Point-Source Decay of Ground-Motion Amplitudes at <10 km

The amplitudes and decay rate of ground motions from small to moderate earthquakes are important to the assessment of induced seismicity hazards and useful as input components to finite-fault models of larger events. The decay rate of mining events (M < 3) recorded on hard rock is consistent with a geometric spreading rate of R−1.3, with no apparent saturation effects in the hypocentral distance range from 0.15 to 8 km. Response spectral acceleration often exceeds 1000  cm/s2 at high frequencies (>10 Hz) for events of M < 3 at <0.5 km. Underground motions in rock are less than those on surface at low frequencies, and greater at high frequencies, in a way that is explained by the product of free surface amplification and near-surface attenuation (kappa) effects.