Generic finite-fault model for ground-motion prediction in eastern North America

Ground-motion models based on the Brune point-source approximation have an underlying omega(2) spectrum, with a single corner frequency. These models overpredict observed spectral amplitudes at low to intermediate frequencies (similar to 0.1 to 2 Hz), for earthquakes with moment magnitudes M of 4 or greater. The empirical spectra of moderate to large events tend to sag at these frequencies, relative to the level suggested by the Brune point-sourc e model. A model that accounts for the finite extent of the fault plane correctly de scribes the observed spectral shapes. The model represents seismic radiatio n as a sum of contributions from several subfaults. Each subfault may be re presented as a paint source, and each subevent has an omega(2) spectrum. Wh en contributions to ground motion at an observation point are summed over a ll subfaults, the resulting spectral shape has two corner frequencies and m ore closely matches observed spectra. The more realistic spectral shape obt ained through finite-fault modeling reflects the underlying reality that th e radiation from real faults is formed by ruptures of their smaller parts, whose corner frequencies are higher than those implied by the full fault di mension. The two corners appear naturally as a result of subevent summation . We use the stochastic finite-fault methodology to simulate the recorded gro und-motion data from all significant earthquakes in eastern North America ( ENA). These data include eight events of M > 4 recorded on modern digital i nstruments (regional seismographs and strong-motion instruments), and three historical events of M 5.8 to 7.3 recorded on analog instruments. The good ness of fit of synthetics to the data is defined as simulation bias, which is indicated by the difference between the logarithms of the observed and t he simulated spectrum, averaged over all recordings of an earthquake. The f inite-fault simulations provide an unbiased fit to the observational databa se over a broad frequency range (0.1 to 50 Hz), for all events. A surprising conclusion of these simulations is that the subfault size that , best fits the observed spectral shape increases linearly with moment magn itude, in an apparently deterministic manner. This strongly suggests that t he subfault size can be unambiguously defined by the magnitude of the simul ated earthquake. In this case, the radiation-strength factor(s), which is p roportional to the square root of the high-frequency Fourier acceleration l evel, remains the only free parameter of the model. Its value is related to the maximum slip velocity on the fault. The strength factors for all model ed ENA events are within the range of 1.0 to 1.6, with the exception of the Saguenay mainshock (s = 2.2). This suggests a remarkable uniformity in ear thquake slip processes.