Correlations of earthquake focal mechanisms

We performed a statistical analysis of the Harvard catalogue of seismic moment tensor solutions. We investigated the distribution of hypocentres on focal spheres of earthquakes. The hypocentres are concentrated along fault planes; the hypocentre distribution does not significantly differ for earthquakes in different depth ranges. To study the rotation of focal mechanisms, we have solved an inverse problem of a 3-D rotation of double-couple earthquake sources, i.e., for each pair of focal mechanisms we find all four 3-D rotations which rotate one mechanism into another. The stochastic disorientation of focal mechanisms is well approximated by the rotational Cauchy distribution which has previously been identified through theoret-ical arguments and simulations as the result of stress perturbations caused by random defects in the medium. The Cauchy distribution is characterized by one parameter, K; K = 0 means no rotations, and K> 0.5 corresponds to approximately random rotation. For earthquake focal mechanisms the value of K is between 0.05 and 0.1 in a fault zone, and it increases with distance between pairs of events. In directions other than a fault plane, K reaches the value 0.5; the Cauchy distributio