台灣地區T波的生成研究

T-waves generation around the coast of Taiwan

凃道霖(Tao-Lin Tu)

指導教授 ： 林正洪

共同指導教授 ： 吳逸民(Yih-Min Wu)

國立臺灣大學/理學院/地質科學研究所/碩士(2008年)

https://doi.org/10.6342/NTU.2008.02342

摘要

T波（Tertiary wave）為一種能量經海水低速層傳遞後所形成的特殊波相，其以海洋為主要的傳播介質。當能量以適當的角度進入海洋聲波低速帶(SOFAR channel)之後，會因全反射而使能量侷限其中不易散失，以至可以傳播至數千公里遠，進一步被海洋中的水聲儀或是島嶼、大陸沿岸的地震儀所接收紀錄。
一般而言，生成T波的成因很多，如海底火山活動、地震以及水下試爆等都會形成所謂的T波。然而在台灣地區，T波主要多為地區性地震所引起。雖然易見，但是由於缺乏整體的研究，因此對於T波的傳播模式、路徑以及影響因子都不甚清楚。為了對此現象做更進一步的探討，本研究選用中央氣象局地震觀測網（Central Weather Bureau Seismic Network, CWBSN）2003年1月至2005年12月短週期地震站之記錄，從中挑選出規模大於4的區域地震共689筆，並利用濾波器(5-10HZ)來刪除其中非T波波相之雜訊，之後再運用T波的走時與傳播特性設計一傳播模型，並使用格點搜尋法（Grid search）來找出適當的轉換點和傳播路徑，進而求得T波整體的傳播模式。
從資料結果中發現，台灣東部沿岸由於緊鄰太平洋，海水深度大，聲波低速帶發展完全，所以東部測站之地震儀器極易收到T波訊號，其中又以宜蘭、成功、大武和蘭嶼四站最為明顯；西部沿岸則因台灣海峽深度不足，不利於聲波低速帶的發展，因此台灣西部之測站不易記錄到T波訊號。在能量經由地震波改變成水中聲波的轉換點上，其主要多集中於花蓮外海的大陸斜坡、琉球島弧南側、呂宋島弧和沖繩海槽等地，且隨著測站的地理位置不同而呈現出強烈的區域變化。另外，本研究依據轉換點與震央、測站三者之間的空間關係以及地震深度的空間分佈得知，地震的震源位置並非決定是否引發T波的主要關鍵。除此之外，由蘭嶼站所記錄到的大振幅T波和宜蘭站所記錄到的小振幅T波，其成因都為不同介質而其能量衰減率迥異所導致。最後，綜合上述結果，本研究認為T波的傳播模式應做以下修正：無論地震震源位於何處，當地震發生後，震波經傳遞到達地表並使陸地產生振動，如震動點剛好位於海洋聲波低速帶附近，且能量能夠順利進入低速帶的話，各測站就有相當大的機率記錄到T波訊號，而非只有位於轉換點後方之地震才有機會形成T波。

關鍵字

T波 ； 聲發波道 ； 台灣 ； 海洋聲波

並列摘要

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.

並列關鍵字

T-waves ； T-phase ； SOFAR channel ； acoustic waves ； ocean

參考文獻

Bath, M. and M. Shahidi (1974). T-phase from atlantic earthquake. Pure Appl. Geophys., 92,

systems. Bull. Seismol. Soc. Am., 40, 53-58.

Ewing, M. and J. L. Worzel (1948). Long-range sound transmission. Geol. Soc. Am. Mem.,

27, 35 pp., 1948.

autonomous underwater hydrophones. AGU fall meeting.