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

Abstract

The 2011 off the Pacific coast of Tohoku Earthquake occurred at the plate boundary between the Pacific plate and the landward plate on March 11, 2011, and had a magnitude of 9. Many aftershocks occurred following the mainshock. Obtaining a precise aftershock distribution is important for understanding the mechanism of earthquake generation. In order to study the aftershock activity of this event, we carried out extensive seafloor aftershock observations using more than 100 ocean-bottom seismometers just after the mainshock. A precise aftershock distribution for approximately three months over the whole source area was obtained from the observations. The aftershocks form a plane dipping landward over the whole area, nevertheless the epicenter distribution is not uniform. Comparing seismic velocity structures, there is no aftershock along the plate boundary where a large slip during the mainshock is estimated. Activity of aftershocks in the landward plate in the source region was high and normal fault-type, and strike-slip-type, mechanisms are dominant. Within the subducting oceanic plate, most earthquakes have also a normal fault-type, or strike-slip-type, mechanism. The stress fields in and around the source region change as a result of the mainshock.

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

“…Previous seismic observations Lon. along the Japan Trench forearc region show that hypocenters relocated using OBS data tend to be shallower than those determined using only land-based station data (e.g., Shinohara et al 2012a), and that is also the case in this study. The earthquakes with depths of 40-60 km that are clustered around 140.6°E are exceptionally deep relative to those determined by JMA.…”

“…Few earthquakes were located in the sedimentary layers represented by the red and yellow layers. An absence of earthquakes in sedimentary layers along the Japan Trench forearc region has also been reported previously (e.g., Shinohara et al 2012a;Obana et al 2013). …”

“…Two of the BBOBSs were equipped with differential pressure gauges (DPGs) (Araki and Sugioka 2009). To cover the northern part of the Boso offshore area, we used seismic data from 50 OBSs of the ERI (Shinohara et al 2011(Shinohara et al , 2012a for the same period as the JAMSTEC OBSs were deployed. In addition to the OBS stations, we also used seismic data from 15 permanent land-based seismic stations on the Boso Peninsula operated by the National Research Institute for Earth Science and Disaster Prevention (NIED) (http://www.bosai.…”

“…The Earthquake Research Institute (ERI) of the University of Tokyo conducted aftershock observations following the 2011 Tohoku-Oki earthquake across a broad area of the Japan Trench forearc region (Shinohara et al 2011(Shinohara et al , 2012a), but they did not cover the majority of the region off the Boso Peninsula (south of 35.5°N). In this study, we determine hypocenter distributions and focal mechanisms of earthquakes off the Boso Peninsula, as close to the Japan Trench and Sagami Trough as possible, by using data from both OBSs and land-based stations.…”

Determination of Mmax from Background Seismicity and Moment Conservation

“…We also remove the M w 8.3 Hokkaido event of 2003 and its aftershocks. With a dip of 13°and seismogenic depth of 35 5 km, the interface is coupled to within about 200 km of the trench, which agrees with the limits of the aftershock distribution after the Tohoku-Oki earthquake (Shinohara et al, 2013). As with other subduction zones, we initially take the coupled fraction to be 45%.…”