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

摘要

Recent developments of ocean‐bottom pressure gauges (PG) have enabled us to observe various waves including seismic and tsunami waves covering periods of T?～?100–103?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 az for ～3?hr from the origin time. We also found a relationship between p and vertical velocity vz 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 earthquake early warning and disaster mitigation.