蛇纹石脱水与大洋俯冲带中源地震（70-300km）的关系

Relationship between dehydration of serpentine and intermediate-focus earthquakes in oceanic subduction zones.

蛇纹石脱水致裂作用是诱发大洋俯冲带中源地震（70～300km）的一种重要成因机制，它与中等深度双地震带的形成有很密切的关系。双地震带在冷俯冲带中是一种常见现象，它由上下相距20～40km的两个平行地震层组成。上地震层位于俯冲洋壳中，可能是洋壳蓝片岩脱水形成榴辉岩的系列脱水反应诱发了地震；下地震层位于大洋俯冲地幔中，可能是部分交代的地幔橄榄岩脱水控制着中源地震的分布。蛇纹岩在高温高压条件下的变形实验证实蛇纹石在脱水过程中引起岩石弱化和脆性破裂，这已经得到了对蛇纹石脱水过程中岩石物理性质和变形后样品的显微构造等理论研究上的支持。在蛇纹石脱水过程中，产生的流体与固体残留物分离，形成了大量的Ⅰ型（张性）微裂隙，最终导致岩石破裂和形成断层。根据叶蛇纹石脱水反应相图，理论上在大洋俯冲带中蛇纹石脱水位置会出现双层结构，但只有平行于俯冲板块顶层等温线的一支才可能脱水诱发地震，并对应于双地震带的下地震层。下地震层所处的位置具有低的vp／vs值，暗示岩石圈大洋地幔顶层发生了部分交代。但它的交代机制尚不清楚，可能是海水通过洋底转换断层和／或沿着在外海沟隆起中形成的断层渗入大洋地幔顶层，并发生了洋壳和大洋地幔交代。双地震带在120-200km深度合一以后，冷俯冲带中所发生的中源地震可能与蛇纹石脱水有关，在热俯冲带中更可能与“湿”榴辉岩脱水有关。

Dehydration embrittlement of serpentine is an important trigger of intermediate-focus earthquakes （70-300 kin） in oceanic subduction zones, which is closely related to the formation of a double seismic zone at the depth range of 50-250 km. A double seismic zone in which two parallel seismic planes are usually separated by 20-40 kin, is commonly observed in cold subduction zones; whereas the upper seismic plane is located in subducting oceanic crust and the earthquakes result from blueschists dehydration and formation of eclogites; the lower seismic plane is formed in suhdueting oceanic mantle and the loci of earthquakes are probably controlled by dehydration of locally hydrated uppermost mantle. Deformation experiments of serpentinite at high temperatures and pressures have showed dehydration weakening and embrittlement. Petrophysics theory suggests that water-assisted faulting occurs when the fluid is separated from solid residue during the process of serpentine dehydration and ModeⅠ（tensile） microfractures are generated. The dehydration loci of serpentine should produce a double-layered structure in an oceanic subduction zone according to the phase diagram of antigorite, but only the lower layer which parallels isotherms corresponds to the lower plane of double seismic zones. It is associated with very low vp/vs ratios （1. 60-1.70） that imply a partially hydrated oceanic uppermost mantle. However, the hydrated mechanisms of the mantle have not been well understood. One suggestion is that seawater can be injected deep into the uppermost oceanic mantle along transform faults and/or major faults in a subducting slab that are ruptured by outer trench rise events. When the two planes of a double seismic zone merge into one below depths of 120-200 km in a cold subduction zone, the initiating mechanism of earthquakes should still be the dehydration embrittlement of serpentine. Alternatively, it might be related to ＂wet＂ eclogite dehydration, especially in hot subduction zones with only one seismic plane if the earthquakes occur in the appropriate depth range for such dehydration.