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題名 | 海上人工震源在花蓮地區之初達P波走時的分析解釋 |
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作者 | 王正松; 李焜發; 黃慧華; 黃志峰; | 書刊名 | 臺灣海洋學刊 |
卷期 | 34:4 1995.12[民84.12] |
頁次 | 頁61-72 |
分類號 | 351.94 |
關鍵詞 | 人工震波; P波走時曲線; 視速度; 波速構造; Artificial waves; P-wave travel-time curve; Apparent velocity; Wave velocity structure; |
語文 | 中文(Chinese) |
中文摘要 | 為求取臺灣東部花蓮地區之地下波速之地下波構造,海洋大學於1993年9月在臺灣 東部外海施放28個人工震源,藉由中央研究院地球科學研究所在花蓮地區所設置之遙記式臨 時地震網接收震波。本研究分析其中通過花蓮南邊一條約呈N60°W走向(約與臺東縱否垂直 )地帶內之八個測站和六個震源所得之初達P波走時曲線,並利用傾斜地層界面首波之觀念 解釋此地帶下之二維波速構造。 分析此八個測站與六個震源之震波走時特性,我們發現走時曲線大部分為線性,且可將 測站和震源分為兩組。其中一組包含在海岸山脈上之兩個測站和靠近海岸的兩震源,另一組 包含在中央山脈東緣之六個測站和遠離海岸的四個震源。由第一組測站和震源之P波走時數 據可求得與海岸山脈及近海底下一個界面有關之西北和向東南之初達P波平均水平視速度。 所求得之初達P波平均水平視速度向西北為5.4km/s,向東南為3.7km/s。由此可知此界面向 東傾斜且其上之速度必小於3.7km/s。另一組測站和震源可求得與花蓮附近之中央山脈東緣 、海岸山脈和近海底下其他界面有關之初達P波走時數據。這些走時數據很明顯地可依震源 分成四條斜率不同之線性走時曲線,其代表之向西北之水平視速度分別為6.0,6.2,8.3和 7.0km/s。這些走時數據若依測站來分,則可形成與靠近臺東縱谷的四個測站相關之四條線 性走時曲線,其餘走時數據無法形成明顯的直線。此四條直線代表之向東南之水平視速度分 別為4.5,4.6,4.8和7.1km/s。整體而言,由第二組測站和震源所求得之向西北的水平視速 度比向東南的大許多,顯示花蓮地區底下有向東傾斜之界面和地震存在。 |
英文摘要 | In order to study the crustal velocity structure in the Hualien area in eastern Taiwan, dynamite explosions were set off in September 1993 by the National Taiwan Ocean University in the offshore area of Hualien, to generate artificial seismic waves to be recorded by a portable on-land PANDA-2 seismographic array deployed by the Institute of Earth Science, Academia Sinica (Taipei, Taiwan). In this study, we analyze travel times of the first-arrival P- waves which were emitted by six explosions at sea and recorded by eight receiving stations in the belt to the south of the Hualien city, which is almost perpendicular to the Longitudinal Valley in eastern Taiwan. In the analysis, the concept of refraction in structure with dipping layers is used to account for linear travel-time curves with different apparent wave velocity. The travel-time curves of the waves emitted by the two sources near the coastline or received by the two receiving stations on the Coastal Range are much behind the other curves . Thus, we divide the sources and receivers into two groups, with the first group including these two sources and two receivers, and the second group including the other sources and receivers. From the travel times obtained in the first group, we calculated the average apparent velocities of the waves going in the NW and SE directions to be 5.4 and 3.7 km/s, respectively. From these average apparent velocities, we conclude that, beneath the Coastal Range and its nearby offshore area, there is a interface dipping to the east, the velocity above which is less than 3.7km/s. The travel times from the second group from four linear travel-time curves for the four individual sources. The corresponding noresponding northwestward apparent velocities are 6.0, 6.2, 8.3, and 7.0km/s. The travel times also form four linear travel-time curves for four individual stations on the eastern edge of the Central Range. The corresponding southeastward apparent velocities are 4.5, 4.6, 4.8 and 7.1km/s. Since the northwestward apparent velocities are significantly greater than the southeastern ones in general, we infer that beneath the Hualien area which extands from the eastern edge of the Central Range to offshore area, there are structures with layers dipping to the ease. |
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