Crustal Deformation in the Isthmus of Tehuantepec, Mexico Observed from Microseismic Data recorded by the VEOX Experiment

Abstract:

An array of 45 broad band sensors was installed across the 300-km-long Isthmus of Techuantepec, in the narrowest part of Mexico. This array was deployed from August of 2007 to March of 2009. Data was recorded in a continous manner in all seismographs throughout the whole deployment period. We used an LTA/STA algorithm to automatically detect earthquakes recorded by the norhernmost stations near the coast of the Gulf of Mexico. Earthquakes recorded by one station were searched in adjaecent sites. This allowed a thorough and systematic search of the seismic traces in order to identify seismic events recorded by the northern segment of the array. The earthquakes culled from the algorithm were filtered in order to include only those with crustal or upper mantle depths that reflect deformation of the overriding plate. Intermediate depth earthquakes occurring within the subducted slab were filtered out. In total, during the 18 month duration of the experiment, 34 earthquakes were clearly identified and located within the crust and the upper mantle. Tests on the location accuracy were conducted using different velocity models proposed for this region. A selection criteria of the best located earthquakes was applied observing the stability of the hypocentral locations to changes in the velocity model. The resulting subset of the best-located earthquakes indicate that the deformation of the upper North American plate is concentrated in the northern part of the Isthmus of Tehuantepec, along the coast of the Gulf of Mexico. Only a few events occurred farther inland, to the south of the coast of the Gulf of Mexico. The location of these microseismic events coincides with the location of moderate events recorded in the past. In particular, the largest cluster of microearthquakes occurs near where the 29 August 1959 earthquake. The location and depth of the microearthquakes located further demonstrates that the overriding North American plate is being deformed, probably by the subduction taking place to the south. Considering that this region hosts the largest oil refining facilities in the country, the presence of these microearthquakes demonstrates the need to better understand the tectonic processes present and their impact on the seismic hazard of this region.