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Measurements of Upper Mantle Shear Wave Anisotropy from a Permanent Network in Southern Mexico

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Measurements of Upper Mantle Shear Wave Anisotropy from a Permanent Network in Southern Mexico 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\ZRUGVSKS 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 DWLPHZLWKIUDQJLQJEHWZHHQDQG)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 HJ6LOYHU6DYDJH3DUN 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 = IbPHDQVWKDW 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
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