Detection of Ocean Internal Tide Source Oscillations on the Slope of Aogashima Island, Japan

Abstract

Barotropic tidal currents over bottom topography force density surfaces to oscillate vertically and thereby to act as quasi‐stationary internal tide sources. Deploying a seafloor pressure gauge array with an aperture of 30 km for a year (2014–2015), we detected the low‐mode semidiurnal internal tidal waves propagating with a horizontal phase speed of ~1 m/s in the onshore and offshore directions over the array along the eastern slope of Aogashima Island, south of Japan. The amplitudes of the offshore propagating waves were greater than those of the onshore propagating waves, and both were positively correlated with the amplitudes of the local semidiurnal tide, which peaked in September and March. A tide‐resolving ocean circulation model (JCOPE‐T) well reproduced the observed onshore and offshore internal tidal wave propagation. The model indicated a standing wave region on the slope, where offshore propagating waves interact with standing waves locally pinned to the slope. Along the same profile over a distance of 100 km, we conducted seismic‐oceanographic analysis of the legacy multichannel seismic reflection data to retrieve vertical cross sections of the reflecting layers, which indicated sharp temperature changes in the ocean. Many of the slant reflecting layers were subparallel to the contour lines of the semidiurnal internal‐tide‐associated temperature anomalies in the JCOPE‐T model, suggesting a causal link between the fine reflection layering structure and the semidiurnal low‐mode internal tidal field.

Ultrabroadband Seismic and Tsunami Wave Observation of High‐Sampling Ocean‐Bottom Pressure Gauge Covering Periods From Seconds to Hours

“…Utilizing the array of high-sampling PGs will enable us to analyze the ultrabroadband geophysical wave propagation, 10.1029/2020EA001197 including tsunamis and seismic waves. Developments of the offshore PG networks will facilitate the analysis of ultrabroadband pressure signals covering periods of 10 0 -10 3 s. PGs can further be employed for observing much longer time scale phenomena (e.g., Baba et al, 2006;Fukao et al, 2019;Inazu et al, 2012;Tonegawa et al, 2018;Wallace et al, 2016;Figure 12). Using these new PG networks will create new possibilities for understanding the geophysical wave propagation processes in the solid-fluid coupled system and the generation processes of earthquakes and tsunamis.…”

“…• High-sampling rate is important for pressure gauges to record seismic body waves, Rayleigh waves, and tsunamis and their dispersive feature • Theoretical relation between pressure and vertical acceleration (p = ρ w Η 0 a z ) is valid for a long time (∼3 hr for the 2010 Chile earthquake) • A relationship between pressure and vertical velocity (p = ρ w c ¯0vz) holds only at the first P wave arrival, but not for later phases longer time scale phenomena, such as infragravity waves (e.g., Tonegawa et al, 2018) and oceanographic and geodetic phenomena (e.g., Baba et al, 2006;Fukao et al, 2019;Inazu et al, 2012;Wallace et al, 2016).…”

“…Using these new PG networks will create new possibilities for understanding the geophysical wave propagation processes in the solid-fluid coupled system and the generation processes of earthquakes and tsunamis. Several studies have already begun to use an array of the high-sampling PGs for the broadband geophysical wave propagation analyses (Fukao et al, 2018(Fukao et al, , 2019Mizutani & Yomogida, 2019;Sandanbata et al, 2018).…”

“…Owing to this advantage, PGs have significantly contributed to our understanding of tsunami generation and propagation processes derived from solid and fluid dynamics (e.g., Allgeyer & Cummins, 2014; Inazu & Saito, 2013; Kubota et al, 2020; Lay et al, 2016; Maeda et al, 2013; Poupardin et al, 2018; Rabinovich et al, 2013; Saito et al, 2010; Sandanbata et al, 2018; Satake et al, 2005; Watada et al, 2014; and references therein). PGs have further been utilized for observing much longer time scale phenomena, such as infragravity waves (e.g., Tonegawa et al, 2018) and oceanographic and geodetic phenomena (e.g., Baba et al, 2006; Fukao et al, 2019; Inazu et al, 2012; Wallace et al, 2016).…”

Sub-decadal Volcanic Tsunamis Due to Submarine Trapdoor Faulting at Sumisu Caldera in the Izu-Bonin Arc

“…Ocean bottom pressure data of the array off Aogashima Island (Fukao et al, 2019) and the Deep Sea Floor Observatory off Muroto Cape (Momma et al, 1997) Zheng, Y., Blackstone, L., & Segall, P. (2022). Constraints on absolute magma chamber volume from geodetic measurements of trapdoor faulting at Sierra Negra volcano, Galapagos.…”