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

Abstract

We estimated centroid moment tensors of earthquakes that occurred from 2003 to 2011 in and around the focal area of the 2011 M w 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 thrusttype 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.

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

“…There is low seismicity in the region where a large slip during the mainshock is estimated , which is consistent with the result of Asano et al (2011). Especially there was little seismicity at the plate boundary.…”

“…After the mainshock, seismic activity became higher and normal fault type, or strike-slip type, events were dominant in the landward plate region. Occurrence of normal fault type, or strike-slip type, events in the landward plate have also been reported by Asano et al (2011) using landbased data. A change of focal mechanisms in the landward plate is thought to correspond with a stress change caused by a large slip at the plate boundary Kato et al, 2011;Imanishi et al, 2012;Yoshida et al, 2012).…”

Seismicity near the hypocenter of the 2011 off the Pacific coast of Tohoku earthquake deduced by using ocean bottom seismographic data

“…Most of the off-plane earthquakes identified from the focal depth distribution were not thrust type but tended to have normal-fault type mechanisms (Fig. 5(b)), as reported by Asano et al (2011). Other non-thrust type earthquakes included a large intraslab earthquake (M 7.1, April 7) and a normal faulting shallow earthquake (M 6.5, March 28) followed by their own aftershocks.…”

Shear-wave splitting in a region with newly-activated seismicity after the 2011 Tohoku earthquake

“…This earthquake occurred along a plate boundary where the Pacific Plate is being subducted to the WNW beneath the Japanese Islands along the Japan Trench at a velocity of 3-5 cm/year (e.g., Seno et al, 1993). The 2011 Tohoku earthquake was a subduction-type earthquake that occurred along the entirety of the Japan Trench, and was associated with a 400-kmlong and 200-km-wide fault (e.g., Asano et al, 2011). In Japan, a high-spatial-density seismic network was recently constructed to cover the entirety of the country at an average spacing of about 20 km (Okada et al, 2004), and the 2011 Tohoku earthquake was the largest seismic event since the establishment of this network.…”