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智利北部俯冲板片双地震带成像研究
陆攀1, 张海江1, 高磊1, Diana Comte2
1.中国科学技术大学地球和空间科学学院;2.Departamento de Geofísica,Advanced Mining Technology Center,Facultad de Ciencias Físicasy Matemáticas,Universidad de Chile,Blanco Encalada ,Santiago,Chile
摘要:
在全球范围内俯冲板片存在双地震带是一个普遍的规律,为研究中等深度地震的成因机制提供了很好的研究窗口。本文利用双差地震成像算法,对智利北部存在双地震带的俯冲区域进行了地震重定位和速度成像研究。研究结果表明,智利北部俯冲带在大约50-140km深度范围内存在双地震带,两地震带之间间隔约20km。双地震带上层地震带大约~90公里以浅位于低Vp(约7.8-8.0km/s)、低Vs(约4.4-4.5km/s)和高Vp/Vs(约1.85)异常区域,在大约90公里以深位于较高的Vp(~8.2km/s),Vs(~4.8km/s)和比正常略高的Vp/Vs(~1.76),可能与俯冲洋壳含水矿物的系列脱水反应相关。双地震带下层地震带虽然局部存在相对较高的Vp/Vs异常(~1.8),但整体上位于低Vp/Vs异常区(约1.7),可能与蛇纹岩的脱水反应有关。根据Vp、Vs、Vp/Vs的异常,并结合前人岩石学实验和热力学模型的综合分析,我们认为中等深度地震的产生主要与俯冲板片内部含水矿物的脱水有关。含水矿物的脱水导致板片内部孔隙流体压力升高,有效正应力降低,从而导致脆性破裂,产生俯冲带内的中等深度地震。智利北部俯冲带的成像结果进一步支持了俯冲板片内双地震带的存在与不同含水矿物的脱水反应有关。
关键词:  双差地震成像  双地震带  脱水致脆
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Seismic Imaging of the Double Seismic Zone in the subducting slab in Northern Chile
Lu Pan1, Zhang Haijiang1, Gao Lei1, Diana Comte2
1.University of Science and Technology of China;2.Departamento de Geofísica,Advanced Mining Technology Center,Facultad de Ciencias Físicasy Matemáticas,Universidad de Chile,Blanco Encalada ,Santiago,Chile
Abstract:
It is common that there exist double seismic zones in the subducting slab around the world, which provides a good window for studying the seismogenetic mechanism of the intermediate-depth earthquakes. In this study, we have relocated earthquakes and determined seismic velocity models using the double-difference seismic tomography method for the subduction zone in northern Chile where double seismic zones exist. The results suggest that there are double seismic zones in northern Chile at a depth of about 50-140km, with an interval of about 20km between them. The upper seismic zone is associated with relatively low Vp (~7.8-8.0km/s), low Vs (~4.4-4.5km/s) and high Vp/Vs (~1.85) above the depth of ~90 km, while below ~90 km it is associated with relatively high Vp (~8.2km/s),high Vs (~4.8km/s) and slightly high Vp/Vs(~1.76),which may be related to a series of dehydration reactions of hydrous minerals in the subducted oceanic crust. In comparison, lower seismic zone of double seismic zones is associated with the anomaly of low Vp/Vs (~1.7), although some local areas may appear relatively high Vp/Vs values (~1.8), which may be related to dehydration reaction of serpentinite. Based on Vp, Vs, Vp/Vs anomalies, combined with previous petrological experiments and thermodynamic models, it can be derived that intermediate-depth earthquakes are mainly related to the dehydration of various hydrous minerals in the subducting slab. By relasing the water into the subducting slab, dehydration of hydrous minerals can lead to the increase of pore fluid pressure and the decrease of effective normal stress, thus causing the correnence of brittle failure and the occurrence of intermediate-depth earthquakes in the subducted zones. The imaging results of subduction zone in northern Chile further support that the existence of double seismic zones in the subducting slab is related to the dehydration of different hydrous minerals.
Key words:  Chile subduction zone, double difference seismic tomography  double seismic zone  dehydration embrittlement