1996 年 49 巻 3 号 p. 375-387
Seismograms from the 1923 Kanto earthquake (M=7.9) and its aftershocks at the Yamagata observatory of JMA (The Japan Meteorological Agency) in Tohoku district, Japan, are examined. They were recorded by the Imamura-type strong motion seismograph. Horizontal-component records from the main shock and the 1924 Tanzawa earthquake (M=7.3), one of the largest aftershocks, are digitized and the instrumental characteristics of the seismographs are examined. Natural period To and damping ratio v of the instrument are evaluated to be 4.5s and 1.5 for both the NS and EW components from the free oscillation records and documents for the results of testing the instrumental response. The maximum displacement in EW component of 11.2cm is obtained for the main shock in the period range from 2 to 20s, after the instrument correction.
On the other hand, uncertainties of the instrumental characteristics remain for the seismograms from the 1923 Kanto earthquake observed at the Mukaiyama observatory of the Tohoku Imperial University in Sendai, [TAKEMURA et al. (1995)]. The Sendai city is located about 40km east from the Yamagata city. The epicentral distance and azimuth of the Mukaiyama observatory is not so different from those of the Yamagata observatory for the 1923 Kanto earthquake. It is found that the displacement records at Sendai and Yamagata have mostly the same amplitude for the recent moderately large earthquakes with almost the same location of epicenter as the 1923 Kanto earthquake. All the records were observed by the strong motion displacement seismographs of To=6s and v=8 both at the Yamagata observatory and at the Sendai district meteorological observatory of JMA. This fact indicates that the displacement at the Mukaiyama observatory in Sendai ought to show almost the same amplitude as one at the Yamagata observatory during the 1923 Kanto earthquake. Then, we redetermined To of the instrument at Mukaiyama observatory so that the amplitude of the displacement after the instrument correction is the same as that at the Yamagata observatory. Redetermined To is 5s in EW component, being meaningfully longer than the results estimated by TAKEMURA et al. (1995).