Here we briefly introduce 13 papers that exemplify the role of Quaternary research in disaster prevention and mitigation from multiple approaches ; this is a compilation from the open symposium “Multiple approach to reduce and mitigate disaster from Quaternary studies” held at Fukuoka University on August 27, 2017. These studies cover a variety of topics, from recent natural disasters, to historical and paleo- disasters that were revealed from historical records, and geomorphological, stratigraphical, and archaeological investigations, in which we demonstrate the significant contributions from Quaternary research to improving hazard assessment.

The 2016 Kumamoto Earthquake sequence with magnitude 6.5 (April 14) and magnitude 7.3 (April 16) brought severe damage in the central Kyushu area. Here, the outline of the earthquake and the related Quaternary tectonics in central Kyushu area are introduced and summarized from the viewpoint of geophysical and geological studies. Especially, the distribution of active faults in Kyushu (right-lateral strike-slip activity of the Median Tectonic Line, densely distributed E-W-trending normal faults associated with volcanic activity in the central region, and strike-slip activity of the Futagawa and Hinagu faults in western Kyushu) provides us with significant information on the relationship between the 2016 Kumamoto earthquake and the Quaternary tectonics in central Kyushu.

Damage due to liquefaction in Kumamoto plain has occurred by the 2016 Kumamoto earthquake in Japan. Topographically, liquefaction occurred in former stream channels, floodplains, reclaimed land, natural levees, and so on. Especially, areas of liquefaction in a natural levee is characterized as a 50-100-m wide and ～5-km-long band, spanning from the Shirakawa River to the Kasegawa River. Other two similar bands of liquefaction have been recognized around the Shirakawa River. The band of liquefaction seems to be a former stream channel, but aerial photographs and the historical record do not support that hypothesis. In general, the natural levee, which is relatively short, has a very high liquefaction potential similar to a former stream channel. However, it should be noted that liquefaction did not occurred in the whole of the natural levee, but did occur in the characteristic bands. In addition to the characteristics of topography, the others are that liquefaction area expanded from “foreshock” to “main shock” of the earthquake from comparing with aerial photographs after each shock. The liquefaction damage has been enormously increased due to the influence of characteristic volcanic ash sand.

Numerous surface cracks appeared with displacements on the gentle slope in a residential area at the western foot of Takano-obane volcano in the western part of Aso caldera, central Kyushu, Japan, due to the 2016 Kumamoto Earthquake. The surface of the slope is composed of thick loam layer of approximately 10m in thickness. The ground in the residential area has been artificially modified for cultivated land, an ice-skating rink, and housing in the last three decades. Both horizontal and vertical displacements are recognized along most of the cracks. The former are several to thirty centimeters ; the latter are one meter or less. They are mainly E-W to ENE-WSW in strike, and a remarkable one extends more than 150m long with northward sinking in the central portion of the area. Cracks with vertical displacements tend to appear in the northward zone of this major crack. The zone almost coincides with the outer periphery of the previous skate rink, so it is highly likely that sliding of embankment materials along the cut slope of the rink occurred at the time of construction of the residential area. Therefore, such cracks might have been formed as settlement or displacement due to earthquake vibration.

Inland large earthquakes occur not only on major active faults but also in areas no active fault and/or minor short fault mapped. It leads a conservative evaluation that seismogenic fault up to ～20km is hidden or slightly truncated by the surface, and an M～7 earthquake is assigned on each short fault. Based on field investigation and InSAR analysis for the 2016 Kumamoto earthquake, we here counter-argue that some of such minor and low-slip-rate faults might have been developed by insignificant but frequent slips triggered by nearby large earthquakes. Another unique case possibly contributes to better evaluation of such short faults is the recent repeating M～6 earthquakes occurred on March 19, 2011 (M6.1) and November 22, 2016 (M6.3) at Ibaraki-ken-hokubu, in northern Kanto region, Japan. Both shocks have shared the same 4-km-long fault based on InSAR images and field survey. It enables us to interpret some of the short active faults might have been also developed by more frequent slip at only upper seismogenic layer, not involving the entire seismogenic layer, due to M～6 earthquakes. Together with other cases, we discuss the classification of short active faults with regards to slip rate, erosion rate, and seismogenesis, and then tentatively conclude that short active faults not only generate an independent single M～7 earthquake but also behave aseismically or seismically influenced by neighboring major faults.

After the landslide disasters in Hiroshima City in 2014, I confirmed the necessity to display landform information of the micro-landform or the watershed, and to overlay regional disaster prevention units on the hazard map. Based on this confirmation, four concepts from the viewpoint of Quaternary research and creating a hazard map for landslide disasters were summarized in the case study area including the volcanic fan of Mount Unzen.

The history of landform development with landform processes, such as deposition and erosion within a period of the order of 10² years, can be used as a basic material for area classification to landslide disasters.

Spatial analysis consisting of interpreting of elevation changes as landform process and overlaying the results from the oldest to the youngest means the construction of landform development.

Spatial analysis, reclassifying the geographic information interpreted as a set of landform processes including its type, size, and sequence means the classification of micro-landforms.

From the results of landform analysis using DEM on the volcanic slope, it is easy to understand the area safety based on micro-landforms and the area in which to be aware of debris and water movement from watersheds.

Colluvium deposits distributed at the foot of the caldera wall of the Aso volcano are estimated to be piled-up materials derived from steep slopes. They are composed of poorly sorted gravel beds including andesite, welded tuff, and tephra blocks, and form sharp alluvial cones. Therefore, numerous shallow landslides caused by heavy rainfall and/or large earthquake vibrations may have brought about these deposits after the completion of the caldera wall.

In order to determine the ages of depositional events of such materials, we carried out the dating of three charred materials and four paleosols collected from the colluvium deposits. Calendar-calibrated AMS ¹⁴C dating indicates that more than 15 depositional events occurred during the past 26,000 years at five investigation points. Based on this relation, the recurrence interval of such events is estimated to be on the order of 10³ years in this area.

Beppu Bay, occupying the western portion of an arc-bisecting dextral fault system, is a tectonic depression that has existed since ca. 5Ma. Funai Fault is a major normal fault along the southern margin of the tectonic basin. In order to estimate the activity of this fault, sedimentary facies, diatom assemblages, and radiocarbon ages of 17 terrestrial plant fragments and 4 marine carbonate samples were determined from seven drill cores. Based on the analysis, three sedimentary facies, namely delta front, delta plain, and artificial soil, were identified. These accumulation and deformation structures were imaged by ground penetrating radar profiling. Based on the results, the latest seismic event was identified at 800 to 400calBP based on the depositional ages and diatom assemblages from the uppermost mud layers in the delta plain sediment. The previous event was also identified at ca 2,100calBP on the basis of the vertical displacement of the lower mud layer in the delta front sediment. The recurrence interval was calculated at ca 1,700 years.

Before the 2011 Tohoku-oki earthquake, long-term tsunami history along the Sanriku Coast was not well understood. After the 2011 tsunami event, exhaustive tsunami deposit research revealed the number and distribution of tsunami deposits on the Sanriku Coast, which was thought not to be an appropriate site for paleo-tsunami deposit survey. Especially, tsunami deposits related to two historical tsunamis, the 1611 Keicho Sanriku tsunami and the 869 Jogan tsunami, were widely identified on the coast. Moreover, distribution and ages of older tsunami deposits were also revealed, and some tsunami deposits show similar age. In the future, for understanding magnitude and source of paleo-tsunamis, we need to establish a method for correlation of paleo-tsunami deposits from each site and to study the details of paleo-tsunami deposits.

Numerical modeling of tsunami sediment transport is a powerful tool to understand tsunami-induced sediment erosion and deposition. Application of the modeling to paleotsunamis will improve our ability for assessing hazards from earthquakes and tsunamis. Assessment of uncertainties due to incomplete inputs will be an inevitable challenge in the sediment transport modeling of paleotsunamis. Uncertainties from various factors, such as paleo-topography, vegetation, sediment source and tide level, will inhibit systematic comparison of the modeling results and field data, and will obscure the picture of the earthquake and tsunami. Datasets of long-term change in coastal environment, annual and seasonal variation of beach topography and anthropogenic modification of the landscape, as well as framework for quantitative assessment of the uncertainties, will be the keys for effective application of sediment transport modeling to paleotsunamis.

Speleothems in limestone caves are precipitated during CO₂ degassing from groundwater supersaturated with calcite. Stalagmites, a type of speleothem that develops from the cave floor, grow upward at around 10-100µm a year. Paleoenvironmental information such as the Asia-monsoon activity and the vegetation changes on the upper cave surface can be obtained from detailed geochemical analysis of stalagmites along a growth direction. Episodic events like earthquakes are indicated by flaws in the speleothems. Absolute ages of the events can be obtained by U-Th dating and UV-fluorescent annual lamina counting in speleothems. We reviewed possible records remaining in limestone caves during large-scale earthquakes and tsunamis around the world, and discussed the effects of a large scale tsunami on 500-year records of Mg/Ca, δ¹³C and Sr/Ca ratios of two stalagmites from the Shiraho-Saonetabaru Cave Site, near the Shiraho beach, Ishigaki, Okinawa.

Kaimondake volcano is an active volcano located on the southeastern end of the Satsuma Peninsula, Kyushu Island. Its volcanic activity started about 4,400 years ago and erupted in the Heian period about 1,150 years ago. The eruptions of the Heian period occurred in year 16 of the Jogan era (874 AD) and the first year of Nin-na era (885 AD). The houses and farmland of the Hashimuregawa and the Shikiryo archaeological sites located in Ibusuki city were significantly damaged by the eruption of Kaimondake volcano, in year 16 of the Jogan era. Based on the results of excavations at the two archaeological sites, herein, we document the eruption of Kaimondake volcano and the subsequent human responses to this disaster in year 16 of the Jogan era. In addition, the ejecta (Km12a3～4, Murasaki-Kora) from these two sites appears to support details from the historic document “Nihon-Sandai-Jitsuroku”. This document noted that it rained during the eruption followed by a Lahar deposit. Ejecta from year 16 of the Jogan era eruption appears in the Hashimuregawa archaeological site as a 50cm to 80cm thick deposit ; consequently, people abandoned the village instead of trying to remove the ejecta. In the Shikiryo archaeological site, located about 1km north of Hashimuregawa, the ejecta deposition is about 30cm thick. At this site, people attempted without success to remove the deposits, but ultimately abandoned the village. Restoration was unsuccessful at Hashimuregawa and Shikiryo archaeological sites because the ejecta deposits and rainwater created a very firm matrix. People attempted restoration because public facilities and paddy fields were essential to life in this area at this time.

Low-frequency events, such as earthquakes or eruptions, that can cause large-scale disasters are a major issue for society. These events have a long occurrence interval, and their event deposits, which are preserved for a long time, are the key to revealing their real image. In Beppu Bay, event deposit research began with active fault studies in the 1980s, and it has subsequently been conducted by many researchers and from several perspectives. This accumulated knowledge has been revealing not only the paleo-event records but also their complicated recording processes. It is important for the development of event deposit analysis to understand all recording processes, by combining these event deposit records with paleo environmental information, ancient document records, and prospective simulations.

We analyzed the sedimentary facies of deposits from lacustrine events in the Middle Pleistocene Miyajima Formation to test the accuracy of recognition of event deposits induced by floods and slope failures using sedimentary processes taking into account lateral changes in the Middle Pleistocene Hiruzenbara Formation. The Miyajima and Hiruzenbara formations are lacustrine varved and intercalate event deposits, respectively. The event deposits of the lower section of the study area can be observed horizontally. The paleo-Shiobara Lake, which contains the Miyajima Formation deposits, is smaller and closer to the source than the paleo-Hiruzenbara Lake, which contains the Hiruzenbara Formation deposits and many intercalate event deposits. It is difficult to recognize these Miyajima Formation event deposits because varve deposits include inflow material. Thus, the accuracy of recognition of event deposits is verified. Flood event deposits continue laterally, while the thicknesses and sedimentary facies of slope failure deposits change laterally. Hyperpycnite, hypopycnite, and homopycnite are affected by paleo lake environments and can be distinguished by sedimentary processes taking into account lateral changes. Hyperpycnite erode a lower layer and relative lateral changes are observed in its layer. Hypopycnite and homopycnite do not erode, and the thicknesses and sedimentary facies of their layers remain approximately laterally continuous and uniform.