地幔流体及其在地震孕育中的作用

Mantle Fluids and Their Role in Earthquake Generation

摘要:

依据地球物理宏观资料、行星化学和地球化学微观分析以及高温高压实验结果,阐明了地幔中存在丰富的流体,扼要介绍了若干流体引发地震的假说,简述了隐爆产生地震的机制。地幔矿物流体包裹体、高温高压相含水硅酸盐矿物与地球物理探测资料表明,整个地幔内普遍存在高能密流体,地幔过渡带和D″层是流体富集的地带,地幔中存在的总水量比海水量高出几个数量级。地幔高能密流体流动产生热对流,造成了固体地球内温度、电导率、地震波速度以及地球化学组成的不均一性。高能地幔流体向外逃逸,不仅改变了壳幔岩石的物理和化学性质,且为岩浆形成和地震孕育提供了能量。流体在地震孕育中的作用包括两个方面:降低断层带摩擦力以及岩石强度触发地震和流体爆炸或相变产生地震。流体爆炸产生地震是地核与下地幔流体以“运移—局部聚集—爆炸的幕式循环”反复进行,产生不同震源深度、不同震级的各类地震。然而,地幔流体的组成、行为及其在孕震过程中的作用机制还需深入研究。

Abstract:

Analysis of the geophysical,planetary-geochemical and high pressure and temperature experimental data indicates that plenty of fluids exists in the mantle. In this paper,some hypotheses of fluid-induced earthquakes were briefly introduced and the prospects were suggested. The data of the fluid samples in mantle rocks,water-bearing high-pressure phases of silicate and geophysical exploration indicate that fluids exist in the whole mantle,mantle transition zone among which D″ layer are enriched in the fluids that are high energy density,and total amount of water in the mantle is estimated being several magnitudes higher than that of sea water on the earth. Thermal convection caused by migration of the high energy density fluids results in heterogeneities of temperature,seismic wave velocity,electrical conductivity and chemical compositions in the solid earth. The upwards migrating fluids alter the physical and chemical properties of the crust and mantle rocks,and provide huge amount of heat energy for producing magma and earthquake. The functions of mantle fluids in the earthquake generation involve two aspects that are inducing earthquake by reducing friction between two walls of a fault,and producing earthquake by explosion or phase transfer of the fluids. The cryptoexplosive mechanics means that the high energy density fluids derived from the core and mantle migrate repeatedly in the way of episodic penetration-inflate-cryptoexplosion cycle,resulting in different types of earthquakes at the different depths. However,more work should be done further to understand both behavior of mantle fluids and their role in earthquake generation.