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Miyaji N.,Nihon University | Kan'no A.,Nihon University | Kanamaru T.,Nihon University | Mannen K.,Hot Springs Research Institute of Kanagawa Prefecture
Journal of Volcanology and Geothermal Research | Year: 2011

The Hoei eruption of Mount Fuji in 1707 caused the worst ashfall disaster in Japanese history. Despite the availability of numerous historical documents describing the eruption, the detailed eruption sequence has not been verified because the correlation between these descriptions and geological sequences remains unclear. In this study, we reconstruct the sequential change in the column height using newly established stratigraphy and a detailed timeline obtained from historical documents.The eruptive deposit was subdivided into 17 units on the basis of their facies, with the mass of each unit established using isopach maps. The eruption column height is a function of the magma discharge rate; hence, the column height of each unit was estimated from its erupted mass and duration, which were inferred from the historical documents. The correlation of each unit with the actual time was based on the tephra color and grain size, an unconformity caused by rainfall, and ashfall distribution.While reconstructing the unit boundaries, we found that not all of them represented a hiatus or relenting phase of ashfall. We detected only six obvious quiet intervals from the historical documents. Therefore, many of the unit boundaries may represent a hiatus or relenting phase that was too subtle to have been recorded in the historical documents. We define an eruptive pulse as a period of continuous ashfall followed by an obvious quiet interval. We divided the 17 units of the Hoei eruption into 6 pulses, and into 3 stages on the basis of the patterns of the eruptive pulses. The characteristics of the three stages are described as follows.Stage 1 had two energetic eruptive pulses (pulses 1 and 2; at least 20. km high column), each showing an intense initial outburst, followed by a decrease in intensity. The eruption sequence indicates ruptures of highly overpressured dacite and andesite magma chambers. The initial silicic eruption was followed by basaltic magma withdrawal from a deep and voluminous magma chamber. Stage II consisted of discrete subplinian pulses of relatively degassed basaltic magma. Although the eruption rate throughout the stage decreased, the magma supply from depth appears to have been sustained because extensive intrusion near the surface created the Mt. Hoei cryptodome near the vent. Stage III was principally characterized by sustained column activity without a clear repose time. During this stage, the column height appears to have been more than 13. km, and we recognized at least two distinct periods of increased activity in which the column height is presumed to have exceeded 16. km. The Cu-rich vesicular scoria and continuous eruption in stage III indicate a stable supply of volatile-rich magma from depth. No significant decay was observed in the magma discharge rate; hence, the eruption could have been halted by a sudden process such as conduit collapse, rather than decompression of the magma chamber. © 2011 Elsevier B.V. Source

Yukutake Y.,Hot Springs Research Institute of Kanagawa Prefecture | Takeda T.,Japan National Research Institute for Earth Science and Disaster Prevention | Yoshida A.,University of Shizuoka
Earth and Planetary Science Letters | Year: 2015

To investigate the applicability of frictional reactivation theory to active faults, we evaluated the slip tendency of active faults in Japan. Slip tendency is defined as the ratio of shear stress to the frictional resistance acting on a fault plane. To estimate the stress field near active faults, we determined focal mechanisms for numerous shallow earthquakes in the intraplate region of Japan, using data from dense seismic observation networks. The stress fields were estimated by applying the stress inversion method to the focal mechanisms. We found that most active faults are well oriented with respect to the stress field, having slip tendencies of ≤0.6, which indicates that fault reactivation theory is applicable to active faults, and that the present day tectonic stress field has contributed substantially to the development of active faults. Conversely, several steeply dipping active faults in northeast Japan, as well as the source faults of the 1995 Hyogoken-Nanbu earthquake, are mis-oriented, with slip tendencies of <0.6, which may indicate that highly pressurized fluids have contributed to earthquake triggering on these mis-oriented faults. We also discuss the applicability of slip tendency analysis to earthquake hazard assessment; i.e. estimating the probability of a future rupture on active faults. Our results indicate that slip tendency does not correlate with elapsed time since the most recent earthquake event. This observation shows that slip tendency may not be an efficient parameter with which to assess the risk of an earthquake occurring in the near future on a specific fault. © 2014 Elsevier B.V. Source

Mannen K.,Hot Springs Research Institute of Kanagawa Prefecture
Journal of Volcanology and Geothermal Research | Year: 2014

A new way to deduce particle segregation in an uprising eruption column is developed. This method subdivides an eruption column into horizontal slices, and particle dispersal from each column slice is calculated using an advection-diffusion model named Tephra2. A grid search is employed to obtain the particle segregation from each column slice and other unknown parameters (vent position, diffusion coefficient and particle density). The initial conditions for the calculation were mass loading data collected on the ground, wind profile, and minimum empirical parameters. This method is applied to the 1986B eruption of Izu-Oshima volcano, Japan, which was a sub-Plinian eruption with a vertically uprising column of ≈. 10. km high. A previous study of this eruption employed a gravity current model, which assumes that most of the tephra segregation took place at the column-umbrella transition and at the base of the umbrella cloud. The gravity current model was used to calculate the base height of the umbrella cloud from the mass loading distribution on the ground, and the calculated base height (7-9. km) is consistent with the observed column height. However, the present study shows that most of the particles segregated from the lower parts of the column (1-3. km high) and no significant fallout from the upper part of the column was observed. Even though the segregation pattern is different between the present and past studies, the total mass and total grain size distribution are similar. This coincidence is attributed to the use of the same decay function of mass loading in both models-both assume the exponential decay of mass loading as a function of the area enclosed by the isopleth, although the physical meaning of the decay rate is different in the two models. This conclusion also implies that the shape of the decay function itself cannot be used to determine which model is appropriate in describing the source column. However, the applicability of the gravity current model should be justified when the decay rate is low and when an unreasonably high diffusion coefficient is obtained by inversion techniques that use advection-diffusion models. The new approach of this study also enables the calculation of the total mass and total grain size distribution without hand-drawn isopleth maps, and uses all of the point data with error evaluations for each point data. The calculation results (e.g. the erupted mass) are shown with associated errors. © 2014 Elsevier B.V. Source

Yukutake Y.,Kyoto University | Yukutake Y.,Japan National Research Institute for Earth Science and Disaster Prevention | Iio Y.,Kyoto University | Horiuchi S.,Hot Springs Research Institute of Kanagawa Prefecture
Journal of Geophysical Research: Solid Earth | Year: 2010

We estimate the stress field after the 1984 western Nagano earthquake from numerous focal mechanisms. To precisely determine the focal mechanisms, we analyze the earthquakes that occurred around the eastern part of the main shock fault, where station coverage is fairly good. Most of the earthquakes that occurred in this part, except the main shock fault, are reverse fault types. In contrast, earthquakes that occurred near the main shock fault have T axes distributed in a belt (strike slip, oblique slip, and reverse fault types occurred equally). Using the stress inversion method to quantitatively estimate the stress field of these earthquakes revealed that the σ2 axis near the main shock fault is close to the vertical direction, whereas it is close to the horizontal direction in other regions. At the western edge of the study area, we observe that the σ1 axis rotates toward the NS direction and that the stress ratio (σ1 - σ2)/ (σ1 - σ3) is low. We estimate that the magnitude of shmin decreases near the main shock fault. Spatial variation of the stress field around the eastern part of the main shock fault is not generated by static stress changes caused by the main shock. Local stress anomalies might have occurred around the main shock fault before the main shock. Copyright © 2010 by the American Geophysical Union. Source

Honda R.,Hot Springs Research Institute of Kanagawa Prefecture | Yukutake Y.,Hot Springs Research Institute of Kanagawa Prefecture | Yoshida A.,University of Shizuoka | Harada M.,Hot Springs Research Institute of Kanagawa Prefecture | And 2 more authors.
Journal of Geophysical Research B: Solid Earth | Year: 2014

Hakone volcano, located at the northern tip of the Izu-Mariana volcanic arc, Japan, has a large caldera structure containing numerous volcanic hot springs. Earthquake swarms have occurred repeatedly within the caldera. The largest seismic swarm since the commencement of modern seismic observations (in 1968) occurred in 2001. We investigated the anisotropic structure of Hakone volcano based on S wave splitting analysis and found spatiotemporal changes in the splitting parameters accompanying the seismic swarm activity. Depth-dependent anisotropic structures are clearly observed. A highly anisotropic layer with a thickness of ~1.5 km is located beneath the Koziri (KZR) and Kozukayama (KZY) stations. The anisotropic intensity in the region reaches a maximum of 6-7% at a depth of 1 km and decreases markedly to less than 1% at a depth of 2 km. The anisotropic intensity beneath Komagatake station (KOM) decreases gradually from a maximum of 6% at the surface to 0% at a depth of 5 km but is still greater than 2.5% at a depth of 3 km. At KZY, the anisotropic intensity along a travel path of which the back azimuth was the south decreased noticeably after the 2001 seismic swarm activity. During the swarm activity, tilt meters and GPS recorded the crustal deformation. The observed decrease in anisotropic intensity is presumed to be caused by the closing of microcracks by stress changes accompanying crustal deformation near the travel path. ©2014. American Geophysical Union. All Rights Reserved. Source

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