Active Volcanoes In Japan Research Articles

Inferring phases of thermal unrest at Mt. Asama (Japan) from infrared satellite observations

On 1 September 2004 a vulcanian eruption, the strongest since 1983, occurred at Mt. Asama, one of the most active volcanoes in Japan. An ash plume that rose 2km above the crater was emitted, together with incandescent blocks ejected several kilometers away from the summit crater. Subsequently, after a short quiescence period of about 2weeks, a series of minor eruptions took place from 14 September. They were considered unusual for Mt. Asama for the presence of juvenile material dominant among fallout. Small and moderate eruptions continued even in the following months to end in December 2004. On the basis of GPS observations, a dike intrusion occurred under the western flank of Mt. Asama about 6weeks before 1 September eruption, triggering the magma ascension towards the surface. This phenomenon produced many signs of unrest including a thermal activity observed on the ground from the end of July 2004. The retrospective analysis presented in this paper, performed applying the RSTVOLC algorithm to Moderate Resolution Imaging Spectroradiometer (MODIS) data, shows the occurrence of pre-eruptive hot spots at Mt. Asama during 1–31 August and 11–12 September 2004. These results extend observations of a previous MODIS-based independent study and show the occurrence of a distinct phase of thermal unrest, of short-time duration, a few days before the 14 September minor eruption. Moreover, by the temporal trend of radiative power from infrared data, time of occurrence of dome extrusion is also estimated, in good agreement with independent radar observations. These outcomes seem to be compatible with different mechanisms of upward propagation of magma speculated by previous authors, confirming the potential of infrared satellite observations in identifying thermal precursory signals of volcanic eruptions, provided that proper detection algorithms are used.

Analysis of Topographic Change at Mount Sakurajima, South Kyushu, Japan, using JERS-1 SAR Interferometry

Mount Sakurajima, situated in the southern Kyusyu district of southwest Japan, is known as one of the most active volcanoes in Japan. It has erupted intermittently, ejecting large volumes of volcanic materials including lava, scoria, and ash. We have adopted a SAR interferometry method using JERS-1 images to monitor volcanic activity and estimate accompanying topographic changes in order to mitigate hazard from the inevitable eruptions and debris flows. A map expressing the phase-difference distribution for pairs of images recorded on two acquisition dates is produced for this purpose. This map, obtained by subtracting orbital and topographic fringes from the initial interferogram, is capable of quantifying vertical displacements at each point on the SAR image. Considering the acquisition-date intervals for the images used, three phase-difference maps are generated for the period from January 1996 to March 1997. These show that relatively large displacements occurring along the valleys on the northern slopes of the volcano are common to the three pairs. Because the volcanic activity was low during the period, the displacements may be caused chiefly by the erosion of the surface materials. Differential SAR interferometry is thus thought to be capable of estimating temporal fluctuations of topography during arbitrary periods, detecting regions that are readily aggraded or eroded, and helping to determine the position and dimensions of debris barriers.

Temporal changes in electrical resistivity at Sakurajima volcano from continuous magnetotelluric observations

Continuous magnetotelluric (MT) measurements were conducted from May 2008 to July 2009 at Sakurajima, one of the most active volcanoes in Japan. Two observation sites were established at locations 3.3 km east and 3 km west–northwest of the summit crater. At both observation sites, the high-quality component of the impedance tensor (Zyx) showed variations in apparent resistivity of approximately ± 20% and phase change of ± 2°, which continued for 20–180 days in the frequency range between 320 and 4 Hz. The start of the period of changes in apparent resistivity approximately coincided with the start of uplift in the direction of the summit crater, as observed by a tiltmeter, which is one of the most reliable pieces of equipment with which to detect magma intrusion beneath a volcano. A 2D inversion of MT impedance suggests that the resistivity change occurred at a depth around sea level. One of the possible implications of the present finding is that the degassed volatiles migrated not only vertically through the conduit but also laterally through a fracture network, mixing with shallow groundwater beneath sea level and thereby causing the observed resistivity change.

Depth and Evolution of a Silicic Magma Chamber: Melting Experiments on a Low-K Rhyolite from Usu Volcano, Japan

We estimated the depth of the magma chamber system beneath Usu volcano, one of the most active volcanoes inJapan, by melting experiments on the eruptive product of the AD 1663 plinian eruption (Us-b pumice).This pumice is a low-K rhyolite with a K2O content of only 1wt %. The experiments were conducted using internally heated pressure vessels at 98^392MPa and 700^9008C under nickel^nickel oxide (NNO)-buffered and water-saturated conditions. The pre-eruptive conditions of the 1663 eruption were estimated to be 780 208C and around 200^250MPa. The phase equilibria, mineral compositions (plagioclase, orthopyroxene, and Fe^Ti oxides), melt composition, and melt fraction are all in agreement with this conclusion. This pressure corresponds to a depth of 8^10 km, assuming a lithostatic condition with an appropriate crustal density structure.The magma was at near water-saturated conditions. Deviation between the estimated pressure and the true total pressure was estimated to be less than 50MPa. The Us-b magma was under very uniform conditions with a temperature variation of less than c. 208C, deduced from the narrow range of the phase equilibrium conditions (crystallizing orthopyroxene without quartz) and the homogeneous composition of plagioclase (c. An 41^45).We also estimated the pre-eruptive conditions of post-1663 eruptions, where it has been proposed that two magmas (high-P and low-P members) mixed prior to each eruption. The pressure of the deep magma was also around 200^250MPa, whereas that of the shallow magma was estimated to be 100^150MPa (4^6 km depth), according to the pressure dependence of the plagioclase composition obtained in our experiments.Thus, there appear to be two magma chambers beneath Usu volcano. Our estimate is consistent with geophysical observations made during the 2000 eruption, including identification of an inflation and deflation source at a depth of 10 km, a low-frequency (12 s) tremor source at a depth of 5^6 km, and an earthquake swarm at a depth of less than 4 km. The depth of the rhyolitic magma chamber (8^10 km) is deeper than the level of neutral buoyancy for the magma, and is comparable with that of mafic magma that existed beneath the rhyolitic magma.This suggests that the depth of the 8^10 km deep magma chamber was controlled by the emplacement of the mafic magma and that the rhyolitic magma was produced at that depth. When compared with other silicic magma systems, the Usu magma system has similar characteristics, including a similar pre-eruptive pressure of around 200MPa and a pressure decrease with time, suggesting upward magma movement towards a neutral buoyancy level. The low potassium content of the Usu magma prohibited crystallization of biotite, K-feldspar or hornblende, which are common phases in magmas with intermediate to high potassium contents. The absence of hydrous minerals at near-liquidus conditions may cause a rapid increase in water content

Magmatic fluids of Tatun volcanic group, Taiwan

Two distinctive magmatic fluids were recognized in the Tatun volcanic group (TVG), Taiwan. One is a relatively reduced fluid represented by the fumarolic gases at Hsiao-you-ken (HYK) geothermal field. Another is an oxidized fluid containing high concentrations of HCl represented by the fumarolic gases at Da-you-ken (DYK). An intermediate gas was recognized at Gung-tze-ping (GTP) and She-hung-ping (SHP). The fumarolic gases at HYK and GTP possess the features of so-called primary steam generated on mixing of magmatic gas and meteoric groundwater. The fumarolic gases at DYK are a simple mixture between magmatic gas and water vapor of meteoric origin. The CO2/H2O molar ratio of the magmatic component in the fumarolic gases at DYK was estimated to be 0.018, meanwhile it was estimated to be 0.027 for the fumarolic gases at HYK and GTP, suggesting the magma beneath DYK is depleted in volatiles relative to the magma beneath HYK and GTP. The estimated CO2/H2O ratio for the magmatic component is comparable to that of some active volcanoes in Japan, suggesting the enrichment of volatiles in the magmas beneath TVG.

Chemical monitoring of volcanic gas using remote FT-IR spectroscopy at several active volcanoes in Japan

Abstract Chemical compositions of volcanic gases of several Japanese active volcanoes have been monitored from distant safe places since the beginning of the 1990s using an FT-IR spectral radiometer. For absorption measurements, an infrared light source behind volcanic gas emissions is necessary in a volcanic environment. In the early observations, infrared radiation from hot lava domes (Unzen volcano) and hot ground heated by high-temperature fumaroles (Usu, Aso, and Satsuma-Iwojima volcanoes) were used as infrared light sources. However, these sources were not available in many cases. This remote FT-IR method became more commonly applied to chemical monitoring of volcanic gases emitted from the summit or slopes of active volcanoes using scattered solar infrared light as infrared light sources (Sakurajima, Miyakejima, and Asama volcanoes). To date, eight species have been measured using this method: SO2, HCl, HF, CO, CO2, COS, SiF4, and H2O. The observations indicate that volcanic gases for each volcano have different chemical composition on a SO2–HCl–HF ternary diagram in spite of similar tectonic settings, suggesting that vapor/melt volume ratios during volcanic gas formation differ among volcanoes. During more than 15 years of monitoring, chemical changes in volcanic gases attributable to ascent of magma were observed only at Asama, where HCl/SO2 and HF/HCl ratios in the eruptive period were higher than those in non-eruptive period because of scrubbing of more soluble components in surface hydrothermal systems in the non-eruptive stage or solubility-controlled fractionation processes. Results show that these parameters are the most prospective ones among the various parameters measured using the remote FT-IR method to monitor volcanic activities.

Post-eruptive gravity changes from 1999 to 2004 at Unzen volcano (Japan): A window into shallow aquifer and hydrothermal dynamics

We report on results from post-eruptive time-lapse gravity investigations performed between 1999 and 2004 at Unzen volcanic complex, Shimabara Peninsula, Kyushu Island, southwest Japan. One of the most active volcanoes in Japan, Unzen saw its most recent eruption between 1990 and 1995. Here we demonstrate that annual residual gravity variations reflect subsurface mass changes most likely coupled to changes in shallow aquifer levels on the order of several meters. We find a positive correlation between rainfall and gravity data with a 3 to 4 month lag time. We interpret this link as a delayed aquifer response to local rainfall and show that high permeability rocks of the Unzen complex are indicative for low aquifer storativity, which in turn results in substantial water table fluctuations upon deviation from average rainfall budgets. While aquifer dynamics may represent a first order explanation to the observed gravity data, hydrothermal fluid circulation in the feeder system of the recent eruptive episode is also likely to contribute to the gravity signals by deteriorating the signal-to-noise ratio compared to hydrothermally quieter volcanoes. This study shows that shallow hydrological processes may dominate temporal changes in the gravity field after dome-building eruptions identifying a clear need for a better understanding of the interaction between magmatic and hydrological systems at arc volcanoes.

Detection of aeromagnetic anomaly change associated with volcanic activity: An application of the generalized mis-tie control method

Repeat aeromagnetic surveys may assist in mapping and monitoring long-term changes associated with volcanic activity. However, when dealing with repeat aeromagnetic survey data, the problem of how to extract the real change of magnetic anomalies from a limited set of observations arises, i.e. the problem of spatial aliasing. Recent development of the generalized mis-tie control method for aeromagnetic surveys flown at variable elevations enables us to statistically extract the errors from ambiguous noise sources. This technique can be applied to overcome the spatial alias effect when detecting magnetic anomaly changes between aeromagnetic surveys flown at different times. We successfully apply this technique to Asama Volcano, one of the active volcanoes in Japan, which erupted in 2004. Following the volcanic activity in 2005, we conducted a helicopter-borne aeromagnetic survey, which we compare here to the result from a previous survey flown in 1992. To discuss small changes in magnetic anomalies induced by volcanic activity, it is essential to estimate the accuracy of the reference and the repeat aeromagnetic measurements and the probable errors induced by data processing. In our case, the positioning inaccuracy of the 1992 reference survey was the most serious factor affecting the estimation of the magnetic anomaly change because GPS was still in an early stage at that time. However, our analysis revealed that the magnetic anomaly change over the Asama Volcano area from 1992 to 2005 exceeded the estimated error at three locations, one of which is interpreted as a loss of magnetization induced by volcanic activity. In this study, we suffered from the problem of positioning inaccuracy in the 1992 survey data, and it was important to evaluate its effect when deriving the magnetic anomaly change.

Mechanism of explosive eruption revealed by geophysical observations at the Sakurajima, Suwanosejima and Semeru volcanoes

A common sequence of phenomena associated with volcanic explosions is extracted based on seismic and ground deformation observations at 3 active volcanoes in Japan and Indonesia. Macroscopic inflation-related ground deformations are detected prior to individual explosions, while deflations are observed during eruptions. Precursory inflation occurs 5 min to several hours before eruption at the Sakurajima volcano, but just 1–2 min at Suwanosejima and 3–30 min at the Semeru volcano. The sequence commences with minor contraction, which is detected by extensometers 1.5 min before eruption at Sakurajima, as a dilatant first motion of the explosion earthquakes 0.2–0.3 s before surface explosions at Suwanosejima, and as downward tilt 4–5 s prior to eruption at the Semeru volcano. The sequence is detected for explosive eruptions with > 0.1 μrad tilt change at Sakurajima, 90% at Suwanosejima and 75% at Semeru volcanoes. It is inferred that the minor contraction is caused by a volume and pressure decrease due to the release of gas from a pocket at the top of the conduit as the gas pressure exceeds the strength of the confining plug. The subsequent violent expansion may be triggered by sudden outgassing of the water-saturated magma induced by the decrease in confining pressure.

Time‐series data server optimized for multichannel and real‐time processing

AbstractSince there are a considerable number of earthquakes and active volcanoes in Japan, it is important to install high‐accuracy seismic monitoring equipment and to develop systems that can process observed data efficiently. In the field of seismic data processing, specific requirements such as multichannel and on‐demand/real‐time processing are essential, whereas they are not so significant in other time‐series data. Since conventional systems cannot fulfill such requisites, a client/server system is newly developed. It provides a unified solution for delivering the seismic data to the client side, whether it is used for real‐time monitoring or for on‐demand printing. © 2006 Wiley Periodicals, Inc. Electron Comm Jpn Pt 3, 89(7): 8–18, 2006; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/ecjc.20257

The 1969-1985 Pozzuoli event and active volcanisms.

Pozzuoli is located at the center of the Campi Flegrei caldera, near Naples and is famous for its anomalous subsidence and upheaval documented since the Roman period. Its secular and gradual subsidence can be interpreted as self-loading compaction of the caldera fills while abrupt upheavals are geologically suspected to be caused by magmagenic movements or steam forces. In order to interpret the origin and the process of the Pozzuoli upheavals, they are compared with active volcanisms represented by the 1977-1982 eruption of Usu volcano in Hokkaido. Usu volcano outburst in 1977 in major pumice eruptions and repeated magmatic and phreatomagmatic eruptions, and manifested remarkable ground deformations accompanying earthquake swarms. In 1969, the ground of Pozzuoli began to upheave with increases in seismicity but finally failed to cause any eruptive phenomena at the surface; nevertheless there are common characteristics of their motives and processes between the two events. The motive of the Usu deformation is clearly due to magma movements while that of the Pozzuoli upheaval has not been completely settled. A quantitative relationship between seismicity and deformation gives a clue for discussing the motive of the Pozzuoli deformations. The discharge rates of seismic energy and the deformation rates are compared between the two events and a certain similarity is found. This suggests that the origin of the Pozzuoli event may be partly magmatic as well as the Usu eruption, but its behavior largely depends on the property of the caldera deposits. When their deformation volumes are taken into consideration, their characteristics become quantitatively conspicuous. The ground at Pozzuoli is much more easily deformed by the upward motive force than Usu volcano. This is due to the rheological property of the caldera deposits of Campi Flegrei, and agrees to the theory that interprets the secular subsidence observed in historical times, as self-loading compaction. It is interesting that there is a point of contact between anomalous movements of the ground along the seashore in Italy and remarkable magmatic movements at the active volcano in Japan.

Basaltic pyroclastic flows of Fuji volcano, Japan: characteristics of the deposits and their origin

Fuji volcano is the largest active volcano in Japan, and consists of Ko-Fuji and Shin-Fuji volcanoes. Although basaltic in composition, small-volume pyroclastic flows have been repeatedly generated during the Younger stage of Shin-Fuji volcano. Deposits of those pyroclastic flows have been identified along multiple drainage valleys on the western flanks between 1,300 and 2,000 m a.s.l., and have been stratigraphically divided into the Shin-Fuji Younger pyroclastic flows (SYP) 1 to 4. Downstream debris flow deposits are found which contain abundant material derived from the pyroclastic flow deposits. The new14C ages for SYP1 to SYP4 are 3.2, 3.0, 2.9, and 2.5 ka, respectively, and correspond to a period where explosive summit eruptions generated many scoria fall deposits mostly toward the east. The SYP1 to SYP4 deposits consist of two facies: the massive facies is about 2 m thick and contains basaltic bombs of less than 50 cm in size, scoria lapilli, and fresh lithic basalt fragments supported in an ash matrix; the surge facies is represented by beds 1 to 15 cm thick, consisting mainly of ash with minor amount of fine lapilli. The bombs and scoria are 15 to 30% in volume within the massive facies. The ashes within the SYP deposits consist largely of comminuted basalt lithics and crystals that are derived from the Middle-stage lava flows exposed at the western flanks. SYP1 to SYP4 were only dispersed down the western flanks. The reason for this one-sided distribution is the asymmetric topography of the edifice; the western slopes of the volcano are the steepest (over 34 degrees). Most pyroclastic materials cannot rest stably on the slopes steeper than 33 degrees. Therefore, ejecta from the explosive summit eruptions that fell on the steep slopes tumbled down the slopes and were remobilized as high-temperature granular flows. These flows consisted of large pyroclastics and moved as granular avalanches along the valley bottom. Furthermore, the avalanching flows increased in volume by abrasion from the edifice and generated abundant ashes by the collision of clasts. The large amount of the fine material was presumably available within the transport system as the basal avalanches propagated below the angle of repose. Taking the typical kinetic friction coefficient of small pyroclastic flows, such flows could descend the western flanks where scattered houses are below 1,000 m a.s.l. A similar type of pyroclastic flow could result if explosive summit eruptions occur in the future.

Deep low-frequency earthquake swarm in the mid crust beneath Mount Fuji (Japan) in 2000 and 2001

Beneath Mount Fuji, the highest active volcano in Japan, deep low-frequency (DLF) earthquake activity has been monitored since the early 1980s. The DLF earthquakes occurred in the mid-crustal depth range, and burst-type activity lasting from several minutes to 30 min was detected 10 to 20 times in an ordinary year. The DLF earthquake activity increased sharply in the period from October 2000 to May 2001, showing swarm-like activity. The occurrence rate during the DLF earthquake swarm was approximately 20 times higher than the usual activity, and the wave energy released during the swarm period was twice as high as the total wave energy during the past 20 years. The DLF earthquakes in the period from 1987 to 2001 were relocated by estimating station corrections in order to reduce the effect of the change of seismic station distribution. The epicenters of most DLF earthquakes occurred in an elongated region with a long axis of about 5 km, whose center is located 2–3 km NE from the summit. A few percent of the DLF earthquakes, however, occurred around the summit area, significantly apart from the main epicenter region. The focal depths of well-located DLF events range from 10 to 20 km. During the high activity period in 2000 and 2001, most DLF events occurred within this main hypocenter area. The sharp increase of DLF earthquake activity at Mount Fuji started immediately after magma discharge and intrusion events in the Miyake-jima and Kozu-shima regions in July and August 2000. The tectonic and volcanic activity changes around the area suggest that the DLF earthquake swarm at Mount Fuji was triggered by the change of state of the deep magmatic system around Mount Fuji.

Risk Perception, Risk-Taking Attitude, and Hypothetical Behavior of Active Volcano Tourists

ABSTRACT To help understand what determines an individual's risk-taking attitude and behavior of tourists visiting a volcano, an interview survey of 523 adults was undertaken in Mt. Aso, an active volcano in Japan where guidance is designed to prohibit visitors with cardio-pulmonary disorders from ascending to the crater. The survey included the individual's knowledge of the prohibition regulation, their risk perception of life-threat to volcanic gas and risk-taking attitude toward the prohibition. Their hypothetical risk-taking behavior assuming their being accompanied by a health risk companion was also investigated. A logistic regression model was used to assess the effects of various factors on the specific risk perception, attitude, and behavior. In a different model, how the risk perception and knowledge would affect attitude and behavior was also assessed. Those having knowledge of the guidance significantly employed a high perception of the risk (OR, 0.45: 95% CI, 0.27–0.73). Those with low risk perception significantly opposed to the current regulation (OR, 2.56:95% CI, 1.63–4.03). However, if subjects possessed health problems, they were more likely to visit the crater when they were asked to do so by their accompanying health risk subjects (2.89:1.28–6.52). Improving the specific risk perception might have beneficial effect on risk-taking attitude and behavior.

Monitoring eyes on Indium Phosphide

The 16th international conference on Indium Phosphide and Related Materials 2004 was held between 31 May and 4 June, at Kagoshima, a stone's throw from Sakurajima; one of the best known active volcanoes in Japan. In his opening, conference chairman Dr Yuichi Matsushima, said that 350 participants from 16 countries would be involved in three plenary talks, 25 technical sessions, and 199 presentations during the conference. This review is of proceedings that covered the six areas of characterization and bulk materials, epitaxy, processing and materials integration, electron devices, optoelectronics and nanostructures and novel materials.

Nature and origin of volcanic ash in the 2000 eruption of Usu volcano, southwestern Hokkaido, Japan

Usu volcano, one of the most active volcanoes in Japan, erupted at regular intervals of ca. 30 years during the 1900s. The 2000 eruption took place on March 31 following 23 years of dormancy. An increase in seismic activity and remarkable ground deformation preceded the eruption. Many small-scale craters were successively formed in two areas at the foot of the volcano and ash was ejected intermittently from them. Ten samples out of the 2000 ash collected around the volcano during half a year were analyzed. They were depleted in SiO2 but enriched in FeO compared with the essential ejecta of the five major historic eruptions and all were highly altered as a whole. XRD analysis revealed smectite as a major clay mineral in the ash and kaolin as a minor one. The clay mineral assemblage and the trend of change in chemical composition of the 2000 ash are quite similar to those of the 1977–1978 ash. No temporal changes in chemical composition and clay mineral assemblage of the 2000 ash were detected. The percentages of the LOI and the relative proportions of SiO2 in the ashes and the essential ejecta showed a linear correlation. These results indicate that the constituents of the 2000 ash were alteration products of the volcaniclastics of the historic eruptions in the subsurface.

Distributed control for tele-operations

A distributed tele-operating system has been designed and simulated in our laboratory with model-based supervisory control to enable reduced human attendance. Dual sensory feedback channels, top-down image processing, and a global positional system permit robust vehicle pose recovery. The distributed network component design and model three-dimensional display are based on currently available technologies. The most difficult design aspect was to achieve smooth integration between web technologies and the hardware physical entities. Constraints from the remote environment were carefully considered, since the system will be used to build a lava-diverting dam at the base of an active volcano in Japan.

Unzen volcano from 1990 to 1995 observed by satellite

This is a record of the phenomena at Mt. Unzen, an active volcano in Japan, seen by the earth observation satellites during the active period from 1990 to 1995. Unzen volcano started the series of activities in November 1990 after 198-year dormancy and ceased them in the spring of 1995. Conditions of the area affected by ashfall, pyroclastic or debris flows and topographic deformation are described.

Thermal and crustal structure of the Aso volcano and surrounding regions constrained by gravity and magnetic data, Japan

The Aso volcano forms a large caldera that is one of the most active volcanoes in Japan. Much evidence suggests the existence of a high-temperature zone beneath the active central cones of the Aso volcano and a recent study using seismic observations revealed an attenuation in amplitude of seismic waves that pass through the central region of the Aso caldera. The SWW-NEE-trending Futagawa active fault, a part of the Oita-Kumamoto Tectonic Line, which dips north, lies to the west of the caldera and joins the caldera rim but there are no scars within the caldera. An elongate magnetic positive anomaly from the central cones to the west exists close to the Futagawa fault, this anomaly connects the central cones and the two lateral volcanoes west to the caldera. Magnetic analyses through spectral method and forward modeling delineate a highly magnetized two-dimensional buried prism source dipping north beneath the fault. The lower bounds are about 10 km depth outside the caldera and quite shallow depths of about 3 km inside the caldera. The area to the north of the Oita-Kumamoto Tectonic Line forms a gravity depression whereas the area to the south is a regional gravity high. A two-layered two-dimensional gravity model yields a normal fault dipping north with a throw of 1.3 km, whereas a multi-layered two-dimensional gravity model gives a depression of high-density basement within the caldera. The greatest depth is about 2 km below the surface. Available temperature-depth data suggest the possibility of a sudden change between the inside (high-temperature zone) and the outside (low-temperature zone) of the caldera. Therefore, the difference in the lower boundary of magnetic sources between the area inside and outside the caldera is possibly attributed to the difference of depth of the Curie point. Magnetic sources along the Futagawa fault are likely to be related to materials produced by the volcanic activity. If so, the passage for magma for Aso volcanism could be along the Futagawa fault.

Cutoff depth of seismicity and large earthquakes near active volcanoes in Japan

Abstract Cutoff depths of seismicity in the earth's crust change from place to place over a range of 5–25 km. These variations are closely related to the values of the surface heat flow. In order to examine regional variations in greater detail, available cross sections of well-determined focal depths have been collected for five areas located near active volcanoes, where the thermal structure is considered to change abruptly. The seismic-aseismic boundary is located at depths of about 5–8 km immediately below the craters, with increasing depth towards the flanks of the volcanoes. The boundary is found at depths of 10–15 km at distances of about 10 km from the center of the volcanoes. Further, an examination was performed of the magnitudes of earthquakes versus the distances from the craters for seven volcanoes, near which large earthquakes ( M ≥ 6) occurred during the past 30 years. All of the large earthquakes were found to have occurred at distances of approximately 10 km or greater from the center of the volcanoes. Moreover, the distances between active volcanoes and earthquakes were calculated for all large intra-plate earthquakes in Japan that have occurred during the period 1885–1990. It was found that all the large earthquakes occurred at distances of more than 10 km from active volcanoes. These facts suggest that the seismogenic layer near the center of an active volcano is thin and not strong enough to accumulate sufficient stress for the occurrence of large earthquakes.