南海东北部洋陆过渡区域地震折射波衰减特征

Attenuation characteristics of refracted seismic wave in the continent-ocean transition zone of the Northeastern South China Sea

地震波衰减研究是了解岩石圈构造特征的有效方法.本文基于OBS2016-2测线的海底地震仪(OBS)数据,首次对南海东北部洋陆过渡区域地震波衰减特征进行研究,通过正演模拟获得该区域二维纵波衰减(Q_(P))结构.结果表明,下陆坡的上地壳存在一个宽约40 km,厚度约为4~5 km的高衰减区,其特征为低纵波和横波速度(V_(P)为5.5~6.3 km·s^(-1)和V_(S)为3.1~3.6 km·s^(-1))以及较低的波速比(V_(P)/V_(S)为1.72~1.80),对应较低的Q_(P)(280~410),推测与断裂发育有关,且受到火山活动的影响.洋陆过渡及洋壳区域的上地壳高衰减区具有低Q_(P)(300~400)和高V_(P)/V_(S)(1.90~1.96)特征,可能对应较多的断裂发育及流体运移.洋陆过渡区域下地壳高速异常体表现为相对低的Q_(P)(550~600),对应较高的V_(P)(7.0~7.8 km·s^(-1))和V_(S)(3.5~3.8 km·s^(-1))以及较高的V_(P)/V_(S)(1.85~1.96),推测与蛇纹石化作用有关.蛇纹石化可能进一步增加岩石的孔隙度并导致更多的流体运移,使得洋陆过渡及洋壳区域存在较高的地震波衰减.Q_(P)结构有助于我们分析南海大陆边缘的地震波衰减特征,结合V_(P)、V_(S)以及V_(P)/V_(S),可以更好地了解该区域地质结构和岩石属性,对进一步挖掘OBS数据信息有重要参考价值.

Seismic attenuation can be used to investigate the structural and compositional characteristics of the lithosphere.We studied the seismic attenuation characteristics of the continent-ocean transition zone in the Northeastern South China Sea using ocean bottom seismometer(OBS)data from the survey line OBS2016-2.A P-wave attenuation(Q_(P))transect was obtained by the Q_(P)forward modeling.The results reveal a high attenuation zone in the upper crust of the continental slope that is about 40km wide and 4~5km thick.This low Q_(P)(280~410)and the high-attenuation zone are also low in V_(P)(5.5~6.3km·s^(-1)),V_(S)(3.1~3.6km·s^(-1)),and V_(P)/V_(S)(1.72~1.80),which may be related to faults and volcanic activity in this area.The high attenuation area with the Q_(P)of 300~400in the upper crust from the continent-ocean transition zone to the oceanic domain is characterized by high V_(P)/V_(S)(1.90~1.96),possibly related to faults and fluid migration.Moreover,the lower crust high-velocity anomaly zone in the continent-ocean transition area is characterized by relatively low Q_(P)(550~600),high V_(P)(7.0~7.8km·s^(-1))and V_(S)(3.5~3.8km·s^(-1)),and high V_(P)/V_(S)(1.85~1.96),which is related to possible serpentinization.Serpentinization can increase the porosity of rocks and lead to more fluid migration,causing high seismic wave attenuation in the ocean-continent boundary area.The Q_(P) model,P-and S-wave velocity,and V_(P)/V_(S)ratio can help us better understand the geological structure and petrophysical parameters around the continental margin,which has important reference value for further mining OBS data information.