A New Adjoint-state Full Waveform Tsunami Source Imaging Method

海啸是由海洋中大量水的位移产生的海洋重力波，通常由复杂的地球物理过程引起，包括破坏海洋地壳的浅层地震和海底滑坡。海啸可能在沿海地区造成大量人员伤亡和经济损失，而这些地区居住着世界上三分之一以上的人口。因此，海啸预警是减少和减轻海啸灾害的一个重要系统。为了更快速、更准确地进行海啸预警，并更好地了解海啸源的复杂性，研究人员建议应用伴随状态方法来求解海啸源的初始海底变形。该方法提高了震源过程的成像分辨率，并且以相对较小的计算成本产生较少的伪影。该项目充分利用了海底压力传感器和海岸验潮仪等海啸仪器的不断增加，并支持了一名女学生的博士工作。研究成果通过会议和期刊共享，并在公立学校和本科生课堂上推广。传统的海啸波震源反演是基于有限断层滑动建模或时间反演成像。这种反演方法受到断层参数或地壳刚度的不确定性的影响。此外，计算绿色函数的计算负担沉重，导致有限的空间分辨率，阻碍了实时适用的海啸预警的传统方法。在这项工作中，我们移植了最先进的伴随态全波形反演方法从勘探地震海啸源成像。伴随状态法以较小的计算量求解初始水位模式，有望提高海啸预警的速度，减少预警盲区。我们的新方法不依赖于预定义的故障参数，适用于海啸生成地震与未知的故障几何形状。该方法还有效地处理了密集网格，并能够解决小规模的次级海啸源，如海底滑坡或扇断层上的次级破裂。我们的研究成果将推动海啸科学和震源动力学的发展，并为改善海啸预警等实时应用奠定基础。该奖项反映了NSF的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Tsunamis are oceanic gravity waves generated by the displacement of a large volume of water in the ocean, typically resulting from complex geophysical processes including shallow earthquakes that break the oceanic crust and seafloor landslides. Tsunamis can cause large number of casualties and economic loss in coastal regions, which are home to over a third of the world population. Tsunami warning is thus a crucial system to reduce and mitigate tsunami hazards. In order to perform a more rapid and accurate tsunami early warning as well as better understanding the complexity of tsunami source, the investigators propose to apply the adjoint-state method to solve for the initial seafloor deformation of the tsunami source. The method improves the imaging resolution of the earthquake source process and produces little artifacts with a relatively small computational cost. This project takes full advantage of increasing tsunami instrumentations including ocean-bottom pressure sensors and coastal tide gauges and supports the Ph. D work of a female student. The research results are being shared via conferences and journals, as well as outreach to public schools and in undergraduate classes.Traditional source inversion using tsunamis waves is based on either the finite-fault slip modeling or the time-reversal imaging. Such inversion methods suffer from the uncertainty of fault parameters or crustal rigidity. Moreover, the heavy computational burden of calculating Green's functions result in limited spatial resolution and hinders the real-time applicability of the traditional methods to tsunami early warning. In this work, we transplant the state-of-art adjoint-state full-waveform inversion method from exploration seismology to tsunami source imaging. The adjoint-state method solves the initial-water-elevation pattern with less computational cost, which potentially can improve the speed of tsunami early warning and reduces the blind zone. Our new method does not rely on pre-defined fault parameters and is suitable for tsunami-generating earthquakes with unknown fault geometry. This method also efficiently handles dense mesh grid and is capable of resolving small-scale secondary tsunami sources, such as the seafloor landslide or secondary ruptures on splay faults. Our results will advance tsunami science and earthquake source dynamics and set the stage to improve real-time applications such as tsunami early warning.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.