HR: 1400h
AN: G33A-01    [Abstracts]
TI: Earthquake cycle implications of the Jalisco/Colima GPS project, 1995-2007
AU: * DeMets, C
EM: chuck@geology.wisc.edu
AF: Dept of Geology and Geophysics, University of Wisconsin-Madison, 1215 W Dayton St, Madison, WI 53706, United States
AU: Marquez-Azua, B
EM: bmarquez@cencar.udg.mx
AF: Departamento de Geografia y Ordenacion Territorial, Universidad de Guadalajara, Av. de los Maestros y Mariano Barcena, Planta Alta, Guadalajara, JAL 44260, Mexico
AU: Sanchez, O
EM: osvaldo@ollin.igeofcu.unam.mx
AF: Instituto de Geofisica-UNAM, Ciudad Universitaria, Mexico, DF 04510, Mexico
AU: Stock, J
EM: jstock@gps.caltech.edu
AF: Seismological Laboratory, California Institute of Technology, 1200 E California Blvd, Pasadena, CA 91125, United States
AB: This talk will summarize results from modeling of campaign and continuous GPS measurements in a 30+ station network that extends 350 km along the Pacific coast and 300 km inland in the states of Jalisco, Colima, and Michoacan. Measurements since March of 1995 have recorded in detail the Oct 9, 1995 M=8.0 Colima/Jalisco and Jan 22, 2003 M=7.4 Tecoman earthquakes, constituting the only earthquakes with magnitudes above 6.5 that have ruptured the northwestern end of the Middle America trench since 1932. Modeling of coseismic offsets for both earthquakes indicate that coseismic slip extended no farther downdip than the Pacific coastline, defining a shallower seismogenic zone than is typical for subduction zone. Near-term postseismic measurements demonstrate that both earthquakes were followed within hours to days by aseismic propagation of slip along the subduction interface to deeper areas beneath the continent, decaying logarithmically with time after the earthquakes. These are diagnostic of fault-centered afterslip, representing a transient frictional response to the earthquake. Measurements months to years after each earthquake clearly show protracted, decaying transient deformation, consistent with viscoelastic flow of the upper mantle and possibly lower crust to relax the seismically-elevated stresses at depth. The afterslip and viscoelastic transients are further superimposed on linear landward motions of the GPS sites, which are an elastic response to relocking of shallow seismogenic areas of the subduction interface. The site motions are poorly fit by models that exclude steady interseismic strain accumulation, afterslip, or viscoelastic transients, strongly suggesting that all three contribute to surface deformation. Interestingly, no episodic transient slip has been recorded in Jalisco or Colima since continuous GPS recording was initiated by INEGI in early 1993. Unlike the rest of western and southern Mexico, where ETS is pervasive and presumably caused by the existence of a wide region of relatively flat Cocos plate subduction, the absence of ETS in areas characterized by steeper Rivera plate subduction may indicate that the area of transitional slip associated with ETS is narrow or possibly non-existent along the northwesternmost 300 km of the trench. A new 10-station continuous network operated jointly by the Universidad de Guadalajara and the University of Wisconsin will improve the detection threshold for such transients if they occur.
DE: 1242 Seismic cycle related deformations (6924, 7209, 7223, 7230)
SC: Geodesy [G]
MN: 2007 Joint Assembly