俯冲带大断层的强度问题

俯冲带大断层的强度是地球动力学的一个重要问题.过去20年来，学界对这一问题的认识有了根本性的改变.本文将简单介绍近期这一研究的核心科学内容、主要方法和结果.宏观估计这种断层大尺度平均强度的有效方法只有两种：一是用弧前地壳应力观测约束横向挤压力从而推算大断层强度，二是用弧前地表热流观测约束断层摩擦生热量从而推算大断层强度.目前的研究结果表明：首先，这些断层极弱，其有效摩擦系数一般在0.03左右，有些可能略大于0.1，这一结果也解释了为什么俯冲带不是造山带.其次，发生特大地震的大断层是最弱的，充分表明大地震不需要高强度断层和高应力，而需要易于破裂传播扩展的地质条件.另外，大地震发生时断层的应力降与强度相比，既不是微乎其微，也远不到百分之百.导致大断层极弱的地质原因还尚不明确.一般认为需要相当高的孔隙液压，但是断层泥中的软弱含水矿物可能也起关键作用.

On the strength of subduction megathrusts

The strength of subduction megathrusts is an important problem in geodynamics. Over the past two decades, the scientific community has made fundamental progress in understanding this problem. In this article, I briefly introduce the core scientific issues in this research, the primary methods used, and the main results. There are only two effective methods to provide macroscopic estimates of these faults' strength at large spatial scales. One method is to use forearc crustal stress observations to constrain the level of margin-normal compression which helps to estimate the strength of the megathrust. The other is to use forearc heat flow measurements to constrain the amount of frictional heating along the megathrust which also helps to estimate its strength. Research results show that these faults are extremely weak. Their effective coefficients of friction are often around 0.03, although some may be slightly higher than 0.1. The weakness of the megathrusts explains why subduction zones are not orogenic belts. Research results also show that the weakest megathrusts are those that produce very large earthquakes, strongly indicating that large earthquakes do not need strong faults or high stress but only need geological conditions that facilitate the propagation and expansion of seismic rupture. The strength estimates further imply that stress drop in great earthquakes, when compared with the strength of the faults, is neither negligibly small nor nearly 100%. The geological reasons for the weakness of the megathrusts are not yet clear. Pore fluid pressure is generally expected to be rather high, but the presence of weak hydrous minerals in the fault gouge may also play a critical role.