For example, the volcano has been inflating as new magma moves into the plumbing system beneath it think of burying a balloon in the sand and then inflating it. Increasing thermal activity has been melting more ice and there has also been a recent increase in earthquake activity.
So what happens next? Again, based on the pattern observed at past eruptions, an intense swarm of earthquakes lasting a few hours one to ten hours will signal that magma is moving towards the surface and that an eruption is imminent. In cases where the hidden subglacial lake drains and triggers the eruption, the earthquakes occur after the lake has drained and just before the eruption.
That means the resulting ash gets wet and sticky and so falls from the sky relatively quickly. Ash clouds therefore only travel a few tens of kilometres from the eruption site. This is a good scenario for Icelanders and also for air travel, as it prevents the formation of substantial ash clouds that could drift around and close off airspace.
But will it be a small eruption? Such magma batches could be located below 1 km b. S -wave ray paths grey lines are mostly confined to the southwestern and western rims of the main caldera. Multiple north-component record section of events at the western rim of the caldera reduced traveltime with a reduction velocity of 4. No bandpass filter is applied.
An 80 yr record of heat release through measurements of meltwater accumulation and drainage testifies that the geothermal areas are being frequently replenished by dyke intrusions into the shallow crust. This body is essentially nonmagnetic, indicating that its temperature may be above the Curie point or that it has undergone severe hydrothermal alteration. Single station GPS measurements by Sturkell et al.
The GPS data indicates that the eruption was fed from a magma body located within the northern margin of the low-velocity region in the final LET model Fig. However, the location of this magma body should be treated with caution since data from only one GPS station was available and a single source of inflation and deflation was assumed. A more complicated magma storage region than can represented by a single Mogi source may be present and crustal heterogeneity can deflect the displacement vector.
Based on the GPS data, increased microseismicity within the caldera during the months preceding the December eruption was most likely generated by strain changes within the region above and adjacent to the inflating magma chamber. The relatively minor increase in seismicity prior to the December eruption may indicate that the crust within the caldera is only moderately seismogenic, i. A total of 35 events, with magnitudes ranging from 1 to 3.
Based on the seismic distribution and increase in geothermal activity along the southern caldera rim following the eruption, the feeder dyke may have been 4—6 km long and 2—3 km high. A marked decrease in microearthquake activity following the eruption can also be taken as an indicator of a relaxed state of stress.
Einarsson Delays from the two shots and from the regional and teleseismic earthquakes are 0. The region where ray paths from these different events intersect Fig.
Microearthquakes within the main caldera were mostly confined to 1—3 km depth during V98 Fig. An additional observation is the region of attenuated shear waves from local earthquakes and passive sources within the caldera Figs 11— Clear S -waves are observed from shot 1 at all stations east of and including station V23 Fig.
Within and to the west of the caldera, however, these S -waves are completely attenuated. Possible attenuated S m S phases are also observed between stations V05 and V These observations of clear shear energy on either side of the caldera, together with the high attenuation observed within and immediately west of the caldera, are best explained by a melt lens centred beneath the main caldera, and a zone of high percentage partial melt Fig.
The clear shear energy observed at V23 and the attenuated S m S phase at V03 constrain the maximum lateral extent of any pure melt lens to be 7—8 km in an east—west direction between these two stations. The source of the delays observed in Fig. The velocity model shown is a hybrid model, containing a low-velocity body representative of a sill of pure melt with an underlying 1. See text for further discussion. Grey-shaded areas indicate zones of negative velocity contrast relative to the background velocity of the surrounding crust at that depth.
Dark grey indicates a high negative velocity contrast, lighter shades of grey indicate progressively smaller negative velocity contrasts. Using the distribution of local seismicity and shear wave attenuation we estimate the maximum lateral extent of the magma chamber to be 7—8 km E—W and 4—5 km N—S. If there is also a region of partial melt extending over a greater depth zone, then the magma chamber containing pure melt could be correspondingly smaller.
A somewhat larger 12—54 km 3 and thicker 0. This situation is represented by P -wave velocities of 2. The seismic data currently available is not sufficient to allow us to discriminate between these two end-member models. A thin pure melt lens, perhaps — m thick, could be underlain and flanked by a zone of high percentage partial melt. Such a structure would account for all observed P -wave delays as well as the attenuation of shear energy from the explosive shots and the gravity and magnetic data.
It is also broadly consistent with models of magma plumbing and crustal accretion at both the Mid-Atlantic Ridge Henstock et al. The low velocities are inferred to be caused by higher amounts of unconsolidated material with recent intrusives and are flanked by high velocities along the caldera rim. The small magnitudes and the spatial concentration of earthquakes recorded during the V98 experiment, a period of inflation, indicate that localized rather than regional sources of stress are responsible for the microearthquake activity.
Predominantly normal faulting focal mechanisms represent a tensional regime around the inflating magma chamber. We thank S. Husen, E. Kissling, F. Haslinger, D. Eberhart-Phillips and C. Thurber for providing their tomography software and various other assistance during this project. University of Cambridge contribution number ES.
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Earthquake epicenters — and volcanic systems in Iceland. Guomundsson M. Sigmundsson F. Gunnarsson K. Seismic crustal structure in Iceland and surrounding area , Tectonophysics , , 1 — Milsom J. Internal structure of active volcanoes in Iceland deduced from gravity modelling , The 25th Nordic Geological Winter Meeting , Abstracts, p. The dense root of the Iceland crust , Earth planet. Gudmundsson O. Sigvaldason G. The crustal magma chamber of the Katla volcano in south Iceland revealed by 2-D seismic undershooting , Geophys.
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Sparks D. Compressional and shear velocity structure of the lithosphere in northern Iceland , Bull. Miller A. Julian B. Foulger G. Three-dimensional seismic structure and moment tensors of non double couple earthquakes at the Hengill-Grensdalur volcanic complex , Geophys.
Murase T. McBirney A. Properties of some common igneous rocks and their melts at high temperatures , Geol. Crustal structure of Iceland from Explosion Seismology , pp, Soc. Podvin P. Lecomte I. I myself have observed most of them since Unfortunately, the weather is bad these days for conventional photographing; hence, aerial thermal imagery would no doubt prove useful in this case.
They melt a large volume of water which in some historical cases has amounted to as much runoff as the Amazon River yields in the course of a few days.
Geological Survey. Source Contact: S. Thorarinsson , Univ. Total discharge is estimated at 1. Information Contacts: S. Thorarinsson and H. Rist , National Energy Authority.
The new eruption was preceded by an intense earthquake swarm that began at about on 28 May. The largest earthquakes were in the magnitude range Earthquake activity declined at about and soon after that bursts of volcanic tremor began to appear on the seismograms. The tremor amplitude increased at about and intense bursts of tremor were recorded for the rest of that day and the next day.
During the following days the tremor gradually decreased in amplitude. This lake was oval-shaped, about m in diameter, and during 29 May covered by raft ice from the overhanging caldera wall. Explosions were observed in the lake at varying time intervals. The highest explosions reached about 50 m but the accompanying steam columns reached km. During the next few days, weather conditions prevented direct observations except the height of the steam column.
On 30 May, a maximum height of 6,, m was observed, on the 31st 7,, m, and on 1 June about 5, m. The steam column was intermittent, never continuous. After that, no activity has been observed, but on 5 June a small island was observed in the steaming lake.
Further Reference. Einarsson, P. Information Contacts: K. Einarsson and H. Abrupt subglacial fissure eruption fills caldera lake with meltwater; glacier burst expected. The eruption was preceded by an unusual sequence of earthquakes. One, at on 29 September, was Ms 5. Similar earthquakes have occurred beneath Bardarbunga many times during the last 22 years.
Unlike this event, however, none of the previous large earthquakes had either significant aftershocks or preceded magmatic activity. In the two hours following the M 5. Shortly after on 30 September, Science Institute seismologists informed Civil Defense authorities and the scientific community about this unusual seismicity and the possibility of impending eruptive activity.
The seismic swarm continued throughout 30 September, with increasing intensity. Hundreds of earthquakes were recorded each day, including over 10 events larger than M 3. The Civil Defense Council issued a warning of a possible eruption at on 30 September. The sudden decrease in earthquake activity and the onset of tremor may be taken as evidence that an eruption began between and on September Tremor amplitude increased very slowly during the next hours, reaching a maximum at about on 1 October.
The eruption site was spotted from aircraft in the early morning of 1 October. By that time two elongate, km wide and N23E-trending subsidence bowls or cauldrons had developed in the ice surface.
The bowls one of which is shown in figures 2 and 3 appeared in the glacial ice above a km-long NNE-trending fissure; ice in this location had been considered m thick, though some later estimates put the ice thickness more precisely at m.
The eruption was most powerful under the northernmost bowl, causing it to subside 50 m over 4 hours. The lake was covered by m of ice and held in place by an ice dam. Widening and deepening of the bowls during the day added an estimated 0.
By 1 October the lake's surface had risen m to 1, m. N-directed floods were also expected if the eruptive fissure continued to propagate N. At on the morning of 2 October a vent on the floor of one bowl broke through the ice and the eruption began a subaerial phase. At vigorous explosive activity was observed in the crater with the eruption column rising to km altitude. One account noted that rhythmic explosions resulted in black ash clouds rising m while the buoyant eruption column rose to 3 km.
In the afternoon the opening in the ice was several hundred meters wide. The eruptive fissure apparently extended 3 km farther N, because on the ice surface observers saw a new, elongated, N-trending ice cauldron.
By this time the glacier had subsided over an area km long and km wide. Subaerial eruptions pulsated, alternating between quiet periods and explosive activity. Ash mainly dispersed N but also SSW.
The opening at the eruption site grew larger. Eruptive intensity began to decline on this day but tremor continued. A TV photographer captured footage of two lightning strikes traveling along the ash cloud that was widely shown on news reports. Initial electron microprobe analysis of the ash indicated that it was basaltic andesite in composition.
The eruption continued on 4 October. It was noted that the caldera lake was higher than at any point in this century. On 9 October J-M. Bardintzeff and a visiting French team saw a 4-km-high plume as well as violent phreatic ash emissions between and On 10 October eruptive intensity appeared similar to the low levels seen since 3 October.
An 11 October flight confirmed that emissions continued, but lacked rooster-tail-shaped explosions seen previously and may have declined in intensity. The eruptive crater was still water covered. In this image they found increased backscatter compared to earlier in the month; they suggested that this may have been due to cooler ice caused by a return to stability around the crater.
In accord with this observation, on 18 October NVI announced that the eruption had apparently stopped on 13 October. The eruption left material piled up to form a subglacial ridge; the highest part of this ridge supported an eruptive crater that reached a few to tens of meters out of meltwater at the eruptive site. Cooling eruptive materials continued to melt significant volumes of ice.
Increased CO 2 and H 2 S in N-flowing river water suggested some flow of meltwater from the eruptive site. GPS measurements in October documented the lake's rise on the 12th 1, m , 15th 1, m , and 17th 1, m. In both cases, after the water reservoir drained, distinct tremor episodes occurred.
Presumably, these pressure releases triggered small eruptions. In February there was an intense, week-long earthquake swarm centered on Hamarinn volcano figure 1. The Laki eruption produced Sulfur and other gases released produced an acid haze aerosol that perturbed the weather in Western Eurasia, the North Atlantic, and the Arctic.
An estimated 9, Icelanders died in the "haze famine" from , an interval that included two severe winters, crop failures, livestock and fish deaths, and various illnesses, including fluorine poisoning Stothers, Gudmundsson, M.
Stothers, R. Thordarson, T. Worsley, P. Bardintzeff , Lab. Eruptive activity. The eruption began at on 18 December. The plume could be seen from Reykjavik, km W. Winds deflected the plume, causing tephra fallout onto the glacier up to 50 km SE. The London Volcanic Ash Advisory Center issued aviation notices later that day and throughout the eruption. The eruption was preceded by a mild increase in seismicity for several weeks.
A small earthquake swarm began at on 17 December and a sharp increase in earthquake activity began at on 18 December. This latter activity was replaced by continuous tremor at , marking the beginning of the eruption. The Icelandic Meteorological Office and the Science Institute monitored seismicity during the eruption. Vents were located along a 1,m-long E-W oriented fissure on the S caldera fault, similar to eruptions in and , at the foot of Mt.
Activity was most vigorous at one crater, but several other craters on the short eruptive fissure were also active with less frequent explosions. The eruption was slightly less vigorous on 19 December. The plume was continuous, but somewhat lower, rising to km. Tephra continued to fall SE. Activity was mostly limited to one crater. An overflight on 20 December from to revealed variable activity.
The eruption plume extended to 7 km altitude. Initially the plume was light-colored, and narrow at its base. Later the ash content of the plume greatly increased, and the plume turned black. It collapsed down to km, created a base surge, and Mt. Photographs from 27 December showed intermittent eruptive activity between and The plume was discontinuous, fed by intermittent crater activity.
It rose to a maximum of 4. The eruption has resulted in the formation of a tephra ring that lies partly on ice, but its inner part is likely to be made completely of ash overlying bedrock. The eruption ended on 28 December. Continuous tremor recorded at the Grimsfjall seismograph, 3 km from the eruption site, stopped at on 28 December. Small tremor bursts were recorded for another 3 hours, but activity stopped completely at Chemical analyses of ash. The ash was well sorted with an average grain size of 0.
Table 1. The analyses of the hyaloclastites are from Heikki Makipaa All analyses are in weight percent. Courtesy NVI. The potential chemical pollution of the fallout ash was tested by leaching a batch of ash with 6.
The pH of the leachate was 5. Gudmunsson, M. Subglacial eruption penetrates ice cover and sends ash far as Finland. Discharge declined quickly after the peak. No damage occurred to roads or bridges. Seismicity increased at the volcano in mid, about the same time that uplift exceeded a maximum reached in Additional uplift and expansion of the volcano since mid heralded the latest activity.
About 3 hours before the eruption an intense swarm of volcanic earthquakes started, changing to continuous low-frequency tremor at the onset of the eruption. The amount of drop in water level in the caldera at the onset of the eruption was uncertain, but was probably on the order of m, corresponding to a pressure change of 0. This modest pressure change triggered the eruption because pressure in the shallow magma chamber was high after continuous inflow of magma since Figure 5 shows the epicenters from 18 October to 1 November , along with preliminary locations of the eruption site.
By there were 12 earthquakes; at the largest, an event of M 3 occurred. At on 1 November an eruption warning was sent to the Civil Defense, earthquake magnitudes had increased and around that time the swarm intensified. About earthquakes with magnitudes up to 2. Initially under ice m thick, the eruption melted its way through to the surface in about 1 hour.
An eruption plume was detected by radar around midnight on 1 November. Radar estimates of plume altitude stood at km numerous times during November. A plot of altitude versus time showed two cases where plume heights were almost 13 km; each occurred about on 2 and 3 November. The ash plume was driven to the N by southerly winds during the whole eruption. Accordingly, both the scatter and SE extension of the lightning were judged likely artifacts of imprecision in estimates of lighning locations figure 6.
Regarding the lightning data, geophysicist Pordur Arason described the three systems used. First, the Icelandic lightning location system consists of three LLP direction finder stations, each measuring time, direction, polarity, intensity and multiplicity.
The stations discriminate lightning and record only cloud-to-ground CG lightning. The location system is old produced pre and unfortunately only one station Sydri-Neslond gave useful measurements. He noted that almost all of this CG lightning showed negative polarity lightning polarity is determined by the charge of the cloud compared to Earth.
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