Seismicity and geodetic anomalies in a wide area preceding the Niigata-Ken-Chuetsu earthquake of 23 October 2004, central Japan

The ordinary short-term occurrence rate of earthquakes in a region is predicted using the Epidemic Type Aftershock Sequence (ETAS) model of triggering seismicity. Any anomalous seismic activity, such as quiescence and activation, can be quantified by identifying significant deviation from the predicted rate. Such anomalies are revealed to have occurred during the three year period leading up to the 2004 Chuetsu earthquake of M6.8 (M(w)6.6), central Japan. The quiescence and activation, which took place in the zone of negative and positive increments of the Coulomb failure stress, respectively, were possibly caused by aseismic slip on or near the focal fault plane. Such slip is further supported by transient crustal movement around the rupture source. Time series records of the baseline distances between permanent GPS stations deviated from the predicted trends, with the deviations mostly consistent with the coseismic horizontal displacements of the stations due to the Chuetsu earthquake. The cumulated preceding slip is estimated to be roughly equivalent to M(w)6.