Numerical Assessment of Coastal Multi-hazard Vulnerability in Tokyo Bay

Many bays in the world are threatened by coastal hazards such as storm surge, river flood and tsunami. Since most of the existing studies have been focused on one or two of them, in this study, the assessment of coastal vulnerability caused by the three hazards was the research target. Inundation simulation is a widely used and straightforward way in coastal vulnerability assessments; however, it is computationally expensive, and considering an increase in the number of cases in multi-hazard analysis, an efficient method was proposed using an estimated overflow volume without computing inundation, which was validated by comparing with inundation simulation. It shows that when free overflow is dominant, this method is consistent with inundation simulation approach. Using Tokyo Bay as a study area, the efficient method was then applied to multi-hazard vulnerability assessment. By comparing the overflow volume maps and maximum anomaly distribution along the coasts for four types of hazards (worst storm surge; worst concurrent storm surge and river flood; worst concurrent storm surge, river flood and Tokai-Tonankai earthquake tsunami; worst concurrent storm surge, river flood and Tokyo inland earthquake tsunami), we investigated the characteristics of different types of hazards and identified the difference between single hazard and multi-hazards. The characteristic of overflow volume along the coasts is similar to that of maximum anomaly distribution, especially for only storm surge case, the multi-hazard case combining storm surge and river flood, and the multi-hazard case combining storm surge, Tokyo inland earthquake tsunami and river flood. However, for multi-hazard case combining storm surge, Tokai-Tonankai earthquake tsunami and river flood, only by the maximum anomaly distribution, it cannot reflect the real overflow volume condition. For only storm surge case and multi-hazard case combining storm surge and river flood, the head of the bay suffers the highest vulnerability while for multi-hazard cases combining storms surge, tsunami and river flood, the difference of vulnerability in the north and south of the bay is not significant. The difference of superposing method and concurrent method for computing multi-hazards was also compared. It was found that the linear superposing method tends to overestimate the total water elevation in coastal region; however, in the coasts where superposing method underestimates the multi-hazard anomalies, upgrading dikes needs to be considered by policymakers.

Development and Preliminary Analysis of a VLF-Band Electromagnetic-Wave Observation System for Short-Term Earthquake Precursory Monitoring

Preseismic VLF electromagnetic pulses occasionally increase a few days before large earthquakes, especially inland earthquakes. More than two decades ago, the Tokai University group developed a digital recording system for collecting the preseismic electromagnetic pulse data and showed remarkable results. However, due to the limitations of personal computers’ data storage and CPU power during that time, they discontinued the observation. We relaunched this research using current technology. This paper shows the development of the new observation system and presents preliminary results. In addition, we introduce an electromagnetic-wave arrival discrimination algorithm that combines the autoregressive model and the Akaike information criterion, which are commonly used for automatic waveform reading in seismology, to obtain accurate data on the time of arrival (TOA) of electromagnetic waves. Then, source positioning was performed using TOA of electromagnetic waves. Seven electromagnetic pulses near the epicenter were observed 2 days before the largest inland earthquake (M = 5.6) that occurred near the observation network during the observation period (2016–2020). These VLF pulses may be a seismic precursory phenomenon because they were not electromagnetic pulses originating from lightning. These results encourage future observations.

Nucleation process of the 2011 northern Nagano earthquake from nearby seismic observations

The 2011 magnitude (M) 9.0 Tohoku-oki earthquake was followed by seismicity activation in inland areas throughout Japan. An outstanding case is the M6.2 Northern Nagano earthquake, central Japan, occurred 13-h after the megathrust event, approximately 400 km away from its epicenter. The physical processes relating the occurrence of megathrust earthquakes and subsequent activation of relatively large inland earthquakes are not well understood. Here we use waveform data of a dense local seismic network to reveal with an unprecedented resolution the complex mechanisms leading to the occurrence of the M6.2 earthquake. We show that previously undetected small earthquakes initiated along the Nagano earthquake source fault at relatively short times after the Tohoku-oki megathrust earthquake, and the local seismicity continued intermittently until the occurrence of the M6.2 event, being likely ‘modulated’ by the arrival of surface waves from large, remote aftershocks off-shore Tohoku. About 1-h before the Nagano earthquake, there was an acceleration of micro-seismicity migrating towards its hypocenter. Migration speeds indicate potential localized slow-slip, culminating with the occurrence of the large inland earthquake, with fluids playing a seismicity-activation role at a regional scale.

Wide-area seismicity anomalies before the 2011 Tohoku–Oki earthquake

Summary This study investigates various types of seismicity changes that occurred in several regions in and around the Tohoku District, prior to the 2011 M9.0 Tohoku–Oki earthquake. In particular, we focus on the seismicity anomalies that were revealed not only in inland local areas but also in a wide area for several years before the 2008 M7.2 earthquake in the inland Tohoku District. We reconsider these seismicity anomalies in nearly identical regions, which persisted in the extended period up until the M9 mega event. This suggests that the stress changes due to transient slow slips on the Pacific Plate boundary are more likely to be the cause of the wider seismicity changes than the slips beneath the inland earthquake. To confirm the significance, we use the two-stage stationary epidemic-type aftershock sequence model and explore the relationship between seismicity changes and stress rate changes due to slow slip by means of global navigation satellite system geodetic observations.

The Advancement of Research on Inland Earthquake Generation 2014–2018

The 2011 Tohoku-Oki Earthquake (M9.0) significantly affected inland areas of Japan. The crust and mantle response to the magathrust earthquake induced changes in the mechanical conditions of the seismogenic zone. Here we present important progress in the research into the seismogenesis of inland earthquakes. Stress, strain, strength, and structures are key parameters affecting the occurrence of earthquakes. In particular, both the spatial and temporal changes in these parameters around the focal areas of the large inland earthquakes have been detected and modeled. These results have provided spatial potential evaluation in terms of future inland earthquake occurrence. However, we clearly recognize that, in order to understand and predict the inland earthquake generation process, it will inevitably be necessary to unify the research on various spatial and temporal scales, from problems related to long-term stress loading from plate-relative motion to instant fault response.

The Role of Japan DMAT in Tokyo Inland Earthquake

Introduction:An inland earthquake is expected to occur in Tokyo in the near future, and disaster preparedness and response measures have been put in place by the government of Japan and local authorities.Methods:Japan Disaster Medical Assistant Teams (DMATs) conducted two large-scale drills for the first time in preparation for a Tokyo inland earthquake, in collaboration with the following participants: the Tokyo Metropolitan Government, disaster base hospitals in Tokyo, three Staging Care Units (SCUs), and neighboring prefectures. One of the scenarios was a north Tokyo Bay earthquake affecting the Tokyo wards and had 142 Japan DMATs participation. Another scenario was Tama inland earthquake affected mid-west of Tokyo and 110 DMATs participated. The drill included headquarters operation, affected hospital support operation, patient transportation within the area and to the wider region, SCU operation, collaboration with associated organizations, and logistics operation.Results:Post-drill assessments identified the following areas that need to be addressed: review of Japan DMAT implementation strategies; improvement of SCUs; establishment of a patient air transportation framework; securing means of patient transportation; improvement of communication systems; strengthening of disaster response of all hospitals in the Tokyo Metropolis; and preparations for survival in the event of isolation caused by the disaster.