Abstract

This paper summarizes the characteristics of focal depths of intracontinental earthquakes based on a compilation of reliably determined depths reported in the past five years. Since the study of Chen and Molnar (1983), several additional moderate-sized sub-crustal events (mb ≥ 5) have occurred in southern Tibet, the Tien Shan, and the Karakorum where such events are known to exist. Furthermore, three events have been identified near the Moho beneath the Brazilian shield, the coldest part of the Baltic shield, and the passive margin of Newfoundland. Since these events took place in stable parts of the continent, they must be an integrated part of the deformation of the intracontinental lithosphere at depth, not that of subducted material. Therefore there seems to be little doubt that earthquakes near the Moho is an incessant element of intracontinental seismicity. Due to the lack of detailed information on the crustal and mantle structures in the source regions, many of the events near the Moho seem to be located in the uppermost mantle but some might have occurred in the lowermost crust. In any case, the events near the Moho in most regions seem to be separated from the familiar seismicity in the upper part of the crust, forming a pattern of two seismogenic zones straddling an aseismic region in the lower crust. With data from South America and the Baltic shield, the maximum depth of earthquakes in cratons can be correlated with tectonic age within a single continent, and with low heat flow of the source region in a given tectonic province, respectively. These observations reinforce the view point that to a first approximation, the distribution of focal depth serves as an indicator of temperature or mechanical strength of the lithosphere, a result consistent with extrapolation of data from experimental rock mechanics. The aseismic zone in the lower crust is interpreted as a relatively weak region of the lithosphere sandwiched between two layers of more competent material where sufficient strain can accumulate to produce earthquakes.

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