HR: 0830h
AN: S51E-0103    [PDF]
TI: Realistic three-dimensional sensitivity kernels for local tomography
AU: * Zhao, L
EM: zhaol@usc.edu
AF: Department of Earth Sciences, University of Southern California, Los Angeles, CA 90089-0740 United States
AU: Jordan, T H
AF: Department of Earth Sciences, University of Southern California, Los Angeles, CA 90089-0740 United States
AU: Olsen, K B
AF: Department of Geological Sciences, University of California, Santa Barbara, CA 93106-1100 United States
AB: The frequency-dependent travel-time and amplitude anomalies measured at stations in the Southern California Seismic Network from local earthquakes carry rich information on the 3-D subsurface structure in and around the LA Basin (P. Chen et al., this meeting). So far tomography inversions for the LA Basin have been based on travel-time data using approximate Fr\'{e}chet kernels obtained under high-frequency assumption that breaks down when the spatial dimensions of the structural heterogeneities are comparable to the Fresnel-zone size of the seismic waves. We have developed an algorithm to compute the exact 3-D Fr\'{e}chet kernels for the frequency-dependent travel-time and amplitude anomalies without relying upon the assumptions of a smooth heterogeneity and a simple reference structure. The algorithm uses a staggered-grid finite-difference method to solve the wave equation in 3-D media and the reciprocity to reduce the number of finite-difference simulations. Numerical results for the travel-time and amplitude Fr\'{e}chet kernels reveal a number of properties: 1) As expected, the sensitivities of both travel time and amplitude of a finite-bandwidth signal such as the usual P or S wave do not concentrate only on the ray path, but spread out in a volume surrounding it. 2) If the measurement is obtained from the entire waveform of a given arrival, e.g. through the cross-correlation of two waveforms, the travel-time kernel exhibits the counter-intuitive banana-doughnut behavior with minimum on the ray path; whereas the amplitude kernel takes on an intuitive shape of a sausage with maximum on the ray path. 3) In the first Fresnel zone, the travel-time sensitivity is mostly negative, indicating an advance in arrival time for a velocity increase, whereas the amplitude sensitivity is positive, suggesting an amplitude reduction for a wave-speed increase. 4) At stations away from the nodal points, the kernels for measurements obtained on different components of the same phase are similar. 5) Finite-difference synthetics show that the 3-D SCEC Community Velocity Model (Magistrale et al. 2000) can serve as a reference model for linearized inversions for the LA Basin, and the Fr\'{e}chet kernels in this model are much more complicated than those in simple (e.g. 1-D layered) reference models. Rapid velocity transitions can generate new phases and change the distribution of the sensitivities. The algorithm can be parallelized on both SMP and MPI systems for practical tomography inversions for the LA Basin.
DE: 7205 Continental crust (1242)
DE: 7260 Theory and modeling
SC: Seismology [S]
MN: 2003 Fall Meeting