EAGER: Collaborative Research: Can Low-Angle Normal Faults Produce Earthquakes? A Paleoseismic Perspective

低角度正断层（即倾角小于30度的断层）的识别与我们目前对断层形成机制的理解有关。根据经典的安德森断层力学，这种断层在大多数地质环境中不应该形成。 尽管存在这个问题，但它们在许多地质环境中被观察到，但它们形成的方式仍然存在争议。 解释这些故障的假设分为两类：提供解释的假设（例如，断层薄弱，有效正应力降低），以了解断层如何以次优方向滑动，以及那些表明它们在旋转到当前方向之前以更高角度移动的断层。一个关键问题是这些相对断层是否会引发地震。具有根本重要性的是，低角度正断层通常以假玄武玻璃（摩擦熔体）脉的形式显示出过去地震活动的明确记录（但研究甚少）。 这些的存在？地震？对试图解释低角度正断层起源的结构模型提出了一个重要的限制：它们在历史上的某个时刻显然是地震孕育的。 我们的目标是确定在美国西南部和新西兰的低角度正断层产生地震的方向，通过使用保存在pseudotachylte的剩磁来量化地震成因以来的后续倾斜的潜在影响。根据明确定义的表观极移路径，通过将样本的磁矢量与研究区域的预期参考方向进行比较，确定倾斜度（如有）。假玄武玻璃岩形成的年龄（以及地震成因的年龄）将通过40 Ar/39 Ar定年法确定，该定年法结合了增量加热分析和基于紫外激光的原位方法，以新结晶材料区域为目标，避免玻璃和残余碎屑对40 Ar/39 Ar年龄的有害影响。这项研究将有助于解决构造地质学和构造学界长期存在的争议。如果我们的结果表明，这些断层在倾斜之前只能产生更高角度的地震，那么它们将证实一个长期建立的地震和断层力学理论。然而，如果我们的研究结果表明，假玄武玻璃岩是由30度或更小角度的断层形成的，那么我们目前的地震力学概念要么是不完整的，要么是有缺陷的，这些数据将需要重新考虑对地震力学的基本控制。无论结果如何，我们的研究将有助于更好地了解从假玄武玻璃中可以收集到多少信息，假玄武玻璃是古地震的唯一岩石记录。除了研究目标外，该项目还将支持在威斯康星大学麦迪逊分校培训两名女研究生。研究结果也将用于共同设计和评估“地震在一个盒子”的教育活动与重点小组的六年级女孩。这些活动将用于威斯康星大学麦迪逊分校的课堂和正式的外展计划？美国地质博物馆致力于向公众传播研究成果，并鼓励10至14岁的女孩继续参与课程和企业，这将使她们能够在以后的岁月里灵活地走上STEM领域的职业道路。在我们的焦点小组的女孩将在我们的推广工作的全面合作伙伴，每个女孩将被列入作为共同作者的论文将提交给同行评审的期刊在地球科学教育。该项目是威斯康星大学麦迪逊分校和明尼苏达大学之间的合作努力。

The identification of low-angle normal faults, i.e. faults that have dips less than 30 degrees, are problematic with respect to our current understanding of the mechanics by which faults form. According to classic Andersonian fault mechanics, such faults should not form in most geologic settings. Despite this problem, they are observed in many geologic settings but the means by which they form remain controversial. Hypotheses explaining these faults fall into two categories: those that offer explanations (e.g., fault weakness, reduced effective normal stress) for how the faults slip in suboptimal orientations, and those that suggest they move at higher angles before rotating into their current orientations. A key question is whether or not these relatively faults can generate earthquakes. Of fundamental importance is the observation that low-angle normal faults commonly display an unequivocal - yet poorly studied - record of past seismic activity in the form of pseudotachylyte (frictional melt) veins. The existence of these ?fossilized earthquakes? places an important constraint on structural models that seek to explain the origin of low-angle normal faults: they were clearly seismogenic at some point in their history. Our goal is to determine the orientations at which low-angle normal faults from the southwestern U.S. and New Zealand produced earthquakes by using the magnetic remanence preserved in pseudotachylyte to quantify the potential effects of subsequent tilting since seismogenesis. The degree of tilt (if any) will be determined by comparing the magnetic vector of the sample with the expected reference direction for the study area based on well defined apparent polar wander paths. The age of pseudotachylite formation (and hence the age of seismogenesis) will be determined using 40Ar/39Ar dating using a combination of incremental heating analyses and UV laser-based in situ methods that will target areas of neocrystalline material and avoid the deleterious effects of glass and relict clasts on 40Ar/39Ar ages. This research will contribute to the resolution of a long-standing controversy in the structural geology and tectonics community. If our results show that these faults can only produce earthquakes at higher angles prior to tilting, they will confirm a long-established theory of the mechanics of earthquakes and faulting. If, however, our results show that the pseudotachylites have resulted from faults that formed at angles 30 degrees or less, our current concepts of earthquake mechanics must be either incomplete or flawed, and the data will require a reconsideration of the fundamental controls on earthquake mechanics. Regardless of outcome, our research will contribute a much greater understanding of how much information can be gleaned from pseudotachylytes, the sole rock record of paleoearthquakes. In addition to the research objectives, the project will support the training of two female graduate students at the University of Wisconsin-Madison. The research results will also be used co-design and evaluate "earthquake in a box" educational activities with a focus group of 6th grade girls. The activities will be used in both classroom and formal outreach programs of University of Wisconsin-Madison?s Geology Museum in an effort to both disseminate results to the public and encourage 10- to 14-year-old girls to remain involved in classes and enterprises that will allow them the flexibility to follow career paths in STEM fields in later years. The girls in our focus group will be full partners in our outreach effort, and each girl will be included as a co-author on a resulting paper to be submitted to a peer-reviewed journal in geosciences education. This project is a collaborative effort between the University of Wisconsin-Madison and the University of Minnesota.