Equivalent Point-Source Modeling of Moderate-to-Large Magnitude Earthquakes and Associated Ground-Motion Saturation Effects

We modeled the source and attenuation attributes of well-recorded M6� earthquakes based on the equivalent point-source approach, with the goal of determin- ing 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 R �� D 2 � h 2 � 0:5 , in which Drup is the closest distance to the rupture and h is a pseudodepth term that accounts for sat- uration 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 logh �� −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 M6� earthquakes can be modeled using a simple Brune point-source model. For a few of the M6� earthquakes, notably those in California, we observed a spectral sag at intermediate frequencies. For such earth- quakes, 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).