We have detected an event of pore pressure change (hereafter, we refer it to “pore pressure event”) from borehole stations in real time in March 2020, owing to the network developed by connecting three borehole stations to the Dense Oceanfloor Network System for Earthquakes and Tsunamis (DONET) observatories near the Nankai Trough. During the pore pressure event, shallow very low-frequency events (sVLFEs) were also detected from the broadband seismometers of DONET, which suggests that the sVLFE migrated toward updip region along the subduction plate boundary. Since one of the pore pressure sensors have been suffered from unrecognized noise after the replacement of sensors due to the connecting operation, we assume four cases for crustal deformation component of the pore pressure change. Comparing the four possible cases for crustal deformation component of the volumetric strain change at C0010 with the observed sVLFE migration and the characteristic of previous SSEs, we conclude that the pore pressure event can be explained from SSE migration toward the updip region which triggered sVLFE in the passage. This feature is similar to the previous SSE in 2015 and could be distinguished from the unrecognized noise on the basis of t-test. Our new finding is that the SSE in 2020 did not reach very shallow part of the plate interface because the pore pressure changes at a borehole station installed in 2018 close to the trough axis was not significant. In the present study, we estimated the amount, onset and termination time of the pore pressure change for the SSE in 2020 by fitting regression lines for the time history. Since the change amount and duration time were smaller and shorter than the SSE in 2015, respectively, we also conclude that the SSE in 2020 had smaller magnitude that the SSE in 2015. These results would give us a clue to monitor crustal deformation along the Nankai Trough directly from other seafloor observations.
We constructed a real-time tsunami prediction system using the Dense Oceanfloor Network System for Earthquakes and Tsunamis (DONET). This system predicts the arrival time of a tsunami, the maximum tsunami height, and the inundation area around coastal target points by extracting the proper fault models from 1,506 models based on the principle of tsunami amplification. Since DONET2, installed in the Nankai earthquake rupture zone, was constructed in 2016, it has been used in addition to DONET1 installed in the Tonankai earthquake rupture zone; we revised the system using both DONET1 and DONET2 to improve the accuracy of tsunami prediction. We introduced a few methods to improve the prediction accuracy. One is the selection of proper fault models from the entire set of models considering the estimated direction of the hypocenter using seismic and tsunami data. Another is the dynamic selection of the proper DONET observatories: only DONET observatories located between the prediction point and tsunami source are used for prediction. Last is preparation for the linked occurrence of double tsunamis with a time-lag. We describe the real-time tsunami prediction system using DONET and its implementation for the Shikoku area.
The damage and loss of life caused by tsunamis can be reduced by timely warnings, which predict the arrival time and maximum height of tsunamis, to support evacuations and other mitigating actions. We have developed a real-time tsunami prediction system based on data from the Dense Oceanfloor Network system for Earthquakes and Tsunamis (DONET) that has been implemented in some local governments along the Pacific coast of Japan. The system generates estimates of tsunami arrival times and the height, inundation areas, and worst case using selected fault rupture models. The main objective of this paper is to show the possibility of applying the above system for a complicated topography area, and we report a successful application of the system in Sakaide, a city on the Shikoku coast of the Inland Sea, using a simulated great plate-boundary earthquake in the Nankai Trough. The simulated tsunami propagates to Sakaide by complicated routes between several islands. According to calculated tsunami waveforms of 1,506 cases, waveforms of tsunamis propagating to the Inland Sea have a relatively uniform frequency, regardless of the magnitude of the causative event, after running through the narrow straits in the Inland Sea. At the same time, waves are amplified as they pass between the islands of Shodoshima and Shikoku by an interaction with reflected waves. These effects are compatible with this prediction system, and we confirmed that our predicted tsunami is consistent with the final result from a model of a magnitude 9 Nankai Trough earthquake.
Mega-thrust earthquakes are anticipated to occur in the Nankai Trough in Southwest Japan. In order to monitor seismicity, crustal deformations, and tsunamis in earthquake source areas, we deployed the seafloor seismic network DONET (Dense Ocean-floor Network System for Earthquakes and Tsunamis) in 2010 (Kaneda et al., 2015; Kawaguchi et al., 2015). The DONET system consists of a total of 20 stations that are composed of multiple types of sensors, including strong-motion seismometers and quartz pressure gauges. These stations are densely distributed at an average distance of 15‐20 km and cover from near the trench axis to coastal areas. Observed data are transferred to a land station through a fiber-optic cable and then to the Japan Agency for Marine-Earth Science and Technology (JAMSTEC) data management center through a private network in real time.After the 2011 earthquake off the Pacific coast of Tohoku, each local government close to the Nankai Trough sought to devise a disaster prevention scheme. These local governments requested that JAMSTEC disseminate the DONET data along with other research capabilities so that they could exploit this important earthquake information. In order to provide local government access to the DONET data, which are recorded ostensibly for research purposes, we have developed a web application system, REIS (real-time earthquake information system), that provides seismic waveform data to some local governments close to the Nankai Trough. In the present paper, we introduce the specifications of REIS and its system architecture.
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