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

Abstract

According to currently available seismicity catalogues, seismicity (for example, the number of events with Ms more than or equal to 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 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. -Authors