Faulting under High Confinement: Experimental Investigation and Model Development

地球物理学和地震学特别感兴趣（和争议）的是在深部地震发生的数百公里或更深处控制脆性破坏的机制。在这些深度，摩擦滑动（在较浅深度存在的下限限制下运行的基本失效机制）名义上受到抑制。我们提出了新的实验和模型，旨在阐明高约束下局部脆性压缩破坏的特征和基本物理机制。在为解释高约束下的脆性破坏而提出的机制中，脱水脆化和相变断层是最常被引用的两种。它们的作用是减少局部约束，实际上允许低约束摩擦断层继续进入高约束状态。然而，更深处地震特征的变化可能与最深地震破裂物理机制的变化一致。我们建议研究的另一种机制是塑性断层。这种机制与其他两种机制根本不同：它是一种由绝热加热辅助的非摩擦剪切不稳定性。受到过去使用冰作为岩石模型材料的成功的鼓舞——Kirby's [1987]发现相变断层，以及我们自己发现[Renshaw and Schulson, 2001]似乎是低约束脆性破坏的通用机制——并受到其矿物学简单性和由于其与岩石和矿物的行为显着相似，我们建议对颗粒冰进行一系列系统压缩实验，以确定约束、晶粒尺寸和应变率对最终破坏应力和破坏模式的影响（如果有的话），并关注微观结构细节。初步实验表明，随着限制的增加，类脆性失效模式会发生转变，并且高限制断层似乎不受摩擦控制。它们似乎也与 II 型裂纹、脱水脆化或相变无关。我们的工作假设是塑性断层在起作用。此外，我们建议开发高约束压缩破坏的定量模型，使用物理过程的直接观察作为输入。

Of particular interest (and controversy) in geophysics and seismology are the mechanismscontrolling brittle-like failure at depths of several hundred km or more where deep earthquakesoccur. At these depths frictional sliding, the fundamental failure mechanism operating under thelower confinements present at shallower depths, is nominally suppressed. We propose newexperiments and modeling aimed at elucidating the characteristics and the fundamental physicalmechanism(s) that underlie localized brittle compressive failure under high confinement.Among the mechanisms that have been proposed to explain brittle-like failure under highconfinement, dehydration embrittlement and transformational faulting are the two most oftencited. They act to reduce confinement locally, in effect allowing low-confinement frictionalfaulting to continue into the high-confinement regime. However, changes in earthquake seismiccharacter at greater depths are possibly consistent with a change in the physical mechanism ofrupture for the deepest earthquakes. An alternative mechanism, and the one we propose toinvestigate, is plastic faulting. This mechanism is fundamentally different from the other two: itis a non-frictional shear instability aided by adiabatic heating.Spirited by past success in using ice as a model material for rock--Kirby's [1987] discoveryof transformational faulting, and our own discovery [Renshaw and Schulson, 2001] of whatappears to be a universal mechanism of low-confinement brittle failure--and encouraged by itsmineralogical simplicity and by its marked similarity to the behavior of rocks and minerals, wepropose a series of systematic compression experiments on granular ice to determine the effects,if any, of confinement, grain size and strain rate on both the terminal failure stress and the failuremode, with attention to microstructural detail. Preliminary experiments demonstrate a transitionin brittle-like failure mode with increasing confinement and that the high confinement faultsappear not to be friction controlled. Nor do they appear to be related to either mode II cracking,dehydration embrittlement, or phase transformations. Our working hypothesis is that plasticfaulting is at play. In addition, we propose to develop quantitative models of high confinementcompressive failure, using as inputs direct observations of the physical processes.