Improved Stress Drop Estimates for M 1.5 to 4 Earthquakes in Southern California From 1996 to 2019

Abstract

We estimate Brune-type stress drops for over 70,000 southern California M 1.5–4 earthquakes from 1996 to 2019 using a P-wave spectral decomposition approach. Based on our recent work documenting hard-to-resolve trade-offs between absolute stress drop, stress drop scaling with moment, high-frequency falloff rate, and empirical corrections for path and attenuation terms, we adopt a new approach in which the average corner frequency of the smallest earthquakes within a short distance from each target event is fixed to a constant value. This removes any true coherent spatial variations in stress drops among the smallest events but ensures that any spatial variations seen in larger event stress drops are real and not an artifact of inaccurate path corrections. Applying this approach across southern California, we document spatial variations in stress drop that agree with previous work, such as lower-than-average stress drops in the Salton Trough, as well as small-scale stress drop variations along many faults and aftershock sequences. We observe an apparent increase in Brune-type stress drop with moment for M 3–4 earthquakes, but their spectra can be fit equally well with self-similar models with a high-frequency falloff rate shallower than f−2.