P-wave velocity structure in the southernmost source region of the 2011 Tohoku earthquakes, off the Boso Peninsula, deduced by an ocean bottom seismographic survey

Abstract

We present the result of a seismic experiment conducted using ocean bottom seismometers and controlled sources in the region off Ibaraki and the Boso Peninsula. This region is the southern edge of the rupture zone of the 2011 off the Pacific coast of Tohoku Earthquake. We estimated the P-wave seismic velocity structure beneath the profile using a 2-D ray-tracing method. The crustal structure in the southern area is more heterogeneous than that of the northern area. This heterogeneity is thought to be related with subducting the Philippine Sea plate (PHS). The plate boundary between the landward plate and the Pacific plate (PAC) is positioned at depths of 20 km at a distance of 170 km from the southern end of the profile. The subducting PHS is imaged on the southern part of the profile. However, we could not obtain a distinct image of the contact zone of PHS and PAC. The contact zone of PHS and PAC is estimated to have a large heterogeneity resulting from strong deformation due to the collision of the two plates. We infer that the termination of the rupture, and the large afterslip in the collision region, are caused by this strong heterogeneity.

Precise aftershock distribution of the 2011 off the Pacific coast of Tohoku Earthquake revealed by an ocean-bottom seismometer network

“…Although our location of the aftershocks was based on a one-dimensional velocity structure, systematic errors in hypocentral positions are estimated to be comparable to a few kilometers because the velocity structures for the location were derived from the results of these seismic surveys. In a cross-section of the hypocentral distribution projected on a vertical plane along the seismic surveys (Miura et al, 2003;Takahashi et al, 2004;Nakahigashi et al, 2012;Shinohara et al, in preparation), we can clearly see the relation between the aftershock distribution and the seismic velocity structure (Fig. 6).…”

“…In the southern region, the Philippine Sea plate is considered to be subducting below the landward plate (e.g. Nakahigashi et al, 2012). Before the mainshock, several seismic gaps near the edge of the Philippine Sea plate were found .…”

“…The velocity structures for the hypocenter location were derived from the results of Takahashi et al (2004) for the northern region, Miura et al (2003) for the middle region, and Nakahigashi et al (2012) for the southern region. The profiles were selected to be situated in the regions for hypocenter location.…”

“…The models were derived from the refraction experiment carried out using OBSs in the each region for the northern area; Miura et al, 2003 for the middle area; and Nakahigashi et al, 2012 for the southern area. )…”

Upper boundaries of the Pacific and Philippine Sea plates near the triple junction off the Boso Peninsula deduced from ocean-bottom seismic observations

“…For the deeper part of the PAC mantle, we assumed a V p of 8.35 km/s based on results of tomographic study near the Boso Peninsula ). The overlying crust and mantle consist of the NA and PHS plates, and we assumed crustal V p to be 5.5 km/s, which is an average velocity above the Moho based on results of a refraction study off the Boso Peninsula (Nakahigashi et al 2012). The boundary between the crust and mantle was assumed to remain at a constant depth of 20 km, as described by Nakahigashi et al (2012).…”