni.mum estimate of the S-P interval because the recorder may not be triggered by the first P-wave arrival. If we adopt our main-shock location and a ratio of P-velocity to s-velocity of 1. 7, we obtain the follCMing differences between carputed S-P and = 34° 22.2' N., 119° 43.0' W.

= Mr. = 5 .1 (average of five CIT Wood-Anderson stations} Maximum Strong-notion station eration tine Santa Bal:bara

(USGS) Univ. calif. Santa Bal:bara North Hall

'lbese ti..ne differences indicate that ou":" main-shock location and its error estilnates are reasonable. In addition, the station S~ (at

z

Figure 2.-Crustal structure rrodels.

Figure 2 .-Crustal structure rrodels. accel- Trigger

'.... ___ 1 S-P Difference tine

-0.4s [YFAR, IDN, DAY, HR., MN, SEC, origin time in Greenwich Civil Time (GCI') • IAT N1 I..l:NG W, location of epicenter in degrees and minutes of north latitude and west longitude. DEPTH, depth of focus in k.il.areters. MAG, local magnitude <Mr) of the earthquake estimated fran signal durations .. NO, number of stations used in locating earthquake. GAP, largest azimuthal separation in degrees between stations .. r:MrN, epicentral distance in kilarreters to the nearest station. RM>, root-mean-square error of the time residuals: RMS = [E. (R . /NO]\ where R. is the observed seismic-wave arrival ti.Ire minus the cx::rnputed time at the i th station. Em, standard error of the epicenter in kilarreters: ERH =[SDX + SDY respectively, of the epicenter. When NO <5, ERH cannot be CX>ItpUted and is left blank. ERZ, standard error of the focal depth in kilaneters. When NO <5, ERZ cannot be carrputed and is left blank. If ERZ ~ 20 km, it is also left blank. Q, solution quality of the hypaneter (table 3)]

1978 AUG 13 22 54

Table 2.--£ist of Santa Barbara earthquakes, August Z3-Z8, Z978

13 23 18 10.1 13 23 18 32.0 34-25.5 13 23 18 56.9 34-23.5 13 23 19 42.4 lr3 23 22 50.6 34-23.3

13 23 35 53.7 34-24.4 13 23 40 13 23 52 13 23 54 52.3 13 23 56 SEC

3.0 34-24.5 ]~. SDX and SDY are the standard errors in latitude arxl longitude,

2.3 12 73 2.6 13 75 2.0 10 163 2.9 15 70 3.1 13 70 NO GAP

7.7 0.06 0 t::• 5.3 0.05 0.3 5.7 0.04 0.5 0.7 B 5.8 11.7 0.05 0.2

2.9 10.4 0. 15 1.5 2.3 c 3.4

2.0 0.02 0.2 0.2 B

6.6 0.02 0.1 2.9 0.01 9.5 0.05 0.3 2.6 6.7 0.04 0.3 0.5 A

5.8 0.08 0.5 11.7 0.08 0.4 2.6 0.04 0.3 0.3 B 9.4 0.04 0.2 0.3 A 12.9 0.06 0.3 0.03

0.03 0.2 0.3 B 0.10

0.02 0.2 0.2 B ERZ Q

0.4 A 0.3 0.2 B 2.2 0.8 c 0.5 A 0.1 c

0.5 B 0.6 A

0.5 0.6 B 0.2 B 0.3 B 0.5

1. 1 A
2. 1 0.2 B 0.4 A

0.8 A 0.7 B

2.4 B 0 14 0 21 0 36 0 40 1 2

0.04 0.01 0.09 0.25 11.2 10.0 D 0.04

0.06 0.2 A 0.2 0.2 B 0.2 0.6 A 0.2 0.4 B 0.2 0.2 A 0.1

0.2 A 0.3 0.6 A 0.3 0.2 A 0.2 A 0.2

0.2 A 0.3 0.4 B 0.3 A 0.2 0.4 A 0.5 0.2 A 0.1

0.3 0.4 A 0.1 0.1 A 0.6 0.7 A

1. 1 B

0.8 0.4 B 0.3 0.8 B 0.4 0.2

2.8 It 4.1 0.3 0.3 B 0.9 B 0.6 0.4 0.3 A 0.3 B 0.3

0.3 0.6 A 0.7 B 0.3 0.3 A 0.4 0.2 0.3 A 0.2 A 0.2

1. 6 A

0.7 6.4 5.7 It 0.5 A 0.2 0.8 B 0.5

c

Santa BaJ::bara Musemn of Natural Histocy and operated by CIT) gives S-P ti.ne of 2.0 s fran its torsion and strong-notion instrurrents. If we include the SBC data and the above S-trigger tines as S-P intervals in our location of the IPain shock, the epicenter differs by about 1 km and the focal depth by about 2 km from our preferred location.

Figure 3.-EPicenters of Santa Baxbara earthquake and its major aftershocks.

Figure 3.-EPicenters of Santa Baxbara earthquake Table 3.-Griteria for the four quality grades of Q

FOCAL MECHANISM OF THE

Fault-plane solutions of an eart'lquake can be detennined directly fran the first-notion pattern of P-waves provided that (1) the earthquake is well located, (2) the energe:"lt angles

its major aftershocks.

MAIN SHOCK < s.o > s.o > s.o

of seismic waves to the stations are c:mput.ed cmrectl.y fran an adequate crustal IICdel, and (3) the true :polarity of the first P-notions is properly identified fran seisirograns. 'Ihe nodel we used to locate the eari:hJuakes approximates the travel tiires well, but it has too many arl>itrarily discrete layers. Consequently, it may give an erroneous first-notion pattern. 'lherefore, we used a sinpler crustal mxlel (see fig. 2) for cntputing the first-notion pattern. Figure 4 sha-18 our fault-plane solution of the main shock. '!he two possible fault planes are (1) strike N. 66° w., dip 40° N., and (2) strike N. 22° w. , dip 60° s. The focal nechanism indicates reverse faulting with a minor strikeslip cxmponent. 'Ihe local geology and spatial distribution of the aftershocks d:>viously favor the N. 66 w. -40° N. fault plane, which has a minor left-lateral cxmponent. Our poor :Jmor,.rledge of the crustal structure precludes determination of the dip to better than ±10°. 'Ihe strike of the north-dipping fault plane is reasonably well constrained, but that of the south-dipping plane is poorly constrained. If, as in the present case, we intei:pret the crustal structure in tents of a sinple multilayer node!, then the dip of the fault plane is controlled by the first critical refraction angle of seismic rays to the intermadiate-distance stations. rnris in tum depends on the velocity contrast between the rocks in the focal area and those i.rmediately bela.r. 'Ihe fault dip (<I>) is determined approximately by:

<1> =90° - sin-

where v1 is the layer velocity containing the earthquake focus and v 2 is the velocity of the layer next bela.r. It is unlikely that the dip will be larger than 60° because that would require v2 to be nore than twice v 1 , which is unlikely because v 2 cannot be greater than 8 km/s in the lCMer crust, and v 1 is probably not less

CORRELATION

In an atterpt to identify the fault or faults that generated the August 13, 1978 earth-

quake hypocenters along line 'h-A' (fig. ~), which is perpendicular to the trend of the aftershock pattern. '!he result is shCMn in figure 5. 'Ihe present data are inadequate to m'liquely identify the generative fault. First, the fault may not be a uniformly dipping surface. Second, the hypocenters could be fitted to a surface dipping between 30° and 75° N. '1he fault-plane solution of the main shock suggests that faulting occurred on a plane dipping about 40° N. but not greater than 60° N. ; if that plane is projected to the surface it can be correlated with fault X. However, geologic data in the eastern Santa Bai:bara Olannel indicate that sarre structural features south of well num-Figure 4.-Fault-plane solution of

Figure 4.-Fault-plane solution of S~ta

ber 5 (fig. 3) dip south. 'lbe Pitas Point fault dips steeply north near the surface and could be the generative fault if it dips nore gen+:ly at depth.

CONCLUSIONS

PreUmina.ry results on the Santa Barbara

that reverse faulting took place on a ncrthdipping fault at a depth of about 12 km and that the rupture propagated northwest torrlard Goleta. '!his is consistent with 0. 44g maximum ac~lera tion recorded by crM; in Goleta at North Hall, university of ca.lifon1ia at Santa Barbara; nost of the damage occurred in Goleta also. In addition, extension of the trend of the aftershock oottem westward intersects the shorelin~ at the nouth of Telecote Canyon; severe shaking there caused failure of a long segnent of 1:mSU'?lJ?Orted railroad fill and the resulting derail.:rre"'lt and wreck of a freight train about 7 minutes after the earthquake.

The Santa Barbara earthquake was relatively small, and there was no onshore surface rupture .. The subsurface rupture progagated to the northwest. Had the earthquake been larger a"l.d rupture propagated to the southeast or a greater distance to the northwest, it could have posed a hazard to oilfield operations. It is interesting to note that the Jnne 30, 1941 Sa"l.ta Barbara earthquake was preceded by an eatthquake swann in Februai:'.f of 1941. Similarly, t~ August 13, 1978 Santa Barbara earthquake w~ preceded by an earthquake swann located abo\lt 20 km taYard the southeast in March and April of 1978. Ten years before, the 1968 earthquake sw:n:m in

s .....

en

Figure 5..-cross section of area of figure 3, showing hypocenter distributions and faults.

Figure 5 ..-cross section of area of figure 3, showing hypocenter distributions and faults.

the Santa Barbara Chmnel was located in the sane general area as the March-April 1978 swann and included a magni tu:1e 5. 2 event (fig. ll (Sylvester and others, 1970}.. IJa.rever, this swarm was not follO!Ned by any larger earthquake. 'lberefore, it is not clear that ea.rth}uake swanns are reliable precursors to larger earthquakes in the Santa Barbara Channel.

REFERENCES CITED

Allen, c. R., St. Amand, P., Richter, c. F., and Nor~t, J. M. , 1965, Relationship between seismicity and geologic structure in the southern california region: Seisnol. Soc. Anerica Bull., v. 55, p. 753-797.

Eaton, J. P. , 1969 , Hypolayer-a carputer program for determining hypocenters of local earthquakes in an earth consisting of unifonn flat layers over a half-space: U.s. Geol. Survey q?en-file report, 155 p.

Ellsworth, W. L., Cantbell, R. H., Hill, D. P., Page, R. A. , Alewine, R. w. , III, Hanks, T. C., Heaton, T. H., Hileman, J. A. , Kanartnri, H. , Minster, B. , and Whit.cx:nb, J. H. , 1973, Point Mugu, California, earthquake of 21 Februaey, 1973, and its aftershocks: Science, v. 182, p. 1127-1129.

IS' 34°20'

i A'

34°2fi'

Geiger, L. , 1912, PrOOability nethod for the detel::mination of earthquake epi~.nters fran the arrival tine only (translate-1 fran Geiger•s 1910 Gernan article): Bull. St. IDuis Univ., v. 8, p. 56-71. Hamilton, R. M. , Yerkes, R. F., Browr , R. D., Jr., Burford, R. 0., and De Neyer, J. M., 1969, Seismicity and associated effects, Santa Bazbara region, pt. D of Geology, petroleum developnent, and seisnliCity of the Santa Ba.rl:>ara Olannel region, Califon1ia: u.s. Geol. Survey Prof. Paper 679, p. 47-68. Healy, J. H. , 1963, Crustal structure. along the ooast of california fran seisnrl..c refraction neasu:renents: J. Geophys. Res. , v. 68, p. 5777-5787. Hileman, J. A., Allen, c. R., and Nordquist, J.

M. , 1973, Seisnrl..city of the southem Califomia region: Contribution no. 2385, Div. of Geological and Planetary Sciences, California Institute of Technology. lee, W. H. K. , Bennett, R. E. , and .Me>"tgher, K. L., 1972, A nethod of estimating magnitude of local earthquakes frar signal duration: u.s. Geological S~r open-file report, 28 p. lee, W. H. K., Eaton, M. S., and BraJ:l'>, E. E., 1971, '!he earthquake sequence ne::rr Danville, california, 1970: Seisnological Society of Anerica Bulletin, v. 61, p. 1771-1794.

Earthquake activity in the Santa Bal:bara Olannel region, in Draft enviromrent ~ nent, Oil and gaS"developrent in the Santa Barbara Cll.armel outer oontinental shelf off california: U.s. Geological SUrvey, p. II-80 - II-137.

(Revised): A carputer program for determining hypocenter, magnitude, and first notion pattern of local earthquakes: U.S. Geological Survey Open-file Report 75-311, 114 p. Lee, W. H. K., Yerkes, R. F., and Simirenko, M., (in press) , Recent eartb:Juake activity and focal nechaniSDB in the western Transverse Ranges, California: U.s. Geological Survey

Ridlter, C. F. , 19 58, Elenenta:cy seisnology:

San Francisco, W. H. Freedman and carpany, 768 p. Stierman, D. J., and Ell.sworth, W. L., 1976, Aftershocks of the February 21, 1973 Point a:tgu, california earthquake: SeiSltl-:>logical Society of Anerica Bulletin, v. 66, no. 6, p. 1931-1952. Sylvester, A. G., Smith, s. W., and Sdlolz, c. H., 1970, Farthquake swann in the Santa Barbara Cll.annel, california, 1968: Seisnological Society of Anerica Bulletin, v. 60, p. 1047- 1060. Yeats, R. S., Lee, W. H. K., and Yerkes, R. F., (in press) , Geology and seismicity of the eastern end of the Red lobuntain fault, Ventura Cotmty, california: u.s. G~logi cal Survey Professional Paper. Yerkes, R. F., and Lee, W. H. K., (in press), Late Quatema:ry defo:rnation in the western Transverse Ranges, california: U.s. Geological Survey Circular 799B.

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