Decadal-Scale Decoupling of the Japan Trench Prior to the 2011 Tohoku-Oki Earthquake from Geodetic and Repeating-Earthquake Observations

Abstract:

We report geodetic evidence for decadal-scale decoupling of the Japan Trench prior to the 2011 M9 Tohoku earthquake, with supporting evidence from repeating earthquakes. We build on the results of Mavrommatis et al. (2014, GRL), where we investigated GPS time series for 15 years before the M9 event, correcting for the copious postseismic deformation following numerous M ~ 6.5+ earthquakes between 2003 and 2011. We modeled interseismic deformation as linear trends plus constant accelerations and corrected for coseismic and postseismic effects of M 6.3+ quakes during the period 1996 to 2011. We find spatially coherent and statistically significant accelerations throughout northern Honshu.

While the accelerations in northern Tohoku can be explained by decaying postseismic deformation following the 1994 M7.7 Sanriku quake, the accelerations in south-central Tohoku cannot be explained simply by postseismic effects. Time series in south-central Tohoku reveal an approximately continuous transient spanning the period 1996 to 2011, without any obvious onset. The transient is consistent with a decrease in the average shear strain rate by ~1/3 in 15 years, and can be explained by increasing average slip rate and/or updip migration of the locked-to-creeping transition depth. MCMC inversions of instantaneous velocities in 1996 and 2011 imply a shallowing of the transition depth by 13.5 +/- 9.5 km and an increase in the average slip rate in the depth range from 22 km to the transition depth by 48.5 +/- 10.5 mm/yr.

To test these predictions and to provide additional constraints on the changes in slip rate, we use independent observations of repeating earthquakes during the same period. We find that several sequences of repeaters offshore south-central Tohoku exhibit decreasing recurrence intervals, possibly implying increasing slip rate, while sequences offshore Sanriku exhibit increasing recurrence intervals, consistent with decaying afterslip from the 1994 M7.7 Sanriku event. This along-strike dichotomy is consistent with an inversion of the GPS accelerations and suggests that some change in slip rate in depths shallower than ~60 km is required by the data. Further analysis of the repeating earthquakes will provide additional constraints to discriminate between such models.