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

EAGER：合作研究：低角度正断层能否产生地震？

• 批准号: 1237105
• 负责人: Laurel Goodwin
• 金额: $ 7.22万
• 依托单位: University of Wisconsin-Madison
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2012
• 资助国家: 美国
• 起止时间: 2012-04-01 至 2014-09-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1237105&HistoricalAwards=false
• 关键词: EAGER; Collaborative; Research; Low; Angle

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.

就我们目前对断层形成机制的理解而言，低角度正断层（即倾角小于 30 度的断层）的识别是有问题的。根据经典的安德森断层力学，此类断层不应在大多数地质环境中形成。 尽管存在这个问题，它们在许多地质环境中都被观察到，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：一类为断层如何以次优方向滑动提供解释（例如断层薄弱、有效法向应力降低），另一类则表明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产生地震。最重要的是观察到低角度正断层通常以假速石（摩擦熔体）脉的形式显示出过去地震活动的明确（但研究很少）记录。 这些“化石地震”是否存在？对试图解释低角度正断层起源的结构模型施加了一个重要的限制：它们在历史上的某个时刻显然是地震发生的。 我们的目标是通过使用伪速晶石中保存的剩磁来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，以量化地震发生以来后续倾斜的潜在影响。倾斜程度（如果有）将通过将样本的磁矢量与基于明确定义的表观极漂移路径的研究区域的预期参考方向进行比较来确定。假速石形成的年龄（以及地震发生的年龄）将通过 40Ar/39Ar 测年法确定，结合增量加热分析和基于紫外激光的原位方法，该方法将瞄准新晶材料区域并避免玻璃的有害影响以及40Ar/39Ar年龄的残骸碎屑。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只能在倾斜之前以更高的角度产生地震，那么它们将证实长期以来建立的地震和断层力学理论。然而，如果我们的结果表明假速石是由角度为 30 度或更小的断层形成的，那么我们当前的地震力学概念一定是不完整的或有缺陷的，并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了解可以从古地震的唯一岩石记录假速石中收集多少信息。除了研究目标外，该项目还将支持威斯康星大学麦迪逊分校两名女研究生的培训。研究结果还将用于与六年级女生的焦点小组共同设计和评估“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地质博物馆的课堂和正式外展项目，旨在向公众传播研究成果，并鼓励 10 至 14 岁的女孩继续参与课堂和企业活动这将使他们能够在以后的几年中灵活地遵循 STEM 领域的职业道路。我们焦点小组中的女孩将成为我们外展工作的全面合作伙伴，每个女孩都将作为合著者参与撰写论文，并提交给地球科学教育领域的同行评审期刊。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项目。