Seismicity and Crustal Structure in the Orozco Fracture Zone'

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JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 87, NO. B10, PAGES 8501-8507, OCTOBER 10, 1982 Seismicity and Crustal Structure in the Orzc Fracture Zne' Prject Rse Phase II TORU OUCHI, 1 ABOU-BAKR K.

1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 87, NO. B10, PAGES , OCTOBER 10, 1982 Seismicity and Crustal Structure in the Orzc Fracture Zne' Prject Rse Phase II TORU OUCHI, 1 ABOU-BAKR K. IBRAHIM, 2 AND GARY V. LATHAM 3 Gephysics Labratry, Marine Science Institute, University f Texas at Austin, Galvestn, Texas A ttal f 301 earthquakes were recrded in the vicinity f the Orzc fracture zne by seven Texas cean bttm seismgraph statins during the 2-week perid f the Rivera Ocean Seismic Experiment (ROSE) (phase II). Using data frm the entire ROSE array, hypcenters f 50 earthquakes were determined. These revealed tw distinct znes f seismic activity within the fracture zne. In additin t these earthquake families, many very small events were detected by a statin lcated very clse t the spreading center f the East Pacific Rise. The magnitudes f these earthquakes, defined by their duratin times, were s small that mst f them were recrded nly at this statin (statin 14) in cntinual r swarmlike ccurrences. The slpe f the frequency-magnitude distributin f these events is significantly larger than thse f ther earthquake grups detected during the experiment, i.e., they appear t have an unusually high b value. These results suggest that this new ppulatin f earthquakes may be assciated with vlcanic activity r stress release within highly fractured crustal material. Refractin studies in the fracture zne reveal the presence f a rather high-velcity crustal layer ( km/s) beneath the experiment zne. The Mh velcity and the crustal thickness are estimated at 7.8 km/s and.2 km, respectively. INTRODUCTION In this decade, a number f micrearthquake and refractin surveys have been carried ut in the ridge-fracture zne areas f the East Pacific Rise and the Mid-Atlantic Ridge. These studies have revealed high but cmplicated seismic activity in the vicinity f these features [e.g., Hyndrnan et al., 1978; Prther et al., 197; Francis et al., 1977]. Hwever, because relatively few statins were available fr these studies, detailed delineatin f the patterns f seismic activity was nt pssible. Previus refractin surveys have als shwn remarkable variatins in the crustal structure f these regins [e.g., Lewis and Snydsman, 1979], but very few measurements have been made within the fracture zne itself [e.g., Derrick and Purdy, 1980]. The Rivera Ocean Seismic Experiment (ROSE) was carried ut t address these issues by cmbining the instruments frm many grups t frm a dense netwrk f cean bttm seismgraph (OBS) statins surrunding and within a prtin f the East Pacific Rise. In this paper, we reprt preliminary results n the seismicity and crustal structure in the Orzc fracture zne btained during prject ROSE phase II, primarily frm the Texas OBS statins. OBSERVATIONS Twelve Texas OBS's were deplyed in the junctin areas f the Orzc fracture zne with the East Pacific Rise during prject ROSE phase II between February 27 and March 1, Eleven statins were recvered, and tape recrder malfunctins prevented the recrding f useful data by fur f these statins. Results frm the remaining seven statins Nw at Department f Earth Sciences, Faculty f Science, Kbe University, Nda, Kbe 57, Japan. 2 Nw at U.S. Nuclear Regulatry Cmmissin, Washingtn, D. C Nw at Chevrn Oil Field Research C., La Habra, Califrnia Cpyright 1982 by the American Gephysical Unin. Paper number 2B /82/002B are presented in this paper. The lcatins f these statins are shwn in Figure l a. The statin crdinates and perids f peratin are listed in Table 1. In additin t micrearthquake bservatins, fur refractin prfiles were sht in the experiment zne. Results frm tw f these lines are presented in this paper. Each statin cntains a single, vertical cmpnent gephne with a resnant frequency f 8 Hz. A detailed descriptin f the Texas OBS statins has been presented in a previus paper by Latham et al. [1978]. SEISMICITY IN THE OROZCO FRACTURE ZONE A ttal f 301 micrearthquakes were recrded by the seven OBS's f the University f Texas in the vicinity f the Orzc fracture zne during a 2-week bservatin perid. Fifty f these earthquakes were large enugh t als be recrded by the statins f ther participating grups. These events, which delineate tw prminent znes f seismic activity, were reprted in a previus team paper [Prject ROSE Scientists, 1981]. As shwn in Figure 2, ne grup ccurred within a narrw, east-west trending zne, presumable defining a branch f the transfrm fault system that was active during the experiment. The secnd grup is mre diffuse and falls within the central prtin f the fracture zne [Prject ROSE Scientists, 1981]. Hwever, seismicity in this area is als) characterized by the frequent ccurrence f very small earthquakes in the near vicinity f individual instruments. The magnitudes f these earthquakes were s small (see Figure 4) that mre than 80% f them were recrded at a single statin nly. The ttal number f earthquakes recrded by each statin and the duratins f recrding are given in Table 2. The spatial variatins in seismicity were large during the experiment. Average activity levels f earthquakes with S-P times f 3 s r less, detected by the varius statins, ranged between 0 and 20 per day, as indicated in Figure 2. Seismic activity was greatest in the vicinities f statins 2 and 14, which were clse t eastern and western junctins between the spreading centers and the fracture zne, whereas the regins arund statins 4, 5, and 10 were relatively quiet. Sharp 8501

2 8502 OUCHI ET AL' OROZCO FRACTURE ZONE SEISMICITY 10 ø 1 ø 105 ø 104 ø 1 ø 15 ø ø 105 ø 104 ø Fig. la. Bathymetric map (J. Mammerickx, persnal cmmunicatin, 1980; cnturs in meters) f the ROSE phase II area. Stars indicate OBS (cean bttm seismgraph) r OBH (cean bttm hydrphne) lcatins. Large stars and numbershw OBS's f the University f Texas. Slid line A and dashed line B shw refractin lines analyzed in this paper. 14 ø._._, v, ST3 ST4 ST5 ST13 28 I 5 10 FEB. MAR peaks in the S-P time histgrams fr statins 2 and 14, shwn in Figure 3, indicate high spatial clustering f earthquakes in the vicinity f bth statins. Typical seismgrams fr earthquakes that riginated at shrt epicentral distances are shwn in Figure 4. Circles at distances crrespnding t the peaks f the S-P time distributins suggesthat the earthquakes recrded at statins 2, 3, and 5 are mstly assciated with the seismic activity f the narrw transfrm fault and thse detected by statin 13 riginate within the active zne lcated in the central prtin f the fracture zne, as described in the earlier team paper [Prject ROSE Scientists, 1981]. It is ntable that statin 14 recrded many events with S-P times f 2.1 s, althugh mst f them were nt detected by any ther statin and hence culd nt be lcated. Apparently, these events reflect a different ppulatin f earthquakes, ccurring in the near vicinity f statin 14. During the bservatin perid, there were tw peaks in seismic activity: the first n March 2 and the secnd n March 8, as shwn in Figure 5. The largest shck recrded Fig. lb. Backgrund nise level at individual statins. Vertical scales indicate the recrdin gain level, G. Actual gain is btained as 0 + x gain (db). High nise levels n February 27 and 28 are due t the shts fired fr the refractin surveys. Except fr this perid, the gain f individual instruments, crrespnding t the backgrund nise level at each statin, seems t be rather stable (within +- db). It is ntable that the recrding gain at statin 13 is slightly lwer than during the experiment ccurred at 1214 UT n March 8 in the at ther statins. This wuld reduce the relative detectin capability narrw east-wes trending seismic zne and was fllwed by f this statin smewhat. Shts might have reduced the detectin f earthquakes during the refractin survey. Dashed lines dente a gain many aftershcks. It is nted that the rate f activity level f 4 (84 db). detected by statin 14 is much mre unifrm than that f the TABLE 1. Statin Crdinates and Perids f Operatin f Texas OBS Statins' ROSE Phase II Statin Latitude Lngitude Depth, fm Perid f Operatin, UT ' 105ø15.1 ' ø18.1 ' ' ø0.1' 105ø27.1' ' 105ø11.3' ' ' ' ' ' ; ' 1550 Feb. 27, 1542:00-March 1, 2050:00 Feb. 27, 1201:30-March 1, 151:00 Feb. 27, 1352:00-March 14,!808:00 Feb. 27, 1547:00-March 15, 1915:00 Feb. 27, 2239:00-March 1, 1024:00 Feb. 27, 1808:00-March 8, 1833:00 Feb. 27, 1325:00-March 8, 0208:00

OUCHI ET AL' OROZCO FRACTURE ZONE SEISMICITY 8503 20.0 _,,,,,. _ ' Fig. 2. Epicenter distributin f micrea hquakes (data are frm Prject ROSE Scientists [1980]).

3 OUCHI ET AL' OROZCO FRACTURE ZONE SEISMICITY _,,,,,. _ ' Fig. 2. Epicenter distributin f micrea hquakes (data are frm Prject ROSE Scientists [1980]). Large slid circles dente eachquakes dete ined by mre than 10 statins, and smaller slid circles shw the events dete ined by less than 10 statins. Underlined numbers shw the d ly frequency f eachquakes with S-P times 3 s detected by each statin. The l ge circles centered n each statin c espnd t the peaks f the S-P time histgram shwn in Figure 3. Omfi ce cients K are assumed as 7.0 k s fr statins 2, 3, 13, and 14 and 8.0 k s fr statin 5 in determining the radii f these circles. ther statins (Figure 5) and that the seismic activity was characterized by cntinual r swarmlike ccurrences f very small earthquakes. An additinal peak in activity ccurred in the central area f the transfrm n March 4. This may be assciated with the peak seen in the data f statin 13 fr that date. SEISMIC ACTIVITY IN THE VICINITY OF STATION 14 We have investigated the earthquakes in the vicinity f statin 14 in mre detail t understand the ppulatin f earthquakes detected primarily by this statin. The mst ntable feature f this seismic activity, as mentined abve, is the cntinual r swarmlike ccurrences f very small earthquakes. A sample recrding is shwn in Figure. We tried t determine the lcatins f the hypcenters f these events. Fur f these events were recrded at ther statins and the frmal slutins fr three f these placed them clse t statin 13 (Figure 7) at abut the right distance frm statin 14 based upn S-P times at statin 14. Readings frm statin 13 were nt used because f bad timing. Hwever, the signals recrded at statin 13 are very weak, and the S-P times are lnger than 2 s, which indicates that the hypcenters are mislcated. Using S-P times at statins 13 and 14, a lcatin either 12 km t the nrth r 12 km t the suth f statin 14 is indicated. T define this new ppulatin f earthquakes mre clearly, we investigated the magnitude-frequency distributin f the events recrded at the varius statins with S-P times f 3 s r less. Since it is difficult t determine the abslute magnitudes f the events, we assume a cumulative numberduratin relatin f the frm given by equatin (1). Here D is the duratin time f an earthquake measured frm the first break t the time when the signal reaches a certain multiple f the backgrund nise. The reference level is equal t, 2, r 4 times the backgrund nise depending upn recrding gain at the time f the earthquake. We nte here that the recrding gain is autmatically adjusted every 10 min t TABLE 2. Recrding Intervals and Numbers f Earthquakes Recrded by Texas OBS Statins Cmmn Statin Events Days Events

4 , 8504 OUCHI ET AL.' OROZCO FRACTURE ZONE SEISMICITY n 2O 2O l ST3 20 2,,, ST5 ST10 l ST ST13._.[,T T, TT],,, 4O 2O ST 14 0 i s - P (SEC) Fig. 3. S-P time histgrams f earthquakes at varius statins. Nte that many earthquakes were recrded with $-P times f 2.1 s at statin 14. maintain a fixed level f ambient nise, and the gain is recrded as part f the clck cde. Since statin gains varied little during the experiment, usually within db, the cmpensatin f duratin times fr variatins in recrding gain is nt f great imprtance in the present case. Statin gain histries are shwn in Figure lb. Fllwing previus investigatrs, we assume that the effects f variatins in epicentral distance n the duratin time f cda waves is negligible in the near range [e.g., Tsurnura, 197; Crssn, 1972]. Since the events analyzed were limited t thse with S-P times f 3 s r less, this wuld 10'8rn P S 1SEC,_,[l}=;,,,l,, 28 I FEB MAR Fig. 5. Tempral variatins f the earthquakes recrded by the Texas OBS statins. The height f each vertical bar terminated by a dt indicates the number f earthquakes detected by a statin in a given 24-hur perid. Absence f a dt indicates an incmplete 24- hur recrding perid. Tw peaks in seismic activity are seen n March 2 and 8. appear t be a reasnable assumptin in the present case. Fllwing the Gutenberg-Richter frmula fr the frequency-magnitude distributin f earthquakes, we assume a relatinship f the frm lgn=a- blgd (1) where N is the cumulative number f events with duratins equal t r greater than D. As shwn in Figure 8, significant differences are seen in the cumulative number-duratin distributins between statins 2, 3, 13, and 14. The earthquakes in the vicinity f statin 14 have a rather high b value f 1.84, whereas thse f ther earthquake families in the fracture zne range frm 0.9 t 1.1. Althugh it is nt a simple matter t relate these b values with thse btained by cnventinal methds, the relative variatins are significant and imply a relatively high b value fr the ppulatin f earthquakes recrded by statin 14. A high b value indicates a prepnderance f earthquakes f small magnitude. CRUSTAL STRUCTURE IN THE OROZCO FRACTURE ZONE 1 SEC 10'8m P S R T I Fig. 4. Examples f earthquakes recrded at statin 2. The seismicity f this area is characterized by the frequent ccurrence f very small earthquakes riginating in the near vicinity f each instrument. R is a sea surface reflected phase. We have analyzed refractin data recrded at statins 5 and 10 (Figure l a). Line A was sht frm the R/V Cnrad f Lamnt-Dherty Gelgical Observatry. The sht size was kg. Line B was sht by the RN Kana Keki f the Hawaii Institute f Gephysics. The sht size was kg. Sht distances were calculated frm water wave travel ST14 MAR 1 21:48 PS P S PS 10-8m II I I 5SEC I I Fig.. Swarm activity recrded at statin 14.

5 OUCHI ET AL.' OROZCO FRACTURE ZONE SEISMICITY 8505 Fig. 7. Estimated epicenters f three f the largest events frm the set nrmally recrded by statin 14 nly (slid circles). Readings f P and S arrivals frm Oregn, Lamnt, and Hawaii statins (ne each) were used, in additin t thse f statins 10 and 14. Nte that n readings frm statin 13 were used in the calculatin because f clck prblems at this statin. Crustal velcities btained in this study were used. V,/Vs value is assumed as 1.7. Alternate lcatins based upn S-P times at statins 13 and 14, assuming surface fcus, are indicated by pen circles. times. Since a triggering system is used fr the Texas OBS's, and the recrd length is 80 s, sme shts were nt recrded, i.e., grund arrivals were nt large enugh t trigger the system n. In successful cases, recrds frm lng-range shts were truncated. Hwever, analysis f the available data set shws sme interesting features in the crustal structure in this area (Figures 9a-9c). First, relatively high crustal velcities (.9-7 km/s) are fund within a few kilmeters beneath the basement. Secnd, the estimated Mh velcity is lw (7.8 km/s) and the crustal thickness is.2 km (Figure 10) in the fracture zne. We might als nte that the efficiency f prpagatin bserved in this case suggests that the structure is nt as hetergeneus as we might have expected. CONCLUSION AND DISCUSSION Prject ROSE revealed a rather cmplicated pattern f seismic activity cnsisting f at least three different znes f activity in the vicinity f the Orzc fracture zne. Tw f loo loo 000 ST14 C =1.82 lo \ ''' ' C =0.99 lo ST13 C=1.07 ST3 C= 0.92 I I I I, I i lg D lg D Fig. 8. Cumulative number-duratin distributins f earthquakes at statins 2, 3, 13, and 14. Calculated b values are (statin 2), (statin 3), (statin 13), and _ 0.0 (statin 14), respectively. Measurement f the duratin time D was limited t 72 s r lg D < 1.8.

850 OUCHI ET AL.' OROZCO FRACTURE ZONE SEISMICITY ST10 LINEA SOUTH 44 ß..9 0 10 20 0 0 km 0 km 10 Fig. 9a Fig. 9b ST10 LINE B ST5 LINE A. _ 30 km, 0 20 3'0 km 20 lo Fig. 9c Fig. 9d Fig. 9. Recrd sectins f sht prfiles.

6 850 OUCHI ET AL.' OROZCO FRACTURE ZONE SEISMICITY ST10 LINEA SOUTH 44 ß km 0 km 10 Fig. 9a Fig. 9b ST10 LINE B ST5 LINE A. _ 30 km, '0 km 20 lo Fig. 9c Fig. 9d Fig. 9. Recrd sectins f sht prfiles. All data were passed thrugh a 4-8 Hz band-pass filter. All arrivals were crrected fr bathymetric variatins assuming a velcity f 5 km/s. these znes were described in the first team paper n the subject [Prject ROSE Scientists, 1981]. In this paper, we have defined a new ppulatin f earthquakes that riginate in the vicinity f statin 14. This statin is lcated clse t the axis f the spreading center f the East Pacific Rise. The earthquakes f the new zne are characterized by cntinual r swarmlike ccurrences and by I- 5- Q 10- ST10 LINE A NORTH 5.5,9 7,8 km/sec SOUTH km/sec Fig. 10. Estimated crustal structure in the Orzc fracture zne. Depths are measured frm tp f basement. the steep slpe f their frequency-magnitude distributin, i.e., by a high b value. Rather high b values fr earthquakes ccurring within mid-cean ridges, bserved by OBS's r Shbuys, was suggested by Francis and Prter [1973] and Jhnsn and Jnes [1978]. These features suggest that the earthquakes riginating in the vicinity f statin 14 may be assciated with vlcanic activity r stress release within the highly fractured crustal material f the East Pacific Rise. Results frm refractin experiments reveal a relatively high crustal velcity ( km/s, crustal layer III) at a few kilmeters depth. The crustal thickness is estimated as.2 km and the Mh velcity btained is 7.8 km/s. These values are quite similar t results btained fr the Ccs plate (line, Lewis and Snydsmann [ 1979]), and fr the Kane fracture zne [see Detrick and Purdy, 1980, OBH1 and OBH8 in Table 1] f the Mid-Atlantic Ridge. Acknwledgments. The authrs gratefully acknwledge the supprt f the OffSee f Naval Research under cntract N C The University f Texas, Institute fr Gephysics, cntributin 529. REFERENCES Crssn, R. S., Small earthquakes, structure, and tectnics f the Puget Sund regin, Bull. Seisml. Sc. Am., 2, , Detrick, R. S., Jr., and G. M. Purdy, The crustal structure f the Kane fracture zne frm seismic refractin studies, J. Gephys. Res., 8:5, , 1980.

7 OUCHI ET AL.: OROZCO FRACTURE ZONE SEISMICITY 8507 Francis, T. J. G., and I. T. Prter, Median valley seismlgy: The Mid-Atlantic Ridge near 45øN, Gephys. J. R. Astrn. Sc., 34, , Francis, T. J. G., I. T. Prter, and J. R. McGrath, Ocean bttm seismgraph bservatins n the Mid-Atlantic Ridge near 37øN, Gel. Sc. Am. Bull., 88, 4-77, Hyndman, R. D., G. C. Rgers, M. N. Bne, C. R. B. Lister, U. S.Wade, D. L. Barnet, E. E. Davis, T. Lewis, S. Lynch, and D. Seeman, Gephysical measurements in the regin f the Explrer Ridge ff western Canada, Can. J. Earth, Sci., 15, , Jhnsn, S. H., and P. R. Jnes, Micrearthquakes lcated n the Blanc fracture zne with snbuy arrays, J. Gephys. Res., 83, , Latham, G., P. Dnh, K. GrifiSths, A. Rberts, and A. K. Ibrahim, The Texas cean-bttm seismgraph, a paper presented at the Offshre Technlgy Cnference, Sc. f Explr. Gephys., Hustn, Tex., Lewis, B. T. R., and W. E. Snydsman, Fine structure f the lwer ceanic crust n the Ccs plate, Tectnphysics, 55, , Prject ROSE Scientists, Micrearthquake activity n the Orzc fracture zne: preliminary results frm prject ROSE, J. Gephys. Res., , Prther, W. A., I. Reid, M. S. Reichle, and J. N. Brune, Ocean bttm seismic measurements n the East Pacific Rise and Rivera fracture zne, Nature, 22, , 197. Tsumura, K., Determinatin f earthquake magnitude frm ttal duratin f scillatin, Bull Earthquake Res. Inst., Tky Univ., 15, 7-18, 197. (Received July 14, 1981; revised June 21, 1982; accepted June 23, 1982.)

Introduction This auxiliary material contains a section called Supplementary Text, eight Supplementary figures, and their figure captions.

Introduction This auxiliary material contains a section called Supplementary Text, eight Supplementary figures, and their figure captions. Auxiliary material fr Paper 21GL45462 Remtely triggered micrearthquakes and tremr in Central Califrnia fllwing the 21 Mw 8.8 Chile Earthquake Zhigang Peng1, David P. Hill2, David R. Shelly2 and Chastity