Active Faulting, Submarine Surface Rupture, and Seismic Migration Along the Liquiñe-Ofqui Fault System, Patagonian Andes

The intra-arc Liquiñe-Ofqui Fault System (LOFS) is an active transpressive fault zone located in the Patagonian Andes of Chile. In 2007, a seismic sequence occurred in the Aysén Fjord region of Chilean Patagonia along the LOFS, with a M_w 6.2 main earthquake that triggered dozens of landslides, some of which induced tsunami waves that caused severe damage and casualties. Through the analysis of high-resolution seismic reflection and bathymetric data, we identify six submarine faults cutting the Late Quaternary postglacial sedimentary infill of the fjord. The most conspicuous are the dextral-normal NE-SW striking Quitralco fault (QF) and the N-S striking strike-slip Río Cuervo (RCF) and Punta Cola faults (PCF). Our paleoseismological analysis reveals at least seven paleolandslide events buried in the fjord sediments that were triggered by local paleoearthquakes, which occurred since local ice sheet retreat, that is, circa 12 kyr. By combining tectonic observations with local seismicity data, we propose a seismotectonic model for the evolution of the 2007 seismic sequence where three structures were progressively activated from the depth toward the upper continental crust, causing surface rupture along the PCF and with earthquakes, suggesting only partial ruptures along other faults. Because the other faults did not rupture to the seafloor they remain important sources of seismic hazard. Thus, the last seismic sequence was a consequence of a stress transfer from the lower-ductile toward the upper-brittle continental crust, close to the triple junction of the Nazca, South American, and Antarctica Plates. Our results emphasize on the potential synergies between multiple geological and geophysical methods to assess complex events.