Adjoint Inversion of Near-Field Pressure Gauge Recordings for Rapid and Accurate Tsunami Source Characterization

We explore the potential of the adjoint-state tsunami inversion method for rapid and accurate near-field tsunami source characterization using S-net, an array of ocean bottom pressure gauges. Compared to earthquake-based methods, this method can obtain more accurate predictions for the initial water elevation of the tsunami source, including potential secondary sources, leading to accurate water height and wave run-up predictions. Unlike finite-fault tsunami source inversions, the adjoint method achieves high-resolution results without requiring densely gridded Green's functions, reducing computation time. However, optimal results require a dense instrument network with sufficient azimuthal coverage. S-net meets these requirements and reduces data collection time, facilitating the inversion and timely issuance of tsunami warnings. Since the method has not yet been applied to dense, near-field data, we test it on synthetic waveforms of the 2011 Mw 9.0 Tohoku earthquake and tsunami, including triggered secondary sources. The results indicate that with a static source model without noise, using the first 5 min of the waveforms yields a favorable performance with an average accuracy score of 93%, and the largest error of predicted wave amplitudes ranges between -5.6 and 1.9 m. Using the first 20 min, secondary sources were clearly resolved. We also demonstrate the method's applicability using S-net recordings of the 2016 Mw 6.9 Fukushima earthquake. The findings suggest that lower-magnitude events require a longer waveform duration for accurate adjoint inversion. Moreover, the estimated stress drop obtained from inverting our obtained tsunami source, assuming uniform slip, aligns with estimations from recent studies. Issuing timely tsunami warnings is essential in allowing at-risk populations to have adequate evacuation time. However, traditional frameworks for issuing warnings rely on accurate models for the earthquake that caused the tsunami. These models can be inaccurate in the immediate aftermath of an earthquake and can lead to underestimated wave heights and late wave arrival time predictions. To address this issue, we developed a method that can yield the tsunami source without needing an earthquake model, simply relying on the wave height data recorded by pressure sensors at the bottom of the ocean. This can also be useful when tsunami-causing events other than earthquakes (e.g., volcanic eruptions) occur. This method has not yet been tested on a dense ocean bottom instrument network, so we tested it on recorded data from the 2016 Fukushima earthquake's associated tsunami and synthetic waveforms from the 2011 Tohoku tsunami. Our results for the former show accurate predictions of wave arrival times and heights when we use the first 25 min of the wave height data. For the latter, predictions are accurate when we use the first 5 min of the data. We use an adjoint-state tsunami inversion method that predicts tsunami sources directly from near-field pressure gauge recordingsThe pressure gauge recordings of S-net provide sufficient azimuthal coverage within a short time interval for accurate adjoint inversionsThe results of our applications show that accurate tsunami predictions can be obtained using recordings as short as 5 min