Three distinct regimes of volcanic tremor associated with the eruption of Shishaldin Volcano, Alaska 1999

Tremor signals associated with the eruption of Shishaldin Volcano on 19 and 23 April 1999 were the strongest recorded anywhere in the Aleutian Arc by the Alaska Volcano Observatory (AVO) in its 10-year history. Reduced displacements (D_R) reached 23 cm² on 19 April and 43 cm² on 23 April. During the activity, D_R and spectral data with a frequency resolution of 0.1 Hz were computed and put on the World Wide Web every 10 min. These data are analyzed here. The general temporal patterns of seismicity of these eruption events were similar, but the eruptions and their effects quite different. The 19 April event is known to have culminated in a sub-Plinian phase, which ejected ash to an altitude of 16 km. Despite higher amplitudes and the largest hotspot from satellite data, the 23 April event produced little ash reaching only 6 km altitude. For several hours prior to the sub-Plinian phase on 19 April, tremor with a peak frequency of 1.3 Hz intensified. During the sub-Plinian phase the peak frequency increased to 4-8 Hz. However, in 15 h after the eruption, three episodes of stronger tremor occurred with a lower 1.0-Hz peak, alternating with weaker tremor with a 1.3-Hz peak. These transitions correspond to D_R=~8 cm². Although these strong tremor episodes produced higher D_R levels than the sub-Plinian phase, data from a pressure sensor show that only strong Strombolian explosions occurred. The suite of observations suggests three distinct tremor regimes that may correspond to slug flow, bubbly flow, and sustained strong eruptions, or a cyclic change in source parameters (e.g., geometry, sound speed, or ascent rate). This behavior occurred at Shishaldin only during the April 1999 sequence, and we are not aware of similar behavior at other volcanoes.     

Volcanic activity in the Azores. Report for 1959–1964

During the period 1959 to 1964 recorded volcanic activity in the Azores islands (Atlantic Ocean) was manifested by fumarolic fields in S. Miguel, Terceira, Graciosa, Pico and Faial and by two submarine eruptions off Pico and S. Jorge. Fumarolic activity increased in the Faial Capelinhos vent (in eruption 1957–58) after December 1963; it is concluded that the phenomenon is related to the 1963 and 1964 eruptions in the S. Jorge Channel. During February 1964 an important seismic swarm of more than 500 earthquakes, with volcanic tremor, occurred in S. Jorge.

     

Seismicity of Ruapehu volcano,New Zealand, 1971–1996: a review

From 1971 until 1995, the style of seismicity at Ruapehu changed little, reflecting a period of relatively low eruptive activity and consequent long-term stability within the vent system. Volcanic earthquakes and volcanic tremor were both dominated by a frequency of about 2 Hz. Volcanic earthquakes accompanied all phreatic and phreatomagmatic eruptions, but not small hydrothermal eruptions that originated within Crater Lake. Furthermore, more than half of the M_L>3 volcanic earthquakes and changes in the reduced displacement of 2 Hz volcanic tremor by as much as a factor of 20 occurred without any accompanying eruptive activity. Three and 7 Hz volcanic tremor were also recorded, although never at lower-elevation seismometers. At times, this tremor was stronger at the summit seismometer than the 2 Hz tremor. Their source regions were independent of the 2 Hz source, and located at shallower depths. Volcano-tectonic earthquakes were generally unrelated to eruptive activity. The seismicity accompanying the 1995–1996 eruptive activity was significantly different from that of the period 1971 to 1995, and included volcanic tremor with a frequency of less than 1 Hz, simultaneous changes in the amplitude of the previously independent 2 Hz and 7 Hz volcanic tremor, and finally a change in the frequency content of volcanic earthquakes and volcanic tremor from 2 Hz to wideband. Path transmission effects play an important role in determining the characteristics of seismograms at Ruapehu. The presence of Crater Lake affects both the style of eruptions and the accompanying seismicity.     

Vent conditions for expected eruptions at Vesuvius

Determining consistent sets of vent conditions for next expected eruptions at Vesuvius is crucial for the simulation of the sub-aerial processes originating the volcanic hazard and the eruption impact. Here we refer to the expected eruptive scales and conditions defined in the frame of the EC Exploris project, and simulate the dynamics of magma ascent along the volcanic conduit for sub-steady phases of next eruptions characterized by intensities of the Violent Strombolian (VS), Sub-Plinian 2 (SP2), and Sub-Plinian 1 (SP1) scale. Sets of conditions for the simulations are determined on the basis of the bulk of knowledge on the past history of Vesuvius [Cioni, R., Bertagnini, A., Santacroce, R., Andronico, D., Explosive activity and eruption scenarios at Somma–Vesuvius (Italy): towards a new classification scheme. Journal of Volcanology and Geothermal Research, this issue.]. Volatile contents (H₂O and CO₂) are parameterized in order to account for the uncertainty in their expected amounts for a next eruption. In all cases the flow in the conduit is found to be choked, with velocities at the conduit exit or vent corresponding to the sonic velocity in the two-phase non-equilibrium magmatic mixture. Conduit diameters and vent mixture densities are found to display minimum overlapping between the different eruptive scales, while exit gas and particle velocities, as well as vent pressures, largely overlap. Vent diameters vary from as low as about 5 m for VS eruptions, to 35–55 m for the most violent SP1 eruption scale. Vent pressures can be as low as less than 1 MPa for the lowest volatile content employed of 2 wt.% H₂O and no CO₂, to 7–8 MPa for highest volatile contents of 5 wt.% H₂O and 2 wt.% CO₂ and large eruptive scales. Gas and particle velocities at the vent range from 100–250 m/s, with a tendency to decrease, and to increase the mechanical decoupling between the phases, with increasing eruptive scale. Except for velocities, all relevant vent quantities are more sensitive to the volatile content of the discharged magma for the highest eruptive scales considered.     

Volcanic eruption clouds and the thermal power output of explosive eruptions

The maximum height attained by a volcanic eruption cloud is principally determined by the convective buoyancy of the mixture of volcanic gas + entrained air + fine-sized pyroclasts within the cloud. The thermal energy supplied to convection processes within an eruption cloud is derived from the cooling of pyroclastic material and volcanic gases discharged by an explosive eruption. Observational data from six recent eruptions indicates that the maximum height attained by volcanic eruption clouds is positively correlated with the rate at which pyroclastic material is produced by an explosive eruption (correlation coefficient r = + 0.97). The ascent of industrial hot gas plumes is also governed by the thermal convection process. Empirical scaling relationships between plume height and thermal flux have been developed for industrial plumes. Applying these scaling relationships to volcanic eruption clouds suggests that the rate at which thermal energy is released into the atmosphere by an explosive eruption increases in an approximately linear manner as an eruption's pyroclastic production rate increases.     

Geological and seismological evidence of increased explosivity during the 1986 eruptions of Pavlof volcano,Alaska

We present results of study of the best-documented eruptions of Pavlof volcano in historic time. The 1986 eruptions were mostly Strombolian in character; a strong initial phase may have been Vulcanian. The 1986 activity erupted at least 8×10⁶ m³ of feldspar-phyric basaltic andesite lava (SiO₂=53–54%), and a comparable volume of wind-borne tephra. During the course of the eruption, 5300 explosion earthquakes occurred, the largest of which was equivalent to an M _L =2.5 earthquake. Volcanic tremor was recorded for 2600 hours, and the strongest tremor was recorded out to a distance of 160 km and had an amplitude of at least 54 cm² reduced displacement. The 1986 eruptions modified the structure of the vent area for the first time in over two decades. A possible pyroclastic flow was observed on 19 June 1986, the first time such a phenomenon has been observed at the volcano. Overall, the 1986 eruptions were the strongest and longest duration eruptions in historic time, and changed a temporal pattern of activity that had persisted from 1973–1984.     

The possible effects of large 19th and 20th century volcanic eruptions on zonal and hemispheric surface temperatures

The contribution of volcanic material to the stratosphere from major eruptions within the last two centuries has been estimated using volcanological criteria, including eruption type, eruption column height, volume and duration of eruption, and composition and degree of fragmentation of magma. The chronology of major explosive volcanic eruptions is compared with a record of mean surface-temperature deviation (ΔT) for the same interval constructed from all available temperature data. The temperature records are divided into 6 latitudinal zones, allowing analysis for individual zones where temperature changes induced by aerosol perturbation might be intensified.We focus on the explosive volcanic events which by our estimates injected the most material into the stratosphere. These include Tambora 1815, Krakatau 1883, Santa Maria 1902, Katmai 1912 and Quizapu 1932. Such eruptions appear to have produced a consistent but small temperature decrease on the order of 0.2° to 0.5°C on a hemispheric scale for periods ranging from one to five years, although these changes are similar to background temperature variations. The maximum change in ΔT after some of these explosions appears to lag by up to three years in going from equatorial to polar latitudes.Somewhat smaller eruptions, e.g. Agung 1963 and possibly Cosiguina 1835, seem to have produced about the same perturbation in ΔT as the larger eruptions. This suggests either a limiting mechanism on loading of the aerosol layer after a volcanic eruption or, that the composition of injected material (i.e., the ratio of silicate “dust” to volatiles, and composition of the volatiles) may significantly effect stratospheric optical depth perturbation. Temperatures do not remain depressed for a longer period after a series of closely timed eruptions (e.g., the 1881–1889 or the 1902–1903 sequences) than after single events.     

Continuous monitoring of volcanic eruption dynamics: a review of various techniques and new results from a frequency-modulated radar Doppler system

 In situ measurement of volcanic eruption velocities is one of the great challenges left in geophysical volcanology. In this paper we report on a new radar Doppler technique for monitoring volcanic eruption velocities. In comparison with techniques employed previously (e.g., photographic methods or acoustic Doppler measurements), this method allows continuous recordings of volcanic eruptions even during poor visibility. Also, radar Doppler instruments are usually light weight and energy efficient, which makes them superior to other Doppler techniques based on laser light or sound. The proposed new technique was successfully tested at Stromboli Volcano in late 1996 during a period of low activity. The recorded data allow a clear distinction between particles rising from the vent and particles falling back towards the vent. The mean eruption velocity was approximately 10 m/s. Most of the eruptions recorded by radar were correlated to seismic recordings. The correlation between the magnitude of the volcanic shocks and the eruption force index defined in the paper may provide new insights into magma transport in the conduit. Received: 15 May 1998 / Accepted: 15 December 1998     

Reventador Volcano 2005: Eruptive activity inferred from seismo-acoustic observation

Reventador Volcano entered an eruptive phase in 2005 which included a wide variety of seismic and infrasonic activity. These are described and illustrated: volcano-tectonic, harmonic tremor, drumbeats, chugging and spasmodic tremor, long period and very long period events. The recording of this simultaneous activity on an array of three broadband, seismo-acoustic instruments provides detailed information of the state of the conduit and vent during this phase of volcanic eruption. Quasi-periodic tremor at Reventador is similar to that observed at other volcanoes and may be used as an indicator of vent aperture. Variations in the vibration modes of the volcano, frequency fluctuations and rapid temporal fluctuations suggest the influx of new material, choking of the vent and possible modification of the conduit geometry during explosions and effusion over a period of six weeks.     

Statistical analysis of intermittent volcanic tremor associated with the September 1989 summit explosive eruptions at Mount Etna, Sicily

The pattern of volcanic tremor accompanying the 1989 September eruption at the south-east summit crater of Mount Etna is studied. In specific, sixteen episodes of lava fountaining, which occurred in the first phase of the eruption, are analysed. Their periodic behaviour, also evidenced by autocorrelation, allows us to define the related tremor amplitude increases as intermittent volcanic tremor episodes. Focusing on the regular intermittent behaviour found for both lava fountains and intermittent volcanic tremors, we tried an a posteriori forecast using simple statistical methods based on linear regression and the Student’ t-test. We performed the retrospective statistical forecast, and found that several eruptions would have been successfully forecast. In order to focus on the source mechanism of tremor linked to lava fountains, we investigated the relationship between volcanic and seismic parameters. A mechanism based on a shallow magma batch ‘regularly’ refilled from depth is suggested.     

The May 1915 eruptions of Lassen Peak, II: May 22 volcanic blast effects, sedimentology and stratigraphy of deposits, and characteristics of the blast cloud

The May 22, 1915 eruptions of Lassen Peak involved a volcanic blast and the emplacement of three geographically and temporally distinct lahar deposits. The volcanic blast occurred when a Vulcanian explosion at the summit unroofed a shallow magma source, generating an eruption cloud that rose to an estimated height of 9 km above sea level. The blast cloud was probably caused by the collapse of a small portion of the eruption column; absence of a flank vent associated with these eruptions argues against it originating as an explosion that has been directed by vent geometry or location. The volcanic blast devasted 7 km² of the northeast flank of the volcano, and emplaced a deposit of juvenile tephra and accidental lithic and mineral fragments. Decrease in blast deposit thickness and median grain size with increasing distance from the vent suggests that the blast cloud lost transport competence as it crossed the devastated area. Scanning electron microscope examination of pyroclasts from the blast deposit indicates that the blast cloud was a dry, turbulent suspension that emplaced a thin deposit which cooled rapidly after deposition. Lahar deposits were emplaced primarily in Lost Creek, with minor lahars flowing down gullies on the west, northwest and north flanks of the volcano. The initial lahar was apparently triggered early in the eruption when the blast cloud melted the residual snowpack as it moved down the northeast flank of the peak. The event that triggered the later lahars is enigmatic; the presence of approximately five times more juvenile dacite bombs on the surface of the later lahars suggests that they may have been triggered by a change in eruption style or dynamics.     

Multifractal characterization of Vesuvio lava-flow margins and its implications

The digitized lava-flow margins of well-defined extended eruptions occurring at Vesuvio in 1760, 1794, 1861, 1906, 1929 and 1944 are found to follow fractal behaviours inside a scaling region enclosed between 50 and 400 m. Although the invariance region is well respected, the fractal dimension D varies from one lava flow to another: the more irregular the lava-flow margin, the larger the value of D. The ascertained dependence of D on the duration of premonitory activity, preceding the emission of lavas, might provide some insight into the inner volcanic processes before the eruption and into the dynamical processes operating during flow emplacement.     

Damage mechanics preceding the September–October 1989 flank eruption at Mount Etna volcano inferred by seismic scaling exponents

Seismicity preceding the September–October 1989 flank eruption at Mt. Etna volcano has been analysed by means of the fractal two-point correlation dimension D_c relative to epicentres distribution and the scaling exponent D inferred from the seismic b-value. Time evolution of D_c and D preceding the eruption onset revealed: a general decrease of D_c and increase of D (9–4 months prior the eruption onset), intermittent rates of D_c increase and D decrease (4 to 1 month), steady high values of D_c and low values of D (1 month) and sharp decreases of D_c and D marking the eruption onset. Results allowed to constrain the increment of seismic damage by recognising negative (mechanical hardening) and positive (mechanical softening) feedback prior the eruption onset.     

Doppler radar sounding of volcanic eruption dynamics at Mount Etna

Besides their common use in atmospheric studies, Doppler radars are promising tools for the active remote sensing of volcanic eruptions but were little applied to this field. We present the observations made with a mid-power UHF Doppler radar (Voldorad) during a 7-h Strombolian eruption at the SE crater of Mount Etna on 11–12 October 1998. Main characteristics of radar echoes are retrieved from analysis of Doppler spectra recorded in the two range gates on either side of the jet axis. From the geometry of the sounding, the contribution of uprising and falling ejecta to each Doppler spectrum can be discriminated. The temporal evolution of total power backscattered by uprising targets is quite similar to the temporal evolution of the volcanic tremor and closely reproduces the overall evolution of the eruption before, during and after its paroxysm. Moreover, during the sharp decrease of eruptive activity following the paroxysm, detailed analysis of video (from camera recording), radar and seismic measurements reveals that radar and video signals start to decrease simultaneously, approximately 2.5 min after the tremor decline. This delay is interpreted as the ascent time through a magma conduit of large gas slugs from a shallow source roughly estimated at about 500 m beneath the SE crater. Detailed analysis of eruptive processes has been also made with Voldorad operating in a high sampling rate mode. Signature of individual outburst is clearly identified on the half part of Doppler spectra corresponding to rising ejecta: temporal variations of the backscattered power exhibit quasi periodic undulations, whereas the maximum velocity measured on each spectrum displays a sharp peak at the onset of each outburst followed by a slow decay with time. Periodicity of power variations (between 3.8 and 5.5 s) is in agreement with the occurrence of explosions visually observed at the SE vent. Maximum vertical velocities of over 160 m s^–1 were measured during the paraoxysmal stage and the renewed activity. Finally, by using a simplified model simulating the radar echoes characteristics, we show that when Voldorad is operating in high sampling rate mode, the power and maximum velocity variations are directly related to the difference in size and velocity of particles crossing the antenna beam.Editorial responsibility: A. Woods     

Measurement of bubble size distributions in vesiculated rocks with implications for quantitative estimation of eruption processes

This paper outlines methods for determining a bubble size distribution (BSD) and the moments of the BSD function in vesiculated clasts produced by volcanic eruptions. It reports the results of applications of the methods to 11 natural samples and discusses the implications for quantitative estimates of eruption processes. The analysis is based on a quantitative morphological (stereological) method for 2-dimensional imaging of cross-sections of samples. One method determines, with some assumptions, the complete shape of the BSD function from the chord lengths cut by bubbles. The other determines the 1st, 2nd and 3rd moments of distribution functions by measurement of the number of bubbles per unit area, the surface area per unit volume, and the volume fraction of bubbles. Comparison of procedures and results of these two distinct methods shows that the latter yields rather more reliable results than the former, though the results coincide in absolute and relative magnitudes. Results of the analysis for vesiculated rocks from eleven subPlinian to Plinian eruptions show some interesting systematic correlations both between moments of the BSD and between a moment and the eruption column height or the SiO₂ content of magma. These correlations are successfully interpreted in terms of the nucleation and growth processes of bubbles in ascending magmas. This suggests that bubble coalescence does not predominate in sub-Plinian to Plinian explosive eruptions. The moment-moment correlations put constraints on the style of the nucleation and growth process of bubbles. The scaling argument suggests that a single nucleation event and subsequent growth with any kind of bubble interaction under continuous depressurization, which leads to an intermediate growth law between the diffusional growth ( ) at a constant depressurization rate and the Ostwald ripening ( ) under a constant pressure, where R_m and t are the mean radius of bubble and the effective time of diffusion respectively, occurred in the eruptions. It is emphasized that the BSD in vesiculated rocks from terrestrial volcanoes can be used to estimate quantitatively eruption processes such as the initial saturation pressure and magma ascent velocity in a volcanic conduit.     

A multidisciplinary effort to assign realistic source parameters to models of volcanic ash-cloud transport and dispersion during eruptions

During volcanic eruptions, volcanic ash transport and dispersion models (VATDs) are used to forecast the location and movement of ash clouds over hours to days in order to define hazards to aircraft and to communities downwind. Those models use input parameters, called “eruption source parameters”, such as plume height H, mass eruption rate Ṁ, duration D, and the mass fraction m₆₃ of erupted debris finer than about 4 or 63 μm, which can remain in the cloud for many hours or days. Observational constraints on the value of such parameters are frequently unavailable in the first minutes or hours after an eruption is detected. Moreover, observed plume height may change during an eruption, requiring rapid assignment of new parameters. This paper reports on a group effort to improve the accuracy of source parameters used by VATDs in the early hours of an eruption. We do so by first compiling a list of eruptions for which these parameters are well constrained, and then using these data to review and update previously studied parameter relationships. We find that the existing scatter in plots of H versus Ṁ yields an uncertainty within the 50% confidence interval of plus or minus a factor of four in eruption rate for a given plume height. This scatter is not clearly attributable to biases in measurement techniques or to well-recognized processes such as elutriation from pyroclastic flows. Sparse data on total grain-size distribution suggest that the mass fraction of fine debris m₆₃ could vary by nearly two orders of magnitude between small basaltic eruptions ( 0.01) and large silicic ones (> 0.5). We classify eleven eruption types; four types each for different sizes of silicic and mafic eruptions; submarine eruptions; “brief” or Vulcanian eruptions; and eruptions that generate co-ignimbrite or co-pyroclastic flow plumes. For each eruption type we assign source parameters. We then assign a characteristic eruption type to each of the world's  1500 Holocene volcanoes. These eruption types and associated parameters can be used for ash-cloud modeling in the event of an eruption, when no observational constraints on these parameters are available.     

Environmental hazards of fluoride in volcanic ash: a case study from Ruapehu volcano, New Zealand

The vent-hosted hydrothermal system of Ruapehu volcano is normally covered by a c. 10 million m³ acidic crater lake where volcanic gases accumulate. Through analysis of eruption observations, granulometry, mineralogy and chemistry of volcanic ash from the 1995–1996 Ruapehu eruptions we report on the varying influences on environmental hazards associated with the deposits. All measured parameters are more dependent on the eruptive style than on distance from the vent. Early phreatic and phreatomagmatic eruption phases from crater lakes similar to that on Ruapehu are likely to contain the greatest concentrations of environmentally significant elements, especially sulphur and fluoride. These elements are contained within altered xenolithic material extracted from the hydrothermal system by steam explosions, as well as in residue hydrothermal fluids adsorbed on to particle surfaces. In particular, total F in the ash may be enriched by a factor of 6 relative to original magmatic contents, although immediately soluble F does not show such dramatic increases. Highly soluble NaF and CaSiF₆ phases, demonstrated to be the carriers of ‘available’ F in purely magmatic eruptive systems, are probably not dominant in the products of phreatomagmatic eruptions through hydrothermal systems. Instead, slowly soluble compounds such as CaF₂, AlF₃ and Ca₅(PO₄)₃F dominate. Fluoride in these phases is released over longer periods, where only one third is leached in a single 24-h water extraction. This implies that estimation of soluble F in such ashes based on a single leach leads to underestimation of the F impact, especially of a potential longer-term environmental hazard. In addition, a large proportion of the total F in the ash is apparently soluble in the digestive system of grazing animals. In the Ruapehu case this led to several thousand sheep deaths from fluorosis.     

Constraining tephra dispersion and deposition from three subplinian explosions in 2011 at Shinmoedake volcano,Kyushu, Japan

Constraining physical parameters of tephra dispersion and deposition from explosive volcanic eruptions is a significant challenge, because of both the complexity of the relationship between tephra distribution and distance from the vent and the difficulties associated with direct and comprehensive real-time observations. Three andesitic subplinian explosions in January 2011 at Shinmoedake volcano, Japan, are used as a case study to validate selected empirical and theoretical models using observations and field data. Tephra volumes are estimated using relationships between dispersal area and tephra thickness or mass/area. A new cubic B-spline interpolation method is also examined. Magma discharge rate is estimated using theoretical plume models incorporating the effect of wind. Results are consistent with observed plume heights (6.4–7.3 km above the vent) and eruption durations. Estimated tephra volumes were 15–34?×?10⁶ m³ for explosions on the afternoon of 26 January and morning of 27 January, and 5.0–7.6?×?10⁶ m³ for the afternoon of 27 January; magma discharge rates were in the range 1–2?×?10⁶ kg/s for all three explosions. Clast dispersal models estimated plume height at 7.1?±?1 km above the vent for each explosion. The three subplinian explosions occurred with approximately 12-h reposes and had similar mass discharge rates and plume heights but decreasing erupted magma volumes and durations.     

Characterization of tephra deposits with limited exposure: the example of the two largest explosive eruptions at Nisyros volcano (Greece)

Explosive eruptions associated with tephra deposits that are only exposed in proximal areas are difficult to characterize. In fact, the determination of physical parameters such as column height, mass eruption rate, erupted volume, and eruption duration is mainly based on empirical models and is therefore very sensitive to the quality of the field data collected. We have applied and compared different modeling approaches for the characterization of the two main tephra deposits, the Lower Pumice (LP) and Upper Pumice (UP) of Nisyros volcano, Greece, which are exposed only within 5 km of the probable vent. Isopach and isopleth maps were compiled for two possible vent locations (on the north and on the south rim of the caldera), and different models were applied to calculate the column height, the erupted volume, and the mass eruption rate. We found a column height of about 15 km above sea level and a mass eruption rate of about 2 × 10⁷ kg/s for both eruptions regardless of the vent location considered. In contrast, the associated wind velocity for both UP and LP varied between 0 and 20 m/s for the north and south vent, respectively. The derived erupted volume for the south vent (considered as the best vent location) ranges between 2 and 27 × 10⁸ m³ for the LP and between 1 and 5 × 10⁸ m³ for the UP based on the application of four different methods (integration of exponential fit based on one isopach line, integration of exponential and power-law fit based on two isopach lines, and an inversion technique combined with an advection–diffusion model). The eruption that produced the UP could be classified as subplinian. Discrepancies associated with different vent locations are smaller than the discrepancies associated with the use of different models for the determination of erupted mass, plume height, and mass eruption rate. Proximal outcrops are predominantly coarse grained with ≥90 wt% of the clasts ranging between −6ϕ and 0ϕ. The associated total grainsize distribution is considered to result from a combination of turbulent fallout from both the plume margins and the umbrella region, and as a result, it is fines-depleted. Given that primary deposit thickness observed on Nisyros for both LP and UP is between 1 and 8 m, if an event of similar scale were to happen again, it would have a significant impact on the entire island with major damage to infrastructure, agriculture, and tourism. Neighboring islands and the continent could also be significantly affected.     

Tsunamis of volcanic origin: Summary of causes,with particular reference to Krakatoa, 1883

Known tsunamis of volcanic origin are reviewed and classified according to their causes. Earthquakes accompanying eruptions (excluding tectonic events which apparently triggered eruptions), pyroclastic flows, and submarine explosions have each accounted for about 20% of cases. Ten causes of volcanic tsunamis are discussed. From the risk point of view, those due to landslides are particularly dangerous. Eruptions at calderas are more likely to generate tsunamis than eruptions elsewhere. Of those killed directly by volcanic eruptions, nearly a quarter have died as a result of tsunamis. By transfer of energy to sea waves, a violent eruption, which would be comparatively harmless on land, extends greatly the radius over which destruction occurs. Krakatoa, 1883, is the only eruption sequence for which sufficient data exist for a detailed study of tsunamis. The times at which air and water waves generated by this sequence were recorded have been reread, and new origin times have been calculated and compared with observations made at the time. Origin times of successive pairs of air and water waves agree closely, except in some cases in which the tsunami arrived up to 15 minutes early, thus giving an apparent origin time 15 minutes before that of the corresponding air wave. This is explained by postulating that these tsunamis did not originate at the focus of the explosions, but at distances along the path towards the tide gauge, equivalent to those which would be covered by a tsunami in the time interval observed. The calculated point at which the largest recorded tsunami originated coincides with the outer edge of a bank of volcanic debris laid down during the eruption. This is interpreted as part of an unwelded ignimbrite deposit, the violent emplacement of which, within a minute or so of the explosion, generated the tsunami. A satisfactory correlation is established between explosions and deposits laid down by the eruptions, as described from a geological section close to the source vent. An outline is given of a proposed numerical index to define tsunamigenic potential at a given volcano. Such an index could be used to calculate the expected amplitudes of tsunamis at particular places in the vicinity, and hence could serve as a basis for tsunami risk contingency planning.