Development of observation system for tsunami and crustal deformation

The 2011 off the Pacific coast of Tohoku earthquake (M9) occurred at 11 March in last year and huge tsunami brought severe damage around the Tohoku area. The huge earthquake brought large crustal movement. The movements were 24m on the forearc and 50m near the trench (Sato et al., 2011; Fujiwara et al., 2012). The observation, however, was measured after the occurrence and not in real-time. Therefore, we are developing new real-time observation system for tsunami and crustal movement developed by Japan Agency for Marine-Earth Science and Technology (JAMSTEC) and Tohoku University. Our system is composed of seafloor stations and a buoy on sea surface, and these data are sent via satellites in real-time. We selected the m-TRITON buoy developed by JAMSTEC as the platform on sea surface. Because our target is the future Tonankai and Nankai earthquake, the buoy has to adapt slack mooring to stand strong sea current like the Kuroshio with speed of over 5 knots around Japan. We will attach recording systems for tsunami and geodetic movement and the transfer system to send data to satellites in real-time on the buoy. The power supply for them is covered with lithium batteries and the solar power generation. The seafloor station is composed of an equipment to transfer pressure data for tsunami detection and some transponders to detect geodetic movement. Tsunami data recorded by the seafloor pressure sensor is sent acoustically with interval of 15 minutes normally, but the interval changes to 15 seconds in tsunami occurrence. The geodetic movement data is collected with interval of one week. As positioning to monitor geodetic movement with high accuracy less than 10 cm, we adapt the Precise Point Positioning (PPP) technical scheme developed by Japan Aerospace Exploration Agency and have a plan to construct total system to four years.Abstract AOGS/AGU (WPGM) Joint Assembly at Singapore, 13-17 August, 201

An approximate travel time calculation and a robust GNSS-acoustic positioning method using an MCMC technique

Abstract It is important to consider horizontal heterogeneity in an underwater sound speed structure to accurately estimate positions of GNSS-acoustic sites. Although large amounts of moving survey data (a sea-surface platform moves around when acoustic signals are transmitted) are required to accurately detect a sloping sound speed structure, the actual observational data do not necessarily include sufficient moving survey data due to sea conditions or observational time. To treat these insufficient data, it was assumed that a shallow sound speed gradient was continuously present up to a fixed water depth (gradient depth). However, the validity of this assumption has not been investigated, and the gradient depth has not been optimized. In this study, we developed a new GNSS-acoustic array positioning method that optimizes the gradient depth using an MCMC technique. To employ this technique, we also developed an approximate technique for rapidly calculating travel time, because the conventional travel time calculation requires high computational cost for integration into the MCMC technique. We assessed the performance of the approximate travel time calculation technique and demonstrated its sufficient accuracy and precision for estimating array positions. Then, we applied the new GNSS-acoustic array positioning method to the actual observational data collected by the Japan Coast Guard and Tohoku University. Using enough amounts of the moving survey data, our method demonstrated the results comparable with the conventional GNSS-acoustic positioning method estimating a sloping sound speed structure; thus, the assumption of the sound speed gradient with the fixed water depth was valid. Moreover, due to the physical restriction of this assumption, our method provided robust solutions even when the observational data contained small quantities of moving survey data with a simple sea-surface track. Although our method still cannot be used in the scenario, where no moving survey data are available, it can work robustly compared with the conventional methods. Graphical Abstrac

The anticipated Nankai Trough earthquake and tsunami in Japan: Determinant factors of residents’ pre-event evacuation intentions

As a countermeasure against M8-9 class Nankai trough earthquakes, the Japan Meteorological Agency started a service to release “Nankai Trough Earthquake Extra Information (Megathrust Earthquake Alert).” This alert is released after an M8.0 or higher earthquake occurs, and the possibility of having a subsequent earthquake is evaluated to be higher than usual. This is an innovative attempt at disaster mitigation in Japan as it encourages residents in the predefined area to pre-evacuate for a one week period when tsuanmi risk is higher. However, the factors influencing the evacuation behavior of residents are unknown. In this study, we investigated factors that contribute to residents’ pre-event evacuation intentions using the hierarchical multiple regression analysis method. We focused on the extent to which the recognition of the hazard and risk of the Nankai trough earthquake and the response to the Extra Information, which are changeable by the local governments’ public relations activities, contributed to pre-event evacuation intentions after controlling for disaster-related general attitude and socio-demographic factors. Further, we paid special attention to residents’ degree of recognition of this information by checking the accuracy of their understanding of whether they lived within the pre-event evacuation area. The results showed that the recognition factors were relevant, but less so than the general attitude toward disaster and more so than the sociodemographic factors. In addition, residents’ recognition accuracy was found to be low. Our results suggest that it is important for local governments to make adequate efforts to encourage residents to evacuate

Frontal wedge deformation near the source region of the 2011 Tohoku-Oki earthquake

We report an uplift of 5 m with a horizontal displacement of more than 60 m due to the 2011 Tohoku-Oki earthquake. The uplift was measured by an ocean-bottom pressure gauge installed before the earthquake on a frontal wedge, which formed an uplift system near the Japan Trench. Horizontal displacements of the frontal wedge were measured using local benchmark displacements obtained by acoustic ranging before and after the earthquake. The average displacements at the frontal wedge were 58 m east and 74 m east-southeast. These results strongly suggest a huge coseismic slip beneath the frontal wedge on the plate boundary. The estimated magnitude of the slip along the main fault was 80 m near the trench. Our results suggest that the horizontal and vertical deformations of the frontal wedge due to the slip generated the tremendous tsunami that struck the coastal area of northeastern Japan