Abstract
Strong ground motions observed in the Kanto basin during the Tonankai Mw8.1 earthquake in 1944 have been estimated from historical seismographs at Togane, Otemachi (Tokyo) and Yokohama. The reproduced seismograms demonstrate large (>10 cm) and long-time (>600 s) shaking of long-period ground motions at a dominant period of about 7 to 12 s in the center of Tokyo during the Tonankai earthquake. Such long-period signals are strongly radiating from large earthquake and then amplified significantly in thick sediments of the Kanto basin overlying rigid bedrock. The velocity response spectrum of strong ground motions shows large and sharp peaks at a dominant period of about 12 s and amplitude of about 60 cm/s at Togane and about 30 cm/s at Yokohama. A large velocity response is found at Otemachi in the period of 8 s with amplitude of about 25 cm/s. These amplitude levels are roughly 2-2.5 times larger than those observed in Tokyo during the SE Off Kii Peninsula (Mw7.4) earthquake in 2004.
