A Trial Estimation of Frictional Properties and Propagation Process By Focusing on Temporal Aperiodicity Off Kamaishi Just after the 2011 Tohoku Earthquake

At about 50 km depth on the subduction plate boundary offshore of Kamaishi, NE Japan, the M ∼ 4.9 earthquake sequence is composed of nine events that occurred since 1957 which have a strong periodicity (5.5 ± 0.7 yr) and constant size (M4.9 ± 0.2), probably due to stable sliding around the source area (asperity). Motivated by the fact that temporal earthquake aperiodicity was observed off Kamaishi just after the 2011 Tohoku earthquake, we perform numerical simulations of chain reactions due to postseismic slip of large earthquakes on the basis of rate- and state-dependent friction laws. Our results show that, (i) temporal aperiodicity can be well reproduced in the case of the aging-law rather than the slip- and Nagata-laws, (ii) the reason why the greater and smaller earthquakes off Kamaishi temporarily occurred may be explained by the difference between overall and partial rupturing of an asperity, (iii) the observed perturbation of the activated earthquake hypocenters mostly in the ESE-WNW direction may reflect the fact that the large postseismic slip of the 2011 Tohoku earthquake propagated from east-southeast to west-northwest off Kamaishi, (iv) the observed region of repeating earthquake quiescence may reflect strong plate coupling of megathrust earthquakes.