← Back to Gallery

Session: Earthquake Source Parameters: Theory, Observations and Interpretations [Poster]

Type: Poster

Date: 4/25/2019

Time: 06:00 PM

Room: Fifth Avenue

[Withdrawn] Stress Drops for Microseismicity in Asperity-Like Dynamic Fault Models: Actual Values vs. Estimates From Spectral Fitting and Second-Moment Approaches

Stress drop, an averaged difference between the shear stress on the fault before and after an earthquake, is an important source parameter. The values estimated by typical seismological methods (e.g., the spectral fitting approaches) suggest that the stress drop is moment-invariant with large scatter and that the on-fault rupture duration tw obeys the scaling of tw ∝ M0^(1/3). However, several observations show that microseismic events from the same location can have similar source durations but different seismic moments, violating the commonly assumed scaling. We use numerical simulations of earthquake sequences to demonstrate that strength variations over seismogenic patches provide an explanation of such behavior, with the event duration controlled by the patch size and event magnitude determined by how much of the patch area is ruptured. We find that stress drops estimated by the spectral fitting analyses for the sources simulated in an asperity-like fault model significantly increase with the event magnitude, ranging from 0.006 to 8 MPa. However, the actual stress drops determined from the on-fault stress changes are magnitude-independent at ~3 MPa. Our findings suggest that fault heterogeneity results in local deviations in the moment-duration scaling and in earthquake sources with complex shapes of the ruptured area, for some of which stress drops may be significantly (~100-1000 times) underestimated by the spectral fitting methods. We further apply the stress-drop estimation approach using second moments, which aims to account for rupture directivity and elliptical sources. The second-moment approach indeed works better, providing close estimates of stress drop for sources with rupture areas similar to elliptical ones, but still significantly underestimates, by a factor of up to 30, stress drops for more complex rupture shapes such as ring-like sources. Hence the estimated stress drops still overall increase with the event magnitude, ranging from 0.1 to 2 MPa, but are overall closer to the actual stress drop of ~3 MPa.

Presenting Author: Yen-Yu Lin

Additional Authors