Megathrust earthquake complexity from inversion of regional seismic waveforms: the Jan. 2011 Mw 7.1 Araucania, Chile earthquake

Posted by pkm at Mar 19, 2015 01:07 PM |
Stephen Hicks & Andreas Rietbrock - Liverpool Earth Observatory, University of Liverpool, UK

Many of the Earth’s most damaging earthquakes rupture the shallow plate boundary of a subduction zone. Large ruptures along these faults can involve complex static and dynamic stress transfer on the subduction megathrust itself, or between separate faults in the case of an earthquake doublet. We focus on the 2010 Mw 8.8 Maule, Chile earthquake: the sixth largest earthquake ever recorded. In the earthquake’s immediate aftermath, the UK took part in a multinational seismic deployment to record the aftershock sequence, allowing a unique view of structural complexity along a subduction megathrust fault.

We examine the largest interplate aftershock of the Maule sequence: the 2011 Mw 7.1 Araucania earthquake. The earthquake occurred at the boundary between the rupture zones of the 1960 Mw 9.5 Valdivia and 2010 Maule earthquakes, a region with widespread upper plate faulting. We perform a multiple point-source inversion of local waveforms from this earthquake. We find that two sub-events, both with similar moment release (Mw 6.7 – 6.8), occurred within 12 seconds of each other and 35 km apart. The later sub-event, with a normal faulting mechanism, occurred seaward of the initial rupture. A comparison of this source configuration with characteristics of its ensuing aftershocks reveals that the second sub-event ruptured an extensional fault at the base of the overriding plate.

Coulomb stress modelling indicates that the rupture of the first sub-event brought the normal fault closer to failure. These findings provide evidence for a rare and previously undocumented instance of a rupture along the subducting plate interface immediately triggering slip in the overriding plate. Due to the small time gap between the two sub-events and their similar depth, the second sub-event is likely hidden from low frequency teleseismic waveforms. Importantly, this masking of a shallow earthquake rupture presents a challenge for assessing tsunami hazard from megathrust earthquakes because such doublets may only be seen by near-field data. This study sheds light on mechanisms controlling damaging tsunamis in subduction zones, and their relationship with megathrust barrier regions.

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