application of stochastic simulation to co2 flux from soil mapping and quantification of gas release 1 conditional sequential gaussian simulations sgs have been applied for the first time to the study of soil diffuse degassing from different volcanic and nonvolcanic systems the application regards five data sets of soil co2 fluxes measured with the accumulation chamber methodology at the volcanic areas of solfatara of pozzuoli italy vesuvio cone italy nisyros greece and horseshoe lake california and at the nonvolcanic degassing area of poggio dell olivo italy the sgs algorithm was used to generate 100 realizations of co2 flux for each area probabilistic summaries of these simulations together with the information given by probability plots were used 1 to draw maps of the probability that co2 fluxes exceed thresholds specific for a background flux i e to define the probable extension of the degassing structures 2 to calculate the total co2 output and 3 to quantify the uncertainty of the estimation the results show that the sgs is a suitable tool to model soil diffuse degassing producing realistic images of the distribution of the co2 fluxes that honor the histogram and variogram of the original data moreover the relation between the sample design and the uncertainty of estimation was investigated leading to an empirical relation between uncertainty and the sampling density that can be useful for the planning of future co2 flux surveys

paleoseismology and seismic hazards evaluations in extensional volcanic terrains extensional structures in volcanic terrains are the surface expression of shallow dike intrusion and can be misinterpreted as structures associated with major tectonic faults dike induced structures can be distinguished from their tectonic counterparts by their association with cogenetic volcanic rocks and by several geometric relationships tensile fissures with little or no vertical displacement fissure swarms flexural monoclines and normal faults are commonly symmetrically distributed about a central eruptive fissure sometimes forming a graben above shallow dikes the structures typically occur within broad zones not narrow belts near a main fault zone reflecting their origin by repeated dike injection colluvial wedges containing records of single large earthquakes generally do not form instead fault scarps with several meter vertical displacements in volcanic bedrock may reflect the cumulative effects of many decimeter scale displacement events from several dike injection episodes the mechanism of dike intrusion and the nature of cointrusive seismicity have important implications for determination of the maximum magnitude and recurrence of earthquakes in extensional volcanic terrains observational seismicity from volcanic rift zones worldwide suggests the maximum magnitudes of dike induced earthquakes are 3 8 0 8 earthquakes are generally small to moderate because downdip extents of faults and fissures are controlled by the depth to the top of the associated dike usually 5 km permitting only small rupture areas also rupture and displacement on faults and fissures migrate incrementally at about the velocity of propagating dikes 0 5 m s as dike dilation stresses the zone above and ahead of the dike earthquake recurrence is tied to recurrence of volcanic cycles based on the geochronology of the associated volcanic materials based on these concepts innovative approaches have led to estimates of the maximum magnitude and recurrence for magma induced seismicity in eastern snake river plain esrp volcanic rift zones the most conservative approach uses surface length fault width downdip extent and rupture areas of normal faults and fissures produced by dike injection to estimate a maximum m5 5 earthquake for an episode of rift zone volcanism chronology of volcanic rocks suggests annual probabilities of 10 4 to 10 5 for volcanic rift zones near idaho national engineering laboratory facilities probabilistic assessments show that ground motion hazard due to volcanic rift zone seismicity is lower than the hazard from other earthquake sources this is because of the relatively low magnitudes and long recurrence intervals of cointrusive earthquakes in volcanic rift zones

diffuse emission of carbon dioxide methane and helium 3 from teide volcano tenerife canary islands diffuse emission of co2 ch4 and he was investigated in the summit crater of teide volcano tenerife canary islands the results indicate that teide volcano releases abundant co2 not only from its active crater but also from its flanks as diffuse soil emanations the spatial distribution of these emanations correlates quite closely with that of geothermal anomalies and manifestations our flux measurements indicate a total output of 380 ton day 1 of co2 and 1 6 on day 1 of ch4 by diffuse degassing from the he co2 ratios and the co2 flux we estimate a diffuse he flux of 0 052 mol year 1 isotopic analysis of carbon in soil gas samples from sites of high soil co2 flux indicates a magmatic origin for the co2 also reported by high he 4he ratios of 5 50 and 5 87 ra the origin of methane is more uncertain with low 13c values suggesting a biogenic contribution

recent activity of nisyros volcano greece inferred from structural geochemical and seismological data this study summarizes the results of structural geochemical and seismological surveys carried out at nisyros volcano aegean sea greece during 1999 2001 field mapping and mesostructural measurements at the summit caldera lakki plain indicate that faults follow two main strikes ne sw and n s the n s striking fault depicts extensional features accommodating the left lateral component of motion of the ne sw striking main faults the ne sw preferred strike of the lakki faults and of the mineral filled veins as well as the distribution and ne sw elongation of the hydrothermal craters indicate that tectonics plays a major role in controlling the fluid pathway in the nisyros caldera the same ne sw trend is depicted by co2 anomalies revealed through detailed soil co2 flux surveys thus indicating a structural control on the pattern of the hydrothermal degassing degassing processes account for a thermal energy release of about 43 mw most of which occurs at lofos dome an area that was affected by hydrothermal eruptions in historical times the seismic study was conducted in june 2001 using a deployment specifically aimed at detecting signals of magmatic hydrothermal origin our instruments recorded local and regional earthquakes a few local long period events lp and bursts of monochromatic tremor local earthquake activity is concentrated beneath the caldera at depths generally shallower than 6 km plane wave decomposition of tremor signal indicates a shallow 200 m source located in the eastern part of the caldera conversely lp events depict a source located beneath the central part of the caldera in the area of lofos dome at depths in the 1 2 km range in agreement with geochemical and structural measurements these data suggest that both the deeper and shallower part of the hydrothermal system are subjected to instability in the fluid flow regimes probably consequent to transient pressurization of the reservoir these instabilities may be related to input of hot fluids from the deeper magmatic system as suggested by the variations in geochemical parameters observed after the 1997 1999 unrest episode the significance of seismological and geochemical indicators as precursors of hydrothermal explosive activity at nisyros is discussed

seismological and sar signature of unrest at nisyros caldera greece abstract nisyros island a quaternary volcanic center located at the se of the aegean volcanic arc has been in the past characterized by periods of intense seismic activity accompanied sometimes by hydrothermal explosions the last one being in 1887 the recent long lasting episode of unrest 1995 1998 in the area is the first instrumentally documented providing information on the behavior of the volcano evidence from seismicity and sar interferometry suggests that the presently active part of the kos nisyros volcano tectonic complex is located at the nw coast of nisyros island defining an area much smaller than the whole volcano tectonic area seismicity patterns vary both temporally and spatially consistently with different rates of vertical ground deformation inferred from sar interferometry these observations help us to discuss the different elements controlling the behavior of the volcanic system such as the existence location and timing of magma chamber inflation the occurrence of tensile failure at the boundaries of the chamber and the possibility of magmatic fluids being expelled to form a shallow magmatic intrusion the seismic failure and migration of hypocenters indicating shallow magma transport

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

the 1995 kobe japan earthquake a long delayed aftershock of the offshore 1944 tonankai and 1946 nankaido earthquakes the 1995 kobe earthquake produced predominantly right lateral slip along the northeast southwest trending nojima fault approximately 200 km landward of the nankai trough southwest japan the last major earthquakes along the nankai trough the m 8 0 1944 tonanakai and m 8 2 1946 nankaido earthquakes involved about 4 m of interplate slip each releasing about 100 years of accumulated stress from the northwest southeast convergence of the philippine sea phs plate by calculating the change in coulomb failure stress on the nojima fault due to 1944 46 to 1995 postseismic relaxation of a thin low viscosity channel beneath southwest japan we show that stress diffusion from the 1940s interplate earthquakes increased the likelihood of future failure on the nojima fault by slowly but progressively unloading the normal stress on the fault over a 50 yr period

fluid geochemistry of nisyros island dodecanese greece abstract two distinct hydrothermal aquifers are present beneath the lakki plain a a deep aquifer characterized by temperatures higher than 290 c and chloride rich fluids and b a shallow aquifer with a chloride content close to that of sea water and temperatures in the range 170 255 c the vapor phase boiled from the shallow aquifer feeds the fumarolic vents in the southern half of the lakki plain and its liquid phase flows northeast and southwest contributing to the thermal springs located along the northern and southern coasts of the island this conceptual hydrogeochemical model of nisyros island can be used to reconstruct past hydrothermal eruptions and to estimate the risk of such events happening again in the future

water rock interaction in the magmatic hydrothermal system of nisyros island greece abstract in this work we investigated the water rock interaction processes taking place in the hydrothermal reservoir of nisyros through both 1 a review of the hydrothermal mineralogy encountered in the deep geothermal borehole nisyros 2 and 2 a comparison of the analytically derived redox potentials and acidities of fumarolic related liquids with those controlled by redox buffers and ph buffers involving hydrothermal mineral phases the propylitic zone met in the deep geothermal borehole nisyros 2 from 950 to 1547 m total depth is characterised by abundant well crystallised epidote adularia albite quartz pyrite chlorite and sericite muscovite accompanied by less abundant anhydrite stilpnomelane wairakite garnet tremolite and pyroxene these hydrothermal minerals were produced in a comparatively wide temperature range from 230 to 300 c approximately hydrothermal assemblages are well developed from 950 to 1360 m whereas they are less developed below this depth probably due to low permeability based on the r h values calculated for fumarolic gases and for the deep geothermal fluids of nisyros 1 and nisyros 2 wells redox equilibrium with the feo feo 1 5 rock buffer appears to be closely attained throughout the hydrothermal reservoir of nisyros this conclusion may be easily reconciled with the nearly ubiquitous occurrence of anhydrite and pyrite since r h values controlled by coexistence of anhydrite and pyrite can be achieved by gas separation the ph of the liquids feeding the fumarolic vents of stephanos and polybote micros craters was computed by means of the eq3 code based on the cl d relationship which is constrained by the seawater magmatic water mixing occurring at depth in the hydrothermal magmatic system of nisyros the temperature dependence of analytically derived ph values for the reservoir liquids feeding the fumarolic vents of stephanos and polybote micros craters suggests that some unspecified ph buffer fixes the acidity of these reservoir liquids at values of 4 72 4 85 and 4 88 5 23 respectively many of these ph values are lower than those expected for the full equilibrium condition although they are close to those of the reservoir liquids of nisyros 1 5 16 and nisyros 2 4 87 it is likely that this excess of acidity producing species chiefly co 2 promotes release of fe ii and fe iii to the reservoir liquids through rock dissolution permitting the attainment of redox equilibrium with the feo feo 1 5 rock buffer as already suggested by the late werner giggenbach

young pumice deposits on nisyros greece the island of nisyros aegean sea consists of a silicic volcanic sequence upon a base of mafic andesitic hyaloclastites lava flows and breccias we distinguish two young silicic eruptive cycles each consisting of an explosive phase followed by effusions and an older silicic complex with major pyroclastic deposits the caldera that formed after the last plinian eruption is partially filled with dacitic domes each of the two youngest plinian pumice falls has an approximate dre volume of 2 3 km3 and calculated eruption column heights of about 15 20 km the youngest pumice unit is a fall surge flow surge sequence laterally transitional fall and surge facies as well as distinct polymodal grainsize distributions in the basal fall layer indicate coeval deposition from a maintained plume and surges planar bedded pumice units on top of the fall layer were deposited from high energy dry steam propelled surges and grade laterally into cross bedded finegrained surge deposits the change from a fall to a surge flow dominated depositional regime coincided with a trend from low temperature argillitic lithics to high temperature epidote and diopside bearing lithic clasts indicating the break up of a high temperature geothermal reservoir after the plinian phase the transition from a maintained plume to a surge ash flow depositional regime occurred most likely during break up of the high temperature geothermal reservoir during chaotic caldera collapse the upper surge units were possibly erupted through the newly formed ringfracture

models of tremor and low frequency earthquake swarms on montserrat recent observations from soufriere hills volcano in montserrat reveal a wide variety of low frequency seismic signals we discuss similarities and differences between hybrid earthquakes and long period events and their role in explosions and rockfall events these events occur usually in swarms and occasionally merge into tremor an observation that can shed further light on the generation and composition of harmonic tremor we use a 2d finite difference method to model major features of low frequency seismic signatures and compare them with the observations a depth dependent velocity model for a fluid filled conduit is introduced which accounts for the varying gas content in the magma and the impact on the seismic signals is discussed we carefully analyse episodes of tremor that show shifting spectral lines and model those in terms of changes in the gas content of the magma as well as in terms of a time dependent triggering mechanism of low frequency resonances in this way we explain the simultaneous occurrence of low frequency events and tremor with a spectral content comprising integer harmonics

chemical and isotopic equilibrium between co2 and ch4 in fumarolic gas discharges generation of ch4 in arc magmatic hydrothermal systems the chemical and isotopic composition of fumarolic gases emitted from nisyros volcano greece and of a single gas sample from vesuvio italy was investigated in order to determine the origin of methane ch4 within two subduction related magmatic hydrothermal environments apparent temperatures derived from carbon isotope partitioning between ch4 and co2 of around 340 c for nisyros and 470 c for vesuvio correlate well with aquifer temperatures as measured directly and or inferred from compositional data using the h2o h2 co2 co ch4 geothermometer thermodynamic modeling reveals chemical equilibrium between ch4 co2 and h2o implying that carbon isotope partitioning between co2 and ch4 in both systems is controlled by aquifer temperature n2 3he and ch4 3he ratios of nisyros fumarolic gases are unusually low for subduction zone gases and correspond to those of midoceanic ridge environments accordingly ch4 may have been primarily generated through the reduction of co2 by h2 in the absence of any organic matter following a fischer tropsch type reaction however primary occurrence of minor amounts of thermogenic ch4 and subsequent re equilibration with co existing co2 cannot be ruled out entirely co2 3he ratios and 13cco2 values imply that the evolved co2 either derives from a metasomatized mantle or is a mixture between two components one outgassing from an unaltered mantle and the other released by thermal breakdown of marine carbonates the latter may contain traces of organic matter possibly decomposing to ch4 during thermometamorphism

geology chronology fluid inclusions and h o s isotopic compositions of the hongyuntan magnetite deposit eastern tianshan nw china abstract the hongyuntan magnetite deposit which is located in the aqishan yamansu belt of eastern tianshan xinjiang is hosted within carboniferous submarine volcanic sedimentary sequences the deposit is characterized by five metallogenic stages stage i garnet diopside stage ii magnetite tremolite chlorite pyrite stage iii magnetite quartz albite actinolite stage iv pyrite quartz epidote and stage v hematite limonite pyrite associated with magnetite formed during the stage ii has a re os isochron age of 324 31 ma four types of fluid inclusions were observed in the mineral assemblages namely two phase liquid and vapor inclusions multi phase daughter mineral bearing inclusions multi phase co2 bearing inclusions and monophase vapor inclusions microthermometry data reveal that the ore forming fluids evolved from high temperature average 532 5 c and high salinity average 26 9 wt nacleqv fluids to moderate low temperature average 203 6 c and low salinity 5 07 wt nacleqv fluids magnetite precipitation may have been greatly assisted by mixing and boiling in stage ii and stage iii respectively the hydrogen and oxygen isotopic values of stage i garnet dv smow 107 9 to 76 7 18ofluid 6 4 10 7 and stage iii quartz dv smow 85 5 to 83 2 18ofluid 3 8 4 7 indicate that the ore forming fluids were dominated by magmatic water whereas fluid mixing occurred during stage ii dv smow 117 3 to 112 6 18ofluid 3 3 5 7 and stage iv dv smow 115 2 to 58 5 18ofluid 1 3 to 4 5 the variable 34s values of pyrite 3 8 to 4 7 and 18omgt values of magnetite 0 6 to 4 1 suggest that they were not sourced from a single reservoir the hongyuntan magnetite deposit is spatially and temporally associated with volcanism and may be genetically related to it the fluid inclusions and isotopic compositions of this deposit indicate that its ore forming fluid was derived from the exsolution of deep seated magmas and that fluid mixing could have effectively triggered the deposition of ore forming materials multiple periods of volcanic activity could have incorporated additional heat and ore metals into the ore forming system based on our data we conclude that the origin of the hongyuntan magnetite deposit is related to the volcanism in the carboniferous period

simulation of strong ground motions recorded during the 1985 michoacan mexico and valparaiso chile earthquakes abstract the purpose of this study is to develop and test a procedure for simulating acceleration time histories of large subduction earthquakes the ground motions of the large event are obtained by summing contributions from fault elements to simulate the propagation of rupture over the fault surface the radiation from the fault elements is represented by empirical source functions derived from near source strong motion recordings of m w 7 aftershocks and gross aspects of wave propagation are modeled by calculated green9s functions the procedure has been tested against the recorded strong ground motions of the m w 8 0 michoacan mexico and valparaiso chile earthquakes of 1985 we find that models of heterogeneous slip in these events derived by other investigators from the analysis of teleseismic and near source velocity seismograms also explain the shorter period motions of the recorded accelerograms the response spectra of the simulated motions generally have little or no significant bias in the period range of 0 05 to 2 sec for both earthquakes also the peak accelerations durations and envelope shapes of the time histories are in good agreement with the recorded motions the uncertainty associated with the modeling procedure estimated from the misfit between recorded and simulated response spectra shows a dependence on period and site conditions this uncertainty estimate includes the uncertainty involved in using empirical source functions derived from one subduction zone to simulate strong ground motions in another subduction zone the procedure is applied in a companion paper cohee et al 1991 to estimate strong ground motion characteristics in the pacific northwest region of the united states from hypothesized m w 8 subduction earthquakes on the cascadia plate interface

ground motions from subduction zone earthquakes a total of 258 horizontal components of accelerogram data recorded at soil sites during thrust normal and strike slip earthquakes occurring in seven subduction zones around the pacific ocean were analyzed the results of statistical analyses of 5 per cent damped pseudo velocities psv computed from these accelerograms at 10 periods t between 0 1 and 4 sec indicated no significant differences in the average psv levels of the thrust normal and strike slip data from the northern honshu zone each of these groups of northern honshu data which together comprised one half of the total data base were fit equally well by the same attenuation model the analyses also revealed that at intermediate periods 0 8 t 3 sec the psv observed at stiff soil sites within the northern honshu nankai kuril mexico and alaska zones were on the average significantly greater than the psv observed at similar sites in the peru northern chile and new britain bougainville zones this observation indicates that differences in the source and or travel path characteristics between the two groups accounted for the differences in psv independent evidence supporting source differences is the correlation noted between psv in this period band and the characteristic source complexities for these zones which hartzell and heaton inferred from the teleseismic data of the larger magnitude earthquakes certain anomalous tectonic characteristics of the new britain bougainville zone were noted that may have contributed in some systematic manner to the relatively unusual spectral characteristics of the ground motions recorded on stiff soil sites within this zone some of the differences in the psv among the zones at intermediate and long periods were probably the result of differences in local geologic characteristics geology greatly influenced the mexican psv data from soft soil sites to a lesser extent geology probably affected the alaskan data which were recorded mostly on deep stiff soils and the new britain bougainville data recorded at yonki which is underlain by softer soil deposits

diffuse emission of carbon dioxide from cerro negro volcano nicaragua central america we report the first detailed soil co2 efflux survey carried out at cerro negro volcano nicaragua central america soil co2 efflux values ranged from 0 5 to 35 000 g m 2 d 1 spatial distribution of co2 efflux shows that volcanic gas is mainly released from the summit crater and the fissures opened during the august 1999 eruption the total co2 output of cerro negro is conservatively estimated to be 2 800 t d 1 based on a study area of 0 58 km contour maps of soil co2 efflux ground temperature and 13c co2 distributions support an important magmatic contribution for the total output of co2 at cerro negro volcano

observations of loma prieta aftershocks from a dense array in sunnyvale california a small aperture 300 m four station array was deployed in sunnyvale california for 5 days to record aftershocks of the loma prieta earthquake of october 1989 the purpose of the array was to study the seismic response of the alluvium filled santa clara valley and the role of surface waves in the seismic shaking of sedimentary basins strong motion records of the loma prieta mainshock indicate that surface waves produced the peak velocities and displacements at some sites in the santa clara valley we use the recordings from the dense array to determine the apparent velocity and azimuth of propagation for various arrivals in the seismograms of four aftershocks with magnitudes between 3 6 and 4 4 apparent velocities are generally observed to decrease with increasing time after the s wave in the seismograms phases arriving less than about 8 sec after the s wave have apparent velocities comparable to the s wave and appear to be body waves multiply reflected under the receiver site or reflected by crustal interfaces for times 10 to 30 sec after the direct s wave we observe long period 1 to 6 sec arrivals with apparent velocities decreasing from 2 5 to 0 8 km sec we interpret these arrivals to be surface waves and conclude that these surface waves produce the long duration of shaking observed on the aftershock records much of the energy in the 40 sec after the s wave is coming approximately from the direction of the source although some arrivals have backazimuths as much as 60 different from the backazimuths to the epicenters two of the aftershocks show arrivals coming from 30 to 40 more easterly than the epicenters this energy may have been scattered from outcrops along the southeastern edge of the basin in contrast the deepest aftershock studied d 17 km displays later arrivals with backazimuths 30 to 40 more westerly than the epicenter a distinct arrival for one of the aftershocks propagates from the southwest possibly scattered from the western edge of the basin synthetic seismograms derived from a plane layered crustal model do not produce the long period love waves observed in the waveforms of the ml 4 4 aftershock these love waves may be generated by the conversion of incident s waves or rayleigh waves near the edge of the basin

soil diffuse degassing and thermal energy fluxes from the southern lakki plain nisyros greece two diffuse soil co2 flux surveys from the southern lakki plain show that co2 is mainly released from the hydrothermal explosion craters the correspondence between high co2 fluxes and elevated soil temperatures suggests that a flux of hot hydrothermal fluids ascends towards the surface steam mostly condenses near the surface and the heat given off is conductively transferred to the atmosphere through the soil accompanied by a large co2 flux it was calculated that 68 t d 1 of hydrothermal co2 are released through the total surveyed area of 1 3 km admitting that a steam flux of 2200 t d 1 accompanies this co2 flux the thermal energy released through steam condensation amounts to 58 mw

the volcanoseismic crisis of 1996 97 in nisyros se aegean sea greece after a repose time of several years intense shallow seismic activity started at the end of 1995 in the volcanic region of nisyros island by the end of october 1997 hundreds of shocks had been recorded the largest one being that of 27 august 1997 with ms 5 3 the activity is highly clustered in space and time while the b value of the magnitude frequency relation was found equal to 1 67 similar b values were determined from nisyros seismicity registered between 1911 and 1980 and from a 1997 microearthquake dataset these properties seem to be a characteristic of the region rather than precursor of a forthcoming eruption and make nisyros similar to other large calderas of the earth where the episodic unrest with local swarm like earthquake sequences separated by months or years is a typical pattern that do not culminate in a volcanic eruption

hydrothermal eruptions of nisyros dodecanese greece past events and present hazard abstract the detailed analysis of the craters of hydrothermal eruptions and related products present on nisyros island demonstrates the ephemerality of these morphological forms in other words the mere recognizable existence of the craters and associated deposits implies recency of hydrothermal activity the minimum temperature required to cause the explosive phenomenon and possibly the depth of the reservoir which can be evaluated on the basis of the correlation between the diameter of the crater and the depth of explosion as proposed by fytikas and marinelli 1976 are therefore closely representative of the current hydrothermal circulation both field evidence and historical records indicate that all the deposits of hydrothermal eruption recognized on nisyros island were emplaced as debris flows almost all the ballistic ejecta were entrained in these debris flows and either redeposited far from their landing sites or involved in later crater collapse and erosion this emplacing mechanism implies that the original products were characterized by a water content higher than about 5 by weight steam driven hydrothermal eruptions one of which took place in 1871 originated deposits of limited dispersion as no sign of these erodible products can be found in the field today surface geology and fluid geochemistry together with subsurface information e g primary and hydrothermal lithologies distribution of temperature with depth physical chemical characteristics of deep water bearing zones indicate that two distinct hydrothermal aquifers are present underneath the southeastern part of the caldera floor both aquifers were probably involved in the most important historically documented hydrothermal eruptions which occurred in 1873 at that time violent earthquakes fractured the brittle aquiclude separating the two aquifers and caused a sudden transfer of fluids from the deep to the shallow aquifer thus triggering the hydrothermal eruptions hydrothermal eruptions will probably occur in future and this hazard must be taken into serious consideration the southern half of lakki plain where all past eruptions took place and active fumaroles are concentrated is the zone at highest risk at present gas geochemistry represents an effective tool to detect changes in the p t conditions of the shallow aquifer and particularly the phenomena of pressure build up that may lead to a hydrothermal eruption

diffuse emission of co2 from the fossa crater vulcano island italy numerous measurements of co2 degassing from the soil carried out with the accumulation chamber method indicate that in the period april july 1995 the upper part of the fossa cone released a total output of 200 t d 1 of co2 which corresponds to approximately 1000 t d 1 of steam these large amounts of fluids are of the same order of magnitude as those released by the high temperature fumarolic field located inside the crater the spatial distribution of soil gas fluxes shows that the main structures releasing co2 are the inner slopes of the crater and a nw se line located ne of the crater rim which correspond to the main direction of vulcano island active faults the comparison of the co2 maps with the soil temperature distribution derived from both direct measurements and airborne infrared images indicates the occurrence of extensive condensation of fumarolic steam within the upper part of the fossa cone whose total amount is comparable to the rainfall budget part of the condensate which originates from this process contributes to the recharge of the phreatic aquifer of porto plain modifying the chemical and isotopic composition of the groundwater

modelling gravity variations consistent with ground deformation in the campi flegrei caldera italy abstract in recent years 1970 72 and 1982 84 two inflation episodes took place in the campi flegrei caldera italy characterized by significant ground uplift and gravity variations an elastic half space model with vertical density stratification is employed to compute the displacement field and the gravity variations produced by the deformation of buried layers following the inflation of a spherically symmetric deformation source contributions to gravity variations are produced by dilation contraction of the medium by the displacements of density interfaces the free surface and subsurface layers and of source boundaries and possibly by new mass input from remote distances into the source volume three cases were examined in detail in case i the magma chamber is identified as the deformation source and volume and pressure increase in the magma chamber is due to input of new magma from remote distances in case ii deformation is due to magma differentiation within the magma chamber deformation source with constant mass in case iii the geothermal system is identified as the deformation source and a pressure increase possibly driven by the exsolution of high temperature and high pressure volatiles in the magma chamber is assumed to play a dominant role from the comparison between measured and computed gravity residuals free air corrected gravity variations we can assess that in case i an inflation source with constant density would predict gravity residuals compatible with observations whereas an expansion at constant mass case ii would predict gravity residuals much lower than observed the resolving power of gravity data however prevents accurate assessment of the density of the emplaced material in case iii the pervasive density increase of the geothermal fluids induced by pressure increase is assumed to be the main source of gravity variations the average porosity value required for this model to match both the ground deformation and the gravity residuals is found to be 10 a value which is compatible with measured porosity values at campi flegrei in deep wells the subsidence phases following both inflation episodes and the gravity residuals during subsidence lead us to consider case iii as more plausible even if a suitable combination of cases i and iii cannot be discarded

soil efflux and total emission rates of magmatic co2 at the horseshoe lake tree kill mammoth mountain california 1995 1999 abstract we report the results of eight soil co 2 efflux surveys by the closed circulation chamber method at the horseshoe lake tree kill hltk the largest tree kill on mammoth mountain the surveys were undertaken from 1995 to 1999 to constrain total hltk co 2 emissions and to evaluate occasional efflux surveys as a surveillance tool for the tree kills hltk effluxes range from 1 to 10 000 g m 2 day 1 grams co 2 per square meter per day they are not normally distributed station efflux rates can vary by 7 35 during the course of the 8 to 16 h surveys disturbance of the upper 2 cm of ground surface causes effluxes to almost double semivariograms of efflux spatial covariance fit exponential or spherical models they lack nugget effects efflux contour maps and total co 2 emission rates based on exponential spherical and linear kriging models of survey data are nearly identical similar results are also obtained with triangulation models suggesting that the kriging models are not seriously distorted by the lack of normal efflux distributions in addition model estimates of total co 2 emission rates are relatively insensitive to the measurement precision of the efflux rates and to the efflux value used to separate magmatic from forest soil sources of co 2 surveys since 1997 indicate that contrary to earlier speculations a termination of elevated co 2 emissions at the hltk is unlikely anytime soon the hltk co 2 efflux anomaly fluctuated greatly in size and intensity throughout the 1995 1999 surveys but maintained a n s elongation presumably reflecting fault control of co 2 transport from depth total co 2 emission rates also fluctuated greatly ranging from 46 to 136 t day 1 metric tons co 2 per day and averaging 93 t day 1 the large inter survey variations are caused primarily by external meteorological processes operating on time scales of hours to days the externally caused variations can mask significant changes occurring at depth a striking example is the masking of a degassing event generated at depth and detected by a soil gas sensor network in september 1997 while an efflux survey was in progress thus occasional efflux surveys are not an altogether effective surveillance tool for the hltk and making them effective by greatly increasing their frequency may not be practical

observations of high frequency scattered waves using dense arrays at teide volcano abstract a study of the seismic coda of seven small earthquakes recorded on the teide volcano canary islands spain was carried out using a temporary small aperture 12 station seismic array the purpose was to measure backazimuth apparent velocity and the type of waves that compose the coda in the frequency range 4 to 6 hz we used the zero lag cross correlation zlc method to obtain the components of the wave vector and three component analysis techniques based on the covariance matrix of the signal in the time domain to investigate the polarization properties of the signals the results show that a great part of the coda signals in the analyzed frequency range are almost uncorrelated while a low number of isolated correlated arrivals show an apparent slowness between 2 10 3 and 2 5 10 4 sec m and an almost random distribution of backazimuths the correlated arrivals have been interpreted as generated by strong scatterers probably related to the presence in the study area of surface topography irregularities the volcanic cone and the caldera rim the wave type varies from pure s type waves to mixed surface waves with some rayleigh components