NSFGEO-NERC: Earthquake nucleation versus episodic slow slip: what controls the mode of fault slip?

该项目由美国国家科学基金会地球科学理事会（NSF/GEO）和英国国家环境研究委员会（NERC）通过NSF/GEO-NERC牵头机构协议共同资助。该协议允许美国/英国提交一份联合提案，并由其调查员拥有最大比例预算的机构进行同行评审。在成功地共同确定一项奖励后，每个机构为预算的比例和与自己的调查人员有关的调查人员和工作的组成部分提供资金。地震通常是由预先存在的构造断层上的快速滑动或动态破裂引起的。断层受构造板块运动的影响。其他形式的断层滑动也会发生，包括幕式慢滑动事件（sse）。在这种情况下，断层滑动自发地加速，但永远不会达到快速的地震滑动速度。间歇性的sse可以释放与地震相同的应变能量，但持续几天到几周，而不是几秒钟到几分钟。了解sse是至关重要的，因为它们可以缓解断层上的应力积聚，减少地震危险。然而，它们也将应力从断层的一个部分转移到另一个部分，这可以促进大型破坏性地震的形成和传播。到目前为止，sse背后的机制还没有得到很好的理解。目前尚不清楚是什么机制减缓了一些滑动不稳定（没有地震），却允许其他滑动转变为动态破裂（地震）。在这里，一个由美国和英国科学家组成的国际团队探索了SSEs背后的机制。研究人员将最先进的实验室实验和数值模拟相结合。值得一提的是，该模型可以将在小样本上获得的实验室结果外推到大型构造断层的规模。项目成果包括基于物理的模拟，改进了自然地震和诱发地震活动的地震危险性评估。该项目还在跨学科的背景下为一些学生和早期职业科学家提供支持和培训。其中包括加州理工学院的一名研究生和一名博士后。在这里，研究小组测试了三个关键假设，这些假设可以解释加速断层滑动稳定为偶发性ssi，而不是地震破裂：1)摩擦的演化速率依赖，从速度减弱到速度增强，稳定了滑动；2)孔隙流体效应引起的膨胀强化稳定了滑移；3)断层性质的空间变化有助于断层滑动模式的确定。关键的可交付成果是对条件和物理范围的限制，在这些条件和物理范围下，间歇性的慢滑、断层蠕变或地震可能发生。对地震物理认识的提高最终提高了地震灾害预报的水平。英国调查人员对岩石材料和断层条件进行了实验室实验，这些岩石材料和断层条件与sse和地震成核高度相关，但大部分尚未开发。并对实验进行了数值模拟。美国研究人员对这些实验进行了额外的详细建模，以改进控制断层滑动的本构定律。他们还进行了由改进的摩擦定律控制的断层模型的数值模拟，以确定实验结果对大规模自然断层的影响。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

This is a project that is jointly funded by the National Science Foundation’s Directorate of Geosciences (NSF/GEO) and the National Environment Research Council (NERC) of the United Kingdom (UK) via the NSF/GEO-NERC Lead Agency Agreement. This Agreement allows a single joint US/UK proposal to be submitted and peer-reviewed by the Agency whose investigator has the largest proportion of the budget. Upon successful joint determination of an award, each Agency funds the proportion of the budget and the investigators associated with its own investigators and component of the work.Earthquakes are typically generated by rapid slip – or dynamic rupture – on pre-existing tectonic faults. The faults are loaded by tectonic plate motions. Other forms of fault slip occur as well, including episodic slow slip events (SSEs). In that case, fault slip spontaneously accelerates but never reaches rapid earthquake slip speeds. Episodic SSEs can release the same amount of strain energy as earthquakes, but over days to weeks rather than seconds to minutes. SSEs are vital to understand as they relieve the stress buildup on faults and reduce seismic hazard. Yet they also transfer stress from one part of the fault to another, which can promote the nucleation and propagation of large destructive earthquakes. To date, the mechanisms underlying SSEs is not well understood. It is unclear what mechanisms slow down some slip instabilities (no earthquakes) yet allow others to turn into dynamic rupture (earthquakes). Here, an international team of scientists from the US and the UK explores the mechanisms underlying SSEs. The researchers use a combination of state-of-the-art laboratory experiments and numerical modeling. The modeling allows notably to extrapolate laboratory results, obtained on small specimens, to the scale of large tectonic faults. The project outcomes, which include physics-based simulations, improve earthquake hazard assessment in natural and induced seismicity. The project also provides support and training in an interdisciplinary context to several students and early career scientists. These include one graduate student and one postdoctoral associate at the California Institute of Technology. Here, the team tests three key hypotheses that may explain the stabilization of accelerating fault slip into episodic SSEs, rather than earthquake ruptures: 1) evolving rate dependence of friction, from velocity weakening to velocity strengthening, stabilizes the slip; 2) dilatant strengthening due to pore fluid effects stabilizes the slip; 3) spatial variations in fault properties contribute to determining the mode of fault slip. Key deliverables are constraints on the range of conditions and physics under which episodic slow slip, fault creep, or earthquakes can occur. The improved understanding of earthquake physics ultimately improves seismic hazard forecasting. The UK investigators conduct laboratory experiments for rock materials and fault conditions highly relevant to SSEs and earthquake nucleation, yet largely unexplored. They also carry out some numerical modeling of the experiments. The US investigators conduct additional detailed modeling of the experiments, to improve the constitutive laws governing fault slip. They also conduct numerical simulations of fault models governed by the improved friction laws to determine the implications of the experimental results for large-scale natural faults.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.