Phytoliths as an indicator of change in vegetation related to the huge volcanic eruption at 7.3 ka in the southernmost part of Kyushu, southern Japan

Volcanic eruptions can have a significant influence on adjacent ecosystems; however, little is known about the long-term vegetation change related to eruptions. In this study, we examined phytolith records in paleosols at multiple sites in the southern Kyushu District, Japan, to assess the influence of the Kikai caldera eruption 7300 years ago on vegetation. Our results show the vegetational difference before and after the eruption in the study region. Specifically, in the area where the pyroclastic flows distributed more thickly, the original evergreen forest was replaced by Andropogoneae grasslands after the eruption, which has been dominating the landscape in this area for at least 900 years. By contrast, in areas only mildly affected by pyroclastic flows, despite the temporary replacement of forest by grassland, the forest developed and flourished within several hundreds of years of the eruption. This is because a large amount of pyroclastic flow would have devastated all of the vegetation, whereas smaller amounts would have left some untouched forest sites within refugia. Our findings suggest that the vegetation varied significantly depending on the amount of pyroclastic flow reaching the area, even within the pyroclastic flow distributed region.

A difference in a transfer function of two horizontal components between the ground level and the borehole at KiK-net Mashiki Station Anisotropy of shear wave propagation in Kyushu area based on seismic interferometry

In this study, we evaluated the travel time of S-wave between the vertical array stations based on seismic interferometry, focusing on the difference in transfer function due to two horizontal components at the KiK-net Mashiki station (KMMH16). At that time, we surveyed the differences by back azimuth (BAZ) and the polarization direction of seismic waves. Furthermore, we expanded the survey to all KiK-net stations in the Kyushu district, to confirm whether the phenomena seen at KMMH16 is specific to this location. The result shows that the difference by the polarization direction in the travel time was larger than the difference by the BAZ. This result suggests that the difference in transfer function at KMMH16 were affected by the anisotropy of the S-wave velocity. We evaluated the leading S-wave polarization directions (LSPDs) and the strength of anisotropy (ΔV) for all KiK-net stations in the Kyushu district. The LSPDs roughly correspond to the results of previous studies. The LSPDs in the forearc area are nearly perpendicular to the crustal deformation whereas those in the back-arc area are nearly parallel to it. This characteristic is similar to one found by the previous research in the Tohoku district. We examined the change in anisotropy before and after the Kumamoto earthquake at two stations, KMMH16 and KMMH14 that are located near the source region. The changes in the LSPD and the ΔV before and after the earthquake were not notable. At stations that observed weak anisotropy, transfer functions of two horizontal components show similar shape. At stations that observed strong anisotropy, however, the shape of the transfer function differs greatly, depending on the horizontal direction. This suggests that an evaluation of site amplification using a single velocity model may reduce the reproducibility of ground motions.

A difference in a transfer function of two horizontal components between the ground level and the borehole at KiK-net Mashiki Station Anisotropy of shear wave propagation in Kyushu area based on seismic interferometry

In this study, we evaluated the travel time of S-wave between the vertical array stations based on seismic interferometry, focusing on the difference in transfer function due to two horizontal components at the KiK-net Mashiki station (KMMH16). At that time, we surveyed the differences by back azimuth (BAZ) and the polarization direction of seismic waves. Furthermore, we expanded the survey to all KiK-net stations in the Kyushu district, to confirm whether the phenomena seen at KMMH16 is specific to this location. The result shows that the difference by the polarization direction in the travel time was larger than the difference by the BAZ. This result suggests that the difference in transfer function at KMMH16 were affected by the anisotropy of the S-wave velocity. We evaluated the leading S-wave polarization directions (LSPDs) and the strength of anisotropy (ΔV) for all KiK-net stations in the Kyushu district. The LSPDs roughly correspond to the results of previous studies. The LSPDs in the forearc area are nearly perpendicular to the crustal deformation whereas those in the back-arc area are nearly parallel to it. This characteristic is similar to one found by Nakajima and Hasegawa (2008) in the Tohoku district. We examined the change in anisotropy before and after the Kumamoto earthquake at two stations, KMMH16 and KMMH14 that are located near the source region. The changes in the LSPD and the ΔV before and after the earthquake were not notable. At stations that observed weak anisotropy, transfer functions of two horizontal components show similar shape. At stations that observed strong anisotropy, however, the shape of the transfer function differs greatly, depending on the horizontal direction. This suggests that an evaluation of site amplification using a single velocity model may reduce the reproducibility of ground motions.

Special Issue on NIED Frontier Research on Science and Technology for Disaster Risk Reduction and Resilience 2018

The National Research Institute for Earth Science and Disaster Resilience (NIED) is working on three tasks: predicting disasters, preventing damage, and realizing speedy reconstruction and recovery efforts in the event of natural disasters such as earthquakes, tsunamis, volcanic eruptions, landslides, torrential rains, blizzards, and ice storms. In the last two years of the NIED’s fourth mid/long term plan period, which began in 2016, the 2016 Kumamoto earthquake (M6.5 and M7.3), the heavy rainfall in the Northern Kyushu District in July 2017, and the heavy rain event of July 2018 are listed as “named” disasters, named by Japan Meteorological Agency. In addition, there were other disasters: an avalanche accident on Nasudake in 2017, an earthquake (M6.1) with its epicenter in northern Osaka, an eruption of Kirishimayama (Shinmoedake and Ioyama) and a phreatic eruption of Kusatsu-Shiranesan in 2018. The results of research done on the above-mentioned disasters and the latest results of ongoing projects in each research division and center were compiled as the second NIED special issue of the Journal of Disaster Research (JDR). In this special issue, we are delighted to present ten papers on three topics: climatic disasters, seismic disasters, and integrated research on disaster risk reduction. In particular, this special issue contains three papers on the above-mentioned heavy rainfall in the Northern Kyushu District in July 2017 and two papers related to the Kumamoto earthquake. Although the achievements detailed in these papers are the results of individual research, the NIED hopes that these results as a whole will be fully utilized to promote science and technology for disaster risk reduction and resilience. The NIED hopes that this special issue awakens the readers’ interest in new research and, of course, creates an opportunity for further collaborative works with us.

Spatial Analysis of the Landslide Characteristics Caused by Heavy Rainfall in the Northern Kyushu District in July, 2017 Using Topography, Geology, and Rainfall Levels

The heavy rain in Northern Kyushu District on July 5, 2017 caused a sediment disaster, resulting in the loss of many lives and damage to buildings. In this study, the primary causes (topography and geology) and trigger factors (rainfall) for the sediment disaster were spatially analyzed to examine factors contributing to slope failure. As a result, it was found that the number of slope failures was highest in metamorphic rock areas and the occurrence density of the landslides was highest in plutonic rock areas. In addition, the slope angle of the slope-failure source point was sizable in volcanic rock areas and many landslides occurred in the valley-formed areas. A rainfall analysis showed that the Akatani, Shirakitani, Sozu, Kita, Naragaya, Myoken, Katsura river basins and Ono, Ohi, Sata, Inaibaru river basins are different rainfall distributions, which significantly affected the slope-failure occurrence density.

Desired Robot Technology in Consideration of the Records on Natural Disasters in Kyushu Region

<div class=""abs_img""><img src=""[disp_template_path]/JRM/abst-image/00260004/05.jpg"" width=""300"" />Mount Sakurajima near Kagoshima City</span></div> This paper addresses records of natural disasters occurring in Japan’s Kyushu region. Kyushu, from north to south, is the site of frequent natural disasters – typhoons, heavy rains, volcanic eruptions, etc. The first half of this paper reviews volcanic disasters focusing on Mount Sakurajima in Kagoshima Prefecture. The second half reviews water disasters caused by typhoons and torrential rains, such as the August 1993 Heavy Rainstorm in Kagoshima Prefecture, the July 2009 Chugoku-Northern Kyushu District Heavy Rainstorm, and the July 2012 Northern Kyushu District Heavy Rainstorm. The possibility of using robot technology to reduce damages is considered, as is that for disaster measures. </span>