Earth Planets Space, Vol. 63 (No. 7), pp. 583-588, 2011
Ryou Honda1, Yohei Yukutake1, Hiroshi Ito1, Masatake Harada1, Tamotsu Aketagawa1, Akio Yoshida1, Shin'ichi Sakai2, Shigeki Nakagawa2, Naoshi Hirata2, Kazushige Obara2, and Hisanori Kimura3
1Hot Springs Research Institute of Kanagawa Prefecture, 586 Iriuda, Odawara, Kanagawa 250-0031, Japan
2Earthquake Research Institute, University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo 113-0032, Japan
3National Research Institute for Earth Science and Disaster Prevention, 3-1 Tennodai, Tsukuba, Ibaraki 305-006, Japan
(Received April 10, 2011; Revised May 18, 2011; Accepted May 29, 2011; Online published September 27, 2011)
Strong ground motions from the 2011 off the Pacific coast of Tohoku Earthquake, the most powerful earthquake to have occurred in and around Japan after the installation of a modern seismic network, were recorded for more than 300 seconds by a dense and wide-span seismic network, the Metropolitan Seismic Observation Network (MeSO-net), installed around the Tokyo metropolitan area about 200 km away from the epicenter. We investigate the rupture process of the earthquake in space and time by performing semblance-enhanced stacking analysis of the waveforms in a frequency range of 0.05 to 0.5 Hz. By projecting the power of the stacked waveforms to an assumed fault plane, the rupture propagation image of the large and complex earthquake has been successfully obtained. The seismic energy was mainly generated from the off-shore areas of about 100 km away from the coast in Miyagi and Fukushima Prefectures. The shallow and eastern part of the fault along the Japan trench off Miyagi Prefecture released strong seismic energy which might have been related to the excitation of gigantic tsunami. In contrast, the southern shallow part of the fault plane, off Ibaraki Prefecture, released only minor seismic energy. Our analysis suggests that the focal areas combining both the officially-forecasted Miyagi-oki earthquake and those of historical earthquakes that occurred off the coast of Fukushima Prefecture in 1938 were broken, resulting in the 2011 great M 9 earthquake.
Key words: The 2011 Tohoku Earthquake, rupture images, back projection, semblance, MeSO-net.