Collaborative Research: Experimental Study of the Origin and Nature of High Pressure Faulting Relevant to Earthquakes in Subducting Lithosphere

常规的脆性破坏在地球的深层是不可能发生的，因为压力和温度的结合保证了岩石的流动强度在断裂强度之前被超过。然而，地震大量发生在俯冲带的海洋岩石圈下降板块内，有几次发生在深达680公里的地方，在那里地震突然停止。虽然这些地震发生的机制是未知的，但它们的地震信号确保它们代表了沿断层岩石的突然破坏。目前已知有两种机制可以使断层在高压下发生。第一种，脱水脆化，涉及通过分解含水矿物产生自由流体相；自由流体有助于微裂纹的打开，这是导致剪切破坏的关键步骤。就目前所知，如果地幔中存在适当的含水相，并且它们的稳定场在适当的条件下脱水，从而产生观测到的深度分布，那么这种机制就可能解释所有的地震。第二种是相变诱发断层，发生在橄榄石向密度较大的多晶体相变过程中，这种相变随深度增加而发生。如果在过渡带深度的俯冲带冷核中保存亚稳橄榄石，这一机制可以潜在地解释地震的双峰深度分布及其在上地幔底部的突然终止。然而，根据目前对俯冲岩石圈内相位分布的了解，这两种机制都不能令人满意地解释深度超过500公里的特大地震的发生。研究人员将运用他们在高压技术、实验变形和地震学方面的集体专业知识，实现以下目标：(1)验证水合断层再活化的假设；(2)利用声发射探测与定位进行脱水脆化与变形断裂监测实验；(3)开发改进的高压实验装置设计，用于原位同步加速器剪切破坏实验；(4)通过脱水脆化和相变诱发断裂实验，对两种断裂机制进行新的约束。

Green EAR-0125938 Conventional brittle failure is impossible at significant depth within Earth because the combination of pressure and temperature ensures that the flow strength of rocks is exceeded before the fracture strength. Nevertheless, earthquakes occur abundantly within descending slabs of oceanic lithosphere in subduction zones, in several cases to depths as great as 680 km, where they stop abruptly. Although the mechanism by which these earthquakes occur is unknown, their seismic signals ensure that they represent sudden failure of rock along a fault. There are two mechanisms presently known that can enable faulting to occur at high pressure. The first, dehydration embrittlement, involves generation of a free fluid phase by breakdown of hydrous minerals; the free fluid assists in opening of microcracks which is a crucial step leading to shear failure. So far as is known, this mechanism could potentially explain all earthquakes if appropriate hydrous phases are present in the mantle and if their stability fields are such that they dehydrate under appropriate conditions to yield the observed depth distribution. The second, phase-transformation-induced faulting can be triggered during the phase transformations of olivine to its denser polymorphs, which occur with increasing depth. This mechanism can potentially explain the bimodal depth distribution of earthquakes and their abrupt termination at the base of the upper mantle if metastable olivine is preserved in the cold cores of subduction zones at transition-zone depths. However, given current understanding of the phase distribution within subducting lithosphere, neither mechanism can comfortably explain the occurrence of very large earthquakes at depths exceeding 500 km. The investigators will apply their collective expertise in high-pressure technology, experimental deformation, and seismology to achieve the following: (1) test the hypothesis of reactivation of hydrated faults; (2) monitor experiments on dehydration embrittlement and transformation-induced faulting by detection and location of acoustic emissions; (3) develop improved high-pressure experimental assembly designs for in situ synchrotron experiments on shear failure; (4) use faulting experiments on both dehydration embrittlement and phase-transformation-induced faulting to place new constraints on these two faulting mechanisms.