S31A-2736
­Intense Microseismicity Associated with a SSE at La Plata Island in the Central Subduction Zone of Ecuador

Wednesday, 16 December 2015
Poster Hall (Moscone South)
Monica Segovia1, Yvonne Font1,2, Marc M Regnier3, Philippe Charvis2, Jean Mathieu Nocquet4, Audrey Galve5, Yann Hello1, Anthony Ogé1, Paul Jarrin6 and Mario Calixto Ruiz6, (1)GeoAzur, Valbonne, France, (2)IRD Institute for Research and Development, Marseille Cedex 02, France, (3)IRD, Valbonne, France, (4)Institut de Recherche pour le Développement - IRD, Valbonne, France, (5)UMPC, UNSA, CNRS, IRD, Géoazur,, Valbonne, France, (6)Instituto Geofísico, Escuela Politécnica Nacional, Quito, Ecuador
Abstract:
No large historical earthquakes affected the central subduction zone in Ecuador. Instead, this region around La Plata Island experienced periodic seismic swarms as those in 1998, 2002, 2005 and 2010.

During the 2010 swarm, a permanent GPS at La Plata Is. detected a 4-5 days long slow slip event (SSE) with a magnitude 6-6.3 Mw. It was accompanied by an intense microseismicity located downdip of the locked patch centered in the island (Vallée et al., 2013).

An onshore-offshore seismic network consisting of 7 BB and 5 OBS deployed in this area recorded the background seismicity during 2 years starting in Nov. 2011.

In mid-Jan. 2013, the network recorded an increase in the seismicity around the island (~1000 eqs. from which ~400 were well localized). At the same time La Plata GPS showed a trenchward displacement during 3-4 days. The seismicity lasted ~6 days and possibly started ~0.5 day before the onset of the motion. It started south of the island and it propagated bilaterally northeastward and southwestward. It was organized in 3 lines almost parallel to the tectonic lineations in the subducting oceanic crust observed west of the trench. These lines may mark topographic highs in the plunging plate related to inherited and/or extrados-type fault due to the bending during the subduction, which are acting as seismic asperities.

The earthquakes were located in the vicinity of the interface in the lower third of the locked zone, whereas, before the SSE, the seismicity was mostly deeper inland along the plate interface but also along crustal faults. The magnitudes ranged from 2-4.5 (Ml) lower than those of the background seismicity during our experiment (4.7 Ml).

This SSE likely occurred at shallow depth (10-15 km) along the megathrust fault. It triggered seismicity in areas that were quiet in the period immediately before. A bilateral migration and a possible control by the topography of the plunging plate are observed.

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