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波是一種地震波相，其傳播路徑至少部份會經由水體介質傳播。這種聲速波可以傳至很遙遠的距離，因此更小規模的地震也能被偵測得到。但是目前對於控制T波的振幅和到時都還不甚了解。此研究希望能夠更了解T波經由固體地殼介質進入流體海洋介質的轉換點和在海洋中的傳播路徑。我們用了一顆海底地震儀和一個位於蘭嶼的陸域地震儀，在這兩顆地震儀共同的佈站時間記錄到了60個臺灣區域規模大於3.3的地震。其中這60個地震都有被海底地震儀接收到，另外只有大約90%的地震事件被蘭嶼的地震儀記錄到。我們利用這些被臺灣地震網精確定位的地震，去計算理論走時和理論S波-T波，P波-T波的振幅轉換，結合了理論走時和理論振幅轉換，我們可以去模擬T波的振幅。模擬的T波波形與實際上觀測到的波形相符。從理論的到時，地震波的固體彈性波轉成流體聲速波的確與臺灣東部海域的海底地形有關，且我們觀察到部份T波波形缺口與地形的轉換缺口相吻合。最後我們也提出了一個全新的方法，利用地動訊號和海水壓力變化的比值，可去分辨海底地震儀接收到的T波是從海洋傳播抵達或是經由測站底下的地殼抵達。

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.