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 Aiken1 1. Schl f Earth and Atmspheric Sciences, Gergia Institute f Technlgy, Atlanta, Gergia 2. United States Gelgical Survey, Menl Park, Califrnia Gephys. Res. Lett., 37, L24312, di:1.129/21gl45462, 21. Intrductin This auxiliary material cntains a sectin called Supplementary Text, eight Supplementary figures, and their figure captins. Supplementary Text 1. Data The seismic data used in this study cme frm the High Reslutin Seismic Netwrk (HRSN) perated by Berkeley Seismlgical Labratry, University f Califrnia, Berkeley, the Nrthern Califrnia Seismic Netwrk (NCSN) perated by the U.S. Gelgical Survey, Menl Park, and the Suthern Califrnia Seismic Netwrk (SCSN) perated by Caltech, and are distributed by the Nrthern and Suthern Califrnia Earthquake Data Centers. Fr the study in Cs we first dwnlad the SCSN catalg arund the 21 Chile mainshck frm the Suthern Califrnia Earthquake Data Center (SCEDC) within a rectangular area (lngitudes between!118.25 and!117.5, and latitudes between 35.75 and 36.25 ) that bunds ur study regin. We als emply seismgrams f the Chile earthquake recrded at the bradband statin JRC2 (sensr type CMG-3ESP with a natural perid f 12 s). This statin is a replacement f (and clse t) the statin JRC that was used in identifying triggered earthquakes fllwing the 22 M w 7.8 Denali Fault earthquake [Prejean et al., 24]. Fr the triggered tremr arund the Parkfield-Chlame sectin f the San Andreas Fault (SAF), we examine cntinuus wavefrms arund the Chile mainshck recrded at nearby surface statins that belng t the NCSN and Berkeley Digital Seismic Netwrk (BDSN), and the brehle statins that belng t the HRSN. The statin PKD belnging t the BDSN is equipped with STS-2 bradband sensr with a natural perid f 12 s. 2. Seismicity rate change at Cs We test the hypthesis that these events are triggered by the Chile mainshck by the fllwing tw prcedures. First, we cmpare the seismicity rate immediately befre and after the P wave f the Chile mainshck and cmpute the "-statistic value [Matthews and Reasenberg, 1988; Kilb et al., 22], which is a measure f the difference between the bserved number f events after the main shck and the expected number frm the averaged rate befre the main shck. We calculate the "-value based n 15 days f seismicity befre and within 1 hur after the theretical arrival time f the P wave f the Chile mainshck. The resulting "-value is 21.9 fr events with magnitude larger than.9, the magnitude f cmpleteness based n the 9% gdness-f-fit test [Wiemer and Wyss, 2]. If we change the pre-mainshck time windw t be 9, 6, 3, 5, 1, and.25 day 1
(6 hur) befre the P waves, the crrespnding "-value are 2.7, 15.6, 26.7, 22., 11.45, and 12.82 respectively. The variatins in the "-values riginate frm the fluctuatins in the backgrund seismicity. The reductin f the "-value with shrt pre-mainshck time is likely because the triggered activity ccurred during an extended perid f swarm activity in the CGF immediately befre the mainshck. Nevertheless, because all these values are clearly abve 2, we suggest that the increase f seismic activity immediately after the teleseismic P wave f the Chile mainshck is statistically significant [Matthews and Reasenberg, 1988; Hill and Prejean, 27]. As shwn abve and mentined in Hill and Prejean [27], the "-statistic relies largely n the chice f the time windw and culd vary due t fluctuatins in backgrund seismicity. As an alternative apprach, we use earthquakes listed in the SCSN between 1997/1/1 (apprximate beginning time f the bradband statin JRC) and 21/8/25, and determine the likelihd f an M l! 3.45 event and 4 M l! 2 events within 1 hr ccurring by randm chance arund Cs. The ttal numbers are 86 and 57, respectively, which crrespnd t the dds f seeing them fr a given hur t be.7% and.42%, respectively (Figure S1). These numbers are calculated based n the assumptin f randm ccurrence. Instead, the seismicity arund Cs shws clear clustering arund 1998, 2-22, and 21. In particular, the seismicity rate arund Cs in early 21 was high, with tw swarm-like sequences ccurred arund 21/1/15 and the Chile mainshck. If we fix the time windw t be between 1/12/21 and 3/12/21 (the apprximate start and end f the mst recent swarm-like sequences), the ttal numbers fr an M l! 3.45 event and 4 M l! 2 events within 1 hr are 9 and 14, respectively, which crrespnd t the prbability f ccurring in any given hur t be.64% and.99%, respectively. Based n this, we can reject the hypthesis that these events ccur by randm chance at the 99% cnfidence level. 3. Seismicity rate change at Parkfield Similar t the study in Cs, we calculate the "-value based n the 15 days f the lw-frequency earthquakes (LFEs) arund Parkfield befre and within 1 hur after the arrival time f the P wave f the Chile mainshck. The resulting "-value is 18.3. If we change the pre-mainshck time windw t be 9, 6, 3, 5, 1, and.25 day (6 hur) befre the P waves, the crrespnding "-value are 8.9, 8.6, 18.2, 26.1, 43.4, and 11., respectively. Again, the variatins in the "-values riginate frm the fluctuatins in the backgrund LFEs. The increase f the "-value with shrt pre-mainshck time is likely because the rate f LFEs arund Parkfield immediately befre the Chile mainshck is relatively small. Nevertheless, because all these values are clearly abve 2, we suggest that the increase f LFE activity immediately after the teleseismic P wave f the Chile mainshck is statistically significant [Matthews and Reasenberg, 1988; Hill and Prejean, 27]. If we keep the pre-mainshck time windw as 15 day and increase the pstmainshck time windw t 15 day, the "-value decreases t -.48, again suggesting that the seismicity increase fllwing the Chile mainshck is statistically significant nly within a shrt time windw. 4. Crrelatins between the surface waves and triggered activity We shift bth the surface waves and the lcally triggered signals back t the surce regin t evaluate their crrelatins. Fr the Cs case, the distance between the M l 3.5 2
event and the statin JRC2 is 11.3 km. Assuming the Lve wave velcity f 4.3 km/s, we shift the transverse cmpnent seismgram frward by ~2.6 s t reflect the timing at the epicenter f the M l 3.5 event. The time difference between the peak f the Lve wave (pen circle) and the rigin time f the M l 3.5 event is 15.7 s (Figure 3b). Figure S7 shws that the first tw earthquakes with M l = 2.9 and 2.1 ccurred ~51 s befre and ~249 s after the S arrival predicted frm the iasp91 glbal velcity mdel [Kennett and Engdahl, 1991], respectively. The last M l = 2.3 event ccurred ~436 s after the lngperid Rayleigh waves with the velcity f 3.8 km/s. We did nt apply any time shifts in calculating these time differences. Fr the SAF case, we first cmpute the average lcatin f 33 lw-frequency earthquakes (LFEs) ccurred between 22 and 3 s: (!12.2489 ±.4, 35.683 ±.4, and depth 23.6 ± 1.5 km). Next, we cmpute the S-wave travel time based n the 1D velcity in this regin [Peng et al., 29], and shift the 15-3 Hz band-pass-filtered seismgram at statin GHIB backward by 9.25 s t the tremr surce regin. The transverse cmpnent seismgram has been time shifted backward by 9.15 s with the Lve wave velcity f 4.3 km/s, and the radial and vertical cmpnent seismgrams are time shifted backward by 1.3 s with the Rayleigh wave velcity f 3.8 km/s. 5. Theretical assessment f the triggering ptential We mdel the triggering ptential f the Rayleigh and Lves fllwing the prcedure f Hill [28, 21]. Fr the Cs case, we chse the fcal depth t be z ~ 2. km, clse t the fcal depth f the 4 triggered micrearthquakes as listed in the SCSN catalg. We assume an intermediate value f cefficient f frictin µ # =.4, althugh the actual value culd be lwer due t the existence f elevated fluid pressure at shallw depth [Bhattacharyya and Lees, 22]. The fcal mechanism fr the M l = 3.5 Cs earthquake triggered by surface waves frm the Chile earthquake is nt well cnstrained. In the main text we calculated the ptential fr Lve and Rayleigh wave triggering dextral slip n a nrth-striking vertical fault cnsistent with the mapped faults and seismicity patterns in the vicinity f the epicenter [Rquemre, 1982; Feng and Lees, 1998]. In Figure S5 we cmpute the triggering ptential fr fault rientatins based n three fcal mechanisms: ne frm the Suthern Califrnia Seismic Netwrk (W. Yang, persnal cmmunicatin, 21) and tw frm the lcal Cs seismic netwrk perated by the Naval Weapns Center (W.-C. Huang, persnal cmmunicatin, 21). The Lve wave triggering ptential is fur times the Rayleigh wave ptential fr the SCSN slutin (Figure S5a), and cmparable t that fr Rayleigh wave fr the mechanism frm the Naval Weapns Center netwrk (Figure S5b,c). In all three cases, hwever, the $CF stresses crrespnding t wave incidence frm Chile are less than half thse fr the vertical fault in Figure 4a. We d nt use these mechanisms in the main text because f the large uncertainties in the slutins (W. Yang and W.-C. Huang, persnal cmmunicatin, 21). Fr the SAF case, we calculate the triggering ptential n a vertical strike-slip fault (with strike directin f 14 clckwise frm nrth) at the depth f z ~ 25 km with a lw apparent cefficient f frictin µ # =.2, which is likely reasnable fr the surce regin where tremr and LFEs ccur [Peng et al., 29; Thmas et al., 29]. We have als tested ther µ # values and fund that as the frictin cefficient decreases, the triggering ptential curves fr bth the Lve and Rayleigh waves becme mre symmetric abut the 3
-degree incident angle (Figure S6). In all the cases, hwever, the Lve wave has larger triggering ptential than the Rayleigh wave fr incidence n the SAF at depths f ~25 km assuming equal Lve- and Rayleigh-wave displacement amplitudes as the surface (z = km). 6. References that are cited here but nt in the main text: Kennett, B. L. N., E. R. Engdahl (1991), Traveltimes fr glbal earthquake lcatin and phase identificatin, Gephys. J. Intl., 15, 429-465. Matthews, M. V., and P. A. Reasenberg (1988), Statistical methds fr investigating quiescence and ther tempral seismicity patterns, Pure Appl. Gephys., 126, 357-372. Prejean, S. G., D. P. Hill, E. E. Brdsky, S. E. Hugh, M. J. S. Jhnstn, S. D. Malne, D. H. Oppenheimer, A. M. Pitt, and K. B. Richards-Dinger (24), Remtely triggered seismicity n the United States west cast fllwing the Mw 7.9 Denali fault earthquake, Bull. Seisml. Sc. Am., 94, S348 S359, di:1.1785/12461. Rquemre, G. R. (1982), Recnnaissance gelgy and structure f the Cs Range, Califrnia; Naval Weapns Center Tech. Pub. 636, 26 p., 2 plates. Thmas, A. M., R. M. Nadeau and R. Bürgmann (29), Tremr-tide crrelatins and near-lithstatic pre pressure n the deep San Andreas fault, Nature, 462, 148-151. Wiemer, S. and M. Wyss (2), Minimum magnitude f cmpleteness in earthquake catalgues: examples frm Alaska, the western United States, and Japan, Bull. Seisml. Sc. Am., 9, 859 869. Supplementary Figures Figure S1. (a) Magnitudes versus ccurrence times fr all earthquakes ccurred arund the Cs regin since 1997. The red circles mark the events with M l! 3.45 event, and the green dashed lines mark the time when at least 4 M l! 2 events ccurred within 1 hr. The green circle marks the M l = 3.5 event ccurred during the Lve wave f the Chile mainshck. (b) A zm-in plt f (a) arund 29-21. The tw gray dashed lines mark the apprximate start (1/12/21) and end (3/12/21) f the mst recent swarm sequences. (c) Magnitudes versus the time relative t the rigin time f the Chile mainshck (slid vertical line) fr all earthquakes. Figure S2. A recrd sectin f the 2-8 Hz band-pass-filtered vertical seismgrams shwing reginally triggered earthquakes and lcally triggered tremr by the 21 M w 8.8 Chile earthquake in the Parkfield regin, and the bradband three-cmpnent velcity seismgrams recrded at statin PKD. The seismgrams are pltted accrding t the alng-strike distances n the SAF, which are marked n the left hand side tgether with the statin and channel names. The vertical dashed lines mark the riginal times f the fur micrearthquakes ccurred near Cs. The gray and pen vertical arrws mark the predicted arrivals f the Lve (with the phase velcity f 4.3 km/s) and Rayleigh waves (with the phase velcity f 3.8 km/s) at statin PKD. Figure S3. A recrd sectin f 2-16 Hz band-pass-filtered vertical seismgrams shwing the mveut f the M l 3.5 earthquake near Cs. The blue and red lines crrespnd t the seismic recrdings at statins JRC2 and PKD, respectively. The green lines crrespnd t 4
the seismic recrdings at statins arund the Parkfield-Chlame sectin f the San Andreas Fault ther than the PKD statin. Figure S4. A zm-in plt f Figure S1 shwing lcally triggered tremr mdulated with the Rayleigh waves f the 21 M w 8.8 Chile earthquake. Figure S5. Triggering ptential fr 2-secnd Lve and Rayleigh waves in terms f the dynamic Culmb-failure stress, $CF(%) fr Lve (, slid line) and Rayleigh (, dashed line) waves as a functin f incidence angle with respect t fault strike frm the Mw 8.8 Chile earthquake fr incidence n three first-mtin fault-plane (FP) slutins fr the M 3.4 earthquake at at a depth f ~ 2 km beneath Cs. (a) FP slutin frm the Suthern Califrnia Seismic Netwrk, (Wenzheng Yang, persnal cmmunicatin, 21). (b) and (c) alternative FP slutins frm the Cs GPO netwrk (Wei-Chuang Huang, persnal cmmunicatin 21). The peak dynamic stresses at 2 km are based n bserved transverse and vertical displacement amplitudes fr 2-s surface waves f 3.7 and 1.3 cm n statin JRC2. The vertical line indicates the incidence angel f waves frm the Chile earthquake with respect t the respective fault strikes. Rake is assumed t be parallel with the maximum reslved shear stress cmpnent f the reginal stress field n the respective fault planes. Figure S6. Influence f variatins in the cefficient f frictin n the Lve (slid line) and Rayleigh wave (dashed line) triggering ptentials, P(%), as a functin f incidence angle n the San Andreas Fault (SAF) cmputed fr a depth f 25 km as in Figure 4. Here, the dimensinless ptential P(%) is nrmalized by the peak Lve wave dynamic stress $CF(%) = 3.3 kpa. Nte that the peaks and nulls in the Lve wave ptential are shifted t the right (increasing incidence angles) with increasing frictin and Rayleigh wave ptential becmes prgressively mre asymmetric abut zer (strike-parallel) incidence with increasing frictin. The vertical line marks the incidence angle fr waves frm the Chile earthquake. Figure S7. (a) A zm-in plt f Figure 3a shwing the relatinship between the teleseismic bdy waves f the Chile mainshck and the lcal earthquakes near Cs. The rigin times f these tw lcal events are marked by the gray lines. The vertical dashed line mark the predicted S arrival f the Chile earthquake. (b) A zm-in plt f Figure 3a shwing the relatinship between the surface waves f the Chile mainshck and the lcal earthquakes near Cs. Figure S8. A zm-in plt shwing the relatinships amng the Lve waves f the Chile mainshck, the reginal seismic signals frm the M l 3.5 earthquake near Cs, and highfrequency lcal tremr signals. All the traces have been time shifted t reflect their relatinship at the tremr surce regin. 5
(a) Magnitude 5 4 3 2 1 1998 2 22 24 26 28 21 (b) Magnitude 5 4 3 2 1 29.6 29.8 21. 21.2 21.4 21.6 Year (c) Magnitude 5 4 3 2 1-72 -6-48 -36-24 -12 12 24 36 Hurs relative t the Chile mainshck Figure S1 48 6 72
Chile_21227: 35. km, M8.8, 9363.79 km, 142.8 PLO.SHZ. -51.8 km PBW.SHZ. -51.1 km PHR.SHZ. -49.2 km PCC.SHZ..8 km PSA.SHZ. -24.1 km PHSB.EHZ -17. km PHP.SHZ. -3.7 km SCYB.DP1-2.1 km PL11B.EH. km LCCB.DP1 1.7 km RMNB.DP1 2.1 km PKD.HHZ. 3.1 km PMM.EHZ. 4.7 km PPO.SHZ. 5.3 km PVC.EHZ. 5.4 km VCAB.DP1 5.5 km PST.EHZ. 5.9 km B73.EHZ 7. km FROB.DP1 9.2 km VARB.DP1 1.2 km B76.EHZ 1.4 km JCSB.DP1 11.4 km PHOB.EHZ 13.3 km EADB.DP1 14.3 km PWK.EHZ. 15.8 km PHF.EHZ. 16.6 km PHA.EHZ. 2.6 km PCM.EHZ. 22.6 km B78.EHZ 23.7 km GHIB.DP1 24. km B72.EHZ 24.2 km PSR.EHZ. 25.9 km PSN.EHZ. 3.8 km PTA.SHZ. 39.7 km PSC.SHZ. 39.8 km PPB.SHZ. 4.2 km B79.EHZ 42.2 km PBS.EHZ. 42.9 km B91.EHZ 48.1 km PBM.SHZ. 6.5 km PABB.SHZ 63.4 km PBI.SHZ. 73. km PML.SHZ. 87.5 km.2 cm/s PKD HHT PKD HHR PKD HHZ 15 Figure S2 2 Time (s) 25 3
4 35 Epicentral distance (km) 3 25 PKD 2 15 1 5 JRC2 Figure S3 5 Time (s) 1 15
Chile_21227: 35. km, M8.8, 9363.79 km, 142.8 PLO.SHZ. -51.8 km PBW.SHZ. -51.1 km PHR.SHZ. -49.2 km PCC.SHZ..8 km PSA.SHZ. -24.1 km PHSB.EHZ -17. km PHP.SHZ. -3.7 km SCYB.DP1-2.1 km PL11B.EH. km LCCB.DP1 1.7 km RMNB.DP1 2.1 km PKD.HHZ. 3.1 km PMM.EHZ. 4.7 km PPO.SHZ. 5.3 km PVC.EHZ. 5.4 km VCAB.DP1 5.5 km PST.EHZ. 5.9 km B73.EHZ 7. km FROB.DP1 9.2 km VARB.DP1 1.2 km B76.EHZ 1.4 km JCSB.DP1 11.4 km PHOB.EHZ 13.3 km EADB.DP1 14.3 km PWK.EHZ. 15.8 km PHF.EHZ. 16.6 km PHA.EHZ. 2.6 km PCM.EHZ. 22.6 km B78.EHZ 23.7 km GHIB.DP1 24. km B72.EHZ 24.2 km PSR.EHZ. 25.9 km PSN.EHZ. 3.8 km PTA.SHZ. 39.7 km PSC.SHZ. 39.8 km PPB.SHZ. 4.2 km B79.EHZ 42.2 km PBS.EHZ. 42.9 km B91.EHZ 48.1 km PBM.SHZ. 6.5 km PABB.SHZ 63.4 km PBI.SHZ. 73. km PML.SHZ. 87.5 km.2 cm/s PKD HHT PKD HHR PKD HHZ 25 Figure S4 Time (s) 3
a) SCSN CF( kpa CF( kpa CF( kpa 3. 3. 2. 2. 1. 1.. -9 9 b) Cs-1 3. c) Cs-2. -9 45 9 2. 1.. -9 45 9 1.. -9 9 3. 2. 1. Strike ~ 17 Dip ~ 6 Rake ~ -133 Depth ~ 2. km Strike ~ 4 Dip ~ 6 Rake ~ -7 Depth ~ 2. km Strike ~ 184 Dip ~ 36 Rake ~ -121 Depth ~ 2. km ~ -155 135 18 Incidence angle, 135 18 45-135 Incidence angle, ~ 2.4 n ~ -141 =.4 =.4 45-135 ~ 5.2 n 9-9 ~ 75 ~.7 n 9-9 =.4. -9 9. -9 45 9 135 18 45-135 9-9 Figure S5
1. 1. 1. 1. P( ).8.8.6.6.4.4 Chile =..8.8.6.6.4.4 Chile =.2.2.2.. -9 45 9 9 135-135 -9.2.2. -9 45 9. -9 9 135 45-135 9-9 P( ) 1. 1..8.8.6.4 Chile =.4 1. 1..8.6.4 Chile =.6.2.. -9 45 9 9 135-135 -9 Incidence angle,.2.. -9 45 9 9 135-135 -9 Incidence angle, Figure S6
Velcity (cm/s).4.2. (a) M2.9 M2.1 JRC2 S 2!16 Hz Transverse Radial Vertical Velcity (cm/s).6.4.2!.!.2 12 13 14 15 16 17 (b) M3.5 M2.3 JRC2 2!16 Hz Lve Transverse Radial Rayleigh Vertical 2 25 3 Figure S7
Velcity (cm/s).8.6.4.2!.!.2 GHIB 2!16 Hz 15!3 Hz PKD Transverse Radial Vertical M3.5 Lve Tremr 2 21 22 23 24 25 26 Time (s) Figure S8