Exploring the geological signature of Slow Earthquakes through legacy experiments and field analysis

地球的外层是由相互滑动、相互移动或拉开的板块组成的。世界上有很大一部分人口生活在这样的边界附近。过去二十年的遥感研究表明，我们对板块如何沿这些边界变形的看法需要修正。我们现在认识到慢地震(SE)，它比常规地震滑动得慢，但比正常板块运动快得多。有趣的是，这些SES似乎是由相当于一个充满的浴缸重量的非常小的力触发的，它们的观察到的特征目前无法与我们目前对岩石变形的理解相一致。板块尺度的变形是由在微观尺度上活跃的过程控制的，然而，使观测到的SE滑移率的过程的性质仍然是个谜。由于SE可能会影响破坏性地震的可能性，如果我们要更好地评估与板块边界运动相关的危险和风险，就需要了解它们。目前应对这一挑战的方法在很大程度上依赖于远程探测非常细微的地面运动的敏感仪器提供的数据。因为自爆事件发生在我们脚下许多公里的岩石中，所以没有直接的方法来检验已经提出的解释自爆事件的理论。这种对推论而不是直接证据的依赖构成了我们对SE的理解的主要障碍。幸运的是，地球为我们提供了另一组独立的观测结果。SES应该在地质记录中留下印记；它们容纳了许多俯冲带中100公里宽的区域的大部分变形，在这些区域中，一个板块潜入另一个板块下方的地球内部。因此，原则上，对在曾经受环境影响地区发现的出土岩石进行深入分析，应该追究过程的责任。但到目前为止，关于这些标志还没有达成共识。我们假设，SES在地质记录中是看不见的，因为我们目前不知道要寻找什么。在我们的探索项目中，我们使用(1)SE的地球物理来帮助我们缩小可能产生SE的过程的范围(2)可以重现SE行为的实验室实验--得到的样本可以向我们展示并告诉我们岩石在SE之后会是什么样子。(3)从合适的野外地点出土的SE岩石，使我们能够检验我们的理论是否正确(即所识别的过程确实是SE的原因)地球物理数据告诉我们，SE的过程必须使岩石像面团一样流动，突然破裂和快速移动。有趣的是，数十年的实验室变形实验恰恰显示了这种行为。这种“慢裂缝”非常类似于SE性质，在使富含流体的岩石变形时发生。然而，这些实验被认为是“失败的”，因为它们不能用来评估被解释为控制板块变形的岩石的缓慢流动行为。我们将利用这些迄今基本上被忽视的实验来检验我们的假设。我们将只使用目前可用的分析技术进行纳米级的分析。配备了我们新训练的识别预测的东南过程的眼睛，我们将在目前被认为保存着缓慢地震指纹的挖掘区域进行实地工作，并对自然样本进行分析。如果我们的假设是正确的，我们将第一次能够利用地质记录来帮助约束以前只有地球物理数据才能看到的过程。随着团队的深入讨论，遥感专家将确保推断的地质过程可以解释观测到的SE遥感数据。如果我们是正确的，我们的“概念验证”项目将为对SE的基本理解奠定基础，并允许团队建立知识库，以提出一个有充分依据的、更大的SE重点研究项目。

Earth's outer layer is made up of plates that slide past each other, move into one another or pull apart. A large percentage of the World's population is living close to such boundaries. The last two decades of remote sensing has shown that our view of how plates deform along these boundaries needs to be revised. We now recognise Slow Earthquakes (SEs), which slip more slowly than regular earthquakes, but significantly faster than normal plate movements. Intriguingly, these SEs seem to be triggered by very low forces equivalent to the weight of a filled bathtub and their observed characteristics cannot, at present, be reconciled with our current understanding of how rocks deform. Plate-scale deformation is governed by processes active at the microscopic scale, however the nature of the processes enabling observed SE slip rates remains enigmatic. Because SEs can influence the likelihood of damaging earthquakes, they need to be understood if we are to better assess hazards and risks associated with plate boundary motions. The current approach to this challenge relies heavily on data from sensitive instruments detecting remotely very subtle ground movement. Because the SEs are occurring in rocks that are many kilometres beneath our feet, there is no direct way to test the theories that have been put forward to explain SEs. This reliance on inferences rather than direct evidence poses a major barrier to our understanding of SEs. Luckily, the Earth presents us with another set of independent observations. SEs should leave an imprint in the geological record; they accommodate most of the deformation over 100-km-wide regions in many subduction zones, where one plate dives into the Earth's interior beneath another. Thus, in principle, in-depth analyses of exhumed rocks found in regions that were once subject to SEs should hold the signature of the processes responsible. But to date there is no consensus on these signatures.We hypothesise that SEs have remained invisible in the geological record because we currently do not know what to look for. In our exploratory project, we use (1) geophysics of SEs to help us to narrow down the processes likely to produce them(2) laboratory experiments that can reproduce SE behaviour - the resulting samples can show and teach us what rocks would look like following SEs. (3) exhumed SE rocks from suitable field locations, allowing us to test if our theory is correct (namely that the identified processes are indeed responsible for SEs)Geophysical data tells us that the process responsible for SE must enable rocks to flow like a dough and to suddenly crack and move fast. Interestingly, decades of laboratory deformation experiments have shown exactly this behaviour. Such "Slow Fractures" closely resemble SE behaviour and occur when deforming a fluid-rich rock. However, such experiments have been regarded as "failed" as they could not be used to assess the slow flow behaviour of rocks interpreted to govern plate deformation. We will test our hypothesis by utilising these so far largely neglected experiments. We will perform analyses down to the nanometer scale using only-now available analytical techniques. Equipped with our newly trained eye to recognise the predicted SE process, we will carry out field work in an exhumed area currently presumed to hold Slow Earthquake fingerprints and perform analyses on natural samples. If our hypothesis is right, we will for the first time be able to use the geological record to help constrain processes that have previously only been visible to geophysical data. Close discussion with the team's slow slip, remote sensing expert will ensure inferred geological processes can explain observed SE remote sensing data. If we are correct, our "proof of concept" project will lay the foundation for fundamental understanding of SEs and allow the team to build-up the knowledge-base to propose a well-founded, larger SE focused research project.