Aftershock Distribution of the 2005 Miyagi-oki Earthquake (M 7.2) Revealed by Ocean Bottom Seismological Observation: Preliminary Results

Abstract

Hypocenter distribution of the 2005 Miyagi-oki earthquake (M 7.2) and its aftershocks were relocated by using ocean bottom seismographic (OBS) data. The mainshock was observed by 14 OBSs which were deployed in the focal area about one month prior to the occurrence. Additional 16 OBSs were deployed for aftershock observations. Relocated hypocenters concentrate along a landward dipping plane whose location and dip angle correspond well to the plate boundary defined by a previous study. Although the most significant aftershock activity concentrates around the rupture area of the mainshock where substantial amount of coseismic slip was retrieved, two moderate (M 6.3) earthquakes with numerous aftershocks were observed in the trenchward region. The hypocenters of those earthquakes were also located along the plate boundary and the epicenter locations correspond to the aftershock area of the 1981 inter plate earthquake (M 7.0). Most of the aftershocks near the rupture area form a very thin plane and have focal mechanism solutions of a thrust fault type almost identical to that of the mainshock, suggesting these aftershocks occurred along the rupture plane of the mainshock, the plate boundary. There is a conspicuous linear cluster of the aftershocks at the eastern edge of the rupture area. The focal depth distribution of this cluster is slightly thicker than those of the aftershocks around the mainshock hypocenter. The cluster contains a number of events with focal mechanisms other than the thrust type. These observations suggest that a number of aftershocks occurred not only along but also off the plate interface at the up-dip end of the mainshock rupture. The b-value of the aftershocks in the linear cluster is significantly larger than that of the aftershocks near the mainshock hypocenter. This difference may reflect the difference in characteristic scales of structural heterogeneity along the plate interface and that within the plates.