台灣東岸及外海海底地震儀周遭噪訊分析：噪訊來源及岩石圈構造速度逆推

Abstract

本文利用周遭噪訊法，分析2007-2009年TAIGER計畫在花東海盆布放的海底地震儀的連續紀錄，配合台灣東岸陸上測站數據，分析台灣東部海域噪訊來源和特性，以及台灣東部及花東海盆的岩石圈構造。周遭噪訊法近期已被用來擷取台灣全島測站間的表面波群速度與相速度頻散曲線，並透過多重尺度小波逆推技術，成功求得高解析度的速度層析成像，並進一步透過非均向性隨深度的變化，推論台灣造山分層耦合的變形機制。台灣本島之外，過去對台灣近海海域的類似研究則較為不足。本研究採用相同方法分析花東海盆，提供更完整的台灣及相鄰海域構造解釋。我們首先探討台灣東部及海域噪訊特性。透過長時間噪訊疊加，比較陸上及海上噪訊能量大小及主要頻段差異，分析台灣周邊噪訊的可能來源。並發現在2008年，颱風經過台灣顯著提高了海陸測站的噪訊。另一方面，反演表面波群速度與相速度頻散曲線，求得花東海盆二維相速度及非均向性變化。結果發現，短週期(4-8秒)表面波相速度變化平緩，非均向性不強，但快軸呈南北走向，與磁力條帶方向一致，應代表海盆發育時的擴張方向，另一方面，較長週期(12-20秒)的相速度則呈現顯著空間變化，反映的是地幔岩石圈的特徵。在海盆中央及北呂宋島弧的相速度偏高，非均向性在海盆中央區域增強，且快軸呈西北-東南走向，恰與板塊聚合方向一致，應與軟流圈地幔流場的影響有關。

We apply ambient noise analysis to continuous seismic waveform data recorded by ocean-bottom seismometers (OBS) deployed in the Huatung Basin and adjacent regions off the east coast of Taiwan. Taiwan is a young and active orogenic belt resulting from the oblique subduction and collision between the Eurasian Plate and the Philippine Sea Plate. Sitting on the westernmost edge of the Philippine Sea Plate, the Huatung Basin is directly involved in the subduction-collision processes. The structural characteristics of the basin provide important constraints not only on its own history but the tectonic evolution in this complex region. We first analyze the noise signals by stacking cross-correlation functions from station pairs between OBS stations and between land and OBS stations. We find that, unsurprisingly, the noise power of the offshore OBS stations is larger than that of onshore stations. Further, we integrate the OBS data with selected land station data along the east coast, deriving Rayleigh wave Green’s functions from cross-correlation between all available station pairs. We measure phase velocity dispersion at periods from 4 to 20 sec, and invert for 2-D anisotropic phase velocity maps based on a wavelet-based multi-scale inversion scheme. Our results show that, at periods 12-20 s, significant seismic anisotropy is present in the Huatung Basin close to Taiwan’s southeast coast, with fast direction sub-parallel to the direction of convergence. On the other hand, to the north across the Ryukyu trench, anisotropy becomes weaker and the fast direction appears to rotate clockwise toward NW-SE to N-S directions. These characteristics represent the property of the mantle lithosphere of the Huatung Basin, implying for current influence of asthenospheric flow due to currently the PSP plate motion.