Collaborative Research: Can Low-Angle Normal Faults Produce Earthquakes? Reading a Pseudotachylyte 'Rosetta Stone'

产生地震的断层破裂的物理学已经研究了一百多年，这一时期的标志是仪器和分析技术的巨大进步，这些技术需要了解断层破裂引起的地面震动（地震活动性）和断层岩石的物理性质。本研究解决了古代地震的长期岩石记录与短期历史地震记录之间的差异。具体来说，它侧重于一类断层的代表，根据当前的地震力学范式和安德森岩石破裂力学理论，这些断层似乎不适合断裂。根据现有的理论，形成于低角度（即小于30度）的正断层不应该产生重大地震，因为导致断裂的构造力与断层表面的角度很大。历史上的地震活动记录一般支持这一论点，表明这样的断层只产生微地震。然而，地质记录中包含了许多正常断层的例子，这些断层似乎在低倾角（小于30度）下滑动。伪水晶石的存在，是一种极快冷却的玻璃状熔融岩石，通常是地震滑动期间摩擦加热的结果，已被发现与一些低角度断层有关，因此保存了“化石地震”的记录。在这项研究中，主要研究人员在亚利桑那州南山的一个低角度正断层上发现了异常丰富的“化石地震”记录。这项研究正在探索这些化石记录，目的是确定这些断层是否会在非安德森方向产生重大地震，从而解决断层力学中的一级问题。这项研究不仅将加深对地震记录和“定向错误”断层潜在地震危险性的认识，而且有可能改变我们对断层力学的认识。除了本研究的科学目标外，该项目还将有助于培养STEM（科学、技术、工程和数学）学科的研究生和本科生，从而为一个更具科学素养和活力的社会做出贡献。它代表了来自两所研究型公立大学的研究人员之间的合作努力。研究成果将在国家地球科学会议上发表，并通过同行评议的科学文献传播。通过与威斯康星大学地质博物馆合作，制作参与式练习、视频和在线教学模块，研究结果还将用于参与和教育公众关于地质概念和地震危害的知识。根据安徒生断层力学，低角度正断层（LANFs）的滑移取向较差。地质记录通常支持当前低倾角（小于30度）的滑动；然而，地震记录通常表明，这样的断层不会产生超过5.5级的地震。这些数据集之间存在差异的一种解释是，lanf的大尺寸和更高的效率导致其重复间隔比地震记录更长。第二种解释是，当它们被挖掘出来时，由于均衡反弹而重新定向，这使得它们在旋转到最终的浅方向之前，可以在更陡的地方滑动。第三种解释调和了地质和地球物理记录，即低角度正断层因蠕变而失效，产生微震活动，但不会产生大地震。虽然这是对两条明显记录蠕变的活动断层的一个令人信服的解释，但这并不能解释在世界范围内已发掘的低角度正断层带中普遍出现伪岩的原因。这项研究的目的是利用南山变质核杂岩的暴露来探索地震的化石记录。之所以选择这个地点，是因为伪岩断层脉丰富，适合岩石磁分析和地质年代学分析。本项目将通过综合构造、热年代学和古地磁分析和建模来验证以下假设：(1)最大地震的重复周期足够长，可以在40Argon/39Argon同位素年龄（+/-约25万年）的误差范围内进行区分。(2)古地磁残余表明地震发生在现今断层倾角（小于30度）。(3)南山断脉记录了一系列地震震级，最大的地震震级超过5.5级。主要研究人员将使用保存在伪岩中的磁记录来量化断层旋转（倾斜），如果有的话，地震成因。将每个样品记录的剩磁矢量与期望的地磁场方向进行对比。s 40Argon/39Argon年龄及其伴随的地磁北位以及与北美视极移路径的比较。两个矢量之间的任何分歧将表示系统自地震滑动以来的旋转，最终使我们能够量化LANF活跃的角度。围岩的冷却历史也将用几个温度计来确定。建模将纳入这些数据，并根据伪岩断层脉厚度约束地震震级。该项目解决了断层力学中的一个一级问题，并将对美国西部变质核杂岩的古代地震活动性记录提供更深入的了解

The physics of earthquake-producing fault rupture has been studied for over a hundred years, a period marked by dramatic advances in the instrumentation and analytical techniques required to understand both the ground shaking (seismicity) caused by fault failure and the physical properties of fault rocks. This research addresses a discrepancy between the long-term rock record of ancient earthquakes and the short-term historical seismic record. Specifically, it focuses on representatives of a class of faults that appear to be poorly oriented for breakage according to the current earthquake mechanics paradigm and Andersonian mechanical theory for the rupture of rocks. According to established theory, normal faults that form at low angles (i.e., less than 30 degrees) should not produce significant earthquakes because the tectonic forces that cause failure are oriented at a high angle to the fault surface. The historical seismicity record generally supports this argument, suggesting that such faults produce only microearthquakes. However, the geologic record contains numerous examples of normal faults that appeared to have slipped at low (less than 30 degree) dips. The presence of pseudotachylite, an extremely quickly cooled, glassy melt rock that generally forms as the result of frictional heating during seismic slip, has been found to be associated with some of these low angle faults, and thus preserves a record of 'fossil earthquakes.' In this study, the principal investigators have identified an unusually rich record of 'fossil earthquakes' on a low angle normal fault in the South Mountains, Arizona. This research is exploring this fossil record with the goal of determining whether or not such faults produce significant earthquakes in non-Andersonian orientations, thus addressing a first-order question in fault mechanics. The research will not only result in a deeper understanding of the earthquake record and the potential seismic hazard of 'misoriented' faults, but also has the potential to transform our understanding of fault mechanics. In addition to the scientific objectives of this research, the project will contribute to the training of graduate and undergraduate students in a STEM (science, technology, engineering, and mathematics) discipline, thus contributing to a more scientifically literate and vibrant society. It represents a collaborative effort between investigators from two research-intensive public universities. Research results will be disseminated by presentation at national geoscience meetings and through the peer-reviewed scientific literature. The results will also be used to engage and educate the public regarding geologic concepts and earthquake hazards through a partnership with the University of Wisconsin Geology Museum to produce participatory exercises and videos and online teaching modules. Low-angle normal faults (LANFs) are poorly oriented for slip according to Andersonian fault mechanics. The geologic record generally supports slip at current low (less than 30 degrees) dips; however, the seismic record generally suggests such faults cannot produce earthquakes greater than magnitude 5.5. One explanation for the discrepancy between these data sets is that the large size and greater efficiency of LANFs result in recurrence intervals longer than the seismic record. A second is that LANFs were re-oriented by isostatic rebound as they were exhumed, allowing them to slip at steeper dips before rotating into their final, shallow orientations. A third explanation, one which reconciles the geologic and geophysical records, is that low angle normal faults fail by creep, producing microseismicity but not substantial earthquakes. Although a convincing explanation for the two active faults that demonstrably record creep, this does not account for the common occurrence of pseudotachylyte in exhumed low-angle normal fault zones worldwide. The purpose of the proposed research is to explore this fossil record of earthquakes using exposures from the South Mountains metamorphic core complex. This site was chosen because pseudotachylyte fault veins are plentiful and amenable to both rock magnetic and geochronologic analyses. This project will test the following hypotheses by integrated structural, thermochronologic, and paleomagnetic analyses and modeling: (1) Recurrence intervals of the largest earthquakes are sufficiently long that they can be distinguished within the error of 40Argon/39Argon isotopic ages (+/- ca. 0.25 million years). (2) Paleomagnetic remanence indicates that earthquakes occurred at current fault dips (less than 30 degrees). (3) Fault veins in the South Mountains record a range of earthquake sizes, the largest of which are greater than magnitude 5.5. The principal investigators will use the magnetic record preserved in pseudotachylyte to quantify fault rotation (tilting), if any, seismogenesis. The remanence vector recorded by each sample will be compared with the expected geomagnetic field direction by correlating the sample?s 40Argon/39Argon age with its concomitant geomagnetic north location and comparison with the North American apparent polar wander path. Any divergence between the two vectors will represent rotation of the system since seismic slip, ultimately allowing us to quantify the angle at which a LANF was active. The cooling history of the wall rock also will be determined using several thermochronometers. Modeling will incorporate these data and constrain earthquake magnitude based on pseudotachylyte fault vein thickness. The project addresses a first-order question in fault mechanics and will provide a deeper understanding of the record of ancient seismicity in the metamorphic core complexes of the western U.S.