An Examination of Hypocenter of the 1933 Showa-Sanriku Earthquake

Abstract

There are large discrepancies between the epicenters of the 1933 Showa Sanriku earthquake (M_W 8.4) reported by the Japan Meteorological Agency (JMA) and those reported by other studies. The JMA hypocenter (39.13˚N, 145.12˚E, with depth fixed at 0 km) is more than 30 km east of the other reported epicenters. The JMA location is based on local P and S wave arrival data, whereas the others are based on global arrival data. I examined the cause of the discrepancies and found that the earthquake has a multiple process and consisted of a preliminary rupture and a main rupture that occurred about 4-6 s apart; the JMA location represents the epicenter of the preliminary rupture, whereas the other studies used both preliminary rupture and main rupture arrivals to locate epicenters that made the shift of epicenter to the west and nearer the Japan Trench. I carefully selected the arrival data and estimated hypocenter of the main rupture at 39.2˚N, 145.0˚E, with depth fixed at 0 km. This result shows that the mainshock originated in the outer rise area more than 60 km east of the trench axis which is consistent with the fact that the locations of all recent M>7 normal fault earthquakes in the outer rise area are similarly distant from the trench axis in the study area. The 2011 Tohoku earthquake was also a multiple-shock, with the JMA epicenter located by using wave arrivals of the preliminary rupture, whereas the epicenter of the international agencies, ISC and NEIC, which use global arrival data, were located using arrivals both the preliminary and the main rupture, causing a significant shift of the epicenter to the west. If I also refer to the results of other studies that take into account 3D structures and other factors, the true epicenter is still presumed to be in the vicinity of the JMA epicenter.

Although it is now clear that the main shock of the 1933 Showa Sanriku earthquake occurred in the outer rise area outside of the Japan trench, it is difficult to explain the preceding seismic activity on the northern side of the aftershock region in January 1933, the epicenter of the main shock, the distribution of aftershocks covering both sides of the trench, the estimated tsunami source area extending west of the trench, and the upward tsunami observed on the coast in a unified manner using only normal faulting of the main shock; the aftershock activity suggests the existence of a hidden thrust earthquake along the plate boundary. However, the upward tsunami on the coast cannot be explained by a thrust earthquake, but may be explained by the high angle splay fault that was activated by the thrust earthquake. From the above, we do not know which occurred first, but it is highly likely that the 1933 Showa Sanriku earthquake was a doublet, consisting of a normal fault main shock and a thrust earthquake with slip of splay faults.