Evidence of Subducting Seamounts in Copiapo, Northern Chile

The Copiapo, northern Chile region (27-28.5°S) is located near the transition between normal subduction (that dips eastward with an angle of about 30°) and a subhorizontal subduction geometry. This change is well correlated with the end of the Quaternary volcanic chain, which is absent as far south as latitude 33°S. The Copiapó region is characterized by its ability of nucleate large interplate earthquakes; two historical earthquakes M>8.0 have been reported on 1819 (Mw=8.3) and 1922 (Mw=8.5), both with associated destructive tsunamis. Moreover, this region has also experienced at least two clear seismic swarms, one observed during July and August 1973, corresponding to an increase of earthquakes with magnitudes between 5.0 and 6.0 and the second was observed on April 2006, any of them was associated with a large earthquake. The seafloor morphology in the Copiapo area suggests the presence of seamounts that are being subducting in this region. In this work we procesing locally recorded data, that was used to determine a 3D body-wave velocity structure in the study area. The Vp, Vs, Vp/Vs anomalies are well correlated with the trace of the subducting seamount that can be observed with the high resolution bathymetry previously obtained, and with the area involved with both, the 1973 and the 2006 swarms. This is also in agreement with surface neotectonic results showing an extensional regime in this region. Therefore, the seismicity observed can be interpreted as the response of the overriding plate to the heterogeneities associated with the subducting seamounts. The presence of such heterogeneities does not necessarily lead to significant variations in the seismic coupling of the Copiapo region, because it would depend on the size of the subducted seamounts; they could be related to small earthquakes which rupture isolated unstable patches during the interseismic period. It is also interesting to note that the highest anomalies observed in the region coincide with the southern rupture end of the 1922 rupture area. Finally, the area that participated in the 1922 large earthquake and did not break with the1983 earthquake is an important seismic gap, where it is expected to have a large earthquake in the near future.