利用接收函數法分析遠震寬頻資料推估宜蘭平原地殼厚度;Determination of Moho depths in Ilan plain by receiver function analysis

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题名:	利用接收函數法分析遠震寬頻資料推估宜蘭平原地殼厚度;Determination of Moho depths in Ilan plain by receiver function analysis
作者:	孫元晟;Sun,Yuan-Cheng
贡献者:	地球科學學系
关键词:	宜蘭平原;莫荷深度;接收函數;Ilan plain;Moho depth;receiver function
日期:	2015-07-30
上传时间:	2015-09-23 11:30:24 (UTC+8)
出版者:	國立中央大學
摘要:	宜蘭平原位於台灣島的東北部，西北接雪山山脈，南接中央山脈，地理位置上處於沖繩海槽的西南端。沖繩海槽本身是菲律賓海板塊向西北隱沒至歐亞板塊之下形成之琉球島弧，在其後方張裂歐亞大陸板塊邊緣造成之弧後盆地。在本研究中，我們在2013年11月至宜蘭平原設置了10部臨時寬頻地震儀，用來研究分析此宜蘭平原地區的莫荷不連續面分佈。所以我們將得到的遠震地資料選取進行接收函數法處理，以對應之轉換波相包括Ps轉型波、PpPms及PsPms+PpSms兩個複反射波與直達P波到時差來推估莫荷不連續面深度。我們使用時間域迭代解迴旋法運算得到接收函數，其中時間域迭代解迴旋法是利用垂直向與水平向波形進行交相關運算得到對應之脈衝函數，再以此脈衝函數與垂直向波形進行迴旋 (convolution)，直到對應之徑向波形與觀測之徑向波形誤差小於一定值，脈衝函數總和即為所求之接收函數結果，再以此結果進行H-κ疊加法運算而得對應之莫荷不連續面深度 (H) 及Vp/Vs比 (κ)。接下來我們以H-κ疊加法有無得到結果可以大致將宜蘭平原分為南北區域。南區5個測站 (SSE、DJE、LLK、SAJ、MSE) 由西北、東北、西南象限入射遠震事件之接收函數結果皆缺乏清晰之轉換波相，以轉換點檢驗後發現這些遠震事件的轉換點多位於雪山山脈、中央山脈靠近內陸及宜蘭平原底下，導致H-κ疊加法得不到最佳莫荷面深度及κ值。北區5個測站 (SJY、DTE、NCH、NCE、DHE) 四個象限的入射波轉換點與南區5個測站非東南象限入射的轉換位置有一致的結果，所以也無法得到最佳莫荷面深度及κ值。 H-κ結果只有南區5個測站由東南象限入射遠震事件之接收函數疊加得之。我們認為能夠得到結果的主因是由於東南象限入射的轉換位置多位在測站的東南方，顯示中央山脈底下近海處或靠近琉球島弧底下的位置所轉換的入射波之接收函數有較清楚的轉換波相，故能得到最佳之H-κ結果。莫荷不連續面深度介於24至31公里之間，其中以測站DJE轉換點位於蘇澳附近之莫荷面深度最深。北區5個測站之接收函數雖然大致上能觀測到Ps轉型波訊號，但PpPms及PsPms+PpSms兩個複反射波訊號較難從接收函數中觀察到，所以無法以H-κ疊加的最大值得到最佳之莫荷不連續面 (H) 及κ值。但如果已知κ值，可從Ps轉型波的到時求出對應的莫荷不連續面深度。故我們將南區5個測站的κ值取平均，以推得一代表宜蘭平原北區5個測站轉換點位置的代表κ值，再以此平均κ值分別帶入宜蘭平原北區的5個測站的H-κ結果，找出各測站入射波平均轉換點的莫荷面深度。得到的莫荷不連續面深度介於27至29公里之間。此種做法雖然有一定的誤差存在，但是在資料有限制的情況下，所能得到的最合理推測。 ;The Ilan Plain, abutting the Hsueshan range to the northwest and the Central range to the south, is a triangular alluvial delta in northeast Taiwan most likely representing the southwest end of the Okinawa Trough, a back-arc basin due to the subduction of the Philippine Sea Plate underneath the Eurasia Plate. In this study, we deployed an array of ten broadband stations since Nov. 2013. The array is roughly distributed evenly on the Ilan Plain near mountain hills, which is designed to investigate the spatial variations of Moho depths. We conduct receiver function analysis on data of teleseismic events, mostly from the southeast quadrant. The P converted to S phase (Ps) and PpPms, PsPms+PpSms multiple reflection phases at the Moho are examined. The analysis is done by time-domain iterative deconvolution, which progressively subtracted from the radial-component seismogram with the convolution of vertical-component seismogram and updated receiver function. Results show that stations with or without starkly contrasts of H-κ stacking can roughly be divided into those in the south and those in the north Ilan Plain. For the five stations in the south Ilan Plain (SSE, DJE, LLK, SAJ, MSE), teleseismic events from the northwest, northeast and southwest quadrants are mostly unable to resolve receiver functions with clear Ps and/or multiple reflection later phases (PpPms, PsPms+PpSms). Having examined the convergent points of those events, we conclude that Moho discontinuities beneath the Hsueshan range, the Central range inland side, and the Ilan Plain are not sharp enough to generate all phases necessary for the H-κ analysis. For the five stations in the north Ilan Plain (SJY, DTE, NCH, NCE, DHE), the convergent points of events from the four quadrants are all fallen among the aforementioned regions and, for similar reasons, unresolvable of H-κ stacking. The only scenario with starkly contrasts of H-κ stacking to be determined is those teleseismic events to the southeast quadrant of the five stations in the south Ilan Plain. They contribute the most abundant observations with derived receiver functions consistently exhibiting arrivals of Ps and PpPms, PsPms+PpSms phases. As a result, H-κ results are able to individually determine the Moho depths convergent beneath southeast of the Ilan Plain with depths ranging from 24 to 31km and the deepest one occurred near Suao. We conclude that the Moho discontinuities to the southeast of the Ilan Plain (the Central Range shore side, offshore Suao) are sharp enough to generate all phases necessary for the H-κ analysis. Using the average κ as determined from the five stations in south Ilan Plain, we manage to estimate the Moho depths for the five stations in the north Ilan Plain and the results range from 27 to 29 km. Although the results are subject to certain level of uncertainty, it is the optimal approach under conditions of limited data.
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