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Session: Earthquake Source Parameters: Theory, Observations and Interpretations

Type: Oral

Date: 4/25/2019

Time: 04:00 PM

Room: Cascade I

Uncertainties in Stress Drop Estimates and Their Tectonic Consequences

Stress drop, although ideally the difference in stress before and after faulting, is actually a kinematic parameter commonly calculated for earthquakes. Estimating stress drop involves a combination of observable and assumed parameter values. It depends on the cube of a factor reflecting the fault dimension, which is inferred in two ways. One method uses the corner frequency of an earthquake’s spectrum, whereas the other relies on rupture duration calculated from an earthquake’s source time function. Both require scaling by the cube of an assumed rupture velocity and by a factor reflecting the assumed fault shape. If the model assumptions are appropriate, the two methods should produce similar stress drop estimates for the same earthquake.

We explore the uncertainties involved by comparing stress drops inferred from corner frequency and rupture duration for ~900 earthquakes. Although both yield stress drops generally in the 1-1000 bar range, we find little correlation between stress drops estimated by the two methods, even for the same assumed rupture velocity and fault shape. This situation arises because the corner frequency and the time function methods often give inconsistent estimates of the fault dimension.

The weak correlation between the stress drop estimates from the two methods would be enhanced by the variability of the shape factor and rupture velocity between earthquakes. These uncertainties in stress drop values need to be considered when making any tectonic interpretation from them, such as differences between interplate and intraplate earthquakes.

Presenting Author: James S. Neely

Additional Authors