tomographic image of melt storage beneath askja volcano iceland using local microseismicity 1 we use p wave and s wave arrivals from microseismic earthquakes to construct 3 d tomographic vp and vs images of the magma storage region beneath askja s central volcano in the northern volcanic zone of iceland a distinctive ellipsoidal low velocity anomaly with both vp and vsvelocities 8 12 below the background is imaged at 6 11 km depth beneath the caldera the presence of a shallow magma chamber is corroborated by geodetic and gravity studies the small vp vs anomaly suggests a lack of pervasive melt we interpret this anomaly as a region of multiple sills some frozen but hot others containing partial melt a second smaller low velocity anomaly beneath the main magma storage region may represent a magma migration pathway this interpretation is supported by the close proximity to the anomaly of clusters of deep magmatically induced earthquakes however the location and shape of this deep anomaly are poorly constrained by the current data set

glacial rebound and plate spreading results from the first countrywide gps observations in iceland iceland is one of the few places on earth where a divergent plate boundary can be observed on land direct observations of crustal deformation for the whole country are available for the first t

ground deformation in askja iceland its source and possible relation to flow of the mantle plume abstract the volcano askja in east central iceland erupted several times between 1920 and 1930 following a very large caldera forming eruption in 1875 the last eruption occurred in 1961 measurements of the vertical component of ground deformation in askja were commenced in 1966 and made once each year 1966 to 1972 and 1983 to 1987 distance measurements with a geodimeter were first made in 1982 the measurements have shown alternating uplift and subsidence of the central part of the askja caldera two periods of uplift and three periods of subsidence have been identified during the 21 years of observations observed vertical displacements agree fairly well with a single point source model at a depth of 1 5 3 5 km below the surface near the center of the askja caldera deviation from a single point source model can be explained by a secondary source of subsidence beneath the site of the 1961 eruption this secondary source has been diminishing in intensity and appears to have been exhausted by 1983 1987 askja has been subsiding at a rate of 6 10 cm per year during the period 1983 1987 but in 1970 1972 rapid inflation was observed possibly as much as 20 cm per year the period of no levelings 1972 to 1983 is characterized by deflation but at a slower average rate than observed in 1983 1987 this suggests continuation of the 1970 1972 inflation into 1973 or 1974

plate dynamics near divergent boundaries geophysical implications of postrifting crustal deformation in ne iceland the bulk of a tectonic plate is thought to move continously at a rate consistent with the geologic average on the other hand movements are highly episodic at plate boundaries we study the plate dynamics that relate to these two different modes by modelling the displacements observed using the global positioning system in northeast iceland 1987 1990 these observations were made about 10 years after an episodic divergent movement between the north american and eurasian plates 1975 1981 the horizontal displacement field fits well a two dimensional model of postrupture stress relaxation assuming a thin elastic layer overlying a layer of newtonian viscosity this analysis indicates values of about 10 m2 s for the stress diffusivity and 0 3 2 1018 pa s for the newtonian viscosity of the lower layer however no significant correlation exists between the observed and modeled vertical displacements probably because of the relative inaccuracy of the vertical component observations assuming that contemporary plate motion is the sum of many displacements that have diffused from boundaries where episodic displacements occur periodically we simulate the spatial transition from episodic to continuous plate movements the plate boundary zone where movements are episodic or quasi episodic is of the order of 100 km wide depending on the stress diffusivity which may be fairly uniform throughout the world and the frequency of episodic movements

lower crustal earthquakes caused by magma movement beneath askja volcano on the north iceland rift the lower crust of magmatically active rifts is usually too hot and ductile to allow seismicity the icelandic mid atlantic rift is characterized by high heat flow abundant magmatism generating up to 25 30 km thick crust and seismicity within the upper 8 km of the crust in a 20 seismometer survey in july august 2006 within the northern rift zone around the askja volcano we recorded 1700 upper crustal earthquakes cutting off at 7 8 km depth marking the brittle ductile boundary unexpectedly we discovered 100 small magnitude ml 1 5 earthquakes occurring in swarms mostly at 14 26 km depth within the otherwise aseismic lower crust and beneath the completely aseismic middle crust a repeat survey during july august 2007 yielded more than twice as many lower crustal events geodetic and gravimetric data indicate melt drainage from crustal magma chambers beneath askja we interpret the microearthquakes to be caused by melt moving through the crust from the magma source feeding askja they represent bursts of magma motion opening dykes over distances of a few meters facilitated by the extensional setting of the active rift zone

formation of oskjuvatn caldera at askja north iceland mechanism of caldera collapse and implications for the lateral flow hypothesis the oskjuvatn caldera at askja volcano north iceland was formed as a result of an explosive eruption at askja on 28th 29th march 1875 oskjuvatn is one of the youngest collapse calderas on earth and its initiation and growth were observed and documented by explorers and geologists working in the askja region between 1875 and 1932 it is an example of caldera formation where the volume of the caldera collapse far exceeds the volume of associated erupted products the discrepancy between the collapse volume and associated erupted products has been used by previous workers as a justification for the hypothesis that the injection of lateral dykes from askja fed the february to october 1875 n jahraun fissure eruptions at the sveinagja graben some 60 km north of askja however historical accounts documenting the caldera formation in sufficient detail show clearly that oskjuvatn formed by piecemeal collapse taking over 40 years to reach its current form we use these accounts to undertake a detailed examination of the stages of caldera collapse and to compare them with the known magmatic output of askja in the years following the 1875 eruption arcgis software was used to calculate the volume of oskjuvatn during the various stages of collapse and the volume of material erupted after 1875 while a dyke extending between askja and sveinagja may be implied to account for the missing volume our results demonstrate that the volume of oskjuvatn in july 1875 closely corresponds to the volume of rhyolitic material erupted on 28th 29th march 1875 in addition geochemical evidence indicates that the n jahraun magmas were sourced from a deeper reservoir rather than by lateral injection from askja s shallow crustal magma chamber we therefore suggest that the injection of sills and dykes in a coherent intrusion complex beneath askja central volcano combined with background deflation of the magma storage zone beneath oskjuvatn caldera over 30 to 40 years following 1875 satisfactorily accommodates the volume required to form oskjuvatn caldera

deformation in the northern volcanic zone of iceland 2008 2014 an interplay of tectonic magmatic and glacial isostatic deformation gps measurements spanning 2008 to 2014 are used to derive the surface velocity field across the northern volcanic zone nvz of iceland a subaerial part of the divergent boundary between the north american and eurasian plates no volcanic activity nor magmatic intrusions were detected in the zone during this time period we infer an extensional rate of 17 4 0 3 0 2 mm yr in direction 292 0 0 6 0 5 consistent with the results of previous studies and current plate motion models including morvel2010 and geodvel2010 the horizontal velocity field reveals about 50 km wide stretching zone caused by the divergent plate movements glacial isostatic adjustment gia induces uplift of over 20 mm yr at the northern edge of vatnajokull ice cap and 3 4 mm yr horizontal motion directed away from the ice cap deformation in the nvz between 2008 and 2014 can be reproduced by a combination of models relating to several different processes i mogi sources for volcanic and geothermal deformation at the askja and krafla volcanoes ii scaled version of a velocity field derived from a glacial isostatic model and iii simple arctangent based model for secular plate spreading we find the approximate location of the plate boundary spreading axis as well as its locking depth the spreading axis lies through the krafla fremrinamar and askja central volcanoes the most active ones in the nvz it does not appear to follow the general direction of each fissure swarm but rather to change direction at the central volcanoes the locking depth is on average within the 7 9 km range

gravity and elevation changes at askja iceland ground tilt measurements demonstrate that askja is in a state of unrest and that in the period 1988 1991 a maximum 48 3 rad tilt occurred down towards the centre of the caldera this is consistent with 126 mm of deflation at the centre of the caldera with a 2 5 3 0 km depth to the source of deformation the volume of the subsidence bowl is 6 2x106 m3 when combined with high precision microgravity measurements the overall change in sub surface mass may be quantified after correction for the observed elevation change using the free air gradient of gravity measured for each station the total decrease in mass is estimated to be less than 109 kg a small residual ground inflation and net gravity increase in the southeastern part of the caldera may be caused by dyke intrusion in this region the minimum dimensions of such an intrusion or complex of intrusions are 1 m width up to 100 m deep and up to several hundred metres thick

implications of fracturing mechanisms and fluid pressure on earthquakes and fault slip data in the east iceland rift zone comparison of the eroded off rift zone left inactive by plate motion with the inner active seismic rift zone allows us to constrain the fracturing mechanisms in eastern off rift zone we measured 423 fault slips including normal and strike slip faults inversion of fault slip data reveals the parallelism of the minimum stress 3 computed for the normal and for the strike slip faulting and consistency with the direction of plate divergence north of the vatnajokull in the active rift zone we analysed 14 250 earthquakes recorded by the icelandic meteorological office imo between 2004 and 2009 especially northeast of the askja volcano two main average focal depths were determined at 5 km her ubrei table mountain and 15 km upptyppingar hyaloclastite ridge the double couple focal mechanisms determined by imo revealed that more than half of the mechanisms are strike slip faulting type both in active and off rift zone reveals an unusual importance of strike slip regime in such an extensional tectonic context this can be explained by stress permutations 1 2 similarities in terms of stress orientations and type of faulting are observed both in the old and present day rift zones we assume that the seismic events may be generated by rapid deep magma intrusion also associated with shallower hydrothermal activities we thus propose that the presence of fluids shear failure double couple focal mechanism and stress permutations in both active and off rift zones are closely linked assuming a drucker prager failure criterion we evaluated analytically that a state near of lithostatic pore pressure is a necessary condition for shear failure at shallow 5 km and deep 15 km depth in a simple context of crustal extension that allows for stress permutations however processes favouring stress permutations cannot be further constrained from our observations since the dynamics of fluid materiel heterogeneity and post glacial rebound can also play a significant role

deep magma storage at hekla volcano iceland revealed by insar time series analysis 1 hekla volcano is one of the most active volcanoes in iceland the most recent eruption occurred from 26 february to 8 march 2000 when about 0 19 km3 of magma was erupted we present deformation data from multitemporal analyses of synthetic aperture radar sar images acquired between 1993 and 2008 focusing on pixels with low phase variance using persistent scatterer and small baseline approaches prior to and after the 2000 eruption we find a broad area of inflation around the volcano radius about 20 km with satellite line of sight los shortening of up to 5 mm yr we interpret this signal as the result of pressure increase in a deep seated magma chamber which we model as a spherical source at 14 20 km depth increasing in volume by 0 003 0 02 km3 yr within a 6 km radius of the summit of the volcano a los lengthening is superimposed on the broad inflation signal which correlates partly with recent lava flows we interpret this signal as the result of thermally contracting lava flows combined with viscoelastic yielding due to the load of the volcano and its lavas coeruptive deflation during the 2000 eruption was similar to the cumulative inflation from 1993 to 2000 and is consistent with a spherical magma chamber at 14 18 km depth that decreases in volume by 0 04 0 08 km3 interferograms spanning the 2000 eruption show a local coeruptive deformation signal near the eruptive fissure this is consistent with a dike opening from the surface to depths up to 5 8 km with a volume of 0 005 0 006 km3

geodetic data shed light on ongoing caldera subsidence at askja iceland subsidence within the main caldera of askja volcano in the north of iceland has been in progress since 1983 here we present new ground and satellite based deformation data which we interpret together with new and existing micro gravity data to help understand which processes may be responsible for the unrest from 2003 to 2007 we observe a net micro gravity decrease combined with subsidence and from 2007 to 2009 we observe a net micro gravity increase while the subsidence continues we infer subsidence is caused by a combination of a cooling and contracting magma chamber at a divergent plate boundary mass movements at active volcanoes can be caused by several processes including water table lake level movements hydrothermal activity and magma movements we suggest that here magma movement and or a steam cap in the geothermal system of askja at depth are responsible for the observed micro gravity variations in this respect we rule out the possibility of a shallow intrusion as an explanation for the observed micro gravity increase but suggest magma may have flowed into the residing shallow magma chamber at askja despite continued subsidence in particular variable compressibility of magma residing in the magma chamber as well as compressibility of the surrounding rock may be the reason why this additional magma did not create any detectable surface deformation

iceland rising solid earth response to ice retreat inferred from satellite radar interferometry and visocelastic modeling a broad uplift occurs in iceland in response to the retreat of ice caps which began circa 1890 until now this deformation signal has been measured primarily using gps at points some distance away from the ice caps here for the first time we use satellite radar interferometry interferometric synthetic aperture radar to constrain uplift of the ground all the way up to the edge of the largest ice cap vatnajokull this allows for improved constraints on the earth rheology both the thickness of the uppermost earth layer that responds only in an elastic manner and the viscosity below it the interferometric synthetic aperture radar velocities indicate a maximum displacement rate of 24 4 and 31 4 mm yr at the edge of vatnajokull during 1995 2002 and 2004 2009 respectively the fastest rates occur at outlet glaciers of low elevation where ice retreat is high we compare the observations with glacial isostatic adjustment models that include the deglaciation history of the icelandic ice caps since 1890 and two earth layers using a bayesian approach we derived probability density functions for the average earth model parameters for three satellite tracks based on our assumptions the three best fit models give elastic thicknesses in the range of 15 40 km and viscosities ranging from 4 10 1018 pa s

the fissure swarm of the askja volcanic system along the divergent plate boundary of n iceland divergent plate boundaries such as the one crossing iceland are characterized by a high density of subparallel volcanic fissures and tectonic fractures collectively termed rift zones or fissure swarms when extending from a specific volcano volcanic fissures and tectonic fractures in the fissure swarms are formed during rifting events when magma intrudes fractures to form dikes and even feeds fissure eruptions we mapped volcanic fissures and tectonic fractures in a part of the divergent plate boundary in northern iceland the study area is 1 800 km2 located within and north of the askja central volcano the style of fractures changes with distance from askja close to askja the swarm is dominated by eruptive fissures the proportion of tectonic fractures gets larger with distance from askja this may indicate that magma pressure is generally higher in dikes close to askja than farther away from it volcanic fissures and tectonic fractures are either oriented away from or concentric with the 3 4 identified calderas in askja the average azimuth of fissures and fractures in the area deviates significantly from the azimuth perpendicular to the direction of plate velocity as this deviation decreases gradually northward we suggest that the effect of the triple junction of the north american eurasian and the hreppar microplate is a likely cause for this deviation shallow tectonic earthquakes in the vicinity of askja are often located in a relatively unfractured area between the fissure swarms of askja and kverkfjoll these earthquakes are associated with strike slip faulting according to fault plane solutions we suggest that the latest magma intrusions into either the askja or the kverkfjoll fissure swarms rotated the maximum stress axis from being vertical to horizontal causing the formation of strike slip faults instead of the dilatational fractures related to the fissure swarms the activity in different parts of the askja fissure swarm is uneven in time and switches between subswarms as shown by a fissure swarm that is exposed in an early holocene lava nw of her ubrei but disappears under a younger 3500 4500 bp lava flow we suggest that the location of inflation centres in askja central volcano controls into which part of the askja fissure swarm a dike propagates the size and amount of fractures in the kollottadyngja lava shield decrease with increasing elevation we suggest that this occurred as the depth to the propagating dike s was greater under central kollottadyngja than under its flanks due to topography

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

geodetic gps measurements in south iceland strain accumulation and partitioning in a propagating ridge system 1 gps observations in south iceland between 1994 and 2003 are compared with two dimensional elastic half space and viscoelastic coupling models for two parallel rift zones representing the western and eastern volcanic zones wvz evz gps data from the hreppar block between the wvz and evz fit a rigid block model within uncertainties spreading rates across the wvz increase from 2 6 0 9 mm yr in the northeast to 7 0 0 4 mm yr in the southwest conversely spreading rates in the evz decrease from 19 0 2 0 mm yr in the northeast to 11 0 0 8 mm yr in the southwest the direction of ridge propagation summed extension rates across the two rift zones are approximately constant and equal to the total plate rate 18 20 mm yr consistent with a simple propagating ridge model whereby the wvz is deactivating in the direction of evz propagation the coupling model confirms results from the simple elastic half space model including relatively shallow locking depths 5 km beneath the rift zones and allows for an estimate of mean viscosity 1019 1020 pa s beneath the elastic layer the location of maximum surface velocity gradient in the evz presumably the locus of subsurface magma accumulation and future rifting does not coincide with the 1783 1784 lakagigar fissure eruption but is 20 km to the west on the bar abunga veidivotn fissure swarm this had a small volume eruption in 1862 1864 but a major eruption in 1477 a d

1983 2003 decaying rate of deflation at askja caldera pressure decrease in an extensive magma plumbing system at a spreading plate boundary new deformation data from the askja volcano iceland show that the volcano s caldera has been deflating continuously for over 20 years and confirm that the rate of subsidence is slowing down the decay in subsidence rate can be fitted with a function of the form e t where is 39 years reanalysis of gps data from 1993 1998 show that these data can be fitted with a model calling for two mogi point sources one shallow and another one much deeper 16 2 km depth pressure decrease occurs in both sources the deeper source is responsible for observed horizontal contraction towards askja at distances that cannot be explained by the shallower source plate spreading of 19 mm year distributed evenly over about 100 km wide zone is also favoured by the data

the ups and downs of the tvz geodetic observations of deformation around the taupo volcanic zone new zealand the 300 km long taupo volcanic zone tvz formed as a result of back arc rifting associated with subduction of the pacific plate beneath the australian plate with current extension rates of 8 15 mm yr 1 using gps and interferometric synthetic aperture radar insar data collected by the european esa and japanese jaxa space agencies we present ground deformation observations from 2003 to 2011 both insar and vertical gps data show widespread subsidence across the central tvz at rates of up to 20 mm yr 1 using simple elastic dislocation models to represent the contraction of a sill like body at 6 km depth we predict an annual volume change of 0 011 0 016 km3 beneath the central tvz we suggest that the majority of the observed subsidence is a result of the cooling and subsequent contraction of magma within the shallow crust

petroleum geochemistry of the lower and middle jurassic in atlantic margin basins of ireland and the uk potential hydrocarbon source rocks of lower and middle jurassic age have been reported from outcrop shallow boreholes and exploration wells in atlantic margin basins of the uk hebrides west of shetlands and flanking the ne rockall trough and recently in the continuation of this trend offshore ireland slyne erris and porcupine basins previously these organic rich mudrocks were considered to be of little economic importance due largely to their perceived limited areal distribution and low maturity however recent geochemical studies of oils and shales from exploration drilling of these basins shows the lower and middle jurassic to have considerable potential as effective hydrocarbon source rocks supplanting the late jurassic early cretaceous kimmeridge clay formation equivalents as the only viable oil source rock in the region flanking the atlantic margin in the irish and uk sectors rich oil source potential occurs in two transgressive mudrock cycles of lower jurassic age these are the sinemurian pliensbachian interval and the overlying toarcian section present in basins such as the solan minch hebrides slyne north celtic sea st george s channel and central english channel the middle jurassic source rocks have a more limited areal distribution and occur in the faroe shetland solan west lewis west flannan hebrides slyne and north porcupine basins with oil source potential in regressive marginal marine to lacustrine facies mudrocks geochemical studies were undertaken on mudrocks from the lower and middle jurassic sections in atlantic margin basins outcrop shallow borehole core and exploration well cores and cuttings samples and on oils from drill stem test and shows core and cuttings extracts detailed analyses using gc gc ms and carbon isotopes allowed both characterization of the source rocks and oil to source correlation biomarker and carbon isotope studies of oils from the faroe shetland basin foinaven and schiehallion fields the porcupine basin connemara accumulation the wessex basin wytch farm and kimmeridge oil fields and wells in the slyne basin show strong correlations to the various source rock developments in the lower and middle jurassic the mixed biodegraded foinaven and schiehallion oils have a major waxy component and correlate with lacustrine middle jurassic source rocks in the solan and west lewis west flannan basins middle jurassic sourcing of the connemara oils is also suggested while oils in the slyne basin appear to have been largely sourced by the lower jurassic pabba shale formation oils in the wessex basin wytch farm and kimmeridge appear to have been sourced by hettangian sinemurian mudrocks and those in the north celtic sea basin by toarcian source rocks the results from this study in combination with previously published data show that rich effective oil prone source rocks occur in both the lower and middle jurassic of the atlantic margin basins offshore ireland and the uk these source rocks can be correlated with indigenous oils indicating the existence of a previously under evaluated petroleum system

correction to transient rift opening in response to multiple dike injections in the manda hararo rift afar ethiopia imaged by time dependent elastic inversion of interferometric synthetic aperture radar data 1 interferometric synthetic aperture radar insar data spanning the time intervals separating thirteen dike intrusions in the manda hararo dabbahu rift afar ethiopia from 2005 to 2009 show that transient deformation occurs in the inter diking period this deformation can be explained by the presence of seven inflating or deflating pressure sources by combining the data acquired on four different insar tracks through time dependent elastic models we are able to track these deformation modes with a time resolution smaller than 1 month sustained deflation of a deep magma reservoir at dabbahu in the 6 months following the main rifting event of 2005 and slow decelerating post eruptive re inflation of two shallow magma reservoirs below dabbahu and gabho volcanoes are monitored a deflation signal of deep origin on the neighboring rift system is also detected possibly caused by outflow of material from a preexisting reservoir into the deep plate boundary in contrast rapidly evolving deformation is observed at the center of the manda hararo rift segment transient deformation events are monitored in the weeks months following the diking events with pulses of localized rift opening after the dike intrusions followed by an exponential like decay of opening rate this signal may be associated with the replenishment of the central magma reservoir involved in feeding the 2005 2009 dikes alternatively the predominantly horizontal mode of deformation suggests an interaction between the response of the lithosphere to tectonic strain accumulation and the process of hydraulic connectivity within the central magma plumbing system

plate boundary deformation and continuing deflation of the askja volcano north iceland determined with gps 1987 1993 gps geodetic measurements were conducted around the askja central volcano located at the divergent plate boundary in north iceland in 1987 1990 1992 and 1993 the accuracy of the 1987 and 1990 measurements is in the range of 10 mm for horizontal components the accuracy of the 1992 and 1993 measurements is about 4 mm in the horizontal plane regional deformation in the askja region is dominated by extension points located outside a 30 45 km wide plate boundary deformation zone indicate a displacement of 2 4 0 5 cm a in the direction n 99 e 12 of the eurasian plate relative to the north american plate in the period 1987 1990 within the plate boundary deformation zone extensional strain accumulates at a rate of 0 8 strain a displacement of control points next to askja 7 km from the caldera center in the periods 1990 1993 and 1992 1993 show deflation and contraction towards the caldera these results are in accordance with the results obtained by other geodetic methods in the area which indicate that the deflation at askja occurs in response to a pressure decrease at about 2 8 km depth located close to the center of the main askja caldera a mogi point source was fixed at this location and the gps data used to solve for the source strength a central subsidence of 11 2 5 cm in the period 1990 1993 is indicated and 5 5 1 5 cm in the period 1992 1993 the maximum tensional strain rate according to the point source model occurs at a horizontal distance of 2 5 6 km from the source at the same location as the main caldera boundary discrepancies between the observed displacements and predicted displacements from the mogi model near the askja caldera can be attributed to the regional eastwest extension that occurs at askja

volcano deformation at active plate boundaries deep magma accumulation at hekla volcano and plate boundary deformation in south iceland volcano deformation at active plate boundaries deep magma accumulation at hekla volcano and plate boundary deformation in south iceland

interaction between central volcanoes and regional tectonics along divergent plate boundaries askja iceland activity within magmatic divergent plate boundaries mdpb focuses along both regional fissure swarms and central volcanoes an ideal place to investigate their mutual relationship is the askja central volcano in iceland askja consists of three nested calderas namely kollur askja and oskjuvatn located within a hyaloclastite massif along the nne ssw trending icelandic mdpb we performed an extensive field based structural analysis supported by a remote sensing study of tectonic and volcanic features of askja s calderas and of the eastern flank of the hyaloclastite massif in the massif volcano tectonic structures trend n 10 e to n 40 e but they vary around the askja caldera being both parallel to the caldera rim and cross cutting on the western side structural trends around the oskjuvatn caldera are typically rim parallel volcanic vents and dikes are preferentially distributed along the caldera ring faults however they follow the nne ssw regional structures when located outside the calderas our results highlight that the askja volcano displays a balanced amount of regional fissure swarm related and local shallow magma chamber related tectonic structures along with a mutual interaction among these this is different from krafla volcano to the north of askja dominated by regional structures and grimsvotn to the south dominated by local structures therefore askja represents an intermediate tectono magmatic setting for volcanoes located in a slow divergent plate boundary this is also likely in accordance with a northward increase in the spreading rate along the icelandic mdpb

integration of micro gravity and geodetic data to constrain shallow system mass changes at krafla volcano n iceland new and previously published micro gravity data are combined with insar data precise levelling and gps measurements to produce a model for the processes operating at krafla volcano 20 years after its most recent eruption the data have been divided into two periods from 1990 to 1995 and from 1996 to 2003 and show that the rate of deflation at krafla is decaying exponentially the net micro gravity change at the centre of the caldera is shown using the measured free air gradient to be 85 gal for the first and 100 gal for the second period after consideration of the effects of water extraction by the geothermal power station within the caldera the net gravity decreases are 73 17 gal for the first and 65 17 gal for the second period these decreases are interpreted in terms of magma drainage following a mogi point source model we calculate the mass decrease to be 2 1010 kg year reflecting a drainage rate of 0 23 m3 s similar to the 0 13 m3 s drainage rate previously found at askja volcano n iceland based on the evidence for deeper magma reservoirs and the similarity between the two volcanic systems we suggest a pressure link between askja and krafla at deeper levels at the lower crust or the crust mantle boundary after the krafla fires co rifting pressure decrease of a deep source at krafla stimulated the subsequent inflow of magma eventually affecting conditions along the plate boundary in n iceland as far away as askja we anticipate that the pressure of the deeper reservoir at krafla will reach a critical value and eventually magma will rise from there to the shallow magma chamber possibly initiating a new rifting episode we have demonstrated that by examining micro gravity and geodetic data our knowledge of active volcanic systems can be significantly improved

multiple volcano deformation sources in a post rifting period 1989 2005 behaviour of krafla iceland constrained by levelling tilt and gps observations abstract the krafla rifting episode which occurred in north iceland in 1975 1984 was followed by inflation of a shallow magma chamber until 1989 at that time gradual subsidence began above the magma chamber and has continued to the present at a declining rate pressure decrease in a shallow magma chamber is not the only source of deformation at krafla as other deformation processes are driven by exploitation of two geothermal fields together with plate spreading in addition deep seated magma accumulation appears to take place with its centre 10 km north of the krafla caldera the relative strength of these sources has varied with time new results from a levelling survey and gps measurements in 2005 allow an updated view on the deformation field deformation rates spanning 2000 2005 are the lowest recorded in the 30 year history of geodetic studies at the volcano the inferred rate of 2000 2005 subsidence related to processes in the shallow magma chamber is less than 0 3 cm yr whereas it was 5 cm yr in 1989 1992 currently the highest rate of subsidence takes place in the leirbotnar area within the krafla caldera and appears to be a result of geothermal exploitation