Abstract:
A "double dipole" model with a zero moment was first used to describe faults in an earthquake focus by Honda and Vvedenskaya [l]. But until recently, publications dealing with alternative models continued to appear [2]. Generally, the uncertainty in choosing a model of the fault in the earthquake focus results from the fact that the body of input data (the signs of the arrivals in the first P-wave displacements) is too small to allow the construction of nodal lines. The development of a world network of digital seismic stations has made possible a fuller analysis of earthquake recordings. A group of scientists led by A.M. Dziewonski at Harvard has introduced the routine practice of evaluating the centroid moment tensor (CMT) of earthquake foci 131 . The ratio of the sma lles t (fml n) and 1 a rges t (Fmax) e igenva lues of the tensor glves an objective indication of which model the given seismic focus fits. For the double dipole, the quantity F ~ 1Fm;n/Em.,1 should be zero, while for a linear dipole vector it should be 0.5. Dziewonski's grour, has systematically published catalogs of the centroid moment tensor in the journal 'Physics of the Earth and Planetary Interiors" (1982-88), which has enabled us to develop a catalog of 109 CMT measurements for the Kamchatka-Komandor lslands region between 1977 and 1978. The range of moments is 10 1 6 to 10'9 N-m, and the focal depths range from O to 600 km. KRmchatka and the Komandor Islands liP in the zone of intersection of two im-mense structures, the Kurile-Kamchatka rtnd Aleutian island ares. There are large extensional structures in the reginn, e.g., the Kurile-Kamchatka and Aleutian deep-~ater trenches, chains of active volcanoes, and the sloping Benioff seismic focue zone. As a result, the Kamchatka-Komandor zone is an excellent area for testing a variety of geotectonic hypotheses.
