SEISMIC COUPLING ALONG THE CHILEAN SUBDUCTION ZONE

Underthrusting at subduction zones can cause large earthquakes at shallow depths but is accommodated by aseismic creep below a certain depth. The maximum depth of the seismically coupled zone (or seismogenic zone) is a transition from unstable to stable sliding. We have determined the maximum depth of the coupled zone and its variability along the Chilean subduction zone. The maximum depth of seismic coupling is defined by the depth of large (M > 6) underthrusting earthquakes that have occurred at the downdip edge of the coupled plate interface. Earthquake depth is determined with omnilinear waveform inversion of long-period P waves. The statistical uncertainty in the depth is estimated using bootstrapping. Omnilinear inversion formally accounts for the scaling incompatibility between the P waveforms and decreases the uncertainty in the depth estimate. We have found the depths of 27 earthquakes in the time period from 1961 to 1987. Seismic coupling in Chile extends down to 48-53 km. There is a resolvable change in the maximum depth of coupling around 28-degrees-S. The region immediately north of this latitude has a coupled zone that extends no deeper than 36-41 km, while south of this latitude coupling extends down to 48-53 km. This transition is not simply related to the oceanic lithospheric age and plate convergence rate but coincides with a change in several geophysical and geological phenomena. With respect to shallower (north) and deeper (south) coupling there is a change from active volcanos to no volcanos, the dip of the deep slab changes from steep to shallow, the thickness of trench sediments changes from thin to thick, and the depth of the oceanic basement changes from deeper to shallower at 28-degrees-S. Due to the multiplicity of changes at 28-degrees-S there is no clear unique interpretation of what physical mechanism controls the variability in the maximum depth of the seismically coupled zone in Chile.