Distribution and migration of aftershocks of the 2010 Mw 7.4 Ogasawara Islands intraplate normal-faulting earthquake related to a fracture zone in the Pacific plate

We describe the aftershocks of a Mw 7.4 intraplate normal‐faulting earthquake that occurred 150 km east Ogasawara (Bonin) Islands, Japan, on 21 December 2010. It occurred beneath the outer trench slope of the Izu‐Ogasawara trench, where the Pacific plate subducts beneath the Philippine Sea plate. Aftershock observations using ocean bottom seismographs (OBSs) began soon after the earthquake and multichannel seismic reflection surveys were conducted across the aftershock area. Aftershocks were distributed in a NW‐SE belt 140 km long, oblique to the N‐S trench axis. They formed three subparallel lineations along a fracture zone in the Pacific plate. The OBS observations combined with data from stations on Chichi‐jima and Haha‐jima Islands revealed a migration of the aftershock activity. The first hour, which likely outlines the main shock rupture, was limited to an 80 km long area in the central part of the subsequent aftershock area. The first hour activity occurred mainly around, and appears to have been influenced by, nearby large seamounts and oceanic plateau, such as the Ogasawara Plateau and the Uyeda Ridge. Over the following days, the aftershocks expanded beyond or into these seamounts and plateau. The aftershock distribution and migration suggest that crustal heterogeneities related to a fracture zone and large seamounts and oceanic plateau in the incoming Pacific plate affected the rupture of the main shock. Such preexisting structures may influence intraplate normal‐faulting earthquakes in other regions of plate flexure prior to subduction. Key Points OBS aftershock observations for a Mw 7.4 outer‐trench normal‐faulting earthquake Aftershock area oblique to the trench axis expanded after the main shock Structure heterogeneities related to a fracture zone affect the outer‐trench EQ