Collaborative Research: Analysis of AIDA (Aftershock Imaging with Dense Arrays) data from the the 2011 M5.8 Virginia earthquake

合作研究：2011 年弗吉尼亚州 M5.8 地震的 AIDA（密集阵列余震成像）数据分析

• 批准号: 1215739
• 负责人: John Hole
• 金额: $ 7.52万
• 依托单位: Virginia Polytechnic Institute and State University
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2012
• 资助国家: 美国
• 起止时间: 2012-07-15 至 2015-06-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1215739&HistoricalAwards=false
• 关键词: Collaborative; Research; Analysis; AIDA; Aftershock

The August 23, 2011, magnitude 5.8 earthquake in Louisa Co., Virginia, provided an opportunity to test a novel type of high-density aftershock deployment utilizing EarthScope Flexible Array instruments. This event had a NE-striking reverse faulting focal mechanism, a hypocentral depth of 6 km (USGS), and occurred in a previously recognized seismic region known as the ¡§Central Virginia Seismic Zone¡¨. Beginning August 27, AIDA deployed 201 stations in three phases, including lines with a 200-m station spacing above the aftershock zone and a 60-km long regional profile. The survey was designed to record wavefields at sufficiently dense spacing to minimize spatial aliasing and lower the event detection threshold. This use of array methods will allow the location the aftershocks with high precision and the imaging of geologic structure with resolution on par with typical controlled-source crustal surveys. Preliminary work has focused on joint tomography for seismic velocity and hypocenter locations, synthesizing high resolution seismic reflection (CRP) profiles from Vertical Seismic Profile (VSP) processing of earthquake records and body wave and surface wave imaging using ambient noise techniques. In addition, this unique dataset will allow the investigators to test seismic interferometric methods for synthesizing virtual seismograph records at the physical location of the earthquake sources. Preliminary aftershock locations define a best-fitting plane striking ~25?a and dipping 55?a E, consistent with the moment tensor solution for the main shock. Initial application of seismic interferometry has recovered both virtual body waves and surface waves, and VSP to CRP techniques can be effectively used to produce reflection imagery of structure throughout the crust. Ongoing work includes locating more of the 1000 events with high signal-to-noise, event imaging using reverse-time waveform migration, and using seismic interferometry/CRP stacking to produce high quality images of earth structure in 3D . The resulting high-resolution subsurface images and aftershock characterization will not only constrain key geologic relationships at depth for this important, intraplate hypocentral region but should provide a template any future ¡§High Definition¡¨ aftershock deployments (e.g. 1000+ channels).The use of high density arrays the the one that is prototyped with AIDA represents a major advance in a) our ability to reconstruct in detail what happens at depth during an earthquake and b) to use the signals from earthquakes to provide imagery of the surrounding earth volume in unprecedented detail. Such ¡§high definition¡¨ imaging will greatly aid in probing/monitoring other important subsurface processes, from volcanism to hydraulic fracturing for energy and other resources. In both cases, these techniques represent an important new tool in detecting, understanding and mitigating natural hazards. These results should have a major impact on how we study earthquakes and related tectonic phenomena.

2011 年 8 月 23 日，弗吉尼亚州路易莎县发生的 5.8 级地震为利用 EarthScope 柔性阵列仪器测试新型高密度余震部署提供了机会。该事件具有北东向反向断层震源机制，震源深度为 6 公里（美国地质调查局），发生在先前公认的地震区“弗吉尼亚中部地震带”。从8月27日开始，AIDA分三期部署201个站点，其中包括余震区以上站距200米的线路和区域剖面长60公里的线路。该勘测旨在以足够密集的间距记录波场，以最大限度地减少空间混叠并降低事件检测阈值。阵列方法的使用将能够高精度地定位余震，并以与典型的受控源地壳勘测相同的分辨率对地质结构进行成像。初步工作重点是地震速度和震源位置的联合层析成像，通过地震记录的垂直地震剖面（VSP）处理以及使用环境噪声技术的体波和表面波成像来合成高分辨率地震反射（CRP）剖面。此外，这个独特的数据集将使研究人员能够测试地震干涉测量方法，以在震源的物理位置合成虚拟地震仪记录。初步余震位置定义了一个最佳拟合平面，撞击~25?a，倾斜55?a E，与主震的力矩张量解一致。地震干涉测量的初步应用已经恢复了虚拟体波和表面波，并且VSP到CRP技术可以有效地用于产生整个地壳结构的反射图像。 正在进行的工作包括以高信噪比定位更多的 1000 个事件、使用逆时波形偏移进行事件成像，以及使用地震干涉测量/CRP 叠加来生成 3D 地球结构的高质量图像。由此产生的高分辨率地下图像和余震特征不仅会限制这个重要的板内震源区域在深度的关键地质关系，而且应该为任何未来的“高清晰度”余震部署（例如 1000 多个通道）提供模板。使用 AIDA 原型化的高密度阵列代表了以下方面的重大进步：a）我们详细重建深度发生情况的能力 b) 在地震期间使用地震信号以前所未有的细节提供周围地球体积的图像。这种“高清”成像将极大地有助于探测/监测其他重要的地下过程，从火山活动到能源和其他资源的水力压裂。在这两种情况下，这些技术都是检测、理解和减轻自然灾害的重要新工具。这些结果将对我们研究地震和相关构造现象的方式产生重大影响。