Long waves observed in the Kamchatka earthquake of November 4, 1952

Abstract

The responses of conventional seismographs are such that earthquake waves in the period range of about 5 to 20 seconds are written with such large amplitudes an the seismograms that longer period waves cannot be observed. Consequently, in order to increase the observable range to substantially longer periods, I developed an electromagnetic strain seismograph having a recording galvanometer of three minutes period and an equivalent pendulum magnification factor for surface waves of 20. The first great earthquake which has occurred since this seismograph combination was put into service was the Kamchatka shock of 1952, November 4. Studies of the aftershock sequence of this earthquake have indicated that in the principle shock faulting extended approximately 1,000 km with an average slip of about 15 feet. Figure 1 shows a portion of the seismogram of this earthquake written with the new instrument. The most prominent event on this record is the G1 wavelet, which is a horizontally polarized surface wave with no vertical component. Although the G wavelet is a Love wave, its frequency components · all lie in that part of the dispersion curve which is essentially flat, and, consequently, the wavelet propagates without substantial change of shape. Thus, although the duration of G2 which has traveled over the longer arc has been increased to 180 seconds owing to differentially greater absorption of the higher frequencies, its shape is not changed. The apparent change in shape is a result of the instrument response characteristics. The response to the 60-second pulse of G1 is proportional to ground particle displacement, since the pulse frequency components are mainly greater than the galvanometer frequency. With G2, the frequencies cluster about the frequency of the galvanometer, and the over-all response is thus approximately proportional to the ground particle velocity. The G2 pulse as written on the seismogram is thus the first time derivative of the G1 pulse, indicating identity of shape for the two. The time of travel for a complete circuit around the earth for the G wave was measured on the seismogram by comparing the arrival time intervals of G4–G2 and G3–G1 . It is 152.3 minute This corresponds to a surface velocity of 4.38 km per second, which is lower than the value to be expected for a shear wave which penetrates as deeply as the G wave. Figure 2 is a portion of the seismogram containing G1 and R1.