The aftershock sequence of the Kamchatka earthquake of November 4, 1952

Abstract

Aftershock epicenters of the Kamchatka earthquake of November 4, 1952, are distributed over an area approximately 1,030 kilometers in length by 240 kilometers in width. Assuming that this distribution represents the active strain zone, the total average strain, average elastic energy, and average stress of the rocks before slip were 11.9 × 10^(−5), 1.35 × 10^2 ergs/cm.^3, and 12.6 kg/cm.^2, respectively. The strain-release curve of the sequence has been constructed using observations from Uppsala and Kiruna. The data include more than 400 shocks with magnitudes 6.0 and greater which have occurred up to December, 1956. The curve exhibits three segments each of the form ΣJ^(1/2) = A + B log t, where J is the energy and t is the time measured from the time of the principal earthquake. The slope B changes abruptly at t = 0.4 days and at t = 195 days, the latter change being particularly pronounced. Moreover, this was accompanied by other evidence suggesting a change in mechanism. The coefficients B have almost the exact ratio of 1 : 2 : 5 in the three intervals 0-0.4, 0.4—195, and after 195 days. The aftershock activity has its highest concentration in the vicinity of the principal earthquake and tapers off toward both ends of the active fault segment. The majority of the aftershocks have clear pP impulses occurring generally 9 to 13 sec. after P, indicating that the foci were in or close to the Mohorovičić discontinuity. The rate of strain accumulation and release for the time interval from 1897 to 1956 for the entire Kamchatka-northern Japan stress system shows a slow decrease with time. Comparison of the rate of the entire system with that of the aftershock sequence leads to an approximate estimate of the possible duration of the sequence.

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