藉由高密度地震網之監測資料探討新竹地區地殼構造的特性; Crustal Structures Revealed by a Dense Seismic Network in the Hsinchu area of Taiwan

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题名:	藉由高密度地震網之監測資料探討新竹地區地殼構造的特性;Crustal Structures Revealed by a Dense Seismic Network in the Hsinchu area of Taiwan
作者:	蒲新杰;Hsin-chieh Pu
贡献者:	地球物理研究所
关键词:	慢速滑動;軟橋斷層;火成侵入岩體;中新世沈積盆地;新竹;地震網;微震網;三維速度構造逆推;Hsinchu;Seismic network;Tomography;Miocene sedimentary basin;Intrusive igneous rocks;Raunqiao fault;Slow slip
日期:	2010-03-31
上传时间:	2010-06-11 15:48:14 (UTC+8)
出版者:	國立中央大學圖書館
摘要:	台灣地區受板塊擠壓，產生了極複雜的地下構造與頻繁的地震活動。但在本研究的新竹地區中具有密度極高的活動斷層，但地震活動卻相對較少。為了更深入瞭解此區域的地下構造與活動特性，本研究在此地區設置了高密度的地震監測網對微震進行監測，簡稱微震網。此微震網是由25個連續記錄的地震站所構成的，運作時間為自2004年04月至2006年12月。此微震網監測到了1,938個地震活動，我們利用了這些地震訊號，逐一重建了此地區的一維速度模型、三維速度構造與MD-ML的規模轉換式，並將地震重新定位。此外，也利用地震波形的初動方向，求解出282震源機制。並進一步利用這些震源機制求解出小區域的應力場方向。整合這些觀測結果，本研究發現，在研究地區的西北側，由於具有較深厚的沈積盆地和相對的波傳低速區，地震活動較少。因此，此地區的斷層形貌僅能由速度構造推估之。而於波傳的低速區外，地震活動的範圍則相對較廣。以地震分佈和三維速度構造，並配合震源機制，則可較清楚的看到高傾角的新城與大平地斷層和其活動性。此外，於三維速度構造中，也發現到一些呈垂直分佈的異常高速區域，與中新世的火山活動分佈比對後，發現這些高速區可能代表了冷卻後的火成侵入岩體。其成因可能與沈積盆地相仿，應是中新世時期的拉張環境所致。另外，利震源機制逆推應力形態後，發現在研究區內的應力方向有明顯的變化。這些變化反應著一條約為東南-西北（Wrench fault；亦是由地震能量分佈中發現之）與和另一條東北-西南（軟橋斷層）走向的斷層之活動。最後，更深入的分析所求解出的震源機制後，發現正斷層與走向滑移斷層的震源機制，其發生性在時間上有輪替的現象。配合GPS的觀測與前述的研究，認為這個輪替現象可能是由軟橋斷層深部的抬升型式錯動所致。於抬升停止期，此地區受大地應力的擠壓而形成走向滑移的震源機制;但若於抬升活躍期時，軟橋斷層上盤的深部抬升並於其上部地層形成了正斷層的機制與構造。而於研究區內並無足夠大的地震可以表現此一錯動，但卻可發現整個軟橋斷層系統南側的有感地震發生與震源機制的輪替有很高的相依性。因此，推測軟橋斷層於本研究地區的活動可能為有別於一般地震的慢速滑動。 Strong plate convergence in the Taiwan area often produces a lot of earthquakes as well as complicated crustal deformation. However, seismic activity in the Hsinchu area is significantly lower than that in most of other areas where many earthquakes have been taken place in the Taiwan area. In order to improve the understanding of subsurface structures and their geological properties in the Hsinchu area, we deployed a dense micro-seismic network with 25 seismic stations between April 2004 and December 2007. In total, 1938 earthquakes were located in the period of 2.5 years. We used these seismic data to construct one-dimensional, three-dimensional velocity structures and the empirical relationship between MD and ML. We also estimated stress field according to 282 focal mechanisms inverted from the first motions of P-waves. Based on those results, some of interesting results have been obtained. First, low seismicity was largely found beneath the northwestern part of our study area, where has a low P- and S-wave velocity and may be associated with a large scale of the Miocene sedimentary basin. Subsurface structures beneath this part can only be estimated from three-dimensional velocity images without help of seismicity and focal mechanisms. On the other hand, high-angle fault planes of the Hsincheng and Tapingti faults in other parts of the study area can be clearly obtained from seismicity patterns, velocity images and focal mechanisms. Second, some pillar-anomalisms with high velocity are undoubtedly found in inverted velocity profiles. Comparison of the locations between pillar-anomalisms in the upper crust and the Miocene volcanoes near the surface, we ascribe these anomalisms to the intrusive igneous rocks, which were produced of extension environment in the Miocene epoch. Third, significantly spatial variation of stress fields estimated from 282 focal mechanisms shows two active fault systems in the Hsinchu area. One is a wrench fault with a strike of NW-SE; the other is the Raunqiao boundary fault along NE-SW. Finally, the temporal variation of seismic energy released by different faulting mechanisms show a negative correlation between normal and strike-slip sources. Based on the GPS and seismic results, we interpret this phenomenon is might be caused by a deep uplift (deeper than 10 km). As deep strata uplift silently, local stress field predominates over strike-slip faulting. On the contrary, normal faulting might occur in the shallow strata when deep strata uplift actively. Since there was no earthquake in which uplift was found, the deep uplift was a kind of slow slip along the Raunqiao fault plane in the Hsinchu area.
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