The Earthquake and Volcano Hazards Observation and Research Program (2014–2018) carried out comprehensive research to mitigate disasters related to earthquakes and volcanic eruptions. The program selected multidisciplinary research in which earth scientists who study the processes of earthquake generation and volcanic eruptions, historians, archaeologists, human and social scientists, and engineers were all involved. The program aimed to collect pre-instrumental and pre-historical earthquake and volcanic data to understand earthquake and volcano disasters, to find risk evaluation techniques, and to evaluate disaster response and preparedness. Active collaborations between researchers from different science fields inspired new ideas and have driven various research in the program. New findings from the program have also created international collaborations and recognitions. Most of the results and new findings in the program have already been published in various internationally recognized journals and have greatly influenced scientific communities.

We believe that it is important to compile our findings from the last five years of the program and to publish the essence of our findings and published papers in this special issue. We hope that this special issue will be of value to researchers who are interested in multidisciplinary studies of mitigation of disasters such as earthquakes, volcanic eruptions, and related phenomena.

The Earthquake and Volcano Hazards Observation and Research Program was from Japanese fiscal year 2014 to 2018. This national program succeeded the Research Program for Earthquake and Volcanic Eruption Prediction (2009–2013). However, mainly because of the disaster caused by the 2011 earthquake off the Pacific coast of Tohoku, known as the 2011 Tohoku Earthquake, the basic policy of the program changed drastically. It changed from research for predicting earthquakes and volcanic eruptions to comprehensive research for mitigating disasters on the basis of scientific results related to the mechanisms of earthquakes and volcanic eruptions and their forecasts. The program was planned to be multidisciplinary in nature. In addition to Earth scientists working to get a scientific understanding of earthquakes and volcanic eruptions, historians, archaeologists, human and social scientists, and engineers took part in the program aimed at collecting pre-instrumental earthquake and volcanic data, understanding earthquake and volcano disasters, risk evaluation, and research into disaster response and preparedness. In this article, we review the basic concept of the 2014–2018 program and its main achievements. In the end, we summarize the problems left for future studies.

The Great East Japan Earthquake and Tsunami has reawakened people to the reality of large-scale earthquakes that recur in cycles of several hundred to a thousand years. The historical resources and archeology research group, which was established in 2014 within the Coordinating Committee of Earthquake and Volcanic Eruption Prediction Researches, is collaborating with researchers of seismology, history, archeology, and information science to investigate infrequent earthquakes using historical documents that record earthquakes and traces of disasters at archeological sites. To this end, we are creating a database of published historical sources of earthquakes to make the data readily accessible, and reexamining these sources and uncovering new historical material to investigate earthquakes that occurred in pre-modern times. We are also engaged in research on relief efforts for victims of past earthquakes and the post-disaster reconstruction process.

Understanding the occurrence mechanism of subduction zone earthquakes scientifically is intrinsically important for not only forecast of future subduction earthquakes but also disaster mitigation for strong ground motion and tsunami accompanied by large earthquakes. The Program Promotion Panel for Subduction-zone earthquakes mainly focused on interplate megathrust earthquakes in the subduction zones and the research activity included collection and classification of historical data on earthquake phenomena, clarifying the current earthquake phenomena and occurrence environment of earthquake sources, modelling earthquake phenomena, forecast of further earthquake activity based on monitoring crustal activity and precursory phenomena, and development of observation and analysis technique. Moreover, we studied the occurrence mechanism of intraslab earthquakes within the subducting oceanic plate. Five-year observational research program actually produced enormous results for deep understanding of subduction zone earthquakes phenomena, especially in terms of slow earthquakes, infrequent huge earthquakes, and intraslab earthquakes. This paper mainly introduces results from researches on these phenomena in subduction zones.

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.

To mitigate a volcanic eruption disaster, it is important to forecast the transition of the disaster, which depends on the stage of the volcanic phenomena, in addition to forecasting the site, scale, and time of the volcanic activities. To make such forecasts, it is critical to elucidate the evolution of volcanic activity. Accordingly, the Volcano Program Promotion Panel has set the prioritized target as “to forecast volcanic eruption as a cause of disaster by clarifying the branching conditions and theories of volcanic activity and improving volcanic event tree.” The panel promoted a five-year study on the elucidation of volcanic phenomena, including low-frequency and large-scale ones, status of volcanic eruption fields, volcanic eruption modeling, observation method development, and observation system improvement. In this paper, an outline of the main results of this five-year study is presented.

Unusual phenomena sometimes precede a large earthquake and are considered by some as a telltale sign of that earthquake. Judging whether the phenomenon was indeed related to the earthquake is difficult for individual cases. However, the accumulation of data over time allows for statistical evaluation to determine whether there is a correlation between the occurrence of a certain type of phenomena prior to an earthquake. The focus of this study is to review such statistical evaluation. The aspects considered in this study include seismicity, crustal deformation, slow slip, crustal fluids, crustal properties, electromagnetic phenomena, and animal behaviors. The lead times range from minutes to a few decades. The magnitude of the earthquake-preceding tendency can be universally measured by the probability gain G, which is the enhancement ratio of earthquake probability suggested by the occurrence of the phenomenon. A preceding tendency is considered to exist if G is > 1 with reasonable statistical significance. Short-term foreshock activity, that is, temporarily heightened seismicity, produces by far the highest G > 100, sometimes exceeding 10000. While this strongly contributes to empirical forecasting, a considerable part of the predictive power of foreshocks is likely to derive from the mere aftershock triggering mechanism. This enhances the probability of small and large earthquakes by the same factor. It is fundamentally different from traditional expectations that foreshock activity signifies the underlying nucleation process of the forthcoming (large) earthquake. Earthquake-preceding tendency has also been proven significant for a number of other phenomena not ascribable to the aftershock-triggering effect. Some phenomena may be indicators of physical conditions favorable for large earthquakes, while some (e.g., slow slip) may represent triggering effects other than aftershock triggering. Phenomena not ascribable to aftershock triggering have a modest G of < 20 so far. However, these phenomena, including higher-order features of foreshocks, can be combined with the high G from aftershock-triggering effect, sometimes yielding a fairly scaring level of forecast. For example, say ∼10% chance of an M7 earthquake in a week in a few hundred km radius.

In this research area, methodologies for prior predictions of potential hazards and real-time estimations of progressing hazards caused by earthquakes and volcanic eruptions are proved for disaster mitigation. The studies are based on the latest understanding of earthquake processes, volcanic activities, and the crustal structure. The studies have been conducted through the co-operation of the research fields of disaster prevention engineering and social science, in conjunction with the practical services of on-site works, to effectively provide the people with advance and immediately prior predictions. Predicting hazard potentials with high accuracy is important to the planning of disaster countermeasures. The hazards include ground motions, tsunamis, and land slides due to earthquakes as well as flows of volcanic ash and lava from volcanic activities. Real-time estimation of hazards and simultaneous transmission of the estimated results are also help in the mitigation of secondary hazards that followed the main disaster. Typical examples of the results are presented in this review paper.

In order to contribute to the field of disaster science, various research in Japan currently focus on the clarification of the phenomenon called “disaster.” Due to society’s demand for disaster prevention and disaster mitigation, these researches are carried out through collaboration among researchers in science, engineering, humanities, social sciences, etc. These research outcomes are aimed at the following: verification of disaster cases of earthquakes and volcanic eruptions; clarification of the disaster occurrence mechanisms of earthquakes and volcanic eruptions; sophistication of information for disaster mitigation of earthquakes and volcanic eruptions; and development of researchers, engineers, and human resources involved in disaster prevention operations and disaster prevention responses. This article puts these research outcomes together from four points of view: 1) research on earthquakes and volcanic eruptions disaster cases, 2) clarification of disaster occurrence mechanisms of earthquakes and volcanic eruptions, 3) sophistication of information for disaster mitigation of earthquakes and volcanic eruptions, and 4) development of researchers, engineers, and human resources involved in disaster prevention operations and disaster prevention responses.

The objective of the General Research Group for the Nankai Trough Great Earthquake is to obtain a cross-sectional view of research on the Nankai Trough Great Earthquake, conducted by the various working groups of the Earthquake and Volcano Hazards Observation and Research Program, and to thereby promote the Program’s research in a comprehensive manner. In this paper, we report on workshops held during a five-year period (2014–2018), and summarize the findings of the five-year plan following the Research Group’s scheme. We also discuss issues that surfaced during these activities.

Several types of eruptions have occurred at Sakurajima volcano in the past 100 years. The eruption in 1914 was of a Plinian type followed by an effusion of lava. The progression of seismicity of volcanic earthquakes prior to the eruption is reexamined and seismic energy is estimated to be an order of 10¹⁴ J. Lava also effused from the Showa crater in 1946. Since 1955, eruptions frequently have occurred at the Minamidake or Showa craters at the summit area. Vulcanian eruptions are a well-known type of summit eruption of Sakurajima, however Strombolian type eruptions and continuous ash emissions have also occurred at the Minamidake crater. The occurrence rate of pyroclastic flows significantly increased during the eruptivity of Showa crater, with the occurrence of lava fountains. Tilt and strain observations are reliable tools to forecast the eruptions, and their combination with the seismicity of volcanic earthquakes is applicable to forecasting the occurrence of pyroclastic flows. An empirical event branch logic based on magma intrusion rate is proposed to forecast the scale and type of eruption. Forecasting the scale of an eruption and real-time estimations of the discharge rate of volcanic ash allows us to assess ash fall deposition around the volcano. Volcanic ash estimation is confirmed by an integrated monitoring system of X Band Multi-Parameter radars, lidar and the Global Navigation Satellite System to detect volcanic ash particles with different wave lengths. Evaluation of the imminence of eruptions and forecasting of their scale are used for the improvement of planning and drilling of volcanic disaster measures.

The research program titled “Earthquake and Volcano Hazards Observation and Research Program” was started in fiscal year (FY) 2014 as a new five-year project authorized and funded by the Council for Science and Technology of the Ministry of Education, Culture, Sports, Science and Technology. It included a new format of collaborative research called, “Core-to-Core Collaborative Research between Earthquake Research Institute, The University of Tokyo and Disaster Prevention Research Institute, Kyoto University.” In this format, two types of research, “Participation Type Research” and “Subject Proposal Type Research” were conducted from FY2015. A preliminary study was performed in FY2015 for “Integrated Research” of “Participation Type Research,” which developed a framework for seismic risk evaluation at prefectural offices of Osaka and Kochi for an earthquake occurring along the Nankai Trough, considering the epistemic uncertainty. The secondary study was performed from FY2016 through to FY2018, wherein the methodology for the seismic risk evaluation was improved on three aspects: i.e., revision in ground motion prediction models considering the saturation effect, revision in loss models in terms of the fatalities as well as the direct losses of buildings, and extension of target sites to the whole of Osaka and Kochi prefectures. The results suggest that the epistemic uncertainty in the ground motion prediction models is most sensitive to the overall uncertainty of seismic risk. Along with “Integrated Research,” a total of 14 “Research on Specific Topics” related to time-dependent risk analysis, economical risk evaluation, source characterization, structural damage estimation models, ground motion estimation models, soil amplification models, and disaster prevention planning considering the uncertainty of risk assessment, were studied during this period in order to improve the risk assessment studies for “Integrated Research.” With respect to “Subject Proposal Type Research,” a total of 27 individual research themes focused on research to understand hazards/risks by earthquakes and volcanic eruptions and to mitigate disasters by them.

Technological developments in muography have evolved since the first volcano was imaged with muography in 2007. In order to improve the muography technique as a more useful aid to volcano studies, there is a need for the time required to show the resultant images to be shortened. To expedite this process, an automatic real-time visualization system was developed and tested. In conjunction with future scaled-up detectors that will collect muons faster, this visualization system can also offer more practical and efficient tools for volcano muography.

This paper presents the results of a questionnaire survey conducted on those who had difficulty commuting after the 2018 Osaka earthquake. As with the Great Hanshin-Awaji Earthquake, serious traffic congestion occurred in downtown Osaka following the 2018 disaster. Based on the questionnaire survey on those who had difficulty commuting, which is considered to be a factor of traffic congestion, it was found that 60–70% commuted as usual after the earthquake; about half of the commuters who usually take the train changed their method of commuting, one-quarter of whom used automobiles; there were very few who experienced problems in their work because they had not gone to work or their workplace had closed down for the day; and many felt that it would be better to receive instructions on work attendance in the aftermath of an earthquake. The present study points out the need for companies and society to adopt rules so that those who find it difficult to commute will refrain from going to work and remain in their local communities to help others, except for those in certain occupations or positions.

In Japan, many recovery plans were instituted after big disasters such as the 2011 Great East Japan Earthquake occurred. This study focuses on recent big disasters in Japan, namely the 2011 Great East Japan Earthquake and the 2016 Kumamoto Earthquake, and clarifies the trends of the measures mentioned in the recovery plans of damaged municipalities according to their local characteristics (locations and population trends). In order to show the trends, the Genre Mention Rate (GMR) and the Measure Mention Rate (MMR) are calculated, which depict the mention rate – the importance that certain genres or measures have. In the municipalities damaged by the tsunami caused by the 2011 Great East Japan Earthquake, the GMRs of community, infrastructure, and culture are high in rural areas. There is not much difference in terms of population trends. In those damaged by the 2011 Great East Japan Earthquake, the GMRs of lifestyle and industry are high in rural areas, while the GMRs of community and disaster mitigation are low in urban areas. The GMRs of disaster mitigation and community are also high in areas with increasing populations. In the municipalities damaged by the 2016 Kumamoto Earthquake, the GMRs are generally high in urban areas, and low in areas with decreasing populations. In this way, depending on the type of disaster and local characteristics of the affected areas, there are many differences in the trends of the measures needed for recovery.