以TAIGER陸上炸測震源推求臺灣北部之地殼速度構造

Abstract

構造活動是形成臺灣複雜地體構造的主因。台灣東北部的隱沒帶，根據地震活動的分布已可清楚描繪板塊隱沒的形貌。然而，對於地殼構造與莫荷面的分布，雖然曾有學者利用震波走時層析成像法逆推或是重力逆推，企圖獲得速度構造或是莫荷面深度，但是各個團隊提出的解釋無法整合一致。本研究利用台灣大地動力學國際合作計畫(TAIGER)之「陸上主動式震源」實驗的北部測線(T6)，藉由震測實驗短波長訊號的高解析力，嘗試在此區域建立一個二維地殼速度構造。 北部測線(T6)橫跨台灣北部，地理位置由西向東約是新竹至宜蘭南澳間，長約100公里，其上之接收器以200公尺間距佈設。共設有5個人工爆炸震源，炸藥含量約750-3000公斤不等。本研究挑選初達波到時，使用波線追跡法之正推模擬，計算波線在初始速度模型中傳遞之理論走時，比較初達波理論走時和實際走時之差異，再對初始速度模型作修正，反覆地比較走時再修正速度模型，直至初達波理論走時和實際走時可互相擬合，此時的速度模型即可表示接收器下方之速度構造。 受限於測線展距，初達波傳遞深度有限，僅能解析地殼下約10公里內之速度構造。本研究所得之速度模型，可清楚看出P波速度在淺層地殼的側向變化很大。於深度3公里，從西部麓山帶的P波速度4.2 km/s至中央山脈東翼P波速度5.4 km/s。對於區隔地質分區的界限斷層也都有很好的對應，並且在中央山脈東翼下方出現較大的速度梯度。整體來說，本研究的速度模型對於上部地殼(小於10公里)的速度結構獲得良好的解析，和地質構造也相互對應，因此未來可提供作為區域性淺層地殼的速度修正參考。

Taiwan is located along a segment of the convergent boundary between the Eurasian and the Philippine Sea plates. The collision of two plates has generated the ongoing Taiwan Orogeny. Until very recent, seismic studies have given us more information about the velocity structure beneath Taiwan, and gravity data have also been attempted to invert for at least the spatial variation of the depth of the Mohorovičić discontinuity. However, there have been some strong disagreements between them. High resolution images of the crust and detailed seismic velocity structures are still far from well constructed in this region. The experiment of the land seismic refraction from the project of Taiwan Integrated Geodynamic Research (TAIGER) provided a valuable dataset to determine high resolution image of the crust. We take advantage of the data to construct the two-dimensional velocity model across the northern part of the island from P-wave arrival times. We selected data from the Transect 6 line. It is an onshore seismic survey spanned approximate 100-km in northern Taiwan with receiver spacing in 200 meters and consists of five explosions with the dynamite from 750 to 3000 kilograms. Forward ray-tracing is invoked iteratively to adjust the velocity variation until suitable fitting between observed and synthetic travel times are achieved. Because of using the first P-wave arrivals, the deepest ray path may propagate approximate 10-km deep. We used the actual elevations of stations for all of the travel-time calculations. Results from the forward modeling of Pg phases show that strong lateral variations of P-wave velocity in the uppermost crust. At the depth of 3-km, a wide range of velocities from 4.2 km/sec to 5.4 km/sec was obtained. At the distance 30-40 km of our model, there may be a fault bounded Western Foothills and Hsueshan Range. And we observed another boundary separated Hsueshan Range and Backbone Range at the distance about 80 km of our model. Overall, our model coincides with geological observations. To the east, a large velocity gradient appeared under the Eastern Central Range. The results provided a high resolution image beneath Transect 6 and could be very useful for future seismic and tectonic studies.