Small interseismic asperities and widespread aseismic creep on the northern Japan subduction interface

The canonical model of fault coupling assumes that slip is partitioned into fixed asperities that display stick-slip behavior and regions that creep stably. We show that this simple asperity model is inconsistent with GPS-derived deformation in northern Japan associated with interseismic coupling on the subduction interface and the transient response to M-w 6.3-7.2 earthquakes during 2003-2011. Comparisons of GPS data with simulations of earthquakes on asperities and associated velocity-strengthening afterslip require that afterslip overlaps areas of the fault that ruptured in previous earthquakes, including the 2011 M-w 9 Tohoku-oki earthquake. Whereas about 55% of the plate interface ruptured in earthquakes during 2003-2011, we infer that only 9% of the plate interface was fully locked between earthquakes. Inferred locked asperities are roughly 25% the size of rupture areas determined by seismic source inversions. These smaller asperities are consistent with interseismic strain accumulation in 2009, although more extensive locking is required a decade earlier in 1998.