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

NSFGEO-NERC：地震成核与偶发性慢滑移：什么控制断层滑移模式？

• 批准号: 2139331
• 负责人: Nadia Lapusta
• 金额: $ 36.99万
• 依托单位: California Institute of Technology
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2021
• 资助国家: 美国
• 起止时间: 2021-09-01 至 2024-08-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=2139331&HistoricalAwards=false
• 关键词: NSFGEO; NERC; Earthquake; nucleation; versus

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.

这是一个由国家科学基金会地球科学理事会（NSF/GEO）和联合王国国家环境研究理事会（NERC）通过NSF/GEO-NERC牵头机构协议共同资助的项目。该协议允许美国/英国提交一份联合提案，并由研究者拥有最大预算比例的机构进行同行评审。一旦成功地共同确定了赔偿额，每个机构就为预算的一部分和与其自己的调查人员和工作组成部分有关的调查人员提供资金。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。断层受到构造板块运动的影响。其他形式的断层滑动也会发生，包括幕式缓慢滑动事件（SSEs）。 在这种情况下，断层滑动会自发地加速，但永远不会达到快速地震滑动速度。间歇性的SSE可以释放与地震相同数量的应变能，但需要几天到几周，而不是几秒到几分钟。了解SSE是至关重要的，因为它们减轻了断层上的应力积累，减少了地震危险。 然而，它们也将应力从断层的一部分转移到另一部分，这可以促进大破坏性地震的成核和传播。到目前为止，SSEs的机制还没有得到很好的理解。目前还不清楚是什么机制减缓了一些滑动不稳定性（没有地震），但允许其他人变成动态破裂（地震）。在这里，来自美国和英国的国际科学家团队探索了SSE的潜在机制。研究人员使用了最先进的实验室实验和数值模拟的结合。 该模型允许特别是推断实验室的结果，获得小标本，大型构造断层的规模。项目成果包括基于物理的模拟，改善了自然地震和诱发地震活动的地震危险评估。该项目还为一些学生和早期职业科学家提供跨学科的支持和培训。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到幕式SSE的稳定性，而不是地震破裂：1）从速度减弱到速度加强的摩擦率依赖性的演变，稳定了滑动; 2）由于孔隙流体效应引起的渗透性加强稳定了滑动; 3）断层性质的空间变化有助于确定断层滑动的模式。关键的交付成果是限制条件和物理条件的范围，在这些条件和物理条件下，可能发生幕式缓慢滑动、断层蠕动或地震。 对地震物理学理解的提高最终会改善地震灾害预测。英国的研究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，但在很大程度上尚未探索。 他们还对实验进行了一些数值模拟。美国研究人员对实验进行了额外的详细建模，以改进控制断层滑动的本构律。他们还对受改进的摩擦定律约束的断层模型进行数值模拟，以确定实验结果对大规模自然断层的影响。该奖项反映了NSF的法定使命，并通过使用基金会的知识价值进行评估而被认为值得支持和更广泛的影响审查标准。