A PRELIMINARY ESTIMATION OF INTERSEISMIC STRAIN ACCUMULATION ALONG THE COAST OF OAXACA, MEXICO, INFERRED FROM GPS MEASUREMENTS

A GPS (Global Positioning System) network has been established in Oaxaca, Mexico, with the aim of understanding the subduction-related seismotectonics of the area. The network consists of 19 GPS stations (3 permanent and 16 temporal) and spans the entire coast of Oaxaca. The permanent stations have been continuously operating since 2000. The GPS data from temporal stations have been acquired in 2001, 2002, and 2003. The position vectors of the sites show a significant change in the horizontal component during the period 2002-2003 as compared to the period 2001-2002; generally, the amplitudes are greater during the former period. A possible explanation of this difference may be provided by the reported slow slip in the neighboring Guerrero zone that had a duration of about 6 months and coincided with the measurements in Oaxaca during the period 2001-2002. Perhaps this slow slip extended below Oaxaca as well. Some of the position vectors during 2001-2002 may also have been affected by a post-seismic slip on the fault plane of an inslab earthquake which occurred near the coast of Oaxaca in September 1999 (Mw 7.4). In this paper we present preliminary results on the extent and percentage of seismic coupling of the plate interface in Oaxaca using the position vectors of the period 2002-2003.