Spatial distribution and focal mechanisms of aftershocks of the 2011 off the Pacific coast of Tohoku Earthquake

Youichi Asano,Shin Aoi,Tatsuhiko Saito,Katsuhiko Shiomi,Hitoshi Hirose,Shoji Sekiguchi,Takumi Matsumoto,Yoshihiro Ito,Sadaki Hori

doi:10.5047/eps.2011.06.016

Abstract

We estimated centroid moment tensors of earthquakes that occurred from 2003 to 2011 in and around the focal area of the 2011 Mw 9.0 megathrust earthquake in eastern Japan. The result indicates that earthquakes occurring before the mainshock, which included foreshocks off Miyagi, were basically interplate earthquakes with thrust-type focal mechanisms. On the other hand, the aftershocks exhibited a variety of focal mechanisms. Interplate aftershocks with thrust focal mechanisms did not occur within the large coseismic slip area estimated from GPS data but instead occurred in the surrounding regions. This implies that slip could no longer occur in the coseismic slip area due to the large amount of stress release during the mainshock rupture, whereas the aftershocks in the surrounding regions were caused by a stress concentration in these regions due to the large co-seismic slip associated with the mainshock asperity. Normal-fault-type aftershocks were widely distributed in the overriding plate and the outer-rise of the Pacific Plate. These aftershocks may have been due to a tensional stress change caused by the coseismic slip. Thrust-fault-type aftershocks in the subducting Pacific Plate were also interpreted as being due to compressional stress change as a result of the coseismic slip.

Highlights

The 2011 off the Pacific coast of Tohoku Earthquake with a moment magnitude (Mw) of 9.0 (e.g., Japan Meteorological Agency, 2011; United States Geological Survey, 2011) occurred at 5:46 (UTC) on March 11, 2011 along the boundary between the subducting Pacific Plate and the overriding plate
In order to determine the detailed spatial distribution of the different types of aftershocks, we evaluated Kagan’s angles (Kagan, 1991), which are the minimum rotation angles of the focal mechanisms relative to a reference mechanism corresponding to a typical interplate earthquake in this region (Figs. 2(e) and (f))
The results indicate that the Kagan’s angles of earthquakes occurring before the mainshock are mainly within 30 to 40◦, especially for the earthquakes that occurred within 20 km from the plate boundary inferred from the depth distribution of the upper seismic plane along the subducting Pacific Plate (Hasegawa et al, 1994)

Summary

Introduction

The 2011 off the Pacific coast of Tohoku Earthquake with a moment magnitude (Mw) of 9.0 (e.g., Japan Meteorological Agency, 2011; United States Geological Survey, 2011) occurred at 5:46 (UTC) on March 11, 2011 along the boundary between the subducting Pacific Plate and the overriding plate. Ito et al (2006), on the other hand, successfully estimated centroid moment tensor solutions providing centroid locations in addition to mechanism parameters using newly developed grid search and inversion techniques They applied their method to data for inland earthquakes observed at stations distributed in onesided regions to simulate the station coverage of off-shore earthquakes, and compared the obtained centroid locations with the hypocenters precisely determined by dense observations. The Mw 7.3 earthquake that occurred on March 9, 2011, and its 38 aftershocks, which can be considered to be the foreshocks of the Mw 9.0 event, had mainly thrust-type focal mechanisms and were distributed along the plate boundary in an approximately 90 × 70 km region off Miyagi (Fig. 2(a)). An Mw 7.6 normal-fault-type earthquake occurred in the outer-rise of the Pacific plate at 6:26 on March 9, an Mw 7.1 thrust-type earthquake with a larger dip angle occurred in the subducting slab at 14:32 on April 7, and an Mw 6.7 normal-fault-type earthquake occurred in eastern Fukushima at 8:16 on April 11 (Figs. 2(b) and (d))

Detailed Distribution of Interplate and Other Aftershocks

Conclusions