Fault slip velocities inferred from the spectra of ground motion

Better understanding of earthquake-source properties is an important goal in seismology. Dynamic fault theories and practices of ground-motion prediction need independent information about source characteristics obtained directly from recorded data.;The maximum slip velocity on a rupturing fault is the parameter that controls the strength of an earthquake's high-frequency radiation and the properties of its Fourier spectra. We therefore have tested an empirical method for determining the peak slip velocities for a number of well-recorded earthquakes using such spectral information.;High-quality ground-motion data from small-to-moderate earthquakes in Japan were collected, and Fourier transforms of the accelerograms were computed for both horizontal and vertical components of the data. Regional parameters (site effects and path effects) that distort the true source spectra were investigated and separated from the recorded spectra. The obtained source terms following the classic "o2" spectral model were used to determine the corner frequency that carries the information about the fault's maximum velocity.;The results indicate that the maximum slip velocity of the selected Japanese earthquakes ranged from approximately 0.2 to 0.6 m/sec. Direct observation-based slip velocity determinations provide valuable physical information about earthquakes that can be used for constraining dynamics theories of faulting or in ground-motion prediction.