TERRAPUB Earth, Planets and Space
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Earth Planets Space, Vol. 63 (No. 7), pp. 663-667, 2011
doi:10.5047/eps.2011.05.018

LETTER

The 2011 off the Pacific coast of Tohoku Earthquake related to a strong velocity gradient with the Pacific plate

Makoto Matsubara1 and Kazushige Obara2

1National Research Institute for Earth Science and Disaster Prevention, Tsukuba, Japan
2Earthquake Research Institute, the University of Tokyo, Tokyo, Japan

(Received April 10, 2011; Revised May 15, 2011; Accepted May 18, 2011; Online published September 27, 2011)

Abstract: We conduct seismic tomography using arrival time data picked by NIED Hi-net, including earthquakes off the coast, outside the seismic network. For these offshore events, we use the NIED F-net focal depth. We detect two low-V zones in the uppermost subducting oceanic crust. The landward low-V zone with a large anomaly corresponds to the western edge of the coseismic slip zone of the 2011 off the Pacific coast of Tohoku Earthquake. The asperities of the previously known Off-Miyagi and Off-Fukushima earthquakes with magnitudes around 7.0 are also located at the boundary of the low-V and the eastern high-V zones. The initial break point (hypocenter) is associated with the edge of a slightly low-V and low-Vp/Vs zone. The trenchward low-V and low-Vp/Vs zone extending southwestward from the hypocenter may indicate the existence of a subducted seamount. The high-V zone and low-Vp/Vs zone might have accumulated the strain and resulted in the huge coseismic slip zone of the 2011 Tohoku Earthquake. The low-V and low-Vp/Vs zone is a slight fluctuation within the high-V zone and might have acted as the initial break point of the 2011 Tohoku Earthquake.
Key words: Seismic tomography, the 2011 off the Pacific coast of Tohoku Earthquake, high-V coseismic slip zone, seamount, NIED Hi-net, NIED F-net, low-V oceanic crust, asperity.


Corresponding author E-mail: mkmatsu@bosai.go.jp


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