Earth Planets Space, Vol. 58 (No. 12), pp.1587-1592, 2006LETTER
Norihito Umino1, Toshio Kono1, Tomomi Okada1, Junichi Nakajima1, Toru Matsuzawa1, Naoki Uchida1, Akira Hasegawa1, Yoshiaki Tamura2, and Gen Aoki3
1Research Center for Prediction of Earthquakes and Volcanic Eruptions, Graduate School of Science, Tohoku University, Aramaki Aza Aoba, Aoba-ku, Sendai, Miyagi 980-8578, Japan
2National Astronomical Observatory of Japan, Hoshigaoka 2-12, Mizusawa, Iwate 023-0861, Japan
3Sendai District Meteorological Observatory, Gorin 1-3-15, Miyagino-ku, Sendai, Miyagi 983-0842, Japan
(Received November 30, 2005; Revised February 18, 2006; Accepted February 24, 2006; Online published February 2, 2007)
Hypocenters of main shocks and aftershocks of the 1933 M=7.1, 1936 M=7.4, 1937 M=7.1 and 1978 M=7.4 Miyagi-oki earthquakes are relocated using S-P times reported in the Seismological Bulletin of the Japan Meteorological Agency (JMA) and those re-read from original smoked-paper seismograms observed at the Mizusawa station of the National Astronomical Observatory of Japan (NAOJ) and the Mukaiyama station of Tohoku University. In order to reduce the error caused by inaccuracies of the arrival times and the small number of seismic observation stations, we determined the hypocenters by using a grid search method that assumed that the events occurred at the boundary between the subducting Pacific plate and the overriding plate. The main shock epicenters of these four earthquakes were determined to be close to each other, while the distributions of their aftershocks seem to disperse on the upper boundary of the Pacific plate. These distributions show that aftershock areas of the 1933, 1936 and 1937 events partly overlap with that of the 1978 event and occupy its easternmost, central and westernmost portions, respectively. This result suggests that the 1933, 1936 and 1937 events possibly ruptured a part of the source area of the 1978 event, i.e., its eastern, central and western portions, respectively.
Key words: Miyagi-oki earthquake, asperity, interplate earthquake, subduction zone, smoked-paper seismogram.