Abstract

Fluid migration and pore pressure changes within the Earth are key to understanding earthquake occurrences. In this study, we investigated the spatiotemporal characteristics of intensive foreshock and aftershock activity for the 2017 M 5.3 earthquake in Kagoshima Bay, southern Japan, to examine the physical process governing this earthquake sequence. We determined that foreshock hypocenters moved slowly on a sharply-defined steeply-dipping plane, which probably represents the same plane of the mainshock source fault. The mainshock hypocenter was located at an edge of a seismic gap formed by foreshocks along the plane, suggesting that the mainshock ruptured this seismic gap. Aftershock hypocenters, distributed along several steeply-dipping planes exhibited an overall upward migration. Aftershock activity slightly deviated from a simple mainshock-aftershock type, suggesting the existence of an aseismic process behind this earthquake sequence. We propose a hypothesis that consistently explains these observations. First, fluids rose from the deeper portion and intruded into the fault plane, reduced the fault strength, and caused the foreshock sequence, as well as, possible aseismic slips. An area with a relatively high fault strength on the plane existed, where the mainshock rupture finally occurred due to a continuous decrease in the fault strength associated with increasing pore pressure and an increase in the shear stress associated with the aseismic slip and foreshocks. The change in the pore pressure associated with post-failure fluid discharge contributed to the aftershock activity, causing upward fluid migration. These observations show the importance of fluid movement at depth, when attempting to understand the earthquake cycle.