Imaging the Mechanics of Shallow Subduction: the Oaxaca GPS Array

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DeMets 0104299 The Oaxaca GPS study is designed for a detailed geodetic study of the mechanics of subduction along the 350-km-long Oaxaca segment of the Middle America trench, where the trench lies closer to the coast (50-65 km) than for any other continental land mass on Earth. Interseismic loading of the locked subduction interface in this region gives rise to high elastic strain gradients everywhere within 75-100 km of the coast. Surface displacements measured via geodetic techniques are thus sensitive to the distribution of loading and/or slip along the nearby subduction interface. This makes the coastal and near-coastal areas of Oaxaca a superb and possibly unsurpassed region for studying the mechanics of subduction in the upper, seismogenic area of the subduction interface. Forward and inverse modeling of the expected spatial resolution of a 40-site GPS array in Oaxaca indicates it would have a level of resolution 5-12 times better than that of the dense Japanese GPS array. Measurements of site displacements in Oaxaca will allow many questions to be addressed, including at least the following: (1) Is interseismic coupling relatively homogeneous along the fault and if not, what is the spatial scale of the heterogeneity? (2) How does the geodetic coupling coefficient compare to the seismic coupling coefficient of 0.5-0.75 estimated for this trench segment and what does any difference between the two imply about aseismic processes that may accommodate slip along the subduction interface? (3) Does the degree and/or pattern of coupling evolve through time? (4) Is the downdip edge of the locked zone located at a depth of 20 km, as proposed by Suarez and Sanchez (1996) to explain the general absence of large (M 8) earthquakes along the Cocos-North America subduction interface? (5) Is there any evidence for slowly propagating ruptures (slow earthquakes) that help to relieve interseismic loading? Given that large earthquakes tend to recur every few decades along most segments of the Middle America trench, measurements during the interseismic phase will also eventually enable us to attack many additional questions about subduction mechanics, including in particular the comparative patterns of interseismic loading and coseismic slip release. The results of this work will add new and useful observations to the still-modest set of observations that constrain conceptual models for the mechanics of shallow subduction, and should also bear on the applicability of rate- and state-variable friction laws to subduction mechanics. The study, which is co-funded by CONACYT, NSF, and the Vilas foundation of UW-Madison, represents a partnership between Dr. Enrique Cabral at UNAM, Dr. Bertha Marquez at the University of Guadalajara, and Dr. Charles DeMets at UW-Madison.