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对火山中环形断层的认识已有很长时间了,但对其形成机制仍知之甚少。尽管来自相应岩浆房的岩浆在大喷发期间,在一环形断层上的沉陷是容易理解的,但很难解释这种断层本身的产生。环形断层的形成,要求火山处地表的张应力和剪应力必须在距岩浆房中心相对地表点一定的径向距离达到峰值。但是,经验表明:在活火山的大部分不稳定时期,应力场不利于环形断层的产生。我们构制了数十种不同几何形状和经受不同负荷条件的岩浆房边界元模型。结果表明:岩浆房内的压力过大或压力过少作为唯一的负荷条件不可能产生环形断层。对于水平扩展或隆起的球形岩浆房和水平扩展的岩床状岩浆房,在岩浆房中心相对地表点的一定径向距离,火山处地表的张应力和剪应力达到峰值。然而,最大应力出现在岩浆房本身的边界处,这通常导致岩席侵入而不是环形断层的形成。相比之下,经受隆起作用的岩床状岩浆房有利于引起产生环形断层的应力场。
It has been a long time since the understanding of the volcanic annular fault, but little is known about its formation mechanism. Although the magma from the corresponding magma chamber is easy to understand during a large eruption, the subsidence on a circular fault is difficult to explain. The formation of annular faults requires that the tensile and shear stresses at the surface of the volcano must peak at a certain radial distance from the surface of the center of the magma chamber. However, experience shows that during most of the unstable period of active volcanoes, the stress field is not conducive to the production of annular faults. We constructed dozens of magmatic chamber boundary element models with different geometries and with different loading conditions. The results show that it is impossible to produce annular fault in the magma chamber with overpressure or underpressure as the only load condition. For a spherical magma chamber horizontally expanded or uplifted and a horizontally expanded bedrock magma chamber, the tensile and shear stresses at the volcano surface peak at a certain radial distance from the surface point in the center of the magma chamber. However, the maximum stress occurs at the boundary of the magma chamber itself, which usually leads to the intrusion of the mantle instead of the formation of a ring-shaped fault. In contrast, bedrock-like magma chambers that underwent uplift favored the stress field that created ring-shaped faults.