利用海底地震儀震測資料探討南海北部大陸邊緣地殼速度構造

Abstract

本研究處理分析南海北坡被動大陸邊緣上的一條多頻道震測及海底地震儀剖面資料，以探討大陸邊緣的地殼構造。此測線以北北西-南南東走向跨過珠二凹陷與西北次海盆。西北次海盆為南海早期擴張並停止後所形成的海盆，故此區域保留了南海張裂初期的地殼構造。從反射震測剖面中可看出在大陸斜坡上有許多正斷層，推測為西北次海盆在三千萬年前擴張時所形成的，而在西北次海盆中則有許多擴張後所沈積的平坦沈積物與受火成活動影響所形成的隆起。 由於反射震測剖面僅能清楚顯示基盤以上的地殼構造，本研究利用海底地震儀資料來建立此剖面的地殼速度模型。一般利用震波走時信號推求地殼速度構造的方式有兩種，第一種為波線追跡(ray-tracing)，此方法考慮到地層邊界的存在，缺點是每一次正演都須仰賴人工進行模擬，過程較為耗時且結果往往因以人工方式進行模擬的修正所以較為主觀。第二種為層析成像模擬(tomography)，會根據理論走時與觀測走時所得到的誤差來尋找適合的模型，其模擬過程較為快速且結果也較為客觀。但此種方法的觀測走時多為折射波，模擬出來的結果主要是速度在空間上的變化，較無層的概念，因而模型結果可能過於平順。此兩種不同的速度分析方法各有優劣。 在南海北坡已經有許多前人利用震波速度模型來探討此區的地殼構造。有別於前人研究單獨使用層析成像模擬或是波線追跡的方法去求得速度構造，本研究嘗試使用了綜合式速度分析方法：首先利用速度頻譜法分析反射震測資料，獲得淺部沈積層的速度資訊；再使用層析成像模擬與波線追跡來分析模擬海底地震儀的走時資料，以期綜合此三種方法的優勢以獲得比以往使用單一方法更良好的速度模型。 本研究的速度模型結果與震測所辨識出的基盤面對應良好，在半地塹等基盤較崎嶇的地方等速線正確的隨著基盤起伏，且在有火成活動的地方速度也有良好的對應。與前人發表緊鄰本測線的OBS2006相比，OBS2006測線在淺部的等速線較平緩，顯示本研究所使用的速度分析方法能建立更精確的速度構造。在深部地殼構造部份，大陸斜坡下的莫荷面深度約為25公里，往南進入西北次海盆後莫荷面快速上升至11-12公里左右。在西北次海盆演化部份，莫荷面起伏與OBS2006結果相近，顯示西北次海盆確為南海早期擴張所形成。

Multi-channel seismic (MCS) reflection profile and ocean-bottom seismometers (OBS) data along a NW-SE trending profile are analyzed to investigate the crustal structure of the continental margin in the northern South China Sea (SCS). This profile extends from the Zhu II depression southeastward to the Northwest Sub-basin (NWSB) of SCS. The NWSB was formed between 32-29 Ma through sea-floor spreading so it retains the SCS early rifting tectonic structures. The MCS profile shows that there are half grabens on the continental slope and igneous intrusions in the NWSB. Because MCS data can reveal crustal structure clearly mainly above the basement, we use OBS data to constraint deep crustal structure in this study. Many efforts have been made to understand the crustal structures in the northern margin of the SCS in the last thirty years. Generally speaking, there are two approaches for constructing crustal velocity models from seismic traveltime analysis. The first is ray-tracing modeling and inversion. This approach takes layer structures into consideration, but is usually performed manually for model updating thus the final results depend heavily on the experience of the scientist who runs the analyses. The second approach is based on seismic tomography. This approach may take less time to run the inversion and the result is more objective, but the velocity model produced consists of values of velocity grids and usually is very smooth on structural variations. These two approach are different and have their own merits. In this study, we use a hybrid approach to construct crustal velocity models. We extract shallow velocity structure from the MCS profile data, then we conduct travel-time tomographic inversion using the PROFIT software on OBS data to derive a 2D velocity model. Finally, forward modeling using ray-tracing approach is applied to refine the velocity model. We hope by integrating these three methods, we can obtain a better velocity model than using just a single method. The final velocity model we generated matches the MCS profile interpretation well, even in structurally complex area (e.g., half graben, intrusive igneous rocks, etc.). It is better than the velocity model of a nearly profile OBS2006, there the velocity model does not match shallow structural variations shown on the MCS profile. We can see that our approach for constructing crustal velocity model provides a better result. Our crustal velocity model show that the Moho depth is about 25 km below the continental slope, then the Moho depth decreases to 11~12 km rapidly southeastward into NWSB. The Moho depth of the NWSB is very similar to that shown on OBS2006 velocity model, suggesting that the NWSB indeed was formed by sea-floor spreading.