T-waves generation around the coast of Taiwan

T-waves are a kind of acoustic waves propagated within the SOFAR channel within oceans and often recorded at seismic stations near the coast lines and islands. They can be generated by the underwater explosions, submarine volcanic activities and earthquakes. In Taiwan, T-waves are usually excited by local earthquakes, but it is not well known how T-waves are actually generated due to the complicated tectonics and bathymetry in this area. To improve the understanding of T-wave generation, we have examined a number of short-period seismic data generated by 689 local earthquakes (ML>4) recorded at the seismic stations (CWBSN) from 2003 to 2005. A high-frequency band-pass filter (5-10Hz) has been applied to enhance the signal of the T-phases. And possible converted points for most of T-waves are carefully estimated from detailed analyses of the travel-times from the earthquake to the seismic station. The results show that T-waves can be recorded at the seismic stations along the eastern coast of Taiwan, especially at the ILA, TAW and CHK stations, as well as the island of Lanyu. According to the spatial distribution of earthquakes that generated T-waves, earthquake locations were not direct elements to influence the generation of T-phases. All of the converted points from seismic waves to T-waves can be divided into two groups in the Taiwan area. One was in the Okinawa trough; the other was in the southern Ryukyu arc, continental slope and the Luzon arc. Those converted points were strongly dependent on the position of seismic stations and the topography of Taiwan eastern shore. From the geometrical relationship of seismic stations, converted points and the hypocenters of the earthquakes that generated T-waves, it is interesting to note that some converted points were far away from both hypocenters and seismic stations. In summary, no matter where were the hypocenters, the shaking by earthquakes on the slope around 1000 m in depth could transfer seismic energy into the SOFAR channel and then can be recorded at the seismic stations along eastern shore of Taiwan and island of Lanyu.