CAREER: From slow to fast, micro to macro, single events to cascades: A multi-scale study of seismic event triggering in lab and nature

“地震触发”指的是导致其他地震在几秒钟到几年后发生的地震，无论是在附近还是在数千英里之外。大多数地震被认为是这一过程的结果，但根本原因尚不清楚。地震的触发导致地震的“家庭”聚集，有前震、主震和余震。前震可以提供可能即将发生的更大规模主震的信息，主震的大小和位置可以用来预测余震。尽管如此，还有很多东西需要学习，例如，一个断层是什么让它有可能被另一场地震触发，我们能否利用这些知识来改进地震预报？使用最新可用的仪器，戈贝尔和他的团队将在他们的实验室里记录下岩石之间突然断层滑动时的微小地震。他们将在实验室中看到地震的触发，测试触发或多或少发生的条件，并将这些知识应用到现实世界的地震中。这项研究将直接有助于改进对地震危险性的评估和实时预测余震。地震触发在不同的构造和应力状态下普遍存在。触发过程包括静态和动态应力传递、震后蠕变、孔洞弹性效应、亚临界裂纹扩展以及与速率和状态有关的摩擦；然而，放大余震触发的主要机制和条件在很大程度上仍未解决。触发是完全由主震特征控制的，还是也受断层应力和破坏等局部条件控制的？是否有特定的地壳属性来增强触发？是什么控制触发时标的变化，这种变化是否表明应力状态和与大型故障事件的接近程度？为了解决这些问题，本研究将集中在实验室和自然界中关于应力松弛和触发的多尺度调查。只有少数研究集中在模拟地震行为的受控摩擦滑动实验中的地震触发。现在可以使用高速宽带地震仪器记录下亚毫米级实验室裂缝和滑动的完整波形。基于这样的记录，这项工作将提高在实验室中检测自然应力松弛期间触发的能力，并验证触发不仅是源特征的函数，而且是断层应力和损伤状态的函数的假设。建议的研究涉及三个主要问题：1)什么条件和机制放大了地震触发？2)触发时间尺度指示了断层应力状态和与大型断层事件的接近程度？3)哪些实验室过程是尺度不变的，从而有助于推进对断层本构行为的理解？为了解决这些问题，将对实验室产生的中地壳应力断层进行一系列三轴和直剪试验。这些测试将评估i)断层粗糙度；ii)正常应力；iii)断层损害；iv)孔隙流体压力和v)硬度在增强或抑制触发方面的作用。这项研究旨在揭示这些成分是如何联系在一起的，以及实验室结果是如何转移到自然系统中的。预计结果将有助于确定控制触发的关键因素及其对地震可预测性的影响。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

"Earthquake triggering" refers to an earthquake causing other earthquakes to happen seconds to years later, both nearby and up to thousands of miles away. Most earthquakes are thought to result from this process, yet the underlying causes remain unclear. Earthquake triggering results in clustered "families" of earthquakes, with foreshocks, mainshocks and aftershocks. Foreshocks may provide information about possible upcoming, larger-magnitude mainshocks, and mainshock size and location may be used to forecast aftershocks. Still, there is much to learn, for example, what is it about a fault that makes it likely to be triggered by another earthquake, and can we use this knowledge to improve earthquake forecasts? Using newly-available instruments, Goebel and his group will record tiny earthquakes during abrupt fault slip between blocks of rock in their laboratory. They will see earthquake triggering as it happens in the lab, test conditions that make triggering more or less likely to occur, and apply this knowledge to real-world earthquakes. This research will contribute directly to improvements in assessing seismic hazard and forecasting aftershocks in real time.Earthquake triggering is ubiquitous throughout different tectonic and stress regimes. Triggering processes involve static and dynamic stress transfer, post-seismic creep, poroelastic effects, sub-critical crack growth and rate- and state-dependent friction; however primary mechanisms and conditions that amplify aftershock triggering remain largely unresolved. Is triggering solely controlled by mainshock characteristics or also by local conditions such as fault stress and damage, and are there specific crustal properties that enhance triggering? What controls changes in triggering time-scales and are such changes indicative of stress state and proximity to large failure events? To address these questions, this study will concentrate on a multi-scale investigation of stress relaxation and triggering in lab and nature. Only a few studies have focused on seismic triggering in controlled frictional-sliding experiments that mimic earthquake behavior. Full waveforms of sub-millimeter lab-fractures and slip can now be recorded using high-speed, broad-band seismic instrumentation. Based on such records, this work will advance the ability to detect triggering during natural stress relaxation in the lab and test the hypothesis that triggering is not only a function of source characteristics but also of fault stress and damage state. The proposed research addresses three primary questions: 1) What conditions and mechanisms amplify earthquake triggering? 2) Are triggering time scales indicative of fault stress state and proximity to large failure events? 3) Which laboratory processes are scale-invariant and thus help advance the understanding of fault constitutive behavior? To address these questions, a series of triaxial and direct-shear tests on lab-generated faults at mid-crustal stresses will be conducted. These tests will assess the roles of i) fault roughness; ii) normal stress, iii) fault damage, iv) pore fluid pressure and v) stiffness in enhancing or inhibiting triggering. This research aims to unravel how these components are linked and how lab results can be transferred to the natural system. The results are expected to help identify key factors that govern triggering and their implications for earthquake predictability.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.