Inversion of the high-frequency source radiation of M6.8 Avachinsky Gulf, Kamchatka, earthquake using empirical and theoretical envelope Green functions

We study the generation of high-frequency S-wave energy from its source of the 1992 Avachinsky Gulf, Kamchatka, earthquake (M-W = 6.8) by means of the envelope inversion method of Nakahara et al. (1998). Two versions of this technique were employed, the original version and the modified version. In the modified version, to construct the "envelope Green function" we use empirical envelope shapes, constructed by averaging data of small earthquakes from the same area, calibrated to the total energy of the theoretical envelopes. The envelope inversion procedure was applied to six mainshock records, producing estimates of spatial distribution of energy radiation in frequency bands: 1-2, 2-4, 4-8 and 8-16 Hz. In both inversions, the distribution of energy radiation shows a similar pattern: 75-90 percent of the energy was radiated soon after nucleation. Afterwards, the rupture propagated mostly in a westward direction. Similarity of both results demonstrates the robustness of the envelope inversion method. However the version of the inversion procedure that uses empirical envelopes yields more plausible characteristics of the rupture process.