Deterministic Modeling of Regional Waveforms from the Nevada Test Site.

摘要

Time domain waveform modeling studies of short period P (sub nl) and high frequency P (sub n) have been carried out in an attempt to improve event discrimination capabilities with regional data. The purpose was to develop discriminants with a well understood physical basis which could be reliably transported to different areas of the world. The study of P (sub nl) involved adapting methods developed to fit long period data for observations with periods as short as two seconds. To accomplish this, we have modified the wavenumber integration method for efficient computation of P (sub nl). Velocity gradients like those typically observed near the free surface were of great importance. Gradients at lower depths such as at the crust-mantle transition are of much less significance. A simple layered model which accurately predicts regional P (sub nl) propagation from NTS to the surrounding digital stations were developed. It deterministically explains the waveshape of the P (sub n) and approximately the first 15 seconds of high amplitude energy reflecting past critical angle from the mantle. The modeling study of short period P (sub n) has led to the development of an effective and physically based regional discriminant; the underlying purpose of this project. It has been found that the waveshape of the P (sub n) is stable, similar at most stations and straightforward to model. (jhd)