Large-scale natural phenomena are generally infrequent; however, their imprints can still be traced through geological records of past events. Terrestrial archives are one of the sources for the records of past events. Although they have a lower resolution than marine archives, terrestrial areas where human societies develop and where disasters actually occur have the advantage of being easily accessible. This special issue “Environmental Changes and Mega Disasters Unraveled from Terrestrial Archives: Toward Disaster Prevention and Mitigation” includes four papers that deepen our understanding of the characteristics of terrestrial archives to help decipher environmental changes and catastrophes. The first paper is a study that explains the three-dimensional fault displacement structure from underground radar exploration in the Tanna Fault. The second paper is a study on the evaluation method of interlocking earthquakes in inland active fault zones, with the Itoigawa-Shizuoka Tectonic Line fault zone as an example to construct a preliminary method for calculating the probability of occurrence. The third paper introduces the potential of systematic research on cave ruins and the emplaced deposits for clues to understanding paleoenvironments and human history, using the Sakitari Cave and Shiraho-Saonetabaru Cave Sites in the Ryukyu Islands as examples. The final paper involves reconstructing the history of vegetation changes over the past 3,000 years from pollen analysis of wetland sediments on Atiu Island, part of the Cook Islands in the South Pacific, and discussing the process of human immigration and settlement on the island.

Generally, we can estimate the strike-slip displacement of an active fault by detecting the lateral offset of fault-related deformed geomorphic features. However, when the geomorphic features are buried underground due to sedimentation or artificial filling, it is necessary to investigate the subsurface by drilling, trenching excavation, and geophysical exploration. In this study, we attempted to detect buried laterally deformed structures by ground penetrating radar (GPR) profiling, one of the geophysical survey methods to image the near-surface geological structures, at the Tashiro site on the Tanna fault. The GPR survey consists of 41 lines parallel to the fault trace with 40 m length and 0.5 m inline spacing. The GPR data were collected by common-offset modes using 50 MHz GPR systems (pulseEKKO PRO made by Sensors and Software Inc., Canada), and the horizontal scan-point interval was 0.5 m. Furthermore, we carried out wide-angle measurements and acquired common mid-point ensembles to estimate the electromagnetic wave velocity to process the depth conversion of the GPR sections. The results show a north-dipping band structure in the shallow part (less than 2 m depth) and a buried paleochannel structure in the deep part (more than 2 m depth). Both structures show left-lateral deformation, with the deeper structure, which was formed at an older age, being more strongly deformed. Thus, this study demonstrates that the GPR survey effectively detects the lateral offsets caused by active faults and confirms their cumulation.

This manuscript aims to introduce the summary of the newly-developed method for long-term forecast of multi-segment megaquakes on major active fault zones in Japan. Multi-segment megaquakes produced by major active fault zone have potential to cause mega-disaster, in particular, when the source fault is close to densely populated area. The size of multi-segment megaquakes have been evaluated by empirical relations between length of surface or source fault and earthquake magnitude. Whereas, the occurrence probability and frequency of multi-segment megaquakes are relatively difficult to be estimated, due to lack of reasonable evaluation methods in particular identification of past multi-segment earthquakes and their rupture extents. We recently proposed a method to reconstruct multi-segment rupture extent based on the timing and size of slip per event combining with pre-existing time-space diagram along the fault zone. Applying this method to the Itoigawa-Shizuoka Tectonic Line (ISTL) active fault zone, central Japan, we are able to preliminarily calculate the Poisson probability of future megaquake occurrences.

Caves formed in limestone (limestone caves) have a more stable environment than those formed on the Earth’s surface. Stalagmites and deposits formed in limestone caves are known to provide long-term and well-preserved paleoenvironmental records and fossils, respectively. In recent years, many archaeological sites have been excavated in the limestone of the Southwestern Islands (Ryukyu Islands) in Japan, leading to many anthropological and archaeological findings. Conversely, the environment of limestone caves varies greatly depending on their formation process and hydrological environment and formation process. Consequently, the stalagmites and the depositional environment of sediments in which fossils and artifacts are preserved differ between caves. This paper first summarizes the formation process of caves and then outlines the formation process of sediments and stalagmite growth in caves. Then the sediments of the Sakitari-do Cave and Shiraho-Saonetabaru Cave sites are reviewed as examples of cave sites with characteristic event deposits. Finally, the potential use of their sediments as indicators for understanding the paleoenvironment and human history are discussed.

The results of a pollen analysis conducted on Atiu Island, Cook Islands, revealed dramatic vegetation changes at ca. 1,600 cal BP (ca. 350 cal CE). These changes included a decrease in Pandanus pollen and Cyatheaceae spore, increase in herb pollen grains and herbal fern spores, increase in charcoal fragments, and increase in pollen grains of plants probably transported by Polynesians. The results suggest that the grasslands and fernlands expanded due to disturbances caused by large-scale deforestation. However, there are no archaeological data indicating human settlement on Atiu Island during this period. Cultivated plant pollen grains, such as those of sweet potato, and charcoal fragments increased rapidly after ca. 1100 cal CE. This may indicate human arrival on Atiu Island occurred in two stages. This is consistent with previous paleoenvironmental studies, like those from Lake Tiroto. Dynamic reconstruction of human settlement based on paleoenvironmental changes derived using sediments requires analysis of multiple sites from various catchments.

There is a distinct fill terrace along the uppermost course of the Sotoyama River, in northern part of the Kitakami Mountains, northeast Honshu. The terrace deposits are mainly made up of thick stratified and imbricated sand and gravel layers interfingered with lenticular silt and humic silt layers, and they are accompanied with a clay layer in the top part. This terrace is located ca. 700 m above sea level, and in the coolest area in Honshu Island. The Towada-Oofudo tephra (ca. 36 cal ka BP) is found in the upper clay layer. Pollen assemblages in five silty and clayey layers indicate that mixed broadleaf-conifer forest vegetation had been dominant during the whole duration of the deposition of the fill terrace deposits. This paleo-vegetation shows a cooler climate than Holocene but a milder climate than the LGM. The coarse gravelly sediments were deposited not by periglacial processes but by running water from the later stage of the last interglacial age (MIS 5 d) to a stadial in the early last glacial age (MIS 4-3), and supply of them from mountain slopes had stopped in the early stage of the LGM. The volume of sediments of this fill terrace in the uppermost course of the Sotoyama River is estimated to be much less than the volume which is supplied from watershed mountain slope area during the deposition of the fill terrace. It suggests that huge amount of clasts had run off from the uppermost course of this river and formed a fill terrace along middle course of river.