Relocation of the M∼7 Miyagi-oki Earthquakes in the 1930s: Seismic Slips of Asperities that Were Re-ruptured during the 1978 M7.4 Miyagi-oki Earthquake ?

Abstract

Hypocenters of the main shocks and aftershocks of the 1933 M_jma 7.1, 1936 M_jma 7.4, 1937 M_jma 7.1, 1939 M_jma 6.9, 1978 M_jma 7.4, and 1981 M_jma 7.0 Miyagi-oki, NE Japan, earthquakes are relocated by 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 Mizusawa station of National Astronomical Observatory of Japan (NAOJ) and at Mukaiyama station of Tohoku University. In order to avoid large errors caused by inaccuracies in the arrival times of P- and S-waves and the limited number of observation stations, we determined hypocenters by using a grid search method based on the assumption that these events occurred at the boundary between the subducting Pacific plate and the overriding plate. The main shock epicenters of the 1933, 1936, 1937, and 1978 earthquakes are determined close to each other, and distributions of their aftershocks show that aftershock areas of 1933, 1936, and 1937 events partly overlap with that of the 1978 event and occupy its easternmost, central, and westernmost portions, respectively. It is likely that the 1933, 1936, and 1937 events possibly ruptured a part of the source area of the 1978 event, that is the eastern, central, and western portions, respectively. Locations of the main shock and aftershock area of the 1939 event are adjacent to the eastern edge of the source area of 1978 event. After the 1978 event, the 1981 earthquake had occurred there following the slip on the asperities in the presumed Miyagi-oki earthquake source area.