Source Mechanism of an Intraslab Earthquake in Thephilippine Sea Plate; the M5.6 Earthquake under Eastern Mino, Gifu Prefecture, Japan

An M5.6 earthquake occurred in the Philippine Sea plate (PHS) beneath the city of Mizunami, Gifu prefecture, Japan on December 14, 2011. The dense seismic network of the Tono Research Institute of Earthquake Science (TRIES) observed the earthquake and 17 of the aftershocks around the epicenter under eastern Mino. We investigated in detail the distribution and focal mechanisms of the events to discuss the stress field in the PHS and tectonics of subduction. Following the initial shock (M1.6; Focal depth: 48.1 km), the main shock (M5.6) occurred at approximately 6 km below the upper surface of the PHS (Focal depth: 47.7 km). This aftershock activity extended downward to the vicinity of the oceanic Moho on a 54° eastward-dipping plane. The focal mechanisms of most events have characteristics of normal faulting with the T-axis in the E-W (margin-parallel) direction. We concluded that a convex downward shape of the PHS, and the consequent margin-parallel tensional stress field probably cause these normal faulting earthquakes.