The National Research Institute for Earth Science and Disaster Resilience (NIED), the Earthquake Research Institute (ERI) belonging to the University of Tokyo, and Yokogawa Electric Corporation (Yokogawa) have undertaken evaluation of an innovative water pressure gauge for use in the early detection of tsunami. The water pressure gauge used in this evaluation was equipped with a new type of silicon resonant pressure sensor (description of operational principle below), and was installed on the seafloor near the Boso Peninsula at a water depth of 3,436 m. In this evaluation, the gauge successfully identified pressure fluctuations of seven hectopascals that are equivalent to a 7-cm change in sea level. Although it is difficult to obtain data on tsunamis, which are infrequent events, the evaluation allowed the detection of changes in sea level similar to those of a tsunami, and the water pressure gauge is expected to be used in the event of an actual tsunami. The water pressure gauge will be adopted for the Nankai Trough Seafloor Observation Network for Earthquakes and Tsunamis (N-net) to observe water pressure fluctuations on the seafloor caused by tsunamis generated by earthquakes, enabling reliable detection of tsunamis and contributing to damage mitigation.
Details of N-net, the newly developed water pressure gauge, and its evaluation has been presented at the Japan Geoscience Union Meeting 2023, May 21 to 26, 2023. NIED, ERI, and Yokogawa have evaluated the effectiveness of a water pressure gauge equipped with a MEMS silicon resonant pressure sensor for use as a seafloor pressure observation device that enables the acquisition of accurate data during the significant shaking that occurs during an earthquake. In view of the significant ground movement that occurs during an earthquake, this test sought to identify whether the acquisition of measurement data would be impacted by vibrations or attitude changes. It was confirmed that the effect of attitude changes on the water pressure gauge is smaller than conventional water pressure gauges. In addition, in a precision test in which 70 megapascals (MPa) ―equivalent to a water depth of 7,000m― was repeatedly applied, repeatability of 0.005% of 70MPa or less was confirmed to be an excellent characteristic. This water pressure gauge uses MEMS technology, so it offers the advantage of each product having the same quality.
Evaluation phase
To evaluate the performance of the water pressure gauge in an actual seafloor environment, the seafloor observations were performed for a total of 203 days at a depth of 3,436 m in the region off the Boso Peninsula in Chiba Prefecture, Japan. The acquisition of observation data on tsunamis is usually difficult because tsunamis are a rare phenomenon. However, it was observed a 7-cm fluctuation in the sea level due to the January 15, 2022 eruption of the Hunga-Ha'apai volcano in Tonga during the evaluation work. Further data analysis also confirmed that the water pressure gauge was able to observe pressure changes equivalent to a change in sea level of less than 1cm. The confirmed sensitivity indicates that the water pressure gauge has sufficient performance to observe an actual tsunami. The water pressure gauge is a made-in-Japan product with high precision for deep sea applications, and possesses the same level of sensitivity as the most cutting-edge instruments manufactured anywhere in the world.
The observation network for earthquakes and tsunamis is part of the disaster risk reduction (DRR) infrastructure that contributes to the enhancement of DRR information and the development of DRR research on earthquakes and tsunamis. NIED operates the Monitoring of Waves on Land and Seafloor (MOWLAS) covering all land and sea areas in Japan. Starting in 2019, NIED has been developing N-net, a cable-type seafloor earthquake and tsunami observation system. N-net will be installed within the seismic source region in the Nankai Trough where earthquakes are anticipated and where no observation network has been established (from the off coast of Kochi Prefecture to Hyuga-nada). N-net is a network system that directly detects earthquakes and tsunamis and reliably transmits the information to land, enabling observation in real time. This new silicon resonant water pressure gauge, which plays an important role in this system, has been adopted. NIED, ERI, and Yokogawa have conducted numerous tests to ensure the reliability of this water pressure gauge, aiming in the event of a Nankai Trough mega-thrust earthquake to contribute as much as possible in the mitigation of damage.
NIED, ERI, and Yokogawa will continue their efforts to improve DRR science and technology to realize a disaster resilient society.
Resonant Pressure Sensor – Operational Principle
Yokogawa's silicon resonant pressure sensors employ a sensing method based on the pressure-dependent change in resonant frequency of single crystal silicon resonators, and are characterized by low power consumption, compact size, high sensitivity, high stability, and high pressure resistance. The resonator is sealed in a clean vacuum cavity using silicon semiconductor manufacturing technology, which prevents foreign particles from adhering to the resonator and degrading its performance. In addition, there is no change in performance due to gas desorption, which occurs with sensors using quartz crystal resonators, and stable measurement can be achieved.
