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An automated workflow for adjoint tomography—waveform misfits and synthetic inversions for the North Island, New Zealand
Geophysical Journal International ( IF 2.8 ) Pub Date : 2020-08-26 , DOI: 10.1093/gji/ggaa381
Bryant Chow 1 , Yoshihiro Kaneko 2, 3 , Carl Tape 4 , Ryan Modrak 5 , John Townend 1
Affiliation  

We develop and verify an automated workflow for full-waveform tomography based on spectral element and adjoint methods. We choose the North Island, New Zealand as a study area because of its high seismicity, extensive seismic network, and the availability of a candidate ray tomography starting model. To assess the accuracy of this model, we simulated 250 regional earthquakes using a spectral element solver, and compared the resulting synthetics with recorded waveforms. In a 10–30 s passband, reasonable cross-correlation phase and amplitude misfits exist between data and synthetics, whereas at 2–30 s, waveform misalignment is severe enough that meaningful cross-correlation measurements are no longer possible. To improve the velocity model at these short periods, we created an automated inversion framework based on existing tools for signal processing, phase measurement, nonlinear optimization, and workflow management. To verify the inversion framework, we performed a realistic synthetic inversion for 3-D checkerboard structure and analyzed model recovery, misfit reduction, and waveform improvement. The results of this analysis show that the source–receiver distribution within the chosen domain is capable of resolving velocity anomalies in regions of sufficient data coverage, and of magnitudes comparable to those expected in a real seismic inversion. Along with this finding, the relative ease of use and reliability of the workflow motivates future efforts targeting a high-resolution (2–30 s), large-scale (>50 000 measurements) seismic inversion for the North Island. Updated models from such an inversion are expected to improve ground motion predictions, constrain complex velocity structures, and advance understanding of New Zealand tectonics.

中文翻译:

伴随层析成像的自动化工作流程-新西兰北岛的波形失配和合成反演

我们开发和验证了基于频谱元素和伴随方法的全波形层析成像自动化工作流程。我们选择新西兰北岛作为研究区域,因为它的高地震活动性,广泛的地震网络以及候选射线断层摄影起始模型的可用性。为了评估该模型的准确性,我们使用频谱元素求解器模拟了250次区域地震,并将合成的结果与记录的波形进行了比较。在10–30 s的通带中,数据与合成信号之间存在合理的互相关相位和幅度失配,而在2–30 s时,波形失配严重到足以不再进行有意义的互相关测量的程度。为了在这短时间内改善速度模型,我们基于用于信号处理,相位测量,非线性优化和工作流管理的现有工具创建了一个自动反转框架。为了验证反演框架,我们对3-D棋盘格结构进行了逼真的合成反演,并分析了模型恢复,减少失配和改善波形的情况。分析结果表明,选定域内的震源-接收器分布能够解决足够数据覆盖范围内的速度异常,其幅度与实际地震反演中预期的幅度相当。伴随着这一发现,工作流程的相对易用性和可靠性促使未来的工作针对北岛的高分辨率(2–30 s),大规模(> 50,000次测量)地震反演。
更新日期:2020-10-14
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