| 摘要: | 本論文使用2006 至2008 中央研究院、中央氣象局所建置之永久寬頻地震觀測網與TAIGER計畫臨時寬頻地震紀錄，共104個測站，且選取約有五百個遠震地震事件。將遠震記錄進行資料處理，以得到各測站的接收函數，並針對接收函數其莫荷面對應的Ps轉型波相進行探討，根據其極性與到時隨後方位角的變化，進而估算測站下方莫荷面的深度與傾斜方向。 假設一厚度為35公里且向南傾斜的層面模型，模擬當地震發生在不同的後方位角時所產生的模擬地震紀錄與接收函數，並觀察波形隨著後方位角的變化。模擬結果得知，這些合成接收函數在徑向分量之波形，以後方位角為180°為基準呈對稱關係，且P波及Ps轉型波的振幅變化恰相反，即當Ps波的振幅最大時，P波的振幅卻是最小。而切向接收函數波形，則以後方位角180°為基準呈反對稱，且P波及Ps轉型波極性相反。徑向或切向接收函數波形都可觀察得知P波振幅隨後方位角改變的變化較Ps轉型波更為明顯。以上接收函數的波形特性與原則，則可以幫助實際資料的判讀。 根據Ps轉形波到時來分析全台莫荷面的起伏，由北到南的趨勢可以看出，在台灣最東北角有最淺的莫荷面，其深度為泰平分校（TIPB）測站的21公里至五分山（WFSB）測站28公里，其中全台最淺的莫荷面深度為泰平分校（TIPB）測站的21公里。其形成機制可由沖繩海槽的弧後擴張來解釋。根據台灣北部十個測站的分析結果，可以得知在臺灣北部（北緯24.6°~25°,東經121.1°~121.6°）的莫荷面深度為TGN05測站的27公里至TGN12測站的33公里。相對的在中央山脈北段（北緯24°~24.5°,東經121.3°~121.8°）則有較深的莫荷面，其深度為寧安橋（NACB）測站的42公里至TGN09測站的53公里。這是由於菲律賓海板塊向北隱沒，在隱沒邊界的地殼由於擠壓撓曲而增厚，故使得莫荷面加深。並由於擠壓後形成的地殼隆起，造成台灣北部的莫荷面變淺。在臺灣中部（北緯24°,東經121°）則有最深的莫荷面，其深度為TGC06測站的50公里至雙龍（SSLB）測站的56公里，其中值得一提的是，全台最深的莫荷面深度為雙龍（SSLB）測站的56公里，且莫荷面有向東變淺的趨勢。其形成機制可由弧陸碰撞的岩石圈增厚來解釋。台灣南部（北緯23.5°以南）的平均莫荷面深度則約為35公里，與全球平均的莫荷面深度一致。根據台灣北部六個測站其Ps轉形波極性可以得知，台灣北部的莫荷面有偏向南傾斜的趨勢。此結果與利用全台Ps轉型波到時分布，進而估算出的莫荷面深度分布的結果一致。 由接收函數疊加剖面及波形模擬結果得知，金門站其徑向接收函數Ps轉型波到時為4.1秒，PpPmS復反射轉形波到時為13秒，可以估算其測站下方莫荷面深度約為34公里。由切向接收函數其P波與0.5秒處的Ps轉型波極性變化可知，在測站下方三公里處有一向西北傾斜的速度不連續面。澎湖站的徑向接收函數，其初達P波到時不是在0秒處，而是向後移了0.5秒。在約3秒與4.5秒處中Ps1與Ps2轉型波相其所反映的速度構造。可以解釋為由於測站下方有岩漿侵入，使地殼產生部份融熔所形成的鐵鎂質岩漿沉澱之上下界分別對應的速度不連續面。 Receiver function (RF) waveform observation, analysis and simulation approaches are used to investigate the crustal thickness and main crustal structure discontinuity beneath each broadband station in Taiwan. The main goal of this study is to resolve the spatial depth and dip angle variations of Moho discontinuity. The proposed Taiwan Reference Moho Discontinuity Model (TRMDM) is based on the analysis of teleseismic data collected by stations deployed and maintained by the Institute of Earth Sciences (BATS) and Central Weather Bureau (BBCWB) with additional five temporary broadband arrays deployed along east-west and north-south transect lines across island organized under TAIGER (TAiwan Integrated GEodynamic Research) project. More than 500 teleseismic events from Incorporated Research Institutions for Seismology (IRIS) report were recorded by 104 BB stations during 2006-2008. The data selection criteria are that the teleseismic events with mb ≧ 5.5 and epicentral distance is from 30 to 95 degree. From synthetic modeling studies of RF profile for uniform dip, planar interface with sufficient velocity contrast, the radial component shows symmetric waveform variation with back azimuth angle. That is, the amplitude variation with back azimuth angle of P- phase has inverse relationship with Ps phase. For the same polarity, the maximum amplitude of P- phase will corresponds to minimum amplitude for Ps- phase. For transverse component, the anti-symmetric property, coincidence of amplitude variation and inverse polarity change between P- and Ps- phase varying with back azimuth angle can help to determine the dipping direction. In addition, for both components, the amplitude variation with incidence angle for direct P phase is more apparent than the changes in Ps phase. For teleseismic data analysis, interpretation and identification of Moho conversion phase from stacked RF through different stacking criteria and travel-time picks were performed. Time-to-depth conversion on the manually picked Ps phase arrival time, determination of dipping direction and angle variation of TRMDM beneath each station are studied. The single stacked RF trace from all incoming plane waves and stacked four traces from four major directions impinging upon each station are examined and compared their waveform, amplitude, polarity and arrival time variations in order to determined their dipping angle and direction. Ps arrival time information is used to constrain Moho depth. Relative thin crust (21-24-28 km) in the northernmost corner (TIPB-TWBB-WFSB) of the island may correspond to slab budge and/or back-arc opening of Okinawa trough. In northern Taiwan, the Moho depth derived from ten stations indicates that: Moho depth varying from 27 km (TGN05) to 33 km (TGN12) in region covers latitude 24.6o to 25o and longitude 121.1o to 121.6o. In the region close to the northern end of central mountain range, between latitude 24o to 24.5o and longitude 121.3o to 121.8o, significant deeper Moho depth varying from 42 km (NACB) to 53 km (TGN09). In the northern end of the backbone range close to Ilan and Hualien county border, the northeastward subduction and flexure bending of the Phillipine Sea plate causing crustal thickening while thin crust behind the plate bending corresponding to in the northernmost Taiwan. At southwestern end of Shuieh Shan and close to central Taiwan, Moho reaches depth of 50 km (TGC06) to its maximum of 56 km (Shunglong station, SSLB) and become shallow towards east coast. The mechanism involving significant Moho depth variations may attributes to lithospheric thickening due to arc-continent collision in central Taiwan. In southern Taiwan (south of latitude 23.5o), average Moho depth is 35 km which is consistence with average global Moho depth. In northern Taiwan, analyzing polarity variations of six BB stations show clear southward dipping of Moho surface which is consistent with result derived from Ps arrival time. Simulation of common-receiver RF stacking profile at each station provide more detailed information on their spatial azimuthal variation of Moho discontinuity beneath Taiwan island. For Kimen (KMNB) and Matsu (MATB) stations, the clear Ps and PpPmS phases, at 4.1 and 13 sec respectively, show that the average Moho depth is around 34 km. At Kimen station a northwestward dipping shallow interface at depth of 3 km (0.5 sec) is identified from transverse component. At Penhu (PHUB) station, although an obvious 0.5 second shift occurred for P- arrival, the clear arrivals at 3.0 and 4.5 seconds (Ps1 and Ps2) may indicate partial melting of felsic basaltic magma intrusion produce high elastic impedance contrasts in the upper and lower intrusive boundary. Further detail analyses on the conversion phase for other stations are required in the future. The proposed TRMDM can be further constrained base on the broadband data available from TEC data center. |
