Precise focal depth determination from amplitude spectra of surface waves

Abstract

A method of using surface-wave amplitude spectra from shallow earthquakes for focal depth determination is proposed. The theoretical basis of the method is established with the aid of the normal mode theory for surface-wave excitations caused by dislocation sources in a multilayered medium. Various practical aspects, such as the crustal thickness, the spatial and temporal behavior of the source, and the attenuation, are considered and shown to have relatively small effects on amplitude spectral shapes of surface waves as compared to the focal depth. The method requires that the fault-plane solution of an earthquake be known beforehand and also that the length of the fault be no longer than 10 km. An uncertainty of not more than 15° in the fault-plane solution will not seriously impair its applicability. Several earthquakes with known focal depths are used to test the proposed method. The tests show that the focal depths determined from amplitude spectra of surface waves agree excellently with those obtained from body-wave travel times.