ABSTRACT

Funds for a four-year, third and final phase of a GPS-based study of the
Colima-Jalisco region of western Mexico are requested. Since early 1995,
two large magnitude earthquakes (M=8.0 and M=7.6) have ruptured the Middle
America subduction interface directly offshore from a 30-station GPS network,
most recently in January of 2003. These earthquakes have afforded an exceptional
opportunity to study the coseismic and postseismic effects of large
subduction earthquakes, including the gradual decay of postseismic fault
afterslip and viscoelastic flow and re-emergence of interseismic elastic
strain accumulation. Studying these earthquakes and their after effects
have numerous intellectual merits, outlined below. Specific goals for
the proposed project are as follows: (1) Continue annual measurements of the
regional velocity field to characterize spatial variations in viscoelastic
flow and afterslip triggered by the Mw=8.0 Oct. 9, 1995 Colima-Jalisco and
M_w=7.6, 1.22.2003 Tecoman earthquakes, (2) Construct a six-station continuous
GPS network to better characterize the temporal evolution of these viscoelastic
and afterslip responses and better separate them from the underlying linear
elastic strain accumulation. Continuous measurements will enable determine of
whether the Cocos/Rivera subduction interfaces accommodate downdip transient
aseismic slip events similar to those recently described for Cascadia, Japan,
and southern Mexico. (3) Complete modeling of the coseismic and post-seismic
effects of the 2003 Tecoman earthquake using existing and new observations,
including a joint inversion of the geodetic and seismic constraints,
collaborative analysis of local seismic data for aftershock relocations, and
sensitivity analysis for our assumed elastic constants, layer thickness,
and subduction interface geometry. (4) Employ measured variations in
the direction and magnitude of interseismic strain accumulation to
both estimate the degree of locking along the Rivera and Cocos plate
subduction interfaces and relate onshore strain accumulation to
the oceanic plate motion. (5) Determine how the 1995 Colima-Jalisco earthquake
and subsequent processes altered stress levels in the adjacent Tecoman earthquake
rupture zone. (6) Extract information about upper plate deformation associated
with the continental rifts in our field area from GPS, seismic, and Shuttle
Radar Topographic Mission data. As has been the case in previous stages of
this project, the present phase of the project will have numerous broader impacts.
Three Mexican scientists will participate as official collaborators: Osvaldo Sanchez
of UNAM, Bertha Marquez of the Universidad de Guadalajara, and Gabriel Reyes of
the University of Colima, as well as Mexican undergraduate and graduate students
from three universities. In addition, support is requested for a UW graduate student
to continue his finite element modeling of the geodetic data for his dissertation,
a Caltech graduate student who will work with Gabriel Reyes on local seismic data,
and two REU undergraduates who will work under Stock's guidance. Data from the new
continuous sites will constitute a useful legacy for other investigators, particularly
for detecting and characterizing transient signals related to aseismic slip along
the subduction interface.

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