Stress localization, fault activation, and earthquake migration in central and eastern US: A systematic numerical study

这项研究是为了研究美国中部和东部的地震。这些地震不能很容易地用板块构造理论来解释，该理论预测地震将集中在构造板块的边界断层上，如加利福尼亚州的圣安德烈亚斯断层。在板块内部，应该不会发生大地震。据推测，美国中部和东部位于北美板块的稳定内部，但这部分地区的地震一直很普遍，可能会造成灾难性的后果。1811-1812年冬天，密苏里州东南部发生了三次大地震，震动了远至波士顿的地面。地质证据显示，美国中部和东部的许多州都发生了大型史前地震，2011年弗吉尼亚州的地震震动了东部沿海的大部分地区，这提醒人们注意美国中部和东部的地震危险。该项目的研究人员将系统地调查这些地震的原因。利用计算机模型，这项研究将解决三个主要问题：1)是什么导致了美国中部和东部的应力和应变局部化？现有的假说认为，这些地震要么是S地壳软弱带的结果，要么是局部构造力的结果。将开发复杂的计算机模型，以调查观察到的地球结构如何与构造力相互作用，在美国中部和东部引发群集性地震。2)是什么触发了近期的地震活动？密苏里州目前的地震活动显然是地质上最近的断层重新激活的结果，可能是自上一个冰河时代以来。这项研究将测试各种假设的原因，以量化它们在重新激活故障中的作用。3)为什么密苏里州一再发生大地震？这项工作的结果将改善美国中部和东部地震灾害的评估和减轻。这项工作还将有助于了解印度、中国北部、欧洲和澳大利亚等地质相似地区的地震。这个项目将涉及本科生的研究，并加强与中国地震研究人员的联系，那里的板块内部地震很常见。)陆内地震造成的死亡人数比2012年前其他类型地震的总和还要多，但人们对这些地震仍然知之甚少，也不能很容易地用板块构造来解释。CEU位于北美板块假定稳定的内部，尽管如此，它仍然有广泛的潜在灾难性地震。1811年至1812年间，密苏里州东南部新马德里地震带(NMSZ)发生的一系列7.0级地震是美国大陆有记录以来最大的地震之一，2011年弗吉尼亚州地震(Mw 5.8)再次提醒了CEU的地震危险性。这项研究包括三个阶段的地球动力学模拟，使用最先进的粘弹塑性有限元程序，以解决美国中部和东部(CEU)地震的基本问题。现有的CEU聚集地震活动假说包括岩石圈的各种薄弱地带，如断裂裂谷，以及断裂裂谷中的区域和局部驱动力，如脊推覆和镁铁质侵入体。这项研究将在一个三维岩石圈动力学模型中研究岩石圈结构和驱动力的动态相互作用。该模型将通过比较预测和观测到的岩石圈应力方向和应力状态来进行约束和验证。目前NMSZ和CEU其他部分的地震活动显然是最近断层重新激活的结果，可能是自上一个冰河时代以来的结果。断裂重新活动的各种原因，包括冰川均衡调整和侵蚀加剧，已经在不同的模型中得到了探索。这项研究将量化这些原因在同一力学模型中的相对作用，使用GPS测量的CEU地壳垂直运动作为主要约束。这项研究将在岩石圈动力学模型中检验假设的原因，如循环加载和粘性松弛。该模型还将模拟密西西比湾其他断层的破裂如何影响NMSZ的载荷，以及1811-1812年的NMSZ地震如何通过应力迁移影响邻近断层的不稳定性，特别是瓦巴什山谷的断层。

This research is to study earthquakes in the central and eastern United States. These earthquakes cannot be readily explained by the plate tectonics theory, which predicts earthquakes to be concentrated along the boundary faults of tectonic plates, such as the San Andreas Fault in California. Within plate interiors, no large earthquakes are supposed to occur. The central and eastern United States is within the presumably stable interior of the North American plate, yet earthquakes in this part of the country have been widespread and potentially catastrophic. Three large earthquakes occurred in southeastern Missouri in the winter of 1811-1812, shaking ground as far away as Boston. Geological evidence shows large prehistoric earthquakes in many central and eastern states, and the 2011Virginia earthquake, which shook much of the eastern seaboard, is a reminder of earthquake hazard in the central and eastern US. The researchers on this project will systematically investigate the cause of these earthquakes. Using computer modeling, this research will address three major questions: 1) What causes stress and strain localization in the central and eastern US? Existing hypotheses explain these earthquakes as the results of either weak zones in the Earth?s crust or local tectonic forces. Sophisticated computer models will be developed to investigate how the observed Earth structures interact with tectonic forces to cause clustered earthquakes in the central and eastern US. 2) What triggered the recent seismicity? Present seismicity in Missouri apparently has resulted from geologically recent fault reactivation, perhaps since the last ice age. This research will test the various hypothesized causes to quantify their roles in reactivating faults. 3) Why large earthquakes have repeated in Missouri? Results of this work will improve the assessment and mitigation of earthquake hazards in the central and eastern US. This work will also help understand earthquakes in geologically similar regions such as India, North China, Europe, and Australia. This project will involve undergraduate research and strengthen ties with the earthquake researchers in China, where plate-interior earthquakes are common.). Intracontinental earthquakes have killed more people than other types of earthquakes combined prior to 2012, yet they remain poorly understood and cannot be readily explained by plate tectonics. Located within the presumably stable interior of the North American plate, the CEUS nonetheless has widespread and potentially catastrophic earthquakes. A cluster of M~7.0 earthquakes in the New Madrid Seismic Zone (NMSZ), southeastern Missouri, between 1811 and 1812 were among the largest earthquakes ever recorded in continental US, and the 2011 Virginia earthquake (Mw 5.8) is a fresh reminder of earthquake hazard in the CEUS. The research consists of three phases of geodynamic modeling, using the state-of-art viscoelasto-plastic finite element codes, to address fundamental questions of earthquakes in the central and eastern United State (CEUS). Existing hypotheses for the clustered seismicity in the CEUS include various weak zones in the lithosphere such as failed rifts, and regional and local driving forces such as ridge push and mafic intrusions in the failed rifts. This research will investigate the dynamic interplay of lithospheric structures and driving forces in a three-dimensional lithospheric dynamic model. The model will be constrained and validated by comparing the predicted and observed stress orientations and stress states in the lithosphere. Present seismicity in the NMSZ and other parts of the CEUS has apparently resulted from recent fault reactivation, perhaps since the last ice age. Various causes of the fault reactivation, including glacial isostatic adjustment and increased erosion, have been explored in different models. The proposed research will quantify the relative roles of these causes in the same mechanical model, using GPS-measured vertical crustal motion in the CEUS as the primary constraint.This research will test the hypothesized causes, such as cyclic loading and viscous relaxation, in a lithospheric dynamic model. This model will also simulate how rupture of other faults in the Mississippi Embayment may have affected loading in the NMSZ, and how the 1811-1812 NMSZ earthquakes may have influenced the instability of neighboring faults, especially those in the Wabash Valley, via stress migration.