Finite Difference Modeling of T-phase Propagation from Ocean to Land

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. We are continuing this research by performing calculations for paths to the IMS T-Phase stations, using the bathymetry along selected paths to those stations. OBJECTIVE The objective of this project is to better understand the propagation of T-phases from ocean to land, and the performance of T-phase International Monitoring System (IMS) stations, through empirical and numerical methods. The empirical study uses data from pressure sensors in the ocean and coastal and island seismometers to examine the T-phase transfer function. The numerical study uses finite difference calculations to model propagation of Tphases from ocean to land. RESEARCH ACCOMPLISHED The International Monitoring System (IMS) hydroacoustic network is a relatively sparse network consisting of 6 underwater hydroacoustic stations and 5 land-based seismic T-phase stations as shown in Figure 1. The hydroacoustic stations are much more sensitive to underwater signals than the T-phase stations and have a higher sampling rate and broader frequency range. The broader frequency range is important for identifying explosions, which are characterized by higher frequency content than other sources. Because of these limitations of T-phase stations, it is important to understand the efficiency of T-phase conversion in order to assess the capabilities of the IMS network for detection and identification of underwater sources. We are in the third year of a three year project to investigate these issues. In our presentations over the past two years, we described the following research: Report Documentation Page Form Approved