An M_w = 7.7 slow earthquake in 1960 near the Aysén Fjord region, Chile

Abstract

We re-examine the source characteristics of an M_s = 6.9 earthquake in Chile which occurred on 1960 June 6, near the Aysén Fjord region where a remarkable earthquake swarm occurred in 2007 with more than 7000 earthquakes and hundreds of landslides. The June 6 event occurred during the aftershock activity of the 1960 May 22, M_w = 9.5 great Chilean earthquake. A recently found well-calibrated strain seismogram of the June 6 event recorded at Isabella (ISA), California, is the impetus to this study. We confirm that this event is a slow earthquake caused by a source process extending at least 190 s with a seismic moment of M_0 = 4.5 × 10^(20) N⋅m (M_w = 7.7). Although the mechanism cannot be uniquely determined, an NS trending right-lateral strike-slip mechanism is consistent with the ISA record. The depth cannot be constrained well, but the slowness of the event suggests that it may have occurred in a somewhat deeper high-temperature ductile environment caused by nearby subduction of the Chile Rise. The mechanism and the proximity of this event to the Liquiñe-Ofqui fault (LOF) which extends north–south over 1000 km along the Chilean coast suggest that the June 6 event represents a slip on this fault. The large moment of the June 6 event indicates strong interaction between the Nazca-South American plate boundary and the LOF with significant slip partitioning.

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