microearthquakes beneath median valley of mid atlantic ridge near 23 n tomography and tectonics data from a microearthquake experiment in the median valley of the mid atlantic ridge near 23 n in 1982 are used to measure earthquake source parameters to determine the laterally heterogeneous seismic velocity structure across the inner floor and to develop a kinematic tectonic model for this portion of the median valley fifty three microearthquakes occurred over a 10 day period beneath the median valley inner floor and eastern rift mountains twenty of 23 well located inner floor epicenters define a line of activity about 17 km long having a strike of n25 e and located near an along axis depression some 300 400 m deeper than surrounding regions earthquakes with well resolved hypocenters generally have focal depths of 4 8 km beneath the seafloor of both the inner floor and the rift mountains the hypocentral locations are robust with respect to plausible lateral variations in seismic velocity structure composite fault plane solutions for inner floor events indicate normal faulting on planes dipping at angles near 45 normal faulting mechanisms although poorly constrained are also indicated for the rift mountain microearthquakes seismic moments approximate fault dimensions and average stress drops for the largest events recorded are 1019 1020 dyn cm 200 400 m and 1 70 bars respectively a twodimensional tomographic inversion of p wave travel time residuals from microearthquakes and local shots indicates a well resolved lateral heterogeneity in crustal velocity structure across the median valley inner floor p wave velocities at 1 5 km depth within a zone less than 10 km wide beneath the central inner floor are lower by several percent than in surrounding regions the most likely explanation for the low velocities is that the region is the site of the most recent local magmatic injection and remains pervasively fractured as a result of rapid hydrothermal quenching of the newly emplaced crustal column by this view the seismic velocity structure at the ridge axis evolves probably by the sealing of cracks and pores within the first few hundred thousand years of crustal accretion consideration of the detailed sea beam bathymetry in this region of the inner floor the characteristics of large earthquakes that the region has experienced during the past 25 years and the results of the microearthquake and tomography analysis suggests that this section of the median valley has been undergoing continued horizontal extension and block faulting without significant crustal injection of magma for at least the past 104 years

the magmatic plumbing system of the askja central volcano iceland as imaged by seismic tomography the magmatic plumbing system beneath askja a volcano in the central icelandic highlands is imaged using local earthquake tomography we use a catalog of more than 1300 earthquakes widely distributed in location and depth to invert for the p wave velocity vp and the vp vs ratio extensive synthetic tests show that the minimum size of any velocity anomaly recovered by the model is 4 km in the upper crust depth 8 km below sea level bsl increasing to 10 km in the lower crust at a depth of 20 km bsl the plumbing system of askja is revealed as a series of high vp vs ratio bodies situated at discrete depths throughout the crust to depths of over 20 km we interpret these to be regions of the crust which currently store melt with melt fractions of 10 the lower crustal bodies are all seismically active suggesting that melt is being actively transported in these regions the main melt storage regions lie beneath askja volcano concentrated at depths of 5 km bsl with a smaller region at 9 km bsl their total volume is 100 km3 using the recorded waveforms we show that there is also likely to be a small highly attenuating magmatic body at a shallower depth of about 2 km bsl

topographically predicted vertical gravity gradient field and its applicability in 3d and 4d microgravimetry etna italy case study some geophysical or geodynamic applications require the use of true vertical gradient of gravity vgg this demand may be associated with reductions of or corrections to observed gravity or its spatiotemporal changes in the absence of in situ measured vgg values the constant value of the theoretical normal free air gradient fag is commonly used we propose an alternative to this practice which may significantly reduce systematic errors associated with the use of constant fag the true vgg appears to be better approximated in areas with prominent and rugged topography such as alpine or some volcanic regions by a value based on the modelled contribution of the topographic masses to the gradient such prediction can be carried out with a digital elevation model dem of sufficient resolution and accuracy here we present the vgg field computed for mt etna italy one of the most active and best monitored volcanoes worldwide to illustrate how strongly the vgg deviates spatially from constant fag the predicted modelled vgg field is verified by in situ observations we also take a look at the sensitivity of the vgg prediction to the resolution and quality of used dems we conclude with discussing the applicability of the topo predicted vgg field in near surface structural and volcanological micro gravimetric studies

magma reflection imaging in krafla iceland using microearthquake sources the details of magma plumbing beneath active volcanoes remain a major challenge in geochemistry geophysics and volcanic hazard evaluation here we apply a relatively novel variation of seismic interferometry which we call virtual reflection seismic profiling vrsp to produce a high resolution image of a known crustal magma body the technique takes advantage of recent advances in both seismic instrumentation dense arrays and seismic analysis seismic interferometry we have applied this technique to data recently acquired at an iconic volcanic system krafla which lies on the mid atlantic ridge as exposed in northern iceland what make this particular site exceptional are encounters with rhyolitic magma in two drillholes k 39 and iceland deep drilling project 1 iddp 1 these known magma bodies represent a unique calibration opportunity for surface geophysical measurements of magma distribution at depth in this study we produced a stacked seismic reflection section by applying common depth point processing techniques to virtual shot gathers derived from interferometry of p waves from microearthquakes generated by tectonic magmatic and or geothermal activity we observe a strong coherent reflection on the seismic section at a travel time corresponding to the depth at which magma was encountered in the iddp 1 wellbore we interpret this reflection to be from magma or magma related fluids additional coherent reflections may correspond to other components of the magma plumbing beneath krafla these results represent a promising new technique for structural imaging with natural sources that can be applied to a wide array of geologic and energy problems that involve natural or induced seismic clusters

spatiotemporal gravity variations to look deep into the southern flank of etna volcano 1 a 14 year long microgravity data set october 1994 to september 2007 collected along a 24 km east west trending profile of 19 stations was analyzed to detect underground mass redistributions related to the volcanic activity involving the southern flank of mt etna italy a multiresolution wavelet analysis was applied to separate the volcano related anomalies from the unwanted components the residual image having both spatial extension and temporal duration evidenced two complete gravity increase decrease cycles mainly affecting the central and eastern stations of the profile the first gravity increase early 1995 to the end of 1996 and decrease end of 1996 to late in 1998 cycle reached a maximum amplitude of approximately 90 gal the second gravity increase mid 1999 to mid 2000 and decrease mid 2000 to early 2004 cycle attained an amplitude of about 80 gal after about 5 years of a persistent negative gravity anomaly a new semicycle started at the end of 2006 and continued during the last survey carried out in september 2007 the density changes modeled over time since 1994 using a quadratic programming algorithm are mainly located at a depth of 2 4 km bsl in a region recognized to be a preferential pathway of magma rising and an intermediate zone of magma storage withdrawal the computed positive mass variations of about 105 109 kg were interpreted as magma accumulation while negative mass changes of about 120 109 kg were associated with either magma drainage or opening of new voids by tectonic stresses within a source volume where tensional earthquakes occurred

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

microearthquakes at intermediate spreading rate ridges the cleft segment megaplume site on the juan de fuca ridge we discuss models for earthquake generation at intermediate spreading rate ridges and present results from an ocean bottom seismograph obs deployment on the cleft segment of the juan de fuca ridge jdfr although the cleft segment is spreading at approximately 6 cm yr an intermediate spreading rate it has no seismic activity detectable by land networks which provide a detection threshold of ml 4 or by northeast pacific underwater hydrophone arrays with a detection threshold of ml 2 to monitor seismicity four obss were deployed at the cleft segment for 11 days during august 1990 these obss detected 34 regional earthquakes associated with adjacent transform faults in addition 10 local microearthquakes ml 0 to 1 were observed over a 10 km segment of the ridge crest four local events were detected sufficiently well by multiple instruments to allow their hypocenters to be determined all these events occurred within the rift valley of the cleft segment spreading axis three events lie directly beneath a linear surface zone of hydrothermal venting this zone has been identified as the site of a seafloor volcanic eruption dike injection and large scale hydrothermal venting or megaplume event that occurred during 1986 and 1987 these earthquakes were located at depths coincident with the dike injection zone a possible mechanism for earthquake production might be tectonic release of stress acquired during the dike injection either along the dike zone or along the bounding faults of the rift valley an alternative mechanism may be stress release associated with hydrothermal activity and cooling of the dike a fourth event was located southeast of the linear hydrothermal venting and dike injection zone at a depth coincident with the spreading center crustal melt zone these observations suggest that the cleft segment may primarily experience seismic events correlated with periods of volcanic intrusion

review of microgravity observations at mt etna a powerful tool to monitor and study active volcanoes microgravity observations at mt etna have been routinely performed as both discrete since 1986 and continuous since 1998 measurements in addition to describing the methodology for acquiring and reducing gravity data from mt etna this paper provides a collection of case studies aimed at demonstrating the potential of microgravity to investigate the plumbing system of an active volcano and detect forerunners to paroxysmal volcanic events for discrete gravity measurements results from 1994 1996 and 2001 are reported during the first period the observed gravity changes are interpreted within the framework of the strombolian activity which occurred from the summit craters gravity changes observed during the first nine months of 2001 are directly related to subsurface mass redistributions which preceded accompanied and followed the july august 2001 flank eruption of mt etna two continuous gravity records are discussed a 16 month october 1998 to february 2000 sequence and a 48 hour 26 28 october 2002 sequence both from a station within a few kilometers of the volcano s summit the 16 month record may be the longest continuous gravity sequence ever acquired at a station very close to the summit zone of an active volcano by cross analyzing it with contemporaneous discrete observations along a summit profile of stations both the geometry of a buried source and its time evolution can be investigated the shorter continuous sequence encompasses the onset of an eruption from a location only 1 5 km from the gravity station this gravity record is useful for establishing constraints on the characteristics of the intrusive mechanism leading to the eruption in particular the observed gravity anomaly indicates that the magma intrusion occurred passively within a fracture system opened by external forces

very shallow earthquakes and brittle deformation in active volcanic areas the eatnean region as an example abstract some aspects of the stress field acting in active volcanic areas were investigated by studying the relationship between surficial seismicity h 1 2 km and the brittle deformation pattern of the etna volcano the magmatic composition and structural character of mt etna are described together with its geodynamic regional context after a discussion of the problem of the state of stress in active volcanoes and a definition of very shallow seismicity the high level of surficial seismicity the morphology of the sedimentary basement and the kinematic behaviour of the faults suggest that the eastern flank of mt etna is a shallow seismotectonic domain structurally decoupled from the rest of the volcanic body this flank shows a tendency to slide in an eastern direction and is bound to the north by a left lateral oblique slip normal fault trending e w and in the south by an arcuate fault system with right lateral features right lateral oblique slip normal faults

earthquake locations and three dimensional crustal structure in the coyote lake area central california previous work on the simultaneous inversion method has been improved and extended to incorporate iterative solution for earthquake locations and laterally heterogeneous structure approximate ray tracing and parameter separation are important elements of the improved method application of the method to p wave arrival time data recorded by stations of the u s geological survey central california network yields a three dimensional model for the velocity structure of the upper crust in an area encompassing the rupture zone of the coyote lake earthquake of august 1979 very strong correlations between the velocity model and the geology and gravity and magnetic anomalies are observed improved estimates of the locations of earthquakes in the study area are also determined the relocation of explosions indicates epicentral accuracies of the order of a kilometer or better based on the revised hypocentral locations it is concluded that the san andreas fault is vertical in this area with no actual offset between the epicenters and the fault trace in contrast the calaveras has two or more active fault surfaces one nearly vertical and another dipping 75 to the northeast

structure and evolution of the hengill grensdalur volcanic complex iceland geology geophysics and seismic tomography recent geological and geophysical research indicates the presence of three volcanic systems in the hengill grensdalur area and progressive westerly migration of the accretionary plate boundary the hengill system comprises the active hengill central volcano and a nne trending fissure swarm that is the present locus of crustal accretion in the area extinct volcanic systems are identified with the mountain hromundartindur and the grensdalur area and associated nne trending zones the grensdalur system was formerly the locus of accretion in the area a widespread geothermal area encompasses the whole area and is fueled by at least three distinct heat sources associated with the three volcanic systems a tomographic study of the upper 5 km of crust using local earthquakes imaged three bodies with velocities up to 15 higher than the average background velocities and volumes of several tens of cubic kilometers one of these underlies the grensdalur volcano and a second underlies the olkelduhals area within the hromundartindur system these are interpreted as intrusions that are the solidified magma reservoirs of their respective volcanic systems and the heat sources of those parts of the geothermal area above them the third high velocity body underlies the extinct basalt shield husmuli which is not associated with geothermal resources that body is interpreted as a cold intrusion that is the frozen magma conduit that fed the surface eruptive site a low velocity body with a volume of a few cubic kilometers was imaged in the depth range 2 4 km beneath the northern part of the presently active hengill central volcano this volume may contain partial melt and represent the heat source or part of the heat source fueling the hengill field heat balance calculations show that subsurface magmatism continued in the extinct grensdalur volcano long after volcanic activity and crustal accretion migrated from it to the hengill system

seismicity of the askja and barðarbunga volcanic systems of iceland 2009 2015 abstract a large seismic network deployed in the icelandic highlands recorded 100 000 earthquakes from 2009 to 2015 we develop a local magnitude scale appropriate for use in central iceland which is similar to the scale used by the iceland meteorological office using this large catalogue of earthquakes we analyze the spatial and temporal changes in seismicity rates and b values in microearthquakes recorded from the usually ductile lower crust we find that b values are high reflecting the presence of high thermal gradients and low stresses driving seismicity associated with the movement of melt in contrast b values in the upper crust are variable low b values indicative of a high stress environment are observed during seismic swarms such as those around mt her ubrei and around bar arbunga caldera a persistently seismically active area around a geothermal area within askja caldera has a b value around 1 but has a strong annual cycle of seismicity we attribute the annual cycle to varying load from the snow cover modulating the seismicity seismicity driven by the intrusion of a large dyke has a b value well above 1 driven by the high pore fluid pressures and thermal gradients around the dyke

dynamics of the eastern flank of mt etna volcano italy investigated by a dense gps network abstract mount etna has developed at the intersection of two regional tectonic lineaments the nnw sse trending hybleo maltese escarpment which separates the thick inland continental crust of the african platform from the ionian mesozoic oceanic crust and the ne sw messina fiumefreddo fault that marks a rift zone between south calabria and north eastern sicily extending as far as the mt etna area all tectonic features affect with outstanding surface features the eastern side of the volcano the eastern flank of the volcano is affected by a long term motion toward ese in 1997 in order to increase the detail of the ground deformation pattern on the lower eastern flank of mt etna a new gps network the ionica network was installed on this sector of the volcano this gps network consists of 24 stations and covers the lower eastern flank of the volcano from the town of catania to taormina and from the coastline up to an altitude of about 1300 m all the new stations consist in self centring benchmarks this kind of benchmark allows all station set up errors to be avoided before the merging of the ionica network to the frame of the global gps network of mt etna in june 2001 three surveys were carried out on this network in september 1997 august 1998 and january 2001 from the ground deformation pattern it is possible to distinguish two different sectors showing different characteristics of deformation the southern part of the network shows a more uniform distribution of the vertical motion with a mean se ward horizontal component while the northern one shows an heterogeneous vertical motion with a ese ward horizontal component furthermore a higher velocity is detected between 1997 and 1998 due to the additional stress induced by a shallow intrusion on the nw flank of the volcano the model resulting from data inversions defines a wide sliding plane beneath the entire eastern flank of the volcano with a low dip angle the expected velocity vectors fit well the observed ones even if the measured velocities are still quite higher than expected at lowermost stations the vertical inclination of the velocity vectors measured during the 1998 2001 period gradually decreases from west to east suggesting a sort of rotational movement of the south eastern flank interrupted by some anomalous vectors on the lower part that show higher vertical velocities these anomalies being located on a wedge defined by the intersection of the main nnw sse and ne sw fault systems and near the timpe faults are probably due to the activity of the vertical faults cutting the lower eastern flank of mt etna stations lying on the hanging wall and on the footwall of the timpe fault system are affected by similar horizontal displacements meaning that these structures are moving eastwards together with the sliding flank this evidence suggests that the timpe faults are probably second order structures with respect to the detachment surface these results depict a structural framework of the eastern flank of mt etna in which the low angle dislocation can be considered as a first order approximation of an actual listric plane and the current active part of the timpe fault system is confined above the detachment surface

mountain front development by folding and crustal thickening in the internal zone of the betic cordillera alboran sea boundary the boundary between the alboran sea and betic cordillera is a good example of a fold related mountain front in the internal zone of an alpine mountain range since the late miocene nnw sse convergence between the eurasian and african plates has produced shortening and related orthogonal extension to improve the characterisation of the geometry of the deep structure in the region and to establish the recent tectonic evolution of the mountain front well logs and newly acquired geophysical data multichannel reflection seismic and gravimetric surveys have been interpreted and integrated with available surface data the most marked tectonic structure corresponds to large antiforms and synforms of ene wsw trend which are related to mountain ranges and basins respectively the fold belt continues toward the northern continental shelf of the alboran sea the fold vergence is generally northwards and its amplitude decreases progressively towards sse until disappearring in a sharp boundary where the reflectors are undeformed the deep geometry suggests that fold growth started during upper tortonian times and continued its activity up to pliocene or even quaternary times the nnw sse compression produces crustal thickening and a regional and progressive southwards emersion the location of main present day deformation fronts in the internal zones contrasts with classical models where the deformation progresses towards the frontal part of external zones of cordilleras in addition this fold related deformation mountain front has features different from fault related fronts as it does not show a sharp boundary and folds that determine rectilinear mountain boundaries decrease progressively in amplitude or in wavelength up to undeformed areas

volcanomagnetic evidence of the magmatic intrusion on 13thmay 2008 etna eruption 1 during the onset of 2008 volcanic crisis at mt etna the near real time magnetic data provided a continuous updating of the volcano activity state on the northern flank on the morning of 13th may 2008 significant local magnetic field changes marked the resumption of the eruptive activity characterized by the opening of a fracture field on the northern flank and an eruptive fissure in the valle del bove in agreement with the northward propagation of seismic events magnetic signals at 5 stations in the summit area revealed a nearly nnw sse oriented magmatic intrusion which started at about 9 00 gmt propagated northward for about 2 km and stopped at 14 00 gmt before reaching the north east rift magnetic variations with amplitude ranging between 1 8 nt and 6 5 nt are consistent with those calculated from piezomagnetic models where stress induced changes in rock magnetization are produced by the magmatic intrusion

modelling of ground deformation and gravity fields using finite element method an application to etna volcano summary elastic finite element models are applied to investigate the effects of topography and medium heterogeneities on the surface deformation and the gravity field produced by volcanic pressure sources changes in the gravity field cannot be interpreted only in terms of gain of mass disregarding the ground deformation of the rocks surrounding the source contributions to gravity changes depend also on surface and subsurface mass redistribution driven by dilation of the volcanic source both ground deformation and gravity changes were firstly evaluated by solving a coupled axisymmetric problem to estimate the effects of topography and medium heterogeneities numerical results show significant discrepancies in the ground deformation and gravity field compared to those predicted by analytical solutions which disregard topography elastic heterogeneities and density subsurface structures with this in mind we reviewed the expected gravity changes accompanying the 1993 1997 inflation phase on mt etna by setting up a fully 3 d finite element model in which we used the real topography to include the geometry and seismic tomography to infer the crustal heterogeneities the inflation phase was clearly detected by different geodetic techniques edm gps sar and levelling data that showed a uniform expansion of the overall volcano edifice when the gravity data are integrated with ground deformation data and a coupled fem modelling was solved a mass intrusion could have occurred at depth to justify both ground deformation and gravity observations

a comparison on the accumulation characteristics of 7be and 137cs in lake sediments and surface soils in western yunnan and central guizhou china abstract the uplifting of the himalayas has resulted in the development of a three tiered landform in southern china the yunnan guizhou plateau is the middle tier of this landform on the eastern slope of the himalayas lake hongfeng and lake baihua in central guizhou are about 1000 km away from lake erhai in western yunnan sediment cores were collected from lakes erhai and lugu in west yunnan and lakes hongfeng and baihua in central guizhou along with 28 surface soil cores from the watersheds of these lakes the accumulation characteristics of 7xxxx be and 137yyyy cs show that 1 137 cs activities in the soil cores of central guizhou were higher than that in the western yunnan the activities and maximum penetrative depth of 7 be were similar for the two regions the activity ratio of 7 be 137 cs in surface soil reached 100 1000 in western yunnan but only 10 100 for central guizhou region 2 7 be inventories in soil cores at most sampling sites of central guizhou were low reflecting severe soil erosion 7 be inventories in the soil cores of lake lugu watershed in summer autumn were higher than that in lake hongfeng baihua watershed and also higher than that in lake erhai watershed this could be related to high 7 be precipitation in summer and altitude 3 7 be inventories in sediment cores of lake erhai lake lugu and lake baihua were 237 73 322 19 and 783 44 bq m 2 respectively based on activity values that were corrected to deposition prior to 1986 137 cs inventories in these lakes were 519 26 937 13 and 3704 56 bq m 2 respectively 4 modeling indicated that 7 be and 137 cs accumulations in lake hongfeng and lake baihua were dominated by watershed erosion and controlled by the retention factor in lake erhai and lake lugu but the 137 cs inventory ratios obtained in the sediment core and from direct atmospheric fallout were similar and the ratios were smaller for 7 be 5 7 be inventories from direct atmospheric fallout in lake erhai lake lugu and lake baihua were 0 07 0 02 0 29 0 02 and 0 08 0 01 bq m 2 respectively prior to 1986 137 cs inventories were 0 11 0 01 0 22 0 01 and 0 37 0 01 bq m 2 respectively showing that 137 cs deposition has a regional difference this phenomenon may reflect that uplift of the himalayas has a screening effect on the fallout of globally distributed atmospheric pollutants in western yunnan china

magma bright spots mapped beneath krafla iceland using rvsp imaging of reflected waves from microearthquakes abstract the geometry and distribution of magma in the crust remain controversial topics with recent studies questioning the role of large magma chambers in this investigation high resolution 3d reflection images of crustal discontinuities beneath the krafla geothermal field in northern iceland were generated by applying vertical seismic profiling vsp techniques adapted from reflection seismology to microearthquake data exceptionally large amplitude reflections bright spots at a depth of 2 1 km correlate with rhyolitic magma encountered in the iddp 1 borehole although similarly bright reflectors at about 4 km correspond in depth to the top of an inferred magma chamber from previous seismic studies the scattered reflectivity that persists beneath this deeper reflector argues for a distributed magma system rather than a large feeder chamber

magma storage eruptive activity and flank instability inferences from ground deformation and gravity changes during the 1993 2000 recharging of mt etna volcano abstract a long recharging period characterized mount etna volcano during 1993 2000 before the main explosive effusive 2001 and 2002 2003 flank eruptions the joint analysis of ground deformation and gravity data over the recharge period reveals that different phenomena occurred within etna s plumbing system and clearly indicates two phases spanning 1993 1997 and 1997 2000 respectively the first phase was characterized by magma storage and accumulation at an intermediate depth 2 6 km below sea level which provoked overall inflation and positive gravity changes during the second phase the magma started to rise and intrude at shallower levels favoring the movement of the unstable eastern flank which accelerated its sliding toward the east the shallower magma accumulation also caused the gas exsolution associated with increasing explosive activity at the summit craters negative gravity changes were detected during this phase the gravity measurements independent of the same result obtained by geochemical studies suggest that only 20 30 of the magma volumes supplied in the plumbing system were then erupted the complex dynamics of rising magma beneath mount etna makes ground deformation and gravity measurements complementary being able to detect different effects of magma emplacements beneath the surface our results also highlight how the joint use of ground deformation and gravity observations may be crucial in identifying the nature and rate of an impending period of volcanic eruptions

local tomography and focal mechanisms in the south western alps comparison of methods and tectonic implications abstract we investigate how focal solutions and hypocenter locations may depend on the ray tracing algorithm and the strategy of velocity inversion using arrival times from a temporary seismological network in the south western alps a local earthquake tomography has been performed by paul et al paul a cattaneo m thouvenot f spallarossa d bethoux n and frechet j 2001 a three dimensional crustal velocity model of the south western alps from local earthquake tomography j geophys res 106 19367 19390 with the method developed by thurber thurber c h 1993 local earthquake tomography velocity and vp vs theory in seismic tomography theory and practice iyer h m and irahara eds chapman and hall new york 563 583 another inversion of the same data set is performed here using a different tomography code relying on a shooting paraxial method and cubic interpolation of velocities the resulting images display the same main features although thurber s code appears to be more robust in regions with scarce ray coverage and strong velocity contrasts concerning hypocenter location in piemont units one major result is the concentration of hypocenters at the boundary between the mantle wedge of the ivrea body and the european crust forty six focal mechanisms are shown that were computed using both the take off angles in the minimum 1 d model and in the 3 d velocity structures resulting from the two inversions the sets of focal solutions are very similar proving the reliability and the coherency of the focal solutions the widespread extension in the core of the western alps is confirmed whereas a few compressive solutions are found east of the piemont units these results constrain the sharp change of stress tensor and evidence a decoupling of strain beneath the east of dora maira massif up to beneath the north of argentera massif on a geodynamical point of view seismicity and focal mechanism distribution are compatible with the present day models published for the western alps where the major feature is the lithospheric thickening schmid s m and kissling e 2000 the arc of the western alps in the light of geophysical data on deep crustal structure tectonics 19 62 85 implying widespread extension in the core of the western alps sue c thouvenot f frechet j and tricart p 1999 widespread extension in the core of the western alps revealed by earthquake analysis j geophys res 104 25611 25622 however the existence of compressive events dealing at depth with the boundary of ivrea body allows to postulate that this geological structure is still tectonically active even if field work has not shown this so far the insubric line appears to extend toward the south at depth as a blind fault and to play a key role in the dynamics of the south western alps

melt inclusion record of the conditions of ascent degassing and extrusion of volatile rich alkali basalt during the powerful 2002 flank eruption of mount etna italy 1 two unusual highly explosive flank eruptions succeeded on mount etna in july august 2001 and in october 2002 to january 2003 raising the possibility of changing magmatic conditions here we decipher the origin and mechanisms of the second eruption from the composition and volatile h2o co2 s cl content of olivine hosted melt inclusions in explosive products from its south flank vents our results demonstrate that powerful lava fountains and ash columns at the eruption onset were sustained by closed system ascent of a batch of primitive volatile rich 4 wt basaltic magma that rose from 10 km depth below sea level bsl and suddenly extruded through 2001 fractures maintained opened by eastward flank spreading this magma the most primitive for 240 years probably represents the alkali rich parental end member responsible for etna lavas evolution since the early 1970s few of it was directly extruded at the eruption onset but its input likely pressurized the shallow plumbing system several weeks before the eruption this latter was subsequently fed by the extrusion and degassing of larger amounts of the same but slightly more evolved magma that were ponding at 6 4 km bsl in agreement with seismic data and with the lack of preeruptive so2 accumulation above the initial depth of sulphur exsolution 3 km bsl we find that while ponding this magma was flushed and dehydrated by a co2 rich gas phase of deeper derivation a process that may commonly affect the plumbing system of etna and other alkali basaltic volcanoes

earthquakes stress and strain along an obliquely divergent plate boundary reykjanes peninsula southwest iceland earthquakes stress and strain along an obliquely divergent plate boundary the reykjanes peninsula southwest iceland

the katla volcanic system imaged using local earthquakes recorded with a temporary seismic network katla is one of the most active subglacial volcanoes in iceland a temporary seismic network was operated on and around katla for 2 5 years a subset of similar to 800 analyzed local earthquakes cl

focal mechanisms and size distribution of earthquakes beneath the krafla central volcano ne iceland seismicity was monitored beneath the krafla central volcano ne iceland between 2009 and 2012 during a period of volcanic quiescence when most earthquakes occured within the shallow geothermal field the highest concentration of earthquakes is located close to the rock melt transition zone as the iddp 1 wellbore suggests and decays quickly at greater depths we recorded multiple swarms of microearthquakes which coincide often with periods of changes in geothermal field operations and found that about one third of the total number of earthquakes are repeating events the event size distribution evaluated within the central caldera indicates average crustal values with b 0 79 0 04 no significant spatial b value contrasts are resolved within the geothermal field nor in the vicinity of the drilled melt besides the seismicity analysis focal mechanisms are calculated for 342 events most of these short period events have source radiation patterns consistent with double couple dc mechanisms a few events are attributed to non shear faulting mechanisms with geothermal fluids likely playing an important role in their source processes diverse faulting styles are inferred from dc events but normal faulting prevails in the central caldera the best fitting compressional and tensional axes of dc mechanisms are interpreted in terms of the principal stress or deformation rate orientations across the plate boundary rift maximum compressive stress directions are near vertically aligned in different study volumes as expected in an extensional tectonic setting beneath the natural geothermal fields the least compressive stress axis is found to align with the regional spreading direction in the main geothermal field both horizontal stresses appear to have similar magnitudes causing a diversity of focal mechanisms

induced seismicity in the geysers geothermal area california a simultaneous inversion for hypocenters velocities and station delays was performed from the p arrival times of 55 microearthquakes and four explosions distributed throughout a 90 by 100 km region centered at the geysers geothermal area in northern california the resulting one dimensional layered model shows velocity increasing with depth from 4 4 km s at the surface to 5 9 km s at 8 km and deeper we relocated 7215 earthquakes which occurred between may 1975 and february 1982 using the new velocity model these relocations show a significant spatial clustering near production wells in the steam field that was not apparent in earlier studies we infer from the spatial and temporal pattern of seismicity that the seismicity at the geysers is induced by geothermal production activities with the expansion of power production by approximately 70 in 1979 1980 seismic activity has developed near the new production areas seismicity has also expanded northwestward of the present production area although there are no active steam wells in this area statistical cross correlations between the number of earthquakes per month and both steam withdrawal per month and injection volume per month were calculated for the wells studied we find no consistent pattern of correlation between injection and seismicity likewise we find little or no correlation between steam withdrawal and seismicity for steam wells in production longer than 7 years two mechanisms remain plausible for inducing seismicity at the geysers volumetric contraction due to mass withdrawal may perturb the stress field and cause faulting in reservoir rock already near failure due to the regional stress field alternatively aseismic deformation due to regional tectonic stress may be converted to stick slip seismic deformation due to an increase of coefficient of friction along fault surfaces such as could occur from exsolution of dissolved silica into fractures neither mechanism would be likely to show strong correlation between production and seismicity rates however we expect seismicity to continue in the geysers and for seismicity to appear in areas where new production occurs

spatiotemporal gravity changes on volcanoes assessing the importance of topography 1 mass redistribution and deformation cause spatiotemporal gravity changes in active volcano areas the ability of the gravity measurements to detect subsurface magma movement as a precursor to volcanic eruptions is greatly enhanced if gravity changes are analyzed and modeled jointly with ground deformation data one effective tool for this analysis is provided by the gravity and elevation change ratio the proposed numerical formulation allows to simulate a wide range of pressures and mass loads however the assessment of gravity changes is especially important when they occur without measurable ground deformation in such a case it is very difficult to relate the gravity uplift ratio to changes in subsurface mass movements by using classical models in this work we study the role played by pressure and mass sources in the interpretation of observed gravity uplift ratio when deformation is negligible in volcanic areas associated with rough topographic relief

the krafla spreading segment iceland 1 three dimensional crustal structure and the spatial and temporal distribution of local earthquakes the geothermal seismicity of the krafla volcanic system ne iceland was monitored for 3 months in 1985 using a dense local seismometer network the seismicity was continuous and the spatial and temporal distributions were roughly known prior to monitoring the instruments could thereby be deployed in a well positioned array a total of 489 locatable events were recorded within the network and 1771 arrival times were inverted to calculate the three dimensional p wave velocity structure and hypocentral locations low velocity volumes were imaged beneath the krafla and namafjall geothermal areas and indicate zones of hydrothermal alteration high velocity bodies beneath the krafla caldera rim are interpreted as gabbroic intrusions using a three dimensional velocity structure instead of a refraction based one dimensional model to locate the hypocenters significantly improves their location quality and illustrates the shortcomings of using refraction based models to locate earthquakes in local anomalous areas seismic activity was concentrated within the krafla and namafjall geothermal areas and in a narrow zone where dike injections had occurred 8 and 5 years earlier the activity occurred in the depth range 0 3 km the seismic rate for the whole area was one magnitude 3 2 event per year and the b value was 0 77 0 10 most of the seismicity appears to result from geothermal processes in the manner proposed for other icelandic areas foulger and long 1984 foulger 1988b seismicity directly beneath the bjarnarflag well field within the namafjall area is probably induced partly by geothermal exploitation considerable seismicity also occurred immediately below leirhnjukur a site of intense geothermal activity in the center of the krafla caldera that overlies a roof pendant in the magma chamber below this volume of high seismicity is probably highly fractured and may provide a conduit for magma escaping from the magma chamber

ground deformation modeling of flank dynamics prior to the 2002 eruption of mt etna on 22 september 2002 1 month before the beginning of the flank eruption on the ne rift an m 3 7 earthquake struck the northeastern part of mt etna on the westernmost part of the pernicana fault in order to investigate the ground deformation pattern associated with this event a multi disciplinary approach is presented here just after the earthquake specific gps surveys were carried out on two small sub networks aimed at monitoring the eastern part of the pernicana fault and some baselines belonging to the northeastern edm monitoring network of mt etna were measured the leveling route on the northeastern flank of the volcano was also surveyed furthermore an investigation using sar interferometry was performed and also the continuous tilt data recorded at a high precision sensor close to the epicenter were analyzed to constrain the coseismic deformation the results of the geodetic surveys show a ground deformation pattern that affects the entire northeastern flank of the volcano clearly shaped by the pernicana fault but too strong and wide to be related only to an m 3 7 earthquake leveling and dinsar data highlight a local strong subsidence up to 7 cm close to the pernicana fault significant displacements up to 2 cm were also detected on the upper part of the ne rift and in the summit craters area while the displacements decrease at lower altitude suggesting that the dislocation did not continue further eastward three dimensional gps data inversions have been attempted in order to model the ground deformation source and its relationship with the volcano plumbing system the model has also been constrained by vertical displacements measured by the leveling survey and by the deformation map obtained by sar interferometry

magma uprising and flank dynamics on mount etna volcano studied using gps data 1994 1995 1 ground deformation occurring on mount etna from 1994 to 1995 is analyzed in this paper this period was characterized by intense volcanic activity at the four summit craters with frequent strombolian activity fire fountains and emplacement of several new lava flows four gps surveys were carried out during this time two as routinely planned each year and an additional two in 1995 to acquire more data to follow the activity at the ne crater the comparisons between gps surveys are reported in terms of horizontal and vertical displacements of each station and in terms of areal dilatation and principal strain axes during the period considered in this work a trend of increasing areal dilatation of the volcano at a rate of about 5 strain yr was recognized it was briefly interrupted by a small contraction about 2 strain in the autumn of 1995 when volcanic activity at the summit craters began in detail the strain distribution of the network is analyzed it allows the detection of areas showing anomalous behavior such as the summit zone or the pernicana fault inversions of the ground displacement vectors have been performed by appropriately combining numerical and analytical approaches results of the inversions suggest structures defining an eastward and southward sliding of the eastern and southeastern sectors of mount etna

feeding system and magma storage beneath mt etna as revealed by recent inflation deflation cycles 1 ground deformation at mt etna detected by three gps surveys carried out in 2004 2005 and 2006 is analyzed the data set encompasses the 2004 2005 eruptions and foreruns those of 2006 a wide deflation of the entire volcano was detected from 2004 to 2005 accompanying the 2004 2005 eruption conversely an evident inflation phase from 2005 to 2006 followed this eruption and preceded the 2006 one in both cases the deflation inflation cycle was accompanied by a continuous seaward motion of the eastern flank we inverted both data sets 2004 2005 deflation and 2005 2006 inflation using an optimization algorithm based on the genetic algorithm ga in order to detect the ground deformation sources the wide contraction measured during the eruption reveals the drainage of a sill shaped magma reservoir located by data inversions at a depth of about 4 5 km b s l the pressurizing source modeled for the 2005 2006 time interval indicates a refilling of the shallower near vertical plumbing system of the volcano this could indicate a change in the geometry of the feeding system active after the 2004 2005 eruption with a new and shallower magma storage that could have enabled the resumption of volcanic activity that was observed at summit craters in 2006 these results improve the imaging of the plumbing system of mt etna volcano

lucky strike seamount implications for the emplacement and rifting of segment centered volcanoes at slow spreading mid ocean ridges the history of emplacement tectonic evolution and dismemberment of a central volcano within the rift valley of the slow spreading mid atlantic ridge at the lucky strike segment is deduced using near bottom sidescan sonar imagery and visual observations volcano emplacement is rapid 1 myr associated with focused eruptions and with effusion rates feeding lava flows that bury tectonic features developed prior to and during volcano construction this volcanic phase likely requires efficient melt pooling and a long lived crustal magma chamber as a melt source a reduction in melt supply triggers formation of an axial graben rifting the central volcano and the onset of seafloor spreading may eventually split it at lucky strike this results in two modes of crustal construction eruptions and tectonic activity focus at a narrow graben that bisects the central volcano and contains the youngest lava flows accumulating a thick layer of extrusives away from the volcano summit deformation and volcanic emplacement is distributed throughout the rift valley floor lacking a clear locus of accretion and deformation volcanic emplacement on the rift floor is characterized by axial volcanic ridges fed by dikes that propagate from the central axial magma chamber the mode of rapid volcano construction and subsequent rifting observed at the lucky strike seamount is common at other central volcanoes along the global mid ocean ridge system

importance of gravitational spreading in the tectonic and volcanic evolution of mount etna the interaction between gravity and thermal effects largely determines the structural and magmatic evolution of volcanic constructs at scales spanning several orders of magnitude from small cones to the oceanic crust1 3 although gravitational spreading is a direct consequence of this interaction and a fundamental process in volcano growth it is rarely recognized as such here we describe a striking example of this process at etna volcano in italy the volcano and its clay rich substratum are slowly spreading towards the east and south driven by gravity spreading produces extensional structures in the summit region and contpressional structures at the base of the volcano eastward movement of the volcanic edifice over a stationary magma supply may be the cause of an apparent westward migration of volcanic activity as gravitational spreading seems to control the location and magnitude of shallow seismicity and flank eruptions an appreciation of its effects could become an essential element of future volcanic hazard evaluation our model proposed here for etna may also be relevant to a reinterpretation of the geological history of a number of other volcanoes

hengill triple junction sw iceland 1 tectonic structure and the spatial and temporal distribution of local earthquakes the hengill area is an unstable ridge ridge transform triple junction in sw iceland it contains the active central volcano hengill associated with the present accretionary zone and the extinct grensdalur central volcano associated with an accretionary zone that became inactive at about 0 7 m y when the locus of spreading migrated about 5 km w to the hengill zone the dominant tectonic trend of the area is n25 e parallel to the accretionary zones but transverse tectonic topographic features have also developed in the neighborhood of the two central volcanoes these result from local modification of the regional stress field associated with the spreading by radially symmetric stress fields associated with the volcanoes and the migration of volcanism a double high temperature geothermal system mirrors the double volcanic system the transform branch of the triple junction is represented by an ew striking zone of historic destructive earthquakes in the s of the area a seismological study was conducted with the aims of studying the tectonic structure and evaluating the passive seismic method as a geothermal prospecting tool the seismicity may be broadly divided into two groups first infrequent intense episodes of crustal movement occur that are associated with the release of tectonic stress along the plate boundary second continuous small magnitude activity occurs on a day to day basis this activity is mostly associated with the extinct grensdalur central volcano not the present plate boundary statistically significant temporal variations in b accompanied a swarm in the transform zone the continuous small magnitude activity may be geothermal in origin releasing thermal stress and not associated with plate boundary tectonics

toward a realistic deformation model of the 2008 magmatic intrusion at etna from combined dinsar and gps observations abstract the combination of i dinsar data capable of observing deformation pattern at a spatial resolution unachievable with other sparse geodetic measurements ii continuous gps data able to provide temporal constraints on source evolution and iii numerical modeling procedures appropriate to consider a non uniform opening distribution of a source embedded in a 3d heterogeneous medium allowed us to infer a complex and realistic deformation model of the magmatic intrusion that occurred in the northern flank of etna on 13 may 2008 numerical modeling of ground deformation data defines a near vertical dyke intruded for 2 5 km starting from a depth of 1400 m asl right below the summit craters and reaching shallow crust level in the northern flank from the estimated opening distribution of the propagating magma filled crack which reached a maximum value of about 2 m a volumetric expansion of crustal rocks of about 5 3 106 m3 was obtained also we clarified the temporal evolution of the northward magmatic intrusion which lasted just over 5 h with an initial magma propagation velocity of about 1 2 km h and decreased to about 0 24 km h as the driving pressure lowered due to the effusive activity started at southern vents

faroe iceland ridge experiment 2 crustal structure of the krafla central volcano the seismic velocity structure of the krafla central volcano is characterized by large variations in compressional velocity a 40 km wide high velocity dome extends from the lower crust 11 14 km depth beneath the volcano narrowing upward a magma chamber sits at its top near 3 km depth it is defined by both 0 2 0 3 s compressional wave delays and shear wave shadowing to be 2 3 km n s 8 10 km e w and 0 7 1 8 km thick the near surface structure uppermost 2 5 km of the krafla caldera is approximately flat lying with only minor lateral heterogeneities the crust beneath the magma chamber has low shear wave attenuation and anomalously high compressional and shear wave velocities shear waves reflected from a 19 km deep moho are clearly visible for some paths through the crustal volume below the magma chamber even though the more shallow diving s waves are severely attenuated the midcrust beneath the shallow magma chamber cannot contain partial melt or even be at near solidus temperatures the krafla central volcano plays a major role in crustal genesis along the plate boundary the high velocity dome in our view represents crust generated in and around the magma chamber which has subsequently been advected to greater depths

seismic detection of the summit magma complex of kilauea volcano hawaii application of simultaneous inversion of seismic p wave arrival time data to the investigation of the crust beneath kilauea volcano yields a detailed picture of the volcano s heterogeneous structure zones of anomalously high seismic velocity are found associated with the volcano s rift zones a low velocity zone at shallow depth directly beneath the caldera coincides with an aseismic region interpreted as being the locus of kilauea s summit magma complex

three dimensional seismic velocity structure beneath aso volcano kyushu japan tomographic results for p and s wave velocity structure beneath the active aso volcano kyushu japan using 800 well recorded earthquakes and ten shots recorded by an eight station seismic network are presented a 68 variance reduction was achieved upon simultaneous inversion for hypocenter and velocity structure well resolved velocity anomalies associated with the active crater reveal heterogeneity up to 26 slower and 18 faster in p velocity and up to 31 slower and 22 faster in s velocity than the one dimensional model the largest anomaly is seen over the upper 11 km in the central and northern parts beneath the central cones two low velocity regions are imaged the first region a 10 15 km region encompassing the upper 3 km centered near the caldera wall at tateno valley is characterized by p velocities up to 19 slower 20 for s the second low velocity region is associated with the central cones and active magma conduit system at 6 km depth velocities as low as 4 3 km s up to 26 in p and 2 km s 31 slower in s characterize the 7 km wide volume the magma chamber is roughly spherical in shape centered at 6 km depth flattens at 10 km depth and is located between mt kishima mt eboshi and mt naka the present focus of magmatism a sharp velocity contrast at the depth of 3 km with high velocities to the southwest and lower velocities to the northeast characterizes different abutting structures associated with the oita kumamoto tectonic line

evidence of a shallow persistent magmatic reservoir from joint inversion of gravity and ground deformation data the 25 26 october 2013 etna lava fountaining event

3 d density model of mt etna volcano southern italy a detailed density model of mt etna and its surrounding areas has been evaluated using a 3 d inversion of the gravimetric data acquired in the 1980 s several high density and low density bodies are found penetrating from shallow depths as far down as 12 km bsl a positive correlation in terms of location extent density and velocity is established between several anomalies of the density model and features identified in previously published seismic tomographies a prominent high density body extending down to 7 km bsl is recognized in the southern part of the valle del bove and interpreted as a solidified magmatic intrusion on the western boundary of this anomaly a low density body is interpreted as a bubble and liquid magma mixture outside the central area three other high density anomalies are imaged and attributed to the earliest phases of volcanic activity in the area several interesting low density anomalies are also identified and correlated with known fault lines and other structural features of the region

a new dyke intrusion style for the mount etna may 2008 eruption modelled through continuous tilt and gps data after a recharge phase that began in 2007 on 13 may 2008 a new eruption started on mt etna volcano the final intrusion was very fast accompanied by a violent seismic swarm and marked by ground deformation recorded at permanent tilt and gps stations the violence of the eruptive event generated concern that the eruptive fissures might propagate downslope towards populated areas the ground deformation modelling explains both the mechanism of the intrusion as well as the attempt of the dyke to propagate in the shallower part of the northern sector of the volcano we show that the 2008 intrusion was characterized by a mechanism which is new and different to the ones modelled in previous eruptions following the path of the central conduit in the first part of the intrusion below 1 6 km and then breaking off towards the east in the last shallow part

active spreading and regional extension at mount etna imaged by sar interferometry abstract large flank instability has been proposed by most recent authors as a major process in the dynamics of mount etna however several aspects of the models are still disputed such as the boundaries of the moving area and the driving force for the instability in this paper we present synthetic aperture radar interferometric data which allow us to identify two main sectors of active instability where the deformational process differs an eastern sector is bounded to the north by the pernicana provenzana fault to the west by the north rift zone and the south rift zone and to the south by the mascalucia tremestieri trecastagni fault system interferograms provide new data on the activity and extent of the southern boundary which was previously underestimated in this sector the structural features indicate an eastward sliding driven by e w regional extension which can be interpreted as a result of the retreating slab of the ionian subduction a second collapsing sector exists along the southern flank of mount etna where the interferograms show the presence of an active anticlinal ridge this ridge is interpreted as the result of southward gravity spreading over a basal decollement between etna and its plio quaternary basement and the hyblean platform the two sectors first became active in summer 1996 after the beginning of a new cycle of eruptive activity at the summit the activity was ongoing until january 1998 with the deformation rate ranging from 4 6 mm yr for the mascalucia tremestieri trecastagni faults to 12 mm yr for the anticlinal ridge

detecting volcanic eruption precursors a new method using gravity and deformation measurements abstract one of the fundamental questions in modern volcanology is the manner in which a volcanic eruption is triggered the intrusion of fresh magma into a reservoir is thought to be a key component the amount by which previously ponded reservoir magma interacts with a newly intruded magma will determine the nature and rate of eruption as well as the chemistry of erupted lavas and shallow dykes the physics of this interaction can be investigated through a conventional monitoring procedure that incorporates the simple and much used mogi model relating ground deformation most simply represented by h to changes in volume of a magma reservoir gravity changes g combined with ground deformation provide information on magma reservoir mass changes our models predict how during inflation the observed g h gradient will evolve as a volcano develops from a state of dormancy through unrest into a state of explosive activity calderas in a state of unrest and large composite volcanoes are the targets for the methods proposed here and are exemplified by campi flegrei rabaul krafla and long valley we show here how the simultaneous measurement of deformation and gravity at only a few key stations can identify important precursory processes within a magma reservoir prior to the onset of more conventional eruption precursors

implosive earthquakes at the active accretionary plate boundary in northern iceland non double couple earthquake focal mechanisms imply a mode of seismic failure other than simple shear and movements perpendicular to the fault plane reports of such events are rare and it is controversial whether such failure can occur at depth in the earth following reports of such earthquakes from the hengill and reykjanes volcanic systems on the accretionary plate boundary in iceland1 3 a seismological survey was conducted in the krafla system iceland to explore whether such earthquakes occur elsewhere along the boundary here we report the observation of a mixed suite of focal mechanisms at krafla including shear events explosive tensile crack events such as were observed at hengill and reykjanes and implosive events these last represent cavity collapse and constitute an entirely new class of earthquake the krafla system had undergone a crustal spreading episode4 10 during the decade before the observations were made and our results indicate that immediately after such an episode the accretionary plate boundary is characterized by a heterogeneous stress regime with shear extensional and compressional sources in close juxtaposition

volcanic signatures in time gravity variations during the volcanic unrest on el hierro canary islands gravity changes occurring during the initial stage of the 2011 2012 el hierro submarine eruption are interpreted in terms of the preeruptive signatures during the episode of unrest continuous gravity measurements were made at two sites on the island using the relative spring gravimeter lacoste and romberg gphone 054 on 15 september 2011 an observed gravity decrease of 45 mu gal associated with the southward migration of seismic epicenters is consistent with a lateral magma migration that occurred beneath the volcanic edifice an apparently clear precursor of the eruption that took place 25 days later on 10 october 2011 high frequency gravity signals also appeared on 6 11 october 2011 pointing to an occurring interaction between a magmatic intrusion and the ocean floor these important gravity changes with amplitudes varying from 10 to 90 mu gal during the first 3 days following the onset of the eruption are consistent with the northward migration of the eruptive focus along an active eruptive fissure an apparent correlation of gravity variations with body tide vertical strain was also noted which could indicate that concurrent tidal triggering occurred during the initial stage of the eruption

the 1991 eruption of hekla iceland the eruption that started in the hekla volcano in south iceland on 17 january 1991 and came to an end on 11 march produced mainly andesitic lava this lava covers 23 km2 and has an estimated volume of 0 15 km3 this is the third eruption in only 20 years whereas the average repose period since 1104 is 55 years earthquakes as well as a strain pulse recorded by borehole strainmeters occurred less than half an hour before the start of the eruption the initial plinian phase was very short lived producing a total of only 0 02 km3 of tephra the eruption cloud attained 11 5 km in height in only 10 min but it became detached from the volcano a few hours later several fissures were active during the first day of the eruption including a part of the summit fissure by the second day however the activity was already essentially limited to that segment of the principal fissure where the main crater subsequently formed the average effusion rate during the first two days of the eruption was about 800 m3 s 1 after this peak the effusion rate declined rapidly to 10 20 m3 s 1 then more slowly to 1 m3 s 1 and remained at 1 12 m3 s 1 until the end of the eruption site observations near the main crater suggest that the intensity of the volcanic tremor varied directly with the force of the eruption a notable rise in the fluorine concentration of riverwater in the vicinity of the eruptive fissures occurred on the 5th day of the eruption but it levelled off on the 6th day and then remained essentially constant the volume and initial silica content of the lava and tephra the explosivity and effusion rate during the earliest stage of the eruption as well as the magnitude attained by the associated earthquakes support earlier suggestions that these parameters are positively related to the length of the preceeding repose period the chemical difference between the eruptive material of hekla itself and the lavas erupted in its vicinity can be explained in terms of a density stratified magma reservoir located at the bottom of the crust we propose that the shape of this reservoir its location at the west margin of a propagating rift and its association with a crustal weakness all contribute to the high eruption frequency of hekla

combining relative and absolute gravity measurements to enhance volcano monitoring to achieve a balance between uncertainty and efficiency in gravity measurements we have investigated the applicability of combined measurements of absolute and relative gravity as a hybrid method for volcano monitoring between 2007 and 2009 three hybrid gravity surveys were conducted at mt etna volcano in june 2007 july 2008 and july 2009 absolute gravity data were collected with two absolute gravimeters which represent the state of the art in recent advances in ballistic gravimeter technology 1 the commercial instrument fg5 238 and 2 the prototype instrument imgc 02 we carried out several field surveys and confirmed that both the absolute gravimeters can still achieve a 10 gal or better uncertainty even when they are operated in severe environmental conditions the use of absolute gravimeters in a field survey of the summit area of mt etna is unprecedented the annual changes of the gravity measured over 2007 2008 and 2008 2009 provide unequivocal evidence that during the 2007 2009 period two main phenomena of subsurface mass redistribution occurred in distinct sectors of the volcano accompanying different eruptive episodes from 2007 to 2008 a gravity change of 60 gal was concentrated around the north east rift this coincided with a zone affected by strong extensional tectonics and hence might have been related to the opening of new voids between 2008 and 2009 a north south elongate feature with a maximum gravity change of 80 gal was identified in the summit craters area this is interpreted to indicate recharge of a deep intermediate magma storage zone which could have occurred when the 2008 2009 eruption was still ongoing

the initial phases of the 2008 2009 mount etna eruption a multidisciplinary approach for hazard assessment accepted for publication in journal of geophysical research copyright 2010 american geophysical union

spatiotemporal gravity changes at miyakejima volcano japan caldera collapse explosive eruptions and magma movement 1 calderas are ubiquitous topographic features of volcanos yet caldera formation itself has not been recorded intensively by modern measurement techniques here we report the spatiotemporal gravity changes before and after caldera collapse at the miyakejima volcano japan in 2000 a gravity decrease of as much as 145 gal 1 gal 10 8 m s2 at the summit area since june 1998 was detected 2 days prior to the collapse interpreted as reflecting the formation of a large void beneath the volcano gravity changes detected after the initiation of collapse can mostly be corrected by the effect of collapsed topography from which a rapid rate of collapse of more than 1 6 107 m3 d can be inferred correcting for the effect of topography change we identified a temporal decrease in gravity from the middle of july to late august despite ground subsidence the gravity decrease is interpreted as a reduction of the density in a cylindrical conduit attributed to water inflow from an ambient aquifer that also promoted intensive magma water interaction and subsequent explosive eruptions from september to at least november 2000 gravity values at all sites increased significantly by a degree that cannot be explained by ground displacement alone we interpret this temporal evolution as primarily due to magma ascent and refilling of the conduit

tomographic inversion of local earthquake data from the hengill grensdalur central volcano complex iceland parameters is generally low however a quantitative analysis of resolution provides an unambiguous assessment of well resolved volumes within the better resolved regions of the model the averaging volumes are 1 2 km and 2 4 km in vertical and horizontal extent respectively results of tomographic inversion image three distinct bodies of anomalously high velocity two of these extend from near the surface to a depth of about 3 km these high velocity volumes are located directly beneath the surface expressions of the extinct grensdalur volcano and the extinct husmuli basalt shield the third high velocity structure occurs in the depth range of 3 4 km but does not extend to the surface these three high velocity bodies are interpreted to be solidified magmatic intrusions relatively low velocities underlay limited portions of the trace of the present accretionary axis and a low velocity body is imaged in the roots of the active hengill volcano the volume of lower velocities located beneath the surface expression of the hengill volcano is interpreted to be a region of partial melt

3d integrated geophysical modeling for the 2008 magma intrusion at etna constraints on rheology and dike overpressure abstract we present a 3d numerical model based on finite element method fem to jointly evaluate geophysical changes caused by dislocation and overpressure sources in volcanic areas a coupled numerical problem was solved to estimate ground deformation gravity and magnetic changes produced by stress redistribution accompanying magma migration within the volcano edifice we successfully applied the integrated numerical procedure to image the magmatic intrusion occurring in the northern flank of etna during the onset of the 2008 eruption a multi layered crustal structure of the volcano constrained by geological models and geophysical data was considered geodetic and gravity data provide information on the strain field while piezomagnetic changes give constraints on the stress field therefore the integrated modeling gives insights on mt etna rheology and dike overpressure involved in the magma propagation and improves understanding of dike emplacement in the northern sector of the volcano our fem based approach improves the reliability of model based inference of geophysical parameters obtained during monitoring of the onset of etna lateral intrusions that can prelude to an impending eruption

intrusive mechanism of the 2002 ne rift eruption at mt etna italy modelled using gps and gravity data summary microgravity and gps data collected at mt etna during a 1 yr time interval encompassing the 2002 ne rift eruption are analysed the common gps gravity profile traverses the summit area of mt etna between the summit craters about 3000 m and the northernmost tip of the eruptive fractures 2500 m displacements up to 2 m in both the horizontal and vertical directions and gravity variations up to 350 gal after having removed the effect of elevation changes observed during this period are among the largest ever recorded at etna displacements and gravity changes have been modelled separately assuming a magma influx from the summit feeder pipe to the ne rift models obtained through surface deformation data partially overlap with those explaining the gravity data but in general are narrower and extend to a greater depth the discrepancies found between gravity and deformation models are significant and can be regarded as due to the different structural features encountered by the intruding magma during its downslope propagation along the ne rift in particular on the grounds of our result we infer that both the eastward sliding of the east flank of the volcano and the curved shape of the ne rift influenced the 2002 intrusive process

imaging the magmatic system beneath the krafla geothermal field iceland a new 3 d electrical resistivity model from inversion of magnetotelluric data

dynamics of mount etna volcano inferred from static and kinematic gps measurements 1 ground deformations measured on mount etna from late 1995 to mid 1998 using gps observations are analyzed four gps surveys were carried out during the considered period in 1995 the gps network consisted of 18 stations on the volcanic edifice and 6 stations around it during the 1996 survey 22 new monuments were added along a n s profile crossing the summit craters area this profile was surveyed by a semikinematic method allowing greater spatial detail to be achieved on the uppermost part of the volcano the comparisons between the gps surveys are reported here in terms of horizontal and vertical displacements for each station and also in terms of areal dilatation and strain distribution this last parameter continues its ascending trend at a rate of 5 strain yr 1 already shown in the previous period after the end of the 1991 1993 flank eruption inversion of the ground deformation patterns permits investigation of the evolution of both the position and dynamics of magma reservoirs beneath the volcano consistently associated with a general eastward sliding of its eastern sector this study allows us to define the nonuniqueness of ground deformation sources through the investigated period suggesting that the plumbing system of the volcano is made up of a complex system of single intrusions occurring at different times

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

three dimensional seismic structure and moment tensors of non double couple earthquakes at the hengill grensdalur volcanic complex iceland summary the volcanic and geothermal areas of iceland are rich sources of non double couple non dc earthquakes a state of the art digital seismometer network deployed at the hengill grensdalur volcanic complex in 1991 recorded 4000 small earthquakes we used the best recorded of these to determine 3 d v p and v p v s structure tomographically and accurate earthquake moment tensors the v p field is dominated by high seismic wave speed bodies interpreted as solidified intrusions a widespread negative 4 per cent v p v s anomaly in the upper 4 km correlates with the geothermal field but is too strong to be caused solely by the evect of temperature upon liquid water or the presence of vapour and requires in addition mineralogical or lithological diverences between the geothermal reservoir and its surroundings these may be caused by geothermal alteration well constrained moment tensors were obtained for 70 of the best recorded events by applying linear programming methods to p and s wave polarities and amplitude ratios about 25 per cent of the mechanisms are within observational error consistent with dc mechanisms consistent with shear faulting the other 75 per cent have significantly non dc mechanisms many have substantial explosive components one has a substantial implosive component and the deviatoric component of many is strongly non dc many of the non dc mechanisms are consistent within observational error with simultaneous tensile and shear faulting however the mechanisms occupy a continuum in source type parameter space and probably at least one additional source process is occurring this may be fluid flow into newly formed cracks causing partial compensation of the volumetric component studying non shear earthquakes such as these has great potential for improving our understanding of geothermal processes and earthquake source processes in general

imaging hydraulic fractures in a geothermal reservoir 1 an injection experiment at the coso geothermal field in eastern california in march 2005 caused a swarm of microearthquakes that was recorded by a local network of three component digital seismometers high resolution relative hypocenter locations propagated upward and northward on a 700 600 m plane striking n 20 e and dipping 75 to the wnw this plane is a pre existing fault whose surface projection coincides with an active scarp the earthquakes have similar non double couple mechanisms that involve volume increases and the fault plane bisects their dilatational fields implying a process dominated by tensile failure the source types require the additional involvement of subsidiary shear faulting however events before and after the swarm have variable orientations and volume changes of both signs similar tensile shear failure is observed in some natural microearthquake swarms for example at long valley caldera california its occurrence under low fluid pressure may imply a heterogeneous stress field or the induction of thermal stresses by introduction of cold fluid

the 4d microgravity method for waterflood surveillance part 3 4d absolute microgravity surveys at prudhoe bay alaska the 4d microgravity method is becoming a mature technology aprojecttodeveloppracticalmeasurementandinterpretation techniques was conducted at prudhoe bay alaska from1994through2002 beginningin2003thesetechniques have been systematically applied to monitor a waterflood in the gas cap of the prudhoe bay reservoir approximately 300 stations in a 150 km 2 area are reoccupied in each survey year with sub 5 gal precision absolute gravity and centimeter precision global positioning system gps geodetic measurements the 4d gravity measured over epochs 2005 2003 2006 2003 and 2007 2003 has been successfullymodeledtotrackthemassofwaterinjectedsincelatein 2002 anewandimprovedversionofthea 10field portable absolute gravity meter was developed in conjunction with this project and has proven to be a key element in the success of the 4d methodology the use of an absolute gravity meter in a field survey of this magnitude is unprecedented there aresubstantialdifferencesbetweena4dabsolutemicrogravity survey and a conventional gravity survey in terms of station occupation procedures gps techniques and the 4d elevation correction we estimate that the overall precision of the4dgravitysignalineachepochislessthan10 gal

the 4 5 september 2007 lava fountain at south east crater of mt etna italy abstract continuous monitoring of paroxysmal eruptions at mt etna italy is performed from the istituto nazionale di geofisica e vulcanologia sezione di catania in recent years this activity has allowed us to study fallout deposits and track the evolution of eruptive phases this communication reports on the paroxysm that occurred in the afternoon of 4 september 2007 when a powerful lava fountain started from south east crater at the summit of etna the fountaining formed a plume up to 2 km high for almost 12 h causing abundant tephra fallout in the eastern sector of the volcano and emplacement of a thick about 4 km long clastogenic lava flow extensive area collection of tephra samples from the fallout deposit was performed within a few hours after the end of the paroxysm we calculated a total erupted volume of 3 92 4 9 10 5 m 3 and a total grain size fitted by a gaussian distribution with the mode of 0 and equal to 1 3 the total volume of the clastogenic lava flow deposit that resulted was 2 4 10 6 m 3 the systematic study of paroxysms at etna helps to better investigate etnean type lava fountains and to improve the classification of explosive activity on basaltic volcanoes

multidisciplinary investigation on a lava fountain preceding a flank eruption the 10 may 2008 etna case a multidisciplinary approach integrating a wide data set ranging from bulk rock compositions of the erupted products to volcanic tremor long period events and tilt and gravity signals is used to investigate the source depth and magma dynamics of the 10 may 2008 lava fountain at southeast crater sec of mount etna the investigation was undertaken in the framework of the previous 2007 explosive activity as well as the subsequent effusive eruption beginning 13 may 2008 and lasting up to july 2009 all the data concur in indicating that the 10 may lava fountain was generated by the fragmentation of a foam layer trapped at the top of a shallow reservoir about 1500 1700 m below the summit of sec the shift from the episodic strombolian lava fountain activity occurring in 2007 at sec to the more powerful 10 may 2008 lava fountain is explained by the intrusion of a new more primitive magma into the shallow reservoir data also indicate that an attempted magma intrusion east of the summit area occurred during the 10 may fire fountain this event caused the fracturing and weakening of the surrounding rocks and created a preferential pathway for the penetration of the magma that only 3 days later started to feed the 2008 2009 effusive eruption

crustal structure and magmato tectonic processes in an active rift asal ghoubbet afar east africa 2 insights from the 23 year recording of seismicity since the last rifting event 1 the asal ghoubbet ag rift has sustained a major volcano tectonic rifting episode in 1978 and has been subsequently monitored with continuous geodetic and seismological surveys it is therefore an ideal place to study the transient magmato tectonic processes that operate after a rifting episode we examine the space time evolution of 2500 md 2 8 earthquakes recorded in the rift from 1979 to 2001 we focus on the relationships between this seismic activity and both the three dimensional structure of the rift and its postrifting behavior depicted from geodesy the results highlight the major role of the central magmatic system fieale shark bay on the structure seismic activity and overall behavior of the rift from 1978 to 1986 the rift opens at a fast rate yet mainly aseismically the opening is magmatically driven and accommodated since 1986 when the opening rate abruptly decreased the seismicity is concentrated in the central part of the rift and reveals pulses of activity of the central volcanic system these pulses result from that magmatic zone undergoing alternating stretching and inflating episodes thus while the plate driven induced stresses have been rebuilding in the rift since 1987 the rift opening is still essentially accommodated in the axial magmatic zone the ag rift has thus sustained a postrifting unsteady opening over more than 23 years following its stretching episode that transient opening has essentially occurred aseismically and most tectonic faults remain relaxed and locked

long term dynamics across a volcanic rift 21 years of microgravity and gps observations on the southern flank of mt etna volcano abstract in this paper an integrated analysis of microgravity and ground deformation measurements at mt etna is performed over a 21 years time span 1994 2014 a period encompassing several flank and summit eruptions data were collected along a common gravity and ground deformation network located on the volcano s southern flank to monitor the dynamics of the main volcanic and tectonic processes gravity variations have first been corrected for the free air effect using the vertical deformation and the experimental vertical gravity gradients measured at each station the free air corrected gravity changes were then reduced for seasonal fluctuations mainly imputable to water table fluctuations and for high frequency variations noise this long term dataset constitutes a unique opportunity to examine the behavior of mt etna in a period in which the volcano exhibited different styles of activity characterized by recharging flank eruptions and fountaining episodes at summit craters the joint analysis highlights common periods in which gravity and deformation underwent contemporaneous changes occurring mainly in the central and eastern stations of the profile specifically it was possible to distinguish three different sectors showing significant deformation and gravity patterns a western sector lying west of the south rift a central one between the south rift and the 1989 fracture zone and an eastern one lying east of the 1989 fracture system the data shows that the deflation dynamics accompanying the eruptive phases mainly affect the western portion of the profile gradually disappearing toward east where the more constant sliding dynamics dominates this pattern defines a transition zone corresponding to the sector comprised between the south rift and the 1989 fracture furthermore it was also possible to detect at least three main magma charging phases in 1996 2000 and 2011 which preceded and accompanied the onset of intense flank eruptions and lava fountaining activity at the summit craters