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经济损失的主要自然灾害之一。在板块边界，例如环太平洋的“火环”，通常预计会有很高水平的地震危险。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/天然气勘探的发展，以及相关废水进入深层地层(即，不是生产石油和天然气的地层)。因此，大多数新的地震被称为“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“新地震之都”(更准确地说，是“人造地震的地震之都”)，其3级以上的地震率超过了加利福尼亚州。2016年2月13日，俄克拉何马州费尔维尤附近发生5.1级地震，为该州历史上第三大地震。发生这场地震的断层在过去四个月里经历了七次M4级地震。当地居民反映了强烈的地面震动，能量(前震)逐渐积累，房屋出现裂缝，更重要的是，对安全的担忧。这一地区的土壤主要是沙子(以小撒哈拉国家公园为特色)，这往往会使建筑地基不稳定，并有液化和山体滑坡的潜在危险。这项快速建议旨在记录该地区的地震活动，以更好地了解其原因和最终的潜在危险。根据这项建议，我们将部署10个宽带站，为俄克拉荷马州西部地区提供密集的方位覆盖，特别是在美景和伍德沃德断层周围。这些断裂相距仅8公里左右，但具有明显的地震活动特征--美景断裂以4级地震的余震序列为主，而伍德沃德断裂多为小震，地震活动速率呈渐进式变化，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露头地质绘制的断层和活跃的地震调查之间有什么关系？它们揭示了什么关于断层作用的过程？(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，可以建议采取什么减容措施来减轻人为地震灾害？(3)该地区两条主要断层之间的断层机制是否存在可解决的差异，从而导致对流体注入的地震响应显著不同，例如，相对于地应力场的速度结构、断层强度或断层方向？研究结果将提供重要的参数，帮助评估与地面震动和其他次生灾害相关的地震危险，如液化或山体滑坡。

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.