Asabata flood control basin located in about 5 km north of the central part of Shizuoka city was constructed for water control of the Tomoegawa River. Monitoring was conducted with regards to water temperature, pH, and concentrations of chlorophyll a, suspended solids (SS) and nutrients (nitrate, nitrite, ammonium, phosphate and silicic acid) at three sites in the 3rd building construction area of the Asabata flood control basin every week from April 21, 2014 to January 15, 2015 to record the water quality before improvement works. Silicic acid was highest (86 ~ 944 µmol L−1) among the nutrients through the observation period, and ammonium (0.52 ~ 83.0 µmol L−1) was higher than nitrate (≤ 0.04 ~ 46.2 µmol L−1) and nitrite (0.06 ~ 3.38 µmol L−1). Phosphate concentration was relatively low and at constant (≤ 0.005 ~ 2.91 µmol L−1) compared to nitrate, nitrite and ammonium that increased after heavy precipitations and in winter. Concentrations of chlorophyll a and SS changed little compared with those measured 10 years ago, but their highest values are comparable (SS) or exceed (pH) the environmental limit levels required to the Tomoegawa River.
We try applying a regularized regression method of “elastic net”, which naturally combines L2 regularization (for smoothing) and L1 regularization (for sparsity) with two hyperparameters, to inversion analysis of crustal deformation using on-land GNSS data. For an example of the Tokai slow slip event in the early 2000s, we obtain an overall consistent solution with previous studies for seismic moments. The Tokai slow slip event started shortly after the 2000 Izu islands earthquakes, and ended under screen of the 2004 Kii-Hanto earthquake and their postseismic deformation. For the other example of the 2011 Tohoku earthquake, we also obtain a similar solution with previous studies for seismic moments, but the estimated large slip areas are mainly located near the trench. This point is far from previous inversion studies using on-land GNSS data.
Numerical simulation of isothermal decompression-driven crystallization was performed for the basaltic melt of the 1707 eruption at Fuji volcano (SiO2 ~ 51.5 wt.%) by using “rhyolite-MELTS” program under conditions of temperature of 1184–1094°C, initial melt H2O content of 0.5–3.0 wt.%, initial pressure of 150 MPa and redox state of Ni-NiO oxygen buffer, respectively. During decompression, most abundantly crystallized phase is plagioclase and amounts of crystallized olivine, pyroxenes and magnetite depend on temperature. As initial temperature decreases from 1184°C to 1094°C, the onset pressures of degassing and crystallization (Psat)increase from ~3 MPa to ~90 MPa and from ~2 MPa to ~40 MPa, respectively, and the final weight fraction of melt at 0.1 MPa (F1atm) decreases from 0.93 to 0.4. The relation between melt fraction and pressure is chiefly controlled by Psat and F1atm, and both Psat and F1atm depend on temperature alone. This indicates that decompression-driven crystallization is essentially controlled by temperature of melt. During decompression, residual melt composition is not changed significantly at temperature higher than ~1120°C whereas melt SiO2 contents increase up to ~55 wt.% at 1114°C and ~60 wt.% at 1094°C, respectively, chiefly due to magnetite crystallization. Changes in magma viscosities (ηmagma) during decompression are estimated from melt composition, H2O content, temperatue, and crystal volume fraction; it increases as pressure decreases at all temperature, but –dηmagma /dP drastically increases as temperature decreases. Rapid increases of ηmagma and crystal volume fraction during decompression facilitate viscous fragmentation and preclude outgassing, resulting in more intense explosive eruption of H2O-rich low-T basaltic melt. Present results suggest that preeruptive temperature may be an essential factor to control eruption dynamics of basaltic magma at Fuji volcano.
Hayachine-Miyamori ophiolite is an Ordovician arc ophiolite located in the northwestern margin of the South Kitakami Massif. The ophiolite has been divided into an aluminous spinel ultramafic suite (ASUS) and a chromite-bearing ultramafic suite (CRUS) depending on petrographic and mineral chemical features. The Hayachine complex is mostly composed of ASUS. The Miyamori complex is mostly composed of CRUS peridotites and pyroxenites with 1–2 km-size patchy domains of ASUS. In this study, structural analysis of the Hayachine-Miyamori ophiolite was carried out in order to examine the origin of peridotite. The peridotite samples were taken from the Hayachine ASUS and Miyamori ASUS. These peridotites are harzburgites-lherzolites and show coarse grained textures (1–3 mm), undulatory extinction, irregular grain boundaries and exsolution lamellae in pyroxene crystals. There is no distinct difference in the chemical compositions and olivine crystal-fabrics between the Hayachine ASUS and the Miyamori ASUS peridotites, though they are geochemically distinct. Spinel compositions have relatively low Cr# (0.13–0.31) and lower Ti contents (0.02–0.05). The olivine crystal-fabrics were quantified using VP-Flinn Diagram and show A-type and AG-type patterns (Fabric Index Angle: 7°–53°, VP anisotropy: 5.7–8.9%). J-index values show relatively weak concentrations less than 3.5, possibly due to dynamic recrystallization. It shows that the studied ASUS peridotites preserve textures and crystal-fabrics deformed under higher temperature such as solidus condition in the uppermost mantle.
Diffusion timescale estimated from a chemical zoning profile in a phenocryst mineral indicates the time between formation of chemical heterogeneity in the crystal and eruption. Therefore, diffusion timescales offer important constraints for understanding pre-eruptive magmatic processes. In the last two decades, many studies have reported data of diffusion timescales estimated for phenocrysts of various mineral species in both pyroclasts and lavas from worldwide. In this study, we compiled published data of diffusion timescales with information of eruption characteristics such as eruption style, eruptive volume, and chemical composition of magma, and examine relationship between diffusion timescales and eruption characteristics. Our results suggest that (1) the estimated maximum and minimum diffusion timescales depend on species of mineral and element used for diffusion modeling, (2) estimation error of diffusion time is at least larger than ~±0.4 log unit, which should be considered into account when diffusion time is compared with geophysical observations, (3) each phenocryst records information about its own growth history, not a history of whole magma chamber, as chemical zoning, (4) diffusion timescales estimated for phenocrysts in pyroclasts of caldera-forming eruptions are systematically longer than those of non-caldera-forming eruptions, although number of data is small for non-caldera-forming rhyolitic eruption; this discrepancy may be attributed to differences of magma chamber volumes and/or mechanism of eruption triggering between caldera-forming and non-caldera-forming eruptions.
Volcanic and tectonic history of the Median Uplift Belt (MUB) of the North Fossa Magna region in central Japan is not well understood. Our study focused on the Kirigamine area in the southern part of the MUB, where the Lower Pleistocene (mainly Calabrian) Enrei Formation (Fm.), one of the main constituents of the MUB, is widely exposed. Our work reveals the following events recorded in the Enrei Fm. The basement rocks of MUB, mainly composed of the Miocene volcani-clastic rocks and minor intrusive bodies, were regionally uplifted during the Pliocene and Early Pleistocene. In the late stage of this uplifting, about 2.0 to 1.3 Ma, faulting produced a large inland basin, more than 15 km in N-S length and about 12 km in E-W width with the depth of about 250 m or more. The basin, fringed by irregular, polygonal and high-angled extensional fractures, was filled with mafic lavas and volcaniclastic rocks of the Lower Enrei Fm. forming abut-type unconformities on the fracture surfaces. After the deposition of the Lower Enrei Fm. the NW-trending volcanoes, including the Kirigamine and Yashigamine volcanoes, produced violent bimodal volcanic rocks of the Upper Enrei Fm in the central part of the Kirigamine area. The volcanic rocks form the present-day low-relief highland landform. Regional stress regime changed at about 0.8 Ma which is responsible for the formation of a new collapse basin in the central part of this area, the Daimon-Oiwake Graven (DOG), about 16 km in WNW-ESE length and 3–4 km in ENE width. Both sides of the DOG are now bordered by high-angled normal faults with the vertical-slip displacements of 200 m in maximum. These faults and minor folds in and around the DOG do not support a uniform stress regime during the basin development, but suggest locally changing compressive and extensional stresses in complex manner. The Kirigamine area is bounded by the NW-striking Itoigawa-Shizuoka Tectonic Line (ISTL) to the SW. Previous workers suggested that the Suwa Basin on the SW of ISTL is an active pull-apart basin produced by the Late Quarternary leftlateral strike-slip faulting. Our study shows no evidence supporting the strike-slip faulting in this area. Instead, we propose that a flexure structure of the Enrei Fm. plunged basin-ward (i.e. SW-ward) in response to reverse faulting. Our proposed interpretation is supported by the seismic reflection data in the area.