Inversion of seismic waveform data and its interpretation.

Abstract

Seismic waveforms are a complex combination of the seismic source process, propagation effects and distortions due to mechanical recording. The inversion of long-period, teleseismic body waves has proven quite effective in determining source parameters of moderate to large-sized earthquakes. The focal mechanisms from moderate-sized events also have been recovered using regional waveforms. In this thesis, the waveform inversion method is applied to moderate to large-sized earthquakes at teleseismic distance and extended to small earthquakes at regional and local distance. The moment tensor formalism provides a framework for the seismic source inversion problem. It can be solved when broad band or occasionally band-limited waveform data from a single station or sparse network are available. The signature of the seismic source orientation on the waveform is robust if the P, SV and SH displacement waveforms from a single station are used and the gross crustal structure along the travel path is known. The focal mechanisms of the two small earthquakes, which occurred in the Rio Grande Rift, New Mexico, were determined and are consistent with those from the P-wave first motion data. This moment tensor inversion method was also used to analyze the small aftershocks of the 1991 Costa Rica earthquake (M(s) = 7.6). It was found that the focal mechanisms of the aftershocks can be recovered from three-component waveform data recorded by a sparse mid-period network. The focal mechanism signature on the waveforms is not very sensitive to the details of the crustal structure, which are poorly known in this study. However, the source depths are not well constrained due to the trade-off with the assumed crustal model since the details of the crustal structure strongly affect the body waveforms at regional distances. The focal mechanisms of small aftershocks were used in studying regional tectonics setting. The teleseismic waveform inversion method is used to investigate moderate- to large-sized earthquakes in the Pamir-Karakorum region. Most shallow earthquakes are within the upper crust with focal depths less than 20 km. Nearly horizontal compressional P axes of shallow events trend in an approximately north-south direction, consistent with convergence of the two continental plates. Both the hypocentral distribution and focal mechanisms of intermediate-depth events clearly delineate a southward-dipping Wadati-Benioff zone, representing the subduction of the continental lithosphere of the Eurasian plate in the Pamir-Karakorum region.