Nov 5, 1993 - To the NW of the Colima volcano, Lange and. Carmichael [1991] and Wallace .... de Chile, Casilla 2777, Santiago, Chile. (Received: August 11 ...
GEOPHYSICAL RESEARCH LETTERS, VOL. 20, NO. 21, PAGES 2391-2394, NOVEMBER 5, 1993
STEEP SUBDUCTION
GEOMETRY
OF THE RIVERA PLATE BENEATH THE JALISCO BLOCK IN WESTERN
Mario
MEXICO
Pardo •.2 and Gerardo Sufirez •
reported [Dean and Drake, 1978; LeFevre and McNally, 1985]. In orderto studythe subductionregimeof this small plate, Eissler and McNally [1984] relocatedearthquakeson and near the boundaries of the Rivera plate. They were unable,however,to discernthe geometryof the downgoing
Abstract. The morphologyof the Rivera plate subducted beneath the Jalisco block in western Mexico
is determined
from accurately located hypocentersof locally recorded microearthquakes,and from earthquakeswith magnitude mb>4.5recordedat teleseismic distances. The hypocenters of theselatterearthquakes are relocated,andfor five of themthe focal depth is constrainedby the inversionof long-period body waves. The Wadati-Benioff zone inferred from these data indicates a steep subductionof the Rivera plate that resemblesthe geometry of subductionof the Cocos plate beneathCentralAmerica.It is, however,very differentfrom the shallower
and almost
subhorizontal
subduction
slab.
The pole of relative motion for the last 3.0 m.y. of the Rivera plate with respectto North America is locatedvery close to the Rivera plate (22.8øN, 109.4øW, 1.883 ø/m.y.) [DeMets and Stein, 1990] (Figurel). The rotation vector of DeMets and Stein [1990] predictsa relative velocity of 2.15 crn/yroffshoreColima (18.6øN, 105øW).The plateconsumed at the trenchis of late Miocene age (9 m.y.) and is therefore one of the youngestlithospheresbeing subducted[Klitgord and Mammerickx, 1982]. The slow rate of convergenceand the high temperaturesof this young oceanicplate may be responsible for the low rateof observedseismicity. The purposeof this studyis to determinethe geometryof the subducted Riveraplatefrom reliablehypocentrallocations of microearthquakes and from the relocatedhypocentersand focal mechanismsof earthquakesrecorded at teleseismic distances.Understandingthe geometryof subductionof this young plate is of fundamental importance to unravel the complextectonicstructureof thisregion.
of the
Cocosplate observedin southernMexico, southeastof this region. The Rivera plate is comparableto the Juande Fuca plate in terms of the small areal extent,young seafloorage, low relative velocity, and low teleseismicactivity in the subductionzone. This studyshowsthat the dip of both the Juan de Fuca and Rivera plates are similar once they are decoupled from the overriding continental crust. The downgoingRivera plate initially startswith a dip of-10 ø down to a depth of 20 km and then increasesgraduallyto a constantdip of-50 ø below a depthof 40 km. Intermediatedepth seismicity is low in this zone associatedwith the subductionof the slow (2 cm/yr) and young(9 m.y.) Rivera plate. The maximumdepthextentof earthquakesobservedin the Rivera subduction zone is about 130 km. The andesitic,
calc-alkalineColima volcanoappearsto be directlyrelatedto the subductionof the Riveraplate.To the NW of thisvolcano, the observedQuaternaryvolcanismin theJaliscoblock,which is parallel to the trench, may also be explained by the subductionof the Riveraplate. Introduction
The shapeof the Wadati-Benioff zone of the Rivera plate beneathwesternMexico is poorly known. This is due to the low rateof teleseismically recordedearthquakeactivityandto the poor local seismiccoverage.However,the Jalisco-Rivera plateboundaryis far from being aseismic,for it was the site, in 1932, of the largestinstrumentallyrecordedearthquakein Mexico (Mw=8.2) [Singhet a1.,1985;EisslerandMcNally, 1984].
Because of the lack of teleseismicallylocated seismic activity, Nixon [1982] suggestedthat the Rivera plate subductsaseismically.Few focal mechanismsof shallow earthquakesand only one intermediate-depth earthquake associated apparently to theRiveraplatesubduction havebeen • Instituto de Geoffsica, UNAM, M6xico.
2Departamentode Geoffsica,Universidadde Chile.
Copyright1993 by the AmericanGeophysicalUnion. Paper number 93GL02794 0094-8534/93/93 GL-02794503.00
Data Analysis
The data used to infer the morphologyof the subducted Rivera plate beneath the Jalisco block are accurately determinedhypocentersobtainedfrom severaldatasets:1) Local microearthquakes recorded by five temporary seismological stationsdeployedfor onemonthin 1989 around the Colima graben.These data are complementedwith the permanent seismic stationsoperatedby the University of Colima, Mexico andby the NationalSeismologicalService, UNAM (Figure 2); 2) Twenty teleseismicevents,relocated using the methodof Joint HypocenterDetermination(JHD) [Dewey, 1971]; and 3) Five events (mb>5.0) for which the depthand focal mechanismare constrainedusinga longperiodbodywave inversionscheme[Nfibelek, 1984]. Localmicroearthquake data.Themicroearthquake datawere located using HYPO71 [Lee and Valdes, 1985] and the velocity model reportedby Sufirezet al. [1992], with a ratio
Vp/Vs=1.76determined forthiszone.Hypocentral locations wereconsidered reliableandincludedin thedataset whenthey met the following criteria: 1) A minimumof sevenphases were read with at leastone S-wavereading;2) A root mean squareerror of less than 0.25 s; 3) Estimatedhypocentral errorsof lessthan 10 km; and 4) A hypocentralvariationof lessthan 10 km whentheinitialtrial depthwasvariedfrom 10 to 150 km. A final subsetof 100 reliablehypocenters were obtainedfroma totalof 162microearthquakes. Relocatedearthquakes.The largestaftershockof the 1973 Colima earthquake(Ms=7.5) wasusedasthe calibrationevent in a JHD scheme[Dewey, 1971]. This aftershock(mb=5.6, Ms=6.2) was located with a temporary seismic network
2391
2392
PardoandSu•rez:RiveraPlateSubduction Geometry
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