Collaborative Research: Mechanisms of transient deformation following great 2006-2007 Kuril earthquakes: Frictional afterslip or viscoelastic relaxation?

Columbia University, New York NY

Investigators

Abstract

The earthquake cycle in subduction zones includes strain accumulation caused by subduction, sudden strain release by the earthquake rupture, and postseismic time-dependent deformation. GPS observations of postseismic transients after large subduction earthquakes provide a natural experiment to study rheology of the megathrust and of the sublithospheric mantle. Scientists still disagree on what mechanism prevails in the postseismic transients: frictional afterslip on the coseismic rupture, viscoelastic relaxation in the asthenosphere, or poroelastic rebound. In 2006-2007, a doublet of great earthquakes (Mw > 8) struck in the center of the Kuril subduction zone at the northwest Pacific Ocean: a thrust event at the subduction interface followed by an extensional event beneath the oceanward flank of the Kuril trench. It is probably the greatest doublet that has been observed in the era of satellite geodesy and broadband seismology. In 2006, the Kuril GPS Array was installed, which allowed to measure all components of the earthquake cycle. The data collected in 2006-2009 before and after the 2006-2007 earthquakes outlined a broad zone of postseismic deformation with initial horizontal velocities as fast as 100 mm/a, and a regional uplift. Most of the postseismic signal after the great Kuril doublet is caused by the viscoelastic relaxation of shear stresses in the weak asthenosphere. Viscoelastic relaxation was discriminated from other candidate mechanisms by the pattern of horizontal and vertical motions. We predict that the postseismic deformation will die out in about a decade after the earthquake doublet, so the preferred mechanism can be tested with observations continued for several years. Our initial results suggest large variations among subduction zones in the upper mantle viscosity, one of the most important parameters that governs the stress distribution. The focus of work during this 1-year proposal period will be to: (1) Guarantee continuity of the postseismic time series, without which all future work would be handicapped. (2) Analyze and develop models of postseismic transient deformation for a period 4?5 years after the great 2006?2007 Kuril earthquakes, revisiting or testing several assumptions made in the work so far. (3) Compare and assess geodetic and seismologic models for the two events, and include the effect of the outer rise event in models. (4) Map the space-time distribution of frictional afterslip, and in particular study how stress changes from the second event affected ongoing afterslip. (5) Model the deformation caused by the eruption of Sarychev volcano (Matua Island in the central Kurils). The project continues collaboration between the scientists of the US and Russia. The results of the project will have an impact on the region subject to disastrous earthquakes. This work and the methods we will develop are expected to lead to generally improved understanding of the earthquake potential of subduction zones and may thus ultimately lead to improved hazard estimation and mitigation elsewhere.