Global variations of stress drop for moderate to large earthquakes

TLDR

The study investigates global variations in earthquake stress drops using spectra from ~2000 events with mb ≥ 5.5 recorded between 1990 and 2007. Stress drops were estimated by isolating source displacement spectra with an iterative least‑squares method, correcting P‑wave spectra with a global empirical correction, and applying a Brune‑type model to derive corner frequencies and compute stress drops. Stress drops ranged from 0.3 to 50 MPa with a median of ~4 MPa that is independent of moment, indicating self‑similarity, while regional and focal‑mechanism variations were observed, with strike‑slip events showing 3–5× higher and intraplate events 2× higher stress drops than interplate events.

Abstract

We investigate the global variation of earthquake stress drops using spectra of about 2000 events of m b ≥ 5.5 between 1990 and 2007. We use an iterative least squares method to isolate source displacement spectra from travel path and receiver contributions, based on a convolutional model. The observed P wave source spectra are corrected with a globally averaged empirical correction spectrum and estimates of near‐source attenuation. Assuming a Brune‐type source model, we estimate corner frequencies and compute stress drops. Stress drop estimates for individual earthquakes range from about 0.3 to 50 MPa, but the median stress drop of about 4 MPa does not vary with moment, implying earthquake self‐similarity over the M w = 5.2 to 8.3 range of our data. A comparison of our results with previous studies confirms this observation over most of the instrumentally observable magnitude range. While the absolute values of our estimated stress drops depend upon the assumed source model, we identify relative regional variations of stress drop that are robust with respect to the processing parameters and modeling assumptions, which includes an inherent assumption of constant rupture velocity. We find a dependence of median stress drop on focal mechanism, with a factor of 3–5 times higher stress drops for strike‐slip earthquakes and also find a factor of 2 times higher stress drops for intraplate earthquakes compared to interplate earthquakes.