Influence of Lithostatic Stress on Earthquake Stress Drops in North America

Earthquake stress drop is a critical parameter for estimating seismic hazard. This parameter can have a strong effect on ground motion amplitudes above ~1Hz and is especially important in Oklahoma and Kansas where earthquake rates have increased sharply since 2008. We estimate stress drops for 1121 earthquakes greater than ~M3 in and near the conterminous United States using spectral ratios between collocated events at given stations. We find that the average stress drop for the few eastern United States (EUS, 26340 Bars) tectonic main shocks studied, which tend to be deeper thrusting events with few foreshocks and aftershocks, is about three times greater than tectonic main shocks in the western United States (WUS, 1077 Bars), which tend to be shallower but have a wide range of focal mechanisms and moderate numbers of foreshocks and aftershocks, and six times greater than potentially induced main shocks in the central United States (CUS, 241 Bars), which tend to be shallow and strike-slip to normal faulting mechanisms with numerous foreshocks and aftershocks. With the possible exception of CUS aftershocks, we find that differences in stress drop among all events can be accounted for, within one standard deviation of significance, by differences in the shear failure stress for an average coefficient of friction of about 0.68. After correcting for fault style and depth dependence, we find an average stress drop of about 3% of the failure stress. These results suggest that high frequency shaking hazard (> ~1 Hz) from potentially induced events and aftershocks is reduced to some extent by lower stress drop. However, these events tend to occur at shallower depths, which will increase hazard in the near field.