Ocean-Wave Gradiometry: Visualizing and Extracting Propagation Features of the 15 January 2022 Tsunami Wavefield with Dense Ocean-Bottom Pressure Gauge Arrays


 Dense geophysical observation networks have recently enabled monitoring the wavefield of sea-bottom pressure changes. Significant sea-bottom pressure disturbances were recorded by ocean-bottom pressure gauge (OBPG) arrays around Japan on 15 January 2022, the day of the massive eruption of Hunga Tonga–Hunga Ha’apai volcano in the Tonga Islands. At the same time, sea-surface height disturbances and atmospheric pressure disturbances were recorded by tide gauges around the Pacific Ocean and barometers around the world. Because the atmospheric disturbances may have affected the propagation of the sea-surface height changes, we investigated the propagation properties of the sea-bottom pressure disturbances recorded by the OBPG arrays around Japan using wave gradiometry. We found that the leading pressure disturbances propagated from southeast to northwest with a velocity expected from linear long-wave theory for ocean waves, that is, tsunamis. We also detected several later coherent sea-bottom pressure disturbances that propagated at the velocity of tsunamis. In addition, we detected anomalous short-period later phases of pressure disturbances with propagation directions more nearly north–south than those of the leading disturbances at the coast of southwestern Japan. These results indicate that the pressure disturbances recorded at the OBPG arrays propagated as tsunamis rather than sea-surface disturbances excited by atmospheric Lamb waves, although atmospheric pressure disturbances might have affected the amplitude of sea-surface height changes. This study demonstrates that wave gradiometry can be successfully applied to data from a dense OBPG array and may be suitable for real-time monitoring of sea-bottom pressure wavefields.