Zengxi Ge

Investigating Explosion Source Energy Partitioning and Lg-Wave Excitation Using a Finite-Difference plus Slowness Analysis Method

A finite-difference modeling plus slowness analysis method is devel- oped to investigate near-source explosion energy partitioning and Lg-wave excita- tion. The finite-difference method is used to calculate seismic wave excitation and propagation, and an embedded array slowness analysis is used for quantifying how energy will be partitioned into the long-range propagation regime. Because of its high efficiency, the method can simulate near-source processes using very fine struc- tures. A large number of source and model parameters can be examined for broad- frequency ranges. As examples, P-pS-to-Lg and S*-to-Lg conversions in the presence of near-source scattering are tested as mechanisms for Lg-wave excitation. The nu- merical results reveal that the depth of the source and the depth of the scattering process have strong effects on P-to-S conversion and partitioning of energy into trapped or leaking signals. The Lg-wave excitation spectra from these mechanisms are also investigated. The modeling shows that S*-to-Lg excitation is generally stronger for low frequencies and shallow source depths whereas P-pS-to-Lg scatter- ing is stronger for high frequencies.

Wave Propagation in Irregularly Layered Elastic Models: A Boundary Element Approach with a Global Reflection/Transmission Matrix Propagator

A direct boundary element method that uses the full-space Green’s function is proposed for calculating elastic wave propagation in two-dimensional irregularly stratified models. The global matrix equation becomes larger as the number of layers increases. These equations are usually solved by improved block Gaussian elimination, conjugate gradient algorithms, or other approaches based on different approximations. In this article, we adopt the global generalized reflection/transmission matrix method (Chen, 1990, 1995, 1996) to solve this problem. This method can prevent excessive requirement of both computer memory and CPU time. The method is validated by comparing its results with those obtained using the finite- difference method.

An Efficient Approach for Simulating Wave Propagation with the Boundary Element Method in Multilayered Media with Irregular Interfaces

The boundary element method (BEM) is a useful method for seismic-wave simulation in a stratified medium with irregular interfaces. However, the central processing unit (CPU) time required for the traditional BEM method increases exponentially as the number of layers increases. Ge and Chen (2007) presented a BEM with global reflection/transmission matrix propagators that can prevent excessive requirement of both computer memory and CPU time. In this article, we present a more efficient approach for this method. In the new approach, the global matrix propagators can be calculated directly, which can further increase the efficiency by about 40%.

P-SV Wave-Field Connection Technique for Regional Wave Propagation Simulation

A boundary element (be) method is developed to calculate the two-dimensional P-SV elastic response for crustal wave guides with irregular topographic features. To simulate long-range propagation of regional waves, a connection technique is proposed to avoid large matrix inversions that become formidable for long-range, high-frequency problems. By using this technique, a long crustal wave guide can be divided into relatively shorter sections, and the be method can be used section by section to model the effects of rough topography on wave propagation at extended regional distances. The validity of the technique is tested by comparison with a direct calculation. Numerical simulations with this scheme show that rough topography can scatter the P and Rayleigh waves and attenuate the energy propagating in the wave guide. This method can be used in computing the site effects on sites such as canyons, mountains, and valleys. The connection technique expands this method to deal with large earth models with irregular topography.