GEOFÍSICA INTERNACIONAL (2013) 52-4: 385-402 ORIGINAL PAPER Measurements of upper mantle shear wave anisotropy from a permanent network in southern Mexico
Steven A. C. van Benthem, Raúl W. Valenzuela* and Gustavo J. Ponce
Received: November 13, 2012; accepted: December 14, 2012; published on line: September 30, 2013
Resumen Abstract
Se midió la anisotropía para las ondas de cortante 8SSHU PDQWOH VKHDU ZDYH DQLVRWURS\ XQGHU en el manto superior por debajo de estaciones VWDWLRQVLQVRXWKHUQ0H[LFRZDVPHDVXUHGXVLQJ en el sur de México usando fases SKS. Las records of SKS phases. Fast polarization directions direcciones de polarización rápida donde la placa ZKHUHWKH&RFRVSODWHVXEGXFWVVXEKRUL]RQWDOO\ de Cocos se subduce subhorizontalmente están are oriented in the direction of the relative orientadas aproximadamente paralelas con el PRWLRQEHWZHHQWKH&RFRVDQG1RUWK$PHULFDQ movimiento relativo entre las placas de Cocos y plates, and are trench-perpendicular. This América del Norte y además son perpendiculares SDWWHUQLVLQWHUSUHWHGDVVXEVODEHQWUDLQHGÀRZ DODWULQFKHUD3RUORWDQWRVHLQ¿HUHTXHODSODFD and is similar to that observed at the Cascadia VXEGXFLGD DUUDVWUD HO PDQWR TXH VH HQFXHQWUD subduction zone. Earlier studies have pointed SRUGHEDMR\ORKDFHÀXLU HQWUDLQHGÀRZ 8QD out that both regions have in common the young situación similar existe en la zona de subducción age of the subducting lithosphere. Changes in the GH&DVFDGLD(VWXGLRVSUHYLRVKDQVHxDODGRTXH RULHQWDWLRQRIWKHIDVWD[HVDUHREVHUYHGZKHUH estas dos regiones tienen en común la subducción the subducting plates change dip and/or are torn, GH OLWRVIHUD MRYHQ (Q DTXHOORV OXJDUHV GRQGH DQGDUHWKXVLQGLFDWLYHRI'ÀRZDURXQGWKHVODE las placas subducidas muestran un cambio en HGJHV7KH\DUHFRQVLVWHQWZLWKVODEUROOEDFNDV el buzamiento o bien están rotas, se observa SUHYLRXVO\VKRZQE\RWKHUDXWKRUV6RPHVWDWLRQV un cambio en la orientación de los ejes rápidos. ORFDWHG DZD\ IURP WKH SODWH ERXQGDULHV KDYH 'LFKRV FDPELRV VXJLHUHQ TXH VH SURGXFH XQ their fast directions controlled by the absolute ÀXMR WULGLPHQVLRQDO DOUHGHGRU GH ODV RULOODV GH motion of the North American plate. The fast axis las placas subducidas, el cual es consistente con for station ZAIG, located in the Mesa Central, is HOUHWURFHVRGHODSODFDVXEGXFLGD VODEUHWUHDW oriented WNW-ESE and is different from all the RU UROOEDFN FRPR \D VH KDEtD REVHUYDGR HQ other measurements in this study. algunos trabajos anteriores. En algunas de las estaciones instaladas lejos de los límites de placa .H\ZRUGV SKS splitting, upper mantle anisotropy, el movimiento absoluto de la placa de América PDQWOH ÀRZ VXEGXFWLRQ ]RQHV 0LGGOH $PHULFD del Norte controla las direcciones rápidas. El eje Trench, Cocos, Rivera, and North American plates. rápido de una estación ubicada en la Mesa Central se orienta ONO-ESE y es diferente de todas las demás mediciones en este estudio.
Palabras clave: Partición de ondas SKS, anisotropía G. J. Ponce GHO PDQWR VXSHULRUÀXMR GHO PDQWR ]RQDV GH Departamento de Sismología subducción, Fosa Mesoamericana, placas de Instituto de Geofísica Universidad Nacional Autónoma de México Cocos, Rivera y América del Norte. Circuito de la Investigación S/N Cd. Universitaria Del. Coyoacán 04510. México, D.F., México
Also at Departamento de Geofísica Facultad de Ingeniería Universidad Nacional Autónoma de México Mexico D.F., México S. A. C. van Benthem Department of Earth Sciences R. W. Valenzuela* Utrecht University Departamento de Sismología Budapestlaan 4, 3584 CD Utrecht Instituto de Geofísica The Netherlands Universidad Nacional Autónoma de México Circuito de la Investigación S/N Formerly at Departamento de Sismología Cd. Universitaria Instituto de Geofísica Del. Coyoacán Universidad Nacional Autónoma de México 04510. México, D.F., México Mexico D.F., México *Corresponding author: [email protected]
385 S. A. C. van Benthem, R. W. Valenzuela and G. J. Ponce
Introduction ZDV VHDUFKHG LQ WKH SHULRG IURP 0D\ WR HDUO\ -DQXDU\ ,W SURYLGHG D WRWDO RI The use of the teleseismic phase SKS to study HDUWKTXDNHVZLWKLQWKHGLVWDQFHDQGPDJQLWXGH upper mantle anisotropy in both seismically FULWHULD /RFDO SUREOHPV ZLWK WKH HTXLSPHQW TXLHWDQGVHLVPLFDOO\DFWLYHUHJLRQVKDVEHFRPH precluded the simultaneous recording of some D VWDQGDUG WRRO 6LOYHU 6DYDJH HYHQWV E\ DOO VWDWLRQV 7KH GDWD ZHUH VDPSOHG 3DUNDQG/HYLQ 7KHPHWKRGLVEDVHGRQ 20 times per second using Streckeisen STS-2 the intrinsic anisotropy of the seismic velocity broadband, three-component velocity sensors, RIWKHPLQHUDOROLYLQHZKLFKLVRQHRIWKHPDMRU H[FHSWDWVWDWLRQ7(,*ZKHUHD*HRWHFK.6 components of the upper mantle. Olivine crystals ERUHKROH VHLVPRPHWHU ZDV LQVWDOOHG )LJXUH become oriented as they are subjected to strain, VKRZVWKHVWXG\DUHDDQGWKHORFDWLRQRIWKH XVXDOO\FDXVHGE\PDQWOHÀRZ$VWKHSKS phase stations used. travels through an anisotropic medium it becomes VSOLWLHDIDVWDQGDVORZZDYHDUHSURGXFHG The procedure used to measure anisotropy 0HDVXUHPHQWVRIVKHDUZDYHVSOLWWLQJ\LHOGWZR ZDVH[SODLQHGE\6LOYHUDQG&KDQ DQGLV parameters: the fast polarization direction, I, SUHVHQWHGKHUHRQO\EULHÀ\7KHVSOLWSKSZDYHV usually referred to geographic north, and the delay DUH REVHUYHG LQ WKH IDVW DQG VORZ KRUL]RQWDO time, Gt. Laboratory experiments are essential to FRPSRQHQWV ZKLFK DUH RUWKRJRQDO ,Q JHQHUDO HVWDEOLVKWKHUHODWLRQVKLSEHWZHHQWKHGLUHFWLRQ these are different from the radial and transverse RIPDQWOHÀRZDQGWKHIDVWSRODUL]DWLRQGLUHFWLRQ FRPSRQHQWVZKLFKDUHDOVRRUWKRJRQDO,QRUGHU in olivine. In general, these experiments have to obtain I and Gt, a time segment containing the VKRZQ WKDW WKH IDVW D[LV EHFRPHV RULHQWHG LQ SKS arrival, or another P to S conversion at the WKHGLUHFWLRQRIPDQWOHÀRZ -XQJet al., FRUHPDQWOHERXQGDU\ &0% LVVHOHFWHGIURPWKH There is one notable exception, for type-B QRUWKVRXWKDQGHDVWZHVWKRUL]RQWDOFRPSRQHQWV olivine the fast axis aligns perpendicular to the 7KHWZRFRPSRQHQWVDUHURWDWHGE\RQHGHJUHHDW GLUHFWLRQ RI PDQWOH IORZ )URP WKHVH UHVXOWV DWLPHZLWKIUDQJLQJEHWZHHQ DQG )RU it is possible to infer the direction of mantle each value of I, the components are time shifted ÀRZ XVLQJ VHLVPLF PHDVXUHPHQWV &RQQHFWLRQV relative to each other using increments of 0.05 s, EHWZHHQ VHLVPLF DQLVRWURS\ DQG WKH WHFWRQLF ZLWKGt ranging from 0 to 8 s. For each combination
HQYLURQPHQW ZKHWKHU LW EH DFWLYH PDUJLQV RU of I and Gt, the eigenvalues O1 and O2 of the stable continental interiors, have been made in FRYDULDQFH PDWUL[ EHWZHHQ WKH WZR RUWKRJRQDO WKHSDVW HJ6LOYHU 6DYDJH 3DUN components are evaluated. For an anisotropic
DQG/HYLQ 3DUWLFXODUO\VXEGXFWLRQ]RQHV medium, O1 and O2ZLOOEHGLIIHUHQWIURP]HUR,Q have come under intense scrutiny; see Long and WKHSUHVHQFHRIQRLVHWKHGHVLUHGVROXWLRQZLOOEH 6LOYHU IRUDUHYLHZ3UHYLRXVZRUN JLYHQE\WKHPDWUL[ZKLFKLVPRVWQHDUO\VLQJXODU in the case of Mexico has relied mostly on data A grid search is then run through all combinations from temporary deployments and has focused RI I, Gt ZLWKLQWKHVSDFHRISRVVLEOHVROXWLRQVLQ min DURXQG WKH *XOI RI &DOLIRUQLD 2EUHEVNL et al., order to choose the minimum eigenvalue, O2 . min 2006; Obrebski and Castro, 2008; van Benthem 7KH DFWXDO YDOXHV RI I, Gt DVVRFLDWHG WR O2 et al., /RQJ DQGRQVXEGXFWLRQRI characterize the anisotropy because the highest the Rivera and Cocos plates at the Middle America FURVVFRUUHODWLRQ ZLWKLQ WKH JLYHQ VSDFH RFFXUV 7UHQFK 6WXEDLORDQG'DYLVDE IRUWKHIDVWDQGVORZZDYHIRUPV6RPHWLPHVWKH Bernal-Díaz et al., 2008; Soto et al., 5RMR measurement returns a null value and splitting of *DULEDOGL ,QWKHSUHVHQWDUWLFOHGDWDIURP WKHVKHDUZDYHFDQQRWEHGHWHFWHGDVLVWKHFDVH DSHUPDQHQWEURDGEDQGQHWZRUNKDVEHHQXVHG IRUDQ\RIWKHWKUHHIROORZLQJVLWXDWLRQV Gt = WKXV FRYHULQJ D ODUJH DUHD RI WKH FRXQWU\ ZLWK VLQGLFDWHVWKHDEVHQFHRIDQLVRWURS\ I =
ZLGHO\VSDFHGVWDWLRQV7KLVDSSURDFKVWDQGVLQ Ib means that the fast axis, I,LVRULHQWHGZLWKWKH
FRQWUDVWWRWHPSRUDU\GHSOR\PHQWVZKLFKXVXDOO\ back azimuth, Ib. I = Ib PHDQVWKDW FRYHUVPDOOHUDUHDVZLWKDGHQVHDUUD\ the fast axis is perpendicular to the back azimuth.
Data and Procedure )LJXUHDVKRZVWKHSKS arrival on the radial and transverse components at SSN station OXIG Most of the anisotropy measurements in this study IRU WKH HDUWKTXDNH RI 0D\ VRXWK RI used the SKS phase at epicentral distances greater WKH )LML ,VODQGV 7KH K\SRFHQWHU ZDV ORFDWHG DW WKDQ $GGLWLRQDOO\ RWKHU FRUHWUDQVPLWWHG NP GHSWK DQG WKH HSLFHQWUDO GLVWDQFH ZDV phases such as sSKS, SKKS, and PKSZHUH 7DEOHOLVWVWKHHYHQWVXVHGLQWKLVVWXG\ used as available. Clear SKS readings at these WR TXDQWLI\ XSSHU PDQWOH DQLVRWURS\(DFK SKS GLVWDQFHV UHTXLUHG D PLQLPXP PDJQLWXGH RI ZDYHIRUP ZDV FKRVHQ E\ YLVXDO LQVSHFWLRQ DQG 6.0, although occasionally smaller events proved D ¿UVW RUGHUEDQGSDVV %XWWHUZRUWK ¿OWHU ZDV XVHIXO 7KH UHFRUGV ZHUH SURYLGHG E\ 0H[LFR¶V DSSOLHG ,Q HYHU\ FDVH DQ DWWHPSW ZDV PDGH 6HUYLFLR6LVPROyJLFR1DFLRQDO 661 EURDGEDQG to retain the broadest bandpass possible, but QHWZRUN 6LQJKet al., 7KH661GDWDEDVH WKH DFWXDO FRUQHU IUHTXHQFLHV ZHUH GHWHUPLQHG
386 VOLUME 52 NUMBER 4 GEOFÍSICA INTERNACIONAL
Figure 1. Average measurements of I and Gt for stations in southern Mexico. The length of the black bars is proportional to GtDVLQGLFDWHGLQWKHOHJHQG7KHJUD\EDUDW&8,*UHSUHVHQWVDSRRUO\FRQVWUDLQHGPHDVXUHPHQW%ODFNDUURZV LQGLFDWHWKHGLUHFWLRQRIDEVROXWHPRWLRQIRUWKH1RUWK$PHULFDQSODWH*UD\DUURZVVKRZWKHGLUHFWLRQRIWKHUHODWLYH SODWHPRWLRQVIRUHLWKHUWKHVPDOO5LYHUD QRUWKZHVW RUWKHODUJHU&RFRV VRXWKHDVW SODWHUHODWLYHWR1RUWK$PHULFD Velocities are given in cm/yr for both the APM and the RPM. The Middle America Trench is represented by the line ZLWKVPDOOWULDQJOHV7KH7UDQV0H[LFDQ9ROFDQLF%HOW 709% LVLQGLFDWHGE\WKHOLJKWVKDGLQJ6ROLGOLQHVUHSUHVHQW WKHLVRGHSWKFRQWRXUVRIK\SRFHQWHUVZLWKLQWKHVXEGXFWLQJ5LYHUDDQG&RFRVSODWHV/LQHVDUHGDVKHGZKHUHQR K\SRFHQWHUVZHUHDYDLODEOH&RQWRXUVZHVWRI :DUHIURP3DUGRDQG6XiUH] ZKLOHFRQWRXUVHDVWRI : DUHIURP5RGUtJXH]3pUH] ,QDOOFDVHVFRQWRXUVGHHSHUWKDQNPDUHIURP5RGUtJXH]3pUH] $OVR VKRZQDUHWKH5LYHUD7UDQVIRUP)DXOW 57) (DVW3DFL¿F5LVH (35 (O*RUGR*UDEHQ (** 2UR]FR)UDFWXUH=RQH 2)= 2¶*RUPDQ)UDFWXUH=RQH 2*)= DQG7HKXDQWHSHF5LGJH 75 )RXUOHWWHUVWDWLRQFRGHVZHUHVKRUWHQHGE\ dropping the -IG ending common to all of them.
EDVHG RQ WKH IUHTXHQF\ RI WKH QRLVH DIIHFWLQJ HDFK UHFRUG 7KH ORZ IUHTXHQF\ FRUQHU ZDV FKRVHQLQWKHUDQJHIURPWR+] SHULRGV EHWZHHQDQGV ZKLOHWKHKLJKIUHTXHQF\ FRUQHU YDULHG EHWZHHQ DQG +] IURP WRV ZKLFKLVDSSURSULDWHIRUWKHSKDVHV XVHG DQG WKH H[SHFWHG GHOD\ WLPHV :ROIH DQG 6LOYHU $WLPHVHJPHQWLQFOXGLQJRQO\WKH VHOHFWHG SKDVH ZDV FXW IURP WKH VHLVPRJUDP In Figure 2 the time series is 33 s long and is typical of the record lengths used throughout WKLV ZRUN )LJXUH VKRZV WKH FRPELQDWLRQ RI min I, Gt SURGXFLQJWKHPLQLPXPHLJHQYDOXHO2 . The black dot means that the delay time, Gt, is 1.05 s and the fast axis, I,LVHDVWRIQRUWK 7KH¿UVWFRQWRXUDURXQGWKHGRWERXQGVWKH FRQ¿GHQFHLQWHUYDOIRUWKHPHDVXUHPHQW$OOWKH
Figure 2. SKSZDYHIURPWKHHYHQWRI0D\ VRXWKRIWKH)LML,VODQGV 6 : h = 586 km, M UHFRUGHGDWEURDGEDQGVWDWLRQ2D[DFD 2;,* 7KHHSLFHQWUDOGLVWDQFHLV D 7KHUDGLDODQGWUDQVYHUVHYHORFLW\FRPSRQHQWVDUHVKRZQ E 7KHUDGLDODQGWUDQVYHUVH FRPSRQHQWVDUHVKRZQDIWHUFRUUHFWLQJIRUVSOLWWLQJXVLQJWKHYDOXHVWKDWZHUHPHDVXUHG
OCTOBER - DECEMBER 2013 S. A. C. van Benthem, R. W. Valenzuela and G. J. Ponce
Table 1.6RXUFHSDUDPHWHUVRIWKHHDUWKTXDNHVXVHGWRPHDVXUHXSSHUPDQWOHDQLVRWURS\
Date Origin time Lat. Long. Depth Mag. Location Y/M/D H:M:S (°N) (°E) (km)
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okkaido region, Japan )LML,VODQGVUHJLRQ 6RORPRQ,VODQGV 1HZ%ULWDLQUHJLRQ3DSXD1HZ*XLQHD 6RXWKHUQ0LG$WODQWLF5LGJH 1HZ,UHODQGUHJLRQ3DSXD1HZ*XLQHD 9DQXDWX,VODQGV 1RUWKHUQ$OJHULD %RXJDLQYLOOHUHJLRQ3DSXD1HZ*XLQHD 6FRWLD6HD 6RXWK,VODQGRI1HZ=HDODQG +RNNDLGRUHJLRQ-DSDQ +RNNDLGRUHJLRQ-DSDQ +RNNDLGRUHJLRQ-DSDQ 0ROXFFD6HD +RQVKX-DSDQ 6RXWKHDVWRIWKH/R\DOW\,VODQGV 6RXWKHDVWRIWKH/R\DOW\,VODQGV
388 VOLUME 52 NUMBER 4 GEOFÍSICA INTERNACIONAL
Figure 3.&RQWRXUSORWVKRZLQJWKHPLQLPXPYDOXHLQ I, Gt VSDFHDVLQGLFDWHGE\WKHGRW,QWKLVFDVHWKH IDVWSRODUL]DWLRQGLUHFWLRQLV1(DQGWKHGHOD\WLPH LVV7KH¿UVWFRQWRXUDURXQGWKHGRWERXQGVWKH FRQ¿GHQFHUHJLRQ
RWKHUFRQWRXUVDUHPXOWLSOHVRIWKH¿UVWRQHDQG are located at higher “elevations” or mountains. The black dot is located at the bottom of a valley. 7KH FRQ¿GHQFHFRQWRXUZDVFDOFXODWHGXVLQJ HTXDWLRQ LQ 6LOYHU DQG &KDQ DQG taking one degree of freedom for each second of the record containing the SKSDUULYDO 6LOYHU DQG&KDQ .0)LVFKHU%URZQ8QLYHUVLW\ SHUVRQDOFRPPXQLFDWLRQ 7KHXQFHUWDLQWLHV are read directly from the contour plots. In this FDVHWKHPHDVXUHPHQWZLWKLWVV uncertainty is I, Gt V ,QWKHHYHQWWKDW WKH FRQ¿GHQFHUHJLRQLVQRWDSSUR[LPDWHO\ V\PPHWULF WKH ODUJHVW RI WKH WZR SRVVLEOH V YDOXHVLVXVHGLHLQJRLQJIURPWRDV RSSRVHGWRJRLQJIURPWR )LJXUH 7KH FRQWRXUSORWVDUHDOVRXVHIXOWRJDXJHWKHTXDOLW\ RIWKHPHDVXUHPHQWV)RUH[DPSOHDODUJH FRQ¿GHQFHDUHDPHDQVWKDWWKHSDUDPHWHUVDUH poorly constrained. If multiple minima occur then the measurement is not reliable. Another SRVVLELOLW\ LV WKDW WKH FRQWRXU GRHV QRW close, thus indicating a null measurement. All the individual splitting measurements are reported in Table 2.
It is important to run a number of checks in order to make sure that the observation of SKS energy on the transverse component is indeed the result of anisotropy and does not arise from D GLIIHUHQW SURFHVV VXFK DV VFDWWHULQJ 6LOYHU DQG&KDQ 6DYDJH /LNHZLVHWKHVH checks mean that the values determined for I and GtDUHUHOLDEOH $Q³XQVSOLWWLQJ´FRUUHFWLRQLV applied to the radial and transverse records using WKHHVWLPDWHGVSOLWWLQJSDUDPHWHUV,I I, Gt GR describe anisotropy, then the SKSZDYHPXVW disappear, or at least its amplitude is decreased, IURPWKHFRUUHFWHGWUDQVYHUVHFRPSRQHQW )LJXUH E 6LPLODUO\WKHDPSOLWXGHRISKS is increased on the corrected radial component, although this HIIHFWLVVPDOO 7KHSDUWLFOHPRWLRQRIWKHSKS arrival in the transverse component is plotted as function of the radial component. Before the unsplitting correction, particle motion must be DSSUR[LPDWHO\ HOOLSWLFDO )LJXUH D DQG DIWHU FRUUHFWLRQLWEHFRPHVQHDUO\OLQHDU )LJXUHE The N-S and E-W records are rotated through the
Figure 4. $ IXUWKHU FKHFN ZDV D FRPSDULVRQ RI WKH particle motion in the transverse component as a IXQFWLRQ RI WKH UDGLDO D %HIRUH FRUUHFWLQJ IRU WKH DQLVRWURS\WKHSDUWLFOHPRWLRQLVHOOLSWLFDO E $IWHUD correction for the measured anisotropy is applied, the particle motion becomes close to linear.
OCTOBER - DECEMBER 2013 S. A. C. van Benthem, R. W. Valenzuela and G. J. Ponce
Table 2. Individual splitting parameters measured at each station.
Station Date ijb Phase ij ıij įW ıįW Y/M/D (°) (°) (°) (s) (s)
&$,* 6.6 V6.6 6.6 6.6 6.6
&&,* 6.6 6.6 6.6
&-,* 6.6 2000/08/21 148.88 SKS -23 12 1.35 0.55 6.6 6.6 2002/10/04 245.10 SKS -41 - - -
&0,* V6.6 2002/06/22 32.68 SKKS 44 - - - 6.6 2002/10/22 248.86 SKS 52 - - - 6.6 6.6 6.6 6.6
&2,* 6.6 6.6 6.6 6.6 6.6 6.6 6.6
CUIG 2000/05/06 261.25 SKS 38 55 1.20 1.25
+8,* 6.6 6.6 6.6 6.6 6.6 6.6 2000/05/06 261.50 SKS 50 43 1.10 0.60 2000/06/14 244.58 SKS 63 - - - 2002/10/04 248.31 SKS 66 - - - 6.6 6.6
/9,* 6.6 6.6 6.6 6.6
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Table 2. Continuation.
Station Date ijb Phase ij ıij įW ıįW Y/M/D (°) (°) (°) (s) (s)
02,* 6.6 6..6 2002/08/15 288.85 PKS 25 - - - 6.6 6.6 6..6 6.6
2;,* 6.6 6.6 6.6 6.6 6.6 2000/03/28 301.82 SKS 32 - - - 3.6 6.6 6.6 6.6 6.6 6.6 6.6
3/,* 6.6 6.6 6.6 6.6 6.6 V6.6 2000/05/06 261.01 SKS 13 10 1.35 0.45 6.6 6.6 6.6 V6.6 6.6 6.6 6.6 6.6 6.6 6.6 6.6 6.6 6.6 6.6 3.6
31,* 6.6 6.6 6.6 V6.6 2002/06/28 325.63 SKS 26 51 0.50 0.40 6.6
33,* 6.6 3.6 6.6 2003/08/04 156.03 SKS 22 56 1.80 2.55
OCTOBER - DECEMBER 2013 S. A. C. van Benthem, R. W. Valenzuela and G. J. Ponce
Table 2. Continuation.
Station Date ijb Phase ij ıij įW ıįW Y/M/D (°) (°) (°) (s) (s)
SCIG 2002/05/08 251.16 SKS -16 - - - 6.6
7(,* 6.6 6.6 6.6 2002/06/28 331.62 SKS 60 - - - 6.6
73,* 6.6 6.6 6.6
78,* 6.6 6.6 6.6 6.6
<$,* 6.6 6.6 2000/03/28 301.22 SKS 56 14 2.15 0.85 V6.6 6.6 2002/06/28 325.52 SKS 82 - - - 6.6 6.6 6.6 6.6 6.6
=$,* 3.6 6.6 6.6 6.6 6.6 6.6 6.6 6.6 6.6 6.6 6.6
=,,* 6.6 6..6 6.6 6..6
'DWHVRIWKHHDUWKTXDNHVDQGWKHSKDVHVXVHGWRPHDVXUHLQGLYLGXDOVSOLWWLQJSDUDPHWHUVDWHDFKVWD- tion. ijb is the back azimuth. Parameter ijLVWKHIDVWSRODUL]DWLRQGLUHFWLRQ PHDVXUHGHDVWRIQRUWK įW is the delay time, and ıij and ıįWDUHWKHıXQFHUWDLQWLHV 1XOOPHDVXUHPHQWVDUHUHSUHVHQWHGZLWKDQRQ]HURYDOXHIRUijDQGGDVKHVIRUWKHQH[WWKUHHFROXPQV 7KHUHIRUHDQ\RIWKHWKUHHIROORZLQJVLWXDWLRQVFRXOGRFFXUIRUWKHSDUWLFXODUHDUWKTXDNHWRVWDWLRQ path being considered: ij§ijb, ij§ijb RUįW§V7KHDFWXDOYDOXHOLVWHGIRUijLVXVHGDVD SRVVLEOHLQWHUSUHWDWLRQRIWKHGDWDDQGLVLQWHQGHGDVDGGLWLRQDOLQIRUPDWLRQZKHUHDSSURSULDWH
VOLUME 52 NUMBER 4 GEOFÍSICA INTERNACIONAL angle IWRREWDLQWKHVORZDQGIDVWFRPSRQHQWVRI LVZRUWKPHQWLRQLQJWKDWWKHDYHUDJHXQFHUWDLQW\ the SKSSXOVH7KHIDVWDQGVORZZDYHIRUPVPXVW LQ7DEOHLV7KHVPDOOHVWGHOD\WLPHLV have roughly the same shape and the fast SKS s and the largest is 2.25 s. The average delay time is 1.20 s. The smallest 1 uncertainty for PXVWDUULYHEHIRUHWKHVORZSKS by an amount VGt DSSUR[LPDWHO\HTXDOWRWKHPHDVXUHGGt )LJXUH the delay time is 0.20 s, and the largest is 4.20 6KLIWLQJWKHVORZZDYHIRUZDUGE\DWLPHGt s at station CCIG, although this is an extreme value. The second largest 1 uncertainty is 2.60 VKRXOGDOLJQLWZLWKWKHIDVWZDYH VGt V 7KH DYHUDJH XQFHUWDLQW\ KRZHYHULV PXFK ORZHU DW V 7KH VWDWLRQ ZLWK WKH ODUJHVW QXPEHU RI REVHUYDWLRQV LV 3/,* ZLWK QLQHWHHQ UHFRUGV VKRZLQJ FOHDUO\ VSOLW ZDYHIRUPV DQG three null measurements given that it had the largest number of records meeting the selection criteria previously explained. Station CUIG had WKHIHZHVWREVHUYDWLRQVRQO\RQHZLWKDFOHDUO\ VSOLWZDYHIRUPZDVDYDLODEOH6LQFHWKLVVWDWLRQLV ORFDWHGZLWKLQ0H[LFR&LW\FXOWXUDOQRLVHLVFHUWDLQO\ a problem. On average, 6.8 measurements are reported per station. Some sites are noisier WKDQ RWKHUV $GGLWLRQDOO\WKH VWDWLRQV ZHUH QRW installed simultaneously and some of them had EHHQUXQQLQJIRURQO\DVKRUWWLPHZKHQWKHGDWD IRUWKLVVWXG\ZHUHJDWKHUHG(YHQWKRXJKUHFRUGV IURP GLIIHUHQW HDUWKTXDNHV ZHUH DQDO\]HG RQO\RIWKHPJHQHUDWHGREVHUYDEOHP to S CMB Figure 5. Once the fast polarization direction is NQRZQWKH16DQG(:KRUL]RQWDOUHFRUGVDUHURWDWHG conversions at the various stations. The individual through the angle I LQ RUGHU WR REWDLQ WKH VORZ DQG VSOLWWLQJSDUDPHWHUVDUHVKRZQLQ)LJXUH fast components of the SKS SXOVH 7KHVH DUH VKRZQ normalized to the same amplitude. $YHUDJHVSOLWWLQJSDUDPHWHUVZHUHFDOFXODWHG for each station using the stacking method of Wolfe DQG6LOYHU 7KHHUURUVXUIDFHDVVRFLDWHGWR Results the contour plot of each individual measurement is min normalized by its minimum eigenvalue, O2,i . The Table 1 lists the events that provided useful data normalized error surfaces from all measurements IRU WKLV VWXG\ (DUWKTXDNH LQIRUPDWLRQ LQFOXGHV DW WKH VWDWLRQ DUH WKHQ VXPPHG ,Q WKLV ZD\ the date, origin time, hypocenter, magnitude, and the best splitting parameters are given by the the geographic location. A total of 136 individual PLQLPXPYDOXHRIWKHVXPDQGDQHZ VSOLWWLQJ PHDVXUHPHQWV ZHUH PDGH DW WZHQW\ FRQ¿GHQFH LQWHUYDO LV REWDLQHG $V WKH QRLVH different stations and are given in Table 2. The SURSHUWLHVYDU\IRUGLIIHUHQWHDUWKTXDNHVVWDFNLQJ parameters are the fast polarization direction and HYHQWV ZLWK D VLPLODU SRODUL]DWLRQ LPSURYHV WKH WKH GHOD\ WLPH ERWK ZLWK WKHLU FRUUHVSRQGLQJ ¿QDOUHVXOW :ROIHDQG6LOYHU 7KHUHIRUH 1V uncertainties. Also provided are the date WKH VL]H RI WKH FRQ¿GHQFH UHJLRQ IRU WKH of the event, the back azimuth, and the phase averaged values is smaller than for the individual XVHG SKS, SKKS, sSKS, or PKS 7KHREVHUYHG measurements. The averaged splitting parameters orientations of the fast polarization direction DUHSUHVHQWHGLQ7DEOHZKLFKLQFOXGHVWKHIDVW SUDFWLFDOO\ VSDQ WKH ZKROH VSDFH RI SRVVLEOH polarization direction and the delay time, both ZLWK WKHLU FRUUHVSRQGLQJ uncertainties, as VROXWLRQV IURP WR 7KH VPDOOHVW VI V XQFHUWDLQW\IRUWKHIDVWSRODUL]DWLRQGLUHFWLRQLV ZHOODVWKHVWDWLRQV¶FRRUGLQDWHVDQGJHRJUDSKLF ZKLOHWKHODUJHVWLV 7KHODWWHUYDOXHLVTXLWH location, and the total number of clearly split and ODUJHDQGVXFKDQHVWLPDWHQRUPDOO\ZRXOGQRW null measurements. In this case, the smallest be reliable. In general, such large uncertainties 1VI uncertainty for the fast polarization direction DULVHIURPDSRRUVLJQDOWRQRLVHUDWLR:HDOORZHG LVZKLOHWKHODUJHVWLV6WULFWO\VSHDNLQJ ODUJHXQFHUWDLQWLHVIRUDIHZPHDVXUHPHQWVLIWKH WKHUHDUHWKUHHXQFHUWDLQWLHVRIDQG GDWD DW WKH VWDWLRQ ZHUH VFDUFH )XUWKHUPRUH at stations CUIG, HUIG, and TUIG, respectively. :ROIHDQG6LOYHU VKRZHGWKDWLQFHUWDLQ These stations only had one split measurement FDVHV VWDFNLQJ PHDVXUHPHQWV ZLWK ODUJH HUURU each and so the individual measurement is regions can lead to improved estimates of the UHSRUWHGDVWKHDYHUDJH$VVKRZQLQ7DEOHDQG VSOLWWLQJ SDUDPHWHUV $V H[SODLQHG EHORZ RXU in Figure 6, the results at stations HUIG and TUIG PHDVXUHPHQWV IRU HDFK VWDWLRQ ZHUH DYHUDJHG are considered reliable because they also had null using the stacking method proposed by Wolfe and PHDVXUHPHQWVFRQVLVWHQWZLWKWKHRQO\VSOLWYDOXH
6LOYHU DQGFRQVHTXHQWO\KDYHVPDOOHUV reported. The average 1VI uncertainty in Table 3 uncertainties than the individual measurements. It LV7KHVPDOOHVWGHOD\WLPHLVVDQGWKH
OCTOBER - DECEMBER 2013 S. A. C. van Benthem, R. W. Valenzuela and G. J. Ponce
ODUJHVWLV V7KHDYHUDJHGHOD\WLPHLV s. The smallest 1 uncertainty for the delay time VGt LVVZKLOHWKHODUJHVWLVV7KHDYHUDJH XQFHUWDLQW\LVV6WDWLRQ6&,*RQO\KDGWZR null measurements from nonorthogonal back azimuths, so it is interpreted to have splitting EHORZ WKH WKUHVKROG RI WKH GDWD 7KH DYHUDJHG VSOLWWLQJSDUDPHWHUVDUHVKRZQLQ)LJXUH
Discussion
)LJXUHVKRZVWKHDYHUDJHGVSOLWWLQJSDUDPHWHUV at each station and the tectonic environment of the VWXG\DUHD7KHEHVWFRYHUDJHLVIRXQGEHWZHHQ DQG : ORQJLWXGH 7KH VWXG\ DUHD ZDV divided into several regions based on variations LQWKHWHFWRQLFHQYLURQPHQWDVZHOODVGLIIHUHQFHV in the measured anisotropy parameters. They are GLVFXVVHGEHORZ
Mexican Subduction Zone Between 96 and 101°W Longitude
,Q WKLV UHJLRQ )LJXUH VXEGXFWLRQ RI WKH Figure 6. D ,QGLYLGXDO PHDVXUHPHQWV RI I and Cocos plate under the North American plate at Gt at stations in southern Mexico. Symbols are as WKH0LGGOH$PHULFD7UHQFK 0$7 LVERXQGHGE\ in Figure 1. Additionally, null measurements from WKH ODQGZDUG H[WHQVLRQ RI WKH 2UR]FR )UDFWXUH WZR QRQRUWKRJRQDO EDFN D]LPXWKV DUH UHSUHVHQWHG =RQH 2)= DQGWKH7HKXDQWHSHF5LGJH 75 7KH E\ WKH ZKLWH EDUV IRUPLQJ WZR FURVVHV IRU VWDWLRQ UHODWLYHYHORFLW\EHWZHHQWKH&RFRVDQG1RUWK 6&,*ZKLFKLVLQWHUSUHWHGWRKDYHVSOLWWLQJEHORZWKH $PHULFDQSODWHVLQFUHDVHVIURPFP\ULQWKH threshold of the data. Other null measurements are QRUWKZHVWWRFP\ULQWKHVRXWKHDVW PRGHO VKRZQ RQO\ IRU VWDWLRQV ZLWK D IHZ ZHOO FRQVWUDLQHG PVEL in DeMets et al. ZKLOH WKH &RFRV splitting measurements. The fast directions for these QXOOPHDVXUHPHQWVZHUHFKRVHQWREHFRQVLVWHQWZLWK SODWH PRYHV LQ WKH GLUHFWLRQ a1( 7KH IDVW the split observations. The box represents the area polarization direction for many stations in this VKRZQDVDQLQVHWLQ E region is approximately the same as the relative
VOLUME 52 NUMBER 4 GEOFÍSICA INTERNACIONAL
Table 3. Averaged splitting parameters measured at each station.
Station Lat. Long. ij ıij įW ıįW N Location (°N) (°E) (°) (°) (s) (s) Split Null
&$,* (O&D\DFR*UR &&,* &RPLWiQ&KLV &-,* &KDPHOD-DO &0,* &RO&XDXKWpPRF2D[ &2,* &ROLPD&RO CUIGa &LXGDG8QLYHUVLWDULD') +8,* +XDWXOFR2D[ /9,* /DJXQD9HUGH9HU 02,* 0RUHOLD0LFK 2;,* 2D[DFD2D[ 3/,* 3ODWDQLOOR*UR 31,* 3LQRWHSD1DFLRQDO2D[ 33,* 3RSRFDWpSHWO7ODPDFDV3XH 6&,* 6DEDQFX\&DPS 7(,* 7HSLFK45 73,* 7HKXDFiQ3XH 78,* 7X]DQGpSHWO9HU <$,* Parameter ijLVWKHIDVWSRODUL]DWLRQGLUHFWLRQ PHDVXUHGHDVWRIQRUWK įW is the delay time, and ıij and ıįWDUHWKHıXQFHUWDLQWLHV$YHUDJHVZHUHFDOFXODWHGXVLQJWKHVWDFNLQJPHWKRGRI:ROIHDQG6LOYHU 1LVWKHQXPEHURIPHDVXUHPHQWVDYDLODEOHIRUDQDO\VLV7KHDEEUHYLDWLRQVDIWHUWKHORFDWLRQV stand for the names of Mexican states. 7KHVWDWLRQZLWKDYDOXHIRUijDQGGDVKHVIRUWKHQH[WWKUHHFROXPQVGRHVQRWH[KLELWVSOLWWLQJIURP DWOHDVWWZRGLIIHUHQWQRQRUWKRJRQDOEDFND]LPXWKV ijb DQGLVLQWHUSUHWHGDVKDYLQJVSOLWWLQJEHORZWKH threshold of the data. aData at station CUIG are noisy, a measurement is provided but it is poorly constrained. SODWHPRWLRQ 530 EHWZHHQWKH&RFRVDQG1RUWK ZLWK VXEVWDQWLDO VXEVODE VSOLWWLQJ DURXQG WKH American plates, and approximately perpendicular ZRUOGKRZHYHUVKRZWUHQFKSDUDOOHOIZKLFKLV to the MAT. Most of these stations overlie the LQWHUSUHWHGDV'UHWXUQÀRZLQGXFHGE\WUHQFK &RFRV SODWH ZKHUH LW VXEGXFWV VXEKRUL]RQWDOO\ PLJUDWLRQ /RQJDQG6LOYHU 3HUKDSVWKH 3DUGRDQG6XiUH] 3pUH]&DPSRVet al., most notable exception is the Cascadia subduction +XVNHUDQG'DYLV DQGOLNHO\UHÀHFW ]RQHZKHUHDWUHQFKSHUSHQGLFXODUSDWWHUQKDV WKHÀRZGLUHFWLRQRIWKHXSSHUPDQWOHEHORZWKH EHHQIRXQG &XUULHet al., 2004; Long and Silver, slab. Given the conditions expected to prevail 7UHQFKSHUSHQGLFXODUIDVWGLUHFWLRQVKDYH EHQHDWKWKHVODEVXFKDVORZVWUHVVORZZDWHU also been reported by other studies in Mexico FRQWHQW DQG UHODWLYHO\ KLJK WHPSHUDWXUH ZH using SKS phases. Soto et al. IRXQG DVVXPHWKDWWKHODWWLFHSUHIHUUHGRULHQWDWLRQ /32 trench-perpendicular I ZKHUH WKH 5LYHUD SODWH RI ROLYLQH LV RI W\SH$ -XQJet al., 2006; Long VXEGXFWV EHQHDWK WKH -DOLVFR EORFN WKH UHJLRQ DQG6LOYHU 7KHUHIRUHWKHIDVWDQLVRWURS\ EHWZHHQVWDWLRQV&-,*DQG&2,*LQ)LJXUHVDQG D[LV DQG PDQWOH ÀRZ DUH RULHQWHG LQ WKH VDPH RIWKLVVWXG\ 'DWDIURPWKH0HVRDPHULFDQ GLUHFWLRQ ,W DOVR IROORZV WKDW ÀRZ LV HQWUDLQHG 6XEGXFWLRQ([SHULPHQW 0$6( DUUD\DOLQHRI under the subhorizontal Cocos slab. Most regions seismometers running north and starting just east OCTOBER - DECEMBER 2013 S. A. C. van Benthem, R. W. Valenzuela and G. J. Ponce Figure 7.0DSVKRZLQJ WKH UHODWLRQVKLS EHWZHHQ measurements made at the 0$56 VWDWLRQV 6RWR et al., DQG WKRVH IURP 661 stations in the present study. The stations enclosed by dashed-line polygons are discussed in the main text. 0$56 VWDWLRQ FRGHV ZHUH shortened by dropping the 0$SUH¿[ RI&$,* )LJXUH VKRZDQDYHUDJHIRI1( .DQMRUVNL %RWK&DVFDGLDDQGWKH0LGGOH IRUVWDWLRQVLQWKHIRUHDUFZKLOHIDYHUDJHV1: America Trench represent the most extreme IRUVWDWLRQVLQWKHEDFNDUF WKHYROFDQLFDUFLV examples of the subduction of young lithosphere. located at the southern edge of the Trans-Mexican &RQVHTXHQWO\/RQJDQG6LOYHU K\SRWKHVL]H 9ROFDQLF%HOW 6WXEDLORDQG'DYLVD that the amount of strain is not yet high enough E 5RMR*DULEDOGL )DUWKHU HDVW for the shear mechanism to reach steady state PHDVXUHPHQWVIURPWKH9HUDFUX]2D[DFD 9(2; and so decoupling has not yet occurred. Farther SUR¿OHDOLQHRIVHLVPRPHWHUVUXQQLQJQRUWK east along the MAT, in Nicaragua and Costa VRXWK DFURVV WKH ,VWKPXV RI 7HKXDQWHSHF MXVW Rica, Abt et al. LQWHUSUHWWKHLUDQLVRWURS\ ZHVWRI&0,*LQ)LJXUH UHYHDOIDVWGLUHFWLRQV measurements using teleseismic SKS and local FRQVLVWHQWZLWKQHDUE\661VWDWLRQV78,*&0,* SSKDVHVDVPDQWOHÀRZSDUDOOHOWRWKHWUHQFK DQG+8,* %HUQDO'tD]et al., both beneath the subducted Cocos slab and in WKH PDQWOH ZHGJH 7KH DJH RI WKH OLWKRVSKHUH ,Q RUGHU WR DOORZ IRU WKH ' UHWXUQ IORZ VXEGXFWLQJDWWKH0$7WKHUHLVEHWZHHQDQG induced by trench migration observed at most 0\U $EWet al., VXEGXFWLRQ]RQHV/RQJDQG6LOYHU propose the existence of a thin decoupling zone The trench-perpendicular fast directions found EHWZHHQ WKH GRZQJRLQJ VODE DQG WKH VXEVODE beneath the Cocos plate in Guerrero and Oaxaca in mantle. The decoupling zone may result from the present study stand in contrast to the trench- the entrainment of a thin layer of buoyant parallel fast directions observed by Stubailo et al. DVWKHQRVSKHUHZKLFKLVSURGXFHGWKURXJKVKHDU XVLQJ5D\OHLJKZDYHV7KH\REWDLQHGSKDVH VWUDLQDQGVKHDUKHDWLQJ 3KLSSV0RUJDQet al., YHORFLW\PDSVIRUZDYHVRIGLIIHUHQWSHULRGVLQWKH /RQJDQG6LOYHU ,QWKLVVWXG\ range from 16 to 100 s. Since phase velocities KRZHYHUWKHDSSUR[LPDWHO\WUHQFKSHUSHQGLFXODU provide a depth-integrated image of the upper fast directions observed beneath the Cocos plate mantle, they inverted the phase velocity maps and in the Mexican states of Guerrero and Oaxaca, GHWHUPLQHGD'PRGHOIRUVKHDUZDYHYHORFLW\ MXVWOLNHREVHUYDWLRQVDW&DVFDGLD /RQJDQG6LOYHU and anisotropy. Their model is parameterized VXJJHVWWKDWWKHWKLQGHFRXSOLQJ]RQHGRHV into three layers: the continental crust, mantle QRWH[LVW7KHUHIRUHWKHGRZQJRLQJVODEDQGVXE OLWKRVSKHUHDQGDVWKHQRVSKHUHGRZQWRDGHSWKRI VODEPDQWOHDUHFRXSOHGDWGHSWKZKLFKUHVXOWVLQ 200 km. Both the lithospheric and asthenospheric HQWUDLQHGÀRZ7KHDJHRIWKHVXEGXFWLQJRFHDQLF OD\HUVVKRZWUHQFKSDUDOOHOIDVWGLUHFWLRQVLQWKH Cocos plate at the MAT offshore Guerrero and IRUHDUF ZLWK YDOXHV UDQJLQJ EHWZHHQ DQG 2D[DFDLVDSSUR[LPDWHO\EHWZHHQDQG0\U \LQDQLVRWURS\)DUWKHUZHVWFRLQFLGHQWZLWK VOLUME 52 NUMBER 4 GEOFÍSICA INTERNACIONAL WKH ODQGZDUG SURMHFWLRQ RI WKH 2UR]FR )UDFWXUH 6LOYHU &DOFXODWLRQZLWKWKHHTXDWLRQJLYHQ Zone, the fast axes become trench-perpendicular. in the caption to their Figure 2 predicts Gt Stubailo et al. LQWHUSUHWWKHVHUHVXOWVDV s for the MAT in Guerrero and Oaxaca. WRURLGDO ÀRZ LQWR WKH PDQWOH ZHGJH FRPLQJ LQ IURPWKHWZRHGJHVWKDWOLPLWWKHVHJPHQWRIWKH Mexican Subduction Zone East of 96°W &RFRVVODEORFDWHGEHWZHHQWKH2)=WRWKHHDVW Longitude DQG WKH 5LYHUD VODE WR WKH ZHVW ,I WKH WUHQFK parallel fast directions observed by Stubailo et al. $VOLJKW a FORFNZLVHURWDWLRQRIWKHDQLVRWURS\ LQ*XHUUHURDQG2D[DFDZKHUHWKHVODE fast axes is observed for stations HUIG, CMIG, is subhorizontal, are interpreted as 3-D return and TUIG relative to nearby stations PNIG, OXIG, ÀRZSURGXFHGE\WUHQFKUROOEDFNWKLVZRXOGEH DQG73,*ZKLFKDUHORFDWHGWRWKHZHVW )LJXUH LQFRQVLVWHQWZLWKRXULQWHUSUHWDWLRQRIHQWUDLQHG 7KHVH GLUHFWLRQV DUH DOVR FRQVLVWHQW ZLWK ÀRZ7KHGHOD\WLPHVPHDVXUHGIURPSKS splitting preliminary SKSUHVXOWVIURPWKH169(2;SUR¿OH LQWKLVUHJLRQUDQJHIURPWR V8VLQJWKH GHSOR\HGMXVWZHVWRI&0,* %HUQDO'tD]et al., FRPPRQDVVXPSWLRQWKDWVKHDUZDYHDQLVRWURS\ 8VLQJ 9(2; GDWD DQG ORFDO HDUWKTXDNHV LV 6LOYHU DQG &KDQ 6DYDJH ZLWKLQ WKH &RFRV VODE /HyQ6RWR et al. a delay time of 1 s corresponds to an effective found that the fast anisotropy axis is trench- thickness for the anisotropic layer of 115 km. SHUSHQGLFXODU LQ WKH PDQWOH ZHGJH 6HYHUDO Therefore, the observed delay times translate changes in subduction zone morphology take LQWRDQLVRWURSLFOD\HUWKLFNQHVVHVEHWZHHQ SODFHLQWKLVDUHD 7KH0LGGOH$PHULFD7UHQFK and 220 km. It is possible that the anisotropic PDNHVDEHQGVRXWKHDVWRIVWDWLRQ+8,* 7KH regions sampled by the Rayleigh and SKSZDYHV Tehuantepec Ridge intersects the MAT. The TR has are different given that their paths are different. long been recognized as a sharp contrast in the The sensitivity kernel for a period of 100 s peaks properties of the Cocos plate. The oceanic crust of DW D GHSWK RI NP 6WXEDLOR et al., the Guatemala Basin in the southeast is older and DQG VR 5D\OHLJK ZDYHV VDPSOH WKH OLWKRVSKHUH GHHSHUWKDQWKHUHJLRQQRUWKZHVWRIWKH75 VHH preferentially. Thus, if the SKS anisotropic layer is Manea et al. IRUDUHYLHZ 7KHFRDVWOLQH SODFHGXQGHUWKH5D\OHLJKZDYHDQLVRWURSLFOD\HU LV IDUWKHU DZD\ IURP WKH 0$7 WKDQ LQ WKH DUHD LWZRXOGVWDUWDWDGHSWKRIaNP$GPLWWHGO\ ORFDWHGWRWKHZHVWZKLFKLPSOLHVWKHH[LVWHQFH WKHYHUWLFDOUHVROXWLRQIRUVKHDUZDYHVSOLWWLQJLV RIDEURDGFRQWLQHQWDOVKHOI 7KHVXEGXFWLQJ poor, but it is generally agreed that anisotropy Cocos plate is not subhorizontal anymore and UHVLGHVLQWKHVKDOORZSDUWRIWKHXSSHUPDQWOH GLSV DW DQ DQJOH RI a 3DUGR DQG 6XiUH] 6LOYHU 6DYDJH ,IWKHDQLVRWURSLF 5RGUtJXH]3pUH] 0HOJDUDQG3pUH] layer in the present study is placed too deep, Campos, 2011; Kim et al., )XUWKHUPRUH KRZHYHULWZLOOQRORQJHUEHLQFRQWDFWZLWKWKH recent results suggest the possibility of a tear in VXEGXFWHGVODEDQGFRQVHTXHQWO\HQWUDLQHGÀRZ WKHVODEVRPHZKHUHLQWKHUHJLRQZKHUHLWFKDQJHV cannot be used to explain the observed anisotropy. IURP VXEKRUL]RQWDO WR D GLS RI a 3pUH] $SRVVLEOHZD\WRUHVROYHWKLVGLVFUHSDQF\PD\EH Campos et al., 7KH URWDWLRQ RI WKH SKS WRFDUU\RXWDMRLQWLQYHUVLRQRIWKHVXUIDFHZDYH fast axes may be caused by the change in dip of and SKS data. Anisotropy in this region has also the Cocos slab. Furthermore, this orientation may been reported by other researchers. Song and be transitional to the orientation of the fast axis .LP E IRXQGWKDWWKHXSSHURFHDQLFFUXVWRI DWVWDWLRQ7(,*LQWKH OCTOBER - DECEMBER 2013 TZHUW\XLRSDVGIJKM O E []4:(57 S. A. C. van Benthem, R. W. Valenzuela and G. J. Ponce ERXQGDU\EHWZHHQWKH&RFRVDQG5LYHUDSODWHVLV DUHVRPHZKDWGLIIHUHQWIURPWKHSKS data in the FRQWLQXHGE\WKHEDWK\PHWULFIHDWXUHNQRZQDV(O present study given that the fast axis at ZIIG is *RUGR*UDEHQZKLFKIXUWKHUH[WHQGVRQODQGDV RULHQWHGVOLJKWO\ZHVWRIQRUWK the N-S trending Colima rift. Additionally, Colima YROFDQR LV ORFDWHG ZLWKLQ WKH ULIW 7KH 5LYHUD Fast Axes Oriented With the Absolute Plate plate subducts more steeply than the adjacent Motion of North America &RFRVVODE 3DUGRDQG6XiUH] DQGDJDS EHWZHHQWKHWZRZDVLPDJHGWRPRJUDSKLFDOO\DW The absolute motion of the North American plate GHSWKVJUHDWHUWKDQNP VOLUME 52 NUMBER 4 GEOFÍSICA INTERNACIONAL %DVLQ DQG 5DQJH IDUWKHU QRUWK 6HGORFN et al., 7UHQFK7KLVREVHUYDWLRQLVFRQVLVWHQWZLWKVXEVODE 7KH0HVD&HQWUDOZDVXQGHUFRPSUHVVLRQ HQWUDLQHGÀRZLQDUHJLPHRIW\SH$ROLYLQH7KH GXULQJWKH/DUDPLGHRURJHQ\ZKLFKFUHDWHGWKH situation is similar to that reported in Cascadia, 602UWRWKHHDVW 1LHWR6DPDQLHJRet al., except that the slab is not subhorizontal. At most ,W ZDV VXEVHTXHQWO\ VXEMHFWHG WR H[WHQVLRQ VXEGXFWLRQ ]RQHV DURXQG WKH ZRUOG KRZHYHU likely related to the Basin and Range. The main ILVWUHQFKSDUDOOHOZKLFKPD\EHFDXVHGE\ episodes of extension occurred during the Eocene, a thin asthenospheric layer that decouples the oriented NE-SW; and during the Oligocene, GRZQJRLQJ VODE IURP WKH VXEVODE PDQWOH DQG ZLWK SUHGRPLQDQW H[WHQVLRQ RI a RULHQWHG DOORZVÀRZWREHGULYHQE\WUHQFKPLJUDWLRQ*LYHQ (:SOXVaH[WHQVLRQRULHQWHG16 1LHWR the young age of the lithosphere being subducted, Samaniego et al., 1HLWKHU WKH HSLVRGHV both in Mexico and in Cascadia, the decoupling of compression nor extension can explain the layer may be absent. The fast directions observed orientation of I. The fast axis is predicted to align around the edges of the Rivera slab, and near the parallel to transpressional structures, and parallel projection of the Orozco Fracture Zone under the WRWKHH[WHQVLRQGLUHFWLRQ 6LOYHU 7KH$30 FRQWLQHQWDUHFRQVLVWHQWZLWKPDQWOHÀRZDURXQG of North America cannot explain the orientation WKHHGJHVRIWKHWRUQVODEV)XUWKHUPRUHWKLVÀRZ of I HLWKHU )LJXUH ,W VKRXOG EH PHQWLRQHG is driven by slab rollback. Stations located east that in the Western Mexican Basin and Range the RIWKHVXEKRUL]RQWDOVODEZKHUHWKHVXEGXFWLQJ direction of ILVFRQVLVWHQWZLWKERWKWKHFXUUHQW SODWH VWDUWV WR LQFUHDVH LWV GLS VKRZ D VOLJKW $30RI1RUWK$PHULFDDQGZLWKH[WHQVLRQGXULQJ FORFNZLVH URWDWLRQ RI WKHLU IDVW D[HV UHODWLYH WR WKH0LRFHQH 2EUHEVNLet al., 2006; van Benthem QHDUE\VWDWLRQVORFDWHGWRWKHZHVW$IHZVWDWLRQV et al., &RPSDULVRQ ZLWK WKH 5D\OHLJK LQ RU QHDU WKH 709% DQG DZD\ IURP WKH SODWH ZDYHPRGHORI6WXEDLORet al. VKRZVWKDW ERXQGDULHVVKRZIDVWGLUHFWLRQVFRQVLVWHQWZLWK =$,*LVORFDWHGLQWKHLUORZUHVROXWLRQDUHD,WLV DVWKHQRVSKHULFÀRZGULYHQE\WKHDEVROXWHPRWLRQ nonetheless interesting that the fast direction in of the North American plate. The same is also true WKHLUPRGHODWSHULRGVIURPWRVDJUHHVZLWK for one station in the Yucatán peninsula. Finally, the fast direction observed in the present study. the fast polarization direction for a single station ZLWKLQWKH0HVD&HQWUDOLVRULHQWHG:1:(6(DQG Another possibility for explaining the anisotropy it is different from measurements at any of the at ZAIG is to look at the evolution of the other stations. Previous episodes of compression subducted slab in the mantle. The PZDYHYHORFLW\ DQG VXEVHTXHQW H[WHQVLRQ LQ WKH UHJLRQ IDLO WR tomographic model of Yang et al. FDQ account for this direction. The fast direction is LPDJHWKHVXEGXFWHG5LYHUDSODWHGRZQWRGHSWKV DOVRLQFRQVLVWHQWZLWKWKH$30RI1RUWK$PHULFD of ~350 km. They speculate that the leading edge of the slab may have detached from the upper Acknowledgments portion and might be found at greater depths. /LNHZLVH WKH DGMDFHQW &RFRV VODE WR WKH HDVW We are thankful to Karen Fischer for providing FDQEHIROORZHGDVDFRQWLQXRXVIHDWXUHIURPWKH the computer code to measure the splitting VXUIDFHGRZQWRDGHSWKRIaNPZKLFKLVWKH parameters; Fernando Terán for the computer ORZHUOLPLWRIWKHLUPRGHO7KHS ZDYH YHORFLW\ code to check the measurements; Manuel tomographic model of van der Lee and Nolet 9HOiVTXH] IRU FRPSXWHU VXSSRUW +DQQHNH D E LPDJHVWKHEURNHQDQGVXEGXFWHG Paulssen, Karen Fischer, Arie van der Berg, Renate )DUDOORQSODWHLQWKHXSSHUPDQWOH7KHVODEVKRZV den Hartog, Vlad Manea, Rob Govers, and Rob URXJKO\D:1:(6(WUHQGZKLFKDJUHHVZLWKWKH Clayton for discussions and suggestions; and fast anisotropy axis at ZAIG. The southernmost -HUHPtDV%DVXUWRDQG$QWRQLR/R]DGDIRUKHOSZLWK extent of the Farallon slab at a depth of ~350 km data analysis in the early stages of this project. is located underneath station ZAIG. Being at such The suggestions made by Raúl Castro, Rob DJUHDWGHSWKLWLVXQFOHDUKRZHYHUKRZWKHVODE Clayton, and Gerardo León greatly enriched the might control the anisotropy observed at ZAIG. On PDQXVFULSW 7KH RSHUDWLRQ DQG GDWD DFTXLVLWLRQ the other hand, the PZDYHYHORFLW\WRPRJUDSK\ IURP 0H[LFR¶V 6HUYLFLR 6LVPROyJLFR 1DFLRQDO RI*RUEDWRYDQG)XNDR VKRZVWKH)DUDOORQ EURDGEDQG QHWZRUN KDV EHHQ SRVVLEOH GXH WR slab under ZAIG at a depth of ~400 km, but it WKHZRUNE\-DYLHU3DFKHFR&DUORV9DOGpV6KUL trends roughly E-W. Krishna Singh, Arturo Cárdenas, José Luis Cruz, Jorge Estrada, Jesús Pérez, and José Antonio Conclusions 6DQWLDJR2QHRIXV 6$&Y% UHFHLYHGSDUWLDO funding from the Molengraaff Fonds and Trajectum The fast anisotropy polarization directions for %HXUVIRUWUDYHOWRDQGOLYLQJH[SHQVHVZKLOHLQ VWDWLRQVORFDWHGZKHUHWKH&RFRVSODWHVXEGXFWV 0H[LFR7KLVZRUNZDVIXQGHGE\0H[LFR¶V&RQVHMR subhorizontally, beneath Guerrero and Oaxaca Nacional de Ciencia y Tecnología through grant VWDWHV DUH RULHQWHG ZLWK WKH UHODWLYH PRWLRQ 77KHFRQWRXUSORWVDQGPDSVLQWKLVVWXG\ EHWZHHQ WKH &RFRV DQG 1RUWK $PHULFDQ SODWHV ZHUHPDGHXVLQJWKH*HQHULF0DSSLQJ7RROV *07 and are perpendicular to the Middle America SDFNDJH :HVVHODQG6PLWK OCTOBER - DECEMBER 2013 S. A. C. van Benthem, R. W. Valenzuela and G. J. Ponce Bibliography Earth Planet. Int., GRL M pepi.2004.08.022. 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