Slip kinematics and dynamics during and after the 1995 October 9 M-w=8.0 Colima-Jalisco earthquake, Mexico, from GPS geodetic constraints

We use horizontal and vertical crustal displacements derived from GPS measu rements at 26 sites in western Mexico to study the coseismic and post-seism ic kinematics and dynamics of the 1995 October 9 (M-w = 8.0) Colima-Jalisco earthquake along the Middle America Trench. The measurements bracket the e ntire landward edge of the approximately 150 km long rupture zone and span a 4 yr period for most sites. We solve for the temporal evolution of slip a long the subduction interface by inverting GPS displacements for the coseis mic and four post-seismic intervals (March 1995-March 1999), subject to the assumption that the crust responds elastically to slip along a shallow-dip ping, curved subduction interface. Coseismic rupture of up to 5 m was large ly focused above depths of 20 km and was limited to a 120-140 km long segme nt of the subduction zone. Within one week of the earthquake, post-seismic slip migrated downdip to depths of 16-35 km, where it has since decayed log arithmically. We also find evidence for shallow aseismic slip during 1996 o r early 1997 northwest of the coseismic rupture zone and increasingly wides pread relocking of shallow regions of the subduction interface after early 1997. The relative lack of afterslip in shallow regions of the subduction i nterface suggests that the interface lies in the unstable frictional regime and hence is strongly coupled between earthquakes. By 1999, the cumulative slip moment associated with post-seismic slip equaled similar to 70 per ce nt of the coseismic moment, with nearly all of this slip occurring downdip from the coseismic rupture zone. The migration of slip after the earthquake to a deeper and presumably velocity-strengthening area of the subduction i nterface and the logarithmic decay of afterslip conform to the qualitative and quantitative predictions of a model in which the fault kinematics are p rescribed by rate- and state-variable frictional. laws. However, misfits to the geodetic displacements exceed the average displacement uncertainties f or all epochs, implying one or more of the following. (1) the elastic respo nse is heterogeneous due to slip along unmodelled upper crustal faults or v ariations in the elastic properties of the crust; (2) other post-seismic me chanisms such as viscoelastic or poroelastic effects contribute to or possi bly dominate the post-seismic response; (3) we have underestimated the unce rtainties in the GPS displacements.