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 minutes 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 to 1.9 meters.
Using the first 20 mins, 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.