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

识别低角度正断层，即倾角小于30度的断层，对于我们目前对断层形成机理的理解是有问题的。根据经典的安德森断层力学，这样的断层不应该在大多数地质环境中形成。尽管存在这个问题，但在许多地质环境中都能观察到它们，但它们的形成方式仍然存在争议。解释这些断层的假设分为两类：一类是对断层如何以次优方向滑动提供解释(例如，断层软弱、降低的有效正应力)；另一类是认为断层在旋转到当前方向之前以更高的角度移动。一个关键问题是，这些相对断层是否会引发地震。最重要的是观察到，低角度正断层通常以假玄武岩(摩擦熔融)脉的形式显示出过去地震活动的明确记录--但研究很少。这些化石地震的存在吗？对试图解释低角度正断层起源的构造模型造成了重要的限制：它们在历史上的某个时候显然是地震孕育的。我们的目标是确定美国西南部和新西兰的低角度正断层产生地震的方向，方法是利用保存在假玄武岩中的剩磁来量化自地震发生以来随后倾斜的潜在影响。倾斜程度(如果有的话)将通过将样品的磁矢量与基于明确定义的视极游走路径的研究区域的预期参考方向进行比较来确定。将结合增量加热分析和基于UV激光的原位方法，使用40Ar/39Ar测年来确定假玄武岩的形成年龄(从而确定地震发生的年龄)，这种方法将瞄准新晶体材料的区域，并避免玻璃和残留碎屑对40Ar/39Ar年龄的有害影响。这项研究将有助于解决构造地质界和构造界长期存在的争议。如果我们的结果表明，这些断层只能在倾斜之前产生更高角度的地震，那么它们将证实一个长期确立的地震和断层力学理论。然而，如果我们的结果表明假粗面岩是由以30度或更小角度形成的断层造成的，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，而且这些数据将需要重新考虑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理解从假青晶石中可以收集到多少信息，假青晶石是古地震的唯一岩石记录。除了研究目标外，该项目还将支持对威斯康星大学麦迪逊分校两名女研究生的培训。研究成果还将用于共同设计和评估以六年级女生为重点群体的“盒子里的地震”教育活动。这些活动将用于威斯康星大学麦迪逊分校S地质博物馆的课堂和正式推广项目，努力向公众传播成果，并鼓励10至14岁的女孩继续参与课堂和企业，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。我们重点小组中的女孩将成为我们外联工作的完全合作伙伴，每个女孩都将被列为一篇论文的共同作者，这篇论文将提交给地球科学教育方面的同行评议期刊。这个项目是威斯康星大学麦迪逊分校和明尼苏达大学的合作项目。

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.