Improving Back-Projection Imaging with a Novel Physics-Based Aftershock Calibration Approach

大地震涉及沿着断层表面的复杂滑动模式，而断层表面本身也很复杂。这些复杂性对预测大地震的预期地面运动有重大影响，因此对此类事件的估计危险也有重大影响。了解这些复杂性是关键，但由于目前成像方法的分辨率有限，朝着这一目标的进展已经放缓。该项目正在通过一种新的方法来提高这类图像的分辨率和精度，该方法使用密集的地震仪集群记录的相对运动来解决大地震的破裂过程。这项技术类似于用天线定位和跟踪移动信号源的程序。建议方法的提高分辨率允许测试改进的基于物理的地震破裂模型，并正在提高我们模拟地震危险的能力。该项目充分利用了全球高密度地震台网日益增多的优势，包括地球镜美国阵列，并支持一名女研究生的博士工作。这些研究成果正在通过会议和期刊，以及通过扩展到公立学校和本科生课堂上分享。地震阵列处理，也被称为反投影(BP)成像，是一种新兴的技术，利用高频(HF)地震波形来提供关于地震震源过程的详细信息。尽管这项技术提供了独特的、高分辨率的观测，但它已经被证明存在缺陷。虽然位于不同大陆的BP图像能够揭示地震破裂过程的细节，但高频源的确切位置并不一致，导致在破裂长度和速度等特性方面存在差异。该团队开发了一种新的余震校准方法，以减轻阵列的震源位置不确定性，并在2015年尼泊尔-戈尔卡7.8级地震中进行了测试。该团队目前正在将他们的方法应用于2004年苏门答腊岛9.2级、2010年智利8.8级和2011年日本9.1级地震的数据，并开发在多个阵列上相互一致的BP图像，以揭示地震过程的更多细节。这项工作正在缩小地震观测和地震模拟之间的差距，并提供关键信息，以改进对大地震地面运动的预测，从而降低地震风险。

Large earthquakes involve complex patterns of slip along fault surfaces that themselves are complex. These complexities have significant impacts on predicting expected ground motions from major earthquakes, and thus on the estimated hazard from such events. Understanding these complexities is key, but progress towards this goal has been slowed by the limited resolution of current imaging methods. This project is improving the resolution and accuracy of such images through a novel method that uses the relative motions recorded by dense clusters of seismometers to resolve the rupture process of large earthquakes. The technique is analogous to procedures used to locate and track moving sources with antennas. The enhanced resolution of the proposed method allows testing of improved physics-based earthquake rupture models, and is improving our capacity to model earthquake hazards. The project takes full advantage of the increasing availability of global dense seismic networks including the Earthscope USArray, and supports the PhD work of a female graduate student. The research results are being shared via conferences and journals, as well as via outreach to public schools and in undergraduate classes.Seismic array processing, also known as back-projection (BP) imaging, is an emerging technique that utilizes high-frequency (HF) seismic waveforms to provide detailed information about source processes of earthquakes. Even though the technique offers unique, high-resolution observations, it has been shown to have shortcomings. While BP images from each array located on various continents are capable of revealing fine details of the earthquake rupture processes, the exact locations of HF sources do not align, leading to discrepancies regarding such properties as rupture length and speed. The team has developed a novel aftershock calibration to mitigate the source location uncertainties of the arrays and tested it on the 2015 M7.8 Nepal-Gorkha earthquake. The team is now applying their method to data from the 2004 M9.2 Sumatra, 2010 M8.8 Chile, and 2011 M9.1 Japan earthquakes, and developing BP images that are mutually consistent across multiple arrays and reveal additional fine detail about the earthquake processes. This work is narrowing the gap between seismic observations and earthquake simulations, and providing key information to improve predictions of ground motions from large earthquakes and thus reduce seismic risk.