Triggered seismic activity in the Liquine-Ofqui fault zone, southern Chile, during the 2007 Aysen seismic swarm

P>We relocated the six large-magnitude (5.2 < M-w < 6.2) earthquakes of the destructive, tsunamigenic Aysen seismic swarm, which occurred from 2007 January-October in Patagonian Chile. We used P and SH arrival times from near-source stations of a temporary seismic network fortuitously deployed in the area when the swarm began, and also traveltimes to stations of the permanent global networks, to locate the 2007 January 23, M-w 5.2 earthquake, the first of the six large magnitude events. This earthquake's hypocentre lies at shallow depth (< 10 km) on the eastern strand of a major intraarc shear zone, the dextral Liquine-Ofqui fault zone. Using the hypocentre of the January 23 earthquake as a fixed location, we relocated the five other large magnitude Aysen earthquakes by joint hypocentral determination. Four of these five events also occurred at shallow depth on the eastern strand Liquine-Ofqui fault, whereas the 2007 April 2, earthquake occurred some 45 km to the west on the Aysen fault, a strike-slip duplex fault that segments the area between the eastern and western Liquine-Ofqui fault strands. The five earthquakes on the Liquine-Ofqui fault were all produced by dextral slip on similar to N-S nodal planes approximately parallel to the mapped trace of the fault. The April 2 earthquake resulted from normal slip on the Aysen fault. Modelling of Coulomb stress changes on the nodal planes of the April 2 earthquake shows that the cumulative slip on the Liquine-Ofqui fault strand could have triggered the April 2 earthquake. Similarly, the April 2 earthquake may have triggered the M-w 6.2 April 21 earthquake, which caused mass wasting into Aysen Fjord, generating a destructive tsunami. The system of channels and fjords in the study region is a major shipping route around South America, and therefore careful evaluation of the seismic hazard is warranted.