a review on phreatic eruptions and their precursors abstract an analysis of the reported information on 132 historical phreatic events indicates that most of these 115 were not followed by a magmatic or phreatomagmatic eruption only in 18 cases precursory phenomena are reportedly lacking whereas they are recognized for the majority 72 of the considered cases these precursors have been analyzed as a function of the kind of phreatic event and of its possible triggering mechanism unfortunately the quality of the available information is poor however data suggest that anomalous seismicity ground deformation changes in the output chemistry and temperature of fumaroles and thermal springs are potential mid term precursors whereas seismic tremor is a common short term precursor

geometrical effects and low attenuation resonance of volcanic fluid inclusions for the source mechanism of long period earthquakes 1 we formulate the resonances of three simple geometrical inclusions a plane a cylinder and a sphere and find several clues to distinguishing the shape of volcanic fluid systems and their relevance to the observation of long period earthquakes the series of eigenfrequencies for a cylindrical and a spherical inclusion have shifts of 4 and 2 respectively in their higher modes from the eigenfrequencies of a planar inclusion the ratios between the higher and fundamental characteristic frequencies can be indicators of the geometry of volcanic fluid inclusions furthermore the eigenattenuation factor of a planar inclusion is a constant value that is determined uniquely by the impedance contrast between the fluid and the surrounding elastic medium and is independent of mode in marked contrast cylindrical and spherical inclusions have smaller or greater eigenattenuation factors particularly in lower modes under certain conditions of p wave velocity contrast and poisson s ratio of the surrounding elastic medium lower modes have less than 0 1 times the attenuation factor of higher modes we name these low attenuation modes lams lams are physically related to the conversion of volumetric changes to shear deformation of the surrounding elastic medium and the radiation of shear waves from the curved boundaries some monotonic waveforms of long period earthquakes can be interpreted by the excitation of these lams crack modes are also simply formulated by the resonance of a rectangular inclusion and also become lams whose attenuation factors are related to the aspect ratio of the crack

geostatistical estimation of the transmissivity in a highly fractured metamorphic and crystalline aquifer man danane region western ivory coast abstract this work is aimed at estimating the transmissivity of highly fractured hard rock aquifers using a geostatistical approach the studied aquifer is formed by the crystalline and metamorphic rocks of the western ivory coast west africa in the man danane area the study area covers 7290 km 2 90 km 81 km the fracturing network is dense and well connected without a marked fracture direction a data base comprising 118 transmissivity t values and 154 specific capacity q s values was compiled a significant empirical relationship between t and q s was found which enabled the transmissivity data to be supplemented the variographic analysis of the two variables showed that the variograms of t and q s which are lognormal variables are much more structured than those of log t and log q s which are normal variables this result is contrary to what was previously published and raises the question whether normality is necessary in geostatistical analysis several input and geostatistical estimations of the transmissivity were tested using the cross validation procedure measured transmissivity data supplemented transmissivity data kriging cokriging the cross validation results showed that the best estimation is provided using the kriging procedure the transmissivity field represented by the whole data sample measured estimated using specific capacity and the structural model evaluated solely on the measured transmissivity the geostatistical approach provided in fine a reliable estimation of the transmissivity of the man danane aquifer which will be used as an input in forthcoming modelling

tornillos at vulcano clues to the dynamics of the hydrothermal system abstract the number of tornillo events has recently increased at the vulcano island italy while only 15 tornillos were recorded during 2004 2006 584 events occurred in 2007 2008 they were located just below la fossa crater at depths ranging between 0 1 and 1 km b s l during two intervals in 2007 2008 increases in the number of tornillos took place at the same time as temperature and geochemical anomalies were observed the spectral content of the tornillos generally characterized by one two dominant spectral peaks near 6 and 10 hz varied over time with changes also noted in the quality factors the simplest source mechanism proposed for tornillos is the free eigenvibration of a fluid volume within a crack or a conduit based on this model we propose a causal relationship between the temperature and geochemical anomalies and the increases in numbers of tornillos as the amount of hydrothermal fluids increases during the anomalies the upward flux of fluids grows the consequent changes in the pressure temperature and dynamics of the system of cracks and conduits result in the generation of tornillos based on the fluid filled crack conduit model the shallow depths of the sources and the values of the quality factors the fluid within the resonant crack conduit was inferred to be an ash gas or water droplet gas mixture moreover the observed variations in the wavefield can be caused by small changes in the location of the source in the source mechanism or in the medium in between the source and the seismic station finally another peculiar feature of tornillos is the amplitude modulation that can be explained as a result of a beating phenomenon

stochastic simulation of volcanic tremor from ruapehu abstract the most common volcanic tremor produced by ruapehu is a continuous signal with a dominant frequency of about 2 hz this signal has a sharply peaked spectrum and an autocorrelation function with a high degree of coherence even for lags of over 20 seconds these characteristics strongly suggest that the cause of this tremor is a single resonator probably a fluid filled cavity resonating in an organ pipe mode the stochastic simulation of such a resonator uses the equation of motion of a simple harmonic oscillator which applies to an organ pipe fundamental resonance with either the characteristics of the oscillator or the forcing function containing a random element a white noise forcing function which would be appropriate for excitation of the cavity by a high pressure gas input gave good agreement with the observed spectra and autocorrelation functions another possible model used an oscillator with a damping factor which varied randomly and was sometimes negative so oscillations built up rather than decayed this also gave a reasonable simulation of ruapehu tremor the third excitation model used a poisson process in which during each time interval there was a certain probability of applying a fixed impulse to the resonator it was found that the impulses had to be frequent i e several times a second to match the characteristics of ruapehu tremor it has been suggested that tremor is composed of a succession of low frequency b type earthquakes the results of this simulation show that at ruapehu tremor could be produced by a resonator with positive feedback just sustaining oscillation or by a resonator excited by external impulses the most promising model for low frequency earthquakes describes them as the result of a major external disturbance of the resonator

long period events the most characteristic seismicity accompanying the emplacement and extrusion of a lava dome in galeras volcano colombia in 1991 abstract since its reactivation in 1988 the principal eruptions of galeras volcano occurred on may 4 9 1989 july 16 1992 and january 14 march 23 april 3 april 14 and june 7 1993 the initial eruption was a phreatic event which clearly marked a new period of activity a lava dome was extruded within the main crater in october 1991 and subsequently destroyed in an explosive eruption on july 16 1992 the eruptions that followed were all vulcanian type explosions the seismicity accompanying the emplacement extrusion and destruction of the lava dome was dominated by a mix of long period lp events and tremor displaying a variety of waveforms repetitive lp events with dominant periods in the range 0 2 1 s were observed in october and november 1991 and visually correlated with short energetic pulses of gas venting through a crack bisecting the dome surface each lp event was characterized by a weak precursory signal with dominant periods in the range 0 05 0 1 s lasting roughly 7 s using the fluid driven crack model of chouet 1988 1992 we infer that two distinct cracks may have acted as sources for the lp and precursor signals spectral analyses of the data yield the following parameters for the lp source crack length 240 360 m crack width 130 150 m crack aperture 0 5 3 4 mm crack stiffness 100 500 sound speed of fluid 880 m s and excess pressure 0 01 0 19 mpa similar analyses yield the parameters of the precursor source crack length 20 30 m crack width 15 25 m crack aperture 2 3 8 7 mm crack stiffness 5 15 sound speed of fluid 140 m s and excess pressure 0 06 0 15 mpa combined with geologic and thermodynamic constraints obtained from field observations these seismic parameters suggest a gas release mechanism in which the episodic collapse of a foam layer trapped at the top of the magma column subjacent to the dome releases a slug of pressurized gas which escapes to the surface while dilating a preexisting system of cracks in the dome structure accordingly the fracture observed on the crystallized dome body is the surface extension of the lp source crack where lp activity is induced by the rapid emission and expansion of gas flowing through this conduit the width and aperture of the crack estimated in the model are in good agreement with the length and aperture of the fracture estimated from visual observations the source parameters of the precursor signal are suggestive of a nozzle like conduit connecting the lp source crack to the underlying magma reservoir excitation of this conduit segment is attributed to the rapid emission and acceleration of the frothy fluid resulting from the collapse of the foam layer at the top of the reservoir the calculated periodicity of foam collapse events is in agreement with the observed average rate of thirteen lp events per hour

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

deep fault plane geometry inferred from multiplet relative relocation beneath the south flank of kilauea dense microearthquake swarms occur in the upper south flank of kilauea providing multiplets composed of hundreds of events the similarity of their waveforms and the quality of the data have been sufficient to provide accurate relative relocations of their hypocenters a simple and efficient method has been developed which allowed the relative relocation of more than 250 events with an average precision of about 50 m horizontally and 75 m vertically relocation of these events greatly improves the definition of the seismic image of the fault that generates them indeed relative relocations define a plane dipping about 6 northward although corresponding absolute locations are widely dispersed in the swarm a composite focal mechanism built from events providing a correct spatial sampling of the multiplet also gives a well constrained northward dip of about 5 to the near horizontal plane this technique thus collapses the clouds of hypocenters of single event locations to a plane coinciding with the slip plane revealed by previous focal mechanism studies we cannot conclude that all south flank earthquakes collapse to a single plane there may locally be several planes perhaps with different dips and depths throughout the south flank volume the 6 northward dipping plane we found is too steep to represent the overall flexure of the oceanic crust under the load of the island of hawaii this plane is probably an important feature that characterizes the basal slip layer below the upper south flank of kilauea volcano differences in seismicity rate and surface deformations between the upper and lower south flank could be related to the geometry of this deep fault plane the present work illustrates how high precision relative relocations of similar events in dense swarms combined with the analysis of geodetic measurements can help to describe deep fault plane geometry systematic selection and extensive relative relocation of similar earthquakes could be attempted in other well instrumented highly seismic areas to provide reliable basic information especially useful for understanding of earthquake generation processes

a synthesis of the recent activity of galeras volcano colombia seven years of continuous surveillance 1989 1995 abstract the current period of re activation since 1988 at galeras volcano colombia has been characterized mainly by the following events 1 a semi continuous series of vulcanian eruptions during 5 9 may 1989 2 emplacement of an andesitic lava dome at the bottom of the main crater in october november 1991 3 six vulcanian eruptions during 1992 1993 the first of which destroyed most of the dome on 16 july 1992 and 4 three volcano tectonic seismic crises in april 1993 november december 1993 and march 1995 during much of this seven year period several small ash and gas emissions also have taken place the 4 9 may 1989 eruptions originated from the secondary crater el pinta and deposited ash lapilli and blocks in the crater area the 1992 1993 eruptions originated from the main crater and were associated with obstruction of the conduit by magma from dome emplacement in late 1991 causing overpressurization of the system for the 1992 1993 eruptions pre eruptive seismicity deformation and so2 flux all exhibited very low levels the eruptions were characterized by their sudden initiation low intensity vei 1 small eruption columns and small volumes of erupted material the source of the volcano tectonic seismic crises is located approximately 3 km north and northeast of the crater some of these events were felt in pasto and other towns located around the volcano on one occasion causing loss of life injuries and damage to buildings

evidence for extreme variations in the permeability of laterite from a detailed analysis of well behaviour in nigeria laterite soils are widespread in tropical africa and have a large impact on the hydrology of the areas they cover the permeability of laterite helps determine the partitioning of runoff and interflow and regulates groundwater recharge to underlying bedrock groundwater within laterite also forms a widespread source of drinking water typically from unimproved hang dug wells despite its importance there is little published information on laterite aquifer properties in this study data from a 6 m deep well in nigeria have been analysed to characterise the hydraulic conductivity of the laterite from repeated pumping tests transmissivity measurements from 40 tests spread out across a hydrological year varied from 0 1 to 1000 m2 d further interpretation of the data demonstrate a strong non linear decrease in horizontal hydraulic conductivity with depth characterised by an upper horizon of extreme permeability 400 m d and a much lower permeability profile beneath 0 1 m d these data are substantiated with observations from other wells throughout the area this non linear permeability structure has several implications the upper laterite can facilitate rapid lateral throughflow in the wet season enabling contaminants to be transported significant distances up to 1 km natural groundwater levels are restricted to a narrow range for much of the year and in the dry season the lower permeability of the deeper laterite restricts the amount of water which can be abstracted from shallow wells leading to well failure the work highlights the need for a wider study to better understand laterite soils and the role they play in regional hydrology 2013 natural environment research council hydrological processes published by john wiley sons ltd

groundwater in fractured bedrock environments managing catchment and subsurface resources an introduction hard rocks including crystalline igneous metamorphic and strongly cemented sedimentary and carbonate rocks cover about 50 of the earth s land surface singhal gupta 2010 globally the volume of groundwater contained in hard rock aquifers is not well constrained comte et al 2012 but locally they can be important aquifers macdonald et al 2012 albeit with low groundwater storage and poor primary porosity and permeability groundwater flow in these hard rocks is commonly observed to be associated with water bearing discontinuities such as fractures joints and faults mazurek 2000 berkowitz 2002 font capo et al 2012 and in the weathered regolith wright 1992 chilton foster 1995 deyassa et al 2014 structural elements such as fault zones also strongly govern the behaviour of these systems forster evans 1991 lopez smith 1995 bense et al 2013 the nature abundance orientation and connectivity of these water bearing features are largely governed by the history and nature of structural deformation of the bedrock and commonly impose strong anisotropic flow and transport parameters on these bedrock aquifers hsieh et al 1985 bour davy 1997 mortimer et al 2011 weathering processes furthermore lead to an alteration of bedrock composition and associated aquifer properties resulting in enhanced fracture connectivity and an overall vertical stratification zonation of bulk aquifer properties ranging from highly altered shallow regolith horizons to more competent sparsely fractured bedrock at depth dewandel et al 2006 krasn sharp 2007 lachassagne et al 2011

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

multiple convection patterns and thermohaline flow in an idealized ogcm abstract this paper investigates how multiple steady states arise in an ocean general circulation model caused by the fact that many different convection patterns can be stable under the same surface boundary conditions two alternative boundary conditions are used in the experiments classical mixed boundary conditions and a diffusive atmospheric heat balance combined with fixed salt fluxes in both cases transitions between different quasi steady convection patterns can be triggered by briefly adding fresh water at convection sites either a large scale freshwater anomaly is used to completely erase the previous convection pattern or a surgical anomaly is added to single grid points to turn off convection there under classical mixed boundary conditions different convection sites can lead to different overturning rates of deep water the dynamics of the convection driven flow is analyzed in some detail with an energy balance atmosphere in contrast the overturning rate is very robust apparently

observations with moored acoustic doppler current profilers in the convection regime in the golfe du lion abstract in the golfe du lion south of france favorable conditions for deep winter convection exist and were documented by the medoc experiments during 1969 75 a renewed investigation of that regime with upward looking moored acoustic doppler current profilers adcps was carried out during 24 january 5 march 1987 to record the three dimensional currents associated with the deep mixing while in the earlier studies initial deep convection did not begin until fairly late in the winter season a very strong mistral around 10 january 1987 had already generated a 1arge deep mixed patch homogeneous down to around 2000 m at deployment time three adcps two working at 150 khz and one at 75 khz were moored in a triangle of 15 km sidelength at 550 780 m depth full records at 1 hour ensemble time intervals 400 pings per ensemble 8 m bin lengths were obtained by the 75 khz and one of the 150 khz adcps in mid february a second mistral hit the region with the onset of strong winds and surface cooling the

topographic preconditioning of open ocean deep convection abstract evidence for oceanic convection over maud rise in the weddell sea suggests that bottom topography may select the location and scale of deep convecting oceanic chimneys forced by seasonal large scale atmospheric cooling in this paper the role of bottom topography in open mean deep convection is studied using an idealized three dimensional primitive equation model a barotropic mean flow impinges on a gaussian shaped seamount in a stratified domain generating a taylor cap a region of topographically trapped fluid uniform surface cooling is applied throughout the domain when the taylor cap is tall enough to interact with the surface mixed layer the local isolation from the advection of heat by the mean flow forms a conduit into the deep water convection within this region is significantly enhanced relative to ambient levels away from the seamount and to similar numerical simulations performed without bottom topography given uniform background stratification domed isopycnals are not importa

inferring the heterogeneity transmissivity and hydraulic conductivity of crystalline aquifers from a detailed water table map abstract estimating the transmissivity or hydraulic conductivity field to characterize the heterogeneity of a crystalline aquifer is particularly difficult because of the wide variations of the parameters we developed a new approach based on the analysis of a dense network of water table data it is based on the concept that large scale variations in hydraulic head may give information on large scale aquifer parameters the method assumes that flux into the aquifer is mainly sub horizontal and that the water table is mostly controlled by topography rather than recharge it is based on an empirical statistical relationship between field data on transmissivity and the inverse slope values of a topography reduced water table map this relationship is used to compute a transmissivity map that must be validated with field measurements the proposed approach can provide a general pattern of transmissivity or hydraulic conductivity but cannot correctly reproduce strong variations at very local scale less than10 m and will face of some uncertainties where vertical flows cannot be neglected the method was tested on a peridotite ultramafic rock aquifer of 3 5 km 2 in area located in new caledonia the resulting map shows transmissivity variations over about 5 orders of magnitude average logt 5 2 0 7 comparison with a map based on measured water level data n 475 shows that the comparison between logt computed values and logt data deduced from 28 hydraulic tests is estimated with an error less than 20 in 71 of cases logt 0 4 and with an error less than 10 logt 0 2 on average in 39 of cases from this map a hydraulic conductivity map has been computed showing values ranging over 8 orders of magnitude the repeatability of the approach was tested on a second data set of hydraulic head measurements n 543 the mean deviation between both logt maps is about 11 these encouraging results show that the method can give valuable parameter estimates and can characterize aquifer heterogeneity the computed logt and logk maps highlight the spatial distribution of parameters that show a pattern clearly controlled by the fault network of this ultramafic massif however the faults are mainly characterized by low permeability zones this differs from results on other crystalline aquifers and may be due to the fact that weathering products of peridotite are clay like materials the resulting transmissivity map can be used as a starting point for modelling or to direct additional fieldwork

localization of deep ocean convection by a mesoscale eddy abstract observations of open ocean deep convection indicate that it is a highly localized phenomenon occurring over areas of tens of kilometers in diameter the cause of this localization has been ascribed to preconditioning the local weakening of the stable density stratification associated with upwardly domed isopycnal surfaces in a surface intensified cyclonic circulation however most numerical and laboratory studies of localized convection have prescribed the localization artificially by confining the surface buoyancy loss to a circular disk in contrast in the numerical simulations described here deep convection forced by horizontally uniform buoyancy loss is localized within a region of initially weaker stratification than its surroundings the preconditioned region is associated with a cold core cyclonic eddy in geostrophic and cyclostrophic balance as in previous studies of disk shaped cooling the localized convection region undergoes baroclinic instability at late times causing the br

gas slug ascent through changes in conduit diameter laboratory insights into a volcano seismic source process in low viscosity magmas seismic signals generated during the flow and degassing of low viscosity magmas include long period lp and very long period vlp events whose sources are often attributed to dynamic fluid processes within the conduit we present the results of laboratory experiments designed to investigate whether the passage of a gas slug through regions of changing conduit diameter could act as a suitable source mechanism a vertical liquid filled glass tube featuring a concentric diameter change was used to provide canonical insights into potentially deep or shallow seismic sources as gas slugs ascend the tube we observe systematic pressure changes varying with slug size liquid depth tube diameter and liquid viscosity gas slugs undergoing an abrupt flow pattern change upon entering a section of significantly increased tube diameter induce a transient pressure decrease in and above the flare and an associated pressure increase below it which stimulates acoustic and inertial resonant oscillations when the liquid flow is not dominantly controlled by viscosity net vertical forces on the apparatus are also detected the net force is a function of the magnitude of the pressure transients generated and the tube geometry which dictates where and hence when the traveling pressure pulses can couple into the tube in contrast to interpretations of related volcano seismic data where a single downward force is assumed to result from an upward acceleration of the center of mass in the conduit our experiments suggest that significant downward forces can result from the rapid deceleration of relatively small volumes of downward moving liquid

representation of convective plumes by vertical adjustment open ocean deep water formation involves the interplay of two dynamical processes plumes 1 km wide driven by upright convection and geostrophic eddies 5 km wide driven by baroclinic instability numerical twin experiments are used to address two questions about the plumes can they be represented by a simple mixing process in large scale models if so is it important that the mixing occurs over a finite time tmix or would instantaneous mixing produce the same effect on large scale properties in numerical simulations which resolve the geostrophic eddies we represent the plumes with a slow convective adjustment algorithm which is broadly equivalent to an enhanced vertical diffusivity of density in statically unstable regions the diffusivity depends on tmix the mixing timescale the fidelity of the plume parameterization is then evaluated by comparison with plume resolving simulations of open ocean deep convection integral properties of the plumes such as the temperature census of the convected water and the strength of the rim current that encircles the convecting region are all accurately reproduced by the slow adjustment scheme the importance of choosing an appropriate finite value for tmix is explored by setting tmix 12 hours in some experiments in accordance with scaling considerations and tmix 0 in others corresponding to instantaneous adjustment the conventional assumption in the case of convection into a moderately or strongly stratified ocean the behavior does not significantly depend on tmix however in neutral conditions the slow adjustment does improve the parametric representation our experiments confirm the picture of plumes homogenizing the water column over a time tmix

seismic precursors of the february 10 1990 eruption of kelut volcano java abstract kelut volcano eastern java indonesia erupted on february 10 12 1990 seismic activity and an increase of temperature and acoustic noise in the crater lake appeared three months before the eruption and were the main precursory signs of this eruption during the pre eruptive stage the number of events was eight per day on average and the cumulative seismic energy released was 5 6 10 15 erg these low values are uncommon for an andesitic volcano seismic activity mainly consisted of swarms of very shallow volcano tectonic earthquakes many of which were part of seismic multiplets in the last three weeks before the eruption seismic activity remained at a very low level this quiescent period was followed 24 hours prior to the eruption initiation by an intense swarm of low energy events which progressively became continuous spasmodic tremor most of the data obtained during the pre eruptive stage are consistent with a process of fracturation of a shallow and low strength plug blocking the eruptive conduit before the eruption several observations suggest that water is an important element in the preeruptive and eruptive phenomena at kelut volcano in particular the monochromatic events observed at kelut are characterized by one narrow peak in their spectra the estimations of q value and impedance contrast for these events suggest that the fluid which fills the source resonator could be water

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

shallow convection and buoyancy equilibration in an idealized coastal polynya abstract the recent theoretical approach of visbeck marshall and jones is used to examine shallow convection and offshore transport of dense water from an idealized coastal polynya a constant negative buoyancy flux is applied in a half elliptical region adjacent to a coastal boundary surrounded by a forcing decay region with uniform width w over which the imposed buoyancy flux decreases smoothly to zero initially the density beneath the forcing increases linearly with time a baroclinically unstable front forms at the edge of the forcing region the width of the front is imposed by the width of the forcing decay region provided this distance is larger than the baroclinic rossby radius baroclinic eddies whose velocities are inversely proportional to w develop along the front and exchange dense water from the forcing region with ambient water eventually reaching an equilibrium in which the lateral buoyancy flux by eddies balances the prescribed surface buoyancy flux the time to reach equilibrium

the role of preconditioning in the evolution of open ocean deep convection large eddy simulation of open ocean deep convection shows that the evolution of convection appears in a fundamentally different pattern depending on the precondition of the ocean and the magnitude of the surface buoyancy flux as the intensity of the cyclonic gyre in the ocean under the cooling increases the pattern of convection is transformed from distributed convection to localized convection in localized convection the typical pattern of open ocean deep convection appears such as the generation of baroclinic instability and large lateral buoyancy transfer secondary circulation restratification and the breakup of the original cyclonic gyre on the other hand in distributed convection small scale convective plumes appear uniformly over the whole surface similarly to the convective boundary layer without generating the typical features of open ocean deep convection hence an enormous difference in the generated eddy kinetic energies exists between the two cases it is also found that a stronger cooling at the sea surface suppresses the transition from distributed convection to localized convection dimensional analysis provides the parameters to characterize the pattern of convection and the critical condition for the transition is estimated by analyzing the numerical results

the formation of labrador sea water part ii mesoscale and smaller scale processes abstract in a previous paper clarke and gascard argued that the formation of labrador sea water was taking place in a cyclonic gyre set up each winter in the western labrador sea within the gyre and at its boundaries a number of different scales of organization are believed to be important in the formation processes the longest of these scales is the mesoscale 50 km which appears to be related to topographic rossby waves generated in the labrador current and propagating offshore the next smaller scale is an eddy scale 20 km believed to arise because the mesoscale is baroclinically unstable as shown by applying a two layer model of tang this instability is believed to promote mixing by generating frontal structures and vertical motions along them thus bringing subsurface t s maxima nearer the surface then within the mesoscale and eddy scale structures intense vertical convective cells take place at scales which are probably of the order of 1 km in three dimensions these events are short liv

stability of a cold core eddy in the presence of convection hydrostatic versus nonhydrostatic modeling geostrophic eddies in a stratified liquid are susceptible to baroclinic instabilities in this paper the authors consider these instabilities when such an eddy is simultaneously cooled homogeneously from above as a linear stability analysis shows the developing convection modifies the background stratification the stability boundaries and the patterns of the dominant modes the coupling between the effects of convection and the largescale flow development of the eddy is studied through high resolution numerical simulations using both nonhydrostatic and hydrostatic models in the latter models several forms of convective adjustment are used to model convection both types of models confirm the development of the dominant modes and indicate that their nonlinear interaction leads to localized intense convection by comparing nonhydrostatic and hydrostatic simulations of the flow development carefully it is shown that convective adjustment may lead to erroneous smallscale variability a simple alternative formulation of convective adjustment is able to give a substantial improvement

source mechanism of long period events at kusatsu shirane volcano japan inferred from waveform inversion of the effective excitation functions abstract we investigate the source mechanism of long period lp events observed at kusatsu shirane volcano japan based on waveform inversions of their effective excitation functions the effective excitation function which represents the apparent excitation observed at individual receivers is estimated by applying an autoregressive filter to the lp waveform assuming a point source we apply this method to seven lp events the waveforms of which are characterized by simple decaying and nearly monochromatic oscillations with frequency in the range 1 3 hz the results of the waveform inversions show dominant volumetric change components accompanied by single force components common to all the events analyzed and suggesting a repeated activation of a sub horizontal crack located 300 m beneath the summit crater lakes based on these results we propose a model of the source process of lp seismicity in which a gradual buildup of steam pressure in a hydrothermal crack in response to magmatic heat causes repeated discharges of steam from the crack the rapid discharge of fluid causes the collapse of the fluid filled crack and excites acoustic oscillations of the crack which produce the characteristic waveforms observed in the lp events the presence of a single force synchronous with the collapse of the crack is interpreted as the release of gravitational energy that occurs as the slug of steam ejected from the crack ascends toward the surface and is replaced by cooler water flowing downward in a fluid filled conduit linking the crack and the base of the crater lake

analyses of unusual long period earthquakes with extended coda recorded at katmai national park alaska usa 1 a swarm of six long period lp events with slowly decaying coda wave amplitudes and durations up to 120 s was recorded by seismic stations located in the proximity of mt griggs a fumarolically active volcano in the katmai national park alaska during december 8 21 2004 spectral analyses reveal the quasi monochromatic character of the waveforms dominated by a 2 5 hz mode frequently accompanied by a weaker high frequency onset 6 0 9 0 hz particle motion azimuths and inclination angles show a dominant wnw ese direction of polarization for all the signals and suggest that seismic energy is radiated by a stable source at shallow depth damping coefficients between 0 0014 and 0 0063 are estimated by fitting an exponential decay model to the signal s coda corresponding quality factors range from 78 to 351 the source of the waveforms is modelled as a resonant cavity filled with a fluid gas mixture

a vortical hot tower route to tropical cyclogenesis abstract a nonhydrostatic cloud model is used to examine the thermomechanics of tropical cyclogenesis under realistic meteorological conditions observations motivate the focus on the problem of how a midtropospheric cyclonic vortex a frequent by product of mesoscale convective systems during summertime conditions over tropical oceans may be transformed into a surface concentrated warm core tropical depression as a first step the vortex transformation is studied in the absence of vertical wind shear or zonal flow within the cyclonic vorticity rich environment of the mesoscale convective vortex mcv embryo the simulations demonstrate that small scale cumulonimbus towers possessing intense cyclonic vorticity in their cores vortical hot towers vhts emerge as the preferred coherent structures the vhts acquire their vertical vorticity through a combination of tilting of mcv horizontal vorticity and stretching of mcv and vht generated vertical vorticity horizontally localized and exhibiting conve

waveform inversion of oscillatory signatures in long period events beneath volcanoes 1 the source mechanism of long period lp events is examined using synthetic waveforms generated by the acoustic resonance of a fluid filled crack we perform a series of numerical tests in which the oscillatory signatures of synthetic lp waveforms are used to determine the source time functions of the six moment tensor components from waveform inversions assuming a point source the results indicate that the moment tensor representation is valid for the odd modes of crack resonance with wavelengths 2l n 2w n n 3 5 7 where l and w are the crack length and width respectively for the even modes with wavelengths 2l n 2w n n 2 4 6 a generalized source representation using higher order tensors is required although the efficiency of seismic waves radiated by the even modes is expected to be small we apply the moment tensor inversion to the oscillatory signatures of an lp event observed at kusatsu shirane volcano central japan our results point to the resonance of a subhorizontal crack located a few hundred meters beneath the summit crater lakes the present approach may be useful to quantify the source location geometry and force system of lp events and opens the way for moment tensor inversions of tremor

unsteady turbulent convection into a rotating linearly stratified fluid modeling deep ocean convection abstract a laboratory experiment has been constructed to model the deep convective processes in a stratified ocean driven by the energetic cooling at the ocean surface for convenience in the laboratory the authors have examined the analogous but inverted problem of the growth of a convective mixed layer generated by a localized source of bottom heating in a rotating thermally stratified fluid the heating is provided by a heat exchanger mounted beneath a central circular plate of a diameter smaller than that of the main cylindrical tank following the initiation of the heating a cylinder of well mixed fluid forms above the heated plate and as it grows upward it steadily erodes the overlying ambient stratification measurements of the root mean square velocities in the mixed layer indicate that rotation affects but does not control the turbulence and the nonrotating turbulent velocity and length scales are thus appropriate to describe the small scale turbulence rotation initially confines the heated f

estimation of groundwater recharge in weathered basement aquifers southern zimbabwe a geochemical approach abstract geochemical techniques have been used to estimate groundwater recharge and its spatial variability in basement terrain in a semi arid area of southern zimbabwe recharge rates estimated by chloride mass balance have been determined in the romwe catchment a small 4 6 km2 headwater catchment underlain by banded gneisses with good hydrological and geological control the results support the findings from piezometric monitoring that there are significant differences in hydrogeological properties of weathered basement derived from different primary lithologies annual recharge estimates for shallow weathered aquifers derived from melanocratic bedrock dominated by pyroxene gneiss 22 mm and leucocratic felsic bedrock 6 7 mm are 3 7 and 1 1 respectively of the long term mean annual rainfall the significant uncertainties associated with the chloride mass balance recharge estimates are discussed groundwater derived from each lithology generally has a distinctive geochemistry na cl k na mg ca na cl b ba the information from the romwe catchment control area was then scaled up using information from remote sensing images which defined areas of dark and light soils above the banded gneiss to confirm the higher recharge rates in the melanocratic lithology in the unexplored greater romwe 225 km2 area it is concluded that properly calibrated remote sensing images could be further regionalized to site groundwater sources in basement terrain providing a relatively inexpensive development tool

a generalized 3 d geological and hydrogeological conceptual model of granite aquifers controlled by single or multiphase weathering summary the weathered layers of crystalline rocks form aquifers that are of prime interest for water supply in hard rock areas these weathering profiles generally develop under both stable geodynamic conditions weathering rate erosion rate and a hydrolysing climate they are composed of thick stratiform layers that follow the paleo landscape paleotopography and thus present a gently dipping sequence at a regional scale the structure and the hydrodynamic properties of the weathering profile of a granitic area 53 km 2 maheshwaram catchment state of andhra pradesh india were characterized in detail and mapped from observations on outcrops 80 vertical electric soundings and lithologs from 45 borewells in which flowmeter measurements and injection tests were also performed to characterize the hydraulic conductivities of the conductive fissure zones the structure of the weathering profile results from a multiphase process an ancient weathering profile was partly eroded down to its fissured layer it was later re weathered more or less parallel to the current topographic surface this peculiar structure is linked to the geodynamic history of the indian peninsula that underwent alternate weathering and erosion dominated phases the profile is thus composed from top to bottom up to a total depth of 35 m of sandy regolith saprolite 1 3 m 10 15 m of laminated saprolite containing unusual preserved fissures and a 15 20 m thick fissured layer by comparing various case studies in similar terrain a generalized 3 d geological and hydrogeological conceptual model of granite type aquifers granites gneisses etc is proposed in granite type rocks single weathering or multiphase weathering and erosion processes induce similar geological structures a few main geological differences arise from the comparison of single phase and multiphase weathering profiles i in the later the respective thicknesses of the various layers can be deeply modified ii the ancient fissured layer can be re weathered and the resulting new laminated layer may contain quite well preserved and conductive ancient fissures and iii the upper part of the fissured layer is more densely fissured as a result of the same weathering processes the fissures from the fissured layer of single phase or multi phase profiles exhibit very similar hydraulic conductivities and show both a higher density of conductive fissures at the top of the layer the preserved ancient fissures within the laminated layer of a multiphase profile are partly obliterated by the recent weathering their hydraulic conductivity is thus significantly reduced nevertheless they do significantly contribute to borewell yield these weathering induced fissures provide most of the aquifer permeability the use of such a conceptual model for the precise geological mapping of the weathering structure appears to be a prerequisite for groundwater development and management in hard rock areas as it answers to several key issues

a numerical study of dense water formation and transport on a shallow sloping continental shelf the circulation and transport of dense water generated by an idealized coastal polynya is studied using a three dimensional primitive equation model starting with a homogeneous quiescent ocean a constant negative buoyancy flux is imposed at the surface over a half elliptical region adjacent to the coastal boundary on a gently sloping continental shelf the flow response can be divided into the following three phases geostrophic adjustment instability and offshore eddy transport during geostrophic adjustment the fluid within the forcing region becomes denser and the flow at the edge of the forcing region accelerates in response to the strong density gradient there eventually the flow at the leading edge of the forcing region relative to kelvin wave propagation becomes unstable and a train of counterrotating eddies develops these eddies then form a complex three dimensional flow field and rapidly transport dense water offshore across isobaths the density within the forcing region reaches a maximum which remains fairly constant after the eddies begin to transport the dense fluid offshore the results are qualitatively insensitive to weakening of the negative buoyancy forcing and to changing the bottom slope eddy scales and velocities are consistent with observations in the arctic the results suggest that instability processes and eddy fluxes are important in transporting dense water off continental shelves and into marginal seas

a model of vulcanian eruptions at galeras volcano colombia abstract a series of six vulcanian eruptions at galeras volcano colombia occurred during 1992 1993 these eruptions followed the emplacement of a lava dome in late 1991 which was accompanied by cooling crystallizing and partial solidification of the column of magma plugging the conduit five of the eruptions were preceded by episodes of monochromatic long period seismicity suggesting cyclic pressurization of the magma the quiescent intervals between eruptions generally decreased with each successive eruption we propose a model of gas release from magma into confined stationary pore spaces by 5 10 crystallization of anhydrous minerals causing repeated overpressurization of the magmatic system using initial lithostatic pressures of 20 100 mpa 800 3900 m depth gas buildup by crystallization indicates overpressures of 3 17 mpa which is the approximate fracture criterion for a magma chamber our galeras data suggest two models of pressurization 1 gas exsolution may have occurred at similar depths but with progressively smaller overpressures being developed before each successive eruption according to this model the comparatively long quiescent periods between the first eruptions resulted in relatively large amounts of gas exsolution from the magma and accumulation in pore spaces whereas the shorter quiescent periods for the later eruptions exsolved less gas the decrease in overpressure with time may have resulted from progressive fracturing of partly solidified magma during the eruptions which progressively decreased the tensile strength and increased the permeability of the magma 2 since overpressures are smaller at shallower depths for a given amount of crystallization the first eruptions of 1992 1993 were caused by gas exsolution at shallow depths while the last eruptions having shorter quiescent intervals resulted from a greater amount of gas exsolution at greater depths in this case the rate of gas exsolution varied as a function of depth the two scenarios for pressurization may be combined into a more general model involving edifice weakening and the development of a shallow hydrothermal system from continued degassing of the magma plug as weakening of the edifice formed an extensive system of fractures the shallow hydrothermal system deepened due to meteoric water input and magmatic degassing with time a greater number of fractures were filled and partly sealed by hydrothermal minerals at progressively deeper levels as a result the source of the overpressure also deepened with time

estimation of hard rock aquifers hydraulic conductivity from geoelectrical measurements a theoretical development with field application summary based on the analogy between darcy s law for groundwater flow and ohm s law for electric current flow a methodology has been developed to estimate the hydraulic conductivity and transmissivity of hard rock granite aquifer from geoelectrical parameters the common parameter aquifer thickness t has been used to combine the two relations and form an analytical equation mathematical relation shows a negative correlation between hydraulic conductivity and aquifer resistivity and a positive correlation between transmissivity and longitudinal conductance the methodology has been calibrated and validated in hard rock granite aquifers in india the good agreement between aquifer hydraulic conductivity k and transmissivity t obtained from the resistivity sounding parameters and those obtained from pumping test analysis proves the potentiality of the methodology it has been applied to estimate the k and t from the surface electrical resistivity parameters and results were utilized to prepare the k and t maps of maheshwaram watershed in hard rock terrain in india

time space variation of volcano seismic events at la fossa vulcano aeolian islands italy new insights into seismic sources in a hydrothermal system we investigated the relationship between volcano seismic events recorded at la fossa crater of vulcano aeolian islands italy during 2004 2006 and the dynamics of the hydrothermal system during the period of study three episodes of increasing numbers of volcano seismic events took place at the same time as geothermal and geochemical anomalies were observed these geothermal and geochemical anomalies have been interpreted as resulting from an increasing deep magmatic component of the hydrothermal fluids three classes of seismic events long period high frequency and monochromatic events characterised by different spectral content and various similarity of the waveforms have been recognised these events clustered mainly below la fossa crater area at depths of 0 5 1 1 km b s l were space distributed according to the classes based on their features we can infer that such events at vulcano are related to two different source mechanisms 1 fracturing processes of rocks and 2 resonance of cracks or conduits filled with hydrothermal fluid in the light of these source mechanisms the increase in the number of events at the same time as geochemical and geothermal anomalies were observed was interpreted as the result of an increasing magmatic component of the hydrothermal fluids implying an increase of their flux indeed such variation caused an increase of both the pore pressure within the rocks of the volcanic system and the amount of ascending fluids increased pore pressures gave rise to fracturing processes while the increased fluid flux favoured resonance and vibration processes in cracks and conduits finally a gradual temporal variation of the waveform of the hybrid events one of the subclasses of long period events was observed likely caused by heating and drying of the hydrothermal system

unusual low frequency volcanic seismic events with slowly decaying coda waves observed at galeras and other volcanoes abstract events with slowly decaying coda waves are observed in active andesitic volcanoes associated mainly with vulcanian type eruptions or large gas emissions however these signals are also recorded in some quiescent volcanoes these unusual signals are considered to be related to magmatic activity and generally occur beneath the active crater the signals have been observed to be a short term precursor galeras volcano colombia 1992 1993 asama yama volcano japan 1983 after eruption tokachi dake volcano japan 1989 during seismic swarms meakan dake volcano japan 1982 and during quiescence puracevolcano colombia 1994 1995 tarumai volcano japan 1970 1971 1975 spectral analysis reveals common characteristics for this type of signal the spectrum is characterized by one or several sharp frequency peaks fundamental frequencies are not affected by epicentral distance azimuth or travel time indicating a source effect the damping coefficient for coda waves ranges from 0 002 to 0 02 and is related to large values of the quality factor q c ranging from 250 to 25 respectively these parameters may be the result of large amounts of gas bubbles in the magma body

tornillos modeled as self oscillations of fluid filling a cavity application to the 1992 1993 activity at galeras volcano colombia abstract tornillos are quasi monochromatic seismic signals with a slowly decaying coda that are observed near active volcanoes and geothermal areas worldwide in this work a lumped parameter model describing the tornillo source process as the self oscillations of fluid filling a cavity is investigated a nonlinear ordinary differential equation is derived that governs the behavior of the model taking into account viscous and nonlinear damping as well as the reaction force of the fluid inside the cavity this equation is numerically integrated both for different cavity sizes and different fluids of volcanological interest such as gas h2o co2 h2o so2 and gas particle mixtures ash so2 water droplets h2o this cavity model predicts that when the filling fluid is a mixture of ash and so2 the signal duration will increase until the mixture becomes enriched in ash and then the duration exhibits a decrease additionally the damping coefficients 1 2q of the synthetic signals are estimated in the range between 0 002 and 0 014 both results agree well with the temporal variation of tornillos duration and the estimated q quality factors damping coefficients observed at galeras volcano in the context of the cavity model tornillo frequency variations from 4 hz to 1 hz observed prior to eruptions can be interpreted as the result of fluid composition changes as more ash particles are added this is in agreement with the observation that gas accumulation at galeras was a steady rather than an episodic process and that tornillos were most likely triggered after a fluid pressure threshold had been exceeded

correlations between so2 flux and long period seismicity at galeras volcano the 14 january 1993 eruption of galeras volcano in colombia which killed six scientists and three tourists1 was followed by a larger eruption on 23 march 19932 both eruptions were preceded by episodes of long period seismicity the source of long period seismic events has been modelled extensively3 8 as the resonance within a fluid filled crack induced by pressure transients in the fluid5 7 here we use the so2 flux from galeras volcano measured remotely to infer the degassing history during the episode of long period events preceding the 23 march eruption so2 flux and long period seismicity have been monitored separately elsewhere to forecast volcanic activity9 13 our results show how the combination and correlation of the two methods can be used to interpret the movement of gases from the magma body to the surface and to monitor the pressure buildup leading to an eruption at explosive volcanoes

tornillo type seismic signals at galeras volcano colombia 1992 1993 abstract unusual low frequency seismic events called tornillos screws at the observatorio vulcanologico y sismologico de pasto ovsp have been observed at galeras volcano during 1992 1993 of six eruptions that occurred between july 1992 and june 1993 five were preceded by episodes of tornillo signals these signals are characterized by 1 a waveform having a homogeneous distribution of frequencies 2 a long coda lasting up to several minutes 3 a small amplitude compared to the duration and 4 a slow decay of the coda the tornillo signals have been grouped into thirteen principal forms the 1992 1993 tornillos showed four main periods of occurrence each of which was followed by an eruption episode i occurred from 11 to 16 july 1992 and consisted of nine events episode ii lasted from 23 december 1992 to 14 january 1993 with twenty events episode iii was observed from 13 february to 23 march 1993 with 74 events episode iv lasted from 10 april to 7 june 1993 with 109 events the seismic activity at galeras in april may 1993 was characterized mainly by the occurrence of tornillo signals the behavior of these signals was similar to that before previous episodes ending with eruptions these observations permitted us to forecast the 7 june 1993 eruption several days to weeks beforehand the occurrence of tornillos is the most important criterion for determining the probability of an eruption at galeras in the current period of reactivation the maximum daily number of tornillos and the longest duration of individual events occur near the end of each episode within an individual episode the durations of single events increase progressively near to the point of eruption a positive correlation is observed between the total number of pre eruptive tornillo signals during an episode and the volume of material ejected by the eruption these observations may suggest that the presence of tornillos is an indication of variations in the physical conditions between the fluid and the surrounding solid material within the volcano and they constitute an important tool for forecasting future eruptions at galeras

response of a hydrothermal system to magmatic heat inferred from temporal variations in the complex frequencies of long period events at kusatsu shirane volcano japan we investigate temporal variations in the complex frequencies frequency and quality factor q of long period lp events that occurred at kusatsu shirane volcano central japan we analyze lp waveforms observed at this volcano in the period between 1988 and 1995 which covers a seismically active period between 1989 and 1993 systematic temporal variations in the complex frequencies are observed in october november 1989 july october 1991 and september 1992 january 1993 we use acoustic properties of a crack filled with hydrothermal fluids to interpret the observed temporal variations in the complex frequencies the temporal variations in october november 1989 can be divided into two periods which are explained by a gradual decrease and increase of a gas volume fraction in a water steam mixture in a crack respectively the temporal variations in july october 1991 can be also divided into two periods these variations in the first and second periods are similar to those observed in november 1989 and in september november 1992 respectively and are interpreted as drying of a water steam mixture and misty gas in a crack respectively the repeated nature of the temporal variations observed in similar seasons between july and november suggests the existence of seasonality in the occurrence of lp events this may be caused by a seasonally variable meteoritic water supply to a hydrothermal system which may have been heated by the flux of volcanic gases from magma beneath this volcano

parameterization of multichromatic tornillo signals observed at galeras volcano colombia in the past decade several of the ash eruptions at galeras volcano colombia have been preceded by tornillos these unusual tremor wavelets have quasi sinusoidal waveforms with screw like envelope profiles and can last up to several minutes a swarm of tornillos occurred at galeras volcano between 8 december 1999 and 12 february 2000 these tornillos appear to be more complex than those previously recorded with the broadband instruments or with the short period network of the observatorio vulcanologico y sismologico in pasto they are multichromatic with a varying number of narrow spectral peaks between 1 and 20 hz we describe a procedure for parameterizing the tornillo signals in the time and frequency domains to determine signal parameters in addition to wavelets like the tornillos the procedure can be applied to random signals such as volcanic tremor we derive distribution and correlation functions for the signal parameters determined from the swarm these provide along with the signal signature constraints for modelling variations of the source process from these observations we derive qualitative conclusions about the characteristics of differential equations which describe the underlying processes and excitation mechanisms as forced or self excited oscillators

observations of vertical currents and convection in the central greenland sea during the winter of 1988 1989 during the winter of 1988 1989 five acoustic doppler current profilers adcps were moored in the central greenland sea to measure vertical currents that might occur in conjunction with deep mixing and convection two adcps were looking up from about 300 m and combined with thermistor strings in the depth range 60 260 m two were looking downward from 200 m and one was looking upward from 1400 m first maxima of vertical velocity variance occurred at two events of strong cold winds in october and november when cooling and turbulence in the shallow mixed layer generated internal waves in the thermocline beginning in late november the marginal ice zone expanded eastward over the central greenland sea reaching its maximum extent in late december in mid january a bay of ice free water opened over the central greenland sea leaving a wedge of ice the is odden curled around it along the axis of the jan mayen current and then northeastward and existing well into april 1989 below the ice a mixed layer at freezing temperatures developed that increased in thickness from 60 to 120 m during the period of ice cover corresponding to an average heat loss of about 40 w m 2 through brine rejection mixed layer salinity increased steadily reducing stability to underlying weakly stratified layers roach et al 1993 during the ice cover period vertical currents were at a minimum after the opening of the ice free bay successive mixed layer deepening to 350 m occurred in conjunction with cooling events around february 1 and 15 accompanied by strong small scale vertical velocity variations upward mixing of more saline waters of atlantic origin during this phase reduced the stability further generating a pool of homogeneous water of 50 km horizontal extent in the central greenland sea preconditioned for subsequent convection to greater depths individual convection events were observed during march 6 16 associated with downward velocities at the 1400 m level of about 3 cm s l one event was identified as a plume of about 300 m horizontal scale in agreement with recently advanced scaling arguments and model results and with earlier similar observations in the gulf of lions western mediterranean the deep convection occurred in the center of the ice free bay hence brine rejection did not seem necessary for its generation plume temperatures at 1400 m were generally higher than that of the homogeneous surface pool suggesting entrainment of surrounding warmer waters on the way down mean vertical velocity over a period of convection events was indistinguishable from zero suggesting that plumes served as a mixing agent rather than causing mean downward transport of water masses however different from the surface pool that was governed by mixed layer physics the water between 400 and 1400 m was not horizontally homogenized in a large patch by the sporadic plumes overall and compared to results from the gulf of lions convection activity in the central greenland sea was weak and limited to intermediate depths in winter 1988 1989

influence of convective adjustment on the stability of the thermohaline circulation abstract three different convective adjustment schemes are employed in the gfdl gcm to investigate if the spontaneous collapse of the thermohaline circulation under mixed boundary conditions as observed by f bryan depends on the parameterization of convective overturning it is found that both a procedure guaranteeing complete static stability and cox s implicit vertical diffusion scheme avoid the spontaneous collapse both schemes are also insensitive to the choice of time step whereas the standard gfdl convection algorithm in conjunction with mixed boundary conditions produces results that differ qualitatively from each other when different time steps are used

convection with rotation in a neutral ocean a study of open ocean deep convection abstract the intensity and scale of the geostrophically adjusted end state of the convective overturning of a homogeneous rotating ocean of depth h at a latitude where the coriolis parameter is f induced by surface buoyancy loss of magnitude b0 are studied by numerical experiment the experiments are related to observations and laboratory studies of open ocean deep convection a numerical model based on the nonhydrostatic boussinesq equations is used the grid spacing of the model is small enough that gross aspects of convective plumes themselves can be resolved yet the domain of integration is sufficiently large to permit study of the influence of plumes on the large scale and geostrophic adjustment of the convected water numerical simulations suggest that cooling at the sea surface is offset by buoyancy drawn from depth through the agency of convective plumes these plumes efficiently mix the water column to generate a dense chimney of fluid which subsequently breaks up through the mechanism of bar

temporal evolution of a hydrothermal system in kusatsu shirane volcano japan inferred from the complex frequencies of long period events 1 we present a detailed description of temporal variations in the complex frequencies of long period lp events observed at kusatsu shirane volcano using the sompi method we analyze 35 lp events that occurred during the period from august 1992 through january 1993 the observed temporal variations in the complex frequencies can be divided into three periods during the first period the dominant frequency rapidly decreases from 5 to 1 hz and q of the dominant spectral peak remains roughly constant with an average value near 100 during the second period the dominant frequency gradually increases up to 3 hz and q gradually decreases from 160 to 30 during the third period the dominant frequency increases more rapidly from 3 to 5 hz and q shows an abrupt increase at the beginning of this period and then remains roughly constant with an average value near 100 such temporal variations can be consistently explained by the dynamic response of a hydrothermal crack to a magmatic heat pulse during the first period crack growth occurs in response to the overall pressure increase in the hydrothermal system caused by the heat pulse once crack formation is complete heat gradually changes the fluid in the crack from a wet misty gas to a dry gas during the second period as heating of the hydrothermal system gradually subsides the overall pressure in this system starts to decrease causing the collapse of the crack during the third period

tornillos beneath tongariro volcano new zealand abstract unusual low frequency seismic events have recently begun to occur beneath tongariro volcano new zealand because the seismic waveforms appear similar to waveforms called tornillos which have been recorded at galeras volcano in colombia and have been found to precede small to medium sized ash eruptions there by several weeks to months we are monitoring them closely at tongariro these events are characterised by small amplitude monochromatic or multichromatic waveforms that decay slowly with time features that suggest a source involving magmatic or hydrothermal fluids within a zone surrounded by hard rock the frequency content of monochromatic events is typically between 1 and 3 hz and is often found to be peaked at f 1 2 hz event durations range from about 10 to 80 s the appearance of the shorter duration events is identical to so called lp or long period events the lps and tornillos appear to be co located so that the only difference between lps and tornillos is the extended duration of the latter these events have been identified in the seismograms of the permanent monitoring station kavz since at least january 2001 occurring at a rate of about one per month during june august 2001 the rate of occurrence gradually increased peaking in september 2001 at several events per day we deployed two broadband and several short period seismometers at close distances to the expected source location the resulting seismograms show a complex mixture of seismic waves originating from shallow depth beneath the area of the te maari craters located approximately 0 5 km from the nearest station the close range broadband records reveal some interesting details about the tornillo source the tornillo waveform is found to consist of two parts a low frequency f 1 2 hz part and a higher frequency f 3 hz part the high frequency energy travels at body wave speeds c 3 0 km s while the low frequency energy travels at the speed of shear or surface waves in the shallow crust c 1 km s interestingly for some events the low frequency waves arrive before the high frequency body waves at the closest station this and comparison with several lp events suggests a spatial separation of the high and low frequency sources with the low frequency energy originating at shallow depth and acting as a trigger for the release of tectonic stress at greater depth in the form of high frequency body waves

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

groundwater flows in weathered crystalline rocks impact of piezometric variations and depth dependent fracture connectivity summary groundwater in shallow weathered and fractured crystalline rock aquifers is often the only perennial water resource especially in semi arid region such as southern india understanding groundwater flows in such a context is of prime importance for sustainable aquifer management here we describe a detailed study of fracture properties and relate the hydraulic connectivity of fractures to groundwater flows at local and watershed scales investigations were carried out at a dedicated experimental hydrogeological park in andhra pradesh southern india where a large network of observation boreholes has been set up twenty height boreholes have been drilled in a small area of about 18 000 m 2 in which borehole loggings and hydraulic tests were carried out to locate the main flowing fractured zones and investigate fractures connectivity several hydraulic tests nineteen slug tests and three pumping tests performed under two water level conditions revealed contrasting behavior under high water level conditions the interface including the bottom of the saprolite and the first flowing fractured zone in the upper part of the granite controls groundwater flows at the watershed scale under low water level conditions the aquifer is characterized by lateral compartmentalization due to a decrease in the number of flowing fractures with depth depending on the water level conditions the aquifer shifts from a watershed flow system to independent local flow systems a conceptual groundwater flow model which includes depth dependent fracture connectivity is proposed to illustrate this contrasting hydrological behavior implications for watershed hydrology groundwater chemistry and aquifer vulnerability are also discussed

a methodology for regionalizing 3 d effective porosity at watershed scale in crystalline aquifers 13 an innovative approach for regionalizing the 3 d effective porosity field is presented and 14 applied to two large overexploited and deeply weathered crystalline aquifers located in 15 southern india the method derives from earlier work on regionalizing a 2 d effective 16 porosity field in that part of an aquifer where the water table fluctuates which is now 17 extended over the entire aquifer using a 3 d approach a method based on geological and 18 geophysical surveys has also been developed for mapping the weathering profile layers 19 saprolite and fractured layers the method for regionalizing 3 d effective porosity 20 combines water table fluctuation and groundwater budget techniques at various cell sizes 21 with the use of satellite based data for groundwater abstraction the structure of the 22 weathering profile and geostatistical techniques the approach is presented in detail for the 23 kudaliar watershed 983 km 2 and tested on the 730 km 2 anantapur watershed at watershed 24 scale the effective porosity of the aquifer ranges from 0 5 to 2 in kudaliar and between 25 0 3 and 1 in anantapur which agrees with earlier works results show that i depending 26 on the geology and on the structure of the weathering profile the vertical distribution of 27 effective porosity can be very different and that the fractured layers in crystalline aquifers are 28 not necessarily characterized by a rapid decrease in effective porosity and ii that the lateral 29 variations in effective porosity can be larger than the vertical ones these variations suggest 30 that within a same weathering profile the density of open fractures and or degree of 31 weathering in the fractured zone may significantly varies from a place to another 32 the proposed method provides information on the spatial distribution of effective porosity 33 which is of prime interest in terms of flux and contaminant transport in crystalline aquifers 34 implications for mapping groundwater storage and scarcity are also discussed which should 35 help in improving groundwater resource management strategies 36 37

resonances of a volcanic conduit triggered by repetitive injections of an ash laden gas 1 a swarm of long period lp seismic events was recorded in december 2001 before heightened eruptive activity of tungurahua volcano an andesitic stratovolcano in ecuador the lp events showed simple decaying harmonic oscillations with frequencies ranging from 2 to 3 5 hz and quality factors q significantly larger than 100 our spectral analysis of the lp events identified systematic temporal variations in both frequency and q of the lp events assuming a crack resonator at the source of lp events the temporal variations can be consistently explained by increasing the ash content within the resonator which may be caused by repetitive injections of an ash laden gas into a pre existing crack in the conduit as a preparatory process for eruptions

dynamics of isolated convective regions in the ocean abstract an initially resting ocean of stratification n is considered subject to buoyancy loss at its surface of magnitude b0 over a circular region of radius r at a latitude where the coriolis parameter is f initially the buoyancy loss gives rise to upright convection as an ensemble of plumes penetrates the stratified ocean creating a vertically mixed layer however as deepening proceeds horizontal density gradients at the edge of the forcing region support a geostrophic rim current which develops growing meanders through baroclinic instability eventually finite amplitude baroclinic eddies sweep stratified water into the convective region at the surface and transport convected water outward and away below setting up a steady state in which lateral buoyancy flux offsets buoyancy loss at the surface in this final state quasi horizontal baroclinic eddy transfer dominates upright plume convection by using parcel theory to consider the energy transformations taking place it is shown that the de

review of seismological studies at mount etna this paper reports the present state of seismological research at mt etna a schematic classification of the earthquakes that occur on the volcano is proposed based on both seismogram and spectrum features we have made both focal solutions and estimates of earthquake source parameters stress drop values between 2 and 20 bars and small source dimensions the crust of etna thus appears as an extremely heterogeneous medium that does not permit great stress accumulation the coexistence of an extensional regime with an older and deeper compressive one seems confirmed at depths greater than about 7 km eruptive and seismic phenomena occur mainly along the principal structural trends of the volcano but often the directions of the eruptive fractures and the earthquake concentration during the same eruption do not coincide tectonics seem to play an important role in controlling seismo volcanic behaviour

causes and consequences of pressurisation in lava dome eruptions abstract high total and fluid pressures develop in the interior of high viscosity lava domes and in the uppermost parts of the feeding conduit system as a consequence of degassing two effects are recognised and are modelled quantitatively first large increases in magma viscosity result from degassing during magma ascent strong vertical gradients in viscosity result and large excess pressures and pressure gradients develop at the top of the conduit and in the dome calculations of conduit flow show that almost all the excess pressure drop from the chamber in an andesitic dome eruption occurs during the last several hundred metres of ascent second microlites grow in the melt phase as a consequence of undercooling caused by gas loss rapid microlite growth can cause large excess fluid pressures to develop at shallow levels theoretically closed system microlite crystallization can increase local pressure by a few tens of mpa although build up of pressure will be countered by gas loss through permeable flow and expansion by viscous flow microlite crystallization is most effective in causing excess gas pressures at depths of a few hundred metres in the uppermost parts of the conduit and dome interior some of the major phenomena of lava dome eruptions can be attributed to these pressurisation effects including spurts of growth cycles of dome growth and subsidence sudden onset of violent explosive activity and disintegration of lava during formation of pyroclastic flows the characteristic shallow level long period and hybrid seismicity characteristic of dome eruptions is attributed to the excess fluid pressures which are maintained close to the fracture strength of the dome and wallrock resulting in fluid movement during formation of tensile and shear fractures within the dome and upper conduit

use of hydraulic tests at different scales to characterize fracture network properties in the weathered fractured layer of a hard rock aquifer the hydrodynamic properties of the weathered fractured layer of a hard rock pilot watershed in a granitic terrain are characterized using hydraulic tests at different scales the interpretation of numerous slug tests leads to characterize the statistical distribution of local permeabilities in the wells the application of flowmeter profiles during injection tests determines the vertical distribution of conductive fracture zones and their permeabilities it appears that the extension of the most conductive part of the weathered fractured layer is limited down to 35 meters depth the partition of drainage porosity between blocks 90 and fractures 10 is determined thanks to the interpretation of pumping tests using a double porosity model the application of anisotropic and single fracture analytical solutions on pumping test data allows to determine respectively the degree of anisotropy of permeability and the radius 4 to 16 meters of the horizontal conductive fractures crossed by the wells two different scales of fractures networks are identified the primary fracture network pfn which affects the matrix on a decimeter scale by contributing to an increase in the permeability and storage capacity of the blocks and the secondary fracture network sfn which affects the blocks at the borehole scale sfn is composed of two sets of fractures the main set of horizontal fractures is responsible for the sub horizontal permeability of the weathered fractured layer a second set of less permeable sub vertical fractures insures the connectivity of the aquifer at the borehole scale the good connectivity of fractures networks is shown by fractional dimension flow solutions the absence of scale effect in the study area suggests that the hydraulic conductivity at the borehole scale is laterally homogeneous finally the analysis and synthesis of the hydrodynamic properties allow to propose a comprehensive hydrodynamic model of the fractured weathered layer many geological and hydrogeological indicators suggest that a continuous and laterally homogeneous weathering process is responsible for the origin of the fractures and permeability encountered in the aquifer these results confirm the major role played by weathering in the origin of fractures and on resulting hydrodynamic parameters in the shallow part of hard rock aquifers

variability and multiple equilibria of the thermohaline circulation associated with deep water formation abstract in this study the variability of the thermohaline circulation on decadal and centennial time scales that is related to the process of deep water formation is investigated this is done within the context of a simple geostrophic three layer ocean model with a rectangular closed basin when slowly varying the atmospheric forcing the model response shows sudden transitions characterized by local changes in convective activity many different equilibria were found within the thermally driven regime in stommel s sense i e downwelling occurs near the poles next the deep water formation process was analyzed with a one dimensional box model in this box model four different regimes can be identified convective nonconvective periodic and a regime where both convection and no convection are possible with the same mixed boundary conditions these regimes were identified in the circulation of the ocean model a region was traced where convection is possible according to the authors analysis but

locating microearthquakes induced by hydraulic fracturing in crystalline rock microearthquakes induced by hydraulic fracturing in crystalline rock at a depth of 3 5 km were located with a precision of better than 30 m to obtain information about the geometry and dimensions of the fracture system produced the induced microseismicity was monitored by a network of five vorehole seismic stations a total of about 800 induced events were reliably located from arrival times event locations show a tabular distribution that strikes 350 sup 0 and dips 65 sup 0 east subparallel to the injection well the injection was intended to produce a fracture system that would hydraulically connect two subparallel wells a lack of fluid communication between them is consistent with a lack of induced microearthquakes near the target wellbore the 150 m thickness of the zone of seismicity far exceeds the relative locational uncertainties the injected fluid appears to have stimulated a zone of rock rather than simply a single fracture the distribution of microseismic events presumably envelopes the zone of fluid paths created by the fracture experiment