A method of simultaneously estimating hypocenter parameters, station corrections, and velocity parameters has been developed for a velocity model with an arbitrary dipping layer boundary. The seismic ray is traced two-dimensionally by defining an equivalent ray path in a flat-layered velocity model for each combination of source and receiver. The partial derivatives of the travel time with respect to the perturbations in the hypocenter parameters and velocity model parameters are found to be expressed in analytical forms. These advantages are utilized in developing a time-saving computational procedure for the simultaneous estimation. The computer program coded according to the above procedure is tested for an artificial data set. The arrival-time data are stably inverted for true hypocenters and a true velocity model. Thus, it is concluded that the method works well, at least for an ideal data set. The present method may be used in estimating the gross image of the lateral variation in the crust of the velocity structure.

抄録全体を表示

The rupture characteristics of the 1983 Nihonkai-chubu (Japan Sea) earthquake (M_JMA 7.7) were investigated using strong motion accelerograms recorded at 10 stations with epicentral distances of between 80 and 280 km. The main shock emitted a large amount of high-frequency seismic energy in two stages, forming two high-amplitude envelopes on the accelerograms. The time difference between S-wave arrival-times of the two events becomes larger as one moves clockwise in azimuth from north to south. Using the time differences, the second event was located 44 km NNE of the first one with the time of origin 26 s after the first event. The azimuthal variation in the amplitude ratio of the two events is consistent with the relative locations. The strong motion accelerograms, combined with the aftershock distribution and the source process time of 63 s obtained from long-period surface waves, suggest the following rupture model. The first event initiated at the southern end of the aftershock area and extended in a direction of N15°E. It stopped near the zone of low after shock activity west of Kyuroku Island. After a pause of about 10 s, the second event started at a place just north of the zone of low aftershock activity and extended in the same direction. A third event initiated near the place where the strike of the aftershock distribution changes from N15°E to N15°W. A low rupture velocity and the paucity of small-scale barriers are possible reasons for the low radiation of high-frequency energy from the third event. The above rupture characteristics appear to be closely correlated with the heterogeneous crustal structure as revealed by other geophysical and geological data in the source region. The observed S-wave accelerations of the first and the second events were jointly inverted for the source acceleration spectra, the attenuation coefficient of Q_β, and the amplification factors of the recording sites. The source spectra of the two events are almost the same, showing a rapid decay of amplitude at frequencies higher than 4 Hz. The Q_β increases in proportion to the frequency from 66 at 1 Hz to 1, 026 at 16 Hz. The local stress drops of subsources in terms of the stochastic source models are estimated to be 380 bars for the first and 340 bars for the second event. The estimates are model-dependent and can vary by as much as a factor of 1.5. The second event being more efficient than the first in radiating high-frequency seismic waves can be interpreted as having smaller subsources.

抄録全体を表示

Recently, activation is increasing. Accordingly, various disasters occur. Among them, a debris avalanche has heavy effect for people who are living around . Therefore, the observation method to surveillant surface is necessary. In this study, we focus on to surveillant amount of ash using excavator. Light and small observation tool is better than heavy tool in case of surface. Because of we would like to move the observation tool to upper position of surface. If the observation tool is light and small, the autonomous carrier like multi copter can move it. Our group develops light small excavator by slider mechanism and rotary drilling.

抄録全体を表示

Nantai , northeast Japan, is a stratovolcano whose recent eruptive activity is poorly known. This paper presents the characteristics and ages of a newly discovered Holocene succession of volcanic and sedimentary units exposed at the northern sector of the present summit crater of Nantai . The stratigraphic relationships and three accelerator mass spectrometry (AMS) ¹⁴C ages for eruptive products and lacustrine deposits of the northern sector of the summit crater show that Nantai was active after the 15-14 cal ka BP eruption, which was thought to be the latest eruption of the . After the 15-14 cal ka BP eruption, the summit crater was filled by a lake in which a water-chilled volcanic and tuff breccias (Goshinbutsunagi Volcanic Breccia) and the overlying lacustrine deposit (Nantai Lacustrine Deposit 1: NLD1) were emplaced. The ¹⁴C age of ∼ 8 cal ka BP obtained from a wood fragment in the bottom of NLD1 defines the initiation of the deposition of NLD1 in the crater lake. The crater lake dried up before 7 cal ka BP, following which a phreatomagmatic eruption occurred in the summit cater, forming Nantai-Yudonoyama Tephra (NYT). NYT is the latest recognized eruption product of Nantai , and it underlies the second lacustrine deposit (Nantai Lacustrine Deposit 2) formed in the renewed crater lake. The young age of 7 cal ka BP obtained from the two charred wood stumps in NYT highlights that Nantai is an active .

抄録全体を表示

We conducted textural and chemical analyses of melt inclusions and their host plagioclase crystals in the scoria of the Izu–Omuroyama monogenetic

, erupted at ~ 4 ka in the Higashi–Izu monogenetic volcanic field, Japan. The groundmass melt was andesitic with ~ 59–61 wt% SiO₂, and it contained abundant microphenocrysts of olivine and plagioclase. In contrast, ~ 59% of the plagioclase–hosted melt inclusions have rhyolitic compositions with ~ 70–75 wt% SiO₂. The host plagioclase phenocrysts have cores with An# of 44.7 ± 4.2 [An# = 100Ca/(Ca + Na) in mol] and rims with An# of 68–78, and the calcic rims have compositions similar to the microphenocrysts. The cores of the host plagioclase phenocrysts have FeO* and K₂O contents that are in equilibrium with the rhyolitic melt inclusions. Using the plagioclase–melt geohygrometers and assuming temperatures of 790–850 °C, we estimated the H₂O contents of the rhyolitic melt inclusions to be ~ 4.4–10.2 wt%, indicating H₂O–saturation depths of >4.5 km. Our results suggest that an inhibited reservoir of plagioclase–bearing rhyolitic melt existed beneath the monogenetic at the time of the scoria eruption, which was ~ 800 years earlier than the first rhyolitic eruption in the volcanic field. Plagioclase content in the silicic reservoir is estimated to be less than 35.8%, suggesting the magma was eruptible. Our results demonstrate the potential usefulness of plagioclase–hosted melt inclusions for indicating the existence of such an inhibited silicic magma.

抄録全体を表示

A synthesis of available geological, petrological, and geophysical data was made to construct a model for explaining the relationships between the late Quaternary volcanism (the Higashi-Izu monogenetic group, HIMVG) and tectonics in and around the Izu Peninsula, Japan. The Izu Peninsula and adjacent areas can be divided into three tectonic blocks: the western Izu (W-Izu), eastern Izu (E-Izu), and Oshima-Zenisu blocks. The vents or intrusions of the HIMVG are formed as open cracks aligning with the nearly NW-SE azimuth of horizontal maximum compression of regional stress in the E-Izu block. This stress field is mainly controlled by the NW-SE compression caused by the strong mechanical coupling of the buoyant Izu-Bonin arc with the Japan arc. In spite of this compression, the upper crust of the E-Izu block is under weak lateral constraint on northeastward extension. Both the plate geometry involving an intra-plate fracture, the West Sagami Bay Fracture, and the existence of block rotations in the northern E-Izu block probably enable the northeastward upper crust extension in the E-Izu block, and make a monogenetic field, where a new dike is formed in the upper crust at each eruptive or intrusive event.

抄録全体を表示

The volcanic island of Kuchinoerabujima recently erupted on Aug. 3, 2014 and on May 29, 2015. After the 2014 eruption, a gradual increase of sulfur dioxide (SO2) flux from the was observed. In response to the increase, we built a UV spectrometer system that can be operated semi-automatically without an experienced observer for repetitive flux monitoring. The monitoring was carried out based on a traverse method using a regular service ferry as a carrier. With the repetitive measurements, we detected a sudden flux increase to over 2500 ton/day at the end of Nov. 2014, a high and variable flux after mid-March to early-May 2015, and a continued decreased flux (<500 ton/day) before the May 29, 2015 eruption. The detected SO2 flux variations showed that a large amount of magma was involved in gas emissions at the , the conduit system became unstable from mid-March, and the conduit system became partially sealed about one week before the 2015 eruption, which was probably the turning point toward the eruption. The repetitive SO2 flux monitoring using a regular public transportation made it possible to detect important changes of flux before the eruption and was very effective for understanding the process toward the 2015 eruption.

抄録全体を表示

Dynamo action of the turbulence which is highly anisotropic under the influence of Coriolis and Lorentz forces, is investigated. The turbulence is taken to be driven by a random body force which is nearly homogeneous, isotropic and mirror-symmetric. Both the rotation and the magnetic field cause the intensity of the turbulent motion parallel to the rotational axis (or the line of force) to become larger than that of the perpendicular components by a factor √2.
The mean electromotive force of the anisotropic turbulence is evaluated separately to isolate the effects of the rotation and the magnetic field. The locally homogeneous turbulence creates two kinds (α-, and β-type) of electromotive forces under the influence of the rotation. The α-type includes a term proportional to the mean field (α-effect), and the β-type a term proportional to Ω×J (Ω is the rotation rate and J the mean current density). It is found that in the highly rotating systems both α-effect and the Ω×J term are effective for the dynamo action. The magnetic field results in only the effect of producing the magnetic helicity which can be retained in derivation of the α-effect for the isotropic turbulence.

抄録全体を表示

At Oosima , Izu, various geomagnetic studies have been continued since its great eruption of 1950-51. And also the results of geophysical and geological investiga-tions of the structure of Oosima have been accumulated. In 1957, the present author discussed the anomalous change in geomagnetic field observed in the 1953-54 eruption. In the present paper the author would revise this discussion reflecting upon the results of 10 years' observation of magnetic declination and the accumulated re-sults of the subsurface structure of the . The conclusion is that the observed anomalous changes are interpretable by the possible thermal processes beneath the . Its quantitative results on heat supply to the may lend a sugges-tion as to energetics of volcanisms.

抄録全体を表示

On August 3, 2014 and May 29, 2015, eruptions occurred at the Shindake summit crater of Kuchinoerabujima , which is located in the Ryukyu Islands in southwestern Japan. The Japan Meteorological Agency upgraded the Volcanic Alert Level (VAL) to 3 (restricted zone within 2 km from the summit) after the 2014 eruption and to 5 (evacuation) after the 2015 eruption. The possibility of implementing early warnings for eruptions and forecasting the area most likely to suffer damage from volcanic eruptions was examined based on monitoring data and the disaster areas of historic eruptions. A long-term process spanning 15 years and a relatively short-term process that could have been implemented immediately before the onset of the 2014 eruption were identified. The seismicity, which mostly represented extremely shallow (less than 0.5 km) -tectonic earthquakes beneath the summit crater, increased from July to December 1999. After the first increase in seismicity, several repeated bursts of seismicity occurred. Repeated ground inflation events around the crater detected by continuous GNSS were associated with the increase in seismicity. Increases in the activity were also accompanied by increases in geothermal and fumarolic activity. An alert zone of 1 or 2 km (VAL 2 or 3) from Shindake crater was established immediately after three bursts of volcanic earthquakes occurred during the precursory period of the 2014 eruption. However, there were no definite indications to trigger an upgrade to the VAL before the 2014 eruption because the relatively high volcanic activity had been stable and the precursory tilt change began only a short time before the eruption. The phenomena prior to the 2015 eruption were more intense than those prior to the 2014 eruption, as demonstrated by the seismicity, which included a felt earthquake six days before the eruption; the larger ground deformation; the higher rate of discharge of SO2 gas; and the higher temperature. Among these precursors, the felt earthquake could be considered an appropriate indication to trigger an upgrade of the VAL from 3 to 5 for evacuation because felt earthquakes suddenly increase the seismic energy, whereas other parameters showed gradual progress. In historical cases, a few felt earthquakes were reported prior to the eruptions in 1931 and 1966. The felt earthquake before the 2015 eruption was induced by the accumulation of other eruption-related parameters, such as an increase in the SO2 discharge rate, the inflation of the , and the appearance of volcanic glow at the summit. Decreases in the seismicity, SO2 gas discharge rate, and geothermal activity led to an initial reduction of the alert zone radius to less than 2.5 km in October 2015, and a further reduction to less than 2 km (VAL 3) was later implemented based on the deflation around the summit area in June 2016.

抄録全体を表示

The remarkable eruption on Kuchierabujima, which occurred on May 29, 2015, had the potential to disrupt land seismic observation if the volcanic activities had expanded. Since relatively deep--tectonic (DVT) earthquakes under the may have extended into the sea region around the island, we deployed four ocean bottom seismometers (OBSs) on the seafloor at distances at approximately 6.5 km from the crater. During the OBS observation period from July 3 to October 9, 2015, 303 volcanic earthquakes were detected. The daily numbers and the temporal amplitude changes indicate that seismic activities declined in the middle of August. However, only nine of the events were DVT earthquakes, and we were unable to identify any clear differences between the hypocenter distribution characteristics of the DVT events discussed in previous studies and those of our observations. However, since the temporal tendency of daily earthquake numbers derived from our OBS data resembled that compiled by the Japan Meteorological Agency using real-time land data, when eruption expansions occur on a small-scale remote island such as Kuchierabujima, OBS observations can provide a useful backup to efforts aimed at grasping the seismicity.

抄録全体を表示

Major dacite pumice eruptions began at Usu on August 7, 1977. The relationship between the change in temperature of one of the associated hot springs and earthquake frequency of the has been statistically examined using the data obtained from 1975 to the onset of the eruption. It is apparent that the coefficient of correlation between them markedly increased with time before the eruption. It may be a useful indicator to predict volcanic eruptions.

抄録全体を表示

Rockmagnetic studies were conducted on three boring cores drilled at Bandai of Fukushima Prefecture. The cores include the debris avalanche deposit of 1888 collapse. Debris avalanche deposits in BD 1 and BD 2 cores, with the thickness of 20m and 80m respectively, show a stable remanent magnetization with fairly constant inclination. It suggests that the 1888 debris avalanche deposit was transported without significant deformation, i.e., slid a distance of 1 to 2km as a block to rest on the area that is BD 1 and BD 2 at present. In the BD 3 core at the center of collapse area, the magnetizations of the samples show appreciable deflected and scattered directions from the geomagnetic inclination, suggesting that both the debris and the underlying volcanic sequence have been severely fractured.
Rockmagnetism was also studied on the hummocky hill of Okinajima debris avalanche, 5km from the crater. Eight samples collected from the outcrop (20m high and 80m wide) showed the concentrated direction of remanent magnetization. It indicates that hummocky hill of the Okinajima debris avalanche (collapse at 80-90ka; Suzuki, 1987) was transported 5km as a sliding block.
Thus, the rockmagnetic method is proved to be useful in the study of transportation and depositional process of mega-block debris avalanches as a huge landslide event.

抄録全体を表示

In this study, airborne ash dispersal from the eruption of Kuchinoerabujima in Japan on 29 May 2015 is simulated using the volcanic ash tracking PUFF model. A major eruption occurred at 9:59 am local time (0:59 UTC), and the ash plume reached 9 km above the ground. According to MTSAT-2 satellite monitoring, the ash plume with a mushroom-shaped distribution drifted in the east-southeast direction. The observed ash fallout over Yakushima Island is in the order of 100 g/m2. Using this observational fact, we estimated that a particle in the numerical model represents 60 tons of ash mass. Based on this number, total emissions are estimated to be 600,000 tons for this event. The result suggests that the ash concentration in the air can also be estimated by this method. According to the aviation safety report, an ash density of 4 mg/m3 and above is considered as a danger zone for commercial airliners. The PUFF model simulation shows that the core region of the plume front at 9 km in height indicates 100 mg/m3 of airborne ash density even for the plume 250 km from the erupting . The satellite images show the exact location of the ash plume. The PUFF model simulation can therefore estimate the airborne ash density over a much wider diffused area to identify the danger zone for aircraft.

抄録全体を表示

Apparent coseismic temperature changes in the groundwater of a well located near the Usu , Hokkaido, Japan, have been observed at the time of occurrence of volcanic earthquakes. These changes were not necessarily associated with all earthquakes but were likely to be associated particularly with earthquakes having the following two characteristics:
1. Earthquakes with large seismic intensity
2. Earthquakes occurring outside the outer rim of the Usu .

抄録全体を表示

 It has been found statistically that the occurrence of thunder clouds in the neighbouring area of the Asama was significantly less frequent in summers when the had frequent eruptions than in summers when the was less active. The average occurrence frequency of thunder clouds in the former case has been about a half of that in the latter case.
 Results of analysis on this fact suggest that frequent eruptions of the have modified the nature of cumulonimbus and suppressed the occurrence of thunder and lightning through the effect of some cloud-physical mechanism induced by seeding of the volcanic dust particles.

抄録全体を表示

The subsurface structure of Oosima is discussed mainly from the geophysical standpoints on the basis of the available data. First the author summarizes the results of gravimetric surveys and explosion-seismic observations carried out on and around Oosima Island to discuss the deeper structure of the and then proceeds to a discussion of the caldera structure utilyzing the results of the drillings, electric prospecting and gravity measurements. Finally a tentative model of the shallow subsurface structure of Oosima is presented.

抄録全体を表示

The structure of the seismic wave attenuation zone beneath the Yakedake in the Hida Mountain range of central Japan was disclosed by a temporary seismic observation array across the mountain range. The amplitudes of P waves from four distant offset explosions showed abrupt decay of seismic wave amplitude under the . However, the amount of decay was dependent strongly on the azimuth. This suggests the existence of a very localized attenuation zone at a shallow depth. To explain consistently these observed data, a vertical wall-like attenuation zone that extends beneath the in the strike of N33°W is suggested. The attenuation ratio that equals the product of the attenuation coefficient and the width of the zone is 0.6. The Q derived from this value is estimated as 30 at 15 Hz if we assume the width of the zone as 2 km. The estimated depth of the top of the zone is about 1 to 2.5 km below the surface. The location coincides well with not only a high geothermal belt near the but also with the active fault that stretches southeast from the .

抄録全体を表示

The large-scale landslide landforms, wish source areas wider than 1 km were mapped by air-photograph interpretation in the Tohoku district between latitude 38°N and latitude 42°N. On the basis of the mapping, characteristics of the distribution of those large-scale landslide landforms are discussed from geomorphological and geological points of view.
Six regions are recognized as those of concentration in the central part of the Ou Mountains, the Shirakami Mountains, the Hinotodake-Kamura Mountains and the northern part of the Asahi Mountains. Those regions mostly coincide with the areas of the maximum uplift during the Quaternary time in the Tohoku district. Those are also underlain by the zones of the Lower and Middle Miocene sedimentary rocks and submarine volcanic rocks, commonly called “Green Tuff”, and the Quaternary volcanic rocks. The topography in those zones are characterized by cap rock structures.
The large-scale landslide landforms of the Quaternary volcanic areas are dominantly found in the old and/or dormant volcanoes, and are roughly divided into three types from the main contributing factors of landsliding, the volume in source area, morphology and so on.
(1) Type 1 is a catastrophic landslide or collapse of a volcanic cone associated with volcanic activity. Its tophographic and structural feature is composed of the so-called the collapse caldera of the source area and hummocky relief of the deposition area. The latter is a typical topographic feature of volcanic dry avala nche deposits composed of megablocks and interstitial fine material. The volume of the source area is the largest of the three types. Examples can be seen in Iwate , Iwaki , Gassan , Cheikai , Zao and Shirataka .
(2) Type 2 shows a typical landslide landform associated with a gentle arc-shaped main scarp and a moving mass. Main scarp of this type is found on the slope of a thick lava flow near the top of the .
The activity of the volcanoes, where the landslides of this type occurred, ceased in the late Pleistocene and seems to be almost dormant at present.
The contributing factor to the landsliding may be a large quantity of water supply related to the late Quaternary climatic change.
Water are prepared in the form of the perennial snow patches and are supplied by the rapid melting of the perennial snow patches with the rising air temperature. The formative age of the perennial snow patches ranges from the late Pleistocene to the eariest Holocene (period about 18, 000y. B. P. to 8, 000y. B. P.).
Because the sea level of this period is lower than that of the present, mountains were vigorously dissected. Then the stability of the mountain slopes rapidly deteriorated and a number of large-scale landslides occured.
Those landslides are found in Hachimantai , Yakeishi , Kurikoma , Funakata , Gassan and Zao .
(3) Type 3 is recognized along the dissected vally wall in the pyroclastic deposits and pumice fall deposits. Type of movement is mostly rotational. The volume in source area is smaller than that of the type 2. However, the same contributing factor may be important for the landslides of type 3 as the type 2.

抄録全体を表示

A series of seismological observations of explosions from 1972 to 1977 has been worked out to study underground structure and possible anomaly of wave propagation beneath and around the Sakurajima and the Aira Caldera as well as regional structure of south Kyushu. Results described in this report are summarized as follows:
1. Name, velocity and bottom depth of the identified layers are, L-1, 3.7-3.8km/sec, 0.7-1.8km; L-2, 4.7-4.9km/sec, 3.3-5.6km; L-3, 5.6-6.1km/sec, 22km; L-4, 7.0km/sec, (40km); L-5, (7.8km/sec).
2. Two fan-shooting observations revealed following anomalies of wave propagation.
1) A large attenuation of the amplitude of seismic waves occurs under the Sakurajima and the Aira Caldera.
2) Wave velocity is likely to decrease under the Sakurajima .
3) These anomalies of wave propagation occurs at the 6km/sec layer.

抄録全体を表示