Abstract

We modeled the source and attenuation attributes of well‐recorded M 6+ earthquakes based on the equivalent point‐source approach, with the goal of determining how to treat ground‐motion saturation effects within this context. We consider ground motions as originating from an equivalent point source and mimic finite‐fault effects by treating the motion as emanating from a virtual point (not a real point on the fault rupture), such that ground motions are correctly predicted at close distances. This is achieved by using an effective distance metric forumla, in which Drup is the closest distance to the rupture and h is a pseudodepth term that accounts for saturation effects. We found that the distance‐saturation effect is magnitude dependent, extending to further distances with increasing magnitude. For earthquakes of M≥6, we model the saturation term as log(h)=−1.72+0.43 M with a standard deviation of 0.19 in log10 units, based on the values obtained from the study earthquakes.

The apparent source spectra of most M 6+ earthquakes can be modeled using a simple Brune point‐source model. For a few of the M 6+ earthquakes, notably those in California, we observed a spectral sag at intermediate frequencies. For such earthquakes, a two‐corner point‐source model provides a better match than the Brune model. We conclude that an equivalent point‐source model based on the effective distance concept can successfully predict the average ground motions from M6+ earthquakes over a wide distance range, including close distances (<20  km).

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