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Montserrat eruption Monitoring on Montserrat: Montserrat, the Emerald Isle of the Caribbean, is not so green any more. Simon Young reports on the eruption so far and prognoses for the future. T he erupting Soufrière Hills volcano has, over the past 33 months, spread ash and lava blocks over much of the southern half of the island of Montserrat. People who have not left the island entirely have been forced to move towards the undeveloped northern tip of the 39 square mile British Overseas Territory (figure 1). Since surface activity began in July 1995, the Montserrat Volcano Observatory (MVO) has provided the base for monitoring operations, using the widest variety of geophysical and geological tools available. The British Geological Survey, in collaboration with UK academia and regional colleagues at the Seismic Research Unit (University of the West Indies) and Institute de Physique de la Globe at volcano observatories in the French Antilles, and with the assistance of international experts from the USA, Japan and Europe, has led efforts to document the eruption and provide hazard and risk assessments. As well as providing scientific data, the monitoring effort has enabled considerable progress to be made in crisis management and the interaction between scientists, authorities and the public in a natural disaster. The current eruptive episode began with seismic unrest beneath the volcano between 1992 and 1994, similar in style but less intense than three previous crises between 1897 and 1967. For the first time in the documented history of the island (colonized in 1632), surface manifestation of this enhanced seismicity began on 18 July 1995 and has gone on ever since. Four months of phreatic activity – explosions without eruption of fresh magma – gave way to continuous dome growth. More than 250 million cubic metres of andesitic magma have since reached the surface. Early seismic activity was dominated by volcano–tectonic earthquakes up to 5 km beneath the volcano but, once the dome began to grow, long-period seismicity 1 to 2 km down has dominated. Magma flux has increased throughout the eruption (figure 2), an unusual pattern. This increasing magma flux from the 5 to 6 km deep magma chamber has led to progressively more violent activity. A chronology of the eruption Violent activity was first manifest during the spring and summer of 1996, when a series of dome collapses occurred, generating pyroclastic flows out of the open side of the crater and into the Tar River valley (figure 3). This series 2.18 of collapses culminated in the first truly explosive event, on the night of 17 September 1996. Pressure in the upper part of the feeder conduit dropped rapidly after about 150 m of dome overburden had been removed, and a short explosion threw metre-sized blocks up to 1.5 km, generating a column of ash and pumice 15 km high. Increased magma flux was matched by increased sulphur dioxide production and an increasing rate of deformation in the upper part of the edifice. Deformation became localized in November and December 1996, with rapid degradation of the southwestern crater wall, accompanied by intense hybrid earthquake swarms. A small explosion and pyroclastic flows in mid-December 1996 marked a switch back to vigorous extrusion, generating more pyroclastic flows in the Tar River valley in January and in the White River valley to the southwest in March and April 1997. Increased rates of deformation on the lower flanks of the volcano, as well as increasing gas and magma flux, culminated in a large dome collapse from the northern flank on 25 June, generating pyroclastic flows and surges which devastated several villages and led to the loss of 19 lives within the declared danger zone. High magma flux was maintained throughout July, accompanied by cyclic inflation and deflation of the lava dome (measured by electronic tiltmeters on the crater rim) with hybrid earthquake and rockfall activity. The capital of the island, Plymouth, was overrun by pyroclastic flows, burying or burning vital infrastructure. Some depressurization from continued dome collapses, with the presence of magma rich in gas high in the feeder system, led to a series of 13 short-lived, vulcanian explosions in early August. These occurred within strong seismic and deformation cycles and were remarkably evenly spaced. Most generated pumiceous pyroclastic flows in valleys around the volcano, as well as ash columns over 10 km high. Similar, though longer-lived and less regular explosive behaviour followed collapse of the northern flank of the dome on 21 September. A total of 75 explosions occurred, on average 9.5 hours apart, all but one generating fountain-collapse pyroclastic flows, and most generating ash columns more than 10 km high (figure 4). Magma crystallization, forming a cap on the conduit, so that pressure builds up is likely to control these multiple explosions. Fig. 3: Pyroclastic flow in the Tar River valley, spring 1996. Photo he current eruption of the Soufrière Hills volcano has lasted three years; it is likely to continue and seems to be increasing in violence. Seismic activity, ground deformation, gas emissions and the composition of the ash and lava erupted have all been monitored, giving a data set valuable for understanding the eruption mechanism as well as assessing the hazards to local people. The eruption has followed a pattern of increasingly explosive T Dome growth switched back to the southwest side of the crater in early November and continued apace until Christmas Day 1997, when frequent, large hybrid earthquakes, merging periodically into continuous tremor, preceded the most violent events yet at the Soufrière Hills volcano. Collapse of the crater wall on the southwest side produced a debris avalanche in the White River valley, rapidly followed by massive dome collapse and one or more laterally-directed blasts generating violent pyroclastic surges which destroyed several villages on the southwestern coast (figure 5). About 45 million cubic metres of dome lava were involved in this collapse, five times larger than any previous single collapse event. A small tsunami was generated as 20 million cubic metres of material entered the ocean April 1998 Vol 39 Montserrat eruption the course of an eruption 1 The island of Montserrat, situated towards the northern end of the Lesser Antilles island arc. N St John's Centre Hills W.H. Bramble Airport Salem Harris Cork Hill St George's Hill S Young, BGS/NERC copyright. Seismology of the eruption Earthquake activity associated with the current eruption began in 1992, reaching a peak in late 1994. Although poorly-located due to a limited seismic network at the time, these events appear to show a progressive upward migration in focal depth. Surface activity began after seven months of seismic quiescence and was associated with swarms of volcano–tectonic earthquakes both beneath the volcano and up to a few kilometres away. Installation of a dense short-period seismometer network around the volcano brought better locations, and hypocentres less than 5 km below the April 1998 Vol 39 Tar River 2 Total volume of magma erupted from the Soufrière Hills volcano, measured by dome growth and deposit accumulation rates. St Patrick's 4 km 2 volume (millions of cubic metres) 4.5 km from the dome; it had an estimated wave-height of 1 to 1.5 m where focused in a bay midway up the west coast of Montserrat. Long Ground Soufriere Hills Volcano Plymouth eruptions, generating larger pyroclastic flows that travel farther from the crater. This appears to be a result of faster ascent of the magma to the surface; more gas is held in the magma until eruption, resulting in more violent explosions. Of particular interest has been the recognition of cyclic behavior of the volcano and the coincidence of dome inflation with enhanced seismicity, followed by rock falls or pyroclastic flows. English's crater 250 200 150 100 50 0 15 Nov 24 Jan 3 Apr 12 Jun 21 Aug 30 Oct 8 Jan 19 Mar 28 May 6 Aug 15 Oct 24 Dec 4 Mar 97 97 97 98 95 96 97 97 97 96 96 96 96 eruption days crater began to dominate. Phreatic explosions and gas venting were associated with sharponset and tremor signals respectively. An array of digitally-telemetered broadband instruments was installed in October 1996 and, despite losses during eruptions, a remarkable set of data has been collected, recording dome-related seismicity in greater detail than ever before. Hybrid earthquakes have dominated the seismicity since before the dome started to grow. These are low-frequency events with impulsive P arrivals, at shallow depth beneath the crater, and probably caused by gas resonance within the magma conduit. Long-period earthquakes, with dominant frequencies of 1–2 Hz and emergent onsets, probably have a similar source mechanism but in a shallower location, perhaps within the dome itself. Long-period earthquakes are not common, but occur more frequently on occasion, with up to 60 events per day. The volcano–tectonic earthquakes that dominated at the start of the eruption have 2.19 Montserrat eruption Formal hazard and risk assessments PHOTOS S YOUNG, BGS/NERC COPYRIGHT Hazard and informal risk assessments have been carried out by the Montserrat Volcano Observatory (MVO) throughout the crisis, but a thorough re-assessment and formalization of the process was requested by the UK Government following the prolonged period of explosive activity at the volcano in September and October 1997. A meeting of 14 scientists, almost all of whom had been intimately involved in operations at MVO, was held in early December. A review of scientific information relevant to the hazard assessment was given and every conceivable scenario for behaviour over the following six months constructed. Through a formal process of expert elicitation, a probability tree was constructed for these scenarios, providing information on how likely a certain event was and conditional probabilities on hazardous phenomena reaching certain areas on Montserrat. Once probabilities were known and some measure of uncertainty was assessed, the formal risk assessment took place. The population distribution of Montserrat at present and in a number of different evacuation situations was used, along with expert assessments of the degree of impact of particular hazardous phenomena on different areas (i.e. the vulnerability of the population and infrastructure to the different phenomena). Through a Monte Carlo simulation process, the number of casualties for different scenarios was evaluated and a probability of one, five, ten etc deaths in the next six months assessed with different population distributions. In this way, the current societal risk exposure for Montserrat was presented and options for reduction of this risk exposure presented by means of altering the population distribution. Individual risk was also assessed for different populated areas. Significant uncertainties surround any quantitative assessment of risk due to the inherent instability of the volcanic system. However, proven statistical methods for assessing low probability events and accounting for large uncertainties can be utilized, and the end product of assessments such as these has proven extremely useful for decision makers in the Montserrat crisis. Fig. 4: Vertical eruption column ascends over Montserrat, as viewed from 15 miles west of the island, October 1997. Fig. 5: Devastation of the southwestern part of Montserrat by debris avalanche, dome collapse and lateral blast, Boxing Day 1997. become less common, but occasional swarms still occur, 2–4 km beneath the crater. These are high-frequency events with P and S phases. Other seismicity includes signals from rockfalls and pyroclastic flows (high frequency, emergent signals with a characteristic cigar-shaped profile) and explosions signals with associated volcanic tremor (both phreatic and magmatic, with dominant frequencies at 1–2 Hz). Hybrid earthquakes have often been recorded in swarms and some show remarkable sequences of near-identical events occurring at regular intervals up to five times per minute for many hours. These repetitive swarms sometimes develop into tremor as the individual events merge into a continuous signal, with the dominant frequency staying constant (figure 6). In the past year, occasional swarms of large hybrid earthquakes have been recorded by seismic stations on neighbouring islands. Hybrid and long-period events are thought to be caused by gas under pressure within the 2.20 dome and upper conduit area, and they are often associated with changes in surface activity on a variety of time scales. Long-period earthquakes often trigger rockfalls from the dome, suggesting that they are caused by gas venting locally. Perhaps the most remarkable aspect of the seismic activity has been the association of hybrid earthquake swarms with near-crater deformation recorded by tiltmeters, and the generation of pyroclastic flows. Electronic tiltmeters: pressure gauges One of the clearest indicators of surface processes on volcanoes is deformation of the ground. The use of electronic tiltmeters to measure ground deformation on volcanoes is well established, since the May 1980 eruption of Mt St Helens. Tiltmeters were installed at three locations on the Soufrière Hills volcano early in the crisis, but all were too far from the growing lava dome. A tiltmeter was first installed in December 1996 on Chance’s Peak, within a few hundred metres of the lava dome, to track deformation specifically associated with degradation of the southwestern crater wall. Harsh conditions at this location meant that only a relatively limited data set could be collected, but deformation associated with the instability of the crater wall was recorded, as were cycles of inward and outward tilt apparently centering on the lava dome and co-incident with swarms of hybrid earthquakes. After replacement of tiltmeters on Chance’s Peak in May 1997, a remarkable two-month record of near-source deformation was collected (figure 7). Strongly cyclic behaviour of the volcano was noted in the few days prior to the large dome collapse on 25 June; cycles comprised a steadily increasing intensity of hybrid earthquakes, which then dropped suddenly from a peak, replaced by intense rockfall or pyroclastic flow activity. Cycles repeated every 8 hours or so, and coincided with steady inflation of the lava dome during the hybrid swarm and rapid deflation as the rockfall activity began. On the basis of these well-developed seismological and deformation cycles, warning sirens were sounded prior to the onset of pyroclastic flows early in the afternoon of 25 June, enabling Plymouth and the airport on the east coast to be evacuated. Unfortunately, many people had ventured too far into the danger zone and could not hear the sirens; 19 lost their lives and many others had lucky escapes. Cyclic activity diminished in early July but picked up again in late July and early August as the next phase of intense activity began. The single remaining tiltmeter was destroyed in a series of vulcanian explosions on 5 August 1997; clear tilt cycles had been recorded prior to this explosion, and it is thought that strong tilt cycles would have matched the cyclic seismicity throughout these explosions. April 1998 Vol 39 Montserrat eruption Mick Murphy at Bristol University (Britain) and Joe Devine at Brown University (USA) and co-workers, have gained useful insights into the volcano from samples collected as part of the monitoring effort. The magma is a typical Lesser Antillean andesite. It has spent some time within the crust below the volcano: experimental work at Bristol and University of California, Berkeley, by Jenni Barclay has defined the magma holding depth as at least 5 to 6 km, with temperatures of between 830 and 850 °C. Lava in older pyroclastic flow deposits at the volcano, very like the lava currently erupting, suggests a long-lived (>30 000 years) magma storage region, periodically reheated and remobilized by injection of mafic magma. If enough new magma arrives over a long enough period, it may start surface activity and dome growth. Petrological studies by Devine and Mac Rutherford at Brown also show increasing magma flux through the eruption. When magma ascends to the vent, the outer parts of hornblende crystals react to form rims of different composition. Rims thinned from 120 µm (implying 30 days ascent) to zero (less than 5 days ascent) between the first erupted magma (December 1995) and that erupted in the summer of 1996. Since then, ascent rate has been too fast for any reaction rims. The increasing propensity for explosive eruptions involving highly vesicular pumice is consistent with less-complete gas loss during the faster ascent of the magma. But there is as yet little firm indication that the storage region has changed. Near-surface phenomena, such as sealing of the conduit, may reduce gas loss; microlite crystallization within the feeder system also affects surface processes. What next? The long-term prognosis for the eruption is for a continuation of similar styles of activity for perhaps several more years, with impacts confined to the southern half of the island. The inconvenience of ash and small clasts falling in the northern, inhabited, part of the island will April 1998 Vol 39 17.12 17.32 –13070 15083 17.52 –18545 22984 18.12 –16700 50510 18.32 Petrology of the Soufrière Hills magma 11620 18.52 A preliminary model for these cycles has been proposed, and it is hoped that two tiltmeters recently installed within a kilometre of the dome will record similar cycles over a longer time period. Gas-charged magma migrates in batches up through the upper part of the conduit and dome, generating hybrid earthquakes and inflating the upper conduit as pressures increase. When there is high dome overburden, the cycle peaks as the magma pulse is injected into the dome, leading rapidly to increased ash and gas venting and rockfalls. If there is little overburden or if the magma is richer in gas, the pressure is released more rapidly, through short-lived vulcanian explosions. –37353 44691 –26260 31933 –30909 2 4 6 8 0 2 time (min) 4 6 8 0 Fig. 6: Repetitive hybrid earthquakes merging into continuous volcanic tremor, December 1996. –50 Chances 3 x-axis –100 –150 –200 –50 –100 –150 Chances 3 y-axis –200 2000 long ground RSAM 1500 1000 500 400 300 gages triggers/hour 200 100 May 25 June 1 June 5 June 10 June 15 June 20 June 25 Fig. 7: Strong cycles of seismicity and dome pressurization recorded by seismometers and electronic tiltmeters close to the lava dome. continue, but recent detailed hazard and risk assessments by MVO suggest that risk in the far north of the island is very low (see box). Monitoring will continue at a high level and constant revision of hazard and risk assessments will take place. MVO continues to provide accurate and timely information and advice to the authorities on Montserrat and in the UK as well as collecting a unique and diverse set of geological and geophysical data which will further volcanological science. ● ● The first collection of papers on the Montserrat eruption is to be published in Geophysical Research Letters later this year. Reports by the MVO on current activity are available on the Web at: “http://www.geo.mtu. edu/volcanoes/west.indies/soufriere/govt”. Dr Simon R Young works at the British Geological Survey, Murchison House, West Mains Road, Edinburgh, and has been one of the Chief Scientists at Montserrat Volcano Observatory since 1992. 2.21