Brune Stress Parameter Estimates for the 2016 M_w 5.8 Pawnee and Other Oklahoma Earthquakes

摘要

I have estimated Brune (single-corner) stress parameters (stress drops) for several Oklahoma earthquakes, including the 2016 M_w 5.8 Pawnee earthquake and the largest aftershocks. My approach is to estimate corner frequency from the peak of the tangential component of the velocity Fourier spectrum using recordings about 50 km or less from the epicenter, where possible. Because Brune stress parameter is for body waves, care has been taken to avoid contaminating spectral peaks from surface waves, nearby building interactions, and soil resonances. For shallow earthquakes, leaky-mode surface waves have been observed overlapping P and S waves, including in Oklahoma. Hence, spectral shape fitting has been avoided in this study. Oklahoma earthquakes (potentially induced) tend to be shallow (<10 km and usually ~5 km or less). In general, Brune stress parameter for the M_w 5.8 Pawnee and other Oklahoma mainshocks ranges between 14 and 22 MPa, whereas aftershocks and smaller magnitude earthquake stress parameters fall in the 1-11 MPa range. These values are typical for the south-central United States potentially induced and natural (shallow and deeper) earthquakes. Other eastern North America regions have natural earthquake stress parameters above 15 MPa, but shallow natural earthquakes can have values in the 2-12 MPa range, similar to potentially induced earthquakes. Comparisons with other estimates of corner frequency using spectral shape fitting suggest that these estimates can be biased by leaky-mode surface waves and/or systematic site responses among recording stations, which affects stress-parameter estimates and interpretations.