An intermediate-depth tensional earthquake (M _W 5.7) and its aftershocks within the Nazca slab, central Chile: A reactivated outer rise fault?

摘要

An intermediate-depth earthquake (7 January 2003, M _w 5.7) occurred within the subducting Nazca plate at longitude 70.3°W, latitude 33.8°S and depth 113km. Its focal mechanism shows normal faulting with a slight strike-slip component. We detected 50 aftershocks within January 2003 using a temporary seismic network installed in the zone. Their local magnitudes M l range between 1.9 and 3.5, with the strongest events occurring around the mainshock. Their spatial distribution, including the mainshock, defines an area of~(35±5)×(10±2) km ~2, cutting through almost half of the slab's total thickness at an angle of ~60° to the slab's surface. This area agrees well with one of the mainshock nodal planes. However, the total seismic area, as defined by the aftershock distribution, is larger than the rupture area normally expected for an earthquake of moderate magnitude. We compare the orientation of the seismic plane with the outer rise fault pattern offshore central Chile and find a correlation with the strike of the seafloor spreading fabric. The seismic sequence shows similarities with other intermediate-depth cases, notably the 13 June 2005 Tarapacá earthquake in northern Chile and similar cases in the Pacific slab beneath Japan. In all these cases, the inferred reactivated fault planes probably originate from the outer rise region, in agreement with the hypothesis that intermediate-depth seismicity is linked to inherited faults. Consequently, even moderate-sized earthquakes can reactivate large areas of inherited faults within slabs at depths >100km. Furthermore, the occurrence of multiple other local events (M _w>5), with similar focal mechanism and depth to the January 2003 event, appear to indicate that the slab becomes mechanically weak ~100km depth. The depth extent in the slab of the reactivated pre-existing faults is likely governed by the slab's bending/unbending stress regime, i.e. the depth to the neutral plane. Dehydration embrittlement is a possible factor for triggering the seismic sequence.