Collaborative Research: Strain accumulation and release at the Kuril subduction zone from postseismic observations on the Kuril GPS array in 2007-2014

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The 1,200-km-long Kuril megathrust in the northwest Pacific Ocean is one of the most seismically active regions on the Earth. It was the last major subduction zone totally unexplored by methods of space geodesy. In 2006?2007, a doublet of great earthquakes in this region partly ruptured one of the most conspicuous gaps in subduction-zone seismic activity (the 15 November 2006 Mw 8.3 thrust event and the 13 January 2007 Mw 8.1 extensional event). A large 15 January 2009 Mw 7.4 thrust earthquake in the same region demonstrated the heightened seismic hazard. The Kuril GPS Array was installed several months before the 2006?2007 earthquakes in collaboration between US and Russian scientists. Observations on the array documented coseismic and postseismic surface deformation following the great Kuril earthquakes. The immediate focus of this final phase of the project is on acquiring irreplaceable, transient postseismic data spanning the time period 2007?2014. This project will continue collaboration between US universities and institutions of the Russian Academy of Sciences.The data from the Kuril GPS Array collected in 2006?2011 allowed us to develop models of coseismic slip and models of postseismic mechanisms. It was shown that most of the postseismic motion was caused by the viscoelastic relaxation of shear stresses in the weak Maxwell viscosity asthenosphere. From experience with other postseismic studies, longer time series will allow us to develop more robust models of postseismic deformation and models of interseismic frictional coupling on the subduction interface. The complete 8-year-long data set is required to address fundamental problems: How much of the strain built up from subduction of the Pacific plate beneath the North American plate (the Sea of Okhotsk) has been released in the 2006 and 2007 Kuril earthquakes coseismically and postseismically? Is it possible to explain postseismic surface deformation at the Kuril subduction zone with the linear viscous mantle or a stress-dependent rheology is required? These problems are especially important after the giant 2011 Mw 9.0 Tohoku earthquake, which made the scientists question the validity of conventional precursors of large earthquakes: location of seismic gaps and location of asperities (patches of high frictional locking at a subduction interface).