Numerical Modeling of Linear and Nonlinear Seismic Wave Attenuation

摘要

CTBT verification requires an understanding of the propagation of seismic phases over complex regional paths which cut across major structural boundaries. The computation of synthetic seismograms by finite difference methods plays an important role in developing such understanding. In order for synthetic seismograms to be realistic, the models must account not only for regional elastic structure of the path, but also for an elastic losses. In addition, high amplitudes near the source require that numerical models take account of nonlinearity. Heretofore, 3D finite difference simulations have generally neglected attenuation, due in large part to the onerous storage requirements entailed by realistic seismic Q models. We describe a novel solution to this problem which we call memory-variable coarse-graining. The coarse-graining method leads to an order of magnitude reduction in computer storage requirements for an elastic memory variables. We then develop and demonstrate a method for modeling nonlinear wave propagation in rock under conditions of intermediate strain, defined as strain levels below the threshold for failure and damage, but above the threshold for onset of nonlinear response. The resulting model reproduces the stress-strain behavior of rock in this strain regime, as measured in quasi-static laboratory tests, and is consistent with key features of laboratory wave propagation measurements.