Earth Planets Space, Vol. 63 (No. 7), pp. 589-594, 2011
Hisashi Nakahara1, Haruo Sato1, Takeshi Nishimura1, and Hiroyuki Fujiwara2
1Department of Geophysics, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan
2National Research Institute for Earth Science and Disaster Prevention, Tsukuba 305-0006, Japan
(Received April 25, 2011; Revised May 31, 2011; Accepted June 2, 2011; Online published September 27, 2011)
A great earthquake of Mw 9.0 occurred on March 11, 2011 off the coast of Tohoku region, Northeast Honshu, Japan. Strong ground motions from the earthquake were recorded at 4 stations of a small seismic array, with an aperture of about 500 m, located 120 km away from the epicenter. Peak ground acceleration exceed the full scale of 2 g on the horizontal components, and was larger than 1 g even on the vertical component. Two prominent bursts and at least two following smaller bursts are identified on the strong-motion records which lasted for longer than 200 s. We have performed semblance analysis to estimate the rupture propagation during the earthquake using coherent seismograms at frequencies of 0.5-2 Hz. The rupture seems to consist of at least four stages. Rupture propagated in a northerly direction in the beginning 50 s forming the first burst, then proceeded to the southwest from the epicenter in the next 50 s during the second burst. The rupture further extended southwests in the following 40 s, and finally migrated to the south for about 30 s. A small seismic array makes it possible to observe rupture propagation during a large earthquake even with a small number of stations.
Key words: The 2011 off the Pacific coast of Tohoku Earthquake, rupture propagation, array, semblance.