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

2011年8月23日发生在路易莎公司的5.8级地震，弗吉尼亚州，提供了一个机会，测试一种新型的高密度余震部署利用EarthScope灵活阵列仪器。这次地震具有北东向逆断层震源机制，震源深度为6公里（美国地质勘探局），发生在以前公认的地震区，即“弗吉尼亚中部地震带”。从8月27日开始，AIDA分三个阶段部署了201个台站，包括余震区上方200米台站间距的线路和60公里长的区域剖面。勘测的目的是以足够密集的间隔记录波场，以尽量减少空间混叠，降低事件探测阈值。这种阵列方法的使用将使余震的定位具有高精度，并使地质结构的成像具有与典型的受控源地壳测量相当的分辨率。初步工作的重点是地震速度和震源位置的联合层析成像，从地震记录的垂直地震剖面处理合成高分辨率地震反射剖面，以及利用环境噪声技术进行体波和表面波成像。此外，这一独特的数据集将使调查人员能够测试地震干涉测量方法，在震源的实际位置合成虚拟地震仪记录。初步的余震位置定义了一个最合适的平面打击~25？a和dipping 55？与主震的力矩张量解一致。地震干涉测量的初步应用已恢复了虚拟体波和面波，VSP到CRP技术可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定位更多的1000个事件与高信噪比，事件成像使用逆时波形迁移，并使用地震干涉测量/CRP叠加，以产生高质量的地球结构的三维图像。由此产生的高分辨率地下图像和余震特征不仅将限制这个重要的板内震源区的深度关键地质关系，而且应该为未来的任何“高清晰度”提供模板。余震部署（例如1000多个通道）。使用高密度阵列（用AIDA原型化的阵列）代表了以下方面的重大进步：我们有能力详细重建地震发生时在深处发生的事情，B）利用地震信号以前所未有的细节提供周围地球体积的图像。如此……高清成像将大大有助于探测/监测其他重要的地下过程，从火山活动到水力压裂，以获取能源和其他资源。在这两种情况下，这些技术都是探测、了解和减轻自然灾害的重要新工具。这些结果应该对我们如何研究地震和相关的构造现象产生重大影响。

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.