利用臺灣東部外海海底地震儀觀測T波︰聲速隨機擾動與地形對T波的影響

Abstract

T 波 (Tertiary wave) 為地震波轉換成聲波的一種特殊波相，並且在聲發聲道 (SOFAR channel) 內傳播，因此有較低的能量損失以致於能夠被數千公里外的陸上測站或水聽器所記錄到。過去T波的觀測以及理論皆建立在陸地測站的資料上，近幾年來，我們同樣在位於臺灣東部外海的海底地震儀 (ocean-bottom seismometers, OBS) 上觀測T波訊號，這樣的發現使我們想要瞭解海水聲速隨機擾動與海底地形對於T波所造成的影響。 本論文使用 2006 年 9 月至 2012 年 7 月的 33 個海底地震儀，主要佈放於沖繩海槽以及花東海盆及呂宋島弧附近，並選取東經 118 至 145 度、北緯 15 至 31 度，規模大於 5 的地震事件共 440 個，依據以下條件︰（ 1 ） 1 – 10 Hz 有明顯能量；（2）較長的持續時間；（3）明顯的紡錘狀波形，共挑選出 88 個含有 T 波的地震事件。我們分析含有 T 波的地震事件之震源位置，發現大部分是淺於 50 公里的淺層地震，僅有一個是來自於深度 225 公里的深層地震。 88 個 T 波事件中，有 32 個事件是由坐落於花東海盆且深度深達 4500 公尺的 3 個海底地震儀所記錄到。為了避免過多變因模糊研究目的，我們聚焦分析兩個同時佈放的海底地震儀測站 S002 與 S004 所接收到的 T 波訊號，並利用此兩測站所在之海洋物理及地形環境進行聲場及聲線的模擬。 S002 坐落於沖繩海槽，此處地形較為平坦，平均水深約 2000 公尺左右。 S004 坐落於花東海盆，此處地形變化較為複雜，且平均水深深達 4500 公尺。 為了瞭解在聲發聲道內的 T 波能量是如何向外傳遞至海床上的海底地震儀，我們使用聲線法模擬在真實物理環境的海底地形及聲速剖面之下 T 波的傳播情形。從模擬中可以得知， T 波不需要在聲發聲道內傳遞，而是在整個水體內傳播，所以可以傳遞至深海的海床上。此外，我們也透過了格點搜尋法，並利用模擬與觀測到的 T 波到時找出可能的 T 波轉換點，接著利用可能的 T 波轉換點所產生的聲線之脈衝響應，與陸上測站所得出的震源時間函數模擬 T 波波形。從模擬波形可以知道， T 波的紡錘狀波形有可能是來自不同方位角及不同深度的轉換點。

T waves excited by earthquakes propagate along the SOFAR channel with low transmission loss, and therefore can be recorded on land-based seismic stations and hydrophones located thousands of kilometers away from earthquake epicenters. Early T-wave observations are mostly based on recordings by land-based stations due to the mechanics of the energy conversion of acoustic waves into seismic phases. Recently, T-wave signals have also been detected by ocean-bottom seismometers (OBS) at deep ocean basin off-shore eastern Taiwan, raising the question of how deep ocean environment and sound speed perturbation affects the generation and propagation of T-waves. In this study, we examined the seismic waveform data recorded at 33 OBSs deployed in Okinawa Trough and Huatung Basin from 2006 to 2012. During this time period, there are 440 regional earthquakes with magnitude larger than 5 in the Western Pacific Ocean. A total of 88 T-wave events are identified using the criteria that significant energy in the dominant frequency of about 1 10 Hz and time duration longer than 100 seconds and spindle shape waveform. Most of these events were generated by shallow-depth (less than 50 km) earthquakes, with only one exception by deep source of 225km. Among these 88 events, 32 events were recorded on 3 OBSs located at 4500-m depth of Huatung Basin, where the depth of minimum sound speed is around 1100 m. The difference in seafloor topography around the OBS sites may influence the characteristics of T-wave signals. The seafloor topography in Okinawa Trough (S002) is relatively flat, with an average depth of 2000 m. In contrast, Huatung Basin (S004) has more complex topography between the OBS and the epicenter of earthquake. To understand how acoustic energy scatters from the SOFAR channel into the ocean bottom, we apply the acoustic ray theory to simulate acoustic propagation in the presence of realistic ocean floor topography and sound speed profile. Our simulations indicate that seafloor topography indeed affects the acoustic propagation pattern, part of which may reach deep ocean regions. We further investigate potential conversion points of T-waves through 2-D grid-search technique. By minimizing the differences between predicted and observed arrival times of T-waves, we reveal possible conversion points around each OBS station. We also simulate seismic energy of T-waves by stacking energy coming from a series of potential conversion points within a specific time-window. The stacked energy distribution expresses a pattern similar to the envelope function of T-waves, indicating that the long-lasting waveform may result from a series of seismic-acoustic conversion processes.