Hypocenter Migration and Seismicity Pattern Change in the Yamagata-Fukushima Border, NE Japan, Caused by Fluid Movement and Pore Pressure Variation

The spatiotemporal distributions of hypocenters and temporal change in seismicity patterns were investigated in details for events in the Yamagata-Fukushima border earthquake swarm, which was a remotely triggered earthquake sequence by the 2011 Tohoku-Oki earthquake. We relocated the hypocenters by applying the double-difference location method to differential arrival time data obtained by waveform cross correlations together with the Japan Meteorological Agency catalogue data. The hypocenter distribution obtained clearly shows that the hypocenters are concentrated on several discrete planes and migrate along those planes from deeper to shallower levels instead of diffusing isotropically. Most of the events have nodal planes of focal mechanisms parallel to those discrete planes, suggesting that ruptures of individual events occurred along those macroscopic planes. The timing of earthquake occurrences is almost random during the initial ~50 days, but it gradually becomes temporally clustered in later periods, with the number of events decaying aftershock-like after relatively large events. These observations suggest that the present swarm is caused by a reduction in frictional strength due to the increased pore pressure of fluids rising from greater depths in response to the decrease in arc-normal compressional stress associated with the Tohoku-Oki earthquake. The fluids permeated into several existing planes, reduced frictional strengths, caused the present earthquake swarm, and migrated upward along the planes with the hypocenters. Our previous observations that stress drops are systematically low and b-values are high during the initial ~50 days can be readily explained if the pore pressure is especially high during that period.