台灣活動斷層構造相關地殼變形監測研究

Abstract

摘 要 台灣位處極為活躍的構造環境，因此地震的發生是台灣自然環境的一部份，也就是有地震的發生，才有台灣島的產生，既然地震是不可免，瞭解地震可能帶來的災害，並設法降低地震可能帶來的災害是我們必須面對的課題；回顧台灣地震災害，主要源自於斷層的活動，近百年來台灣已承受五次斷層活動所導致嚴重地震災害，皆因斷層的活動導致嚴重的地表破裂，造成生命與財產損失慘重。本研究乃以GPS及精密水準等大地測量方法，針對活動斷層監測其地表的三維位移型態，據以建立短期的滑移速率監測系統，提供推估各斷層的活動週期及活動特性，例如其幾何形貌、滑移速率、潛移或鎖定、再現週期、可能產生的地震規模等，希望藉此結果能評估下一次地震可能造成的災害與影響範圍。由於板塊間的擠壓與隱沒，造成台灣複雜的地質構造環境，在台灣西部麓山帶地區以擠壓為主要的變形環境，北部地區以橫移壓縮和橫移伸張為主的變形環境，東北部地區則以隱沒帶弧後伸張變形為環境；本文以中部地區、宜蘭地區、台北地區來探討這三種不同環境下的近期地殼變形。 中部地區在集集地震前大致向西北方向位移，車籠埔斷層下盤位移速率約在10 mm/yr左右，斷層上盤往東側逐漸增大，在10~30 mm/yr之間；集集地震發生前2年，彰化斷層下盤的位移方向由西北西轉為偏北的位移型態，位移速度則由南向北增大，轉為由西向東增大的趨勢；即位移方向與位移速度有明顯的改變，可能為地震前兆的相關資訊。集集地震之同震變形，在車籠埔斷層下盤向東南方向位移0.30~1.5 m，且越靠近斷層位移量越大；車籠埔斷層上盤則向西北方向大幅位移約1.4~7.0 m，濁水溪以南的位移量較小；集集地震以後之震後變形型態，以車籠埔斷層為分界，上盤在震後向西北西方向位移的型態仍與同震時相同，下盤由同震時一致性的向東南東方向，逐步向西北西或西南西方向位移，亦即在車籠埔斷層下盤有逐步依順時針方向回歸至震間的滑移型態；另就位移速率而言，雖然同震變形最大地區在車籠埔斷層北端，但其震後的變形量最大地區，卻集中於震央位置附近；就地震循環而言，震後的6年期間仍處於地層壓密重整階段，尚未回歸至該區震間階段的位移狀態。在垂直變動方面，震後在車籠埔斷層上盤仍有抬升的趨勢。 宜蘭地區GPS結果乃針對現今沖繩海槽西南緣的變形進行詳細地研究，於碰撞和弧後伸張過渡帶中提供對於地殼旋轉及構造脫逸環境下，其應力與應變型態的重要解釋。胡植慶等提出台灣北部地區為側向伸張及側向擠壓環境之過渡帶。區域模型顯示沖繩海槽地區至台灣東北方之最大伸張方向以逆時鐘方向，由南-北向轉成西北-東南向，其量值也隨著接近台灣東北部地區而快速減小。應變速率場的結果也顯示，宜蘭平原主要呈現西北-東南方向之伸張，宜蘭平原現今變形行為之特性為平原中、南部為西北-東南方向之拉張，一般而言，接近山脈之伸張量並不大，且其量值會向南方和東南方快速增加。這與過去十數年的變形型態極為一致，代表位處沖繩海槽西延的宜蘭區域，仍受弧後伸張的影響，屬於伸張變形的環境。 台北地區相對於澎湖GPS監測從1992年至1998年11月期間的位移方向，除了林口台地西南側向西南方向位移外，其餘地區大致向東南東或東南方向滑移，其地表水平位移速率大多在5 mm/yr以下，出現位移速率較大的兩個區塊連線與淡水河出海口的方向平行，另兩個區塊中間則呈現位移速率較小的情形。在金山斷層及山腳斷層兩側有位移型態的變化，但位移型態受宜蘭外海地震較大震度的影響，產生異於震間的變形行為。台北地區1992~2000年之應變率分析結果，大致朝東─西及西北西─東南東方向的伸張變形，另約略呈朝南─北方向的壓縮狀態；2000~2002之應變率比上一時段的累積應變更加明顯，大致朝西北西─東南東方向的伸張變形，另約略呈北北東─南南西方向的壓縮狀態；山腳斷層上盤仍為持續下陷的趨勢，其原因不應只是單純的地層壓密所致，應與山腳斷層的正斷層作用有關；另於下陷區東緣存在反向正斷層的可能性極高。

Abstract Taiwan is located on active plate boundary, and earthquakes are therefore part of people’s everyday life. To clarify the potential effect area and to mitigate possible hazards are always the critical issue for scientists in Taiwan. During the past century, large earthquakes occurred five times and each time caused severe casualties as well as property losses. In our study, we used geodetic method such as Global Positioning System (GPS) and leveling to monitor the surficial 3D deformation of active structures, established short-term slip-rate monitoring system, to calculate the geometry, slip rate, creeping/locking behavior, recurrence interval and prospective earthquake magnitudes of major active structures, and thereby evaluated possible disaster area of each fault. Because of the double subduction system, different strain patterns are observed in different tectonic subdivisions: the western Foothills are compression-dominated; northern Taiwan is the combination of both compression and extension; northeastern Taiwan is subduction-related back-arc extension. We used Taichung Domain, Taipei Domain and Ilan Domain to discuss these three deformation behaviors. The Taichung Domain was moving northwestwards before the 1999 Chi-Chi earthquake: in the footwall of Chelungpu Fault the slip rate was ~10 mm/yr, while in the hanging wall the rate increased eastwards up to 10-30 mm/yr. Two years prior to the earthquake, in the footwall of Changhua Fault the deformation direction changed from WNW to almost due N, and velocity gradient changed from increasing northwards to increasing eastwards, suggesting possible precursor of a large earthquake. As for coseismic deformation of Chi-Chi earthquake, the footwall of Chelungpu Fault moved southeastwards 0.3-1.5 m, and the value was increasing toward the fault; the hanging wall moved northwestwards 1.4-7.0 m, but the value was smaller south of the Choshui River. After the Chi-Chi earthquake, the hanging wall of Chelungpu Fault was still moving west-northwestwards and kept uplifting, while the footwall gradually changed direction from ESE to WSW or even WNW, clockwise resuming to inter-seismic deformation pattern. It is interesting to notice that despite the largest coseismic deformation in the northernmost segment of Chelungpu Fault, the post-seismic deformation peaked around the epicenter of Chi-Chi earthquake. We believe during the six years after the earthquake, most area in Taichung Domain are still undergoing strata compaction and stress reset, and therefore this domain has not yet fully recovered to inter-seismic stage. In the Ilan Domain, we focused on the southwestern margin of Okinawa Trough. Previous studies proposed this margin as the transition between lateral collision and lateral extension, where crustal rotation and tectonic escape could be observed. Our study showed that maximum extension changes direction counterclockwise, from N-S in Okinawa Trough to NW-SE in northeastern Taiwan, and its value also decreases rapidly landwards. The Ilan Plain itself shows NW-SE extension in the middle and southern part, and the extension value rapidly increases southwards and southeastwards. When it gets close to mountain range, the extension gradually diminishes. This result is consistent to most measurements during the past decade, indicating a steadily ongoing back-arc extension from Okinawa Trough to Ilan Plain. The Taipei Domain basically moved east-southeastwards or southeastwards relative to Penghu, at a rate of <5 mm/yr from 1992 to 1998 November. The only exception is the southwest the Linkou Tableland, which moved southwestwards. Two area with larger velocities are distributed roughly sub-parallel to the river mouth of Tanshui River, while the area in between has smaller slip rate. Along the Shanchiao Fault and Chinshan Fault, deformation pattern varies from hanging wall to footwall but is different from general inter-seismic pattern, probably due to the influence of large earthquake in the offshore of Ilan. Strain rate analysis from 1992 to 2000 shows extension in the direction of E-W or WNW-ESE, and some compression in the direction of N-S. Strain rate from 2000 to 2002 strengthens this accumulated pattern, and the compression direction slightly changed to NNE-SSW. The hanging wall of Shanchiao Fault kep subsiding, which could have resulted from not only stratigraphic compaction, but also the normal faulting of Shanchiao Fault. An antithetic normal fault may exist in the east of subsiding area.