Seismic structure between the Pacific coast and Mexico City form the Petatlán earthquake (Ms=7.6) aftershocks

Abstract

P- and S-wave travel times from aftershocks of the M1=7.ó Petatlan, Guerrero, Mexico earthquake of March 14, 1979, have been used to delineate a compressional- and shear-wave velocity structure of the subduction zone between Petatlan and Mexico City and along the coast of the State of Guerrero. The fmal model consists of a two-dimensional ocean-to-continent transition zone composed of a continental, an accretionary and an oceanic crust block. The continental block as modeled consists of three flat crustal layers with the Moho at 45 km depth. The oceanic crust is modeled as two layers with 8 km total thickness, dipping 10° at N34°E, and extends well inland. Seismic waves from aftershocks with hypocenters below the oceanic crust travel principally in the oceanic upper mantle to sites between 50 and 300 1cm inland. The hypocentral location of 792 Petatlan aftershocks was compared with the structure obtained by ray tracing. Three percent of those aftershocks are located within the suggested continental block, 85% are within the oceanic crust and -12.5% are below the oceanic crust. 55% of them are located under the, local array and their hypocenters are constrained to within ±5 km. The hypocenters of the aftershocks define a narrow, 8-10 km wide Wadati-Benioff zone that agrees with the model of oceanic lithosphere based on travel times from aftershocks recorded by profiling arrays along the coast and toward Mexico City. 50% of the aftershocks within the oceanic crust (approximately 350) are concentrated in a thin sheet less than 8 1cm thick with an area of 480 1cm2 and located deeper and away from the trench. This is a region largely under the recording array and has been proposed to be a region of strong coupling in young and fast convergence rate subducting lithospheres (Astiz, 1987; McNally et al., 198ó) and an asperity based on the location of the Petatlan earthquake foreshocks (Hsu et al., 1984) and also aftershocks (Valdes et al., 1982; Hsu et al., 1984).