Seismic activity near the Yake-dake (Mt. Yake) volcano in the Hida mountain range that took place immediate after the 2011 off the Pacific Coast of Tohoku earthquake was investigated. It initiated about ten minutes after the mainshock of the Tohoku earthquake and lasted for about one month. At the beginning, two active swarms were observed. One is at the northern flank of the Yake-dake volcano and the other is located between Yake-dake and Mt. Norikura volcanoes. The latter activity decreased by March 20, and the former activity lasted until early April. It includes two M≥4.5 earthquakes and we could locate more than 9,600 events in the study area during March and April. We mainly focused on the activity near the Yake-dake volcano in this paper. Near the Yake-dake volcano, seismic activity began with M4.7 (JMA) earthquake at 14:57 JST on March 11. This M4.7 event is located 3 km north to the volcano and seismicity increased between the summit of the Yake-dake volcano and the hypocenter. On March 21, an M4.8 (JMA) event took place at 13:15 JST also at 3 km north to the volcano. After this second M≥4.5 earthquake, seismic activity migrated to the north about 1 km. Focal mechanism solutions of these swarm earthquakes show NW-SE compression stress field, which coincides with regional stress field indicated by previous studies. No temporal changes of focal mechanisms are shown during March and April, which probably indicates no magmatic activity such as dyke intrusion related to the Yake-dake volcano took place in this time period.

抄録全体を表示

の火成活動の消長と,山脈の隆起により生産された礫層の形成時期には,よい同時性が認められる.それは,2.5～1.5Ma(Stage I)と0.8～0Ma(Stage III)における火山活動の増大と周辺堆積盆への礫層の供給の時期であり,その間の1.5～0.8Ma(Stage II)には火山活動が低調で,周辺堆積盆における礫層の堆積が停止している.このことから,は,激しい火成活動を伴いながら,3Ma以降に2度大きく隆起しているといえる.
既知の広域テクトニクスの解析結果と隆起モデルから,各時期の隆起メカニズムを考察する.Stage Iでは伸張から中間応力場下での地殻へのマグマの濃集と地殻の厚化によるアイソスタティックな隆起が考えられる.一方,Stage IIIでは,この地域が圧縮場に変化し,マグマの熱によって弾性的厚さが薄くなった地殻が座屈変形し隆起したモデルが考えられる.このようには,大規模なマグマの貫入・定置・熱の影響と,伸張から圧縮場への広域テクトニクスの変化とが合わさって形成された山脈といえる.

抄録全体を表示

We performed ground penetrating radar and global navigation satellite system surveys to investigate the ice thickness and flow of the Karamatsuzawa perennial snow patch, at the village of Hakuba, Nagano Prefecture. The field survey showed that the perennial snow patch contained an ice mass with a thickness greater than 30m and the ice flowed in the slope direction at a rate of 250mm over a 29-day period in the late snow-melting season in 2018. Based on the observed ice thickness and motion, the Karamatsuzawa perennial snow patch was recognised as an active glacier. The observed ice motion was significantly greater than the internal ice deformation estimated using Glenʼs Flow Law. This implies that basal sliding contributes to the motion of the Karamatsuzawa perennial snow patch.

抄録全体を表示

A three-dimensional density structure in the shallower crust beneath the Hida Mountains, central Japan, is estimated by using gravity data. We employed seismic velocity structure models both as an initial condition and constraints of gravity structure models. We estimated that an extremely low-density body (density is smaller than 2.1 g/cm³) exists at 4-8 km depth (2 to 6 km below the sea level) beneath Mt. Tateyama (3,015 m) and also along the Hida mountains. Estimated horizontal extent of the body is about 14 km in east to west, about 28 km in north to south directions, respectively, and about 4 km in thickness. The volume of the body is about 1,000 km³. Spatial distribution of the extremely low-density body is well consistent with a low velocity region estimated from studies of seismic tomography.

抄録全体を表示

1．はじめに

　

北部では，2012年以降七つの氷河が確認されている（有江ら，2019など）．氷河の形成と変化は，降雪を主とする涵養と融解を主とする消耗の上に成り立つ質量収支の結果であり，氷河の質量収支変化の要因を理解するには，冬期収支（涵養）と夏期収支（消耗）の測定が必要である．また，冬期収支と夏期収支の絶対値の総和の半分の値である質量収支振幅は，極地や大陸性気候の氷河で小さく，海洋性気候の氷河では大きくなり，氷河の地理的特徴を示す際に有効である（Braithwaite and Hughes, 2020）．

　福井ら（2018）は， ステーク法で2012〜2016年に御前沢氷河の質量収支の観測をおこなったが，多雪年にステークが雪に埋まってしまい，年間の質量収支だけでなく，冬期収支と夏期収支の観測も実現できていない．そのため，

の質量収支の地理的特徴はいまだ明らかでない．

　そこで本研究では，セスナ空撮画像とSfM-MVS技術を用いた測地学的方法で，

の氷河の2015/16年，2016/17年，2017/18年，2018/19年の計4年間分の年間質量収支，冬期収支，夏期収支を求めた．さらに，の氷河と世界各地の氷河の質量収支振幅を算出・比較し，の氷河の地理的特徴を考察した．

2．方法

　セスナ機空撮によって取得された氷河の空中連続画像とSfM-MVS技術によって，融雪末期（10月）と積雪最大期（3月）の地形表層モデル（DSM）を作成した．氷河の規模が年間で最も小さくなる融雪末期のDSMを一年間隔で比較することで，融雪末期を基準とした氷河の年間質量収支を求めた．また，積雪最大期のDSMと，融雪末期のDSMを比較することで，氷河の冬期収支および夏期収支を求めた．

　また，質量収支振幅に関して，

の氷河の値は，本研究の冬期収支と夏期収支の結果の平均値を用いた．世界各地の氷河の値は，WGMS（2020）に冬期収支および夏期収支が記録されている188の氷河（図1）の冬期収支と夏期収支の値の平均値を用いた．

3．結果

　図2は，

の五つの氷河の年間質量収支，冬期収支，夏期収支の結果である．五つの氷河の年間の冬期収支と夏期収支の大きさは，10m（水当量）程度であった．また，冬期収支は大きな年々変動がみられ，夏期収支はほとんど年々変動がなかった．図3は，地域別の氷河の年間質量収支変動幅，冬期収支，夏期収支の平均値である．に分布する氷河の年間質量収支変動幅の大きさは，世界の氷河と比較して極端に大きかった．

4．考察

　年間質量収支の年々変動と冬期収支の年々変動が同じ傾向で変動していることから，

の氷河は，冬期収支が年間質量収支を決定していると考えらえる．また，の氷河の冬期収支と夏期収支が極端に大きいことから，の氷河は，観測された世界の氷河の中で最も降雪量が多く，温暖な環境に位置する氷河であると考えられる．

引用文献

　有江賢志朗，奈良間千之，福井幸太郎，飯田肇，高橋一徳　2019．

北部，唐松沢雪渓の氷厚と流動．雪氷，81：283-295.

　Braithwaite, R. J. and Hughes, P. D. 2020. Regional Geography of Glacier Mass Balance Variability Over Seven Decades 1946-2015. Front. Earth Sci. 8.

　福井幸太郎，飯田肇，小坂共栄　2018．

で新たに見出された現存氷河とその特性．地理学評論，91：43-61．

抄録全体を表示

の隆起については,鮮新世後期から更新世初頭(2.7～1.5Ma)にかけて,最初の極大期があったとする点では,多くの研究者の意見が一致している.しかし,その後の第四紀の期間にテクトニックな隆起があったかどうかでは,意見が分かれていた.山麓の堆積物から隆起時期を推定する方法では,山脈の隆起がテクトニックなのか,アイソスタティックなのか,判定困難なため,本研究では,爺ヶ岳一帯に分布する鮮新世後期～前期更新世の火山岩類の構造を解析した.この結果,これらの火山岩類はコールドロンをなしており,東に70°前後傾動していることが判明した.南方の高瀬川流域や槍穂高連峰での資料を加味すると,前期更新世後半(1.3Ma～)以降,東半部の広い範囲で,東西圧縮場のもとでの挫屈による傾動・隆起を生じていることが明らかとなり,の2段階にわたるテクトニックな隆起運動が明らかとなった.

抄録全体を表示

Seismic activity near Mt. Hotaka in the Hida Mountains, central Japan was analyzed by using the Matched Filter Method (MFM). In this analysis, MFM was implemented as an automatic hypocenter relocation system. We selected about 30 template earthquakes in the target region that enables us to detect more than 3,000 events and locate about 800 earthquakes in the time period from April 2013 to October 2013. Comparison with manually inspected results indicates that location errors by MFM system is within a couple of kilometers. The seismic activity in the target region started in April 2013 and most intense activity occurred in October 2013. The largest event took place on October 8, 2013 at 19:28 (JST) whose magnitude was 3.9 (JMA). Epicentral area extends about 4km in EW direction with 1km in NS direction at the eastern frank of Mt. Hotaka. Although manually inspected catalogue data is essential to evaluate seismic activity, we suppose MFM is one of the powerful tools to automatically obtain preliminary results for the swarm activity that concentrated in a small area such like this study or volcanic regions.

抄録全体を表示

Seismicity and focal mechanism of earthquakes in the northern Hida region, central Japan, are closely investigated, and seismo-tectonic features of this region are discussed, mainly on the basis of five years' data obtained at seismic network stations of the Kamitakara Geophysical Observatory in cooperation with some other observatories.
More than 5, 800 earthquakes with magnitudes greater than 0.5 have been located. It was found that high seismicity is concentrated along the Atotsugawa fault and around the Ushikubi and Mozumi faults, all of which are typical, Quaternary active faults in this region, and just beneath the Hida mountain range, which is highest in the Japan Islands. Also noticeable is relatively high seismicity around conjugate sets of minor faults northeast of the Atera fault, and in the southwest part of the Toyama plain.
Most of larger earthquakes along and around the Atotsugawa fault indicate strike-slip type mechanisms consistent with right-lateral slip motion that had been continued since Quaternary ages. Earthquakes at shallow depths beneath the northern part of the Hida mountains also show strike-slip mechanisms with a nodal plane nearly parallel to the axis of the mountain range, and sometimes with a small component of normal faulting. In the southern part of the mountains southeast of Mt. Ontake, however, two different types of earthquakes, one with strike-slip, another with reverse faulting, are concentrated in a limited area, suggesting complex structures there. All earthquakes northeast of the Atera fault indicate strike-slip mechanisms.
The maximum pressure axes (P-axes) derived from these focal mechanism solutions are found to be oriented nearly in the ESE-WNW direction (N60°-80°W) over the northern Hida region, but with small, counter-clockwise local deviations in some areas. The direction is parallel to those inferred from the conjugate sets of Quaternary faults in this region and derived from geodetic triangulation surveys, and is also consistent with the direction of relative motion between the Pacific and Eurasian plates rather than between the Eurasian and North American plates. This suggests that the northern Hida region including the Hida mountains appears strongly subjected to the tectonic compressive stress due to the plate movements.

抄録全体を表示