Recent explosive eruptions and volcano hazards at Soputan volcano—a basalt stratovolcano in north Sulawesi, Indonesia

Soputan is a high-alumina basalt stratovolcano located in the active North Sulawesi-Sangihe Islands magmatic arc. Although immediately adjacent to the still geothermally active Quaternary Tondono Caldera, Soputan’s magmas are geochemically distinct from those of the caldera and from other magmas in the arc. Unusual for a basalt volcano, Soputan produces summit lava domes and explosive eruptions with high-altitude ash plumes and pyroclastic flows—eight explosive eruptions during the period 2003–2011. Our field observations, remote sensing, gas emission, seismic, and petrologic analyses indicate that Soputan is an open-vent-type volcano that taps basalt magma derived from the arc-mantle wedge, accumulated and fractionated in a deep-crustal reservoir and transported slowly or staged at shallow levels prior to eruption. A combination of high phenocryst content, extensive microlite crystallization and separation of a gas phase at shallow levels results in a highly viscous basalt magma and explosive eruptive style. The open-vent structure and frequent eruptions indicate that Soputan will likely erupt again in the next decade, perhaps repeatedly. Explosive eruptions in the Volcano Explosivity Index (VEI) 2–3 range and lava dome growth are most probable, with a small chance of larger VEI 4 eruptions. A rapid ramp up in seismicity preceding the recent eruptions suggests that future eruptions may have no more than a few days of seismic warning. Risk to population in the region is currently greatest for villages located on the southern and western flanks of the volcano where flow deposits are directed by topography. In addition, Soputan’s explosive eruptions produce high-altitude ash clouds that pose a risk to air traffic in the region.     

Random patterns of occurrence of explosive eruptions at Colima Volcano, Mexico

Analysis of the patterns of eruption occurrences may improve our understanding of volcanic processes. In this paper, the available historical data of an individual volcano, Colima in México, are used to classify its eruptions by size using the Volcanic Explosivity Index (VEI). The data shows that, if eruptions are only taken into account above a certain VEI level, the stochastic process associated with the explosive volcanic events can be represented by a non-stationary Poisson point process, which can be reduced to a homogeneous Poisson process through a transformation of the time axis. When eruptions are separated by VEI values, the occurrence patterns of each magnitude category can also be represented by a Poisson distribution. Analysis of the rate of occurrence of all eruptions with VEI greater than 1 permits the recognition of three distinct regimes or rates of volcanic activity during the last 430 years. A double stochastic Poisson model is suggested to describe this non-stationary eruptive pattern of Colima volcano and a Bayesian approach permits an estimation the present hazard.     

Pyroclast textural variation as an indicator of eruption column steadiness in andesitic Plinian eruptions at Mt. Ruapehu

Hudson is one of the most active volcanoes in the Southern Andes—it had one of the largest eruptions of the 20th century in 1991 (VEI?=?5) and smaller eruptions in 1971 (VEI?=?3), maybe 1973, and 2011 (VEI of 1-2). We use satellite-based interferometric synthetic aperture radar (InSAR) and thermal imagery to characterize the activity of Hudson between 2004 and 2011 and during the 2011 eruption. InSAR data show that the volcano inflated between 2004 and 2010 with a maximum change rate of between 2 and 3 cm/yr—about half of the deformation rate observed during a previous deformation episode from 1993–1999. Inversion for an inflating point source suggests magma accumulation beneath the SW part of the caldera at an average depth of 10 km. This inferred source is deeper than both the sources estimated for the magma chamber of the 1991 eruption (from petrology) and for the 1993–1999 deformation event. Also, the deformation from 2004–2010 is centered at a slightly different location and has a smaller volume change than that between 1993–1999—further indicating that there is either a large magma reservoir or several separate ones. While the deformation center is a few km from the eruption location near the caldera rim, the two are possibly linked since the predicted static Coloumb stress changes due to the inferred inflation source would encourage unclamping on potential faults in the caldera rim. We also analize nighttime satellite thermal images from MODIS and ASTER. While MODIS did not show any unambiguous evidence for hot spots, ASTER thermal imagery show that at least four months before the eruption there were locations with temperatures 7–8ºK above background. Lahars observed by helicopter overflights on 4 March 2011 and October 2011 suggest that the hotspots may have been caused by lakes or subglacial melting. There is no InSAR data available for the months immediately preceding the eruption, but the ASTER thermal imagery results may indicate an increase in geothermal activity that could have been used to forecast the eruption.     

A methodology for the evaluation of long-term volcanic risk from pyroclastic flows in Campi Flegrei (Italy)

The volcanological history of Campi Flegrei suggests that the most frequent eruptions are characterized by the emplacement of pyroclastic flow and surge deposits erupted from different vents scattered over a 150-km² caldera. The evaluation of volcanic risk in volcanic fields is complex because of the lack of a central vent. To approach this problem, we subdivided the entire area of Campi Flegrei into a regular grid and evaluated the relative spatial probability of opening of vents based on geological, geophysical and geochemical data. We evaluated the volcanic risk caused by pyroclastic flows based on the formula proposed by UNESCO (1972), R=H×V×Va, where H is the hazard, V is the vulnerability and Va is the value of the elements at risk. The product H×V was obtained by performing simulations of type eruptions centered in each cell of the grid. The simulation is based on the energy cone scheme proposed by Sheridan and Malin [J. Volcanol. Geotherm. Res. 17 (1983) 187–202], hypothesizing a column collapse height of 100 m for eruptions of VEI=3 and 300 m for eruptions of VEI=4 with a slope angle of 6°. Each simulation has been given the relative probability value associated with the corresponding cell. We made use of the GIS software ArcView 3.2 to evaluate the intersection between the energy cone and the topography. The superposition of the areas invaded by pyroclastic flows (124 simulations for VEI=3 and 37 for VEI=4) was used to obtain the relative hazard map of the area. The relative volcanic risk map is obtained by superimposing the urbanization maps.     

Late Holocene phases of dome growth and Plinian activity at Guagua Pichincha volcano (Ecuador)

Since the eruption which affected Quito in AD 1660, Guagua Pichincha has been considered a hazardous volcano. Based on field studies and twenty ¹⁴C dates, this paper discusses the eruptive activity of this volcano, especially that of the last 2000 years. Three major Plinian eruptions with substantial pumice discharge occurred in the 1st century, the 10th century, and in AD 1660. The ages of organic paleosols and charcoal from block-and-ash flow and fallout deposits indicate that these eruptions occurred near the end of 100 to 200 year-long cycles of discontinuous activity which was comprised of dome growth episodes and minor pumice fallouts. The first cycle took place from ~ AD 1 to 140. The second one developed during the 9th and 10th centuries, lasted 150–180 yr, and included the largest Plinian event, with a VEI of 5. The third, historic cycle, about 200 yr in duration, includes pyroclastic episodes around AD 1450 and AD 1500, explosive activity between AD 1566 and AD 1582, possible precursors of the 1660 eruption in the early decades of the 17th century, and finally the 1660 eruption (VEI 4). A fourth event probably occurred around AD 500, but its authenticity requires confirmation. The Plinian events occurred at the end of these cycles which were separated by repose periods of at least 300 yr. Older volcanic activity of similar type occurred between ~ 4000 and ~ 3000 yr BP.     

Reconstructing the eruption magnitude and energy budgets for the pre-historic eruption of the monogenetic ∼5 ka Mt. Gambier Volcanic Complex, south-eastern Australia

Understanding explosive volcanic eruptions, especially phreatomagmatic eruptions, their intensities and energy budgets is of major importance when it comes to risk and hazard studies. With only a few historic occurrences of phreatomagmatic activity, a large amount of our understanding comes from the study of pre-historic volcanic centres, which causes issues when it comes to preservation and vegetation. In this research, we show that using 3D geometrical modelling it is possible to obtain volume estimates for different deposits of a pre-historic, complex, monogenetic centre, the Mt. Gambier Volcanic Complex, south-eastern Australia. Using these volumes, we further explore the energy budgets and the magnitude of this eruption (VEI 4), including dispersal patterns (eruption columns varying between 5 and 10 km, dispersed towards north-east to south), to further our understanding of intraplate, monogenetic eruptions involving phreatomagmatic activity. We also compare which thermodynamic model fits best in the creation of the maar crater of Mt. Gambier: the major-explosion-dominated model or the incremental growth model. In this case, the formation of most of the craters can best be explained by the latter model.     

Probabilistic evaluation of the physical impact of future tephra fallout events for the Island of Vulcano,Italy

A first probabilistic scenario-based hazard assessment for tephra fallout is presented for La Fossa volcano (Vulcano Island, Italy) and subsequently used to assess the impact on the built environment. Eruption scenarios are based upon the stratigraphy produced by the last 1000 years of activity at Vulcano and include long–lasting Vulcanian and sub-Plinian eruptions. A new method is proposed to quantify the evolution through time of the hazard associated with pulsatory Vulcanian eruptions lasting from weeks to years, and the increase in hazard related to typical rainfall events around Sicily is also accounted for. The impact assessment on the roofs is performed by combining a field characterization of the buildings with the composite European vulnerability curves for typical roofing stocks. Results show that a sub-Plinian eruption of VEI 2 is not likely to affect buildings, whereas a sub-Plinian eruption of VEI 3 results in 90 % of the building stock having a ≥12 % probability of collapse. The hazard related to long-lasting Vulcanian eruptions evolves through time, and our analysis shows that the town of Il Piano, located downwind of the preferential wind patterns, is likely to reach critical tephra accumulations for roof collapse 5–9 months after the onset of the eruption. If no cleaning measures are taken, half of the building stock has a probability >20 % of suffering roof collapse.     

A remarkable pulse of large-scale volcanism on New Britain Island,Papua New Guinea

New studies of the deposits from the latest caldera-forming eruption (the “Dk” event) at Dakataua Volcano, New Britain Island, Papua New Guinea, help identify an intense space-time concentration of large-scale volcanism during the 7th century AD on New Britain. Radiocarbon dating of charcoal from the Dk deposits yields an age of 1,383 ± 28 BP. Calibration of this result gives an age in the range AD 635–670 (at 1 s. d.). At about the same time, two other volcanoes on New Britain, Rabaul and Witori, also produced very large eruptions. Very high acidity levels in ice cores from Antarctica and Greenland at AD 639 and AD 640 respectively may be linked to either or both of the Dakataua and Rabaul eruptions. Another ice core high acidity level, at AD 692, may be associated with the Witori eruption. Significant volcanic risk within the New Britain region is indicated by its Late Cenozoic history of relatively frequent large-scale eruptions from as many as 8 caldera systems within an arc-parallel zone about 380 km long. Over the last 20 ka the return period for major (VEI 5+) eruptions in this region was about 1.0 ka and individually high frequencies of major eruptive activity were experienced at Witori and Rabaul. The relatively short return period for major eruptions in the region would tend to increase the chance that such events could cluster in time.     

Stochastic modelling at Stromboli: a volcano with remarkable memory

The assessment of potential volcanic eruptions is a critical aspect when evaluating the safety of populated areas. A stochastic approach has been developed for the analysis and simulation of data sampled at active volcanoes. This approach allows for the detection and quantification of time correlation, volcanic event forecasts using Cox model based simulations and volcanic tremor decomposition in order to identify potential precursors of major eruptions. The stochastic approach has been applied to data monitored at Stromboli volcano. Significant time correlation has been detected which makes Stromboli a volcano with a remarkable memory of its recent activity in comparison to other volcanoes. Forecasting of the number of strombolian events for the next few days has been performed by Monte Carlo simulations. Finally, kriging analysis of the tremor intensity has enabled time component estimation which could furnish additional monitoring variables for the forecast of paroxysmal phases at Stromboli.     

The size and frequency of the largest explosive eruptions on Earth

A compilation and analysis of the size and frequency of the largest known explosive eruptions on Earth are presented. The largest explosive events are defined to be those eruptions yielding greater than 10¹⁵ kg of products (>150 times the mass of the 1991 eruption of Mt. Pinatubo). This includes all known eruptions with a volcanic explosivity index (VEI) of 8. A total of 47 such events, ranging in age from Ordovician to Pleistocene, are identified, of which 42 eruptions are known from the past 36 Ma. A logarithmic magnitude scale of eruption size is applied, based on erupted mass, to these events. On this scale, 46 eruptions >10¹⁵ kg are defined to be of magnitude M8. There is one M9 event known so far, the Fish Canyon Tuff, with an erupted mass of >10¹⁶ kg and a magnitude of 9.2. Analysis of this dataset indicates that eruptions of size M8 and larger have occurred with a minimum frequency of 1.4 events/Ma in two pulses over the past 36 Ma. On the basis of the activity during the past 13.5 Ma, there is at least a 75% probability of a M8 eruption (>10¹⁵ kg) occurring within the next 1 Ma. There is a 1% chance of an eruption of this scale in the next 460–7,200 years. While the effect of any individual M8 or larger eruption is considerable, the time-averaged impact (i.e., erupted mass×frequency) of the very largest eruptions is small, due to their rarity. The long-term, time-averaged erupted mass flux from magnitude 8 and 9 eruptions is ~10–100 times less than for M7 eruptions; the time-averaged mass eruption rate from M7 eruptions is 9,500 kg s^–1, whereas for M8 and M9 eruptions it is ~70–1,000 kg s^–1. Comparison of the energy release by volcanic eruptions with that due to asteroid impacts suggests that on timescales of <100,000 years, explosive volcanic eruptions are considerably more frequent than impacts of similar energy yield. This has important implications for understanding the risk of extreme events.Editorial responsibility: R. Cioni     

Paleomagnetic dating of the most recent silicic eruptive activity at Pantelleria (Strait of Sicily)

We report on the paleomagnetism of ten sites in the products of the most recent silicic eruptive cycle of Pantelleria, Strait of Sicily. Previously radiometrically dated at 5–10 ka, our comparison with proxies for geomagnetic field directions allows us to narrow considerably the time window during which these eruptions occurred. The strongly peralkaline composition causes the magmas to have low viscosities, locally resulting in strong agglutination of proximal fall deposits. This allows successful extraction of paleomagnetic directions from the explosive phases of eruptions. One of our sites was located in the Serra della Fastuca fall deposit, produced by the first explosive event of the eruptive cycle. The other nine sites were located in the most recent explosive (pumice fall and agglutinate from Cuddia del Gallo and Cuddia Randazzo) and effusive (Khaggiar lava) products. The (very similar) paleomagnetic directions gathered from eight internally consistent sites were compared to reference geomagnetic field directions of the last 5–10 ka. Directions from Cuddia del Gallo agglutinate and Khaggiar flows translate into 5.9- to 6.2-ka ages, whereas the Fastuca pumices yield a slightly older age of 6.2–6.8 ka. Hence, the most recent silicic eruptive cycle lasted at most a millennium and as little as a few centuries around 6.0 ka. Paleomagnetically inferred ages are in good agreement with published (and calibrated by us) ¹⁴C dates from paleosols/charcoals sampled below the studied volcanic units, whereas K/Ar data are more scattered and yield ∼30% older ages. Our data show that the time elapsed since the most recent silicic eruptions at Pantelleria is comparable to the quiescence period separating the two latest volcanic cycles.     

Impacts of volcanism on pre-European inhabitants of Taveuni, Fiji

 Taveuni is a Fijian ocean-island volcano that sporadically erupted throughout the Holocene. The 437-km² island is an active monogenetic volcanic field with a constantly shifting locus of activity along a single apparent rift axis. Although the eruptions were not large ( ≤VEI 2), unexpected shifts in Taveuni volcanism had the potential to affect habitation sites. Since known human settlement of the Fiji Group (ca. 950–750 BC), there have been at least 58 eruptions on Taveuni. Up to 25 of these eruptions potentially affected pre-European inhabitants of the island and at least four former occupation sites are known to have been affected by volcanic products. Despite apparent earliest settlement of Taveuni post-dating other nearby islands by up to 600 years, volcanism probably did not hinder or stall settlement of Taveuni compared with neighbouring islands. However, a period of voluminous eruptions between 300 and 500 AD covered much of south Taveuni with lava and/or thick tephra, apparently causing abandonment of at least this portion of Taveuni until approximately 1100 AD. Most eruptions were not of catastrophic proportions and, due to their localised effects, re-settlement was rapid in marginal unaffected areas. Localised stories and a relict place name survive to describe former eruption locations and effects since approximately 120–320 AD. Knowledge of the impacts on Taveuni's past inhabitants forms the basis of volcanic disaster-mitigation strategies to minimise future effects on the current 14,500 residents. Received: 9 September 1999 / Accepted: 21 February 1999     

Ubinas: the evolution of the historically most active volcano in southern Peru

Ubinas volcano has had 23 degassing and ashfall episodes since A.D. 1550, making it the historically most active volcano in southern Peru. Based on fieldwork, on interpretation of aerial photographs and satellite images, and on radiometric ages, the eruptive history of Ubinas is divided into two major periods. Ubinas I (Middle Pleistocene >376 ka) is characterized by lava flow activity that formed the lower part of the edifice. This edifice collapsed and resulted in a debris-avalanche deposit distributed as far as 12 km downstream the Rio Ubinas. Non-welded ignimbrites were erupted subsequently and ponded to a thickness of 150 m as far as 7 km south of the summit. These eruptions probably left a small collapse caldera on the summit of Ubinas I. A 100-m-thick sequence of ash-and-pumice flow deposits followed, filling paleo-valleys 6 km from the summit. Ubinas II, 376 ky to present comprises several stages. The summit cone was built by andesite and dacite flows between 376 and 142 ky. A series of domes grew on the southern flank and the largest one was dated at 250 ky; block-and-ash flow deposits from these domes filled the upper Rio Ubinas valley 10 km to the south. The summit caldera was formed between 25 and 9.7 ky. Ash-flow deposits and two Plinian deposits reflect explosive eruptions of more differentiated magmas. A debris-avalanche deposit (about 1.2 km³) formed hummocks at the base of the 1,000-m-high, fractured and unstable south flank before 3.6 ka. Countless explosive events took place inside the summit caldera during the last 9.7 ky. The last Plinian eruption, dated A.D.1000–1160, produced an andesitic pumice-fall deposit, which achieved a thickness of 25 cm 40 km SE of the summit. Minor eruptions since then show phreatomagmatic characteristics and a wide range in composition (mafic to rhyolitic): the events reported since A.D. 1550 include many degassing episodes, four moderate (VEI 2–3) eruptions, and one VEI 3 eruption in A.D. 1667. Ubinas erupted high-K, calc-alkaline magmas (SiO₂=56 to 71%). Magmatic processes include fractional crystallization and mixing of deeply derived mafic andesites in a shallow magma chamber. Parent magmas have been relatively homogeneous through time but reflect variable conditions of deep-crustal assimilation, as shown in the large variations in Sr/Y and LREE/HREE. Depleted HREE and Y values in some lavas, mostly late mafic rocks, suggest contamination of magmas near the base of the >60-km-thick continental crust. The most recently erupted products (mostly scoria) show a wide range in composition and a trend towards more mafic magmas.Recent eruptions indicate that Ubinas poses a severe threat to at least 5,000 people living in the valley of the Rio Ubinas, and within a 15-km radius of the summit. The threat includes thick tephra falls, phreatomagmatic ejecta, failure of the unstable south flank with subsequent debris avalanches, rain-triggered lahars, and pyroclastic flows. Should Plinian eruptions of the size of the Holocene events recur at Ubinas, tephra fall would affect about one million people living in the Arequipa area 60 km west of the summit.Editorial responsibility: D Dingwell     

Volcanic lightning: global observations and constraints on source mechanisms

Lightning and electrification at volcanoes are important because they represent a hazard in their own right, they are a component of the global electrical circuit, and because they contribute to ash particle aggregation and modification within ash plumes. The role of water substance (water in all forms) in particular has not been well studied. Here data are presented from a comprehensive global database of volcanic lightning. Lightning has been documented at 80 volcanoes in association with 212 eruptions. The Volcanic Explosivity Index (VEI) could be determined for 177 eruptions. Eight percent of VEI = 3–5 eruptions have reported lightning, and 10% of VEI = 6, but less than 2% of those with VEI = 1–2. These findings suggest consistent reporting for larger eruptions but either less lightning or possible under-reporting for small eruptions. Ash plume heights (142 observations) show a bimodal distribution with main peaks at 7–12 km and 1–4 km. The former are similar to heights of typical thunderstorms and suggest involvement of water substance, whereas the latter suggest other factors contributing to electrical behavior closer to the vent. Reporting of lightning is more common at night (56%) and less common in daylight (44%). Reporting also varied substantially from year to year, suggesting that a more systematic observational strategy is needed. Several weak trends in lightning occurrence based on magma composition were found. The bimodal ash plume heights are obvious only for andesite to dacite; basalt and basaltic-andesite evenly span the range of heights; and rhyolites are poorly represented. The distributions of the latitudes of volcanoes with lightning and eruptions with lightning roughly mimic the distribution of all volcanoes, which is generally flat with latitude. Meteorological lightning, on the other hand, is common in the tropics and decreases markedly with increasing latitude as the ability of the atmosphere to hold water decreases poleward. This finding supports the idea that if lightning in large (deep) eruptions depends on water substance, then the origin of the water is primarily magma and not entrainment from the surrounding atmosphere. Seasonal effects show that more eruptions with lightning were reported in winter (bounded by the respective autumnal and vernal equinoxes) than in summer. This result also runs counter to the expectations based on entrainment of local water vapor.     

The fractal structure of the sequence of volcanic eruptions worldwide: Order clustering of events and episodic discharge of material

This study of the time-dependent structure of the Earth’s volcanic activity at scales from a few months to a few centuries is based on three sequences of events: two selections from the Smithsonian catalog and a selection from the ice core-volcano index catalog (IVI). This data selection was based on carefully chosen (1) lower thresholds for event size and (2) time intervals, so as to make the resulting data subsets homogeneous. The next step was to analyze the time structure. Three types of clustering trends were revealed, as follows. (1) The rate of events in the sequence is not uniform over time, with their dates forming activity episodes. (2) The time-ordered sequence of eruption size shows that larger events form closely connected clusters. We give the name “order clustering” to this novel phenomenon. (3) The eruption rate shows episodic and spiky behavior. In each of the three cases the tendency to clustering has a multi-scale, fractal (self-similar) character: the sequences of eruptions behave as impulsive 1/f noise. These results suggest the hypothesis that there must be some global mechanism that is responsible for synchronization of bursts of activity on this planet.     

Petrological analysis of the pre-eruptive magmatic process prior to the 2006 explosive eruptions at Tungurahua volcano (Ecuador)

Understanding the processes at the origin of explosive events is crucial for volcanic hazard mitigation, especially during long-lasting eruptions at andesitic volcanoes. This work exposes the case of Tungurahua volcano, whose unrest occurred in 1999. Since this date, the eruptive activity was characterized by low-to moderate explosiveness, including phases with stronger canon-like explosions and regional ash fallout. However, in 2006, a sudden increase of the explosiveness led to pyroclastic flow-forming eruptions on July 14th (VEI 2) and August 16–17th (VEI 3). All magmas emitted from 1999 to 2005, as well as the samples from the 2006 eruptions, have homogeneous bulk-rock andesitic compositions (58–59 wt.% SiO₂), and contain the same mineral assemblage consisting of pl + cpx + opx + mag ± ol. However, during the August 16–17th event, the erupted tephra comprise two types of magmas: a dominant, brown andesitic scoria; and scarce, light-grey pumice representing a subordinate, silica-rich juvenile component. For the andesitic magma, thermobarometric data point to magmatic temperatures ranging from 950 to 1015 °C and pressures in the range of 200 to 250 MPa, which corresponds to 7.5–9.5 km below the summit. Disequilibrium textures in plagioclase and pyroxene phenocrysts, particularly thin overgrowth rims, indicate the recharge of this magma body by mafic magma. Between 1999 and 2005, repeated injections from depth fed the intermittent eruptive activity observed while silica-rich melts were produced by in-situ crystallization in the peripheral parts of the reservoir. In April 2006, the recharge of a primitive magma produced strong convection and homogenisation in the reservoir, as well as pressure increase and higher magma ascent rate after seven years of only moderately explosive activity. This work emphasizes the importance of petrological studies in constraining the pre-eruptive magmatic conditions and processes, as a tool for understanding the fundamental causes of the changes in the eruptive dynamism, particularly the occurrence of paroxysmal phases in andesitic systems with open-vent behaviour.     

Surface deformation at the Krafla volcano,North Iceland, 1982–1992

Extensive measurements of ground deformation at the Krafla volcano, Iceland, have been made since the beginning in 1975 of a series of eruptions and intrusions into the fissure system that extends north and south of the volcano. I concentrate on measurements before and after the eruption of September 1984, the last event of this series when the largest volume of lava was erupted. The patterns of ground deformation associated with the 1984 eruption, determined by precision levelling, electronic distance measurements and lake level observations, were similar to earlier intrusions and eruptions, in that the surface of the volcano subsided and the fissure system widened as magma moved laterally from a shallow central reservoir into the fissure system. The shallow magma reservoir of Krafla continued to expand for about five years after the eruption, but a slow subsidence of the central area began in 1989. Besides the presence of an inflating and deflating shallow magma reservoir at a depth of 2.5 km beneath the Krafla caldera, another inflating magma reservoir may exist at much greater depth below Krafla. The accumulation of compressive strain by numerous rift intrusions and eruptions since 1975 along the flanks of the north-south Krafla fissure swarm is being released slowly and will probably be reflected in the results of deformation measurements near Krafla for the next several decades. The total horizontal extension of the Krafla rift system in 1975–1984 was about 9 m, equal to about 500 years of constant plate divergence. The extension is twice the accumulated divergence since previous rifting events and eruptions in 1724–1729     

Eruption and mass-wasting-induced processes during the late holocene destructive phase of Nevado del Ruiz volcano, Colombia

The November 13, 1985, eruption was characteristic of the Arenas eruptive stage of Nevado del Ruiz, the most recent of a series of twelve eruptive stages that have occurred in the past 11,000 years. Eruptive sequences, deposits and processes similar to that of 1985 have characterized the behavior of Nevado del Ruiz during three major prehistorical and historical eruptive stages: the approximately 3300-3100 yr. B.P. Hedionda, the 16th century Azufrado, and the mid-1800's Lagunillas eruptive stages, that partly destroyed the present Ruiz summit.According to the interpretation of the stratigraphic record of prehistorical eruptions and historical accounts, almost every recent magmatic event was small or short-lived. Nevertheless, rockslide-debris avalanches and catastrophic debris flows were triggered in all the eruptions owing to slope failures related to specific tectonic features of Ruiz volcano and/or to significant interactions between pyroclastic debris and the ice cap. Evidence for headward retreat of avalanche scarps during multiple eruptions reinforce the case that large slope failures can occur repeatedly at a large-volume volcano like Ruiz without reconstruction of the edifice. The latest Ruiz eruptions that involved rockslide-debris avalanches resemble in part the Shiveluch 1964 event for which evidence of lateral blast deposits are lacking, but differ in part from this type because non-eruptive and mass-wasting processes also triggered rockslide-debris avalanches.Many factors render the cluster of domes of the Ruiz summit unstable, including: (1) deeply dissected troughs opened toward the north-northeast (Azufrado), east (Lagunillas), and south (Recio) flanks; (2) strongly hydrothermally altered north and east flanks of the summit; (3) currently glaciated or recently deglaciated, high cliffs; (4) steep unstable margins of the ice cap on the north and east.Thus, in light of its past behavior, a small eruption or an earthquake might trigger catastrophic rockslide-debris avalanches. Furthermore, such avalanches as well as glacial outburst-floods and ice avalanches could induce debris flows by mobilizing weathered, water-saturated, and unconsolidated rocks or deposits.     

诱发前震图象与1995年1月17日日本阪神大地震

地震引起的应力场扰动随距离不断衰减，远程触发很难发生，只有极不稳定状态的断层才有这种可能。利用这一特点，可以预报下次地震的位置，为实施减灾措施和捕捉短临前兆提供机会。所以，紧随大地震之后发生的“诱发前震”就具有了特殊的前兆意义。统计表明，大地震（Ａ事件）可能触发远处临界失稳断层发生前兆地震（Ｆ事件），在Ｆ事件附近发生下次大震（Ｂ事件）的概率比自然概率高十几倍到几十倍。这一结论被从中国大陆、全球主要板块边界及日本列岛发现的众多ＡＦＢ图象所证实。对其发生的可能的物理机制在文末进行了讨论。