Time-dependent earthquake probability calculations for southern Kanto after the 2011 M9.0 Tohoku earthquake

Abstract

S U M M A R Y Seismicity in southern Kanto activated with the 2011 March 11 Tohoku earthquake of magnitude M9.0, but does this cause a significant difference in the probability of more earthquakes at the present or in the future? To answer this question, we examine the effect of a change in the seismicity rate on the probability of earthquakes. Our data set is from the JapanMeteorological Agency earthquake catalogue, downloaded on 2012 May 30. Our approach is based on timedependent earthquake probabilistic calculations, often used for aftershock hazard assessment, and are based on two statistical laws: the Gutenberg–Richter (GR) frequency–magnitude law and the Omori–Utsu (OU) aftershock-decay law. We first confirm that the seismicity following a quake of M4 or larger is well modelled by the GR law with b ∼ 1. Then, there is good agreement with the OU law with p ∼ 0.5, which indicates that the slow decay was notably significant. Based on these results, we then calculate the most probable estimates of future M6–7-class events for various periods, all with a starting date of 2012 May 30. The estimates are higher than pre-quake levels if we consider a period of 3-yr duration or shorter. However, for statistics-based forecasting such as this, errors that arise from parameter estimation must be considered. Taking into account the contribution of these errors to the probability calculations, we conclude that any increase in the probability of earthquakes is insignificant. Although we try to avoid overstating the change in probability, our observations combined with results from previous studies support the likelihood that afterslip (fault creep) in southern Kanto will slowly relax a stress step caused by the Tohoku earthquake. This afterslip in turn reminds us of the potential for stress redistribution to the surrounding regions. We note the importance of varying hazards not only in time but also in space to improve the probabilistic seismic hazard assessment for southern Kanto.