Back-Projection Imaging of a Tsunami Excitation Area With Ocean-Bottom Pressure Gauge Array Data

A back-projection method has been applied to many earthquakes in seismology due to its simple and low computational cost, and it can estimate complex fault rupture processes without any specific a priori information. In this study, we applied the back-projection method to the tsunami records observed using an ocean-bottom pressure gauge array and demonstrated it to be a powerful new tool other than the familiar waveform inversion. The obtained back-projection image was consistent with the initial tsunami height distributions estimated by previous waveform inversions, and its spatial resolution appeared to be even better. Our result suggests that the fault size of the 2016 Off-Fukushima earthquake was about half, different from the scaling law of standard earthquakes. The present tsunami back-projection analysis can also estimate the feature of early tsunami propagations. In addition, the estimated image seems to be reliable even 30 min after the origin time, so the back-projection analysis will be useful in an early detection of the location and spatial extent of a tsunami source. In the present case, the number of available stations in the analysis was found to be affected by the diffraction of tsunami propagation caused by the refraction by a high velocity zone near the Japan Trench. In other words, the further the source is from the coast, the more stations to be analyzed are available. Since most tsunami-generating earthquakes occur near the subduction axis or its outer-rise region, the back-projection analysis should be effective for source estimation of the majority of tsunami-generating earthquakes.