馬尼拉海溝北段隱沒前緣之地震群研究

Abstract

台灣南部的馬尼拉隱沒系統，其板塊相對運動的模式是南海板塊於馬尼拉海溝處向東隱沒至菲律賓海板塊之下，地表上馬尼拉海溝的地形幾何形狀在北緯20度時向西北彎曲，更往北延伸至台灣西南海域時，其海溝的形貌特徵逐漸無法辨認。此區域被認為是海洋地殼與大陸地殼的過渡帶，詳細的地震構造和隱沒板塊的變形行為目前仍不清楚。本研究中使用9套海底地震儀 (OBS, Ocean Bottom Seismometer)，佈放在馬尼拉海溝北段隱沒前緣，偵測該區地震活動特性。本研究所使用的OBS為短週期的地震儀，測站佈放間距大約為15km，佈放時間從2006/11/20至2006/11/25，這段時間內OBS地震網共偵測到兩千多筆區域地震事件，其中多數都沒有被陸地測站記錄到。本研究一開始先辨認地震P-波、S-波的到時，進行地震定位，並以地震初始定位的走時差，逆推該區的平均一維地震速度模型，由一維地震速度模型推得，該區的地殼厚度約16公里。其厚度介於大陸地殼及海洋地殼之間，證明研究區域位於大陸及海洋地殼的過渡帶。接續根據此一維速度模型所定位的地震震源，其地震空間分佈呈現兩個空間叢集分佈。其一為西北東南走向與海溝平行，形成一個由淺層到約30km的斜面分佈；另一群地震位於OBS 測站網的北端，形成一水平的地震群，地震的深度約25~30km之間。此外，為能將偵測到的地震事件與其它地震觀測網相互比較，本研究中利用經驗公式迴歸制定近震規模，其所解析之地震規模多數為-1至1之間，少數事件規模大於2.5。並且，為能夠更進一步了解隱沒帶前緣的地震震源及地殼變形行為，研究中利用較大規模的地震事件進行近場波形模擬，以逆推震源機制解。解算出的震源機制大多以走向滑移和逆衝斷層的破裂型態顯示，但其在空間及時間中的分佈卻不一致。推斷其可能的原因是區域構造所造成的應力隨時間及空間變化，此變化導致震源機制解其破裂型態分佈上差異。

Along the Manila trench, the South China Sea Plate subducts eastward beneath the Philippine Sea Plate in the southern Taiwan. However, the geometrical feature of the Manila Trench becomes indistinguishable as approaching the Taiwan Island. This area is considered as the transition zone between the South China Sea Basin and the southeast continental shelf of the Eurasian Plate. In this study, we used 9 short-period OBS (Ocean Bottom Seismometer) deployed along the northernmost Manila Trench (20.5~21°N, 119.8~120.2°E), from November 20th to November 25th in 2006. Over 2,700 events were relocated during the OBS deployment. The hypocenters reveal focal depths ranging from the seafloor surface to 40 km depth and in two distinct seismic clusters. One distributes vertically with its surface projection tend to be parallel of the neighboring Manila Trench; whereas the other seismic cluster is confined inside an horizontal swarm at a depth of about 30 km. In order to compare our solutions with other seismic observation networks, we constructed an empirical equation to determine the local magnitude for our study. We also determined the 1-D velocity model via the inversion program VELEST; it reveals a thick oceanic crust with of around 16 km. Compared to the typical continental and oceanic lithosphere our model proved that our study area is located at the beginning of oceanic-continental transition zone (oceanic side). Moreover, in order to extract more information on the offshore seismic sources, we performed the near-field waveform inversion program FASTMECA, to determine focal mechanisms with events which ML are larger than 2.5. Focal mechanism solutions are mainly strike-slip in type but secondary normal events are observed to the north of the network while reverse events distribute to the south. We suggested this distribution could be either caused by local structures as the LRTPB (Luzon Ryukyu Transform Plate Boundary) at the scale of our network (but not revealing the tectonic process of the whole LRTPB), or by the process of the subduction of the Eurasian Plate in front of the Manila Trench.