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- Title
Adjoint Synthesis for Trans‐Oceanic Tsunami Waveforms and Simultaneous Inversion of Fault Geometry and Slip Distribution.
- Authors
Takagawa, Tomohiro; Allgeyer, Sébastien; Cummins, Phil
- Abstract
Tsunamis propagate over long distances and can cause widespread damage even after crossing ocean basins. Prediction of tsunamis in distant areas based on observations near their sources is critical to mitigating damage. In recent years, the accuracy of numerical models of trans‐oceanic tsunami propagation has improved significantly due to the incorporation of effects such as the solid earth response to tsunami loading and wave dispersion. However, these models are computationally expensive and have not been fully utilized for real‐time prediction. Here, we derive the adjoint operator for the linear set of equations describing deep‐ocean tsunami propagation and show how a pre‐computed database of adjoint states can achieve rapid synthesis of tsunami waveforms at target sites from nonpoint arbitrary tsunami sources. The adjoint synthesis method allows for an exhaustive parameter search for tsunami source estimation. A method for simultaneous inversion of fault geometry and slip distribution using adjoint synthesis with Sequential Monte Carlo method was proposed and applied to the 2012 Haida Gwaii earthquake tsunami. The influence of model accuracy and the amount of observed data on the estimation of tsunami sources and waveforms was examined. It was found that with a highly accurate propagation model, using only a limited amount of observed data produced source and waveform estimates very similar to the final models obtained with much larger data sets. The final inferred fault model involved megathrust slip distributed between the Haida Gwaii trench and the Queen Charlotte fault. The proposed method can also quantify the uncertainty of the waveform forecasts. Plain Language Summary: Tsunamis can propagate across the ocean for more than 10,000 km and cause damage even areas far from the source. By observing with new instruments and testing theories over the past decade, far‐reaching tsunamis can now be predicted with high accuracy with the consideration of the effects on the seawater compressibility, seafloor deformation due to tsunami loading, gravitational potential change, and Boussinesq dispersion. Reproducing these effects in numerical simulation has been shown to accurately predict tsunami waveforms at distant locations. However, this type of simulation is time‐consuming and has not been used for real‐time tsunami warnings. In this study, we propose a new waveform calculation method named adjoint synthesis. This method can compute waveforms at target points from an arbitrary tsunami source in short time by synthesizing a pre‐computed database. The database can be efficiently computed by adjoint equations. We also propose a method to simultaneously invert fault geometry and slip distribution for earthquake‐generated tsunamis using adjoint synthesis. A case study of the 2012 Haida Gwaii earthquake tsunami showed that source estimation by exhaustive parameter search can provide highly accurate waveform predictions and uncertainty ranges at distant locations from waveform data observed within 1 hr of the source origin time. Key Points: An exact reciprocity approach for linear deep ocean tsunami waveforms is established by defining a tsunami adjoint operatorA method for simultaneous estimation of fault geometry and slip distribution using a sequential Monte Carlo method is proposedExhaustive parameter search enabled by adjoint synthesis improves waveform prediction accuracy for trans‐oceanic tsunami
- Subjects
TSUNAMIS; INVERSIONS (Geometry); MONTE Carlo method; LINEAR operators; GRAVITATIONAL potential
- Publication
Journal of Geophysical Research. Solid Earth, 2024, Vol 129, Issue 6, p1
- ISSN
2169-9313
- Publication type
Article
- DOI
10.1029/2024JB028750