1923年関東地震に対する強震動地震学の成果

The 1923 Kanto earthquake (M=7.9) is one of the most destructive events in Japan. Many studies on seismology and earthquake engineering have been carried out for this earthquake. In one and half decade from 1990, investigations of its source process and strong ground motions had been especially advanced. This paper reviewed the results of the studies in this period. Records from Ewing seismoscope and Imamura seismograph at Hongo were restored and simulated to presume the long-period strong ground motion in Tokyo during the main shock. Other seismograms involving Imamura seismograph records which completely recorded ground motions from main shock and aftershocks at several stations were also revealed in this period. These seismograms were useful for elucidating the source process of the main shock having two big sub-events, whose locations were about 40 km distant on the fault plane, which occurred with time interval of 10 to 15 s after the initial small rupture duration of 3 to 5 s. This complex faulting process of the main shock caused the regional variations of strong ground motions in the near source region, which were estimated from the descriptions of personal experiences. The aftershock sequence was also clarified from these seismograms. Two M=7 class aftershocks were newly found to have occurred at 3 and 4.5 minutes after the main shock, so that total number of aftershocks with more than M=7 was six for the Kanto earthquake. The number of big aftershocks of the Kanto earthquake is meaningfully larger than the other M=8 class subduction events. The seismic intensity on the JMA scale due to the second shaking caused by the first big aftershock was also estimated to be 6 in Tokyo metropolitan area, though the analysis of the descriptions of personal experiences, which was almost the same as the first shaking by the main shock. Besides above studies, the precise seismic intensity distribution of the Kanto earthquake was obtained in this period, after raveling out many confused damage statistics data.