11:30 AM - 11:45 AM
[SSS04-09] Non-tsunamigenic water pressure variations at the ocean bottom due to major earthquakes
Keywords:Ocean bottom pressure, seismic Rayleigh waves, earthquakes
Significant earthquakes generate perceivable ground motions recorded at global seismic networks, which have also shaken the ocean floors and perturbed water mass at deep sea. This study analyzes the bottom pressure data acquired from two sets of the tsunami alarm system deployed in 2018 in east and southwest offshore Taiwan by the Central Weather Bureau (CWB, Taiwan) for regional tsunami hazard monitoring. We statistically evaluated the waveforms of the bottom-pressure time series from 2018 to 2021 (at a sampling rate of every 15 seconds) to investigate the relationship between variations of the ocean bottom pressure with seismic waves. Because the compressibility of the water layer does not play an essential role in low-frequency seafloor oscillations, the seafloor pressure excited by low-frequency seismic waves is proportional to the seafloor acceleration. As the most less response of the bottom pressure is at a frequency of about 60 mHz (17s period), higher than the tsunami frequency band, we considered the transition of the water fluctuation in response to the seismic Rayleigh waves. The power spectral analysis showed that the proportionality coefficient of ground motions and water pressure perturbations is equal to the mass of the water column at the installation of the observatory or, approximately, to the product of the water density and the ocean depth. However, the effect of sediment layer resonances may provide uncertainty in pressure /acceleration transition in seafloor water pressures. This study can address the practical issue of whether the threshold value in the PMEL tsunami monitoring system (Deep-Ocean Assessment and Reporting of Tsunami, DART) is appropriate for regional earthquakes capable of generating dangerous trans-Pacific tsunamis.