Instrumental magnitudes of historical earthquakes, 1892 to 1898

Abstract

In the 1890s of the dawn of instrumental seismology, a variety of primitive but sophisticated seismographs were erected in Europe. The quality varies greatly from instrument to instrument. In use of those records, the instrument magnification needs to be calibrated with extreme caution, because of the lack of damping. From comparison of the trace amplitudes measured by such instruments with those by reliable instruments, the effective gain for surface-wave maxima of distant shocks is determined here for each of the vertical or horizontal pendulums that were operated at Catania, Rocca di Papa, Ischia, Roma, Padua, Nicolajew, Charkow, Strassburg, and Shide. The effective gain is found to be greatly different from the static magnification. The period of surface-wave maxima is on average 17 sec. The results are applied to reported amplitudes for determining the surface-wave magnitude of 53 major earthquakes recorded from 1892 to 1898. The magnitude of the destructive tsunami earthquake that occurred off northeastern Honshu, Japan, on 15 June 1896 is determined to be 7.2. The great Assam, India earthquake of 12 June 1897 has a magnitude of 8.0. The present study demonstrates that despite various difficulties, historical records, if properly interpreted, provide invaluable information.