Numerical simulation of the Kamaishi repeating earthquake sequence: Change in magnitude due to the 2011 Tohoku-oki earthquake

• We conducted numerical simulations of the Kamaishi repeating earthquakes sequence.
• The model assumes afterslip of the 2011 Tohoku-oki event.
• An increase of magnitude of the Kamaishi repeaters was reproduced.
• The model produced occurrence times of the repeaters similar to the observation.

We conducted numerical simulations of a repeating earthquake sequence on the plate boundary off the shore of Kamaishi, northeastern Japan, assuming rate- and state-dependent friction laws. Uchida et al. 2014 reported that the Kamaishi repeating earthquakes showed an increase in magnitude and a decrease in recurrence interval after the 2011 Tohoku-oki earthquake (M9), but an approximately constant magnitude (M ~ 4.9) and a regular recurrence interval (~ 5.5 years) before the Tohoku-oki earthquake. A M5.9 event occurred just after the M9 event, and was followed by a M5.5 event. We considered a fault patch of velocity-weakening friction, with frictional parameters leading to seismic slip confined in its central part of the patch. Afterslip due to the M9 event was involved in the model to increase the loading rate on the patch. The simulation successfully reproduced increasing magnitude and decreasing recurrence time caused by the afterslip. A M6 class event, in which seismic slip spread over the entire area of the patch, occurred just after the M9 event for the aging law and the Nagata law. When we assumed the aging law with frictional parameters which lie near the boundary between leading to slip on the entire patch and slip confined in its central part, the M6 class event was followed by a M5.5 class event. Furthermore we examined a conditionally stable large patch that contained a small unstable patch. This model also reproduced a M6 class event after the M9 event. In these models, stress outside the confined area of the path is released before a dynamic event under a constant low loading rate, whereas the stress perturbation due to afterslip within the seismic cycle induces a dynamic event on the entire patch.