OBSERVATIONS CONSTRAINING NEAR-SOURCE GROUND MOTION ESTIMATED FROM LOCALLY RECORDED SEISMOGRAMS

To estimate the seismic hazard to underground facilities or operations in the environs of a mining-induced tremor or a natural earthquake, it is useful to be able to relate locally recorded seismic waveforms to peak ground velocity and slip at the causative fault. For this purpose, far-field S wave pulses are analyzed to define the faulting slip D and near-fault peak ground velocity D/2 that give rise to the most significant ground motion. This most intense region of faulting, an assumed circular asperity, has radius r within a broader source zone of radius r0, which is traditionally calculated from the corner frequency of the S wave spectrum. In developing relationships between peak far-field velocity v and peak acceleration a, and the source processes of the asperity, D and D, as well as its radius r, the key model assumption is that r = k-beta/omega, where omega is the angular frequency of the sinusoidal velocity pulse of maximum amplitude, beta is the shear wave speed, and k is a constant. Observations in deep-level gold mines of fault slip and slip velocity as well as laboratory observations of slip rate as a function of stress drop for stick-slip failure support a choice of about k = 2.34, the value commonly used for estimating r0 using the Brune model. In particular, observations of fault slip up to 410 mm for mining-induced tremors in the moment magnitude range 4-5 are consistent with D = 8.1 Rv/beta, where R is hypocentral distance. Moreover, estimates based on underground damage of near-fault ground velocities ranging up to 3.5 m/s are in accord with D/2 = 1.28(beta/mu)rho-Ra, where-mu is the modulus of rigidity and rho is the density. Alternatively, the average slip velocity <D> can be expressed in terms of the stress drop DELTA-sigma-a of the asperity as <D> = 0.51 beta-DELTA-sigma-a/mu, and the agreement of this relationship with measurements made during stick-slip failure in the laboratory is good. To the extent that seismic slip exterior to the asperity is a consequence of preevent suppression of slip due to the asperity, the broader-scale (r0) slip can be related to that of the asperity. Just as the asperity radius r can be estimated from r = 2.34-beta-v/a, an alternative estimate for r0 is given by r0 = rho-RaM0/[75.8-rho-mu(Rv)2], the results of which are generally in good agreement with estimates based on the spectral corner frequency method.