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

更广泛的意义这项工作将解决断层力学和地震危险性的主要问题之一：在缓慢爬行的断层上会发生重大地震吗？在2011年日本东北大地震破裂后，这个问题引起了更多的关注。人们认为，该断层只会稳定滑动(而不是作为地震的一部分)。圣安德烈亚斯断层的蠕动段可能也是如此，该项目正在评估该断层段的地震潜力。由于摩擦，当大地震破裂断层时，断层表面会变热。该项目正在利用圣安德烈亚斯断层观测站物理岩石样品中发现的有机分子的深层分解来寻找沿着圣安德烈亚斯断层在缓慢稳定滑动区域的这种加热。如果结果显示有明显的加热脉冲，这可能表明尽管蠕变缓慢，但重大地震可能会破坏这一地区，这意味着地震风险比目前认为的更大。这种探测断层加热的新方法还可能应用于岩石力学和地震物理学中的许多其他问题。该项目正在确定SAFOD钻芯内圣安德烈亚斯断层蠕动段的最大温升，从而确定地震历史的各个方面。尽管该断层目前在构造板块速率附近爬行，但这段断层过去可能曾发生过地震。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，以及更广泛地说，爬行断层是否可以孕育大地震。过去的地震是通过测量活动爬行部分以及残余断层泥和碎裂岩区域内有机分子(生物标记物)的热成熟度来识别的，与变形较小的断层带岩石原岩相比。生物标志物对温度升高很敏感，例如在地震中实现的，因为它们不容易受到逆行反应的影响，所以它们保留了地震的温度上升。除了断裂带的热成熟度外，还通过实验室加热和剪切实验确定了岩心内生物标志物反应的化学动力学。良好的动力学允许对温度历史进行定量限制。由于地震时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是很重要的。热扩散模型与生物标志物反应动力学相结合，将约束圣安德烈亚斯断层蠕动段的力学。

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.