Spatiotemporal Variations of Seismic Quiescence prior to the 2011 M 9.0 Tohoku Earthquake Revealed by an Improved Region-Time-Length Algorithm

The region–time–length (RTL) algorithm, which takes into account the epicenter, time, and magnitude of earthquakes, is an effective technique in detecting seismic quiescence. In order to reduce the possible ambiguity due to the selection of model parameters in the RTL algorithm, we propose an improved technique of search-ing for the optimal model parameters. As a case study, we investigated the spatiotem-poral variations of seismic quiescence prior to the M 9.0 Tohoku earthquake by applying the improved RTL algorithm to the declustered and complete earthquake catalog of the Japan Meteorological Agency (JMA). The temporal variation of the RTL parameter at the epicenter indicated that a seismic quiescence occurred between 2006 and 2008. The spatial distribution showed the consistent time window of the appearance of anomalous quiescence near the epicenter. The significance of the seis-mic quiescence anomaly was supported by the close investigations of model param-eter effects and the stochastic test based on randomized earthquake catalogs.