A mantle wave magnitude for the St. Elias, Alaska, earthquake of 28 February 1979

Abstract

abstract We have investigated surface and mantle waves from the St. Elias earthquake (28 February 1979, 21:27:06.1 UTC) using standard time-domain techniques and highly automated procedure for frequency-domain analysis. Mantle wave spectral densities at periods of 100, 150, 200, and 250 sec were determined from R1 through R5 and G1 through G6 recorded by 10 stations of the Global Digital Seismograph Network using a method similar to one developed by Brune and Engen (1969) for 100-sec Love waves. For comparison we have generated synthetic seismograms by normal mode summation using two published fault plane solutions (Lahr et al., 1979; Hasegawa et al., 1980) and assuming a point source. For the St. Elias event we find that the precise orientation of the focal mechanism has a significant impact on the efficiency of mantle wave generation and hence on moment inference. Further, source finiteness effects, expressed as distortion of the radiation pattern and a disparity between moment estimates made using Love and Rayleigh waves, are clearly visible at all periods we examined. However, these effects decrease dramatically with increasing periods and are gratifyingly small by 250 sec allowing us to make a moment estimate. We have made the following measurements of the size of the St. Elias earthquake 20-sec Rayleigh waves (30 observations) Ms = 7.08 20-sec Rayleigh waves Ms = 7.23 (30 observations azimuthally weighted) Seismic moment (dyne-cm) Mo = 2.36 x 1027 Moment magnitude Mw = 7.52.