monitoring ash emission episodes at mt etna the 16 november 2006 case study abstract mt etna in sicily italy is one of the best monitored basaltic volcanoes in the world due to the frequent eruptions from its summit and flanks routine monitoring carried out by istituto nazionale di geofisica e vulcanologia sezione di catania for surveillance purposes permits following the evolution of volcanic events in this paper a description of the ash monitoring system as occurred during the august december 2006 summit eruption at the southeast crater sec is shown this eruption was characterized by lava flow effusions and vigorous strombolian activity eighteen paroxysmal episodes occurred up to the end of november forming weak ash plumes accompanied by moderate tephra fallout over etna s slopes during these events we applied a multidisciplinary approach to promptly monitor the paroxysmal activity and the associated tephra fallout through analysis from seismic tremor and observation from live cameras sampling operations mapping and analysis of the deposit during the most significant episodes we carried out textural and grain size analysis on tephra samples and evaluated the total grain size deposit and the erupted volume while numerical simulations of tephra dispersal allowed better understanding eruptive dynamics an example of this methodology is applied to the 16 november episode during which seismic tremor furnished important constraints on the chronology this paroxysmal eruption produced light fallout on the north east sector of the volcano for about ten hours and a number of debris avalanches over the slopes of the sec cone the erupted deposit was composed for the most part of lithic components and characterized by a total size distribution centered on 2 2 while its total mass was evaluated 7 10 6 kg on the whole such integrated studies help to obtain information on magma fragmentation and eruptive mechanisms to characterize the explosive styles shown by etna and finally to better approach the monitoring of imminent eruptions

volcanic tremor at mount etna january 1984 march 1985 its relationship to the eruptive activity and modelling of the summit feeding system spectral analyses of volcanic tremor at etna during january 1984 march 1985 have been performed and the relationship between tremor energy and observed volcanic phenomena have been examined the highest energy levels have been observed during the paroxysmal phases of eruptions whereas a gradual decrease was linked to the lowering of eruptive activity amplitude variations with time of some spectral frequency peaks 0 95 1 20 1 45 1 65 1 80 and 2 40 hz have been compared with volcanic activity at the summit craters and on the basis of these results a new schematic diagram for the feeding system of the summit vents is proposed

coupled textural and compositional characterization of basaltic scoria insights into the transition from strombolian to fire fountain activity at mount etna italy strombolian and fire fountain activities represent a common expression of explosive basaltic eruptions however the transition between these two eruptive styles and their source mechanisms are still debated we use textural and compositional studies to characterize pyroclastic material from both the strombolian and hawaiian style fire fountain phases of the january june 2000 etna activity we find that basaltic scoria presents distinctive textural and compositional features that reflect different modes of magma vesiculation and crystallization in the two eruptive regimes overall magma that forms strombolian scoria is far more crystallized less vesicular and more evolved indicating strong volatile depletion and longer residence time before being erupted fire fountain scoria indicates a fast rising magma with evidence of moderate syneruptive volatile exsolution the new textural and compositional data set is integrated with previous volcanological and geophysical investigations to provide further insights into the dynamics of fire fountains and to frame the transition from strombolian explosions to fire fountain activity into a model that may apply to future eruptions at mount etna as well as other active basaltic volcanoes

analytical and 3 d numerical modelling of mt etna italy volcano inflation summary since 1993 geodetic data obtained by different techniques gps edm sar levelling have detected a consistent inflation of the mt etna volcano the inflation culminating with the 1998 2001 strong explosive activity from summit craters and recent 2001 and 2002 flank eruptions is interpreted in terms of magma ascent and refilling of the volcanic plumbing system and reservoirs we have modelled the 1993 1997 edm and gps data by 3 d pressurized sources to infer the position and dimension of the magma reservoir we have performed analytical inversions of the observed deformation using both spheroidal and ellipsoidal sources embedded in a homogeneous elastic half space and by applying different inversion methods solutions for these types of sources show evidence of a vertically elongated magma reservoir located 6 km beneath the summit craters the maximum elevation of topography is comparable to such depth and strong heterogeneities are inferred from seismic tomography in order to assess their importance further 3 d numerical models employing source parameters extracted from analytical models have been developed using the finite element technique the deformation predicted by all the models considered shows a general agreement with the 1993 1997 data suggesting the primary role of a pressure source while the complexities of the medium play a minor role under elastic conditions however major discrepancies between data and models are located in the se sector suggesting that sliding along potential detachment surfaces may contribute to amplify deformation during the inflation for the first time realistic features of mt etna are studied by a 3 d numerical model characterized by the topography and lateral variations of elastic structure providing a framework for a deeper insight into the relationships between internal sources and tectonic structures

volcanic tremor at mt etna italy preceding and accompanying the eruption of july august 2001 the july 17 august 9 2001 flank eruption of mt etna was preceded and accompanied by remarkable changes in volcanic tremor based on the records of stations belonging to the permanent seismic network deployed on the volcano we analyze amplitude and frequency content of the seismic signal we find considerable changes in the volcanic tremor which mark the transition to different styles of eruptive activity e g lava fountains phreatomagmatic activity strombolian explosions in particular the frequency content of the signal decreases from 5 hz to 3 hz at our reference station etf during episodes of lava fountains and further decreases at about 2 hz throughout phases of intense lava emission the frequency content and the ratios of the signal amplitude allow us to distinguish three seismic sources i e the peripheral dike which fed the eruption the reservoir which fed the lava fountains and the central conduit based on the analysis of the amplitude decay of the signal we highlight the migration of the dike from a depth of ca 5 km to about 1 km between july 10 and 12 after the onset of the effusive phase the distribution of the amplitude decay at our stations can be interpreted as the overall result of sources located within the first half kilometer from the surface although on a qualitative basis our findings shed some light on the complex feeding system of mt etna and integrate other volcanological and geophysical studies which tackle the problem of magma replenishment for the july august 2001 flank eruption we conclude that volcanic tremor is fundamental in monitoring mt etna not only as a marker of the different sources which act within the volcano edifice but also of the diverse styles of eruptive activity

velocity and spectral characteristics of the volcanic tremor at etna deduced by a small seismometer array abstract a small l shaped short period seismic array was set up at etna volcano to measure the wave velocity and the incoming azimuth of the volcanic tremor generated by the volcanic activity at etna in non eruptive conditions cross correlation techniques were used to measure the phase shifts between the array stations an approximately n s direction of the propagation direction was estimated coincident with the azimuth of the crater area of etna an apparent velocity of less than 500 m s was estimated suggesting a surface propagation of the tremor waves spectral analysis confirming previous results shows velocity spectra peaked at approximately 2 hz a localized source coincident with the crater area agrees with our data and could generate the volcanic tremor at etna in non eruptive stages

source mechanism of volcanic tremor low frequency 10 hz volcanic earthquakes originate at a wide range of depths and occur before during and after magmatic eruptions the characteristics of these earthquakes suggest that they are not typical tectonic events physically analogous processes occur in hydraulic fracturing of rock formations low frequency icequakes in temperate glaciers and autoresonance in hydroelectric power stations we propose that unsteady fluid flow in volcanic conduits is the common source mechanism of low frequency volcanic earthquakes tremor the fluid dynamic source mechanism explains low frequency earthquakes of arbitrary duration magnitude and depth of origin as unsteady flow is independent of physical properties of the fluid and conduit fluid transients occur in both low viscosity gases and high viscosity liquids a fluid transient analysis can be formulated as generally as is warranted by knowledge of the composition and physical properties of the fluid material properties geometry and roughness of the conduit and boundary conditions to demonstrate the analytical potential of the fluid dynamic theory we consider a single phase fluid a melt of mount hood andesite at 1250 c in which significant pressure and velocity variations occur only in the longitudinal direction further simplification of the conservation of mass and momentum equations presents an eigenvalue problem that is solved to determine the natural frequencies and associated damping of flow and pressure oscillations for a simple constant pressure reservoir conduit orifice fluid system a change in orifice size can cause the source signal duration due to a single disturbance to change from several seconds to in excess of an hour fluid kinematic viscosity 0 412 m2 s is both included and neglected in the analysis to illustrate its effect upon system damping tremor magnitude is related to the magnitude of the conduit wall motion occurring in response to oscillating fluid pressures the entire volcanic fluid system does not generally experience a transient flow condition at a given time closed end and constant pressure fluid system boundaries reflect pressure waves and isolate components of the fluid system total fluid movement may not be accurately monitored by tremor data alone since steady state and slowly changing flows are aseismic

seismic heterogeneity of mt etna structure and activity a dense seismic array maintained temporarily for 7 weeks provides an approach to deducing the 3 d heterogeneity of the structure of the upper crust beneath mt etna volcano from the inversion of p and s wave arrivals of local earthquakes independent data from a different source array geometry using waves generated by distant shots at sea confirm a main lateral variation a high velocity body underlies part of the volcano its contrast with the surrounding thick sedimentary pile and its location extending from the present summit to the southeast suggests that it may have been built by the development and migration of past eruptive centres followed by caldera collapse local low velocity anomalies near its sharp western edge may be associated with presently active features

statistical forecasting of repetitious dome failures during the waning eruption of redoubt volcano alaska february april 1990 abstract the waning phase of the 1989 1990 eruption of redoubt volcano in the cook inlet region of south central alaska comprised a quasi regular pattern of repetitious dome growth and destruction that lasted from february 15 to late april 1990 the dome failures produced ash plumes hazardous to airline traffic in response to this hazard the alaska volcano observatory sought to forecast these ash producing events using two approaches one approach built on early successes in issuing warnings before major eruptions on december 14 1989 and january 2 1990 these warnings were based largely on changes in seismic activity related to the occurrence of precursory swarms of long period seismic events the search for precursory swarms of long period seismicity was continued through the waning phase of the eruption and led to warnings before tephra eruptions on march 23 and april 6 the observed regularity of dome failures after february 15 suggested that a statistical forecasting method based on a constant rate failure model might also be successful the first statistical forecast was issued on march 16 after seven events had occurred at an average interval of 4 5 days at this time the interval between dome failures abruptly lengthened accordingly the forecast was unsuccessful and further forecasting was suspended until the regularity of subsequent failures could be confirmed statistical forecasting resumed on april 12 after four dome failure episodes separated by an average of 7 8 days one dome failure april 15 was successfully forecast using a 70 confidence window and a second event april 21 was narrowly missed before the end of the activity the cessation of dome failures after april 21 resulted in a concluding false alarm although forecasting success during the eruption was limited retrospective analysis shows that early and consistent application of the statistical method using a constant rate failure model and a 90 confidence window could have yielded five successful forecasts and two false alarms no events would have been missed on closer examination the intervals between successive dome failures are not uniform but tend to increase with time this increase attests to the continuous slowly decreasing supply of magma to the surface vent during the waning phase of the eruption the domes formed in a precarious position in a breach in the summit crater rim where they were susceptible to gravitational collapse the instability of the february 15 april 21 domes relative to the earlier domes is attributed to reaming the lip of the vent by a laterally directed explosion during the major dome destroying eruption of february 15 a process which would leave a less secure foundation for subsequent domes

operation of a digital seismic network on mount st helens volcano and observations of long period seismic events that originate under the volcano during 1981 we operated with the cooperation of the u s geological survey a 9 station digital seismic array on mount st helens volcano in washington state one of our stations was placed inside the crater of the volcano six were located on the flanks of the volcano within two km of the crater and two were approximately ten km from the crater four of the instruments recorded three components of motion and the remaining five recorded only the vertical component in addition to the monitoring and recording performed by the digital instruments we carried out a one day experiment during which we complimented our crater monitoring seismometer by the addition of two ink recording instruments during the one day experiment we had six observers including 4 usgs scientists who had significant experience working and making observations in the crater recording times of rockfall felt earthquake occurrences and changes in steam emissions from the dome in the crater using information obtained during the one day experiment we are able to classify seismic events recorded by our digital instruments as earthquakes rockfalls helicopter noise and a type of event that is unique to volcanoes which we call long period waveforms of these long period events have a duration of up to 30 seconds and a spectrum that is peaked at approximately 2 hz the frequency at which the peak in the spectrum occurs is nearly the same at all stations which means that the unique waveform of long period events is due to a source effect not a path effect the peak frequency is fairly insensitive to the amplitude of the signal which means that the size of the source region is constant independent of the signal amplitude long period events were not felt and were accompanied by no visible changes inside the crater which lead us to conclude that they are some sort of seismic disturbance generated inside the volcano

dynamics of a lava fountain revealed by geophysical geochemical and thermal satellite measurements the case of the 10 april 2011 mt etna eruption 1 geophysical tilt seismic tremor and gravity signals geochemical crater so2flux and infrared satellite measurements are presented and discussed to track the temporal evolution of the lava fountain episode occurring at mt etna volcano on 10 april 2011 the multi disciplinary approach provides insight into a gas rich magma source trapped in a shallow storage zone inside the volcano edifice this generated the fast ascending gas magma dispersed flow feeding the lava fountain and causing the depressurization of a deeper magma storage satellite thermal data allowed estimation of the amount of erupted lava which summed to the tephra volume yielded a total volume of erupted products of about 1 106 m3 thanks to the daylight occurrence of this eruptive episode the so2 emission rate was also estimated showing a degassing cycle reaching a peak of 15 000 mg d 1 with a mean daily value of 5 700 mg d 1 the so2 data from the previous fountain episode on 17 18 february to 10 april 2011 yielded a cumulative degassed magma volume of about 10 5 106 m3 indicating a ratio of roughly 10 1 between degassed and erupted volumes this volumetric balance differently from those previously estimated during different styles of volcanic activities with long term years recharging periods and middle term weeks to months effusive eruptions points toward the predominant role played by the gas phase in generating and driving this lava fountain episode

retrieval and intercomparison of volcanic so2 injection height and eruption time from satellite maps and ground based observations abstract syneruptive gas flux time series can in principle be retrieved from satellite maps of so 2 collected during and immediately after volcanic eruptions and used to gain insights into the volcanic processes which drive the volcanic activity determination of the age and height of volcanic plumes are key prerequisites for such calculations however these parameters are challenging to constrain using satellite based techniques here we use imagery from omi and gome 2 satellite sensors and a novel numerical procedure based on back trajectory analysis to calculate plume height as a function of position at the satellite measurement time together with plume injection height and time at a volcanic vent location we applied this new procedure to three etna eruptions 12 august 2011 18 march 2012 and 12 april 2013 and compared our results with independent satellite and ground based estimations we also compare our injection height time series with measurements of volcanic tremor which reflects the eruption intensity showing a good match between these two datasets our results are a milestone in progressing towards reliable determination of gas flux data from satellite derived so 2 maps during volcanic eruptions which would be of great value for operational management of explosive eruptions

an approach to problems on energy sources at mount etna based on seismological and volcanological data based on seismograms from the most continuously operating station of the seismic network at mt etna monte vetore 1665 m a s l the energy release patterns through time have been obtained for volcanic tremors and earthquakes which occurred between 1978 1982 both energies range between 1011 1013 j a but their release patterns are not strictly correlated to each other considering only the eruptions which occurred during the same time span the amount of thermal energy released and the associated potential energy needed to raise the magma to the surface from a 2 km deep hydrostatic equilibrium level were estimated to be about 1017 and 1015 j a respectively the computed energies for earthquakes and tremors are at least one order of magnitude less than what is needed to match a model of magma transport based solely on the jerky propagation of melt filled cracks the energy needed to raise the magma to the surface could be supplied by expanding gases in the upper levels of the magma column and or by tectonic stresses acting on shallow batches of magma

infrasound reveals transition to oscillatory discharge regime during lava fountaining implication for early warning 1 we present the analysis of 4 million infrasonic signals which include 39 episodes of lava fountains recorded at 5 5 km from the active vents we show that each eruptive episode is characterized by a distinctive trend in the amplitude waveform and frequency content of the acoustic signals reflecting different explosive levels lava fountain starts with an 93 min long violent phase of acoustic transients at 1 25 hz repeating every 2 5 s infrasound suddenly evolves into a persistent low frequency quasi monochromatic pressure oscillation at 0 4 hz we interpret this shift as induced by the transition from the slug discrete strombolian to churn flow sustained lava fountain regime that is reflecting an increase in the gas discharge rate we calculate that infrasonic transition can occur at a gas superficial velocity of 76 m s and it can be used to define infrasonic based thresholds for an efficient early warning system

deep volcanic tremor and magma ascent mechanism under kilauea hawaii deep harmonic tremor originating at depths around 40 km under kilauea was studied using records accumulated since 1962 at the hawaii volcano observatory of the u s geological survey the deep source of the tremor was determined by onset times and confirmed by the relative amplitude across the island wide network of seismometers the period of tremor was conclusively shown to be determined by the source effect and not by the path or station site effect because the period would change considerably in time but maintained uniformity across the seismic net during the tremor episode the tremor appeared to be primarily composed of p waves we interpret the observed period and amplitude in terms of the stationary crack model of aki et al 1977 and find that the seismic moment rates for deep tremors are considerably larger than those for shallow tremors suggesting more vigorous transport for the former we propose a kinematic source model which may be more appropriate for deep tremor according to this model a measurable quantity called reduced displacement is directly proportional to the rate of magma flow a systematic search for deep tremor episodes was made for the period from 1962 through 1979 and the amplitude period and duration of the tremor were tabulated we then constructed a cumulative reduced displacement plot over the 18 year period the result shows a generally steady process which does not seem to be significantly affected by major eruptions and large earthquakes near the surface the total magma flow estimated from the reduced displacement is however one order of magnitude smaller than that estimated by swanson 1972 it may be that most channels transport magma aseismically and only those with strong barriers generate tremor

temporal and spatial characteristics of volcanic tremor wave fields abstract detailed volcanic tremor studies were carried out with portable seismic stations installed both in the near and distant zones at klyuchevskoy volcano in 1984 1986 and 1987 it has been determined that the coherence of wave field decreases with distance to 0 5 km and does not change at greater distances based on coherence and phase spectra the frequency dependence of the phase velocities has been determined the dispersion of phase velocity points to the surface character of the wave field the behaviour of volcanic tremor characteristics through time is stationary which implies stability of the parameters of the volcanic tremor source a detailed spectral study has demonstrated that when the intensity of volcanic tremor varies by a few orders of magnitude parts of the low frequency and high frequency spectra change which is likely due to the variations in source dimensions the detailed spectral characteristics of tremor may be used for both determining the source parameters and studying the structure of the medium

stress strain and mass changes at mt etna during the period between the 1991 93 and 2001 flank eruptions abstract during the 8 year period between the 1991 93 and 2001 flank eruptions the eruptive activity of mt etna was confined to the summit craters deformation and tomography studies indicate that this activity was fed by a magma accumulation zone centered ne of the summit at a depth of 5 to 9 km below sea level the most significant gravity changes measured during the same period were induced by mass redistributions at shallower depth below the southeastern flank of the volcano where minor ground deformation was observed i e vertical displacements within 2 cm the mismatch between the position of pressure and mass sources is difficult to explain under the assumption that both are directly related to magma dynamics past studies have suggested that the gravity changes observed during 1994 2001 may primarily reflect changes in the rate of microfracturing along the nnw sse fracture weakness zone fwz that crosses the se slope of etna we use the finite element method to shed new light on the complex relations between stress strain and mass changes that occurred at etna during the studied period in particular following previous results on the degradation of the mechanical properties of rocks we perform a set of simulations assuming that the part of the medium containing the fwz is characterized by a lower young s modulus than would be expected from interpolation of tomographic data we find that the presence of the fwz creates a distortion of the displacement field induced by the deeper pressure source locally resulting in a weak extensional regime this finding supports the hypothesis of a cause effect relationship between pressurization beneath the nw flank and tensile extension beneath the se slope of the volcano we propose that this extensional regime enhanced the propagation of pressurized gas that in turn amplified the tensile strain across the fwz we also find that decreasing the value of young s modulus in the fwz allows for a larger amount of extension at depth with no change in the magnitude of surface displacements this result provides an indication of how the changes in the rate of microfracturing at depth which are needed to induce the observed gravity changes might have occurred without large ground deformation

observations of volcanic tremor at mount st helens volcano digital recordings of ground motion during tremor episodes accompanying eruptions at mount st helens volcano in the state of washington on august 7 and october 16 18 1980 are studied the spectra of the vertical component waveforms contain at least two dominant peaks at 1 0 and 1 3 hz for all events recorded during both eruptions that were studied spectra of horizontal ground motion show peaks at 0 9 and 1 1 hz the relative amplitude of the two peaks changes between tremor episodes and during single tremor episodes and shows no consistent relation to amplitude of ground motion spectra of long period earthquakes are very similar to those of tremor events suggesting that tremor is composed of many long period earthquakes that occur over a period of time the unique waveform of tremor events observed at mount st helens must be due to a source effect since the relative amplitude of the two dominant peaks changes during tremor episodes the path effect on tremor waveforms is small since there are no peaks in the spectra of waveforms recorded during tectonic earthquakes occurring in the vicinity of mount st helens the consistency of the location of the spectral peaks for the wide range of tremor amplitudes means that there must be a physical length at the source that is constant independent of the amplitude of motion at the source variations in amplitude of motion generated during tremor events must be due to variations in the force driving the tremor amplitudes of ground motion varies between 0 11 and 4 7 m seismic moment rates during the two eruptions are calculated using the model of aki et al 1977 and found to vary between 6 1018 and 1 1020 dynes cm s 1 which are larger than values found by aki et al 1977 who studied amplitudes of shallow tremor events recorded during the october 1963 eruption of kilauea volcano in hawaii study of tremor amplitudes recorded at corvallis oregon leads to the conclusion that tremor accompanying the cataclysmic may 18 1980 eruption was at least one order of magnitude larger in amplitude than tremor during august and october

from source to surface dynamics of etna s lava fountains investigated by continuous strain magnetic ground and satellite thermal data we investigated the eruptive episodes that occurred at etna volcano on 15 november 2011 and 18 march 2012 using different types of data we present novel data from two recently installed strainmeters that recorded unique signals during the lava fountain phases of these events the strainmeter data integrated with those recorded by the magnetic network and with satellite and ground thermal data allowed us to follow the path of a gas rich magma batch from the source inside the volcano to the surface and atmosphere the amplitude ratio of the volumetric strain changes constrained the storage depth of the magma feeding the lava fountains above 1 5 km below sea level magnetic data revealed an attempted shallow lateral intrusion whereas ground and satellite thermal data furnished a quantification of the total erupted volumes of 2 2 106 m3 for the 15 november event and 3 0 106 m3 for the 18 march event despite different durations of the explosive and effusive phases of the two lava fountain events the total erupted volume was quite similar suggesting the emptying of a shallow storage system displaying a steady behaviour

modeling of volcanic tremor as explosive point sources in a single layered elastic half space i investigate path effects on the waveform of a volcanic tremor using a model that consist of an impulsive dilatational point source embedded in a single layered elastic half space such a source may arise during a variety of volcanic conditions such as a sequence of rapid explosions at a volcanic vent as well as gas piston activity observed in basaltic lava lakes using acoustic signals recorded during the 1975 tolbachik eruption the 1983 klyuchevskoy eruption and the 1989 tokachi dake eruption i propose a general impulsive time function for the tremor source consisting either of a single impulse or a sequence of identical impulses computed seismograms illustrate the influence of a low velocity surface layer on the shape and spectra of the waveforms for the same layered model a single impulse produces a dispersive wave while a sequence of identical impulses produces a wave of multiple harmonics for a single impulse the peak in the seismograms shifts to higher frequencies when the surface layer is very thin for multiple impulses spectral peaks shift to higher frequencies as the time between successive impulses increases indicating an increasing importance of higher order harmonics

the exceptional activity and growth of the southeast crater mount etna italy between 1996 and 2001 between 1971 and 2001 the southeast crater was the most productive of the four summit craters of mount etna with activity that can be compared on a global scale to the opening phases of the pu u k paianaha eruption of k lauea volcano hawai i the period of highest eruptive rate was between 1996 and 2001 when near continuous activity occurred in five phases these were characterized by a wide range of eruptive styles and intensities from quiet non explosive lava emission to brief violent lava fountaining episodes much of the cone growth occurred during these fountaining episodes totaling 105 events many showed complex dynamics such as different eruptive styles at multiple vents and resulted in the growth of minor edifices on the flanks of the southeast crater cone small pyroclastic flows were produced during some of the eruptive episodes when oblique tephra jets showered the steep flanks of the cone with hot bombs and scoriae fluctuations in the eruptive style and eruption rates were controlled by a complex interplay between changes in the conduit geometry including the growth of a shallow magma reservoir under the southeast crater magma supply rates and flank instability during this period volume calculations were made with the aid of gis and image analysis of video footage obtained by a monitoring telecamera between 1996 and 2001 the bulk volume of the cone increased by 36 106 m3 giving a total 1971 2001 volume of 72 106 m3 at the same time the cone gained 105 m in height reaching an elevation of about 3 300 m the total dre volume of the 1996 2001 products was 90 106m3 this mostly comprised lava flows 72 106 m3 erupted at the summit and onto the flanks of the cone these values indicate that the productivity of the southeast crater increased fourfold during 1996 2001 with respect to the previous 25 years coinciding with a general increase in the eruptive output rates and eruption intensity at etna this phase of intense summit activity has been followed since the summer of 2001 by a period of increased structural instability of the volcano marked by a series of important flank eruptions

stratigraphic isotopic and mineralogical evidence for an early holocene thaw unconformity at mayo yukon territory ice rich glaciolacustrine sediments near mayo yukon territory reveal a thaw unconformity in the form of truncated ice wedges and abrupt changes in cryotexture the unconformity has been radiocarbon dated at 8870 200 years bp which is within the holocene period of optimal climatic conditions in northern yukon and the mackenzie delta area reported by other workers analysis of the mineralogy of the sediments indicates that the material above the unconformity is enriched in minerals that are the products of a more intense weathering environment than those deeper in the profile oxygen isotope ratios of ground ice in the sediments suggest the presence of two genetically distinct ice units above and below the unconformity an average rate of upward permafrost growth in this area of 0 1 0 2 mm year 1 is calculated for the period since the climatic optimum

the coupling between very long period seismic events volcanic tremor and degassing rates at mount etna volcano 1 from december 2005 to january 2006 an anomalous degassing episode was observed at mount etna well correlated with an increase in volcanic tremor and in the almost complete absence of eruptive activity in the same period more than 10 000 very long period vlp events were detected through moment tensor inversion analyses of the vlp pulses we obtained quantitative estimates of the volumetric variations associated with these events this allowed a quantitative investigation of the relationship between vlp seismic activity volcanic tremor and gas emission rate at mount etna we found a statistically significant positive correlation between so2 gas flux and volcanic tremor suggesting that tremor amplitude can be used as a first order proxy for the background degassing activity of the volcano vlp volumetric changes and so2 gas flux are correlated only for the last part of our observations following a slight change in the vlp source depth we calculate that the gas associated with vlp signal genesis contributed less than 5 of the total gas emission the existence of a linear correlation between vlp and degassing activities indicates a general relationship between these two processes the effectiveness of such coupling appears to depend upon the particular location of the vlp source suggesting that conduit geometry might play a significant role in the vlp generating process these results are the first report on mount etna of a quantitative relationship between the amounts of gas emissions directly estimated through instrumental flux measurements and the quantities of gas mass inferred in the vlp source inversion

observations of volcanic tremor at mount st helens in april and may 1980 abstract high amplitude nearly monochromatic volcanic tremor was observed before during and after the 18 may 1980 eruption of mount st helens the tremor often remained stationary for periods of several minutes or more for the period before and immediately following the main eruption spectra calculated from three component seismograms contained a nearly constant dominant frequency of 1 1 to 1 5 hz this contrasts with a dominant frequency of 1 7 to 2 1 hz calculated for the tremor associated with the 25 may eruption amplitude decay with distance obeys the form a r n where a is the tremor amplitude and r is the observation distance from the source the exponent n was determined in a least squares sense to be 1 5 0 4 1 5 0 3 and 1 7 0 3 before and shortly after the main eruption on 18 may 1980 and 24 may 1980 respectively the exponent for low frequency volcanic earthquakes before the main eruption was 1 6 0 3 a value similar to that of the volcanic tremor and suggests that the source of the volcanic tremor is shallow as is the source of the low frequency earthquakes the spectra of the tremor periods are very similar to those of the low frequency volcanic earthquakes there are cases where volcanic tremor appears as a continuation of a low frequency earthquake these observations suggest that the volcanic tremor and the low frequency volcanic earthquakes are strongly interconnected and indeed may well share a common source mechanism based on both signal stationarity and spectral amplitude decay consistency we suggest in common with others that the monochromatic nature of volcanic tremor is mainly due to a source effect while the path and station effects are relatively minor the tremor amplitude changes as a result of variations in the source driving force the frequency shift for the 24 may 1980 tremor may be due to a source dimension change

continuous tracking of volcanic tremor at mount etna italy summary using data from two dense array of short period seismometers we analyse the kinematic properties of volcanic tremor preceding and accompanying the 2004 2005 eruption of etna volcano italy results from slowness analyses indicate the action of at least two distinct sources the first dominates the pre eruptive period and is likely associated with the main plumbing system feeding the summit and southeast craters following the onset of the eruption secondary directions of wave arrival encompass the eruptive fissures located on the lower eastern flank of the southeast crater nonetheless significant energy radiation from this latter source was also occurring prior to the onset of the lava effusion likely suggesting the presence of a resident magma batch in agreement with independent petrologic and geochemical data

detecting imminent eruptive activity at mt etna italy in 2007 2008 through pattern classification of volcanic tremor data abstract volcano monitoring aims at the recognition of changes in instrumentally observable parameters before hazardous activity in order to alert governmental authorities among these parameters seismic data in general and volcanic tremor in particular play a key role recent major explosive eruptions such as okmok aleutians and chaiten chile in 2008 and numerous smaller events at mt etna italy have shown that the period of premonitory seismic activity can be short only a few hours which entails the necessity of effective automatic data processing near on line here we present a synoptic pattern classification analysis based on self organizing maps and fuzzy cluster analysis which is applied to volcanic tremor data recorded during a series of paroxysmal eruptive episodes and a flank eruption at etna in 2007 2008 in total eight episodes were analyzed in six of these significant changes in the dynamic regime of the volcano were detected up to 9 h prior to the onset of eruptive activity and long before changes in volcanic tremor amplitude and spectral content became evident in classical analysis in two cases the state transition was

source mechanism of volcanic earthquakes a description and classification of seismic sources found in active zones of volcanism are given the main sources of seismic wave energy are caldera collapse shear fractures with comparatively low spectral corner frequencies shallow gas explosions hydraulic pressure transients and non stationary magma flew

relationship between tremor and volcanic activity during the southeast crater eruption on mount etna in early 2000 1 the southeast crater of mount etna italy was characterized by a violent eruptive activity between 26 january and 24 june 2000 this activity produced 64 lava fountain episodes with repose periods from 3 hours to 10 days we estimated a volume of about 15 20 106 m3 lava and at least 2 3 106 m3 of tephra we compared the paroxysmal volcanic activity to its associated seismic signature the high number of events highlighted a strict correlation between tremor and volcanic activity seismic and volcanic characteristics such as the frequency of occurrence the duration of lava fountains and the associated tremor energy suggested the subdivision of the studied period into two stages separated by the 20 february event combining volcanic with seismic data we observed some useful relationships among lava fountain height sustained column height and reduced displacement in addition we found that the entire episode was well correlated with the duration of the amplitude increase computing the tremor energy linked to each event the total energy associated with lava fountains episodes results in 76 of the energy released during the whole period finally the different ratios among the overall spectral amplitude of the seismic signals of the stations located at different altitudes suggested to us the elaboration of a simple qualitative model to explain the dynamic behavior of the tremor source during the whole episode

complex magma dynamics at mount etna revealed by seismic thermal and volcanological data 1 three eruptive episodes during the 2006 summit eruptions of mount etna were exceptionally well documented by visual seismic and thermal monitoring the first 16 november was strongly explosive with vigorous strombolian activity and ash emission from multiple vents lava emission and phreatomagmatic explosions generating pyroclastic density currents pdcs the second episode 19 november had a rather weakly explosive component with mild strombolian activity but more voluminous lava emission the third 24 november was a moderately explosive paroxysm with intermittent lava fountaining and generation of a tephra column as well as lava emission and pdcs data recorded by a thermal monitoring camera clearly document the different phases of each paroxysm weather clouds occasionally hampering thermal monitoring the images show a rapid onset of the volcanic activity which during each of the paroxysms reached a peak in eruptive and thermal intensity and then decreased gradually the stronger phreatomagmatic explosions and pdcs on 16 and 24 november did not yield any seismic signature linked to the opening of new vents nor were they associated with peculiar characteristics of the seismic signal nevertheless eruptive styles strombolian activity lava emission and different levels in the intensity of explosive activity were generally well reflected in the amplitude and frequency content of the seismic signal and in the source location of the volcanic tremor centroid throughout the three eruptive episodes this multidisciplinary study therefore not only provides a key to distinguish between endogenous and exogenous origins of the phenomena observed but also documents the complex magma dynamics within the volcano

activity of mount etna 1977 1983 volcanic phenomena and accompanying seismic tremor a period of exceptional eruptive activity for both size and frequency of lateral eruptions and violence of paroxysms at the summit craters characterized etna from 1977 to 1983 the complex fissure eruption of 1971 and the eccentric eruption of 1974 on the w flank appear to be precursors of the present activity which began in july 1977 with the change in style of summit eruptions from moderate persistent activity to intermittent paroxysms the ne crater produced a score of such unusual eruptions up to march 1978 lasting a few days or even a few hours with powerful lava fountaining and flows up to 7 km long with high effusion rates other solely explosive paroxysms occurred in april 1980 in the chasm central crater and later at the se crater a return to a period of gentle persistent activity may august 1980 at the latter vent was interrupted by the recommencement of violent explosive and effusive events at the ne crater in september 1980 and february 1981 particularly remarkable lateral flank eruptions occurred in april june 1978 fracturing of the se slope from 3000 to 2600 m altitude august 1978 se ene 3000 2300 m november 1978 se 3000 1650 m august 1979 se e ne 3000 1600 m march 1981 nnw 2600 1120 m and march august 1983 s 2700 2260 m the first four of these eruptions were preceded and accompanied by violent lava fountaining at the se crater all were accompanied and or followed by phreatic explosions principally from the bocca nuova or w central crater the flow emitted on 4th august 1979 from the eastern flank reached next to the village of fornazzo in half a day that of 17 march 1981 the most destructive cut all communications roads and railways round the northern foot of the mountain in the 14 hours following its appearance it was in places 800 m wide and in less than 5 days 18 106 m3 of lava had been emitted covering 6 km2 of largely agricultural land from 28th march to 6th august 1983 the slow but sustained emission of lava gradually covered and destroyed several tourist installations between 2300 and 1100 m altitude each of these eruptive paroxysms was marked by a substantial increase in the trace amplitude of harmonic seismic tremor the intensity of which being not directly proportional to the observed activity it is likely that turbulence caused by the rising of magma in the main conduits had an important effect in the energy of the tremor this is considerably reduced after the initial paroxysmal phase e g in 1983 yet the lava effusion may continue tremor energy attains relatively high levels from 1 to 5 months before eruptions this may be caused by the recharging of the system of dykes and fissures leading up to an eruptive event after a variable period several distinct periods of recharging may precede one eruption there were three before the 1983 eruption in the few days preceding the start of some of the eruptions aug 1979 sep 1980 feb and mar 1981 a rapid diminution in the amplitude of tremor occurred this may be associated with changes in the stress field caused by the opening of new fractures or the closing opening of pre existing ones

seismic and infrasonic evidences for an impulsive source of the shallow volcanic tremor at mt etna italy during a seismo acoustic experiment we recorded volcanic tremor around the summit craters of mt etna volcano tremor shows amplitude modulation which disappear 900 m from the crater area the infrasonic wavefield is coherent even at distances of 750 m time delay between infrasonic transient is stable around 1 3 s and is consistent with the position of the source in the voragine crater amplitude modulation of tremor is well correlated 0 72 with infrasound amplitude with a time lag of 0 37 s coherent with a shallow position of the source amplitude of volcanic tremor decays over increasing distances according to geometrical spreading of body waves tremor wavefield shows a linear polarization following the same time occourrence as the infrasonic pulses polarization azimuth indicates that wavefield rectilinearity is mostly due to p waves we infer that most of the volcanic tremor we recorded at mt etna is generated by superimposition of small impulsive sources acting at 1 2 s rate caused by pressure instability during magma degassing

approach to the volcanic tremor by covariance analysis application to the 1989 eruption of mt etna sicily polarization of the tremor wavefield recorded during three periods of the 1989 eruption of mt etna at a temporary array of three component stations is analyzed by use of a polarization filter the tremor shows steady horizontal near linear west east polarization at all stations such a behaviour which could be coherent with a p wavefield radiated by a vertical extended source with overall north south alignment excludes confinement of the source to the only active vent and is tentatively linked to activity of the main dyke system of mt etna

history of the greenland ice sheet paleoclimatic insights abstract paleoclimatic records show that the greenland ice sheet consistently has lost mass in response to warming and grown in response to cooling such changes have occurred even at times of slow or zero sea level change so changing sea level cannot have been the cause of at least some of the ice sheet changes in contrast there are no documented major ice sheet changes that occurred independent of temperature changes moreover snowfall has increased when the climate warmed but the ice sheet lost mass nonetheless increased accumulation in the ice sheet s center has not been sufficient to counteract increased melting and flow near the edges most documented forcings and ice sheet responses spanned periods of several thousand years but limited data also show rapid response to rapid forcings in particular regions near the ice margin have responded within decades however major changes of central regions of the ice sheet are thought to require centuries to millennia the paleoclimatic record does not yet strongly constrain how rapidly a major shrinkage or nearly complete loss of the ice sheet could occur the evidence suggests nearly total ice sheet loss may result from warming of more than a few degrees above mean 20th century values but this threshold is poorly defined perhaps as little as 2 c or more than 7 c paleoclimatic records are sufficiently sketchy that the ice sheet may have grown temporarily in response to warming or changes may have been induced by factors other than temperature without having been recorded

long period seismicity at shishaldin volcano alaska in 2003 2004 indications of an upward migration of the source before a minor eruption abstract we have analyzed the long period lp seismic activity at shishaldin volcano aleutians islands alaska in the period october 2003 july 2004 during which a minor eruption took place in may 2004 with ash and steam emissions thermal anomalies volcanic tremor and small explosions we have focused the attention on the time evolution of lp rate size spectra and polarization dip angle along the dataset we find an evolution toward more shallow dip angles in the polarization of the waveforms during the sequence the dip angle is a manifestation of the source location because the lp seismic sources are presumed to reflect the aggregation of gas slug or pockets within the melt we use the polarization dip at the lp onset as a proxy for the nucleation depth of the seismic events within the conduit we refer to this parameter as the nucleation dip and the position along the conduit of the gas aggregation as nucleation depth the nucleation dip changes throughout the dataset it shows a sharp decrease between the end of december 2003 and the end of january 2004 followed by a gradual increase until the onset of the eruption at the same time a general increase of the lp rate occurs we have associated the dip evolution with a sinking and a subsequent decrease of the nucleation depth which would quickly migrate up to about 8 km below the crater rim followed by a slow depth decrease which culminates in the eruption the change in the nucleation depth reflects either a pressure variation within the plumbing system which would affect the confining pressure experienced by the gas aggregations we have imputed such a pressure change to the intrusion of batches of magma from a deeper magma chamber 5 km for a cylindric conduit with rigid walls this leads to a volume of the injected new magma of 10 5 10 7 m 3 compatible with estimates in other areas suggesting that the lp process can be considered a good proxy of the thermodynamical conditions of the shallow plumbing system

proximal lava drainage controls on basaltic fissure eruption dynamics hawaiian basaltic eruptions commonly initiate as a fissure producing fountains spattering and clastogenic lava flows most fissures rapidly localize to form a small number of eruptive vents the location of which may influence the subsequent distribution of lava flows and associated hazards we present results from a detailed field investigation of the proximal deposits of episode 1 of the 1969 fissure eruption of mauna ulu k lauea hawai i exceptional preservation of the deposits allows us to reconstruct vent proximal lava drainage patterns and to assess the role that drainage played in constraining vent localization through detailed field mapping including measurements of the height and internal depth of lava tree moulds we reconstruct high resolution topographic maps of the pre eruption ground surface the lava high stand surface and the post eruption ground surface we calculate the difference in elevation between pairs of maps to estimate the lava inundation depth and lava drainage depth over the field area and along different segments of fissure aerial photographs collected during episode 1 of the eruption allow us to locate those parts of the fissure that are no longer exposed at the surface by comparing with the inundation and drainage maps we find that fissure segments that were inundated with lava to greater depths typically 1 6 m during the eruption later became foci of lava drainage back into the fissure internal drain back we infer that in these areas lava ponding over the fissure suppressed discharge of magma thereby favouring drain back and stagnation by contrast segments with relatively shallow inundation typically less than 1 m such as where the fissure intersects pre eruptive topographic highs or where flow away from the vent outflow was efficient are often associated with sub circular vent geometries in the post eruption ground surface we infer that these parts of the fissure became localization points for ongoing magma ascent and discharge we conclude that lava inundation and drainage processes in basaltic fissure eruptions can play an important role in controlling their localization and longevity

a spherical source model for low frequency volcanic earthquakes a resonant magma chamber is modeled by a spherically symmetric whole space consisting of a small gas filled chamber surrounded by magma which is encased in country rock source boundary conditions are specified on the cavity boundary simulating a possible sudden gas expansion in the magma the simplicity of this model leads to an analytical closed form solution for the fourier transform of radial displacement which predicts a peaked spectrum and an oscillating decaying seismogram for physically reasonable values of the rock and magma elastic properties and dimensions of the model the dominant frequency of the signal is in the range of 1 to 5 hz near the values observed for low frequency volcanic earthquakes and volcanic tremor furthermore the seismograms and spectra bear resemblance to those observed for low frequency earthquakes the fundamental mode of oscillation is a pure radial breathing mode which has markedly different properties from single cavity solutions the fundamental frequency is insensitive to the magma chamber boundary radius when both the inner and outer radii are greater than specified values this may help to explain the apparent similarity of the dominant frequency of volcanic earthquakes over a wide range of physical situations the presence of an inner effective bubble of a certain minimum size is necessary for the existence of the dominant low fundamental frequency spectral peak although highly idealized we believe that this fundamental fluid solid cavity mode may provide an explanation for the observations of tremor and low frequency volcanic earthquakes

volcanic unrest leading to the july august 2001 lateral eruption at mt etna seismological constraints abstract a close relationship between earthquake swarms volcanic eruptions and ground deformation at mt etna was well documented shortly before the beginning of the july august 2001 eruption past experiences at this volcano suggest how magma dike intrusion in the shallow crust or in the upper part of the volcanic pile normally occurs after several years months of internal recharging since seismic investigations provide a means to study the scale and origin of stress perturbations at active volcanoes allowing to better investigating the preparation phase of an eruption in this paper we performed a close examination of the seismic activity recorded at mt etna in the months preceding the 2001 eruption and in particular between november 2000 and july 2001 after integrating data recorded by the two networks operating during that time and run by the istituto internazionale di vulcanologia and sistema poseidon we relocated 522 earthquakes by using the tomodd code in a 3d velocity model and then we computed their fault plane solutions the application of different selection criteria enabled obtaining a good quality revised data set consisting of 111 fault plane solutions the high precision locations identified well defined seismic clusters in different periods suggesting a link with the magma migration from a depth of 8 13 km b s l towards shallower zones moreover the computed maximum compressive stress axis as inferred from earthquake focal mechanisms indicated a roughly w e oriented 1 this findings reflect an overpressure of the mid to shallow crust due to the progressive magma uprising in central portion of the volcano and also highlighted a rotation of the local stress field with respect to the regional one n s trending in addition p axis distribution pointed out the presence of a center of pressure located to the south of the central craters these results provide particularly compelling evidence for a direct causal link between pressurization of the mid level volcanic plumbing system by ascending magma and precursory local stress field reorientations demonstrating that seismological analysis can be used to detect subtle local stress changes that herald eruptive activity

excitation of a buried magmatic pipe a seismic source model for volcanic tremor recent observations of seismic events at various volcanoes suggest that harmonic tremor results from the sustained occurrence of so called long period or low frequency events accordingly we can view the long period volcanic event as the elementary process of tremor and interpret it as the impulse response of the tremor generating system we present a seismic model in which the source of tremor is the acoustic resonance of a fluid filled volcanic pipe triggered by excess gas pressure the model consists of three elements namely a triggering mechanism a resonator and a radiator for simplicity we assume a hemispherical trigger cylindrial resonator and circular radiator set in a vertical configuration with the trigger capping the top of the pipe and the disk shaped radiator shutting off its bottom considering the simple case of a source buried in a homogeneous half space we then apply the discrete wave number method to obtain a complete representation of the ground motion response at near and intermediate distances the results demonstrate that the displacement attributed to the pipe dominates the near field motion while that due to the disk is representative of the intermediate and far fields the trigger itself has a smaller contribution mainly limited to the field in the proximity of the source the characteristics displayed by the free surface response evolve from a strong impulsive signature in the immediate vicinity of the epicenter to a well developed harmonic wave train dominated by rayleigh waves at larger distances no clear shear arrival can be detected in the synthetic seismograms the displacement spectrum reflects the organ pipe modes of the conduit and the bandwidth associated with the dominant spectral peak of motion is controlled by the combined losses due to viscous attenuation in the fluid and elastic radiation into the solid in the case of the cylindrical magma column considered the radiation loss is proportional to the square of the pipe radius while the loss related to viscous damping is inversely proportional to the same factor indicating that the relative importance of the two loss mechanisms is critically dependent on the geometry of the magma reservoir the relative importance of the pipe and disk elements likewise is a function of the conduit cross section this suggests the possibility of determining the geometry of the source as well as the radiation loss and in situ magma viscosity from a comparison of near and far field observations

thirty years of satellite derived lava discharge rates at etna implications for steady volumetric output 1 we present a 30 year long data set of satellite derived time averaged lava discharge rates tadr for mount etna volcano sicily italy spanning 1980 2010 and comprising 1792 measurements during 23 eruptions we use this to classify eruptions on the basis of magnitude and intensity as well as the shape of the tadr time series which characterizes each effusive event we find that while 1983 1993 was characterized by less frequent but longer duration effusive eruptions at lower tadrs 2000 2010 was characterized by more frequent eruptions of shorter duration and higher tadrs however roughly the same lava volume was erupted during both of these 11 year long periods so that the volumetric output was linear over the entire 30 year period increasing at a rate of 0 8 m3 s 1 between 1980 and 2010 the cumulative volume record can be extended back in time using data available in the literature this allows us to assess etna s output history over 5 centuries and to place the current trend in historical context we find that output has been stable at this rate since 1971 at this time the output rate changed from a low discharge rate phase which had characterized the period 1759 to 1970 to a high discharge rate phase this new phase had the same output rate as the high discharge rate phase that characterized the period 1610 1669 the 1610 1669 phase ended with the most voluminous eruption of historic times

seismic precursors to the mount st helens eruptions in 1981 and 1982 six categories of seismic events are recognized on the seismograms from stations in the vicinity of mount st helens two types of high frequency earthquakes occur near the volcano and under the volcano at depths of more than 4 kilometers medium and low frequency earthquakes occur at shallow depths less than 3 kilometers within the volcano and increase in number and size before eruptions temporal changes in the energy release of the low frequency earthquakes have been used in predicting all the eruptions since october 1980 during and after eruptions two types of low frequency emergent surface events occur including rockfalls and steam or gas bursts from the lava dome

application of gravity and deep dipole geoelectrics in the volcanic area of mt etna sicily abstract the structural pattern of the basement of the mt etna volcano is studied by combining results from new gravity and deep dipole geoelectric field data at first the bouguer map was separated into regional and residual contour maps by a 50 km cut off wavelength then by the geoelectric interpretation n s and e w resistivity cross sections were elaborated which were used as guidelines for modelling gravity residuals after observing a close correspondence between bouguer anomalies and resistivity classes by a trial and error adjustment to the relationship between resistivity and density a 3 d block model of the basement was finally constructed which consistently matched with both gravity and geoelectric information content two main conclusions emerged from the combined interpretation the first is the presence of diffuse inhomogeneities in the upper portion of the basement down to an average depth of 2 km below sea level in contrast with the regularity of the explored underlying portion the second is the absence of geophysical markers suggesting the existence of a large magmatic chamber down to about 3 km below sea level

monitoring the december 2015 summit eruptions of mt etna italy implications on eruptive dynamics abstract a lengthy period of eruptive activity from the summit craters of mt etna started in january 2011 it culminated in early december 2015 with a spectacular sequence of intense eruptive events involving all four summit craters voragine bocca nuova new southeast crater and northeast crater the activity consisted of high eruption columns strombolian explosions lava flows and widespread ash falls that repeatedly interfered with air traffic the most powerful episode occurred on 3 december 2015 from the voragine after three further potent episodes from the voragine activity shifted to the new southeast crater on 6 december 2015 where strombolian activity and lava flow emission lasted for two days and were fed by the most primitive magma of the study period activity once more shifted to the northeast crater where ash emission and weak strombolian activity took place for several days sporadic ash emissions from all craters continued until 18 december when all activity ceased although resembling the summit eruptions of 1998 1999 which also involved all four summit craters this multifaceted eruptive sequence occurred in an exceptionally short time window of less than three days unprecedented in the recent activity of mt etna it also produced important morphostructural changes of the summit area with the coalescence of voragine and bocca nuova in a single large crater the central crater reproducing the morphological setting of the summit cone before the formation of bocca nuova in 1968 the december 2015 volcanic crisis was followed closely by the staff of the etna observatory to monitor the on going activity and forecast its evolution in accordance with protocols agreed with the italian civil protection department

volcanic tremors at etna a model for hydraulic origin spectral analysis of volcanic tremors at etna reveals 2 characteristic features 1 the escaping vapor and gas streams drive the magma in random turbulent motion generating a seismic wave emission according toa f n exp m f k wherea amplitude f frequency n m k parameters 2 sudden transients in the boundary conditions of the piping system generate hydraulic pressure pulses which in the seismogram appear as wave packages their monochromatic frequency content suggests that their origin is associated with strong resonance phenomena it is conclusive that volcanic eruptions may be distinguished and classified according to different eigenfrequencies of the fissure and pipe network involved in the volcanic activity furthermore positions of active dykes may be traced following the amplitude distributions associated with pronounced spectral lines in the tremor spectra

a model for internal dynamical processes on mt etna summary results of studies concerning seismic activity earthquakes and tremor and its relation with eruptive events on mt etna are briefly discussed seismic records collected from 1977 up to 1986 have been analysed and special care was given to observation of changes in some seismic parameters such as the rate of microseismic activity the amplitude and dominant spectral peaks of the volcanic tremor which vary significantly in relation to the occurrence of eruptive events the systematic nature of variations in the seismic parameters considered allows us to propose a preliminary model which puts all the observations made so far into a wide context giving an interpretative hypothesis of the magma uprise mechanisms preceding either summit or flank eruptions summit eruptions are modelled in two stages and occur without changes in the seismic parameters considered except for a sharp increase in tremor amplitude almost coincident in time with the eruption onset flank eruptions are modelled in three stages and follow a simultaneous change of all the mentioned parameters with time the onset of these eruptions is in fact preceded by an increase in both the daily number of shocks and the amplitude of the volcanic tremor as well as a shifting from relatively high values in the dominant peaks of the tremor spectra which appear in the pre eruptive stage towards usual lower frequency values 1 0 2 3 hz

ground motion in the near field of a fluid driven crack and its interpretation in the study of shallow volcanic tremor we present a study of the motion of the ground in the near field of a fluid driven tensile crack embedded in a layered half space the source that we consider is the jerky opening of a channel connecting two fluid filled cracks and the cause of this opening is the excess pressure of fluid in one of the cracks we make a complete representation of the three components of ground motion in the space time and frequency domains and analyze the effects of fluid compressibility source depth and medium structure on the ground response the calculations show the presence of a dominant frequency of motion which depends not only on the source geometry and bulk modulus of the fluid but also on medium characteristics receiver position and the component of motion being considered using this source model we view an episode of volcanic tremor as a continuous sequence produced by numerous jerky openings of channels occurring randomly in time along a chain of cracks our results are applied to the october 5 6 1963 east rift eruption of kilauea volcano hawaii and found to be compatible with available seismic data suggesting that magma is transported through an ensemble of cracks with the area of 1 1 km each pair of cracks constituting an individual dike segment which opens in discrete increments at a rate of 1 s we obtain the following parameters for each dike segment opening increase in cavity volume 40 m3 stress drop 0 004 bar seismic moment 1019 dyn cm and force applied by the fluid to open the dike 1014 dyn the total moment integrated over the entire duration of tremor for the eruption is 0 5 1024 dyn cm roughly equivalent to a single magnitude m 5 earthquake the seismic source parameters and other field observations offer constraints on the process of mass transport and are compatible with the excess pressure and viscosity of magma of about 40 bars and 102 p respectively

activity of mount etna preceding the february 1999 fissure eruption inferred mechanism from seismological and geochemical data abstract the february 1999 eruption of mt etna took place through a fissure on the sse flank of the cone of the summit se crater this event was preceded by continuous activity since 1995 sometimes accompanied by violent outbursts from one or more of the three other summit craters ne crater voragine or chasm and bocca nuova and finally by a series of 20 short lived eruptions from the se crater between september 1998 and january 1999 these phenomena could be accounted for by invoking gradual invasion of a shallow small reservoir by more primitive basic and gas rich magma coming from depth the shallow chamber is more likely to be a plexus of dikes which had developed during the previous years 1995 1997 following variations of the local stress field owing to enhanced magma generation and accumulation at the top of the mantle magma injection and mixing is evidenced through geochemistry whereas the state of stress of the volcanic pile and underlying crust is determined using earthquake distributions and focal mechanisms the behaviour of the seismic tremor amplitude appears to be a good indicator of the state of unrest of the volcano although not always directly linked to the relative energy of degassing phenomena

observations and numerical simulations of the braking effect of forests on large scale avalanches although the disaster reduction effects of forest braking have long been known empirically they have not been known in detail down to recent in this study we ascertained forest braking effect by numerical simulations using the avalanche dynamics program titan2d to model large scale avalanches one of these avalanches occurred in the makunosawa valley myoko and damaged a cedar forest the others occurred on mt iwate and damaged a subalpine forest all avalanches damaged many trees and terminated within the forests in our simulations the resistance of the forests to avalanches is simulated using a larger bed friction angle fitting the observations from the makunosawa avalanche by trial and error a bed friction angle of 13 14 in the non forested area and of 25 in the forested area is obtained we conducted simulations of the mt iwate avalanches using the same method as for the makunosawa valley avalanche and obtained good agreement between observations and simulations simulations reveal that without the forest the avalanche would have traveled at least 200 m farther than the forest s actual end in the makunosawa valley and at least 200 m and possibly up to 600 m farther on mt iwate this study therefore clearly shows that forests provide a braking effect for avalanches