Triggered seismic activity in the Liquiñe‐Ofqui fault zone, southern Chile, during the 2007 Aysen seismic swarm

SUMMARY 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 Liquiñe‐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 Liquiñe‐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 Liquiñe‐Ofqui fault strands. The five earthquakes on the Liquiñe‐Ofqui fault were all produced by dextral slip on ∼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 Liquiñe‐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.