NSF-RAPID: Rapid Response for the M5.1 Fairview Earthquake - Detailed Understanding of the Fault Systems in Western Oklahoma

地震仍然是造成人员和财政损失的主要自然灾害之一。在板块边界，例如沿着太平洋的“火环”，通常预计会有高水平的地震危险。然而，在过去的五年里，大陆内部的地震正在迅速增加。大部分增加归因于石油/天然气勘探的开发，以及相关的废水处置到深层地层中（即，除了生产石油和天然气的地层之外）。因此，大多数新事件被称为“诱发地震”，因为它们与人类活动密切相关。俄克拉荷马州被称为“新的地震之都”（更准确地说，是“人造地震的地震之都”），M级以上地震的发生率超过了加州。2016年2月13日，俄克拉荷马州费尔维尤附近发生5.1级地震，是该州历史上第三大地震。在过去的四个月里，承载这次地震的断层经历了七次M4地震。当地居民报告说，强烈的地面震动，逐渐积累的能量（前震），房屋裂缝，更重要的是，对安全的担忧。该地区的土壤主要是沙子（以小撒哈拉国家公园为特色），容易使建筑物地基不稳定，并存在液化和山体滑坡的潜在危险。本RAPID提案旨在记录该地区的地震活动，以更好地了解其原因并最终了解其潜在危险。通过本提案，我们将部署10个宽带站，这些站将为俄克拉荷马州西部地区，特别是Fairview和Woodward断层周围提供密集的方位角覆盖。这些断层仅相隔约8 km，但具有明显的地震活动特征-Fairview断层主要具有M4事件的单个余震序列，而Woodward断层则以地震活动率逐渐变化的小地震为主，它们与注入量变化也有不同类型的相关性。 所收集的数据将有助于解决以下科学问题：（1）由地震活动性绘制的发震断层与由露头和主动地震调查绘制的地质断层之间的关系是什么？它们揭示了断层作用的什么过程？(2)注入量的变化是否影响诱发地震的最大规模？如果是这样的话，对减少人为地震危险的体积程序可以提出什么建议？(3)该地区两个主要断层之间的断层机制是否存在可分辨的差异，这将导致对流体注入的显著不同的地震响应，例如，速度结构、断层强度或断层相对于原地应力场的方位？研究结果将提供重要的参数，有助于评估与地面震动和其他次生灾害（如液化或滑坡）相关的地震灾害。

Earthquakes remain one of the major natural hazards that cause human and financial losses. High levels of earthquake hazards are generally expected at plate boundaries, such as the "Ring of Fire" along the Pacific Rim. However in the last five years, earthquakes in the interior of continents are rapidly increasing. Most of the increases are attributed to the development in oil/gas exploration, and the associated wastewater disposal into deep formations (i.e., other than the formations that produce oil and gas). For this reason, most of the new events are termed as "induced earthquakes" as they are closely related to human activities. The state of Oklahoma has been known as "the new earthquake capital" (more precisely, "the earthquake capital of man-made earthquakes"), with earthquake rates of M larger than 3 exceeding those in California. On February 13, 2016, a M=5.1 earthquake occurred near Fairview, Oklahoma, being the third largest event in the history of the state. The fault that hosted this earthquake had experienced seven M4 earthquakes in the past four months. Local residents reported strong ground-shaking, gradual accumulation of energy (foreshocks), cracks in the house, and more importantly, concerns about safety. The soil in this area is mostly sand (featured with the Little Sahara State Park), which tend to make building foundations unstable, and potential hazard of liquefaction and landslides. This RAPID proposal aims at recording the seismic activity in this region to better understand its cause and ultimately its hazard potential.With this proposal, we will deploy 10 broadband stations that will provide a dense azimuthal coverage to the western Oklahoma area, in particular around the Fairview and Woodward faults. These faults are separated by only about 8 km, but have distinct seismicity characteristics - the Fairview fault has predominately individual aftershock sequences from M4 events, while the Woodward fault has mostly small earthquakes with gradual seismicity rate changes, they also have different types of correlation with injection volume changes. The data collected will help to address scientific questions such as: (1) What is the relationship between seismogenic faults mapped from seismicity and geological mapped faults from outcrops and active seismic survey? What do they reveal about the faulting processes? (2) Does injection volume variations influence the maximum size of induced earthquakes? If so, what could be suggested for the volume reduction procedures to mitigate man-made earthquake hazards? (3) Are there resolvable differences in fault mechanisms between the two major faults in this area, which would lead to significantly different seismic responses to fluid injection, e.g., velocity structures, fault strength or fault orientation relative to in-situ stress field? The results will provide important parameters that would help to assess earthquake hazards associated with ground shaking and other secondary hazards, such as liquefaction or landslides.