Scaling Relationships for Seismic Moment and Average Slip of Strike-Slip Earthquakes Incorporating Fault Slip Rate and Maximum Likelihood Values of Fault Width and Stress Drop

Session: Earthquake Source Parameters: Theory, Observations and Interpretations [Poster]
Type: Poster
Date: 4/30/2020
Time: 08:00 AM
Room: Ballroom
Description: 

A previous study (Anderson et al., 2017: ABW17) proposed a closed-form, constant stress drop equation for the dependence of the magnitude of vertical strike-slip earthquakes on surface rupture length. An added term adjusts the model for the slip rate on the fault. This paper extends and supersedes ABW17. The rupture width originally used as a function of rupture length can be improved by a model that is also optimized to agree with the observed surface slip of large earthquakes. To deal with the large uncertainties, a maximum likelihood method was adopted to estimate the two adjustable parameters: the model stress drop and the poorly- to un-constrained downdip rupture width. The likelihood of the model fitting both magnitude and slip for each earthquake was calculated for an extensive grid of values of these parameters. The events were then grouped by ranges of rupture length and the likelihoods in each group were determined. As expected, the contours show a strong trade-off between width and stress drop: larger stress drop on thinner faults are difficult to distinguish from smaller stress drop on wider faults. The trade-off was resolved by requiring a fixed stress drop for which the most likely width is reasonably consistent with our compiled estimates of rupture widths. The preferred model uses a stress drop (Chinnery model) of 28 bars. The most likely values of width are, unexpectedly, proportional to the logarithm of the rupture length. Slip-rate dependence of the magnitude estimate persists, with reduced uncertainties with both average and actual slip rate.

Presenting Author: John G. Anderson

Authors