Taking the pulse of the Ecuador subduction zone near a locked patch

In Central Ecuador the Nazca Plate, with a major topographic feature, the Carnegie Ridge, is subducting beneath the North Andean Margin at a rate of about 5 cm/yr along a ~N80°E direction. Intense seismic swarms have been regularly observed in 1977, 1998, 2002, and 2005 but no large megathrust earthquake is known in this segment of the margin, south of the rupture zone of the great 1906 Mag 8.8 Ecuador-Colombia earthquake. In 2010, a week-long slow slip event (SSE) was documented beneath La Plata Island, located 40 km east of the trench, at about 8 km depth within a locked patch of the subduction interface. The equivalent moment magnitude (Mw) released during this SSE was in the range of 6.0-6.3. This event unleashed an intense microseismic activity along the plate interface located beneath La Plata Island. The focal mechanisms and the space-time occurrence of these earthquakes suggest that the stress perturbations related to the slow slip event trigger the seismicity (Vallée et al., 2013). This study suggests a posteriori that recurrent seismic swarms, like the 2005 sequence may have been triggered by large-magnitude slow slip events with equivalent magnitude up to 7.5. In order to better observe this microseismic activity associated with SSE we deployed a temporary seismic network (OSISEC for Observación SISmica en ECuador) for a 2 years period (from November 2011 to October 2013). Six broadband land seismometers equipped with Trillium compact sensors and 5 Ocean Bottom Seismometers (3 components Guralp CMG-40T and 1 High-Tech Inc. wideband hydrophone) complement the 3 permanent seismological stations of the national seismic network with an average distance between stations of about 25 km. Automatic and manual analyses of continuous data of the first year yielded 2800 earthquake locations. Hypocentral determinations were computed with the complete network including readings from OBS data. Specific velocity models for the OBS stations were derived from vertical seismic profiles to account for the sea bottom anomalous structures. The seismicity distribution shows an intense activity inland, mostly beneath the land stations where ancient faults have been identified, whereas offshore, the seismicity appears to be sparse in time and space. This distribution is partly due to a much lower detection magnitude threshold inland (about mag. 2) than offshore (about mag. 3.5). Hence we observe a seismic quiescence near the trench, which is probably related to the presence of a strong coupled interplate patch. Hypocenters clusters outlined major crustal faults on-land and active deformation within the subducting slab. Both interplate and intraslab seismic activities show strong lateral variations indicating changes in stress pattern over short wavelengths. This ongoing work will present a better-resolved shape of the shallower part of the plate interface through accurate location of interplate events.