NONLINEAR PROCESSES IN EARTHQUAKE FOCI

The results of laboratory experiments on biaxially compressed physical models of a seismic source are presented, discussed and interpreted i n terms of nonlinear dynamics; the relation is shown between the degre e of seismic pulse coherency (expressed through the amplitude frequenc y spectra development) and energy (or magnitude) in a series of model experiments. It has been ascertained that the degree of radiated waves coherency plays a more important role concerning the seismic energy r elease than the size (radius) of the seismic source. The relations amo ng individual source parameters obtained in the laboratory were tested by the analysis of three series of seismograms of aftershocks which f ollowed the 1988 Spitak earthquake (two series) and the 1975 Oroville earthquake (one series). The fundamental effect observed in the labora tory, i.e., the growth of pulse coherency with increasing stress conce ntration in the focal region (reflected in growing earthquake magnitud e), was clearly manifested in all three earthquake aftershock series. Due to derivation by a comparison of the above results, obtained on th e basis of nonlinear dynamics with the classical source models by Brun e, Madariaga and others, it seems to be namely the self-organizing of the structure itself caused by the increasing stress field in the seis mic source, which can answer the question concerning the degree of ene rgy cumulation in the earthquake focus at a given moment.