Abstract

We consider the resolution of parameters, such as strength excess, ^y - ^o, and slip-weakening distance, d_c, related to fault-constitutive properties, that may be obtained from the analysis of strong-ground motions. We show that waveform inversion of a synthetic strong-motion-data set from a hypothetical M 6.5 event resembling the 1979 Imperial Valley earthquake cannot uniquely resolve both strength excess and d_c. Specifically, we use a new inversion method to find two rupture models, model A having d_c = 0.3 m and high-strength excess, and model B having d_c = 1 m and low-strength excess. Both models have uniform initial stress and the same moment-rate function and rupture time distribution, and they produce essentially indistinguishable ground-motion waveforms in the 0-1.6 Hz frequency band.

These models are indistinguishable because there is a trade-off between strength excess and slip-weakening distance in controlling rupture velocity. However, fracture energy might be relatively stably estimated from waveform inversions. Our Models A and B had very similar fracture energies. If the stress drop is fixed by the slip distribution, the rupture velocity is controlled by fracture energy.

We show that estimates of slip-weakening distance inferred from kinematic inversion models of earthquakes are likely to be biased high due to the effects of spatial and temporal-smoothing constraints applied in such inverse-problem formulations.

Regions of high-strength excess are often used to slow or stop rupture in models of observed earthquakes, but our results indicate that regions of long d_c and lower strength excess might alternatively explain the slowing of rupture. One way to constrain d_c would be to model ground-motion spectra at frequencies higher than those at which waveform modeling is possible. A second way to discriminate between regions of long d_c and large-strength excess might be to assume that d_c is long where there are no aftershocks.

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