| 摘要: | 本研究使用中央研究院地球科學研究所(IES)寬頻地震網(BATS)、中央氣象局(CWB)寬頻地震站及中國福建地震局(EAFP)幾個沿海寬頻地震站的地震資料進行台灣及其鄰近地區的二維雷利波群速成像研究。首先，以多重濾波法分析1996至2015年發生在台灣及其周圍地區芮氏規模大於5.0地震所產生的區域雷利波，最後得到超過1900條的雷利波群速；接著，將研究區域以0.25°×0.25°大小劃分成288個區塊；最後，以層析成像逆推法完成台灣及其鄰近地區週期7-42秒的二維雷利波群速成像。結果顯示，在週期16秒之前，速度側向變化明顯，高、低速的分布符合地質岩性及地體構造的變化；週期16秒前，中央山脈呈現高速，與變質岩帶有關，之後，漸漸轉為低速並持續至週期42秒，顯示中央山脈有厚的地殼，暗示山根的存在，此外，此週期對應的深度約16-17公里，似乎反映中央山脈上、下部地殼可能受到不同板塊變形機制的作用，而中央山脈中段的高速遲至週期20秒才轉為低速，暗示有地殼堀升的作用。在週期14-19秒於西部北緯24°-25°處呈現相對低速，此區可能存在上部地殼低速帶，因此，埔里布蓋重力低區除與淺部地質及中央山脈山根有關外，亦不能忽略此上部地殼低速帶的影響。西部海域(台灣海峽)在週期16秒前，由北至南顯現低-高-低速特徵，與此海域的沉積盆地及澎湖隆起分布十分一致；而東部海域的高、低速分布則與菲律賓海板塊、琉球島弧系統有關；隨著週期的增加，海岸山脈的高速漸往南移，此為菲律賓海板塊在台灣東部向北隱沒的特徵，但此高速往北未過北緯24°，可能是本研究所使用的週期不夠長所致，然沿海岸山脈往北超過北緯24的低速，或許與地函楔有關。
In this study, we used seismograms from broadband stations installed by IES, CWB, and EAFP to examine the 2-D images of Rayleigh-wave group-velocity in Taiwan and its surrounding regions. First, the multiple filter technique was adopted to determine the group-velocity of Rayleigh-wave generated by ML ≥ 5.0 earthquakes, which occurred from 1996 to 2015. At last, more than 1900 Rayleigh waves were obtained. Subsequently, we divided the study region into 288 sub-blocks with each size of 0.25°×0.25° to carry out the tomographic inversion for the 7-42 periods Rayleigh-wave. For periods less than 16 sec, lateral variation in velocity was quite obvious and corresponded to the geological features. In the Central Range (CR), high velocities existed in period less than 16 sec; however, for periods longer than 16 sec, low velocities dominated the region and persisted until the 42-s period. This implied the mountain root beneath the CR. The transition period between high and low velocities in the CR was 16 sec, consistent with the depth of 16-17 km. This probably indicated the deformation mechanism in the upper crust was different from that in the lower crust under the CR. Additionally, the high velocity in the middle of the CR existed until 20-s period, implying the crustal exhumation taking place in the middle CR. At periods of 14-19 sec, there was a relatively low velocity appearing between 24° and 25° north; thus we suggested a low-velocity zone in the upper crust, which also contributed to the Taichung-Puli gravity lows except for from the shallow geology and mountain root. The velocity variations in the eastern waters off Taiwan were related to the Philippine Sea plate (PSP) and Ryukyu arc system; whereas those in western waters of Taiwan were consistent with the features of sedimentary basins and Penghu Platform. The high velocity moved northward along Costal Range with increasing period. This seemed to show the feature of subduction of PSP. However, the high velocity did not cross the position, 24° north. Presumably, the periods used in this study were not long enough to show the deeper variations about the subduction. In addition, the low velocities around 24° north near northeastern Taiwan was probably in connection with the mantle wedge. |
