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

孕震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显著进步，这些仪器和分析技术既需要了解断层破裂引起的地面震动(地震活动)，也需要了解断层岩石的物理性质。这项研究解决了古代地震的长期岩石记录和短期历史地震记录之间的差异。具体地说，它集中在一类断层的代表，根据当前的地震力学范式和安德森的岩石破裂力学理论，这些断层似乎不太容易破裂。根据既定的理论，以低角度(即小于30度)形成的正断层不应产生重大地震，因为导致破坏的构造力与断裂面呈高角度定向。历史地震活动记录大体上支持这一论点，表明这种断层只产生微震。然而，地质记录包含了许多正常断层的例子，这些断层似乎在低倾角(不到30度)时滑动。假玄武岩是一种冷却速度极快的玻璃状熔融岩石，通常是在地震滑动过程中摩擦加热形成的，它的存在被发现与其中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一化石记录，目的是确定这些断层是否会在非安德森方向上产生重大地震，从而解决断层力学中的一个第一级问题。这项研究不仅将加深对地震记录和错位断层潜在地震危险性的理解，而且有可能改变我们对断层力学的理解。除了这项研究的科学目标外，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。这代表着来自两所研究密集型公立大学的研究人员之间的合作努力。研究成果将通过在国家地球科学会议上的介绍和通过同行评议的科学文献传播。这些成果还将用于通过与威斯康星大学地质博物馆合作制作参与性练习、视频和在线教学模块，让公众了解地质概念和地震灾害。根据Andersonian断层力学，低角度正断层(LANF)的滑动方向很差。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，这种断层不会产生5.5级以上的地震。对这些数据集之间差异的一种解释是，LANF的大小和更高的效率导致重复间隔比地震记录更长。第二个原因是，当LANF被挖掘出来时，它们被均衡的反弹重新定向，使得它们在旋转到最终的浅方向之前，可以在更陡峭的坡度中滑动。第三种解释与地质和地球物理记录相一致，即低角度正断层因蠕动而失效，产生微震活动，但不产生大地震。尽管这是对两条明显记录蠕变的活动断裂的令人信服的解释，但这并不能解释世界各地出土的低角度正断裂带中普遍存在的假玄武岩。这项拟议研究的目的是利用南山变质核杂岩的暴露来探索这一地震化石记录。之所以选择这个地点，是因为假玄武岩断层脉丰富，可以进行岩石磁学和地质年代学分析。该项目将通过综合的构造、热年代学和古地磁分析和模拟来检验以下假设：(1)最大地震的重复间隔足够长，可以在40Agon/39Ar同位素年龄(约25万年)的误差内区分它们。(2)古地磁剩磁指示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，最大震级大于5.5级。主要调查人员将使用保存在假玄武岩中的磁记录来量化断层旋转(倾斜)，如果有地震成因的话。通过将S 40Ar/39Ar年龄与其伴生的地磁北极位置进行对比，并与北美视极移轨迹进行比较，将每个样品记录的剩磁矢量与预期的地磁方向进行比较。这两个矢量之间的任何偏离都将代表自地震滑动以来系统的旋转，最终使我们能够量化LANF活动的角度。围岩的冷却历史也将使用几个温度计时器来确定。建模将结合这些数据，并基于假玄武岩断层脉厚度来约束地震震级。该项目解决了断层力学中的一个一级问题，并将提供对美国西部变质核杂岩中古代地震活动记录的更深层次的了解。

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.