Collaborative Research: Dynamic fault rupture in the presence of 3D heterogenous tectonic stress: the case of the San Andreas Fault in Eastern San Gorgonio Pass

合作研究：三维异质构造应力存在下的动态断层破裂：以圣戈戈尼奥山口东部圣安德烈亚斯断层为例

• 批准号: 1623637
• 负责人: Michele Cooke
• 金额: $ 2.8万
• 依托单位: University of Massachusetts Amherst
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2016
• 资助国家: 美国
• 起止时间: 2016-08-01 至 2019-07-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1623637&HistoricalAwards=false
• 关键词: Collaborative; Research; Dynamic; fault; rupture

The San Andreas Fault is the largest and arguably the most dangerous source of earthquakes in Southern California. It has produced very large (magnitude greater than 7.5) earthquakes in the past, and it is likely to do so again, with essentially no warning. One of the key tools that seismologists can use to help anticipate and mitigate the effects of such future disasters is to make numerical (computer) models of potential earthquakes. Current numerical models indicate that an earthquake that propagates from Indio toward Los Angeles, through the San Gorgonio Pass (SGP), can channel ground shaking energy directly toward the Los Angeles Basin, where it is amplified by basin sediments and can lead to very high ground motion. Thus, it is extremely important to determine whether such an earthquake that passes through the SGP is likely (or even possible). This is not a trivial question, as the fault structure in the San Gorgonio Pass is quite complex, with the main San Andreas splitting into a number of smaller faults with different orientations and directions of slip. We will use the most accurate available modeling techniques to address the question of whether an earthquake can propagate through this region, leading to a large, damaging event for the region. We will focus on the eastern SGP, where the Coachella Valley strand of the SAF branches into the Mission Creek, Banning, and Garnet Hill fault strands. Each of these strands has very different structure and apparent activity levels, so an understanding of through-going earthquake potential in the region may hinge on whether there is a preferred rupture path in this region. This project implements numerical models of potential earthquakes in this region to determine which of these strands is the most likely path of an earthquake. The models use fault stresses derived from long-term simulations of fault slip and Earth surface deformation in the region, and give information on potential earthquake rupture path, slip distribution, earthquake size, and ground motion. These innovative earthquake models will also include the effects of rock failure away from the faults, which may have a significant effect on the energy budget of earthquakes. This work constitutes an important advance in the science of earthquake modeling, and it has important broader impacts, including estimates of earthquake size and ground motion in the region, with further implications for engineering, zoning, and emergency response. The question of whether an earthquake can propagate through the San Gorgonio Pass (SGP) region of the San Andreas Fault (SAF) is of tremendous scientific and societal importance. The SGP is a ?pinch point? along this fault system, in which the SAF splits into multiple non-coplanar segments, including both strike-slip and thrust; there may well be no through-going surface to support continuous earthquake rupture in this region [e.g., Yule, 2009]. Numerical models [e.g., Olsen et al., 2008] indicate that an earthquake rupture that propagates through the SGP from southeast to northwest can channel seismic radiation directly toward the Los Angeles Basin, where it is amplified by basin sediments and can lead to very high ground motion. Thus, the question of whether the San Gorgonio Pass serves as a significant barrier to earthquake rupture propagation is of great practical as well as theoretical interest, and it affects ground motion not just locally but in the most populated regions of Southern California. This work aims to help answer this question by performing numerical models of potential earthquakes in this region. Specifically, we will focus on the eastern SGP, where the Coachella Valley segment of the SAF branches into the Mission Creek, Banning, and Garnet Hill fault strands. Each of these strands has very different structure and apparent activity levels, so an understanding of through-going earthquake potential in the region may hinge on whether there is a preferred rupture path in this region. We will use a combination of methods to address this question: coupled long-term and interseismic quasi-static modeling method to determine the on- and off-fault stresses in the region, and a 3D dynamic rupture model that uses these stresses to model potential earthquakes, including rupture propagation, slip, and near-source ground motion. Each of these methods has been well tested individually, but this is the first time that they have been combined in this way. The work will is a significant advance in the field of realistic fault dynamics, in that it is (to the PIs knowledge) the first study to combine a tectonically- and geometrically-consistent heterogeneous stress field with off-fault failure; prior models have had either but not both of these abilities. Earlier homogenous-stress models that incorporate such failure mechanisms [Andrews, 2005; DeDontney et al., 2012; Duan and Day, 2008; Dunham et al., 2011; Gabriel et al., 2013] indicate that off-fault plasticity can have a very strong effect on the energy budget and rupture propagation during an earthquake; we expect the inclusion of these effects in our heterogeneous stress models to be at least as significant. Our work will advance the science of fault dynamics in regions of geometrical complexity, especially fault branches. The methods developed in this project are readily applicable to other earthquake-prone regions with significant fault complexity, such as Northern California. The results have important implications for earthquake occurrence in the SGP, as well as for seismic hazard throughout Southern California. In particular, it has important implications for estimates of potential earthquake size in Southern California, and consequently the generation of strong ground motion and estimation of seismic hazard. This information can be useful for engineering, zoning, and emergency response purposes. In addition, the investigators perform outreach activities to the local Morongo Band Of Mission Indians in San Gorgonio Pass. This funding supports the Ph.D. research of a talented young postdoctoral researcher of Afro-Caribbean ancestry, a significantly under-represented group within the Geosciences.

圣安德烈亚斯断层是南加州最大也是最危险的震源。它过去曾发生过非常大的地震（震级大于7.5级），而且很可能再次发生，基本上没有任何预警。地震学家用来帮助预测和减轻这种未来灾害影响的关键工具之一是建立潜在地震的数值（计算机）模型。目前的数值模型表明，一场从印第奥向洛杉矶传播的地震，通过圣戈尼奥山口（SGP），可以将地面震动能量直接引导到洛杉矶盆地，在那里它被盆地沉积物放大，并可能导致非常高的地面运动。因此，确定这样的地震是否有可能（甚至可能）通过SGP是极其重要的。这不是一个微不足道的问题，因为圣戈尔尼奥山口的断层结构相当复杂，主要的圣安德烈亚斯分裂成许多不同方向和滑动方向的小断层。我们将使用最精确的建模技术来解决地震是否会通过该地区传播，从而导致该地区发生大规模破坏性事件的问题。我们将把重点放在SGP的东部，在那里，SAF的Coachella Valley链分支到Mission Creek， Banning和Garnet Hill断层链。每一股都有非常不同的结构和明显的活动水平，因此，对该地区持续地震潜力的理解可能取决于该地区是否存在首选断裂路径。该项目实现了该地区潜在地震的数值模型，以确定这些股中哪一条是最有可能发生地震的路径。这些模型使用了从该地区断层滑动和地表变形的长期模拟中得到的断层应力，并提供了潜在地震破裂路径、滑动分布、地震大小和地面运动的信息。这些创新的地震模型还将包括远离断层的岩石破坏的影响，这可能对地震的能量收支产生重大影响。这项工作构成了地震模拟科学的重要进展，它具有重要的更广泛的影响，包括估计地震大小和该地区的地面运动，并进一步影响工程、分区和应急响应。地震是否能够通过圣安德烈亚斯断层（SAF）的圣戈尔尼奥山口（SGP）地区传播的问题具有巨大的科学和社会意义。SGP是一个？夹点吗?沿该断裂体系，上覆构造分裂为多个非共面段，其中既有走滑段，也有逆冲段；该地区很可能没有支撑连续地震破裂的贯通面[例如，Yule, 2009]。数值模型[例如，Olsen等人，2008]表明，通过SGP从东南向西北传播的地震破裂可以将地震辐射直接引导到洛杉矶盆地，在那里它被盆地沉积物放大，并可能导致非常高的地面运动。因此，圣戈尔尼奥山口是否作为地震破裂传播的重要屏障的问题不仅具有理论意义，而且具有实际意义，因为它不仅影响当地的地面运动，而且影响南加州人口最稠密地区的地面运动。这项工作旨在通过执行该地区潜在地震的数值模型来帮助回答这个问题。具体来说，我们将关注SGP的东部，在那里，SAF的Coachella山谷部分分支到Mission Creek， Banning和Garnet Hill断层链。每一股都有非常不同的结构和明显的活动水平，因此，对该地区持续地震潜力的理解可能取决于该地区是否存在首选断裂路径。我们将使用多种方法来解决这个问题：耦合长期和地震间准静态建模方法来确定该地区的断层上和断层外应力，以及3D动态破裂模型，该模型使用这些应力来模拟潜在的地震，包括破裂传播、滑动和近源地面运动。每一种方法都单独经过了很好的测试，但这是第一次以这种方式将它们结合起来。这项工作将是现实断层动力学领域的重大进步，因为它是（据PIs所知）第一次将构造和几何上一致的非均匀应力场与断层外破坏相结合的研究；之前的模型只具备其中一种能力，但没有同时具备这两种能力。早期的均质应力模型包含了这种破坏机制[Andrews, 2005；dedonney et al., 2012；Duan and Day, 2008；Dunham et al., 2011；Gabriel等人，2013]表明断层外塑性对地震期间的能量收支和破裂传播有很强的影响；我们期望在我们的异质应力模型中包含这些影响至少同样重要。我们的工作将推动断层动力学的科学在几何复杂的区域，特别是断层分支。本项目开发的方法很容易适用于其他断层复杂的地震易发地区，如北加州。这些结果对SGP的地震发生以及整个南加州的地震危险具有重要意义。特别是，它对估计南加州潜在的地震规模，从而产生强烈的地面运动和估计地震危险性具有重要意义。此信息可用于工程、分区和应急响应目的。此外，调查人员还向圣戈尼奥山口当地的莫朗戈部落印第安人传教团开展外展活动。这笔资金支持一位有才华的年轻非洲裔博士后研究员的博士研究，这是地球科学领域一个明显缺乏代表性的群体。