Sentinel-1時系列解析（Cバンド，昇順トラック124，2018年4月20日-8月24日）をキラウエア火山全域で行なった．Cバンドは，植生による影響が大きく干渉性が失われやすいため，キラウエア火山東側のリフト・ゾーン下部（図中 “LERZ”）の東側はその影響が小さいALOS-2/PALSAR-2の差分干渉画像（Lバンド，トラック089，2018年1月20日-5月22日）を用いて解析した．なお，画像のカラースケール1サイクルは12 cmの変動量を表し，衛星に近づく向きに，シアン–マゼンダ–黄色の順番に変化してもとの色に戻るように表示した．キラウエア山頂南西側の縞模様は，山頂直下とカルデラの圧力源の収縮による沈降を示す．図中 “MERZ“ の縞模様は，山頂とMERZにおけるマグマの収縮による沈降を示す．これは，LERZにおける岩脈貫入と溶岩流出を引き起こした．海岸に向かう粗い縞模様はキラウェア火山南山腹において2018年5月4日に発生したM6.9の地震によるものである．
解析は，米国航空宇宙局（NASA）との契約のもと，カリフォルニア工科大学（Caltech）ジェット推進研究所（JPL）で行われた．Sentinel-1 Copernicusデータは，Copernicus Open Access Hubおよび欧州宇宙機関（ESA）より，ALOS-2/PALSAR-2データは，宇宙航空研究開発機構（JAXA）より提供され，JPLのARIAプロジェクトで処理された．ALOS-2/PALSAR-2データの所有権はJAXAにある．
（表紙：© 2019 JPL-Caltech; データ解析・説明：Paul LUNDGREN，日本語説明：田中明子）
A suitable sampling tool is indispensable for making sediment observation. Eleven portable sediment core samplers using lake sediment observations are introduced: two box core samplers, four dropping core samplers, and five pushing core samplers. Some are popular for making sediment observations. However, others are unique and are not well known, although they have excellent characteristics. Their structures, merits, demerits, and suitable lake conditions are explained. Core samplers introduced are: Ekman-Birge grab sampler, Smith-McIntyre grab sampler, Rigo gravity corer, Limnos corer, Knocking corer, Mackereth piston corer, Proto-type pushing piston corer, Seto-type undisturbed separable piston corer, Suganuma-type portable percussion piston corer, Multisampler, and Russian Peat sampler.
The Mj 6.4 Eastern Shizuoka earthquake occurred on March 15, 2011, which was 4 days after the 2011 off the Pacific coast of Tohoku Earthquake (MW 9.0). The hypocenter of the earthquake was approximately 6 km southwest of Mount Fuji's peak at a subsurface depth of about 15 km. There had not been an earthquake with a magnitude of 6 or higher in the region for the last 80 years, and many experts were concerned that the Eastern Shizuoka earthquake increased the probability of a Mount Fuji eruption. Estimating spatial variations of the b-value of the Gutenberg–Richter relation is important in examining time and spatial changes of volcanic and seismic activity after large earthquakes. Changes in seismic activity after the 2011 Eastern Shizuoka earthquake are evaluated using spatial variations of the b-value. Hypocentral data from July 2001 to December 2017 were obtained from the JMA catalog. Only data for earthquakes with focal depths of 25 km or less were selected for the investigation. As a result, the b-values for earthquakes around Mount Fuji were significantly smaller during the period after the 2011 Eastern Shizuoka earthquake than during the period from July 2001 through February 2011, and continued to gradually recover after the Eastern Shizuoka earthquake. Furthermore, in the source region of the northeastern part of Mount Fuji, the b-value increases from the southwest toward the northeast, and the spatial variations of the b-values may coincide with the density distribution of volcanic fluid.
In order to constrain the onset timing of the Median Tectonic Line in SW Japan, detrital zircon geochronology is examined for pre-Miocene undated sandstones at both sides of the subhorizontal MTL in the Mikawa-Ono area, central Japan, where the 3-D configuration of the primary MTL between the Ryoke and Sanbagawa belts is observed. In addition, another Cretaceous sandstone unit (the Idaira Formation) in the Chichibu belt, ca. 10 km to the south, is analyzed for comparison. The following new results were obtained. The undated sandstone-dominant unit, previously called the Kawachi Formation or the Nanasato-Isshiki Formation as a whole is, in fact, composed of two distinct units with contrasting characteristics of distribution, rock type, and zircon age spectrum, although both are revealed to be of the Cretaceous. The one to the north of the subhorizontal MTL (in the Ryoke belt) is re-defined as the Nanasato-Isshiki Formation sensu stricto, whereas the one to the south (in the Sanbagawa belt) is newly designated as the Rokutazawa Formation. The Lower Cretaceous Idaira Formation forms the third group with a distinct signature in the zircon age spectrum. The Nanasato-Isshiki Formation (together with the associated Atera-Nanataki conglomerate) and the Idaira Formation, overlying the Ryoke granitoids and Chichibu pre-Cretaceous accretionary complex, respectively, form autochthonous units. In contrast, the Rokutazawa Formation alone represents an allochthonous units sitting structurally on the Sanbagawa schists. Age spectra of detrital zircons suggest that both the Nanasato-Isshiki Formation and Atera-Nanataki conglomerate are roughly dated as the Late Cretaceous, and also that they are correlated with the Upper Cretaceous Izumi Group in the Shikoku and Kinki regions of SW Japan. On the other hand, the sandstone of the Rokutazawa Formation demonstrates a totally different age spectrum with full of older zircon grains of the Paleozoic and even of the Neoarchean, and with the youngest grain suggesting the Early Cretaceous age of the formation. Its distribution, rock type, and age spectra of detrital zircons indicate that this unit can be correlated with the Atogura/Maana formations within the allochthonous klippe unit in the Sanbagawa belt in the Kanto Mtn. and western Shikoku, SW Japan. These units were probably transported for a long distance on a hundred-kilometer scale from the primary depositional site in the Hida belt. The correlation of the Idaira Formation and the Lower Cretaceous strata of the Chichibu belt in other areas (e.g. the Ryoseki–Monobegawa Group in Shikoku) is also confirmed by the age spectrum of detrital zircons. All of these data suggest that the low-angle faulting of the primary has started probably some time in the Oligocene, after the deposition of the Nanasato-Isshiki Formation on the hanging wall (the Maastrichtian or later), after the emplacement of the klippe with Lower Cretaceous onto the Cretaceous blueschists of the Sanbagawa belt on the foot wall (after the Eocene), and before the deposition of the neighboring Miocene strata (the Shitara Group). These results constrained for the first time the initiation timing of the MTL, not in the Cretaceous as previously imagined, but in the Paleogene, most probably in the Oligocene.
The Kushiro Marsh in eastern Hokkaido is the largest wetland in Japan. Monitoring changes in environmental conditions is important to support preservation of the wetland. In the summer of 2016, Hokkaido was affected by several typhoons and suffered record-breaking heavy rainfall. ∼50 ALOS-2 SAR interferograms were constructed covering the Kushiro Marsh over the period 2014 to 2018, including the summer of 2016. A time series of vertical displacements of the wetland detected by the interferograms corresponded to water level changes in rivers of the wetland. This implies that the SAR data successfully detected height changes of the water surface in the wetland. Between August 6, 2016 and September 5, 2016, an area of the wetland (∼1 km) to the southeast of Akanuma shifted ∼2.7 m horizontally in the downstream direction, which coincided with a large and rapid increase in the water level caused by the heavy rains. This large displacement remained after the water level fell. Prior to this large horizontal shift, an uplift of ∼10 cm was identified in almost the same region. This uplift might have been caused by groundwater leaking from the basement of the Akanuma pond, and implies that the peat layer, which had a thickness of several meters had floated slightly, like a floating island, possibly causing the large horizontal shift.
The alkaline tephra U-Oki is widely dispersed in and around the Japanese Islands. U-2 tephra, among Holocene tephras (U-2, U-3 and U-4 tephras) on Ulleung Island, was considered to be the first candidate to be correlative with alkaline tephra. Recent studies on stratigraphy, eruption age, and chemical composition of Ulleung Island tephras and Ulleung Island-derived tephras show that U-3 and U-4 tephras, not U-2 tephra, reached the Japanese Islands. Therefore, it is necessary to confirm whether Ulleung Island-derived tephras, such as U-Oki, can be correlative with U-3 or U-4 tephras based on reliable chemical and chronological data. Such reliable correlations provide information on eruption magnitude and dispersal. Alkaline tephra (Hm-2 tephra), which is petrographically similar to U-Oki, is found as an intercalated layer along with numerous other intercalated layers of Hakusan Volcano tephra in a peat layer formed during the last 13,000 years in the summit area of Hakusan Volcano, central Japan. EPMA analyses of major element glass composition and AMS radiocarbon dating have been performed for Hm-2 tephra, showing a correlation between Hm-2 tephra and tephra on Ulleung Island. Volcanic glass shards from Hm-2 tephra have a distinctly high alkali content with high alumina and intermediate silica contents. These chemical characteristics accord with those of Ulleung Island tephra and Ulleung Island-derived tephras. AMS 14C ages obtained for peat layers just under Hm-2 tephra are 7600 BP and 8490 BP, which is consistent with the eruption age of U-3 tephra on Ulleung Island. The ages of the Hm-2 tephra indicate that Hm-2 is correlative with U-3 tephra. The correlation supports the view that tephras correlative with U-3 tephra are distributed widely in and around the Japanese Islands, as well as those correlative with U-4 tephra. A comparison of chemical compositions between Hm-2 tephra and three Units (U-3a, U-3b, and U-3c) of U-3 tephra shows that Hm-2 tephra is most similar to Unit U-3b among the three Units, indicating that a correlation is highly probable. This result suggests that Unit U-3b, in addition to Unit U-3c, also reached central Japan, which is approximately 550 km from Ulleung Island, with the eruption of U-3 tephra.
The Itoigawa–Shizuoka Tectonic Line (ISTL) fault zone is one of the longest and most active fault zones in Japan, extending for approximately 158 km from Otari Village to Hayakawa Town. The southern part of the ISTL fault zone is a ∼48 km long, east-vergent reverse fault zone, and has the potential for causing earthquakes larger than M 7. Slip rates need to be estimated, as well as the timing of faulting and amounts of coseismic displacement, to better understand seismic risk at the southern part of the ISTL fault zone. Geomorphic surveys and tephra analyses were conducted for the middle terrace surface in the Tsukuyama area, Minami-Alps City, Yamanashi Prefecture, to estimate the long-term slip rate. Based on the data obtained, the terrace-surface deposit is at least ∼40 m thick, and is directly covered by Ontake Daiichi tephra (On-Pm1), whose age is ∼100 ka. Thus, it is inferred: (1) the middle terrace in the Tsukuyama area is one of the depositional terrace surfaces developed during the 5d of the marine isotope stage (MIS); (2) dissection of the terrace surface started during the transition period from 5d to 5c of the MIS (∼100-110 ka); and (3) the vertical slip rate of the southern part of the ISTL fault zone is at least 0.9-1.0 mm/yr, because the vertical displacement is estimated at ∼100 m or larger.
Teaching materials on determining epicenters using P-wave lateral polarity data were produced. These materials are intended for use in secondary and advanced education and only require inexpensive tools such as a ruler and a protractor; they do not require specialized software or devices. The materials consist of PDF files containing three-component waveform data before and after P-wave arrivals for 929 small events recorded at Hi-net stations in the Hida region, central Japan, from February 1 to April 30, 2011. A P-wave lateral polarity analysis using the two horizontal components provides information on seismicity in the Hida region before and after the March 11, 2011 off the Pacific coast of Tohoku Earthquake. Students can discuss temporal changes in the underground stress state based on their analyses. Depending on the grade and science education level of students, the time-series waveform data allow them to conduct additional exercises such as error analyses and epicenter determinations using S–P differential travel time, as well as geophysical interpretations of results.