利用高密度地震網分析 台灣近震源地震序列與深部構造

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Title:	利用高密度地震網分析台灣近震源地震序列與深部構造
Authors:	史旻弘;Shih,Min-Hung
Contributors:	地球科學學系
Keywords:	地震序列;雙差分定位法;雙差分層析成像法;桃源;earthquake sequence;hypoDD;tomoDD;Taoyuan
Date:	2014-07-28
Issue Date:	2014-10-15 14:43:35 (UTC+8)
Publisher:	國立中央大學
Abstract:	2008年3月4日17時31分47秒(UT)在高雄市桃源區發生芮氏規模5.2之地震。該地震伴隨著前震與大量之餘震,且餘震主要集中發生於主震後8小時之內,為一個完整之地震序列。地震序列發生的時間適逢為TAIGER 計畫執行期間,該計畫在全台各地設置大量的地震儀以記錄人工爆震源與天然地震。因此該地震序列之波形資料,被鄰近密佈之地震觀測儀完整地記錄下來,提供了多方向研究之可能。首先,本研究利用近震源之高密度地震網,對此地震序列進行完整之初步定位,並使用兩階段重新定位,第一階段先引入測站修正值重新定位,提高其相對定位精準度,第二階段使用hypoDD進行第二次重新定位。由最後之定位結果,得以了解該地震斷層之形貌,確定其斷層走向為東北-西南方向,傾角約為 45°往東南方向傾斜,且地震深度隨著地震走向往西南方向逐漸變深。同時此地震序列之空間分布位置與形貌,以及主震之震源機制,皆與鄰近之內英斷層近似,因此推論此地震序列應與內英斷層有關。本研究進一步再利用tomoDD逆推此地震序列發生之區域,以及鄰近地區之三維速度構造。在Vp與Vs之速度變化以及Vp/Vs分布上,可以看出地震多分佈在高值與低值區域之交界處,而主震發生區域緊臨一個相對低速區域。推斷此區域可能為破裂面,或為斷層滑動之界面所在。同時在地震分布並延伸至地表的方向上,存在一個傾角為45°的低速帶,且此低速帶與地表之內英斷層的位置相當。因此由速度構造中低速帶之存在、震源之空間分布及內英斷層的地表位置,三者間在空間中之關係可佐證內英斷層與地震序列之間的相關性。同時在鄰近區域之速度變化上,西邊之速度也較東邊之速度低,代表地層由更新世的新沉積層,轉變為中央山脈近地表之第三紀變質岩帶。最後,透過使用時間項之方法,利用TAIGER計畫之人工震源資料並聯合天然地震資料,讀取台灣北部測站之 Pn 波到時,推求北台灣下方之深部構造變化之時間項變化。時間項之分佈特徵為,台灣北部至東北部存在一個時間項之相對低區,在台北盆地、宜蘭清水以及宜蘭盆地,皆表現出時間項之相對高區。由透過不同之折射層P波速度、地殼P波速度以及折射層深度的組合,所得到之理論到時與觀測到時的殘差值為最小時,可得到合理的折射層P波速度、地殼P波波速以及地殼厚度組合為 7.6 km/sec、5.6 km/sec 以及35公里。但沉積層或地殼中之異常的速度構造,將對時間項產生影響,使得地殼厚度之估算造成誤差。因此,在東西向之剖線上進行沉積層修正,所求得之莫荷面較淺且變化較為平緩,但變化的趨勢與前人研究一致。其變化為西部平原與麓山帶約30公里,進入中央山脈後深度加深至約34公里,再往東部深度再度變淺為約28公里。;The March 4, 2008, a moderate earthquake (ML 5.2) occurred in the southwestern part of Taoyuan District of Kaohsiung City in southern Taiwan. It was followed by numerous aftershocks in the next 48 hours, including three events with magnitude lager than 4. This sequence occurred during the TAIGER (TAiwan Integrated GEodynamics Research) controlled-source seismic experiment, several seismic networks were deployed in the Taiwan area at this time and many stations recorded this earthquake sequence in the near-source region. We processed the near-source observations and determined 2,340 events in preliminary result. Then we relocated events with station corrections first and relocated again by applying the double-difference relocation algorithm with the differential arrival times obtained by the waveform cross-correlation. The resulting aftershocks are extended along the NE-SW direction and located on a 45° SE-dipping plane. This plane agrees to one of the nodal planes of the focal mechanism determined by BATS (strike=N 37°E, dip=48°SE and rake=96°). Furthermore, we used double-difference tomography to relocate earthquake locations and determine P and S wave velocity structure around the sequence of Taoyuan earthquake. A total of 3,034 events were selected from preliminary results and CWBSN catalog. The relocation results are similar to the prior results and the 3D velocity structures show that obvious velocity variations exist in the crust beneath the Ne-In fault. The sequence occurred on the interface between the low and high velocity area. Based on the hypocentral distribution pattern, focal mechanism and velocity structure variations, we suggest that the Taoyuan earthquake was probably caused by Ne-In fault. The P arrivals of 431 ray paths with the epicentral distances longer than 160 km from 10 explosive sources and 10 earthquakes in Taiwan were analyzed according to the time term method, a simple travel time inversion for seismic re- fraction crustal studies. We used the data to investigate the variation of time term values in northern Taiwan. The results show that relatively high time term values are obtained in the areas of Taipei basin and Chingshui area. The relative high values locate at the northernmost and northeastern area in Taiwan. We also determine the optimal combinations of Pn velocity, crust velocity and Moho depth. The values of 7.6 km/s, 5.6 km/s and 35 km are obtained, respectively.
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