以強地動波形模擬研究台灣中西部S波速度構造  李憲忠

[ 摘要 ]

地震波主要是經由震源效應、傳遞路徑之構造效應及其所在之場址效應所形成,欲了解地震之震源訊息,對於細部速度構造之了解是必須的。台灣地區之速度構造,前人已陸續做出許多模型,這些研究大都以P波初達波走時為依據,逆推求出平均速度構造。本研究所方法乃結合廣義波線法與有限差分法,利用台灣中西部之密集強震網,選取三義-埔里地震密集區之地震,初以廣義波線法模擬波形記錄中之重要反射波相,如沈積層、康拉不連續面、莫氏不連續面之到時及振幅,分析求得震源與每一測站間路徑之一維速度構造,繼之以有限差分法根據一維模型之結果,重新建立各路徑之二維速度模型,探討該區速度不連續面深度之整體分布情形,所獲致之結果如下:
一、台灣中西部S波速度構造
1.
沈積層-中西部之沈積層厚度由西部沿海向中央山脈增加,且與地表地質構造有明顯之相關性,海岸平原平均深2~3公里,西部麓山帶約3~4公里,向東進入雪山山脈與中央山脈其深度則在4公里以上。
2.
明確層-於波形模擬的過程中,此不連續面確有其存在之必要性,深度在9~12公里,大致呈水平分布。兩個明顯高區分別位在地震帶之西北端與東南端附近,前者深度達12公里,後者在11公里以上,其它地方相對較淺,平均深度約10公里
3.
康拉不連續面-由西部海岸平原向中央山脈遞增,海岸平原平均深度18公里,西部麓山帶深約18~20公里,向西至雪山山脈則進入20公里深度,且梯度有增加的趨勢,最大深度約25公里,平均來看康拉不連續面以約5度角向中央山脈傾斜。
4.
莫氏不連續面-朝中央山脈加深的趨勢非常明顯且梯度非常大,最淺約29公里,最深則達45公里,平均以10度角由海岸平原向中央山脈下傾。等深線於地震帶西南側呈倒V字形,其頂點落在埔里盆地附近,顯示於該區Moho深度相對變淺。
二、三義-埔里地震帶
由二維速度構造及地震帶附近之震源深度分布情形我們發現,莫荷面深度於地震帶的兩側有很明顯的變化,其東北側較深而西南側相對較淺,尤其在進入雪山山脈後更為顯著,康拉不連續面亦呈現類似情形但較為緩和。由此結果推論,三義-埔里地震帶的形成機制可能與莫荷面落差有關,由於歐亞大陸板塊與菲律賓海板塊斜向碰撞在台灣地區發生一系列的造山運動並致使地殼厚度加深,此一同時,於造山帶前端的狀逆地塊於西邊因受到北港高區的影響而於台灣西北部往西北方向脫逸,並連帶把加深之地殼隨之引入,因而產生此區莫荷面之落差,且於西南側邊界造成地震構造帶。

[ 英文摘要 ]

Seismic waves are affected by seismic sources, paths of propagation, and structures of underground layers. To understand the seismic sources within a specific region, it is necessary to acquire a more detailed velocity structure. Many studies based on inversions of P-wave arrivals have been made to explain the velocity structure underneath Taiwan area. In this study, a generalized ray theory and finite difference method were applied to analyze seismograms recorded by the strong motion network within the Sanyi-Puli Seismic Zone, to have a closer view of the velocity structure beneath the mid-western Taiwan region. One dimensional velocity structure for each path is constructed by the generalized ray theory with the consideration of the arrival times and amplitudes of the reflected seismic waves, including reflections from the base of the sedimentary layer, Conrad Discontinuity and Moho Discontinuity. Based on these results, finite difference method is then applied to construct a two dimensional velocity structure to analyze the depth distribution of each discontinuity presented in the region. The results from this study are stated below:
A. S-wave velocity structure within the mid-western Taiwanregion:
1. The depth of the sedimentary layer in the region increasedfrom the western coast towards the Central Range, which isstrongly affected by the geological structure near thesurface. The average depths at the coastal plane and the western foothill were 2~3 and 3~
4 km, respectively. To the east, the depth of the sedimentary layer is at least more than 4 km after encountered the Central Range and the Hsuehshan Range.
2. During the construction of the one dimensional velocity structures, an unknown horizontal discontinuity with depth ranges from 9 to
12 km was necessary to explain most of the observed seismograms. The depths of the discontinuity were deeper at the northwestern end and the southeastern end of
the studied region. The average depth is about
10 km.
3. The depth of the Conrad Discontinuity increased from east to west, with the average depth of 18, 18~20 and
20 km beneath the coastal plane, the western foothill, and the Hsuehshan Range, respectively. The depth gradient increased eastwards after encounters the Hsuehshan Range, to a maximum depth as 25 km. As a whole, the Conrad Discontinuity inclined to the
Central Range with an average angle of 5 degrees.
4. The inclination of the Moho Discontinuity towards the
Central Range is very obvious in the studied region. The minimum depth is 29 km, while the maximum depth is up to 45 km, with an average inclination angle of 10 degrees. The depth contour showed an reversed V shape at the southwestern
end of the studied region, with its top point been located near the
Puli Basin, which indicated an shallower depth of the Moho Discontinuity beneath the certain area.

B. Sanyi-Puli Seismic Zone: The two-dimensional velocity structure indicated a sharp variation of Moho depth at the Sanyi-Puli seismic zone. The depth was shallower at the northeastern side, and deeper at the southwestern side. The variation is obvious beneath the Hsuehshan Range. The depth variation of the Conrad Discontinuity had a similar but smoother trend. From these results, we postulated that the formation of the Sanyi-Puli Seismic Zone might have a close relationship with the sharp variation of the depth. As the oblique convergence of the Eurasia Plate and the Philippine Sea Plate, the thrust wedge at the front of the orogenic zone escaped in a northwestern direction by the affection of the existence of the BeiKang Gravity High in the south. The northward movement of the thicken crust was then induced and produced a sharp variation of the Moho depth.