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。板块构造理论预测地震会沿着构造板块的边界断层集中发生，例如加州的圣安德烈亚斯断层。在板块内部，不应该发生大地震。美国中部和东部位于北美板块的稳定内部，但这一地区的地震很普遍，可能是灾难性的。1811-1812年冬天，密苏里州东南部发生了三次大地震，远至波士顿的地面都发生了震动。地质证据显示，在美国中部和东部的许多州都发生过史前大地震，2011年弗吉尼亚地震震动了东海岸的大部分地区，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。使用计算机建模，这项研究将解决三个主要问题：1）是什么原因造成的应力和应变局部化在美国中部和东部？现有的假说解释这些地震的结果，要么在地球上的薄弱地带？地壳或局部构造力。将开发复杂的计算机模型，以研究观测到的地球结构如何与构造力相互作用，从而导致美国中部和东部的地震。2)是什么引发了最近的地震活动？密苏里州目前的地震活动显然是由于地质学上最近的断层复活，也许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，以量化他们的角色，在重新激活故障。3)为什么密苏里州多次发生大地震？这项工作的结果将改善美国中部和东部的地震灾害评估和减灾工作。这项工作也将有助于了解印度、华北、欧洲和澳大利亚等地质相似地区的地震。该项目将涉及本科生研究，并加强与中国地震研究人员的联系，中国板块内部地震很常见。）。在2012年之前，陆内地震造成的死亡人数超过其他类型地震的总和，但人们对它们的了解仍然很少，并且无法用板块构造来解释。CEUS位于北美板块的内部，可能是稳定的，但仍然有广泛的和潜在的灾难性地震。1811年至1812年，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史以来最大的地震之一，2011年弗吉尼亚地震（Mw 5. 8）是CEUS地震危险的新提醒。该研究包括三个阶段的地球动力学建模，使用最先进的粘弹塑性有限元代码，以解决美国中部和东部（CEUS）地震的基本问题。现有的假设，集群地震活动在CEUS包括各种弱带的岩石圈，如失败的裂谷，和区域和本地的驱动力，如脊推和镁铁质侵入失败的裂谷。本研究将在一个三维岩石圈动力学模型中探讨岩石圈结构和驱动力的动态相互作用。通过比较岩石圈中预测和观测的应力方向和应力状态，对模型进行约束和验证。目前在NMSZ和CEUS的其他部分的地震活动显然是由于最近的断层复活，也许是自上一个冰河时代。断层再活化的各种原因，包括冰川均衡调整和增加侵蚀，已在不同的模型中进行了探讨。本研究将利用CEUS中GPS测量的地壳垂直运动作为主要约束条件，在同一力学模型中量化这些原因的相对作用，并在岩石圈动力学模型中检验循环载荷和粘性松弛等假设原因。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不稳定性。

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.