Nigorikawa volcano in southwestern Hokkaido, Japan, has a small caldera approximately 2 km in diameter. We carried out geologic, petrologic, and paleomagnetic studies of pyroclastic deposits on Nigorikawa volcano to reveal its eruption sequence and geologic history with high resolution. Nigorikawa pyroclastic deposits are divided into two eruption stages (1 and 2) separated by a paleosol layer ¹⁴C-dated at 12,901-12,750 calendar years. This suggests a ca. 1000 year hiatus between the Syn-caldera-forming eruption (Stage 1) and post-caldera activity (Stage 2). Stage 1 is composed of 7 units (Ng-1～Ng-7, in ascending stratigraphic order), while Stage 2 (Ng-8) is represented by a small (＞0.01 km³) pyroclastic flow deposit. The minimum total volume of the whole units (Ng-1 - Ng-8) is estimated to be 8.2 km³. Ng-1 consists of a lower (Ng-1 a) 0.11 km³ ash unit and an upper (Ng-1b) 0.53 km³ ash and pumiceous fall deposit, respectively. Ng-2 is a small (＜0.02 km³) pyroclastic flow deposit narrowly distributed in the northeast of the volcano. Ng-3 (0.02 km³) and Ng-5 (0.07 km³) are respectively sub-Plinian to Plinian pumice falls, that sandwich the Ng-4 (0.01 km³) intra-Plinian flow deposit. Ng-6 is a climactic ignimbrite (7.35 km³) further divided into Ng-6 a (lower) and -6b (upper) units based on the existence of a lithic concentration zone (LCZ) between the 2 units. In addition, co-ignimbrite ash (Ng-6c) widely covers the eastern distal area of the volcano. Ng-7 is a lithic-dominated pyroclastic surge deposit (0.07 km³) characterized by cross/parallel laminations. Ng-1 and Ng-7 commonly contain silty ash and blocky glass shards with moss-like morphology suggesting that they were formed by magma-water interaction (phreatomagmatic eruption) that occurred at the initiation and termination of Stage 1. Ng-8 (Stage 2) is a newly discovered eruption unit. Paleomagnetic features demonstrate that Ng-8 is a lateral flow deposit that was emplaced at a high (350-400 °C) temperature. Stage 2 can be stratigraphically and chronologically correlated with the post-caldera activity of the volcano that generated (1) tephra in the caldera-lake (lacustrine) deposits and (2) intrusive rocks (lava and dykes) through the caldera-fill deposits, both of which are described in borehole samples by previous work. Amphibole andesite lava fragments showing oxidization coating and having slightly different chemistry than co-existing juvenile pumice are generally included in all Nigorikawa pyroclastic deposits. We speculate that the older edifice of the andesitic lava dome (1.7 km³) existed before the Nigorikawa caldera formed.

Ohatayama and Ohataike are adjacent volcanoes aligned in NE-SW direction at the northeastern portion of Kirishima Volcano Group, Kyushu, Japan. Ohatayama volcano has three small craters around the summit area, and Ohataike has a summit crater lake with a diameter of 400 m. Both volcanoes are composed mainly of tephra fall deposits by plinian or sub-plinian eruptions, with intercalations of pyroclastic flow deposits. The main volcanic edifice of the highest point of Ohatayama volcano was formed at least 22 cal ka BP. Tsukiyama were formed earlier than the highest point, and there is a possibility that they can be separated from the highest point. The triangulation point of Ohatayama volcano started its activities on the highest point somewhere around 17-11 cal ka BP, and forming craters A and B. Ohataike volcano was formed by erupting again around 17-11 cal ka BP, after the main volcanic edifice was formed during around 22-17 cal ka BP on the northeast side of the triangulation point. This was followed by four different modes of eruption at Ohatayama C crater in the following order: a phreatic eruption (Oy-4) in 7.6 cal ka BP, Ohatayama lava emission, magmatic and phreatic eruptions (Oy-3) in 7.1 cal ka BP, magmatic and phreatic eruptions (Oy-2) in 6.8 cal ka BP, and lastly, phreatic eruption (Oy-1) in 6.5 cal ka BP. Although no volcanic activity occurred from Ohataike volcano since 11 cal ka BP, foamy volcanic gas is currently being detected on the surface of the crater lake.

The southwestern rim of Aira caldera, which is situated at the head of Kagoshima Bay, is critical for examining late Pleistocene and Holocene crustal movements of the caldera with respect to volcanic activity. A suite of Pleistocene and Holocene sea-level and eruption records occurs in combination in exposures on the rim, and so tectonic displacement of the caldera as well as volcanic activity in historical times are both obtainable. Using elevations of coastal landforms and deposits, and with a chronology determined via tephrochronology and archeological remnants, we examined vertical crustal movements of the Aira caldera in the late Pleistocene and Holocene, and compared these movements with historical movement in the light of concomitant volcanic activity. The main conclusions are as follows. Aira caldera has been subjected to distinct uplift, with an average rate of 0.5-0.8 mm per year over the past ~108,000 years. The uplift rate of 0.8-1.1 mm per year, from ~7000 cal BP to the present, appears to be higher than that, 0.4-0.7 mm per year from ~108,000 to ~7000 cal BP. Comparison of these late Quaternary uplift rates with those in historical time clearly suggests that volcanic activities of Aira caldera are responsible for the late Quaternary vertical movements in and around Aira caldera. The results help to evaluate future eruptions of Aira caldera, and to examine the relationships between the late Quaternary crustal movement and volcanic activities in other gigantic calderas without sea-level remnants.

Shinmoedake is the compound volcano in Kirishima Volcano and the most active volcano in Japan, having recorded frequent magmatic eruptions during 1716-1717, 2011, and 2018. The three geological active periods of Shinmoedake in the last 8 ka were recorded by a geological survey (Tajima et al., 2013a). The geological eruptive time category of Shinmoedake is divided into long-term, middle-term, and short-term activities. Short-term activity is captured by monitoring and covers a period of several years or more. The magma eruption rates during middle-term activities were estimated to be several times higher than the long-term magma eruption rate. Moreover, the centers of magma eruptions within each middle-term period had stabilized in terms of location. Additionally, the magma eruption rates during each period of middle-term activity were not constant. Therefore, knowledge regarding the variation in the magma production of Shinmoedake during geologically short-term, middle-term, and long-term activities is required to understand its development and plumbing system. In this paper, we compile recent geological investigation results of Shinmoedake and propose a rational conceptual model of its current state supported by petrological and geophysical data. A well-known conceptual plumbing model of Kirishima Volcano was proposed by Kagiyama et al. (1997). The seismic attenuation spot (reservoir A) is located at a depth of 4-5 km below Karakunidake (Oikawa et al., 1994), and a wide P-wave velocity anomaly area (reservoir W) is situated at a depth of 10-15 km below Kirishima Volcano (Yamamoto and Ida, 1994). Recently, geophysical observations have indicated that magma was supplied from a depth of 8-10 km (reservoir B) to the western area of Shinmoedake during the 2011 magmatic eruption (Nakao et al., 2013). In addition, petrological analysis suggested two different sources of silicic magma from a level of reservoir A and mafic magma from a level of reservoir B (Suzuki et al., 2013a). Therefore, reservoir B might have been connected to reservoir A, where magma mixing occurred during the 2011 eruption. Furthermore, analysis of the deep low-frequency (DLF) earthquake of the 2011 eruption of Shinmoedake revealed that the DLF activities at a depth of 20-27 km (reservoir L1) in the eastern part of Kirishima Volcano were involved (Kurihara et al., 2019). Reservoirs L1 and B may also be connected. These results support the increasing activities of Kirishima Volcano revealed by the geological survey (Tajima et al., 2013a). It is concluded that the complex magma plumbing system of Shinmoedake may cause different magma eruption rates during periods of middle- and long-term activities.

We obtain a three-dimensional seismic velocity structure below the Aira caldera at a depth shallower than 15 km, southwest Japan, applying seismic tomography inversion method to analyze 14,652 P-wave onsets and 10,935 S-wave onsets of natural earthquakes observed by 45 seismic stations, and 3,121 P-wave onsets generated by artificial explosions. An anomalous zone of low S-wave velocity is discriminated at depths deeper than 12 km below the center of the Aira caldera. The S wave velocity is 18-55 % lower than the surrounding area. The volume of the anomalous zone is 139～255 km³ at shallower depths than 15 km, and the anomalous zone includes about 7 % melt (10～18 km³). Accumulation of magma in the anomalous zone activates a pressure source at the top of the zone, where velocity contrast of the S-wave is intense, and the pressure source induces inflationary ground deformation around the Aira caldera.

Ballistic projectiles are large pyroclasts (＞0.1 m in diameter) traveling through the air without being affected by the flow of gas. This phenomenon is harmful (and potentially fatal) when a volcanic eruption suddenly occurs as the ballistic velocity is quite high, sometimes reaching several hundred meters per second. Therefore, it is important to simulate the trajectory of ballistic projectiles in an affected region. We have estimated the ejection conditions of the 1895 Zao eruption by visually comparing simulated results using a numerical model called “Ballista” to actual block distributions obtained from field observations and aerial photographs. Interestingly, around Goshikidake (northeast of the Okama crater) the farther blocks were from the crater, the larger the block size was. The ejection direction was estimated to be 120° from the north (southeast direction), because the deposit blocks are spatially dense in this direction. The ejection angle was estimated to be 10°, and the ejection velocity was estimated to be 110-120 m/s. The estimated eruption velocity of the 1895 Zao eruption was similar to that of the 2014 Ontake eruption and within the range of small vulcanian eruptions. Although we often worry that a magmatic eruption will occur after a phreatic eruption, it is also possible that a vigorous block emission will occur with a considerably high ejection velocity during a phreatic eruption.

Aso volcano produced four huge ignimbrite-forming eruptions named Aso-1, 2, 3 and 4 in ascending order, among which Aso-4 is considered the largest eruption in Japan in the last 1 million years. This paper describes the tephra sequence between the Aso-4 and Aso-3 eruptions (Aso-4/3 tephra group). The reconstruction of the eruptive history for Aso-4/3 tephra group presented here provides a valuable contribution to the understanding of caldera volcanism by outlining the preparatory processes of a catastrophic ignimbrite eruption. The eruption sequence of the Aso-4/3 tephra group, which is composed of at least 37 units of pumice-fall, scoria-fall, and ash-fall deposits, is divided into five stages. Stage 1 is characterized by the eruption of mafic scoria (VEI 3-4) during 133-114.1 ka, after the eruption of Aso-3. Stage 2 is characterized by the frequent eruptions of mafic scoria and ash (VEI 3-4) during 114.1-108.4 ka. The magma composition became more felsic during explosive eruptions (VEI 3-4) from 108.4-104.7 ka (Stage 3). During the most active stage from 104.7-97.7 ka (Stage 4), voluminous felsic pumice-falls erupted (VEI 4-5). The ABCD tephra (97.7 ka) is the largest plinian pumice-fall deposit of Aso volcano. Stage 5 (97.7-88 ka) is a relatively dormant period, during which only a biotite dacite pumice-fall was deposited (VEI 4). The low number of eruptions during stage 5 suggests that the magma supply rate decreased during the 10 thousand years that preceded the Aso-4 ignimbrite eruption. The estimated total tephra volume for the Aso-4/3 tephra group is 23 km³, which corresponds to 10 km³ in dense rock equivalent (DRE). The estimated the long-range tephra discharge rate (0.23 km³ DRE/ky) is similar to that in the post-caldera stage of Aso-4 (0.2 km³ DRE/ky).

Volcanic gas composition provides us a crucial clue to investigate magma plumbing and geothermal systems. Sensor-based instruments named Multi-GAS have been used for monitoring the volcanic gas compositions at volcanoes. A sensitivity of sensors changes with time caused by deterioration, masking volcanic signals especially during long-term monitoring. Frequent calibration of the sensors is desirable for precise monitoring; however, that is pragmatically not easy because a location of a targeted volcano is remote and rural in general. Sophisticated evaluation of the long-term changes in the sensor sensitivity has not been made yet. In this study, we examined the sensitivity change of the chemical sensors within the Multi-GAS during long-term observations by comparing with other methods such as gas detector tubes and gas sampling. The volcanic gas compositions were monitored using Multi-GAS at Kusatsu-Shirane volcano and Kuju volcano, Japan. Intermittent gas composition measurements using gas detector tubes and gas sampling were conducted at fumaroles around where the Multi-GAS stations are installed. Some disagreements of the CO₂/H₂S ratios are observed between those measured using the Multi-GAS from those measured using other methods. In such cases, large decreases of the H₂S sensor sensitivity were found by the sensor calibration after the monitoring. We found a roughly linear behavior of the H₂S sensor sensitivity changes with time based on a long-term sensor sensitivity monitoring in a laboratory and propose a simple linear sensitivity correction of the H₂S sensors using the calibration results obtained before and after the monitoring. The corrected Multi-GAS results agree well with the results of other methods. Our results open up a possibility for extraction of volcanic signals from the long-term volcanic gas data streams monitored using the Multi-GAS that are masked by the changes in the sensitivity of the sensors.