Seismological study of the central Ecuadorian margin: Evidence of upper plate deformation

Abstract

Abstract A seismic study of a segment of the convergent margin of Ecuador is presented. During the SISTEUR campaign a network of 24 Ocean Bottom Seismometers (OBS) was deployed on the Carnegie Ridge, one line along the main axes of the ridge and two lines across the strike of the edge of the ridge, during one month. This marine network was complemented with a land network of 20 stations distributed in two lines: one parallel to the margin and the other perpendicular to it. The seismic event recorded by these networks, were located using different crustal models defined from the wide-angle seismic data modeling. Relative location techniques were used to improve earthquake locations. Seismogram waveform modeling allowed us to constrain hypocentral location for events farther than ∼50 km from the network. This modeling also provided additional information to constrain the focal mechanisms of these events. The upper limit of the Interplate Seismogenic Zone (ISZ) is estimated to be at a 10 km depth in the region. The background seismic activity of the upper plate provided new insights: 1) A seismic cluster that reaches the base of the overriding plate is linked to the Jipijapa-Portoviejo fault. The reactivation of this Quaternary fault is confirmed by focal mechanisms that provide rupture planes parallel to its superficial projection (N10°–N25°). 2) The focal mechanisms presented in this study are compatible with a homogeneous regional stress field corresponding to an E–W to ESE–WNW compression and an NNE–SSW extension. The presence of strike-slip deformation, with a reverse component, corresponds to the NNE escape of the North Andean Block. Normal faulting accommodating this movement suggests that this part of the North Andean Block cannot be considered as a rigid block.