Transient Deformation in Subduction Zones
GPS time series in northeast Japan exhibit nonlinear trends from 1996 to 2011 before the Mw 9.0, 2011 Tohoku-oki earthquake. After removing reference frame noise, we modeled time series as linear trends plus constant acceleration, correcting for coseismic and postseismic effects from the numerous Mw ~ 6.5+ earthquakes during this period. We found spatially coherent and statistically significant accelerations throughout northern Honshu. Large areas of Japan outside the Tohoku region show insignificant accelerations, demonstrating that the observation is not due to network-wide artifacts. While the accelerations in northern Tohoku (Sanriku area) can be explained by decaying postseismic deformation from pre-1996 earthquakes, the accelerations in south-central Tohoku appear unrelated to postseismic effects. The latter accelerations are associated with a decrease in average trench-normal strain rate and can be explained by increasing slip rate on the Japan trench plate interface and/or updip migration of deep aseismic slip in the decades before the 2011 Tohoku-oki earthquake.
Given the unprecedented nature of this transient slip event, independent confirmation of accelerating slip is required. We show that changes in the recurrence intervals of repeating earthquakes on the Japan Trench megathrust in the period 1996 to 2011 are consistent with accelerating slip preceding the Tohoku-oki earthquake. All sequences of repeating earthquakes with statistically significant trends in recurrence interval offshore south-central Tohoku occurred at an accelerating rate. Furthermore, estimates of the magnitude of slip acceleration from repeating earthquakes are consistent with the completely independent geodetic estimates. From a joint inversion of the GPS and seismicity data, we infer that a substantial portion of the megathrust experienced accelerating slip, partly surrounding the eventual rupture zone of the Mw 9 earthquake.