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Reevaluation of the turn-of-the-century seismicity peak

Kanamori, Hiroo and Abe, Katsuyuki (1979) Reevaluation of the turn-of-the-century seismicity peak. Journal of Geophysical Research B, 84 (B11). pp. 6131-6139. ISSN 0148-0227. doi:10.1029/JB084iB11p06131.

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According to currently available seismicity catalogues, seismicity (for example, the number of events with M_s ≥ 8) around the turn of the century, from 1897 to 1906, was significantly higher than in recent years. However, the magnitudes of the earthquakes which occurred during this period were determined by Gutenberg, who used the records obtained by the undamped Milne seismograph with the assumption that the effective magnification is 5. Because of saturation of the Milne seismogram for very large events used by Gutenberg for calibration, the gain (=5) used by Gutenberg could have been underestimated, and therefore the magnitude overestimated. Because of the lack of damping, the magnification of this instrument needs to be calibrated carefully. In order to calibrate the instrument response, a Milne seismograph was constructed and has been in operation side by side with damped seismographs at Pasadena. Eleven events have been recorded since February 1977. On the basis of (1) comparison of the amplitudes measured on the Milne seismograms with those of the standard seismograms, (2) numerical experiments simulating the response of the Milne seismographs to surface waves, and (3) examination of Gutenberg's original materials used for the calibration, we conclude that the average effective gain is as large as 20 for very large earthquakes, resulting in systematic reduction of the magnitude of up to 0.6. This reduction is large enough to suggest that the turn-of-the-century seismicity peak is of marginal significance.

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Kanamori, Hiroo0000-0001-8219-9428
Alternate Title:Re-evaluation of the turn-of-the-century seismicity peak
Additional Information:Copyright 1979 by the American Geophysical Union. (Received November 14, 1978; revised April 11, 1979; accepted May 18, 1979.) Paper number 9B0849. We thank N. Ambraseys of the Imperial College of Science and Technology, London, England; A. McConnell of the Science Museum, London, England; and T. Usami of the Earthquake Research Institute, Tokyo University, for providing us with valuable information regarding the original Milne seismograph. We thank Karen McNally for reviewing the manuscript. We thank David Hadley for providing us with the digitized seismograms used for the numerical experiments. Francis Lehner and the technical staff of the Seismological Laboratory of the California Institute of Technology designed and operated the Pasadena Milne seismograph. This research was supported by the National Science Foundation under grant EAR77-13641. Contribution 3179, Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125.
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Caltech Division of Geological and Planetary Sciences3179
Issue or Number:B11
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Official Citation:Kanamori, H., and K. Abe (1979), Reevaluation of the turn-of-the-century seismicity peak, J. Geophys. Res., 84(B11), 6131–6139, doi:10.1029/JB084iB11p06131.
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:51337
Deposited By: George Porter
Deposited On:05 Nov 2014 23:38
Last Modified:10 Nov 2021 19:09

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