Finite Difference Modeling of T-Phase Propagation From Ocean to Land
MAXWELL TECHNOLOGIES INC SAN DIEGO CA SYSTEMS DIV
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We model the T-Phase transition from ocean to land and evolution of the seismic signal through this process. We model the transition with a composite technique using normal mode based numerical propagation codes to calculate the hydroacoustic pressure field in the ocean, and use this pressure field as input for the elastic finite difference code TRES to calculate the seismic propagation to land-based stations. Animations are created from the finite difference calculations to help visualize the complex conversion process. We have performed a detailed study of the transition from the Point Sur interim IMS station to seismic stations along the California coast. The numerical calculations performed to date are accurate to 9 Hz. An unusual result of the analysis that is observed both in the data and the calculations is that converted surface waves arrive at coastal stations earlier than body waves. This occurs because conversion to surface waves occurs farther offshore than conversion to body waves. For a typical coastal structure, the P-waves arrive in the middle of the surface wavetrain and are obscured at stations near the coast. T-phases propagate primarily as P-waves once they are well inland from the coast and the surface waves have been attenuated. We have performed a number of test cases to assess the robustness of these results. Calculations for slopes varying from 10 degrees to 30 degrees show little difference in the results. Other studies have reported stronger effects for steeper slopes. All of the IMS T-Phase stations, however, are located in areas where the offshore slope is less than 30 degrees. The California calculations have a low velocity surface layer, so we performed a set of calculations with the ocean embedded in a uniform structure. Again, the results are similar with coastal waveforms dominated by surface waves. In a faster, more uniform structure, however, the P-wave may appear ahead of the surface wave for slopes of 30 degrees or higher.
- Numerical Mathematics
- Computer Programming and Software
- Seismic Detection and Detectors