利用P波極化方法估算沖繩海槽淺部地殼S波速度構造

Abstract

由於瞭解地殼的速度結構對於地震危害度分析的評估相當重要，而地震危害度分析的重要參數之一是近地表的S波速度。當有一地震發生時會產生體波，由於S波的振幅較P波的振幅大，導致S波造成的破壞通常比P波造成的破壞更為嚴重。除此之外，近年來，台灣政府開始全力支持可再生能源或相關產業，離岸風電將會是未來的主要能源之一，因此為確保架設離岸風電機組支撐結構的安全性，了解其風場之S波速度結構以確保海床穩定性具有相當重要意義。現今，常用來獲得淺層速度構造的方法為震測以及鑽井，但其缺點為耗時、耗人力，故如要獲得大範圍之速度構造會較為困難。因此Park & Ishii (2018) 發展出體波極化方法為估計淺部地殼速度結構提供了一個簡單有效的方法。此方法顯示出P波極化方向只會受到地層的S波速度影響，而S波極化方向則同時受到地層的P波和S波速度影響。本研究將P波極化方法應用於佈放在沖繩海槽的海底地震儀陣列 (ocean-bottom seismometers, OBS)、琉球島弧的島上測站和日本九州的F-net陸上測站，以估計淺部地殼的S波速度。初步結果顯示，因為OBS主要位於海床上，而海床的材質是屬於較鬆軟、含水量較高，故OBS下方的地層平均S波速度小於島/陸上測站下方地層的平均S波速度。另外，S波速度隨濾波頻率降低而增加，顯示波速隨深度增加。本研究中所獲得之結果可對應到地表到測站下方深度大約100 m處之地層的平均S波速度。因此，未來可以依照不同濾波頻段去加以討論，以獲得不同深度範圍之地層的平均S波速度，進而對地震危害度分析以及海床穩定性評估提供更詳細之參考依據。

A thorough understanding of crustal structures is crucial for earthquake hazard assessments. One of the essential factors in seismic hazard analysis is shear wave velocity near the surface. When an earthquake occurs, the damage caused by the shear wave is usually more severe than that caused by P waves because of its large amplitude and the shear motions. Moreover, offshore wind energy is one of the main future energy because the government of Taiwan supports renewable energy or related industries. It is important to assess the offshore wind farms seismic forces reasonably in order to ensure the safety and stability of the offshore wind turbine support structures. There have been many methods developed to estimate S-wave velocity. A direct approach is to drill. However, drilling is expensive and not practical. A newly developed body-wave polarization method provides an inexpensive way to explore shallow crustal structures. The P-wave polarization direction has no sensitivity to P-wave velocity but only S-wave velocity. In contrast, the S-wave polarization direction is sensitive to both P-wave and S-wave velocities. Thus, we apply the P-wave polarization method to ocean-bottom seismometers (OBS), island stations on the Okinawa trough (OT), and F-net stations in Kyushu, Japan, to estimate the S-wave velocity of the shallow crust. The preliminary results demonstrate that the average S-wave velocity calculated by OBS is lower than those calculated by inland stations. The low S-wave speed may imply that OBS is mainly seated on soft rich-hydrous seafloor sediments. In addition, the results indicate that the average S-wave velocity from the surface to depth of 100 m below the stations. In the future, it can be discussed according to different filters to obtain different depth ranges to provide a more detailed reference for seismic hazard analysis and seabed stability assessment.