Aftershock observation of the 2011 off the Pacific coast of Tohoku Earthquake by using ocean bottom seismometer network

Abstract

The 2011 off the Pacific coast of Tohoku Earthquake occurred offshore of northeast Japan region on March 11th, 2011. In order to study the aftershock activity of this event, we started deployment of seventy-two ocean bottom seismometers (OBSs) four days after the mainshock. In the south of the source region, thirty-four longterm OBSs (LT-OBSs) had been deployed before the occurrence of the mainshock, and we recovered three LTOBSs to clarify the depth distribution of aftershocks. Using the data of OBSs, ninety-nine aftershocks were located. Most of the aftershocks were located in a depth range of 5-30 km and concentrate in the plate boundary region. In addition, aftershocks occurred within the subducting oceanic crust and the 6.2-km/s layer of the landward plate. No aftershocks were found in the mantle of the subducting plate. From the results of a previous seismic survey using OBSs and controlled sources, the subducting Philippine Sea plate is estimated to be in contact with the subducting Pacific plate. The southern end of the seismic activity region of the aftershocks corresponds to the contact region of two subducting plates. We infer that the rupture of the mainshock sequence was terminated at the oceanic plate contact region.

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

“…Lack of seismic activities in the colliding region of the two plates and high seismic activities in the front of a tip of the collision zone are also seen at the Hidaka Collision Zone, central Hokkaido, Japan (Iwasaki et al, 2004). In addition, Shinohara et al (2012) estimated several focal mechanisms of aftershocks, and there are earthquakes with a P-axis in the north-south direction in front of the uncertain region (Fig. 6).…”

“…7. Comparison of the position of the profile in this study with the aftershock epicenter distribution determined by the OBS network, focal mechanisms of aftershocks obtained by the OBS network (Shinohara et al, 2012) and by the automated moment tensor determination using the land seismic network (F-net) (Fukuyama et al, 1998), the slip distribution during the mainshock using strong motion data (Yoshida et al, 2011) and the slip distribution of the afterslip by the land GPS network as of December 2011 (Ozawa et al, 2011;Geospatial Information Authority of Japan, 2011). The red and yellow contours indicate the amount of co-seismic slips and afterslip, respectively.…”

“…Furthermore, Shinohara et al (2012) shows that the aftershock activity of the 2011 Tohoku earthquake is not seen in the region. They also show that there is a high aftershock activity in front of the region, which we could not resolve (Fig.…”

“…6. The obtained P-wave velocity structures and the aftershock distribution (Shinohara et al, 2012) along the profile. The aftershocks within a 20-km-wide box on both sides of the profile are projected onto the seismic survey profile.…”

“…The aftershocks within a 20-km-wide box on both sides of the profile are projected onto the seismic survey profile. Focal mechanisms determined by Shinohara et al (2012) are also projected on the profile. Red circles indicate the hypocenters of the aftershocks, and diameters of circles correspond to magnitudes.…”