Rheological Behaviour of the Deformation of the Back Arc in the Isthmus of Tehuantepec in South-eastern Mexico

The seismicity in the back arc of the Mexican subduction zone is relatively low.  An exception to this is the seismic activity observed in the Isthmus of Tehuantepec, where shallow earthquakes take place mostly along the coast of the Gulf of Mexico.  The largest recorded earthquake occurred on 26 August 1959 (Mw 6.9).  Other moderate earthquakes are recorded also in the southeastern margin of the Gulf of Mexico with magnitudes ranging from 5.3 to 5.9.  Data from the VEOX experiment that registered seismic data continuously on a cross-section across the Isthmus of Tehuantepec were analyzed.  Shallow earthquakes were culled from the continuous records, eliminating events within the subducted slab.  A total of ~40 shallow earthquakes were identified.  The linear geometry of the VEOX array made it difficult to locate the earthquakes.  Thus, additional stations from the Seismological Service of Mexico were used in the analysis.  Hypocentral locations were improved using the double-difference hypocentral algorithm.  The focal mechanisms obtained show consistently reverse faulting, where the axes of maximum compression are oriented NE – SW, like the 1959 earthquake.  This indicates that the lithosphere is deformed by compressive stress oriented in the direction of relative plate motion.  The best-located earthquakes show focal depths ranging from 20 to 50 km.  The depth of the Moho in the Isthmus of Tehuantepec is well controlled by receiver function results.  Therefore, it is possible to identify where the earthquakes occur relative to the depth of the Moho.  Unlike most upper plate deformation in the back arc of the subduction zones, earthquakes in the Isthmus of Tehuantepec occur both in the crust and the upper mantle.    Rheological models suggest that shallow earthquakes occur mostly in the seismogenic part of the upper crust and the upper mantle.  Our observations clearly show that in this region earthquakes reflect lithospheric deformation involving the crust and the upper mantle.  We are currently exploring rheological models that may help explain earthquakes in both the crust and upper mantle. The focal mechanisms suggest that the deformation may be induced by the subduction of the aseismic Tehuantepec Ridge in the Mexican subduction zone.