Do Creeping Faults Ever Host Large Earthquakes?: An Investigation of Thermal Alteration in the SAFOD Drillcore

蠕变断层曾经引发过大地震吗？：SAFOD 钻芯热蚀变的研究

• 批准号: 1358585
• 负责人: Heather Savage
• 金额: $ 34.18万
• 依托单位: Columbia University
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2014
• 资助国家: 美国
• 起止时间: 2014-06-01 至 2019-05-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1358585&HistoricalAwards=false
• 关键词: Do; Creeping; Faults; Ever; Host

Broader Significance This work will address one of the major questions of fault mechanics and seismic hazard: can significant earthquakes happen on faults that are slowly creeping? This question has garnered more attention after the 2011 Tohoku-oki earthquake in Japan ruptured through a shallow section of the fault that was assumed to only slide stably (rather than as part of an earthquake). The same may be true of the creeping section of the San Andreas Fault, and this project is evaluating the earthquake potential on that section of the fault.Due to friction, when a large earthquake ruptures a fault, the fault surfaces heat up. This project is using the breakdown of organic molecules found in physical rock samples from the San Andreas Fault Observatory at Depth to search for such heating along the San Andreas Fault in an area of slow steady slip. If the results indicate significant pulses of heating, this may indicate significant earthquakes can rupture this region, despite the slow creep, which would mean the seismic hazard is larger than currently thought. This novel approach to detecting heating in faults has potential application to many other questions in rock mechanics and earthquake physics as well.Technical Description This project is determining the maximum temperature rise, and therefore aspects of earthquake history, along the creeping section of the San Andreas Fault within the SAFOD drillcore. Although the fault is currently creeping near tectonic plate rates, it is possible that this section of the fault hosted earthquakes in the past. Evidence of previous earthquakes will aid in understanding the seismic potential of this area of the San Andreas fault and more generally whether creeping faults can host large earthquakes. Past earthquakes are identified by measuring the thermal maturity of organic molecules (biomarkers) within both the active creeping sections as well as areas of relict fault gouges and cataclasites compared to the less deformed protoliths of the fault-zone rocks. Biomarkers are sensitive to temperature rise such as achieved during earthquakes, and because they are not susceptible to retrograde reactions, they preserve the earthquake temperature rise. In addition to the thermal maturity of the fault zone, the chemical kinetics for reactions of biomarkers within the core are determined from laboratory heating and shearing experiments. Well-established kinetics allows for quantitative constraints on temperature history. Because temperature rise during earthquakes is dependent on fault shear strength, slip, and thickness, determining the reaction kinetics is important for understanding earthquake mechanics. Thermal diffusion models coupled to biomarker reaction kinetics will constrain the mechanics for the creeping section of the San Andreas fault.

这项工作将解决断层力学和地震危险的主要问题之一：大地震会发生在缓慢爬行的断层上吗？在2011年日本东北大地震（Tohoku-oki）发生后，这个问题引起了更多的关注，人们认为该断层的浅层部分只会稳定滑动（而不是作为地震的一部分）。圣安德烈亚斯断层的爬行部分可能也是如此，该项目正在评估该断层部分的地震潜力。由于摩擦，当大地震使断层破裂时，断层表面会升温。该项目利用圣安地列斯断层观测站深部物理岩石样本中有机分子的分解，在缓慢稳定滑动的圣安地列斯断层沿线寻找这种加热现象。如果结果显示出明显的加热脉冲，这可能表明，尽管缓慢的蠕变，但该地区可能发生重大地震，这意味着地震危险性比目前认为的要大。这种探测断层加热的新方法在岩石力学和地震物理学的许多其他问题上也有潜在的应用。该项目旨在确定SAFOD钻芯内沿圣安德烈亚斯断层爬行段的最高温升，从而确定地震历史的各个方面。虽然该断层目前正在以接近构造板块的速度爬行，但这部分断层在过去可能发生过地震。以往地震的证据将有助于了解圣安德烈亚斯断层这一地区的地震潜力，以及更广泛地了解爬行断层是否会引发大地震。过去的地震是通过测量活动爬行段以及残断沟和碎裂岩区域内的有机分子（生物标志物）的热成熟度来识别的，与断裂带岩石中变形较小的原岩相比较。生物标志物对地震期间的温度上升很敏感，因为它们不容易受到逆行反应的影响，所以它们保存了地震时的温度上升。除了断裂带的热成熟度外，生物标志物在岩心内反应的化学动力学也通过实验室加热和剪切实验确定。成熟的动力学允许对温度历史进行定量约束。由于地震期间的温升取决于断层的剪切强度、滑动和厚度，因此确定反应动力学对于理解地震力学非常重要。结合生物标志物反应动力学的热扩散模型将约束圣安德烈亚斯断层爬行段的力学。