Historical Seismograms - Evidence from the AD 2000 Izu Islands ...

Catalog of Large and Damaging Earthquakes in Japan 151earthquakes from 1885 to the present. By comparing the patterns of isoseismals ofearthquakes of known and unknown hypocenters, we can estimate the location ofearlier earthquakes. But this method sometimes fails for deep earthquakes, whichexhibit anomalous intensity distributions due to the effect of the descending Pacificplate or Philippine Sea plate beneath the Japanese Islands. Actually, few deepearthquakes have been located for the period before 1900.The magnitudes were calculated by the same method as presently adopted by theJMA. The maximum amplitudes measured by old-fashioned seismographs, many ofwhich have no damping devices, may be systematically different from those measuredby modern instruments. However, no appreciable systematic difference isevident between my determinations and the magnitudes given by Gutenberg andRichter (1954) or those by Abe (1979). For earthquakes before about 1900 forwhich few data on the maximum amplitudes were available, the magnitude were estimatedby the comparison of the distributions of seismic intensities. Consequently,the magnitudes of secalled low-frequency earthquakes occurring before about 1900may be considerably smaller. The low-frequency earthquake has a relatively smallfelt area for its instrumental magnitude determined from the amplitudes of mediumor long-period seismic waves (Utsu, 1980).Figure 1 shows an example of the hypocenter and magnitude determination forthe new catalog. The old CMO catalog indicates the epicenter located off the Pacificcoast of eastern Honshu, but the S-P times at several stations clearly indicate thatthis is a deep earthquake beneath the Sea of Japan. This earthquake (08:06, Jan.21, 1916) is listed neither in Gutenberg and Richter (1954) nor in the Bulletin ofthe British Association.I f 1916 Jan. 2108h 06mvPresent determination 7 )I h = 300 kmreaM 0:erFigure 1. An example of the redetermination of hypocenters and magnitudes for thenew catalog. The radii of arcs correspond to the S-P times for h = 300 km from thestandard travel-time tables presently used in JMA.