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

2011年8月23日发生在弗吉尼亚州路易莎公司的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。这次地震具有北东走向的反向断层震源机制，震源深度为6千米(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，包括在余震区上方200米站距的线路和60公里长的区域轮廓。这项调查旨在以足够密集的间隔记录波场，以最大限度地减少空间混叠，并降低事件检测阈值。这种阵列方法的使用将能够高精度地定位余震，并以与典型的可控震源地壳调查相同的分辨率成像地质结构。初步工作侧重于地震速度和震源位置的联合层析成像，从垂直地震剖面(VSP)合成高分辨率地震反射剖面，利用环境噪声技术处理地震记录以及体波和面波成像。此外，这一独特的数据集将使研究人员能够测试在震源物理位置合成虚拟地震仪记录的地震干涉方法。初步的余震位置确定了与主震的矩张量解一致的最佳平面：走向~25A，倾角55A~E。地震干涉测量的初步应用已经恢复了虚拟体波和面波，VSP到CRP技术可以有效地产生整个地壳结构的反射成像。正在进行的工作包括以高信噪比定位1000个事件中的更多事件，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叠加来产生高质量的地球结构3D图像。由此产生的高分辨率地下图像和余震特征不仅将约束这一重要的板内震中区在深部的关键地质关系，而且应该为未来的任何高清晰度余震部署(例如1000个通道)提供一个模板。高密度阵列的使用代表着a)我们详细重建地震期间深层发生的事情的能力和b)使用来自地震的信号以前所未有的细节提供周围地球体积的图像。这种高清晰度成像将极大地帮助探测/监测其他重要的地下过程，从火山作用到水力压裂，以获取能源和其他资源。在这两种情况下，这些技术都是检测、了解和减轻自然灾害的重要新工具。这些结果应该会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.