T-waves Excited by 60 Mw>3.3 Earthquakes in the Taiwan Region During 2006 and 2007: Implications of Their Ray Paths, Amplitudes, and Conversion Efficiency

Abstract:

T-waves are seismic energies that have at least partially propagated through a water body. Such waves can propagate long distances and are thus useful for studying small events. However, factors controlling the amplitude and duration of T-waves remain unclear. In this study, we obtained data by using a broadband ocean-bottom seismometer (OBS) that recorded T-waves produced by 60 Mw>3.3 earthquakes that occurred in Taiwan. In addition, we used T-waves produced by 90% of these events recorded at an island station. For each earthquake, we forward-calculated the travel times of the P- and S-waves converted to T-wave, and correlated them with different paths from the earthquake. The duration of the observed T-wave fits with the paths from the available conversion points in the regional bathymetry. The waveform gaps were correlated with the gaps of the bathymetry contour lines. However, the arrival times of abyssal earthquakes show that parts of the T-waves converted directly from the deep seafloor near the sources. We calculated the synthetic ground motions at the conversion points along the 1000-m bathymetric contour lines. We then calculated the synthetic T-wave amplitude at those conversion points. Using these results, we calculated the T-wave amplitude envelopes and determined that they fit the observed OBS data. Furthermore, we used the ratio of ground motions to water pressure change to determine whether the T-wave arrives at the OBS as ground motions or as acoustic waves from the water column. This method can help determine the different paths and amplitudes of the T-waves recorded using OBSs.