Seismic waveform modeling of regional phases and wavefields from an off-center explosion

Citation

Zhao, Lian-She (1992) Seismic waveform modeling of regional phases and wavefields from an off-center explosion. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/thvj-vd30. https://resolver.caltech.edu/CaltechTHESIS:10032011-142032525

Abstract

This thesis consists of two parts. Part I (Chapter 1) is a theoretical study of wavefields radiated from an off-center explosion in an embedded solid sphere, while Part II (Chapters 2, 3 and 4) deals with seismic waveform modeling of regional phases.

Chapter 1 investigates the effects of asymmetric source regions on the excitation of seismic body waves of explosions. We give an analytic formulation for determining the wave fields from an off-center explosion in an embedded solid sphere in an elastic whole-space. It is shown that SH wave amplitudes can be ten times larger than the P wave amplitudes if the source is properly located.

Chapter 2 discusses the strategies of modeling regional Broadband recordings with the application to Harvard station recordings of November 25, 1988 Saguenay earthquake. We demonstrate that the longer period motions (WWSSN) preceding the direct S arrival can be modeled reasonably well with a crustal model consisting of a layer over half-space. While a generic model assumed to be the same everywhere can be used to model the P_nl waveshape, HeImberger and Engen (1980), some adjustments are required to fit the absolute timing of P_n and S_n for specific paths. In particular, path from the Saguenay event to Harvard is slower than expected for a shield environment and is associated with the roots of the Appalachian Thrust Belt.

Chapters 3 and 4 concern the velocity structures of the Tibetan Plateau and surrounding regions. A fast shield-like velocity model, TIP, is proposed for the structure beneath the Tibetan Plateau as suggested by modeling SH waveforms in Chapter 3. The crustal model is determined by modeling Love waveforms and P_nl waveforms in the time domain while the upper mantle is constrained with S and SS waveform data. Relocations of Tibetan earthquakes are discussed in Chapter 4, assuming model TIP at source region and model JB at receiver regions. A detailed investigation of 100 such events yields a distinctly different picture from a random distribution of events down to a depth of about 50 kms given by ISC. Waveform modeling of depth phases such as _pP indicates that only three or four events from of this population is actually deeper than 25 km. These few events occur near the edges of the Plateau where active subduction is occurring as suggested by the thrust-like nature of their mechanisms. The events occurred earlier than indicated by the ISC by 3 seconds on average which leads to about a 1.5% and 0.5% over estimation of P_n and S_n velocities respectively applying ISC tables and standard flat-layered models.

Thesis Files