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

Fei Ear-0125097常规脆性破坏不可能在地球内部很深的地方发生，因为压力和温度的组合确保了岩石的流动强度在断裂强度之前被超过。然而，地震在俯冲带的大洋岩石圈下降板内频繁发生，有时深达680公里，在那里突然停止。尽管这些地震发生的机制尚不清楚，但它们的地震信号确保了它们代表着沿着断层的岩石突然破裂。目前已知有两种机制可以使断层在高压下发生。第一种是脱水脆化，通过分解含水矿物而产生自由流体相；自由流体帮助打开微裂纹，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，并且其稳定场在适当的条件下脱水，从而产生观测到的深度分布，这种机制可能解释所有地震。第二，相变诱发的断裂作用可以在橄榄石向更致密多晶型的相变过程中被触发，这种相变随着深度的增加而发生。这种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，如果在过渡带深处俯冲带的冷核中保存有亚稳橄榄石的话。然而，根据目前对俯冲岩石圈内阶段分布的了解，这两种机制都不能很好地解释500公里以下深度的特大地震的发生。研究人员将应用他们在高压技术、实验变形和地震学方面的集体专业知识来实现以下目标：(1)检验水化断层重新激活的假说；(2)通过声发射的探测和定位来监测脱水脆化和相变诱发断层的实验；(3)为原位同步加速器剪切破坏实验开发改进的高压实验组件设计；(4)利用脱水脆化和相变诱发断层作用的断层实验，对这两种断层机制施加新的约束。

Fei EAR-0125097 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.