Seismic structure off- and onshore Maule, constraints for megathrust earthquakes in central Chile

Moscoso, Eduardo

During the last decades, the Chilean margin offshore Maule (34±S −36±S) had been reported as a highly locked and seismically quiet zone. The stress-accumulated state finished on the 27th of February 2010, when a megathrust earthquake (with Mw = 8.8) ruptured » 400 km of the Nazca-South America plate boundary. Unfortunately, up to now little was known about the seismic structure offshore Maule. In the frame of the third phase of the project SFB 574 “Volatiles and Fluids in Subduction Zones” of the Christan-Albrechts University of Kiel, seismic data was analyzed in order to obtain detailed images of the deep structure of the margin and of the outer rise. Here are presented constraints on the forearc and the subduction zone structure of the rupture area derived from seismic refraction and wide-angle data. The results show a wedge shaped body » 40 km wide with typical sedimentary velocities interpreted as a frontal accretionary prism (FAP). Landward of the imaged FAP, the velocity model shows an abrupt velocity-contrast suggesting a lithological change, which is interpreted as the contact between the FAP and the paleo accretionary prism (backstop). The backstop location is coincident with the seaward limit of the aftershocks, defining the updip limit of the co-seismic rupture and the seismogenic zone. Furthermore, the seaward limit of the aftershocks coincides with the location of the shelf break in the entire earthquake rupture area (33.5±S−38.5±S), which is interpreted as the location of the backstop along the margin. Published seismic profiles at the northern and southern limit of the rupture area also show the presence of a strong horizontal velocity gradient imaging the seismic backstop at a distance of » 30 km from the deformation front. The seismic wide-angle reflections from the top of the subducting oceanic crust constrain the location of the plate boundary offshore, dipping » 10±. The projection of the epicenter of the Maule earthquake onto our derived interplate boundary yielded a hypocenter around 20 km depth. This implies that the earthquake nucleated somewhere within the seismogenic zone, neither at its updip nor at its downdip limit. The second part of this thesis focuses on the dependency between the incoming plate’s bend faulting, lithospheric hydration and shallow outer rise seismic activity. To support the interpretation, are presented Vp and Vs seismic models obtained from wide angle seismic data and the derived 2D Poisson’s ratio distribution at the outer rise. The oceanic lithosphere shows a high degree of hydration, due to the water infiltration through the bending-related faults exposed to seawater. This process is presumably intensified bythe existence of a seamount in the area. It is concluded that the water infiltrates deep into the lithosphere, triggering shallow earthquakes in the outer rise and likely serpentinization in the mantle, estimated to be about 10%.

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Moscoso, Eduardo: Seismic structure off- and onshore Maule, constraints for megathrust earthquakes in central Chile. 2011.

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