fault location by radon and mercury detection at an active volcano in nicaragua the masaya caldera complex nicaragua with the active crater santiago fig 1 is the site of present day basaltic volcanism1 2 some of the faults that bound the caldera and lie within it can be located geologically others are postulated a possible older caldera rim lies to the south of the present margin to investigate whether one can identify faults in this environment of active volcanism concentrations of radon and mercury in the regolith were determined at 232 selected points in and adjacent to the caldera higher concentrations of hg0 or rn or both occurred over 75 of the known structures studied some hitherto suspected structures were confirmed while others were not lack of a chemical signature may be ascribed to the absence of a structure inappropriate sampling sites post fault lava cover or other sealing of the structure we show here how the use of rn and hg0 determinations has helped in structural analysis at this active volcano

wavelet analysis for the regional residual and local separation of potential field anomalies a method based on the discrete wavelet transform was applied to the regional residual separation of potential fields and to the filtering of local anomalies a specific space scale wavelet analysis called multiresolution analysis allowed decomposition of the signal with respect to a vast range of scales different analysing wavelets were applied to anomalies in both synthetic and real cases but the more appropriate basis needed to be chosen by requiring the maximum compactness for the multiresolution analysis moreover since such analysis was found not to be shift invariant the same criterion was applied to choosing the best signal shift application of the technique to both synthetic and real cases produced an optimal space scale representation of the fields and a consistent regional residual separation furthermore the space localization allowed a variety of filtered signals to be obtained each one with a specific scale and local area content fourier and wavelet analyses were both applied to the filtering out of the intense etna anomaly from the aeromagnetic field of sicily the wavelet method was more powerful suppressing only the etna volcano anomaly and leaving the rest of the map practically unchanged

hydrothermal circulation beneath mount pelee inferred by self potential surveying structural and tectonic implications abstract self potential sp surveys were made on mount pelee volcano martinique island french west indies in 1991 and 1992 in order to recognize its hydrothermal system the associated groundwater channeling and the main superficial structures of the massif almost 70 km of profiles were carried out with an average sample spacing of 50 m measurements essentially reveal negative sp anomalies down to 1700 mv with high gradients 1 83 mv m due to the infiltration of meteoric water into the massif rims of summit calderas morne macouba and etang sec present sharp negative sp anomalies on the western northern and eastern flanks negative sp anomalies indicate no upward water flow beneath mount pelee summit on the southwestern volcano flank a 3 5 6 km horseshoe shaped structure corresponding to a southwest flank collapse event older than 25 000 years bp is clearly identified by the sp mapping high gradients border the inner southern rim from morne calebasse to st pierre town and the caribbean sea along the northern rim of the horseshoe shaped structure the negative sp anomalies give place to a positive sp anomaly up to 200 mv of sw ne trend this zone covers the area of two active hot springs sources chaudes and puits chaud 40 65 c marine magnetic surveys and bathymetry show that the horseshoe shaped structure spreads into the caribbean sea up to about 10 km from the coast buried structural discontinuities are evidenced inside the flank collapse structure the upper one deviates the groundwater flow coming from the summit toward the south flank where the flow finds an indentation to expand again downwards this discontinuity is either an old hypothetical caldera rim partly destroyed by the collapse of the south southwestern flank and covered by recent pyroclastic deposits or more probably the trace of a bulge landslide a circulation model of the hydrothermal waters is proposed rainfall 5 6 m year is partly drained inside the summital calderas and the flank collapse zone through pyroclastic flows down to an impermeable basement there the groundwater constitutes perched aquifers at the contact of the bulge landslide or of the hypothetical old caldera rim along the inner northern border of the flank collapse structure the phreatic water is reheated warm groundwater flows along the northern avalanche structure rim and discharges near the coast in ground and marine outcrops of medium temperature finally the main part of the meteoric water is channeled along the old caldera rim or along the bulge landslide towards the south flank of mount pelee where some gaps in the rim exist there the groundwater finds again a subhorizontal gravitational circulation along mount pelee slopes into the caribbean sea

understanding caldera structure and development an overview of analogue models compared to natural calderas abstract understanding the structure and development of calderas is crucial for predicting their behaviour during periods of unrest and to plan geothermal and ore exploitation geological data including that from analysis of deeply eroded examples allow the overall surface setting of calderas to be defined whereas deep drillings and geophysical investigations provide insights on their subsurface structure collation of this information from calderas worldwide has resulted in the recent literature in five main caldera types downsag piston funnel piecemeal trapdoor being viewed as end members despite its importance such a classification does not adequately examine a the structure of calderas particularly the nature of the caldera s bounding faults and b how this is achieved including the genetic relationships among the five caldera types various sets of analogue models specifically devoted to study caldera architecture and development have been recently performed under different conditions apparatus materials scaling parameters stress conditions the first part of this study reviews these experiments which induce collapse as a result of underpressure or overpressure within the chamber analogue the experiments simulating overpressure display consistent results but the experimental depressions require an exceptional amount of doming seldom observed in nature to form therefore these experiments are not appropriate to understand the structure and formation of most natural calderas the experiments simulating underpressure reveal a consistent scenario for caldera structure and development regardless of their different boundary conditions these show that complete collapse proceeds through four main stages proportional to the amount of subsidence progressively characterized by 1 downsag 2 reverse ring fault 3 peripheral downsag 4 peripheral normal ring fault the second part of this study verifies the possibility that these latter calderas constitute a suitable analogue to nature and consists of a comprehensive comparison of the underpressure experiments to natural calderas this shows that all the experimental structures as well as their progressive development are commonly observed at natural calderas highlighting a consistency between models and nature as the shallow structure of experimental calderas corresponds to a precise architecture at depth it provides a unique key to infer the deeper structure of natural calderas recognizing diagnostic surface features within a caldera will thus allow it to be categorized within a precise structural and evolutionary context the general relationship between the evolutionary stage of a caldera and its d s diameter subsidence ratio allows such a quantification with stage 1 calderas characterized by d s 40 stage 2 by 18 the four stages adequately explain the architecture and development of the established caldera end members along a continuum where one or more end members downsag piston funnel piecemeal trapdoor may correspond to a specific stage while such a continuum is controlled by progressive subsidence specific collapse geometries will result from secondary contributory factors roof aspect ratio collapse symmetry pre existing faults these considerations allow proposing an original classification of calderas incorporating their structural and genetic features

fluid flow in the resurgent dome of long valley caldera implications from thermal data and deep electrical sounding abstract temperatures of 100 c are measured at 3 km depth in a well located on the resurgent dome in the center of long valley caldera california despite an assumed 800 c magma chamber at 6 8 km depth local downflow of cold meteoric water as a process for cooling the resurgent dome is ruled out by a peclet number analysis of temperature logs these analyses reveal zones with fluid circulation at the upper and lower boundaries of the bishop tuff and an upflow zone in the metasedimentary rocks vertical darcy velocities range from 10 to 70 cm a 1 a 21 km long geoelectrical profile across the caldera provides resistivity values to the order of 10 0 to 10 3 m down to a depth of 6 km as well as variations of self potential interpretation of the electrical data with respect to hydrothermal fluid movement confirms that there is no downflow beneath the resurgent dome to explain the unexpectedly low temperatures in the resurgent dome we challenge the common view that the caldera as a whole is a regime of high temperatures and the resurgent dome is a local cold anomaly instead we suggest that the caldera was cooled to normal thermal conditions by vigorous hydrothermal activity in the past and that a present day hot water flow system is responsible for local hot anomalies such as hot creek and the area of the casa diablo geothermal power plant the source of hot water has been associated with recent shallow intrusions into the west moat the focus of planning for future power plants should be to locate this present day flow system instead of relying on heat from the old magma chamber

effect of secondary minerals on electrokinetic phenomena during water rock interaction 1 the main geophysical application of spontaneous potentials sp anomalies i e electrical signal generated by hydroelectric coupling is the mapping of hydrothermal zones laboratory studies of hydroelectric coupling as a function of temperature are scarce because of experimental difficulties up today experiments carried out by ishido and mizutani 1981 with quartz al k no3 system for temperatures ranging from 20 c to 80 c are not well understood in this work we show that ishido and mizutani 1981 s solutions are oversaturated with aluminium and that the precipitation of al oh 3s is expected triple layer model tlm calculations for a gibbsite kno3 system can account for ishido and mizutani 1981 s measurements these results highlight that 1 the precipitation of a secondary mineral can hide the electrical properties of the primary rock 2 the geochemical fluid surveys should be taken into account to grasp the interfacial processes precipitation dissolution and to interpret the sp measurements in the field

the pit craters and pit crater filling lavas of masaya volcano lava flowing into a pit crater will become entrapped to form an inactive lava lake at masaya volcano nicaragua pit filling lavas are exposed in the walls of nindiri santiago and san pedro pits mapping of these lavas shows that fill can involve emplacement of both a a and pahoehoe with single fill units ranging in thickness from 2 to 22 m thick units with columnar joints were emplaced as simple inactive lava lakes during high effusion rate episodes sequences of thinner units which can form pit floor shields or compound lakes were emplaced at lower effusion rates lava withdrawal caused unsupported sections of three 20 m thick units to subside resulting in unit flexure and faulting and viscous peeling features reveal that subsidence occurred while at least one unit was still partially molten where withdrawal has not occurred fill sequences are flat lying and symmetrically distributed around the feeder structures cinder cones and dykes the filled nindiri pit holds 5 107 m3 of lava in a 215 m thick sequence partial fill of santiago pit with 1 107 m3 of lava has filled the pit with a 110 m thick lava sequence of which 50 has been consumed by formation of a secondary pit altogether 6 4 107 m3 of lava was erupted into nindiri and santiago during 1525 1965 with 94 of this volume remaining pit contained the remainder forms a north flank lava flow field pit development and filling is a dynamic and ephemeral process having short lived effects on volcano morphology where pits develop and fill over hours to centuries however pits play an important role in shaping an edifice representing lava sinks and controlling whether lavas are trapped or able to spread onto the flanks

importance of structural history in the summit area of stromboli during the 2002 2003 eruptive crisis inferred from temperature soil co2 self potential and electrical resistivity tomography the 2002 2003 eruptive crisis of stromboli volcano in the aeolian islands raised the question of how to assess the stability of the flanks of this volcanic edifice during such a crisis to provide a response to this question we analyzed a detailed fluid flow mapping plus the reiteration of a profile located in the vicinity of the active vents using the self potential method temperature data soil gas co2 measurements and electric resistivity tomography coupling the interpretation of these methods that are sensitive to the flow of gas and water in the ground indicates the position of areas of mechanical weakness in addition they can be used to monitor the change in the discharge of fluids associated with these features before and during the 2002 2003 eruptive crisis our results emphasize the importance of old structural boundaries such as the large fossa crater in the development of the new set of fractures observed during the 2002 2003 eruptive crisis between october 2002 and january 2003 the use of co2 soil gas technique evidenced an increase in the discharge of co2 outside the large fossa crater boundaries along the failure boundary of the southern sciara del fuoco area self potential and temperature measurements made before the 2002 2003 eruptive crisis reveal significant changes along the main structural boundaries of the fossa area the development of these anomalies is interpreted as an increase of the permeability of the structure from may 2000 to may 2002 between january 2003 and march 2003 the reiteration of self potential temperature and co2 measurements shows an increase of fluid discharge along weakness planes located inside the large fossa crater boundary they evidence no change outside this structural boundary the importance of the large fossa crater boundary in controlling the deformation and fluid flow from january to march 2003 has been attested by the development of the fractures inside the large fossa crater boundary and also with a network of electrooptical distance measurement stations located inside and outside this ancient crater this multidisciplinary approach to fluid flow assessment before and during an eruptive crisis is complementary to geodetic measurements of the deformation of the edifice it demonstrates for the first time the powerful potential of combining electrical resistivity tomography self potential temperature and soil co2 measurements in assessing the position of the planes of mechanical weakness in a volcanic edifice

fluid circulation at stromboli volcano aeolian islands italy from self potential and co2 surveys this work addresses the study of fluid circulation of the stromboli island using a dense coverage of self potential sp and soil co2 data a marked difference exists between the northern flank and the other flanks of the island the northern flank exhibits 1 a typical negative sp altitude gradient not observed on the other flanks and 2 higher levels of co2 the general sp pattern suggests that the northern flank is composed of porous layers through which vadose water flows down to a basal water table in contrast to the other flanks where impermeable layers impede the vertical flow of vadose water in the sciara del fuoco and rina grande le schicciole landslide complexes breccias of shallow gliding planes may constitute such impermeable layers whereas elsewhere poorly permeable fine grained pyroclastites or altered lava flows may be present this general model of the flanks also explains the main co2 patterns concentration of co2 at the surface is high on the porous north flank and lower on the other flanks where impermeable layers can block the upward co2 flux the active upper part of the island is underlain by a well defined hydrothermal system bounded by short wavelength negative sp anomalies and high peaks of co2 these boundaries coincide with faults limiting ancient collapses of calderas craters and flank landslides the hydrothermal system is not homogeneous but composed of three main subsystems and of a fourth minor one and is not centered on the active craters the latter are located near its border this divergence between the location of the active craters and the extent of the hydrothermal system suggests that the internal heat sources may not be limited to sources below the active craters if the heat source strictly corresponds to intrusions at depth around the active conduits the geometry of the hydrothermal subsystems must be strongly controlled by heterogeneities within the edifice such as craters caldera walls or gliding planes of flank collapse as suggested by the correspondence between sp co2 anomalies and structural limits the inner zone of the hydrothermal subsystems is characterized by positive sp anomalies indicating upward movements of fluids and by very low values of co2 emanation this pattern suggests that the hydrothermal zone becomes self sealed at depth thus creating a barrier to the co2 flux in this hypothesis the observed hydrothermal system is a shallow one and it involves mostly convection of infiltrated meteoric water above the sealed zone finally on the base of co2 degassing measurements we present evidence for the presence of two regional faults oriented n41 and n64 and decoupled from the volcanic structures

a model of diffuse degassing at three subduction related volcanoes and 13c in soil gas were measured at three active subduction related stratovolcanoes arenal and poas costa rica galeras colombia in general rn co2 and 13c values are higher on the lower flanks of the volcanoes except near fumaroles in the active craters the upper flanks of these volcanoes have low rn concentrations and light 13c values these observations suggest that diffuse degassing of magmatic gas on the upper flanks of these volcanoes is negligible and that more magmatic degassing occurs on the lower flanks where major faults and greater fracturing in the older lavas can channel magmatic gases to the surface these results are in contrast to findings for mount etna where a broad halo of magmatic co2 has been postulated to exist over much of the edifice differences in radon levels among the three volcanoes studied here may result from differences in age the degree of fracturing and faulting regional structures or the level of hydrothermal activity volcanoes such as those studied here act as plugs in the continental crust focusing magmatic degassing towards crater fumaroles faults and the fractured lower flanks

ζ potential measurement of volcanic rocks from aso caldera 1 we deduced the z potential of various rocks in aso caldera from streaming potential measurements in laboratory in contrast to the conventional observation that the z potential of crustal rocks is mostly negative under typical geologic conditions almost half of the samples equilibrated with dilute kcl solution show positive values of z potential at ph 4 5 5 5 the samples showing positive z are characterized by relatively low sio2 content and abundance of elements having high isoelectric points and found to be localized around the summit of mt takadake where the positive correlation between self potential sp and altitude is observed we emphasize that measurements of z potential of volcanic rocks are very important to give more appropriate interpretations of sp data from volcanic fields index terms 0634 electromagnetics measurement and standards 0925 exploration geophysics magnetic and electrical methods 5199 physical properties of rocks general or miscellaneous 8499 volcanology general or miscellaneous 1832 hydrology groundwater transport citation hase h t ishido s takakura t hashimoto k sato and y tanaka z potential measurement of volcanic rocks from aso caldera geophys res lett 30 23 2210 doi 10 1029 2003gl018694 2003

zeta potential estimation of volcanic rocks on 11 island arc type volcanoes in japan implication for the generation of local self potential anomalies 1 from streaming potential measurements we deduced the zeta potential of 73 volcanic rock samples collected in 11 volcanoes where self potential sp surveys had also been conducted experiments with crushed rock samples and 0 001 mol l nacl solution showed a large variation in streaming potential coefficient which ranged from 2860 to 2280 mv mpa deduced zeta potential ranged from 45 1 to 37 2 mv among the 73 samples 9 samples showed positive values and 11 samples showed small absolute values less than 10 mv even in a volcano the zeta potential values of rocks were sometimes highly variable to investigate the reason for the variation in zeta potential we performed x ray fluorescence xrf measurements on the rock samples however no significant correlations between zeta potential or isoelectric point and weight percentage of major elements were found implying that the zeta potential of the volcanic rocks is not controlled by chemical composition but by mineralogical composition or crystalline structure in volcanic areas rocks with positive or small absolute values of zeta potential which are considered to be unusual frequently exist in areas along with local positive sp anomalies although positive sp anomalies are usually interpreted as an indication of hydrothermal upflow this correlation suggests that in addition to the groundwater flow pattern the polarity of zeta potential should also be considered for interpretation of sp data therefore in cases where detailed zeta potential measurements have not been conducted local positive sp anomalies should be interpreted with caution

deflation of the askja volcanic system constraints on the deformation source from combined inversion of satellite radar interferograms and gps measurements the askja central volcano in northern iceland has been continuously subsiding at least since 1983 gps and optical leveling tilt measurements show subsidence of at least 75 cm from 1983 to 1998 in the center of the askja caldera without any eruptive activity we have performed an interferometric analysis of synthetic aperture radar images insar of the area utilizing data from the ers satellites we observe subsidence of the askja caldera and its fissure swarm up to a distance of 25 km from the volcano we evaluate the geometry of the magma chamber at askja from a combined inversion of satellite radar interferograms and gps measurements several models were tested including a mogi point source as well as an ellipsoidal source the use of an ellipsoidal source instead of a mogi source gives an estimate of the dimensions of the magma chamber and its deflating pressure whereas these parameters are not independently resolved if a mogi source is used two source models were also considered in order to explain the additional subsidence observed along the askja fissure swarm we tested a model using two mogi sources at different depths a shallow ellipsoidal cavity with a deeper mogi source and then a shallow mogi source with a deeper elongated ellipsoid oriented along the fissure swarm results indicate that an ellipsoidal source at about 3 km depth can accommodate most of the deflation occurring in the caldera residual subsidence occurs along the askja fissure swarm suggesting the existence of a deeper source of contraction we interpret this signal in terms of subsidence of the plate boundary

depth determinations of shallow hydrothermal systems by self potential and multi scale wavelet tomography in hydrothermal studies the depth of the hydrothermal system is always required but rarely known via traditional geophysical exploration techniques while previous studies have shown that continuous wavelet transform algorithms applied to self potential data can theoretically determine the depth of the hydrothermal fluids this study uses multi scale wavelet tomography with multiple wavelets field measurements and geophysical models to accurately determine this depth on stromboli waita and masaya volcanoes multi scale wavelet tomography of field measurements gives reproducible depth results supported by independent geophysical measurements and models and accurately locates the main water flow paths at shallow depths unlike other traditional geophysical methods multi scale wavelet tomography using self potential data is a low cost tool to rapidly determine depths of the shallowest hydrothermal structures this approach has the potential to significantly enhance our ability to locate geothermal systems and monitor active volcanoes

rapid response of a hydrologic system to volcanic activity masaya volcano nicaragua hydrologic systems change in response to volcanic activity and in turn may be sensitive indicators of volcanic activity here we investigate the coupled nature of magmatic and hydrologic systems using continuous multichannel time series of soil temperature collected on the flanks of masaya volcano nicaragua one of the most active volcanoes in central america the soil temperatures were measured in a low temperature fumarole field located 3 5 km down the flanks of the volcano analysis of these time series reveals that they respond extremely rapidly on a time scale of minutes to changes in volcanic activity also manifested at the summit vent these rapid temperature changes are caused by increased flow of water vapor through flank fumaroles during volcanism the soil temperature response 5 c is repetitive and complex with as many as 13 pulses during a single volcanic episode analysis of the frequency spectrum of these temperature time series shows that these anomalies are characterized by broad frequency content during volcanic activity they are thus easily distinguished from seasonal trends diurnal variations or individual rainfall events which triggered rapid transient increases in temperature during 5 of events we suggest that the mechanism responsible for the distinctive temperature signals is rapid change in pore pressure in response to magmatism a response that can be enhanced by meteoric water infiltration monitoring of distal fumaroles can therefore provide insight into coupled volcanic hydrologic meteorologic systems and has potential as an inexpensive monitoring tool

rapid fluid disruption a source for self potential anomalies on volcanoes self potential sp anomalies observed above suspected magma reservoirs dikes etc on various volcanoes kilauea hawaii mount unzen japan piton de la fournaise reunion island miyake jima japan result from transient surface electric fields of tens of millivolts per kilometer and generally have a positive polarity these sp anomalies are usually attributed to electrokinetic effects where properties controlling this process are poorly constrained we propose an alternate explanation that contributions to electric fields of correct polarity should be expected from charge generation by fluid vaporization disruption as liquids are vaporized or removed as droplets by gas transport away from hot dike intrusions both charge generation and local increase in electrical resistivity by removal of fluids should occur we report laboratory observations of electric fields in hot rock samples generated by pulses of fluid water through the rock at atmospheric pressure these indicate the relative amplitudes of rapid fluid disruption rfd potentials and electrokinetic potentials to be dramatically different and the signals are opposite in sign above vaporization temperatures rfd effects of positive sign in the direction of gas flow dominate whereas below these temperatures effects of negative sign dominate this suggests that the primary contribution to observed self potential anomalies arises from gas related charge transport processes at temperatures high enough to produce vigorous boiling and vapor transport at lower temperatures the primary contribution is from electrokinetic effects modulated perhaps by changing electrical resistivity and rfd effects from high pressure but low temperature co 2 and so 2 gas flow ripping water molecules from saturated crustal rocks if charge generation is continuous as could well occur above a newly emplaced dike positive static potentials will be set up that could be sustained for many years and the simplest method for identifying these hot active regions would be to identify the sp anomalies they generate

plinian airfall deposits of basaltic composition two widespread scoria fall deposits locally known as the fontana lapilli and san judas formation are demonstrated to have issued from masaya caldera complex nicaragua and to be the first documented plinian airfall deposit of basaltic composition data from maps of median grain size average maximum pyroclast diameter and average maximum diameter of lithic fragments provide conclusive evidence of the plinian nature of both deposits the volume within the 1 mm isopach of the fontana lapilli is 12 km3 3 4 km3 dense rock equivalent and within the san judas formation it is 1 2 km3 0 35 km3 dre theoretical eruption parameters calculated for the fontana lapilli and san judas formation respectively include muzzle velocity 170 m s and 150 m s column height 50 km and 18 km volume eruption rate 2 105 m3 s and 1 104 m3 s and duration 2 h and 10 h these are all similar to the calculated parameters for silicic plinian eruptions basaltic magmas are generally considered incapable of erupting in the manner characteristic of plinian eruptions recognition of them argues that factors other than high viscosity may be of fundamental importance

self potential soil co2 flux and temperature on masaya volcano nicaragua we investigate the spatial relationship between self potential sp soil co sub 2 flux and temperature and the mechanisms that produce sp anomalies on the flanks of masaya volcano nicaragua we measured sp soil co sub 2 fluxes 1 to 5 0 x 10 sup 4 g m sup 2 d sup 1 and temperatures 26 to 80 c within an area surrounding a normal fault adjacent to comalito cinder cone 2002 2003 these variables are well spatially correlated wavelengths of sp anomalies are le 100 m and high horizontal sp gradients flank the region of elevated flux and temperature carbon isotopic compositions of soil co sub 2 delta sup 13 c 3 3 to 1 1 per thousand indicate a deep gas origin given the presence of a deep water table 100 to 150 m high gas flow rates and subsurface temperatures above liquid boiling points we suggest that rapid fluid disruption is primarily responsible for positive sp anomalies here concurrent measurement of sp soil co sub 2 flux and temperature may be a useful tool to monitor intrusive activity

sulfur and halogen gases at masaya caldera complex nicaragua total flux and variations with time santiago crater of masaya caldera complex near managua nicaragua entered a phase of intense magmatic degassing in late 1979 the flux of so2 increased rapidly from a previous average of 380 t d 1 to approximately 1200 t d 1 in february 1980 we report on results of a study of the gas flux and geochemistry conducted as part of a broader interdisciplinary study of the volcano and its impact on the adjacent environment masaya is an unusual volcano because of its low shield like form consistent tholeiitic basaltic composition frequent activity and especially because of the approximately 25 year cycle of major non eruptive degassing crises we have used a combination of techniques involving remote sensing using a correlation spectrometer cospec of the flux of so2 and direct filter sampling of the ratios of the sulfur and halogen gases and aerosols to quantify the flux of all of these species the three stage filters successively trap aerosols halogen acid gases and sulfur dioxide and are analyzed by ion chromatography we have used the ratios of various species with the cospec measurements of the flux of so2 to calculate that the average flux of so2 is 1275 t d 1 hcl is 830 t d 1 hf is about 16 t d 1 and hbr is at most 1 t d 1 h2s and so4 are only minor sulfur species the flux of so2 and hcl represents the largest reported noneruptive sustained volcanic release of these species in the world masaya contributed 7 of the total so2 estimated to have been released by volcanoes worldwide in 1981 the flux of so2 has declined by about 9 each year since 1979 and can be modeled as a first order kinetic rate law with a decay constant of approximately 0 04 yr 1 approximately 2 2 106 tons of so2 has been released between september 1979 and september 1985 the ratio s cl has declined more rapidly than the flux of so2 reflecting the greater solubility of hcl than so2 in the magma the so2 flux at santiago varies more widely within a single day than observed at other degassing volcanoes and may be controlled at least in part by effects of the solid earth tide an area larger than 500 km2 has been severely impacted downwind of the volcano downwind ground level concentrations of so2 of 0 25 ppm are frequently encountered acid rain unusually chloride rich has a ph as low as 2 63 s cl ratios in the gas plume increase downwind probably as a result of the high solubility and dry deposition velocity of hcl preliminary analyses of primary concentrations of sulfur and chlorine in the magma of the present lava lake are used to calculate that at least 1 2 km3 has been completely degassed with respect to sulfur to provide the observed flux this implies that 10 km3 have been degassed by previous crises in the past century and further suggests a magma supply rate of approximately 0 1 km3 yr comparable with kilauea or etna volcanoes during historic time only two lava flows have been extruded implying an unusually low ratio of extrusion intrusion of only 0 0007

a geochemical and geophysical investigation of the hydrothermal complex of masaya volcano nicaragua abstract masaya volcano nicaragua is a persistently active volcano characterized by continuous passive degassing for more than 150 years through the open vent of santiago crater this study applies self potential soil co 2 and ground temperature measurements to highlight the existence of uprising fluids associated to diffuse degassing structures throughout the volcano the diffuse degassing areas are organized in a semi circular pattern and coincide with several visible and inferred surface volcanic structures cones fissure vents and likely consist of a network of buried faults and dykes that respectively channel uprising flow and act as barrier to gravitational groundwater flow water depths have been estimated by multi scale wavelet tomography of the self potential data using wavelets from the poisson kernel family compared to previous water flow models our water depth estimates are shallower and mimic the topography typically less than 150 m below the surface between 2006 and 2010 the depths of rising fluids along the survey profiles remained stable suggesting that hydrothermal activity is in a steady state this stable activity correlates well with the consistency of the volcanic activity expressed at the surface by the continuously passive degassing when compared to previous structural models of the caldera floor it appears that the diffuse degassing structures have an important effect on the path that shallow groundwater follows to reach the laguna de masaya in the eastern part of the caldera the hydrogeological system is therefore more complex than previously published models and our new structural model implies that the flow of shallow groundwater must bypass the intrusions to reach the laguna de masaya furthermore these diffuse degassing structures show clear evidence of activity and must be connected to a shallow magmatic or hydrothermal reservoir beneath the caldera as such the heat budget for masaya must be significantly larger than previously estimated

late cretaceous to early tertiary structural elements of west kalimantan abstract recent geological and geophysical mapping in west kalimantan has allowed the definition of three domains with distinct geological and geophysical characteristics which were imprinted on the region by early cretaceous to eocene convergent tectonic events subduction related granitoids intruded low grade regional metamorphic rocks during the early cretaceous forming the schwaner mountains domain extending east west across the southern and central part of west kalimantan this is flanked in the northwest by a complex sequence of sedimentary and volcanic rocks ranging in age from late carboniferous to cretaceous which comprise the northwest kalimantan domain deformed cretaceous flysch deposits east of the northwest kalimantan domain form an accretionary complex representing the southern continuation of the extensive dominantly tertiary accretionary wedge in sarawak the boundary between the cretaceous accretionary domain and rocks of the northwest kalimantan domain is a fault which we interpret as a transform that marked the western limit of late cretaceous southward subduction beneath west kalimantan growth of the accretionary complex resulted in uplift of a ridge of melange and flysch deposits on which a half graben extensional basin formed and accumulated thick lacustrine deposits a sedimentary basin developed between continental rocks to the south and the emergent ridge comprising accretionary complex rocks and outer shelf deposits to the north in the position of a forearc basin as accretion proceeded the locus of underthrusting migrated northward and a second melange ridge and sedimentary basin developed farther to the north the evidence for accretionary processes and the association of these with sedimentary basin formation supports the hypothesis that an ancient south dipping subduction zone existed in the eastern part of west kalimantan during late cretaceous to early tertiary times

apparent downwind depletion of volcanic so2 flux lessons from masaya volcano nicaragua a series of 707 measurements at masaya in 2005 2006 and 2007 reveals that so2 emissions 15km downwind of the active vent appear to be 33 to 50 less than those measured only 5km from the vent measurements from this and previous studies indicate that dry deposition of sulfur from the plume and conversion of so2 to sulfate aerosols within the plume each may amount to a maximum of 10 loss and are not sufficient to account for the larger apparent loss measured however the so2 measurement site 15km downwind is located on a ridge over which local trade winds and the entrained plume accelerate greater wind speeds cause localized dilution of the plume along the axis of propagation the lower concentrations of so2 measured on the ridge therefore lead to calculations of lower fluxes when calculated at the same plume speed as measurements from only 5km downwind and is responsible for the apparent loss of so2 due to the importance of so2 emission rates with respect to hazard mitigation petrologic studies and sulfur budget calculations measured fluxes of so2 must be as accurate as possible future campaigns to measure so2 flux at masaya and similar volcanoes will require individual plume speed measurements to be taken at each flux measurement site to compensate for dilution and subsequent calculation of lower fluxes this study highlights the importance of a comprehensive understanding of a volcano s interaction with its surroundings especially for low boundary layer volcanoes

hydrothermal system beneath aso volcano as inferred from self potential mapping and resistivity structure abstract we conducted self potential sp surveys sequentially from part to part over the central cones of aso volcano since august 1998 by december 2001 the compiled sp map revealed large sp anomalies on the central cones the main feature of the sp map is a w shaped profile along the ns transect over the central cones it is probable that this characteristic sp profile is produced by the combination of hydrothermal upwelling in the middle and topographic effect a positive anomaly showing a large concentric pattern has appeared after correcting the topographic effect to evaluate this sp anomaly we implemented a numerical code that calculates electric potential produced by arbitrarily positioned current sources and sinks in any three dimensional resistivity structure a layered structure obtained from a time domain electromagnetic tdem field experiment was used for the resistivity model the estimated current source is 300 a being located in a conductive layer around the sea level meanwhile sinks were estimated to sit on a circular area corresponding to the marginal part of the conductive layer water and heat budget study gives a lower limit of water mass transfer from depth to the bottom of the crater lake of nakadake this value was used to estimate the equivalent current in either case of electro kinetic ek mizutani h ishido t 1976 a new interpretation of magnetic field variation associated with the matsushiro earthquakes j geomag geoelectr 28 179 188 or rapid fluid disruption rfd process johnston m j s byerlee j d lockner d 2001 rapid fluid disruption a source for self potential anomalies on volcanoes j geophys res 106 b3 4327 4335 this comparison suggests that the former process is preferable to explain the observed sp anomaly from these results we infer a large scale hydrothermal system beneath the central cones of aso volcano in which the fluid flow initiates from the surrounding area converging to the central vent to transport the heat and materials up to the crater lake of nakadake through a vapor filled conduit

co2 degassing at la solfatara volcano phlegrean fields processes affecting d13c and d18o of soil co2 abstract the soil co2 degassing is affected by processes of isotope exchange and fractionation during transport across the soil which can deeply modify the pristine isotope composition this has been observed in the solfatara volcano upon a field survey of 110 points where the co2 flux was measured together with temperature co2 concentration and oxygen and carbon isotopes within the soil furthermore in some selected sites the measurements were made at different depths in order to analyze vertical gradients oxygen isotope composition appears controlled by exchange with soil water either meteoric or fumarolic condensate due to the fast kinetic of the isotopic equilibrium between co2 and water carbon isotope composition is reliably controlled by transport driven fractionation due to the differences in diffusion coefficients between 13 c 16 o 2 and 12 c 16 o 2 we model the processes affecting co2 transport across the soil in la solfatara volcano by means of the dusty gas model applied to a multicomponent system to evaluate the reciprocal effect on diffusion of involved gases i e 12 c 16 o 2 13 c 16 o 2 n2 and o2 in our case both numerical and simplified analytical solutions of the equations based on the dusty gas model are given the modeling results fit well with the experimental data and put in evidence an isotope fractionation of carbon up to about 4 4 with respect to the source value in the soil gas this fractionation is independent from the entity of the co2 flux and occurs as long as a concentration gradient exists within the soil on these grounds the dusty gas model can be applied to whichever diffusing gas mixture to evaluate the extent of chemical and or isotopic fractionation that can affect ascending gases upon diffusion in any geothermal volcanic or tectonic area

gravity changes and passive so2 degassing at the masaya caldera complex nicaragua an understanding of the mechanisms responsible for persistent volcanism can be acquired through the integration of geophysical and geochemical data sets by interpreting data on micro gravity ground deformation and so2 flux collected at masaya volcano since 1993 it is now clear that the characteristically cyclical nature of the activity is not driven by intrusion of additional magma into the system rather it may be due in large part to the blocking and accumulation of gas by restrictions in the volcano substructure the history of crater collapse and formation of caverns beneath the crater floor would greatly facilitate the trapping and storage of gas in a zone immediately beneath san pedro and the other craters another mechanism that may explain the observed gravity and gas flux variations is the convective overturn of shallow pre existing degassed cooled dense magma that is replaced periodically by lower density hot gas rich magma from depth buoyant gas rich magma rises from depth and is emplaced near the surface resulting in the formation and fluctuation of a low density gas rich layer centred beneath nindiri and santiago craters as this magma vigorously degasses it must cool increase in density and eventually sink five stages of activity have been identified at masaya since 1853 and the most recent data suggest that the system may have been entering another period of reduced degassing in 2000 this type of analysis has important implications for hazard mitigation because periods of intense degassing are associated with poor agricultural yields and reduced quality of life a better understanding of persistent cyclically active volcanoes will allow for more effective planning of urban development and agricultural land use

shallow open system evolution of basaltic magma beneath a subduction zone volcano the masaya caldera complex nicaragua abstract lavas and pyroclastic material from the masaya caldera complex have a number of distinctive geochemical features relative compositional homogeneity low al2o3 and high feo contents a tholeitic differentiation trend and elevated large ion lithophile lil element concentrations e g ba 800 ppm on cmas projections their compositions always fall on or near low pressure cotectics in addition the basalts of masaya have unusually high 87sr 86sr and 10be masaya has exhibited medium term compositional cycles best exhibited by the sawtoothed changes in tio2 and feo mgo there are also a number of longer term compositional changes which are abrupt and generally coincide with caldera formation many of the geochemical characteristics of masaya coupled with a number of volcanological observations indicate masaya is underlain by a large shallow open system magma chamber perhaps on the order of 10 km3 in size although fractional crystallization is a significant magmatic process in masaya s open system chamber magma mixing contamination is equally important magma mixing is necessary to explain the discontinuous stratigraphic changes in magma composition observed at masaya and crustal contamination is necessary to explain their generally elevated 87sr 86sr and lil element concentrations two components therefore have been admixed into the magma chamber of masaya a lil poor basaltic component such has been erupted from the nearby nejapa and granada cinder cones and a lil rich acidic component such has been erupted from the nearby calderas of apoyo and apoyeque admixtures of the former have dominated admixtures of the latter ended with caldera formation ironically open system behavior has exerted fundamental control on the maintenance of relative compositional homogeneity

geophysical and hydrogeological experiments from a shallow hydrothermal system at solfatara volcano campi flegrei italy response to caldera unrest 1 integration of high resolution geophysical and hydrogeological investigations at solfatara volcano campi flegrei italy allowed us to 1 image the shallow and intermediate subsurface to the crater 2 elucidate patterns in the shallow subsurface degassing and 3 refine and upgrade volcano monitoring strategies for this dynamic area our results show that the subsurface to the crater can be divided roughly into two zones a dry outcropping layer overlying a horizon saturated by hydrothermal fluids within this saturated zone intersections of dominant nw and ene striking structural lineaments act as preferential escape conduits for the fluids which generate high microseismic noise amplitudes in the southeastern part of the crater hydrogeological data suggest an uprising of the isotherms below solfatara crater and a marked increment of fluid degassing over the last 40 years sudden variations of both seismic noise level and noise cycling are positively correlated with early stages of ground inflation during the ad 2000 uplift we believe therefore that monitoring of seismic noise can be used for upgrading early warning strategies in this sector of the campi flegrei volcanic system

localization of self potential sources in volcano electric effect with complex continuous wavelet transform and electrical tomography methods for an active volcano 1 this study concerns the fluid flow circulation associated with magmatic intrusion during volcanic eruptions from electrical tomography studies the objective is to localize and characterize the sources responsible for electrical disturbances during a time evolution survey between 1993 and 1999 of an active volcano the piton de la fournaise we have applied a dipolar probability tomography and a multi scale analysis on synthetic and experimental sp data we show the advantage of the complex continuous wavelet transform which allows to obtain directional information from the phase without a priori information on sources in both cases we point out a translation of potential sources through the upper depths during periods preceding a volcanic eruption around specific faults or structural features the set of parameters obtained vertical and horizontal localization multipolar degree and inclination could be taken into account as criteria to define volcanic precursors

identification of sources of potential fields with the continuous wavelet transform complex wavelets and application to aeromagnetic profiles in french guiana a continuous wavelet technique has been recently introduced to analyze potential fields data first we summarize the theory which primarily consists of interpreting potential fields via the properties of the upward continued derivative field using complex wavelets to analyze magnetic data gives an inverse scheme to find the depth and homogeneity degree of local homogeneous sources and the inclination of their magnetization vector this is analytically applied on several local and extended synthetic magnetic sources the application to other potential fields is also discussed then profiles crossing dikes and faults are extracted from the recent high resolution aeromagnetic survey of french guiana and analyzed using complex one dimensional wavelets maps of estimated depth to sources and their magnetization inclination and homogeneity degree are proposed for a region between cayenne and kourou

inferences on the hydrothermal system beneath the resurgent dome in long valley caldera east central california usa from recent pumping tests and geochemical sampling abstract quaternary volcanic unrest has provided heat for episodic hydrothermal circulation in the long valley caldera including the present day hydrothermal system which has been active over the past 40 kyr the most recent period of crustal unrest in this region of east central california began around 1980 and has included periods of intense seismicity and ground deformation uplift totaling more than 0 7 m has been centered on the caldera s resurgent dome and is best modeled by a near vertical ellipsoidal source centered at depths of 6 7 km modeling of both deformation and microgravity data now suggests that 1 there are two inflation sources beneath the caldera a shallower source 7 10 km beneath the resurgent dome and a deeper source 15 km beneath the caldera s south moat and 2 the shallower source may contain components of magmatic brine and gas the long valley exploration well lvew completed in 1998 on the resurgent dome penetrates to a depth of 3 km directly above this shallower source but bottoms in a zone of 100 c fluid with zero vertical thermal gradient although these results preclude extrapolations of temperatures at depths below 3 km other information obtained from flow tests and fluid sampling at this well indicates the presence of magmatic volatiles and fault related permeability within the metamorphic basement rocks underlying the volcanic fill in this paper we present recently acquired data from lvew and compare them with information from other drill holes and thermal springs in long valley to delineate the likely flow paths and fluid system properties under the resurgent dome additional information from mineralogical assemblages in core obtained from fracture zones in lvew documents a previous period of more vigorous and energetic fluid circulation beneath the resurgent dome although this system apparently died off as a result of mineral deposition and cooling and or deepening of magmatic heat sources flow testing and tidal analyses of lvew water level data show that relatively high permeability and strain sensitivity still exist in the steeply dipping principal fracture zone penetrated at a depth of 2 6 km the hydraulic properties of this zone would allow a pressure change induced at distances of several kilometers below the well to be observable within a matter of days this indicates that continuous fluid pressure monitoring in the well could provide direct evidence of future intrusions of magma or high temperature fluids at depths of 5 7 km

a case study of resistivity and self potential signatures of hydrothermal instabilities inferno crater lake waimangu new zealand 1 inferno crater lake waimangu one of the largest hot springs in new zealand displays vigorous cyclic behavior in lake level and temperature it provides a natural small scale laboratory for investigating the geo electrical signature of fluid flows we measured self potential and electrical resistivity to see whether the huge variations of fluid volume approximately 60 000 m3 during a mean cycle period of 40 days could be detected electrical resistivity measurements revealed spectacular changes over time with the medium becoming more conductive as the lake receded this result is consistent with analog models where the vapor phase is replaced by liquid at recession the self potential survey did not detect temporal changes related to fluid movements this can be explained by the ph of the pore water 2 3 which is close to the point of zero charge of silica

relating soil erosion and sediment yield to geomorphic features and erosion processes at the catchment scale in the spanish pre pyrenees erosion and sediment redistribution are important processes in landscape changes in the short and long term in this study the rmmf model of soil erosion and the sedd model of sediment delivery were used to estimate annual soil loss and sediment yield in an ungauged catchment of the spanish pre pyrenees and results were interpreted in the context of the geomorphic features the estana catchment is divided into 15 endorheic sub catchments and there are 17 dolines gullies and slopes were the main erosive geomorphic elements whereas the colluvial alluvial valley floor and doline deposits were depositional elements spatially distributed maps of gross soil erosion sediment delivery ratio sdr and sediment yield sy were generated in a gis severe erosion rates 100 mg ha 1 year 1 were found in gullies whereas mean and maximum erosion rates were very high on slopes developed on keuper facies and high in soils on muschelkalk facies where crops are grown the depositional type geoforms were predicted by the models to have an erosive dynamic those results were consistent with the rates of erosion quantified by 137cs which reflects the significant role of human activities in triggering soil erosion catchment area was positively correlated with erosion rate but negatively correlated with sdr and sy the latter were negatively correlated with the proportion of the surface catchment covered with forests and scrublands the topography of the area influenced the high sdr and sy in the dolines and valley floors near the sinks intra basin stored sediment was 59 2 of the total annual eroded soil in the catchment the combination of the rmmf and sedd models was an appropriate means of assessing the effects of land uses on soil erosion and obtaining a better understanding of the processes that underlie the geomorphic changes occurring in mountainous environments of the mediterranean region

permanent tremor of masaya volcano nicaragua wave field analysis and source location the masaya volcano nicaragua is a basaltic caldera in a subduction zone the permanent source of the volcanic tremor was located inside santiago crater at the lava lake s position and 400 m below the ne rim and therefore corresponds to superficial magma activity we used two tripartite arrays 90 m side one semicircular array r 120 m in 1992 and two semicircular arrays r 60 m and a 2500 m long linear array radiating out from the source and on the flank of the crater in 1993 we used both a cross spectrum method and a correlation method to determine the wave delay time between the reference station and the other stations of an array and to quantify the wave field using the delays therefore by intersecting the back azimuth wave directions from the arrays we could pinpoint the source additionally the correlation coefficients obtained as functions of frequency for the three components of motion confirm the inferred position of the source of tremor the tremor s wave field is composed of comparable quantities of dispersed rayleigh and love surface waves whose phase velocities lie in the ranges 730 1240 m s at 2 hz and 330 550 m s at 6 hz the dispersive phase velocities were inverted to obtain crustal structures with a minimal number of layers the resulting velocity models are similar for the northern and southern parts of the volcano after geometrical spreading corrections q2hz 14 and q3hz 31 were determined along the northern linear array the typical low velocities and low q corresponding to the cone structure and are similar to those of other basaltic volcanoes like puu oo hawaii and klyuchevskoy kamchatka

a total volatile inventory for masaya volcano nicaragua nerc project magma dynamics at persistently degassing basaltic volcanoes a novel approach to linking volcanic gases and magmatic volatiles within a physical model ne f004222 1 and ne f005342 1

sub surface structures and collapse mechanisms of summit pit craters abstract summit pit craters are found in many types of volcanoes and are generally thought to be the product of collapse into an underpressured reservoir caused by magma withdrawal we investigate the mechanisms and structures associated with summit pit crater formation by scaled analogue experiments and make comparisons with natural examples models use a sand plaster mixture as analogue rock over a cylinder of silicone simulating an underpressured magma reservoir experiments are carried out using different roof aspect ratios roof thickness roof width of 0 2 2 they reveal two basic collapse mechanisms dependant on the roof aspect ratio one occurs at low aspect ratios 1 as illustrated by aspect ratios of 0 2 and 1 outward dipping reverse faults initiated at the silicone margins propagates through the entire roof thickness and cause subsidence of a coherent block collapse along the reverse faults is accommodated by marginal flexure of the block and tension fractures at the surface aspect ratio of 0 2 or by the creation of inward dipping normal faults delimiting a terrace aspect ratio of 1 at an aspect ratio of 1 overhanging pit walls are the surface expressions of the reverse faults experiments at high aspect ratio 1 2 reveal a second mechanism in this case collapse occurs by stopping which propagates upwards by a complex pattern of both reverse faults and tension fractures the initial underground collapse is restricted to a zone above the reservoir and creates a cavity with a stable roof above it an intermediate mechanism occurs at aspect ratios of 1 1 1 2 in this case stopping leads to the formation of a cavity with a thin and unstable roof which collapses suddenly the newly formed depression then exhibits overhanging walls surface morphology and structure of natural examples such as the summit pit craters at masaya volcano nicaragua have many of the features created in the models indicating that the internal structural geometry of experiments can be applied to real examples in particular the surface area and depth of the underpressured reservoir can be roughly estimated we present a morphological analysis of summit pit craters at volcanoes such as kilimanjaro tanzania san cristobal telica and masaya nicaragua and ubinas peru and indicate a likely type of subsidence and possible position of the former magma reservoir responsible for collapse in each case

analytic potentials for the forward and inverse modeling of sp anomalies caused by subsurface fluid flow classical interpretation techniques developed primarily for gravity and magnetic data have been adapted to electric self potential sp data from a recently developed wavelet based technique for potential field interpretation we propose a novel approach for the forward and inverse modeling of sp anomalies caused by subsurface fluid flow we use analytic signals and wavelets associated with the electric and fluid flow potentials fluid flow singularities have typical signatures in the wavelet domain sp data that can be used in the inversion scheme for cases where electrokinetics is unambiguously the origin of sp anomalies our interpretation method provides key parameters of the subsurface fluid flow e g position geometry flow rate to illustrate the theory we consider synthetic 2d flows for which analytic solutions exist and actual sp data acquired around the vulcarolo fissure on mount etna that show sources between 3 to 18 m depths