A Mechanistic Laboratory Investigation of Seismic Preslip

这项研究解决了导致断层滑动的原因——为什么是今天而不是明天？这项研究包括在一个高度受控的摩擦断层上进行的实验室实验，这将有助于我们了解自然断层上地震的形成。断层表面是由微小的接触点（即凸起）组成的，它们各自滑动，并在发生断层之前将应力传递给其他凸起。在野外，这些被称为前震。在实验室里，我们可以控制几乎所有的变量，这样我们就可以专注于感兴趣的行为。激光锥形位移传感器将用于非常精确地测量来自粗糙动力学的地震信号，并反演出前震是如何导致地震的。相比之下，在野外几乎不可能观测到导致地震的断层表面的物理相互作用，因此有限源反演等间接观测方法使用地球表面的传感器来推断地下发生的前兆事件。该项目将通过比较实验室中可能的直接观测与地震学家可用的间接方法，提高对震源机制的理解，并将阐明导致地震的原因。这个项目将阐明在理解地震前兆的三个当前重要问题背后的机制：什么是前兆的机制组成部分？减小断层上的正应力对破裂起裂有什么影响？在缓慢滑动地震中以及在愈合期延长的破裂中，是什么机制导致了高频含量的延长？将进行一系列试验，以检查导致大破裂的断层界面的接触机制。这将阐明所谓的不寻常现象，包括间歇性震颤、低频和极低频地震以及慢滑事件。首席研究员之前的工作已经显示出诱人的迹象，表明由于愈合时间延长而导致更高频率内容的粗糙级机制，以及慢滑事件可能得到解释。实验室规模的地震，使用PMMA作为延展性岩石的模拟物，提供的数据暗示，同样的前兆行为出现在实验规模小得多的断层和接触岩石中，例如，持续数分钟的提前。利用高速摄像机和压敏胶片，可以在模拟小地震之前、期间和之后直接观察断层表面。地震学技术被用来解释断层附近的一系列超声波换能器和滑动传感器的数据。这些直接和间接观测的结合将为滑移的成核和传播提供新的见解，重点关注凹凸体的强度和它们之间的相互作用。

This research addresses the question of what leads up to fault sliding - why today and not tomorrow? The research consists of laboratory experiments on a highly controlled frictional fault that will help us understand the formation of earthquakes on natural faults. Fault surfaces are made of of tiny contact points - asperities - and they individually slip and transfer stress to other asperities in the run-up to faulting. In the field these are known as foreshocks. In the laboratory we have control of nearly all variables so we can focus on the behaviors of interest. Glaser conical displacement sensors will be used to very accurately measure the seismic signals from the asperity dynamics and invert back to how the foreshocks lead to earthquakes. In contrast, in the field it is nearly impossible to observe the physical interaction of the fault surfaces leading up to an earthquake, so indirect observation methods such as finite source inversion use sensors at the Earth's surface to infer the precursory events taking place underground. This project will improve the understanding of earthquake source mechanisms by comparing direct observations made possible in the laboratory to the indirect methods available to seismologists, and will shed light on what leads to an earthquake.This project will elucidate the mechanisms behind three currently important questions in understanding earthquake precursors: What are the mechanistic components of preslip? What are the effects on rupture initiation from decreasing normal stress on faults? What are the mechanisms causing extended high frequency content in slow slip earthquakes and in ruptures following extended healing periods? A sequence of tests will be conducted to examine the contact mechanisms of the fault interface leading to gross rupture. This will shed light on so-called unusual phenomena, including episodic tremor, low and very-low frequency earthquakes and slow slip events. Previous work by the principal investigator has shown tantalizing indications that the asperity-level mechanisms that lead to higher frequency content due to extended healing time, and for slow-slip events might be explained. The laboratory-scale earthquakes, using PMMA as an analog of ductile rock, has provided data hinting that the same premonitory behavior appears in the experiment scaled to the much smaller size of our fault and contact asperities, e.g., preslip of minutes duration. Use of a high-speed video camera and pressure sensitive film allows for direct observation of the fault surfaces before, during, and after simulated mini-earthquakes. Seismological techniques are used to interpret data from an array of ultrasonic transducers and slip sensors adjacent to the fault. The combination of these direct and indirect observations will provide new insights as to the nucleation and propagation of slip, with a focus on the strength of asperities and interaction between them.