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

Abstract

Recent developments of ocean-bottom pressure gauges (PG) have enabled us to observe various waves including seismic and tsunami waves covering periods of T ∼ 10 0-10 3 s. To investigate the quality for broadband observation, this study examined the broadband PG records (sampling rate of 1 Hz) around Japan associated with the 2010 Chile earthquake. We identified three distinct wave trains, attributed to seismic body waves, Rayleigh waves, and tsunamis. Clear dispersive features in the Rayleigh waves and tsunamis were explained by theories of elastic waves and gravity waves. Quantitative comparison between pressure change and nearby seismograms demonstrated the validity of the theoretical relation between pressure p and vertical acceleration a z for ∼3 hr from the origin time. We also found a relationship between p and vertical velocity v z holds only at the first P wave arrival, but not for later arrivals. Similar results were confirmed for various earthquakes with different source-station distances and magnitudes, suggesting the robustness of these relations. The results demonstrate that the high-sampling rate (≥1 Hz) is necessary to observe seismic-wave dispersion and PG can record both seismic waves and tsunamis with reasonable quality for waveform analyses, whereas conventional onshore and offshore seismometers or tide gauges can observe either of seismic waves and tsunamis. Utilizing the high-sampling PG in combination with the seismic and tsunami propagation theory for estimating earthquake source process or analyzing wave propagation processes in the ocean will deepen our geophysical understanding of the solid-fluid coupled system in the Earth and contribute toward disaster mitigation. Plain Language Summary Recent developments of offshore ocean-bottom observation networks have enabled us to use high-sampling (one or more samples per second) seafloor pressure gauge (PG) data. This study investigated PG records with broadband period range (seconds to hours) around Japan during the 2010 Chile earthquake. We identified a seismic P wave train arriving ∼20-30 min after the focal time. Another seismic wave train due to the surface Rayleigh wave during ∼70-110 min and tsunamis during ∼24-72 hr was also confirmed, which showed a dispersive feature; long-period waves arrive earlier than short-period waves. The dispersion theories obtained from the elastic and fluid dynamics thoroughly explained these features. We also compared wave amplitudes between the PG and nearby ocean-bottom seismometer and confirmed the validity of the relationship between pressure and vertical acceleration and between pressure and vertical velocity. This study demonstrates PG can record both seismic and tsunami signals clearly with reasonable quality, while conventional seismometers or tide gauges can either. Ultrabroadband observation of PG plays an important role in deepening our understanding of geophysical wave propagation processes in the solid-fluid coupled system in the ocean and enables delivery of essential information for earthquak...

Extracting Near‐Field Seismograms From Ocean‐Bottom Pressure Gauge Inside the Focal Area: Application to the 2011 Mw 9.1 Tohoku‐Oki Earthquake

“…Based on the three‐dimensional theory, the pressure change inside the focal area includes a component related to vertically accelerating seafloor (Saito, 2013, 2017, 2019; An et al., 2017; Filloux, 1982). This component was confirmed by the actual observations and numerical simulations (e.g., An et al., 2017; Filloux, 1982; Ito et al., 2020; Kubota et al., 2020; Kubota, Saito, et al., 2017; Matsumoto et al., 2012, 2017; Mizutani et al., 2020; Nosov & Kolesov, 2007; Saito, 2019; Saito & Kubota, 2020; Saito & Tsushima, 2016; Webb, 1998). Additionally, the pressure change field includes the high‐frequency ocean‐acoustic waves related to the seawater elasticity (Lotto et al., 2019; Nosov & Kolesov, 2007; Saito & Tsushima, 2016).…”

Theoretical Solution and Applications of Ocean Bottom Pressure Induced by Seismic Waves at High Frequencies

“…However, DPGs are known to suffer from uncertainties of pressure amplitude (gain factor). Previous studies have used p = ρha that is valid at low frequencies (≪c 0 /4h) to calibrate the DPG amplitude (e.g., An et al, 2017An et al, , 2020Kubota et al, 2017Kubota et al, , 2020Nosov et al, 2018Nosov et al, , 2021. Zha and Webb (2016) used the same solution to Equation 5 and an average Rayleigh velocity model to calibrate DPGs from teleseismic Rayleigh waves.…”

“…Results show that the solution is valid for body P , S waves as well as Rayleigh surface waves, and for a wide frequency range that can be higher than the resonance frequency ( c 0 /4 h ). Some previous studies proposed a relation of the ocean‐bottom pressure p with the vertical velocity of the seafloor v as p = ρvc 0 (e.g., Kubota et al., 2020; Mizutani et al., 2020; Saito, 2017). It can be derived by only considering the first reflection and transmission at the solid‐fluid interface, and ignoring the reflected waves from the free surface.…”

Meteorological Tsunami Generation Due to Sea‐Surface Pressure Change: Three‐Dimensional Theory and Synthetics of Ocean‐Bottom Pressure Change

“…Tsunami observations have exhibited impressive growth in past decades due to the development of ocean‐bottom pressure gauges, which enabled us to observe tsunamis outside bays and harbors (e.g., Aoi et al., 2020; Bernard & Meinig, 2011; Kanazawa et al., 2016; Kaneda et al., 2015; Kawaguchi et al., 2015; Mungov et al., 2013; Rabinovich & Eblé, 2015; Tsushima & Ohta 2014). Unlike coastal tide gauges, deep‐ocean observations can capture clear tsunami signals (e.g., Grilli et al., 2013; Inazu & Saito, 2014; Kubota, Saito, Chikasada, & Suzuki, 2020; Sandanbata et al., 2018). The high‐quality records in open oceans have driven studies about dispersion and high‐resolution tsunami source estimations (e.g., Baba et al.…”