The possibility of obtaining an estimate of depth for large earthquakes from low-frequency mantle wave data alone is investigated using moment tensor formalism.
After illustrating the sensitivity to depth of the eigenfunctions in the period range of 160 to 350 sec, we give several examples of application of the two-step inversion procedure proposed earlier for shorter period Rayleigh wave data (Romanowicz, 1982). We find that for earthquakes below a depth of about 50 km, we are able to resolve depth even when a spherically symmetric average Earth model is used for propagation corrections. This is particularly interesting in the case of large subduction zone earthquakes for which an estimate of the vertical extent of faulting can thus be obtained, independently of aftershock studies, in a very fast and simple manner which does not involve any synthetic seismogram calculations or lateral heterogeneity modeling.
For shallower earthquakes, especially those with one steeply dipping nodal plane, the resolution of depth is less precise. Taking lateral heterogeneity into account by using available regionalized phase velocities improves the results marginally. We anticipate that accounting more accurately for the low-order harmonics in the worldwide phase velocity distribution should prove most helpful, since these interfere the most with the theoretical radiation patterns of earthquakes. The determination of depth depends more critically on the source process time, which has to be accounted for especially for horizontally propagating faults.