宮崎県北西部の高千穂町と五ヶ瀬町の境界に位置する二上山（ふたかみやま）は，男岳と女岳の二つの峰からなる標高1,082 mの山である．『日本書紀』には天照大神の孫であるニニギノミコトが降臨した山として記されている．二上山は，九州最古級とされる鞍岡花崗岩（約4.5億年前；オルドビス紀）および祇園山層（約4.3～3.6億年前；シルル紀前期～デボン紀後期）からなるレンズ状岩体の北東端に位置する（斎藤ほか, 1958）．二上山の北東に広がる高千穂盆地には，約13万年前および約9万年前に阿蘇カルデラ起源の火砕流が流入し（星住ほか, 2023, 2024），その後，五ヶ瀬川水系による侵食によって高千穂峡に代表される特徴的な峡谷地形が形成された．高千穂盆地の北東には祖母・傾山系が分布し，さらに北方には九重連山を遠望することができる．

（写真・説明：河合研志・飯塚　遼・佐藤友彦・庄司真史；2025年11月7日撮影）

 A caldera is a volcanic depression formed by a magma chamber roof collapse. Active faults extend along the margins of some calderas, displacing post-caldera geomorphic surfaces. Additionally, earthquakes associated with strong ground motions and crustal deformations often occur along caldera margins. Therefore, tectonic-geomorphological investigations on faults along caldera margins are crucial for seismic hazard assessments, because these faults can be seismogenic. In addition, understanding the activities of such faults is necessary to reconstruct a post-caldera geomorphological history. The Onikobe Fault along the southwestern margin of the middle Pleistocene Onikobe Caldera is investigated to provide insights into seismic and volcanic hazard assessments and post-caldera geomorphological evolution. One late Pleistocene lacustrine terrace (Takahata surface) and six late Pleistocene to Holocene fans (Onikobe I-VI surfaces) have developed along the foot of the southwestern caldera rim. The Onikobe Fault is an arcuate fault system comprising two segments: northeastern and southwestern segments. The northeastern segment is a series of uphill-facing scarplets (4 km in length) vertically displacing the Takahata surface. The southwestern segment consists of both downhill and uphill facing scarplets (8 km in length) vertically displacing the Onikobe surfaces. This segment is inferred to be a steep normal fault system because of the linear trace of each scarplet and the presence of narrow grabens. The average offset rates of northeastern and southwestern segments are > 0.4-0.6 m/kyr and 0.3-1.3 m/kyr, respectively. The offset rate of the southwestern segment is among the highest for active faults of less than 10 km in length in Japan. Activity on this segment has increased the height of the somma relative to the caldera floor and has promoted the differentiation of Onikobe surfaces. Based on the amount of caldera subsidence and the distribution of gravitational anomalies, it is inferred that a ring normal fault was formed along the caldera margin during the caldera collapse. The inferred distribution and structure of the southwestern segment of the Onikobe Fault are consistent with those of the ring normal fault, implying that the former was formed by the post-caldera reactivation of the latter. The findings indicate that detailed investigations regarding recent activities on faults along a caldera margin are important for seismic and volcanic hazard assessments because such faults can reactivate during the post-caldera stage. Further investigations on paleoseismic history, slip per event, and fault plane dip directions will improve associated hazard assessments and understanding of post-caldera geomorphological evolution.

 At Hiranezaki on Sado Island, Niigata Prefecture, a coastal karst landscape has developed on a 60-meter-wide slope inclined about 20 degrees toward the Sea of Japan. Such karst features occur mainly in calcareous sandstone, which is not part of an accretionary complex but consists of in-situ cold-water carbonate deposits. Coastal karst has not been reported along the Sea of Japan, nor has karst formed on cold-water carbonate sedimentary rocks been documented in Japan. Hiranezaki is a slope composed of bedding planes of the Hanyugawa Formation, which was deposited during the Middle Miocene. Numerous small depressions can be observed on this slope. Those below 5 meters in elevation are marine potholes, while those above 5 meters in elevation are interpreted as kamenitzas. The distribution of these features closely follows that of the calcareous sandstone. The slope is divided into four parallel zones from the shoreline upward, each with different karst forms: coastal karren, bowl karren, rillenkarren, linenkarren, and kamenitzas. Hiranezaki is the only known site of coastal karst on the Sea of Japan side and may serve as a type locality for such landscapes in Japan. Its karst features are especially notable for forming in in-situ cold-water carbonate sedimentary rocks rather than in accretionary limestone, which is typical elsewhere in Japan.

 波形相関に基づいた震源再決定カタログを用い，2024年11月26日に能登半島西方沖で発生した地震（M 6.6）に先駆した活動について調査した。先駆的活動は，その時空間分布の特徴から4つの期間に分割できる。Period 1：2024年1月下旬～2月，Period 2：3月～9月下旬，Period 3：9月末～10月末，Period 4：11月初め～M 6.6の地震発生まで。この中で，Period 1では短時日の活動の活発化，Periods 3，4では2週間ほど続く活動が観測された。これらのいずれの活動においても，地震発生域が広がっていく拡散現象が観測された。拡散係数はPeriod 1では約1 m²/s，一方，Periods 3，4の活動では約0.1 m²/sと一桁小さく見積もられた。拡散現象の継続時間についてはPeriod 1では数時間～半日程度であったのに対して，Periods 3，4では約2週間と長く継続した。拡散係数と継続時間の負の相関は先行研究の指摘と整合する。Period 1の活動で見られた拡散現象はその拡散速度や数時間～半日程度という比較的短い継続時間からゆっくりした断層すべりに伴ったものであると推定される。震源分布のフラクタル次元が2に近い値を示すこともその見方と整合する。一方，Periods 3，4の活動で観測された拡散速度は，2024年能登半島地震（M 7.6）に先行した群発活動で見られた拡散現象の速度とほぼ同じであり，破砕帯内への水の浸透によって生じたと考えられる。先行的活動のG-R則のb値は低い値を示し，特にPeriods 3，4の活動のb値は0.53 ± 0.06と低い値だった。低いb値は高応力場であることを示しており，また，先行的活動はM 6.6の地震の震源近くで発生したとみられることから，震源付近は地震発生前に高応力場であったと推定される。能登半島西方沖のM 6.6の地震の観測波形からは，その規模としては小さな振幅で始まって，次第に振幅が増大していった様子が見てとれる。震源付近で高応力場を支えていた小アスペリティが壊れることによって，水の浸透などによって全体として断層強度が弱化していた先行的地震活動域ですべり運動が生じ，その後，既存断層に沿って破壊が進展・拡大していったと考えられる。

 In Fugaku Sanjurokkei (Thirty-six Views of Mt. Fuji) by the Japanese Ukiyo-e artist of the Edo (1603-1868) period, Katsushika Hokusai, Bushu Tamagawa depicts a majestic Mount Fuji as seen from the left side of the Tama River. However, the depiction is simplified; only a few objects are drawn and little information is available on the location. Therefore, many previous studies state that the location is unknown. Although there are two candidate locations, Fuchu and Chofu, in the western suburbs of Tokyo, which have long been known as scenic spots along the Tama River, Mt. Fuji is located behind the Tanzawa Mountains, which hide its foot. This makes them unlikely locations for “Bushu Tamagawa” in which the entire foot of the mountain is represented. An attempt is made to identify the location of “Bushu Tamagawa,” as portrayed by Katsushika Hokusai. The shape of Mt. Fuji as viewed from the Ferry of Hino on the Tama River best matches that of Bushu Tamagawa. This theory is consistent with Hokusai's travel history, historical background, and geographic background. Therefore, the location is concluded to be “the Ferry of Hino” on the Koshu Road (a road connecting Edo and Kofu in what is now Yamanashi Prefecture to the west of Tokyo) at that time.