Efficiency, stress drop, apparent stress, effective stress, and frictional stress of Denver, Colorado, earthquakes

This paper discusses the interrelationships of initial stress, average stress, frictional stress, stress drop, and seismic efficiency. These parameters are determined for Denver earthquakes by seismic methods. The stress drops for two earthquakes are estimated on the basis of the seismic moments (obtained from surface waves) and the source dimensions (obtained from the aftershock distributions). These stress drops (3 and 22 bars) were between 0.01 and 0.10 of the initial stress. These small values indicate that only part of the available tectonic stress has been relieved. On the basis of the ratio of the Gutenberg energy estimate, the apparent stresses for four Denver earthquakes were determined to be about 100 bars, and the efficiency was estimated to be less than approximately 0.5 by comparing the apparent stress with the in situ stress of 203 bars obtained from the Rocky Mountain Arsenal pumping records.

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