Earth Planets Space, Vol. 58 (No. 12), pp. 1555-1559, 2006LETTER
Hisashi Nakahara1, Kaoru Sawazaki1, Nobumasa Takagi1, Takeshi Nishimura1, Haruo Sato1, 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 November 28, 2005; Revised May 20, 2006; Accepted June 2, 2006; Online published February 2, 2007)
An earthquake of Mw 7.2 took place on August 16, 2005 at a plate boundary between the Pacific plate and the North American plate off the coast of Miyagi Prefecture, Northeast Japan. During the Miyagi-Ken-Oki event, we succeeded in recording strong ground motions at six stations in a seismograhic array with an epicentral distance of about 70 km, where we have been operating seven strong-motion seismometers in an aperture of about 500 m since April 2004. The predominant period of the ground motion was shorter than 0.3 s. The peak ground acceleration exceeded 1.7 g at a station where non-linear site response may have occurred during the mainshock. The short-period strong ground motions show a large spatial variation, with up to a ten-fold difference in amplitude even within the array. However, there is a similarity between waveforms registered at different stations for periods longer than 0.4 s. Therefore, the difference in the ground motions may be mainly attributed to the difference in the shallow structure just beneath the stations.
Key words: Miyagi-Ken-Oki earthquake, array, strong ground motion, spatial variation, non-linear site response.