In‐Depth Evaluation of Inversion of Coastal Tsunami Waveforms Utilizing Tsunami Source Constrained by Offshore Tsunami Waveforms From a Wide, Dense Observation Network: Application to the 2016 Mw 6.9 Off‐Fukushima Earthquake, Japan

The tsunami generated by the 2016 M w 6.9 off‐Fukushima earthquake, Japan, was recorded by offshore pressure gauges on a wide, dense ocean‐bottom cable network, called S‐net, as well as by offshore GPS buoys, coastal wave gauges and coastal tide gauges. In this work, source of the tsunami was inverted from the offshore tsunami waveforms and coastal tsunami waveforms independently, using the Green's functions based on linear long‐wave theory. We found that inversion of the offshore waveforms returned robust results, more accurate and better resolved than those obtained from the coastal waveforms. Furthermore, nonlinear long‐wave simulation using the offshore‐data‐inverted tsunami source, accurately reproduced the leading waves recorded at offshore and coastal stations. These results also demonstrated that tsunami nonlinearities are non‐negligible, which is the main reason, together with weak constraints from the coastal waveforms, for the relative inaccuracy of the results of inversion from the coastal waveforms. This inaccuracy can be reduced by rearranging the conditions of inversion. Our analysis demonstrated that offshore tsunami waveforms from a wide, dense network are beneficial to deeply evaluate and improve inversion of coastal tsunami waveforms.