研究2006至2007年臺灣區域60個Mw > 3.3的地震造成的T波之傳播路徑、振幅大小和轉換效率

Abstract

T-waves are seismic energies that have at least partially propagated through a water body. Such waves can propagate long distances, thus are useful for studying small events at far distances. However, factors controlling the amplitude and duration of T-waves are still not clear. It is also advantageous if we know where the energy enters into the water column and which parts of the T-wave is arriving from the crust or from the water column near the receiver side. Here, we used data from one broadband ocean bottom seismometer (OBS) that recorded T-waves from 60 Mw > 3.3 Taiwan earthquakes. T-waves from 90% of these events were also recorded by an island seismic station. For these earthquakes that are well-located by a dense land seismic network, we forward-calculated the travel times of the different parts of P- and S-wave convert to T-wave, and correlated them with different paths from the earthquakes. The duration of the observed T-wave fits with the paths from available conversion points in the regional bathymetry, and the waveform gap consistent to the gap of bathymetry contour. However, arrival times of a few abyssal earthquakes show that parts of the T-waves are converted directed from the deep seafloor near the sources. We have 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 such results we calculated the T-wave amplitude envelopes and found that they can fit with the observed OBS data. Furthermore, we have used the ratio of ground motions to water pressure change to determine if the T-wave is arriving to the OBS as ground motions or as acoustic waves from the water column. Such method can help us to determine the different paths and amplitudes of the T-waves recorded by the OBSs.