HR: 1330h
AN: T52B-0262    [PDF]
TI: Outer Rise Stress Changes in Central Chile related to the 1985 Mw 7.8 Valpara”so Earthquake
AU: * Campos, J A
EM: jaime@dgf.uchile.cl
AF: University of Chile Dept. of Geophysics, Blanco Encalada 2085, Santiago, 2777 Chile
AU: Clouard, V
AF: University of Chile Dept. of Geophysics, Blanco Encalada 2085, Santiago, 2777 Chile
AU: Lemoine, A
EM: lemoine@dgf.uchile.cl
AF: University of Chile Dept. of Geophysics, Blanco Encalada 2085, Santiago, 2777 Chile
AU: Kausel, E G
EM: ekausel@dgf.uchile.cl
AF: University of Chile Dept. of Geophysics, Blanco Encalada 2085, Santiago, 2777 Chile
AU: Perez, A
EM: aperez@dgf.uchile.cl
AF: University of Chile Dept. of Geophysics, Blanco Encalada 2085, Santiago, 2777 Chile
AB: Although intraplate seismicity in the outer rise region is less common than seismicity associated with interplate subduction zone earthquakes, a significant level of seismicity has occurred between the trench and JF Ridge (JFR), in Central Chile, during the past 20 years. Our study focuses on the April 9, 2001 (Mw=6.8) event and its aftershocks. We determine its focal mechanism, depth and source time function by waveform inversion from teleseimic broadband data. The results indicate tensional faulting with a strike of $36\deg$, a dip of $43\deg$, and a rake of $-87\deg$. The source time function has a duration of 11 s, with a seismic moment of $2.5x10^26$ Dyne cm. The depth of 15 km, obtained using oceanic velocity model, localizes this event below the Moho. This result, together with the depth distribution of the first 2 days of aftershocks, determined by the permanent local seismic network, indicates that the neutral stress surface must be located below 40 km depth. The strike of this event is very well constraint by SH waveform modeling and is parallel to the main local bathymetric structures including the JF Ridge. A clear SW to NE directivity source effect is also observed, in good agreement with the 2 days aftershock distribution, confirming that the strike of the focal plane is not parallel to the main trench direction. This is a main result that shows the interaction between JF Ridge and the initiation of the subduction process at the trench. Other outer rise events have occurred during the last 20 years in front of the rupture covered by the 1985 Mw 7.8 Valpara”so Earthquake. To understand the relation between these earthquakes and the state of stress of the lithosphere, we use detailed bathymetric data in the hypothesis of a purely elastic plate to model the shape of the outer rise oceanic lithosphere assuming that the deformation is due to the bending prior to subduction and the loading by JF Ridge. The outer rise aftershocks are located in the upper tensional part of the lithosphere, shifted seaward of the trench because of JF Ridge. Previous compression events occurred 20 years before at 30 km depth in the same area. This enables us to propose a temporal migration at depth of the tensional/compressive limit within the lithosphere.
DE: 8100 TECTONOPHYSICS
DE: 8123 Dynamics, seismotectonics
SC: Tectonophysics [T]
MN: 2003 Fall Meeting