Abstract
We digitized horizontal components of historical seismograms due to the 1923 Kanto earthquake and their six aftershocks obtained by an Imamura strong-motion seismograph at Tokushima JMA observatory, which is located at about 450km west of the epicenter. These seismograms were recorded on three smoke-papers. Because of no time marks on the recording papers, we inferred a paper speed from notes of recording start and end times on the recording papers. We estimate the absolute timing from commencement time of the events at Hongo [IMAMURA (1925)] using paper speed and start time. We could not find any free oscillation tests on the recording papers. So based on the note on the paper and previous studies on the same type instruments, we estimated the following instrument constants; natural periods are 4.2s in the west-east component and 5.0s in the south-north component and damping ratios in the both components are 0.1. Finally we obtained main shock ground motions after correcting the instrument response. The maximum displacements are about 14mm in the west-east component and about 46mm in the south-north component.
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