Modeling Finite - Fault Radiation from the o ) n Spectrum by Igor

The high-frequency seismic field near the epicenter of a large earthquake is modeled by subdividing the fault plane into subelements and summing their contributions at the observation point. Each element is treated as a point source with an o92 spectral shape, where co is the angular frequency. Ground-motion contributions from the subsources are calculated using a stochastic model. Attenuation is based on simple geometric spreading in a whole space, coupled with regional anelastic attenuation (Q operator). The form of the co n spectrum with natural n follows from point shear-dislocation theory with an appropriately chosen slip time function. The seismic moment and comer frequency are the two parameters defining the point-source spectrum and must be linked to the subfanlt size to make the method complete. Two coefficients, Aa and K, provide this link. Assigning a moment to a subfault of specified size introduces the stress parameter, Ao-. The relationship between comer frequency (dislocation growth rate) and fault size is established through the coefficient K, which is inherently nonunique. These two parameters control the number of subsources and the amplitudes of high-frequency radiation, respectively. Derivation of the model from sheardislocation theory reveals the unavoidable uncertainty in assigning ogn spectrum to faults with finite size. This uncertainty can only be reduced through empirical validation. The method is verified by simulating data recorded on rock sites near epicenters of the M8.