Unzen Research Articles

P11 雲仙普賢岳 6・3,1991 火砕流の上木場地区における住民の行動

P11 雲仙普賢岳 6・3,1991 火砕流の上木場地区における住民の行動

B39 雲仙・眉山火山の山体内に分布する先眉山堆積物

B39 雲仙・眉山火山の山体内に分布する先眉山堆積物

P08 Results of Time Domain Electromagnetic (TDEM) Sounding in Eastern Part of Mount Unzen Area, Kyushu

P08 Results of Time Domain Electromagnetic (TDEM) Sounding in Eastern Part of Mount Unzen Area, Kyushu

Testing the accuracy of solar‐reflected radiation corrections applied during satellite shortwave infrared thermal analysis of active volcanoes

Shortwave infrared (SWIR) satellite imagery is frequently used to study thermal emission from active volcanoes. When using daytime SWIR observations it is necessary to first isolate the thermally emitted signal component, principally by subtracting contributions due to solar‐reflected radiation. Two differing approaches have been used: (1) A “mean” approach based on the average SWIR reflectance of the nonthermally anomalous volcanic background, and (2) a “per‐pixel” approach which estimates the SWIR reflectance of each pixel using a measure of their near infrared reflectance. We assess the accuracy of these using near‐coincident daytime and nighttime Landsat Thematic Mapper (TM) imagery of the active lava dome of Unzen Volcano. Using the nighttime data as a reference we find the most commonly applied mean daytime correction approach to be seriously inadequate, returning results for thermally radiant area 81–94% smaller than the nighttime values. The per‐pixel correction method appears somewhat more effective but still provides values 47–61% smaller than the reference data set. Daytime underestimation of total SWIR thermal spectral radiance is less severe than that of total thermally radiant area because it is primarily pixels having low thermal signals that pose a problem for the daytime approaches. Total thermal spectral radiance is underestimated by 52–79% using the daytime data with the mean correction approach, and by 28–59% with the per‐pixel approach. When applying the TM “dual‐band” technique to determine subpixel hot spot temperatures from the SWIR thermal signals, we find that surfaces cooler than 400°C are poorly represented in the daytime retrievals. However, when using the per‐pixel daytime data correction method the distribution of retrieved hot spots >400°C corresponds quite well to the pattern derived via nighttime imagery. Due to the significant parameter underestimation found to be inherent when using daytime data, this study further indicates the value of nighttime observations to volcanic SWIR thermal studies. If daytime data are used, then we recommend use of the per‐pixel solar‐reflected radiation correction method but would advise caution when quantitatively interpreting the derived thermal parameters, at least at andesitic and dacitic volcanoes. It is possible that daytime retrievals may be more robust at basaltic volcanoes due to their characteristic higher magmatic temperatures and lower surface reflectances.

Excitation process of infrasonic waves associated with Merapi‐type pyroclastic flow as revealed by a new recording system

We investigate excitation process of the infrasonic waves associated with Merapi‐type pyroclastic flows at Unzen volcano, Japan, using data set obtained by a new system that permits recording of geophysical data with accompanying visual events on a single videotape. The accurate correlation of infrasonic waves with video images reveals that: (1) successive impulsive waves and the low‐frequency wave (<∼0.8Hz) are excited during the collapse of lava blocks falling from the dome, which generates a pyroclastic flow; (2) an impulsive wave is excited by an explosive blowout of the pore‐gases which are released by the breakdown of the falling blocks; (3) the low‐frequency wave is excited by the volumetric change which results from expansion of the pore‐gases released by pulverization of the falling blocks. The comparison between the observed and the synthetic low‐frequency waves shows that the amount of volume change is about 2 times that of the initial falling blocks.

Rapid fluid disruption: A source for self‐potential anomalies on volcanoes

Self‐potential (SP) anomalies observed above suspected magma reservoirs, dikes, etc., on various volcanoes (Kilauea, Hawaii; Mount Unzen, Japan; Piton de la Fournaise, Reunion Island; Miyake Jima, Japan) result from transient surface electric fields of tens of millivolts per kilometer and generally have a positive polarity. These SP anomalies are usually attributed to electrokinetic effects where properties controlling this process are poorly constrained. We propose an alternate explanation that contributions to electric fields of correct polarity should be expected from charge generation by fluid vaporization/disruption. As liquids are vaporized or removed as droplets by gas transport away from hot dike intrusions, both charge generation and local increase in electrical resistivity by removal of fluids should occur. We report laboratory observations of electric fields in hot rock samples generated by pulses of fluid (water) through the rock at atmospheric pressure. These indicate the relative amplitudes of rapid fluid disruption (RFD) potentials and electrokinetic potentials to be dramatically different and the signals are opposite in sign. Above vaporization temperatures, RFD effects of positive sign in the direction of gas flow dominate, whereas below these temperatures, effects of negative sign dominate. This suggests that the primary contribution to observed self‐potential anomalies arises from gas‐related charge transport processes at temperatures high enough to produce vigorous boiling and vapor transport. At lower temperatures, the primary contribution is from electrokinetic effects modulated perhaps by changing electrical resistivity and RFD effects from high‐pressure but low‐temperature CO2 and SO2 gas flow ripping water molecules from saturated crustal rocks. If charge generation is continuous, as could well occur above a newly emplaced dike, positive static potentials will be set up that could be sustained for many years, and the simplest method for identifying these hot, active regions would be to identify the SP anomalies they generate.

GPS and EDM monitoring of Unzen volcano ground deformation

Following 198 years of dormancy, an eruption started at Mt. Fugen, the main peak of Unzen volcano, in Kyushu, Japan, in November 1990. A dacite lava dome began to grow in May 1991. We installed the surveying points of GPS in 1992 around the lava dome in order to observe the ground deformation that accompanied the growth of the lava dome. In the winters of 1993 and 1994, we observed swift ground deformations that radiated from the vent of the volcano. It was presumed that rising magma accumulated and expanded the volcano body. After the lava effusion stopped in 1995, we also installed surveying points on the lava dome. EDM mirrors were permanently fixed to the large rocks with bolts. A GPS survey was carried out 2 or 3 times each year to estimate the 3-dimensional displacement. The result of the EDM survey showed that the baselines from the flank of the volcano were shortening 5 mm per day, and the result of the GPS survey showed that the displacement vector of the dome was parallel to the direction of the steepest slope of the old volcano body. This indicates that the inside of the lava dome is still very hot, and that deformation of the dome is viscous.

Discrimination of lava dome activity styles using satellite-derived thermal structures

Thermal imaging from satellites is one methodology used for the monitoring and scientific investigation of volcanoes, including those characterised by active lava domes. To be most effective, the remote sensing techniques employed must allow the cause of any observed thermal anomaly to be identified, ideally using information contained within the remote sensing data itself, whilst using any ancillary field data to guide analysis and the necessary assumptions. This study investigates a method by which such discrimination maybe accomplished for activity at lava domes. For this purpose we use three Landsat Thematic Mapper (TM) scenes of Unzen Volcano (Japan) to determine the temperature structure of the active lava dome existing during 1991–1993. These data are of particularly high quality since the TM scenes were obtained during night-time overpasses and a large amount of supplementary data are available to parameterise the model used to retrieve the subpixel temperature structure of the dome surface. The results are matched to near-contemporaneous geological sketch maps of the then current situation in order to identify the TM pixel groupings corresponding to the different styles of dome activity, namely fumarolic degassing, exogenous and endogenous dome growth, and collapse deposits resulting from block and ash flows, small scale pyroclastic flows and rockfalls. The spatial and statistical characteristics of the resultant TM-derived temperature distributions are then investigated to determine rule-based criteria that may be used to differentiate the activity styles of lava domes based on their thermal structure within the remote sensing data. Results indicate that fumarolically active locations, regions of active dome growth, and areas of collapse deposition on the Unzen dome can be differentiated using only the statistical distribution of the hotspot temperatures and fractional areas identified via analysis of TM imagery. The statistics derived for hotspots at the fumarolically active areas show them to be, in general, significantly hotter (and smaller) than those found at the areas of active dome growth, whereas hotspots retrieved on the areas of collapse deposition are cooler (and larger). We find that the frequency distributions of hotspot temperatures extracted for the areas of endogenous and exogenous dome growth exhibit statistically inseparable means, but that discrimination between these two dome growth styles is possible using analysis of the spatial arrangement of the retrieved hotspots. Specifically the exogenous lobes are characterised by structures interpreted to be the effusing vent of new magma and the collapsing lobe front. These features are absent at locations showing only endogenous growth. It is hoped that the criteria developed here will prove useful during future quantitative analysis of the extended TM time-series available for Unzen, and for identification of thermal anomalies of uncertain origin at other active domes observed via satellite remote sensing.

P17 1991 年 9 月 15 日, 雲仙火山平成の噴火で発生した火砕流に伴う火砕サージの流体密度

P17 1991 年 9 月 15 日, 雲仙火山平成の噴火で発生した火砕流に伴う火砕サージの流体密度

P16 雲仙火山, 眉山西部, 垂木台地の変動地形

P16 雲仙火山, 眉山西部, 垂木台地の変動地形

A mechanical model for Merapi-type pyroclastic flow

A numerical simulation model of Merapi-type pyroclastic flow for predicting a potential hazardous area is given, and its applicability is examined by numerically reproducing actual phenomena that occurred in 1991 at Unzen Volcano, Kyushu Japan. Merapi-type pyroclastic flow arises from collapse of the lava dome whereby large lava blocks are crushed into smaller particles during movement down the steep slope. The early stage of flow (it may be called the debris avalanche stage), in which the dominant particles are coarser than about 1mm in diameter, is considered as a grain (granular) flow. In an inertial grain flow particle collision stress plays an important role in the mechanism of flow, and the effect of gas emitted from the lava blocks is minimal. Most of the particles composing the inertial grain flow deposit over a comparatively short range on steep slope due to marked resistance within the flow. The remainder which is composed of mainly fine particles smaller than 1mm continues to run down by a support of the upward flow of gas ejected from the material itself, thereby the resistance to flow becomes small. Thus, the pyroclastic flow stage in a narrow sense appears. The main body of the pyroclastic flow is composed of lower insufficiently fluidized layer (bottom layer) and upper fluidized layer in which the entire weight of the particles is supported by the upward gas flow. Sometimes, the fluidized layer cannot exist if gas emission is insufficient. As the slope down which the pyroclastic flow moves becomes milder downstream, the bottom layer deposits some solids because the driving force due to gravity within this layer becomes smaller than the resistance due to inter-particle contact. Then, a part of the fluidized layer, if exists, changes to a part of the bottom layer because of the shortage of the upward gas flow that is caused by deposition of the gas-emitting particles upstream. Thus, the entire main body stops when it arrives at a gently sloping area. Smaller particles and gas escape from the main body, generating a hot ash cloud layer above the main body. The hot ash cloud layer can travel independent of the main body, but its development or attenuation depend on the conditions of the supply of particles and gas from the main body. When the hot ash cloud lacks the supply of particles and gas due to the stopping of the main body upstream, or by its swerving from the course of the main body, it becomes soon weakened and stops. The entrainment of ambient air, the escape of air and ash from the upper boundary as a buoyant plume, and the particle settling also affect the behaviors of the hot ash cloud. Fundamental mechanics of the grain (granular) flow stage, the pyroclastic flow stage and the transition from the former to the latter stages are discussed theoretically and experimentally, and mathematical formulae describing the phenomena are obtained. These one-dimensional equations are extended to fit the planar two-dimensional system. Behaviors of the main body as well as the hot ash cloud are simulated using the obtained system of equations. The results of simulation are very satisfactory in comparison to the actual phenomena.

Volcano monitoring with Landsat TM short-wave infrared bands: The 1990-1994 eruption of Unzen Volcano, Japan

The Unzen Volcano was active from 1990 to 1994 and in total 13 dacite lava lobes were formed successively at the eastern shoulder of Mount Fugen-dake. Surface temperature observation with nighttime Landsat-5 Thematic Mapper (TM) was conducted at the Unzen Volcano. The thermal anomaly areas up to 0.7 km2 were detected in all the 23 cloud-free images of Landsat TM band 7. The patterns of temperature distribution correspond to the lava dome and the pyroclastic flow activity. The hot area change has been found to be closely related to the lava effusion rate.

Magmas in collision: Rethinking chemical zonation in silicic magmas

The heterogeneity of eruptions attributed to protracted fractionation in subvolcanic magma chambers may instead result from chamber recharge. The case of mafic magma intruding andesitic slush is recognized as giving rise to hybrid effusive eruptions such as at Unzen volcano, Japan, and Soufriere Hills volcano, Montserrat. We propose that zoned andesite-rhyolite explosive eruptions are the complement to mafic recharge, where the resident magma is also andesitic but the intruding magma is silicic.

P31 雲仙火山の火山体地質構造発達史の解明 : 科学技術振興調整費「雲仙火山」プロジェクトでの課題

P31 雲仙火山の火山体地質構造発達史の解明 : 科学技術振興調整費「雲仙火山」プロジェクトでの課題

A new model for volcanic earthquake at Unzen Volcano: Melt Rupture Model

Small earthquakes occurred inside the flowing magma during the 1991–1995 activities of Unzen Volcano. This paradoxical phenomenon can be explained when we consider that even a flowing magma fails brittly under the condition that the product of viscosity and strain rate of magma exceeds a certain threshold value. The estimated strain rate of magma in the conduit leads us to the conclusion the brittle failure is expected when the viscosity is over 1011.5 Pa s. Laboratory measurements of samples show that this viscosity range can be attained when the temperature is under 850°C in dry condition, and 800°C with 0.6 wt.% water. Considering the temperature from Fe‐Ti oxide geothermometer (780–880°C) and the gas temperature (800°C), the brittle failure condition could be satisfied inside the conduit. The melt failure by shear flow could be the source of the earthquakes occurred at the summit area.

Geology and eruptive history of Unzen volcano, Shimabara Peninsula, Kyushu, SW Japan

During the past 500 thousand years, Unzen volcano, an active composite volcano in the Southwest Japan Arc, has erupted lavas and pyroclastic materials of andesite to dacite composition and has developed a volcanotectonic graben. The volcano can be divided into the Older and the Younger Unzen volcanoes. The exposed rocks of the Older Unzen volcano are composed of thick lava flows and pyroclastic deposits dated around 200–300 ka. Drill cores recovered from the basal part of the Older Unzen volcano are dated at 400–500 ka. The volcanic rocks of the Older Unzen exceed 120 km 3 in volume. The Younger Unzen volcano is composed of lava domes and pyroclastic deposits, mostly younger than 100 ka. This younger volcanic edifice comprises Nodake, Myokendake, Fugendake, and Mayuyama volcanoes. Nodake, Myokendake and Fugendake volcanoes are 100–70 ka, 30–20 ka, and <20 ka, respectively. Mayuyama volcano formed huge lava domes on the eastern flank of the Unzen composite volcano about 4000 years ago. Total eruptive volume of the Younger Unzen volcano is about 8 km 3, and the eruptive production rate is one order of magnitude smaller than that of the Older Unzen volcano.

The Sr, Nd and O isotopic studies of the 1991–1995 eruption at Unzen, Japan

The magma generation at Unzen volcano may be considered as the product of crustal material mixed with mantle magma accompanied by fractional crystallization (AFC). The magma in the Unzen volcano is estimated to consist of about 50–80% of residual magma ( F) and about 30–70% assimilated crustal material ( A) relative to the original magma. Concerning the 1991–1995 eruption, it is estimated that the magma formed as the result of mixing of about 50–60% crustal material and about 55–65% of residual magma. An alternative magma eruption model for the 1991–1995 eruption is proposed here. In the early stage, the isotopic characteristics of 1991 eruption are defined by AFC process in the deeper magma chamber. Later, the magma ascended through the conduit and quiescently stayed for a long time in a shallow reservoir before eruption. The minerals continuously crystallized as phenocrysts especially at the chilled top and outer margin in the shallow chamber. The crystallized phenocryst mush was reworked into the central part of the magma chamber by means of magma convection and rapid magma ascent. Therefore, the reaction between phenocrysts and melt occurs only in internal chemical disequilibrium in the magma chamber. In contrast, the isotopic compositions of the original magma shall be little influenced by the above processes throughout its eruptive history. The 1991–1995 eruptive rocks of the Unzen volcano show their characteristics in Sr and Nd isotopic values independent of their two previous eruptions. However, the isotopic values of early eruptive product could represent the original magma value. This result also supports the previous work of Chen et al. (1993) [Chen, C.H., DePaolo, D.J., Nakada, S., Shieh, Y.N., 1993. Relationship between eruption volume and neodymium isotopic composition at Unzen volcano. Nature 362, 831–834], that suggested the εNd of early or precursory eruptive products could be a qualitative indicator of the maximum size of a continuing or impending eruption.

The 15 September 1991 pyroclastic flows at Unzen Volcano (Japan): a flow model for associated ash-cloud surges

Large-scale collapse of a dacite dome in the late afternoon of 15 September 1991 generated a series of pyroclastic-flow events at Unzen Volcano. Pyroclastic flows with a volume of 1×10 6 m 3 (as DRE) descended the northeastern slope of the volcano, changing their courses to the southeast due to topographic control. After they exited a narrow gorge, an ash-cloud surge rushed straight ahead, detaching the main body of the flow that turned and followed the topographic lows to the east. The surge swept the Kita-Kamikoba area, which had been devastated by the previous pyroclastic-flow events, and transported a car as far as 120 m. Following detachment, the surge lost its force after it moved several hundred meters, but maintained a high temperature. The deposits consist of a bottom layer of better-sorted ash (unit 1), a thick layer of block and ash (unit 2), and a thin top layer of fall-out ash (unit 3). Unit 2 overlies unit 1 with an erosional contact. The upper part of unit 2 grades into better-sorted ash. At distal block-and-ash flow deposits, the bottom part of unit 2 also consists of better-sorted ash, and the contact with the unit 1 deposits becomes ambiguous. Video footage of cascading pyroclastic flows during the 1991–1995 eruption, traveling over surfaces without any topographic barriers, revealed that lobes of ash cloud protruded intermittently from the moving head and sides, and that these lobes surged ahead on the ground surface. This fact, together with the inspection by helicopter shortly after the events, suggests that the protruded lobes consisted of better-sorted ash, and resulted in the deposits of unit 1. The highest ash-cloud plume at the Oshigadani valley exit, and the thickest deposition of fall-out ash over Kita-Kamikoba and Ohnokoba, indicate that abundant ash was also produced when the flow passed through a narrow gorge. In the model presented here, the ash clouds from the pyroclastic flows were composed of a basal turbulent current of high concentration (main body), an overriding and intermediate fluidization zone, and an overlying dilute cloud. Release of pressurized gas in lava block pores, due to collisions among blocks and the resulting upward current, caused a zone of fluidization just above the main body. The mixture of gas and ash sorted in the fluidization zone moved ahead and to the side of the main body as a gravitational current, where the ash was deposited as surge deposits. The main body, which had high internal friction and shear near its base, then overran the surge deposits, partially eroding them. When the upward current of gas (fluidization) waned, better-sorted ash suspended in the fluidization zone was deposited on block-and-ash deposits. In the distal places of block-and-ash deposits, unit 2 probably was deposited in non-turbulent fashion without any erosion of the underlying layer (unit 1).

Landsat infrared analysis of fumarole activity at Unzen Volcano: time-series comparison with gas and magma fluxes

We analyse shortwave infrared thermal data of the phase 1 eruption of Unzen Volcano (Japan) extracted from eight nighttime Thematic Mapper (TM) images taken from the Landsat 5 satellite between October 1991 and November 1992. We identify two discrete regions of the dome that were heated to high temperature by the ongoing eruptive activity; a fumarolically heated region and an area associated with the effusion of new lava. We concentrate analysis on the fumarolically heated region and investigate the relationships between parameters derived from the infrared radiance data and the nature of the fumarolic gas and magma fluxes. Temporal variations in the parameters derived from the radiance data closely follow those observed in the measured rate of magma effusion. The positive correlation observed between the fumarolically driven shortwave infrared flux and the magma discharge rate ( r 2=0.64) indicates that degassing occurred efficiently and in proportion to the amount of magma supplied. Over our monitoring period, the data suggest that gas accumulation within the edifice did not occur, this conclusion agreeing with a previous finding obtained using correlation spectrometer (COSPEC) analysis of SO 2 flux rates. A positive correlation ( r 2=0.56) was also found between the mean radiance of the pixels in the fumarolically heated region and the overall size of that region. This suggests a potential mechanism whereby, when gas pressure within the edifice increased, excess gas escaped through additional pathways to the surface as well through an increased flux at the main fumarolic vents.

Volcanic deformation at Unzen, Japan, visualized by a time-differential stereoscopy

Using a pair of old and new photographs taken at the same location, a method named as a time-differential stereoscopy is successfully applied to detect visually the deformation at Unzen volcano, Japan. On September 6, 1992, gradual movement of a lava lobe was visible in a stereo image as relief surrounded by immobile areas. In this case, accelerated movement of a lava block was detected immediately before the time of its collapse, which resulted in the occurrence of a pyroclastic flow. In November 1994, growth of a lava spine was also measured with time-differential stereoscopy. A possible fluctuation of the growth rate might have correlated with an abnormal oscillation of tilt, with a period of about 40 h. This convenient photographic method is recommended for various occasions to detect volcanic and any other deformations as a supplement to precise instrumental measurements.