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

大地震包括沿断层表面的复杂滑动模式，而断层表面本身也是复杂的。这些复杂性对预测大地震的预期地面运动产生了重大影响，从而对此类事件的估计危害产生了重大影响。了解这些复杂性是关键，但由于当前成像方法的分辨率有限，这一目标的进展已经放缓。该项目通过一种新颖的方法，利用密集地震仪记录的相对运动来解决大地震的破裂过程，从而提高了这些图像的分辨率和精度。这项技术类似于用天线定位和跟踪移动源的程序。所提出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，并且正在提高我们模拟地震危害的能力。该项目充分利用了包括Earthscope USArray在内的全球密集地震台网日益增加的可用性，并支持一名女研究生的博士研究。研究结果通过会议和期刊分享，也通过向公立学校和本科班级推广。地震阵列处理，也被称为反向投影（BP）成像，是一种利用高频（HF）地震波形来提供地震源过程详细信息的新兴技术。尽管这项技术提供了独特的、高分辨率的观测结果，但它也有缺点。虽然来自各大洲的每个阵列的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，导致破裂长度和速度等特性存在差异。该团队开发了一种新的余震校准方法，以减轻阵列震源位置的不确定性，并在2015年尼泊尔-廓尔喀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9.2级地震、2010年智利8.8级地震和2011年日本9.1级地震的数据，并开发出在多个阵列之间相互一致的BP图像，并揭示了地震过程的额外细节。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.