Hot Springs Research Institute of Kanagawa Prefecture

Odawara, Japan

Hot Springs Research Institute of Kanagawa Prefecture

Odawara, Japan
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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.


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.


Yukutake Y.,Hot Springs Research Institute of Kanagawa Prefecture | Honda R.,Hot Springs Research Institute of Kanagawa Prefecture | Harada M.,Hot Springs Research Institute of Kanagawa Prefecture | Aketagawa T.,Hot Springs Research Institute of Kanagawa Prefecture | And 2 more authors.
Earth, Planets and Space | Year: 2011

Seismic activity in the Hakone volcano at an epicentral distance of 450 km was remarkably activated just after the 2011 off the Pacific coast of Tohoku Earthquake. More than 1600 events were observed in the caldera of the volcano, from 15:00 on March 11 to 12:00 on April 2. To clarify the relationship between the occurrence of the main shock and the induced activity in the Hakone volcano, we investigated the spatial distribution of hypocenters and temporal changes of the seismicity, and we examined seismographs of the main shock to identify small local events during the passage of the surface waves. Hypocenters determined with the double-difference method are mostly distributed in the N-S direction, showing several clusters of seismicity. Focal mechanisms of major earthquakes are predominantly strike-slip having the P axis in the NNW-SSE direction. These features of the hypocenter distribution and the focal mechanisms are consistent with those of earthquakes that occur ordinarily in the Hakone volcano. The onset of the local event was initiated during the Love and Rayleigh waves from the main shock, suggesting that large dynamic stress changes of 0.6 MPa dominantly contributed to initiate the sequence of seismic activity. Copyright © The Society of Geomagnetism and Earth, Planetary and Space Sciences (SGEPSS).


Yukutake Y.,Hot Springs Research Institute of Kanagawa Prefecture | Ito H.,Hot Springs Research Institute of Kanagawa Prefecture | Honda R.,Hot Springs Research Institute of Kanagawa Prefecture | Harada M.,Hot Springs Research Institute of Kanagawa Prefecture | And 2 more authors.
Journal of Geophysical Research: Solid Earth | Year: 2011

A swarm earthquake sequence is often assumed to be triggered by fluid flow within a brittle fault damage zone, which is assumed to be highly permeable. However, there is little seismological evidence of the relation between the fluid flow within the fault damage zone and the occurrence of swarm earthquakes. Here, we precisely determine the hypocenters and focal mechanisms of swarm earthquakes that occurred in the caldera of Hakone volcano, central Japan, using data from a dense seismic network. We demonstrate that the swarm earthquakes are concentrated on four thin plane-like zones, each of which has a thickness of approximately 100 m. One of the nodal planes of the focal mechanisms agrees with the planar hypocenter distribution. The swarm earthquakes that occurred during the initial stage of the activity exhibited a migration of hypocenters that appears to be represented by the diffusion equation. Based on the spatiotemporal distribution of the earthquakes, the hydraulic diffusivity is estimated to be approximately 0.5-1.0 m2/s. The observations imply that swarm earthquakes were triggered by the diffusion of highly pressured fluid within the fault damage zone. A burst-like occurrence of the swarm earthquakes is also observed in the later stage. These swarm earthquakes are thought to have been triggered primarily by local stress changes caused by the preceding activity. The complicated spatiotemporal pattern is thought to have been caused by the effect of the fluid flow within the high-permeability damage zones as well as the stress perturbations generated by the swarm earthquakes themselves. Copyright © 2011 by the American Geophysical Union.


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.


Yukutake Y.,Hot Springs Research Institute of Kanagawa Prefecture | Takeda T.,Japan National Research Institute for Earth Science and Disaster Prevention | Honda R.,Hot Springs Research Institute of Kanagawa Prefecture | Yoshida A.,Hot Springs Research Institute of Kanagawa Prefecture
Earth, Planets and Space | Year: 2012

We investigate the detailed distribution of hypocenters and focal mechanisms beneath the Tanzawa Mountains, central Japan, where the Izu-Bonin arc has collided into the central part of the Honshu arc. Remarkable differences are found to exist between the hypocenter distributions in the western and eastern parts. The hypocenters of earthquakes in the eastern part tend to be distributed in a horizontal zone, whereas those in the western part are distributed in a volume. The focal mechanisms in the eastern part are right-lateral reverse faulting mechanisms, and one of the nodal planes is consistent with the geometry of the Philippine Sea (PHS) plate in the region. These results suggest that most earthquakes in the eastern part occur along the upper surface of the subducting PHS plate. In contrast, the focal mechanisms in the western part, especially deep in the western part, exhibit a different feature. The stress states in these two regions are found to be significantly different. The maximum and minimum principal stress axes in the eastern part are slightly inclined, whereas those in the western part are oriented in approximately the vertical and horizontal directions, respectively. The stress field in the eastern part may be caused by a slab pull force induced from the deeper part of the subducted plate. Copyright © The Society of Geomagnetism and Earth, Planetary and Space Sciences (SGEPSS).


Yukutake Y.,Hot Springs Research Institute of Kanagawa Prefecture | Miyazawa M.,Kyoto University | Honda R.,Hot Springs Research Institute of Kanagawa Prefecture | Harada M.,Hot Springs Research Institute of Kanagawa Prefecture | And 4 more authors.
Earth and Planetary Science Letters | Year: 2013

Immediately after the March 11, 2011, M9.0 Tohoku-Oki earthquake, seismic activity increased remarkably beneath Hakone volcano, central Japan, at an epicentral distance of 450. km. The heightened seismicity was initiated during the passage of the large-amplitude surface waves from the main shock and continued over the subsequent 2 months. We obtained hypocenters and focal mechanisms of the seismic sequence, with the aim of clarifying the physical mechanism responsible for the remotely triggered seismicity. We used data from a dense seismic network containing 56 online permanent and offline temporary stations in and around the Hakone volcano. We found that the earthquakes that occurred during the passage of the surface waves are located at the lower depth limit of ordinary seismicity in the caldera. These earthquakes have larger magnitudes than both the ordinary seismicity prior to the Tohoku-Oki earthquake and the seismicity triggered after the passage of the surface waves. The focal mechanism that we determined is a strike-slip fault type with the P-axis in the NW-SE direction, which is consistent with the focal mechanisms of earthquakes that occurred after the passage of the surface waves and the tectonic stress field in the region. We also tried to detect missing events that occurred immediately after the passage of the surface waves, by using a waveform correlation technique. The detected events are distributed near the hypocenters of the earthquakes that occurred during the passage of the surface waves. The origin times of the first four events after the arrival of surface waves are consistent with the phases of the decrease in normal stress generated by the surface waves. The results suggest that the changes in dynamic stress due to the surface waves from the 2011 Tohoku-Oki earthquake contributed significantly to the initiation of the sequence of triggered seismic activity. Assuming that normal stress changes on the faults did play an important role in the triggering of earthquakes, we propose that fluid flow induced by the oscillation of permeability on the faults is the main mechanism for the initiation of post-Tohoku-Oki earthquakes beneath the Hakone volcano.© 2013 Elsevier B.V.


Yukutake Y.,Hot Springs Research Institute of Kanagawa Prefecture | Tanada T.,Japan National Research Institute for Earth Science and Disaster Prevention | Honda R.,Hot Springs Research Institute of Kanagawa Prefecture | Harada M.,Hot Springs Research Institute of Kanagawa Prefecture | And 2 more authors.
Tectonophysics | Year: 2010

We investigated precise hypocentral distribution and the mechanisms of small earthquakes in the geothermal region of Hakone volcano, central Japan, where swarm activity has been frequently observed. Earthquake swarms were remarkably prevalent in 2001 and 2006, and were accompanied by crustal deformation. We first determined initial hypocenters, using station corrections and one-dimensional velocity structures as estimated by the joint hypocenter determination method. Then, we applied the double-difference method to relocate the initial hypocenters using the differential arrival time obtained by both manual picking and waveform cross-correlation analysis. Subsequently, we determined the focal mechanisms from the absolute P- and SH-wave amplitudes and P-wave polarities. From the relocated hypocenter distribution, we found that most swarm earthquakes are distributed on thin plane-like zones with width/length of 100. m to 1. km. We found that these plane-like hypocentral distributions range from the E-W to N-S strikes and are consistent with one of the nodal planes of the focal mechanism. It is likely that most swarm earthquakes occurred on pre-existing fracture planes, shaping the plane-like hypocenter distributions. Previous studies suggested that a highly permeable fracture system has developed in the fault interaction area, and these highly permeable fractures affect the occurrence of swarm earthquakes and surface geothermal activity. Since Hakone volcano is located in the interaction area of the active Tanna and Hirayama faults, it is suggested that the fracture planes revealed by the relocated hypocenter distribution have been developed by the activity of the two faults. The fracture planes in the caldera might be channels for the hydrothermal water of hot springs in the caldera of Hakone volcano which are rich in sodium chloride and considered to be composed of fluid heated by a deep-seated magma. The strike of the fracture plane on which the swarm earthquakes of the 2001 activity occurred is inconsistent with the orientation of open crack models estimated from geodetic data. It is therefore possible that the hydrothermal fluid aseismically intruded the open cracks during the 2001 activity. © 2010 Elsevier B.V.


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.


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.

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