rift transform junction in north iceland rigid blocks and narrow accommodation zones revealed by gps 1997 1999 2002 summary the current tectonics of north iceland are characterized by rifting episodes along fissure swarms and by ml 6 7 earthquakes in the transform zone between the northern rift zone and the kolbeinsey ridge north of iceland the last rifting period 1975 1984 was associated with an average opening of 5 m along the krafla fissure swarm post rifting deformation has been revealed by gps investigations in northern iceland a gps network was occupied in 1997 1999 and 2002 to quantify present day displacements and their variation with time on

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

influence of cone morphology on dykes stromboli italy abstract the island of stromboli represents the emerged part of a 2 6 km high composite volcano elongated in a ne direction over the period 100 13 ka bp the morphological evolution of the volcano was characterised by a series of summit caldera collapses between major building phases from 13 ka bp onward a series of large lateral collapses occurred northwestwards in one sector of the cone the results of the field study of the total 94 dyke population is discussed in this paper they show that during the last 100 ka the majority of dykes were injected with a nnegssw to ene wsw strike along a ne sw weakness zone crossing the volcano summit this is interpreted as a volcanic rift the geometry of which is controlled by regional tectonic stresses dykes injected prior to 13 ka bp at lower altitudes are generally steeper than those injected at higher elevations after 13 ka bp dykes were injected along the ne sw weakness zone and parallel and close to the margins of the earliest sector collapse at dips of 20 80 inclined towards the collapse depression i suggest that this is linked to unbuttressing of the collapse depression or to seaward gravity instability of the depression infill deposits extensive emplacement of long dykes along the ne sw weakness zone occurred only when the volcano regained a conical shape some of the dykes emplaced close to the original surface of the cone bent to assume a dip direction parallel to the local slope i propose that this local change of dyke geometry can be explained by the direct influence of the free surface of the cone leading to a rotation of the least principal stress which assumes an orientation perpendicular to the slope

accelerated pacific plate subduction following interplate thrust earthquakes at the japan trench interplate thrust earthquakes are usually followed by afterslips and they let the fore arc move slowly trenchward however we do not know if the subducting oceanic plate is accelerated landward after such earthquakes the westward velocity of global positioning system gps stations in ne japan show gradient decreasing from east to west reflecting the e w contractional strain built up by the inter plate coupling here we show that such coupling significantly enhanced 1 5 times after the 2003 tokachi oki earthquake mw 8 0 hokkaido in the segments adjacent to the ruptured fault the coupling seems to be further enhanced 3 times after the 2011 tohoku oki earthquake mw 9 0 it is unlikely that interplate friction suddenly increased over such a large region and relatively strong pre 2003 coupling there would not allow such enhancements even if full coupling is attained hence they are attributable to the temporary acceleration of the pacific plate subduction we propose a simple 2 dimensional model in which down dip acceleration of the slab let the force balance rapidly recover promoted by a thin low viscosity layer on the slab surface the accelerated subduction would account for temporary activations of regional interplate seismicity after megathrust earthquakes

the long term growth of volcanic edifices numerical modelling of the role of dyke intrusion and lava flow emplacement abstract the contribution of intrusive complexes to volcano growth is attested by field observations and by the monitoring of active volcanoes we used numerical simulations to quantitatively estimate the relative contributions to volcano growth of elastic dislocations related to dyke intrusions and of the accumulation of lava flows the ground uplift induced by dyke intrusions was calculated with the equations of okada bull seismol soc am 75 1985 1135 the spreading of lava flows was simulated as the flow of a bingham fluid with realistic parameters for dyke statistics and lava flow rheology we find the contribution of dyke intrusions to the growth of a basaltic shield archetype to be about 13 in terms of volume and 30 in terms of height the result is strongly dependent on the proportion of dykes reaching the surface to feed a lava flow systematic testing of the model indicates that edifices tend to be high and steep if dykes are thick and high issued from a small and shallow magma chamber and if they feed lava flows of high yield strength the simulation was applied to ko olau o ahu is hawai i and piton de la fournaise reunion is volcanoes the simulation of ko olau with dyke parameters as described by walker geology 14 1986 310 u s geol surv prof pap 1350 1987 961 and with lava flow characteristics collected at kilauea volcano hawai i is results in an edifice morphology very close to that of the real volcano the best fit model of the piton de la fournaise central cone with its steep slope and e w elongation is obtained by the intrusion of 10 000 short and thick dykes issued from a very small and shallow magma chamber and feeding only 700 low volume lava flows the same method may be applied to the growth of basaltic shields and other volcano types in different environments including non terrestrial volcanism

longitudinal strike slip faults in oceanic rifting a mesostructural study from western to southeastern iceland abstract mesostructural analysis carried out in several localities of western southern central and southeastern iceland shows an unexpectedly frequent occurrence of strike slip faults parallel to or lying at relatively small angles with the axis of rifting some faults can be interpreted in terms of shear parallel to the rifts whereas others form conjugate systems referable to rift parallel compression axial shear can be explained by the accommodation of differences in the directions of tectonic extension between different segments of the axial rift system and subordinately by lateral rift parallel displacements between the major lithospheric plates axial compression may be attributable to rift oblique transform shear or to mantle transport parallel to the spreading axis compensating local deficiencies in mantle upwelling the data regard 1974 fault surfaces mainly small scale

seismic risk at the rift transform junction in north iceland the current tectonics of north iceland are characterised by max m 6 7 earthquakes in the transform zone between the northern rift zone and the kobleinsey ridge the last rifting period 1975 1984 was associated with an opening of up to 9 m along the krafla fissure swarm this period was followed by the disappearance of microseismicity along the eastern part of husavik flatey fault the main structure of the transform zone a gps network was occupied in 1995 and again in 1997 to study present day deformation on both sides of the on land part of hff and around its junction with the northern rift zone the main deformational features observed are 1 in tjornes the hff is locked above the ductile brittle boundary below which ductile creep is occurring and 2 aseismic opening along the theystareykir and krafla fissure swarms and along a n 120 e superficial fault that marks the southern boundary of tjornes

plate boundary deformation in north iceland during 1992 2009 revealed by insar time series analysis and gps abstract in north iceland extensional plate motion is accommodated by the northern volcanic zone a set of en echelon volcanic systems and the tjornes fracture zone a transform offset in the mid atlantic ridge consisting of two parallel transform lineaments the southern lineament the husavik flatey fault is a 100 km long right lateral strike slip fault that has not ruptured for more than 140 years and poses a significant seismic hazard to husavik a fishing town located by the fault and to other coastal communities we present results of insar time series analysis data spanning almost two decades 1992 2009 that show extensional and interseismic deformation within the northern volcanic zone and the on shore part of the tjornes fracture zone the results also exhibit transient inflation at theistareykir volcano deflation at krafla central volcano and a broad uplift north of krafla the current plate extension is not uniform across the northern volcanic zone but concentrated at the western fissures of the theistareykir volcanic system and the outermost fissures of the krafla fissure swarm we combine a back slip plate boundary model with a set of point pressure sources representing volcanic changes to describe the current extensional plate boundary deformation and update the previous estimations of the locking depth and slip rate of the husavik flatey fault that were based on gps data alone using different combinations of input data we find that the husavik flatey fault has a locking depth of 6 10 km and with a slip rate of 6 9 mm yr is accommodating about a third of the full transform motion we furthermore show that while the insar data provide important constraints on the volcanic deformation within the nvz they do not significantly improve the model parameter estimation for the hff as the dense gps network appears to better capture the deformation across the fault

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

volcano geodesy and magma dynamics in iceland abstract here we review the achievements of volcano geodesy in iceland during the last 15 years extensive measurements of crustal deformation have been conducted using a variety of geodetic techniques including leveling electronic distance measurements campaign and continuous global positioning system gps geodesy and interferometric analysis of synthetic aperture radar images insar results from these measurements provide a comprehensive view of the behavior of icelandic volcanoes between inflation intrusion and eruption episodes volcanoes are likely to deflate or show no sign of seismic activity subsidence rates are often in the range of a few millimeters to a few centimeters a year reducing progressively with time since the last eruption or intrusion at the volcano subsidence can be caused by cooling and contraction of magma outflow of magma it can be related to plate spreading volcano subsidence or lack of deformation is often interrupted by episodic magma flow towards near surface locations such magma recharge has been observed geodetically at hengill hekla eyjafjallajokull katla grimsvotn and krafla volcanoes with inflow inferred to last from a few months up to two decades in the last 15 years five volcanic eruptions three intrusive events and two m6 earthquakes have occurred in recent years the grimsvotn and katla volcanoes have exhibited continuous inflation of a few centimeters per year which at grimsvotn culminated in an eruption on 1 november 2004 hekla and torfajokull volcanoes have inflated at rates an order of magnitude less subsidence is occurring presently at the askja and krafla volcanoes within the period of geodetic measurement signals consistent with no deformation are typical for most of the 35 active volcanoes in iceland

geochemistry of metalliferous hydrothermal deposits in the aeolian arc tyrrhenian sea abstract in shallow water areas of the submerged volcanic complex around the island of panarea aeolian archipelago hydrothermal precipitation of both low temperature fe oxyhydroxide rich red muds and crusts and high temperature sediment hosted massive sulfides was discovered during an integrated high resolution survey iron rich crusts were also found on the bathymetric high of secca del capo north of salina island the exhalative iron rich sediments occur in small closed depressions or in proximity to faults and scarps at water depths ranging from 55 to 285 m the principal chemical characteristics of these deposits are high but variable fe content ranging from 12 2 to 45 and low contents of the transition elements mn cu zn ni and co the low contents of cu ni and co suggest a hydrothermal origin the fe oxyhydroxide deposits are enriched in light rare earth element ree 35 110 chondrite compared to heavy ree 10 25 chondrite their ree patterns are similar to those of associated calc alkaline volcanics negative slope of light ree and a horizontal distribution of the heavy ones this contrasts with the pattern for iron and manganese oxides of hydrogenous origin which have tilted trends of heavy ree paralleling that of seawater the mineralogy of the polymetallic sulfide deposits consists of galena sphalerite pyrite and barite in the form of silt sand grains and decimeter sized fragments disseminated in clay 30 cm below the seabed at a waterdepth of 80 m the chemistry of the aeolian iron rich low temperature deposits and of the high temperature ba rich pb and zn sulfides suggests that they are genetically analogous to respectively the kuroko type iron formation tetsusekiei and the black ore exposed in the miocene hokuroku district of japan

lithosphere strength controls oceanic transform zone structure insights from analogue models summary oceanic transform zones have often been regarded as plate boundaries the origin of their structural variability is poorly constrained a simple observation indicates that the transform zone is narrow and linear when the offset is large while it is wide with a complex faulting pattern in the case of a small offset on the other hand for a given offset large structural differences exist between transform zones located on the fast spreading south east pacific rise and on the slowspreading mid atlantic ridge in general the transform zones in slow spreading environments are linear with a simple fault pattern while in fast spreading systems they are wide with a complex pattern of deformation we perform small scale analogue modelling to constrain the influence of lithospheric strength on the development of deformation above a transform boundary the models are made up of sand and silicone putty as analogues of the brittle layer and the viscous layer of the lithosphere respectively two plastic sheets coming from shifted gashes form a set up of two diverging discontinuities connected by a transform boundary the rheological layering and strength of the model were modified using different shapes of the viscous layer placed on the transform boundary above the divergent discontinuities the faulting pattern is always formed by parallel normal faults when no viscous layer is placed on the transform boundary strong discontinuity the deformed zone is narrow and has few linear faults by adding a narrow and thin viscous layer the deformed zone becomes wider with a complex faulting pattern formed by oblique slip faults on the limits and by pure strike slip faults in the inner part these latter strike slip faults trend oblique to the transform boundary when a viscous layer with a wide lateral extent overlays the transform discontinuity weak strength the faulting is dominated by obliquely normal faults extending over a wide zone and the strike slip is restricted to the inner part of the deformed zone therefore the mechanical strength of the small scale model controls the shape of the deformed zone and the deformation partitioning these results were applied to 24 oceanic transforms zones we point out that the spreading rate and the transform offset are the two dominant parameters controlling the deformation pattern these two factors directly control the lithospheric strength at the transform boundary however the distance to the nearest hotspot which may generate warmer thermal conditions even in slow spreading environments should modify this result

episodic dike swarms inferred from near bottom magnetic anomaly maps at the southern east pacific rise fissure eruption at 1728 0 s similar lows are observed at three other drained lava lake troughs including one which is at least 1800 years old residing 400 m away from the present day axis we attribute these lows to the presence of shallow dike swarms the degree to which other geologic features may contribute to the lows is constrained using geologic geophysical and geochemical observations and forward modeling compositional analyses of alvin samples at 1728 0 s do not support fe or ti variations as a primary source hypotheses requiring hydrothermal alteration and porosity variations are both inconsistent with geologic observations and near bottom gravity data analysis from similar areas previous mappings between paleointensity variations and the observed magnetic field over distances of several kilometers from the axis suggest that such variations do not create the field low the dominant source of the magnetization low is most likely the presence of a 100 200 m wide region of shallow dikes which are poorly magnetized relative to extrusives or a region heated above magnetic blocking or curie temperatures by intrusions during the most recent eruption though the latter interpretation cannot explain the low at the fossil trough in the first case this extrusive thinning implies a change in eruptive behavior over the last 750 1500 years given the local spreading rate for the latter case thermal models suggest the anomaly had to have been created by a dike swarms totaling at least 45 m width during the most recent eruption s corresponding to 300 years of plate spreading models indicate that the source of the low is centered slightly east of the axial trough this offset suggests that the axis has been progressively migrating westward over the past millennium consistent with other studies covering greater length and timescales westward migration provides an explanation for the preferential emplacement of recent lavas flows west of the axis evident in abe bathymetry and submersible observations index terms 3035 marine geology and geophysics midocean ridge processes 1517 geomagnetism and paleomagnetism magnetic anomaly modeling 8499 volcanology general or miscellaneous 3045 marine geology and geophysics seafloor morphology and bottom photography 3005 marine geology and geophysics geomagnetism 1550

transient deformation following the 30 january 1997 dike intrusion at kīlauea volcano hawai i on 30 january 1997 an intrusion on k lauea volcano opened a new fissure within the east rift zone erz at n pau crater 3 km uprift from the ongoing eruptions at pu u the fissure eruption lasted 22 h and opened a 5 1 km long nearly vertical dike 1 9 m extending from the surface to a depth of 2 4 km owen et al 2000b during the eruption the lava pond at pu u drained and eruptions ceased there pu u eventually refilled in late february and eruptions resumed there on 28 march 1997 continuous gps data show a large transient following the 30 january 1997 dike intrusion after lengthening 40 cm during the initial eruption the baseline between two stations spanning the erz lengthened an additional 10 cm over the following 6 months a coastal station kaep also exhibited transient deformation as it continued to move southward 5 cm over the same 6 month period the baseline between two stations spanning k lauea s summit caldera contracted sharply during the eruption but gradually recovered to slightly longer than its previous length 2 months after the intrusion we use the extended network inversion filter mcguire and segall 2003 to invert continuous gps data for volume change of a spherical pressure source under k lauea s summit opening distribution on a nearly vertical dike in the erz and potential slip on a decollement 9 km beneath the south flank following the 30 january intrusion rift extension continued below the initial dike intrusion for the duration of the transient decollement slip regardless of its assumed depth is not required to fit the data the modeled transient summit reinflation and rift opening patterns under n pau crater coincide with changes in observed behavior of pu u s lava pond rift opening accelerated while pu u eruptions paused and began to decelerate after the lava pond reappeared nearly a month after the n pau eruption the transient deformation is interpreted as resulting from shallow accommodation of the new dike volume

insar observations of post rifting deformation around the dabbahu rift segment afar ethiopia summary increased displacement rates have been observed following manylarge earthquakes and mag matic events although an order of magnitude smaller than the displacements associated with the main event the post seismic or post rifting deformation may continue for years to decades after the initial earthquake or dyke intrusion due to the rare occurrence of subaerial rifting events thereareveryfewobservationstoconstrainmodelsofpost riftingdeformation in2005 september a 60 km long dyke was intruded along the dabbahu segment of the nubia arabia plate boundary afar ethiopia marking the beginning of an ongoing rifting episode con tinued activity has been monitored using satellite radar interferometry and data from global positioning system instruments deployed around the rift in response to the initial intrusion using multiple satellite passes we are able to separate the rift perpendicular and vertical displacement fields around the dabbahu segment rift perpendicular and vertical rates of up to 180 and 240 mm yr 1 respectively here we show that models of viscoelastic relaxation alone are insufficient to reproduce the observed deformation field and that a large portion of the observed signal is related to the movement of magma within the rift segment our models suggest upper mantle viscosities of 10 18 19 pas overlain by an elastic crust of between 15 and 30km to fit the observations inflation and deflation of magma chambers in the centre of the rift and to the south east of the rift axis is required at rates of 0 13 and 0 08km 3 yr 1

satellite radar interferometry 1993 1999 suggests deep accumulation of magma near the crust mantle boundary at the krafla volcanic system iceland 1 deep magma accumulation near the crust mantle boundary 21 km depth at the krafla volcanic system is suggested from insar observations a best fit model derived from four interferograms covering 1993 1999 comprises an opening dike representing plate spreading and post rifting deformation and two mogi sources a mogi source deflating at a rate of 0 3 106 m3 yr coincides with the shallow krafla magma chamber while a deeper inflating mogi source further north at 21 km depth inflates at a rate of 26 106 m3 yr the inflating source is at or near the crust mantle boundary as identified by seismic studies and is interpreted as accumulating magma

stress generation at ridge axes by plate divergence and magma rise abstract a model is explored for the rifting process particularly at the divergent plate boundary in iceland based on direct observations of the krafla rifting episode 1975 1984 magma accumulates near the axial crust mantle transition as the plates diverge from each other and compression on the boundary decreases two dimensional finite element modelling is applied to investigate how divergence and buoyant rise of magma interact in triggering rifting both processes are found to be important but long intervals between rifting episodes require the time average deviatoric stress to be compressible normal to the axis

postrifting anelastic deformation around the spreading plate boundary north iceland 2 implications of the model derived from the 1987 1992 deformation field a decade long rifting episode that began in the krafla volcanic system north iceland in 1975 caused substantial regional postevent anelastic deformation this was modeled as viscous relaxation in an elastic viscoelastic structure by hofton and foulger this issue the results from modeling the deformation detected in north iceland have far reaching implications both for local and regional processes and for the fundamental behavior of deformation around spreading plate boundaries in general tilt in the vicinity of the krafla volcano fits the model well after 1988 1989 which suggests that the volcano magma chamber stopped inflating deflating in 1988 a viscosity of 0 8 1018 pa s was required to match the local tilt data less than that predicted for north iceland as a whole vertical motion measured using the global positioning system gps 1987 1992 around the ice cap vatnajokull is inconsistent with isostatic uplift using the elastic viscoelastic model to predict motion in other regions of iceland suggests that the deformation effects of the krafla episode are significant in many parts of iceland and should be taken into account when modeling deformation there though not a realistic plate boundary model interesting complexities of the elastic viscoelastic model are highlighted by deformation modeling of an infinitely long dike this predicts that the amount of horizontal displacement close to the dike may exceed the amount of initial dike opening early in the spreading cycle a more realistic approximation to the plate boundary in north iceland involving five overlapping segments that experience dike emplacements at discrete intervals suggests that the width of the zone within which transient time dependent deformation occurs may be several hundred kilometers wide considerably wider than the neovolcanic zone a kinematic approach to describing plate motions is not appropriate close to spreading plate boundaries and elsewhere where the viscosity of the earth is low

transient deformation in the asal ghoubbet rift djibouti since the 1978 diking event is deformation controlled by magma supply rates the asal ghoubbet rift ag rift in djibouti lies in the subaerial continuation of the aden ridge system thereby constituting a unique location to study rifting processes and mechanisms involved in continental breakup and oceanic spreading continually upgraded and expanded geodetic technology has been used to record the 1978 asal rifting event and postdiking deformation in light of recent results obtained for the manda hararo dabbahu rifting event 2005 2010 we propose that the horizontal and vertical geodetic data can be modeled with a double source involving a dike like inflation component aligned along the rift axis and a spherical pressure source located at midsegment below the fieale caldera by revisiting the codiking data we propose that the reservoir below fieale could have fed at least partially the 1978 injection and the contemporaneous ardoukoba eruption and potentially induced local subsidence due to magma draining out of the central reservoir as an alternative to previously proposed viscoelastic relaxation models we reinterpret postdiking observations using a purely elastic rheology we determine the relative contribution of a midsegment reservoir inflation and a dike like opening component together with their respective time evolutions our results suggest that interactions between steadily accumulating tectonic strain and temporal variations in melt supply to the shallow magma plumbing system below the ag rift may entirely explain the geodetic observations and that viscoelastic deformation processes played a minor role in the 30 years following the 1978 rifting event

viscosity structure beneath northeast iceland the dynamics of crustal rifting in iceland depend strongly on the lower crustal rheology which controls the intensity of upper crustal stress concentration and scale time of heat diffusion from the underlying mantle plume while magnetotelluric surveys suggest the presence of a pervasive hot and highly ductile lowermost crust with possibly high fraction of partial melt observations of low seismic attenuation and strong shear wave transmission suggest a much cooler lower crust and upper mantle since viscosity is also sensitive to the degree of partial melt present viscosity estimates for these regions could shed light on the factors responsible for these observations in this study we utilize horizontal and vertical displacement vectors determined in gps campaigns in northeast iceland since 1986 these are modeled in terms of steady state tectonic loading plus postseismic postdiking relaxation following the 1975 1984 krafla rifting episode as first proposed by foulger and others with the elastic part of the model fixed by external constraints these data have a high sensitivity to the viscosity structure beneath iceland lower crust and upper mantle viscosities of about 3 1019 pa s and 3 1018 pa s respectively yield the closest agreement with the data our lower crustal viscosity estimate is consistent with the low attenuation and low subsolidus temperature for the lower crust inferred in recent studies inversions for fissure opening during the krafla rifting episode yield about 7 m of opening centered on the krafla rift as is observed allowing for contemporaneous deep rifting on vertical faults along the askja segment partially accounts for the observed increase in separation across the rift during 1987 1992 but does not account for large displacements in the southeastern part of the network or the large relative subsidence around the askja rift during 1987 1990 recent deep normal faulting beneath the askja rift and further south might explain all of these remaining features

hydrothermalism in the mediterranean sea abstract hydrothermalism in the mediterranean sea results from the collision of the african and european plates with the subduction of the oceanic part of the african plate below europe high heat flows in the resulting volcanic arcs and back arc extensional areas have set up hydrothermal convection systems most of the known hydrothermal sites are in shallow coastal waters

a new strategy for estimating geophysical parameters from insar data application to the krafla central volcano in iceland 1 we develop validate and apply a new strategy for estimating parameters in a geophysical model from interferometric synthetic aperture radar insar measurements the observable quantity is a particular component of the deformation gradient tensor defined as the derivative of the change in range with respect to the easting coordinate this range change gradient is derived from wrapped phase data by a quadtree resampling procedure since the range change gradient is a continuous function of position the strategy avoids the pitfalls associated with phase unwrapping techniques to quantify the misfit between the observed and modeled values of the range gradient the objective function calculates the cost as the absolute value of their difference averaged over all samples to minimize the objective function we use a simulated annealing algorithm this algorithm requires several thousand evaluations of the fitting function to find the optimum solution the estimate of the model parameters that produces the lowest value of cost for computational efficiency we approximate the fitting function using a taylor series the simulated annealing algorithm then evaluates the approximate and fast version of the fitting function after performing these two steps several times the scheme converges typically in a few iterations we apply the strategy to krafla central volcano in iceland using a data set composed of eight interferometric pairs acquired by the ers 1 and ers 2 satellites over a 6 year interval between 1993 and 1999 we estimate the four parameters in a mogi model results suggest a source at 4 98 0 21 km depth and a deflation rate that decays exponentially over the interval in agreement with previous studies

the lithosphere asthenosphere boundary in the north west atlantic region abstract a detailed knowledge of the thickness of the lithosphere in the north atlantic is an important parameter for understanding plate tectonics in that region we achieve this goal with as yet unprecedented detail using the seismic technique of s receiver functions clear positive signals from the crust mantle boundary and negative signals from a mantle discontinuity beneath greenland iceland and jan mayen are observed according to seismological practice we call the negative phase the lithosphere asthenosphere boundary lab the seismic lithosphere under most of the iceland and large parts of central greenland is about 80 km thick this depth in iceland is in disagreement with estimates of the thickness of the elastic lithosphere 10 20 km found from postglacial rebound data in the region of flood basalts in eastern greenland which overlies the proposed iceland plume track the lithosphere is only 70 km thick about 10 km less than in iceland which is located directly above the proposed plume at the western greenland coast the lithosphere thickens to 100 120 km with no indication of the iceland plume track identified below jan mayen the lithospheric thickness varies between 40 and 60 km

capturing magma intrusion and faulting processes during continental rupture seismicity of the dabbahu afar rift continental rupture models emphasize the role of faults in extensional strain accommodation extension by dyke intrusion is commonly overlooked a major rifting episode that began in 2005 september in the afar depression of ethiopia provides an opportunity to examine strain accommodation in a zone of incipient plate rupture earthquakes recorded on a temporary seismic array 2005 october to 2006 april direct observation of fault patterns and geodetic data document ongoing strain and continued dyke intrusion along the 60 km long dabbahu rift segment defined in earlier remote sensing studies epicentral locations lie along a 3 km wide 50 km long swath that curves into the se flank of dabbahu volcano a second strand continues to the north toward gab ho volcano considering the 8 m of opening in the september crisis we interpret the depth distribution of microseismicity as the dyke intrusion zone the dykes rise from 10 km to the near surface along the 60 km long length of the tectono magmatic segment focal mechanisms indicate slip along nnw striking normal faults perpendicular to the arabia nubia plate opening vector the seismicity insar continuous gps and structural patterns all suggest that magma injection from lower or subcrustal magma reservoirs continued at least 3 months after the main episode persistent earthquake swarms at two sites on dabbahu volcano coincide with areas of deformation identified in the insar data 1 an elliptical northwestward dipping zone of seismicity and subsidence interpreted as a magma conduit and 2 a more diffuse 8 km radius zone of shallow seismicity 2 km above a shadow zone interpreted as a magma chamber between 2 5 and 6 km subsurface insar and continuous gps data show uplift above a shallow source in zone 2 and uplift above the largely aseismic gab ho volcano the patterns of seismicity provide a 3 d perspective of magma feeding systems maintaining the along axis segmentation of this incipient seafloor spreading segment

deformation partitioning inside a fissure swarm of the northern icelandic rift abstract extension in an oceanic rift domain is accommodated by fissuring and faulting processes the fissuring including dike injection involves extension and an increase in crustal volume consistent with magma supply as in oceanic ridge and hot spot contexts the faulting mainly involves thinning and extension the relationship between these two deformation modes is analysed in the northern rift of iceland in the active rift zone deformation and volcanism occur within 2 3 km wide fissure swarms separated by 10 km wide zones where deformation is minor based on topographic and geological mapping the local structure in a major fissure swarm was analysed the fracture density increases near the faults but also beyond their tips the geometrical analysis of structures revealed that for the last 10 000 years block faulting and tilting accommodated about one half of the extension the other half resulting from fissuring this distribution shows that even in an active volcanic zone with widespread magmatism the stretching resulting from normal shear can play an important role as estimated from the ages of eroded surfaces and lava flows the deformation rates range between 1 5 and 15 cm year these values are compared with other rates estimated for different time intervals in the same area

fracture analysis hydrothermal mineralization and fluid pathways in the neogene geitafell central volcano insights for the krafla active geothermal system iceland abstract studies on hydrothermal fluids circulation and tectonic structures improve the geothermal exploration and exploitation in iceland in this paper we present the results of an integrated study structural kinematic fluid inclusions analyses carried out on a fossil and exhumed geothermal system at geitafell se iceland considered an analogue of the active krafla geothermal system ne iceland our work is addressed to the relationships between the main structures and the paleo geothermal fluids circulation at the boundary between the magma chamber with magma of gabbroic composition and host rocks with the aim to get information on those factors having controlled the hydrothermal fluids flow its storage and the rocks fluid interaction at 1 2 km crustal depth the structural study is mostly based on defining the architecture of mineralized brittle shear zones where by measuring length and width of mineralized fractures permeability was computed in 1 3 10 14 m2 as regards the hydrothermal paleo fluids fluid inclusions in hydrothermal quartz and calcite usually associated with epidote and chlorite i e the typical alteration minerals found in icelandic geothermal fields recorded a fluid circulation with temperature ranging from about 255 c to 320 c and boiling at the highest temperature transient pressure drop likely related to fracture opening and propagation during tectonic activity triggered local boiling process testified by fluid inclusions in a calcite sample during a late evolution the system progressively cooled down to 60 90 c paleo geothermal fluids were characterized by low salinity meteoric water although higher salinity fluids up to 10 6 wt nacl equiv entered in the system and mixed with meteoric fluid in the end based on results from fluid inclusions derived fluid density permeability values and modeling of viscosity hydraulic conductivity between 2 8 10 8 to 1 8 10 7 m s was also computed within the fluids channelling brittle shear zones we finally encourage the key study of fossil geothermal systems as a tool for getting parameters commonly obtained after drilling in active systems

architecture of normal faults in the rift zone of central north iceland this work examines the architecture and propagation of normal faults that accommodate extension in the rift zone of central north iceland it is based on a structural study of portions of three grabens their normal faults and dilational fractures in basaltic lavas of the fremri namur and dyngjufjol volcanic systems we analyzed the shape throw and dilation along faults in order to infer their possible evolution the shape of throw profiles departs from elliptical varying along each fault with throw being lower at the fault and segment tips propagation of fault segments is accommodated by dilational fractures nucleating at their respective tips segments link by lateral propagation of one or both tips or by the development of intervening linking segments two orders of fault segmentation indicate that the present throw accumulated by many increments as the fault scarps changed their geometries faults have large dilational components weakly correlated with fault throw and length

subaerial submarine evidence of structures feeding magma to stromboli volcano italy and relations with edifice flank failure and creep abstract structural field data including parameters of 109 sheets strike mostly n10 55 e average dip angle mostly 70 90 average thickness two peaks at 1 2 m and 4 2 m and of 14 eruptive fissures and vents as well as bathymetric and numerical data suggest that stromboli experienced the development of a volcanic rift zone across the cone summit with a tendency to widen towards the lower submarine flanks during the last 100 ka inside this rift zone intrusive sheets mostly strike ne and become more abundant more steeply dipping and thicker on approaching the rift axis this rift persistence reflects the edifice s state of stress with a nw se horizontal least principal stress on the magma intrusion paths after 13 ka ago sheets mainly intruded with a horseshoe shaped geometry in plan view along the boundaries of the northwestern unstable volcano flank where four main lateral collapses occurred during the last 13 ka they also intruded with a ne strike across the volcano summit and the northeastern volcano flank the horseshoe shaped geometry is influenced by the lateral creep of the volcano flank which provides space for intrusion along the transtensional boundaries of the unstable sector where thinner sheets were emplaced when the collapse depression was filled up by new deposits ne striking thicker dykes emplaced across the northwestern volcano flank indicating a rapid re establishment of regional tectonic forces across the cone unbuttressing due to the present sciara del fuoco depression and ongoing flank creeping can account for intrusions also inside the sciara del fuoco the latter in 2007

comparison of geodetic and geologic data from the wasatch region utah and implications for the spectral character of earth deformation at periods of 10 to 10 million years the wasatch fault and adjacent fault zones provide an opportunity to compare present day deformation rate estimates obtained from space geodesy with geologic displacement rates over at least four temporal windows ranging from the last millennium up to 10 myr the three easternmost gps sites of the basin and range geodetic network bargen at this latitude define a 130 km wide region spanning three major normal faults extending east west at a total rate of 2 7 0 4 mm yr with an average regional strain rate estimated to be 21 4 nstrain yr about twice the basin and range average on the wasatch fault the vertical component of the geologic displacement rate is 1 7 0 5 mm yr since 6 ka 0 6 mm yr since 130 ka and 0 5 0 7 mm yr since 10 ma however it appears likely that at the longest timescale rates slowed over time from 1 0 to 1 4 mm yr between 10 and 6 ma to 0 2 to 0 3 mm yr since 6 ma the cumulative vertical displacement record across all three faults also shows time variable strain release ranging from 2 to 4 mm yr since 10 ka to 1 mm yr averaged over the past 130 kyr conventional earthquake recurrence models reid type behavior would require an accordingly large variation in strain accumulation or loading rate on a 10 kyr timescale for which there appears to be no obvious geophysical explanation alternatively seismic strain release given a wide range of plausible constitutive behaviors for frictional sliding may be clustered on the 10 kyr timescale resulting in the high holocene rates with comparatively low uniform strain accumulation rates on the 100 kyr timescale wallace type behavior the latter alternative combined with observations at the million year timescale and the likelihood of a significant contribution of postseismic transients implies maxima of spectral amplitude in the velocity field at periods of 10 myr variations in tectonic loading 10 kyr clustered strain release and of 100 years postseismic transients if so measurements of strain accumulation and strain release may be strongly timescale dependent for any given fault system

seismotectonics of the calabrian arc abstract relocation of intermediate and deep earthquakes of tyrrhenian sea area through joint hypocenter determination for the period 1962 1979 has allowed a more detailed definition of the geometry of this peculiar benioff zone earthquakes dip along a quasi vertical plane to 250 km depth there is a 50 dip in the 250 340 km depth range and a low dip angle to 480 km depth the structure sketched from the hypocenters is almost continuous but most energy has been released in the 230 340 km depth interval an evaluation of fault plane solutions of intermediate earthquakes in this area indicates predominance of down dip compressions in the central part of the slab at the border strike slip motion occurs independent of depth some earthquakes that occurred at intermediate depth less than 100 km along the ionian margin of calabria show predominance of reverse faulting with the p axis oriented se nw however shallow earthquakes in the calabria sicily region indicate a more complex motion with predominance of normal faulting a possible interpretation of these features according to the available geological history which involves subduction of continental lithosphere is discussed

holocene vertical deformation outside the active rift zone of north iceland in iceland the main deformation activity occurs not only in the active rift zone where volcanism and seismicity coexist in fissure swarms but also in the transfer zones north and south where the main part of the seismic activity is located many studies described extension and vertical displacement associated with rifting and magmatism inside this rift zone they have classically assumed that no deformation occurs outside the rift zone however few studies have already been conducted outside the rift zone this work focuses on an area west of the rift zone in the akureyri area which displays an anomalous topography we illustrate an ene tilting of 0 23 since approximately 17 000 years ago using ancient lacustrine shorelines mapped in detail with a differential gps survey and analysis of spot images this degree of tilting corresponds to a vertical displacement of 160 m i e a deformation rate of 1 cm year which is close to values measured in the active zone this long wavelength deformation is compatible both with i the topographic anomaly of the akureyri area and with ii the very diffuse microseismicity that affects northern iceland the origin of this tilting is discussed and the different hypotheses glacial loading response rift uplift transform zone effect thermal doming local magmatic intrusion are reviewed our results structural data and the topography are more consistent with a local magmatic intrusion inducing local doming this work is the first attempt at quantifying long term deformation outside the rift zone in iceland

ages of the steens and columbia river flood basalts and their relationship to extension related calc alkalic volcanism in eastern oregon stratigraphic and chemical correlations of tertiary volcanic units in eastern oregon confirm that the steens basalt represents the earliest eruptions of the columbia river flood basalt province field correlations are supported by major and trace element analyses and confirmed by 4 0 ar 3 9 ar dates within the basalt of malheur gorge situated between steens mountain and the southernmost extent of the previously mapped columbia river basalt group the lowest unit correlates with the steens basalt and the conformably overlying middle and upper units correlate with the imnaha and grande ronde basalt formations of the columbia river basalt group new dates indicate that imnaha and grande ronde basalt formations on the columbia plateau 90 of the columbia river basalt group erupted between 16 1 and 15 0 ma these were immediately preceded by the steens basalt a plagioclase phyric tholeiite that erupted above the calculated position of the yellowstone hotspot at 16 6 ma in eastern oregon the flood basalt tholeiites of steens mountain and malheur gorge form a voluminous but brief interlude 16 6 15 3 ma superimposed on the low volume calc alkalic to mildly alkalic volcanism associated with continuing eocene to present east west extension

deformation across the asal ghoubbet rift djibouti uplift and crustal extension 1979 1986 geodetic networks of different sizes have been established since 1973 in the asal ghoubbet rift in south western afar in order to study crustal movements across an extensive plate boundary partial or entire remeasurements of these networks during the 1979 1986 interval provide detailed data about strain accumulation and vertical movements after a seismo volcanic rifting event that occured in november 1978 along this area similar to an oceanic accreting plate boundary the results of the geodetic monitoring indicate a continuous opening of the rift zone of about 6 cm yr since 1979 the deformation is principally distributed in the inner floor of the rift with a uniaxial extensional n40 e strain rate of about 15 strain yr the results of the 1979 and the 1984 85 levellings define the development of a large uplift across the rift zone amounting 15 cm during this 6 year interval this uplift is accompanied with a relative collapse of the inner floor zone of about 4 cm which is associated with creep on the active faults bounding the rift two stages of the rifting process are successively observed in djibouti an abrupt rifting event in 1978 and then a continuous accumulation stage which is controled by the plate motion and partially by magmatic inflation and active creep faulting

rheologic controls on inter rifting deformation of the northern volcanic zone iceland article i nfo extensional rifts are characterized by significant lateral variations in crustal rheology as lithospheric material cools and advects away from the rift axes nevertheless most models used for modeling of crustal movements consist of horizontal layers with uniform properties this paper explores the role spatial variation of rheological properties play in modifying the style of surface deformation at rifts by comparing finite element model predictions to inferred inter rifting surface deformation in iceland s northern volcanic zone crustal deformation has been observed by satellite radar interferometry insar and gps measurements extension is observed across the entire northern rift zone whereas subsidence occurs in two distinct areas corresponding to mapped fissure swarms where major rifting took place in 1975 1984 and 1874 1876 respectively our models indicate that the observed inter rifting plate spreading deformation field is controlled by rheological variations within the en echelon arrangement of fissure segments and that a central ridge axes model does not apply uniform stretching across a plate boundary zone where fissure swarms are weaker than the surrounding crust and reflect the surface expression of the rift reproduces the characteristics of the deformation field the most realistic fissure swarm structure consists of a wedge of

deformation at krafla and bjarnarflag geothermal areas northern volcanic zone of iceland 1993 2015 abstract the krafla volcanic system has geothermal areas within the krafla caldera and at bjarnarflag in the krafla fissure swarm 9 km south of the krafla caldera arrays of boreholes extract geothermal fluids for power plants in both areas we collected and analyzed insar gps and leveling data spanning 1993 2015 in order to investigate crustal deformation in these areas the volcanic zone hosting the geothermal areas is also subject to large scale regional deformation processes including plate spreading and deflation of the krafla volcanic system these deformation processes have to be taken into account in order to isolate the geothermal deformation signal plate spreading produces the largest horizontal displacements but the regional deformation pattern also suggests readjustment of the krafla system at depth after the 1975 1984 krafla rifting episode observed deformation can be fit by an inflation source at about 20 km depth north of krafla and a deflation source at similar depth directly below the krafla caldera deflation signal along the fissure swarm can be reproduced by a 1 km wide sill at 4 km depth closing by 2 4 cm per year these sources are considered to approximate the combined effects of vertical deformation associated with plate spreading and post rifting response local deformation at the geothermal areas is well resolved in addition to these signals insar shows that deformation at bjarnarflag is elongated along the direction of the krafla fissure swarm 4 km by 2 km while it is circular at krafla 5 km diameter rates of deflation at krafla and bjarnarflag geothermal areas have been relatively steady average volume decrease of about 6 6 10 5 m 3 yr for krafla and 3 9 10 5 m 3 yr for bjanarflag are found at sources located at 1 5 km depth when interpreted by a spherical point source of pressure this volume change represents about 8 10 3 m 3 ton of the mass of geothermal fluid extracted per year indicating important renewal of the geothermal reservoir by water flow

post rifting relaxation in the afar region ethiopia 1 crustal accretion at divergent plate boundaries typically occurs via the periodic intrusion of dikes but their emplacement and the associated deformation are rarely observed the few existing observations at subaerial rifts show that these diking events are followed by a decadal scale period with extension rates faster than the secular divergent plate motion this transient accelerated deformation has been explained by continued subsurface magma injection or by relaxation in the viscoelastic mantle of the stress changes imparted by dike opening for the first time gps measurements were collected within a few months of a rifting event at a major plate boundary the september 2005 60 km long dike intrusion in the dabbahu segment afar ethiopia extension rates for the first 3 years greatly exceed the plate motion nubia arabia secular divergence rate even at sites located more than 60 km from the rift axis here we show that these observations are consistent with stress relaxation in a viscoelastic upper mantle with a viscosity of about 5 1018 pa s overlain by a 12 14 km thick elastic crust the alternative model of continued diking requires continuous opening well below the moho and is therefore unlikely instead magma injection in afar since june 2006 has taken the form of smaller discrete diking events tapping into a mid crustal melt reservoir under the segment center

numerical modelling of quaternary deformation and post rifting displacement in the asal ghoubbet rift djibouti africa over the last three decades a host of information on rifting process relating to the geological and thermal structure long time scale deformation quaternary and holocene and rifting cycle displacement across the asal ghoubbet rift has been made available these data are interpreted with a two dimensional thermo mechanical model that incorporates rheological layering of the lithosphere dyke inflation and faulting active fault locations and geometry are mainly controlled by both thermal structure and magma intrusion into the crust the distributed slip throughout the inner rift is related to the closeness of magma chamber leading to additional stress into the upper thinned crust assuming a constant arabia somalia motion of 11 mm year the variation of subsidence rate between the last 100 and 9 ka is associated with a decrease of the average injection rate from 10 to 5 mm year these values about equal to the regional opening rate suggest that both volcanism and tectonic play an equivalent role in the rifting process our modelled sequence of events gives one possible explanation for both vertical and horizontal displacements observed since the 1978 seismovolcanic crisis although part of the post rifting deformation could be due to viscous relaxation the high opening rate in the first years after the event and the abrupt velocity change in 1984 1986 argue for a large dyke inflation of 12 cm year ending in 1985 the asymmetric and constant pattern of the gps velocity since 1991 suggests that present post rifting deformation is mainly controlled by fault creep and regional stretching this study demonstrates the internal consistency of the data set highlights the role of magmatism in the mechanics of crustal stretching and reveals a complex post rifting process including magma injection fault creep and regional stretching d 2005 elsevier b v all rights reserved

shallow water polymetallic sulfide deposits in the aeolian island arc back arc basins represent the best known present day sites of ongoing massive sulfide deposition in subduction related tectonic settings here we present results obtained from shallow water sea floor sampling of barium rich zinc and lead sulfides genetically analogous to kuroko type black ore on an active island arc the samples come from a newly discovered hydrothermally active depression on the submerged volcanic edifice of panarea in the aeolian island arc tyrrhenian sea hydrothermal activity in the area is represented by extensive gas venting associated with white sea floor aprons of sulfate and iron oxide precipitates a series of barite rich sediment samples containing galena sphalerite and pyrite was recovered 40 cm below the sea bed at a water depth of 80 m the deposits represent a unique example of massive sulfide deposition in an active island arc not yet affected by strong extensional tectonics and reveal that in this environment the formation of kuroko type black ore sulfides can take place at shallow water depths

current plate boundary deformation of the afar rift from a 3 d velocity field inversion of insar and gps extension faulting and magmatism are the main controls on the magnitude and localization of strain at mid ocean ridges however the temporal and spatial patterns of such processes are not clear since the strain distribution has not been resolved in the past at sufficient spatial resolution and over extended areas interferometric synthetic aperture radar insar and gps data with unprecedented resolution are now available to us from the afar rift of ethiopia here we use a velocity field method to combine insar and gps to form the first high resolution continuous three dimensional velocity field of afar we study an area that is 500 km wide and 700 km long covering three branches of the afar continental rift and their triple junctions our velocity field shows that plate spreading is currently achieved in afar in contrasting modes a transient postdiking deformation is focused at the dabbahu rift segment while in central afar spreading is distributed over several overlapping segments and southern afar exhibits an interdiking deformation pattern focused at the asal ghoubbet segment we find that current spreading rates at dabbahu following the 2005 2010 intrusions are up to 110 mm yr 6 times larger than the long term plate divergence a segment centered uplift of up to 80 mm yr also occurs indicating that magma flow is still a primary mechanism of deformation during postdiking on the other hand no vertical displacements are currently observed in central and southern afar suggesting lack of significant magmatic activity at shallow levels

surface deformation and strike slip faulting controlled by dyking and host rock lithology a compendium from the krafla rift iceland abstract in the present work we have obtained a detailed complete view of surface deformation at a volcano tectonic structure belonging to the krafla fissure swarm ne iceland through the integration of field mapping and the aerial structure from motion technique asfm as well as through orthomosaics and digital surface models dsms the documented 2 5 km long deformation zone is marked by a range of features such as topographic bulging parallel extension fractures and narrow grabens affected by local floor uplift which we have explained as the effect of the northward shallow propagation of a dyke from the krafla magma chamber our lithostratigraphic field survey of the area at a 1 5000 scale indicates that major changes in the pattern of surface deformation take place across the contacts between deposits with different rheological properties the transition from very stiff lavas to softer hyaloclastites produces changes from extensional fracturing to normal faulting though the width of the deformation zone does not change we have also documented a number of ne sw left lateral and nnw sse right lateral strike slip fault zones that are rotated in clockwise and anticlockwise sense to the main nne ssw graben trend and extend outwards as much as about 17 m the graben zone terminates to the north by splitting into two strike slip fault zones these were likely generated at the front of the dyke tip during its propagation and were later on bypassed by the advancement of the dyke as the dyke entered the soft hyaloclastites propagation was slowed down and the dyke became thicker

crustal deformation from 1992 to 1995 at the mid atlantic ridge southwest iceland mapped by satellite radar interferometry satellite radar interferometry observations of the reykjanes peninsula oblique rift in southwest iceland show that the reykjanes central volcano subsided at an average rate of up to 13 millimeters per year from 1992 to 1995 in response to use of its geothermal field interferograms spanning up to 3 12 years also include signatures of plate spreading and indicate that the plate boundary is locked at a depth of about 5 kilometers below that depth the plate movements are accommodated by continuous ductile deformation which is not fully balanced by inflow of magma from depth causing subsidence of the plate boundary of about 6 5 millimeters per year

time dependent deformation of the eastern flank of mt etna after slip or viscoelastic relaxation abstract major results of post intrusive gps monitoring of the 5 months encompassing and following the onset of 2002 03 mt etna eruption are presented and discussed here the overall ground deformation pattern is characterized by a time dependent relaxation function we evaluated two different parametrizations of relaxation functions each one linked to a different relaxation process i viscoelastic relaxation and ii after slip mechanism the former indicated that the process occurred within a weak layer characterized by viscosity values ranging between 7 1 1014 pa s and 1 3 1015 pa s the latter evidenced that frictional processes occurred beneath a layer of velocity strengthening having a thickness with values ranging between 0 4 km and 1 7 km either model may explain the observed time dependent deformation both from the statistical point of view and the comparison with geologic and seismic information although we are unable to favour one model rather than the other these results indicate that the mechanism responsible for the observed post intrusive deformation occurs within a sliding zone located inside the clayey sedimentary basement

benthic foraminiferal distribution at middle valley juan de fuca ridge a northeast pacific hydrothermal venting site fourteen benthic foraminiferal bearing surface samples collected from the area of active venting aav in middle valley at a depth of 2430 m at the northern end of juan de fuca ridge northeast pacific ocean yielded 156 identifiable taxa sample sites were selected near active or recently active vents where temperatures up to 274 c have been measured the sea floor beyond hydrothermal mounds is characterized by unaltered soft hemipelagic mud and is dominated by a calcareous foraminiferal assemblage which includes numerous allochthonous species derived from the shelf via turbidite flows into the valley within the aav agglutinated foraminifera dominate with proximity to hydrothermal vent activity species of the family hormosinidae subfamily reophacinae dominate in the hydrothermal habitat because of their success at colonization attached agglutinated foraminifera densely colonize the indurated sediment in close proximity to vent sites however like the macrofauna observed near hydrothermal vents foraminifera do not venture into the extreme habitats where temperatures are greater than 20 c where clam beds are found in association with active hydrogen sulphide venting benthic foraminifera occur rarely these beds occur in loose pyritic mud that is toxic to all but species e g the clam culyp ena that co exist symbiotically with sulphur oxidizing bacteria the sparse distribution of calcareous benthic foraminifera in the vent areas is caused by enhanced carbonate dissolution at ph values of 5 2 low foraminiferal abundance and species diversity at the immediate site of active venting is attributed to rapid changes in physical and chemical conditions caused by circulating hydrothermal vent waters

surface deformation at the krafla volcano north iceland 1982 1992 extensive measurements of ground deformation at the krafla volcano iceland have been made since the beginning in 1975 of a series of eruptions and intrusions into the fissure system that extends north and south of the volcano i concentrate on measurements before and after the eruption of september 1984 the last event of this series when the largest volume of lava was erupted the patterns of ground deformation associated with the 1984 eruption determined by precision levelling electronic distance measurements and lake level observations were similar to earlier intrusions and eruptions in that the surface of the volcano subsided and the fissure system widened as magma moved laterally from a shallow central reservoir into the fissure system the shallow magma reservoir of krafla continued to expand for about five years after the eruption but a slow subsidence of the central area began in 1989 besides the presence of an inflating and deflating shallow magma reservoir at a depth of 2 5 km beneath the krafla caldera another inflating magma reservoir may exist at much greater depth below krafla the accumulation of compressive strain by numerous rift intrusions and eruptions since 1975 along the flanks of the north south krafla fissure swarm is being released slowly and will probably be reflected in the results of deformation measurements near krafla for the next several decades the total horizontal extension of the krafla rift system in 1975 1984 was about 9 m equal to about 500 years of constant plate divergence the extension is twice the accumulated divergence since previous rifting events and eruptions in 1724 1729

surface displacements on faults triggered by slow magma transfers between dyke injections in the 2005 2010 rifting episode at dabbahu manda hararo rift afar ethiopia the rifting episode that occurred in dabbahu manda hararo ethiopia between 2005 and 2010 during which 14 dyke intrusions were emitted was a unique opportunity to study interactions between tectonic deformation and magmatic processes while magmatism has been shown to control primarily the spatial and temporal distribution of dyke intrusions during this accretion sequence the role of faults in accommodating plate spreading in rift segments is poorly understood during interdyking periods transient ground deformation due to magma movement is generally observed investigating such a small scale deformation and in particular the movement along faults during these periods will help understanding the factors that trigger fault movement in magmatic rifts we analyse fault activity during three interdyking periods 2006 december june d0 d1 2007 january july d5 d6 and 2009 november january d10 d11 the time space evolution of surface displacements along 700 faults is derived from pairs of ascending and descending sar interferograms surface slip distributions are then compared with codyking ground deformation fields the results show that faults are mainly activated above the areas affected by magma emplacement during interdyking periods a detailed analysis of brittle deformation during the six months following the 2005 september intrusion shows asymmetric deformation on the rift shoulders with significant opening on faults located to the west of the dyke we explain this feature by the activation of westward dipping pre existing faults with block rotations in between in addition we observe that the strip encompassing the activated faults narrows by 30 per cent from co to interdyking period this suggests that magma keeps migrating to shallower depths after the dyke intrusion during a rifting episode activation of faults in a pre existing fracture network therefore seems to be mainly controlled by deep magma processes

global review of human induced earthquakes the human induced earthquake database hiquake is a comprehensive record of earthquake sequences postulated to be induced by anthropogenic activity it contains over 700 cases spanning the period 1868 2016 activities that have been proposed to induce earthquakes include the impoundment of water reservoirs erecting tall buildings coastal engineering quarrying extraction of groundwater coal minerals gas oil and geothermal fluids excavation of tunnels and adding material to the subsurface by allowing abandoned mines to flood and injecting fluid for waste disposal enhanced oil recovery hydrofracturing gas storage and carbon sequestration nuclear explosions induce earthquakes but evidence for chemical explosions doing so is weak because it is currently impossible to determine with 100 certainty which earthquakes are induced and which not hiquake includes all earthquake sequences proposed on scientific grounds to have been human induced regardless of credibility challenges to constructing hiquake include under reporting which is 30 of m 4 events 60 of m 3 events and 90 of m 2 events the amount of stress released in an induced earthquake is not necessarily the same as the anthropogenic stress added because pre existing tectonic stress may also be released thus earthquakes disproportionately large compared with the associated industrial activity may be induced knowledge of the magnitude of the largest earthquake that might be induced by a project mmax is important for hazard reduction observed mmax correlates positively with the scale of associated industrial projects fluid injection pressure and rate and the yield of nuclear devices it correlates negatively with calculated inducing stress change likely because the latter correlates inversely with project scale the largest earthquake reported to date to be induced by fluid injection is the 2016 m 5 8 pawnee oklahoma earthquake by water reservoir impoundment the 2008 m 8 wenchuan people s republic of china earthquake and by mass removal the 1976 m 7 3 gazli uzbekistan earthquake the minimum amount of anthropogenic stress needed to induce an earthquake is an unsound concept since earthquakes occur in the absence of industrial activity the minimum amount of stress observed to modulate earthquake activity is a few hundredths of a megapascal and possibly as little as a few thousandths equivalent to a few tens of centimeters of water table depth faults near to failure are pervasive in the continental crust and induced earthquakes may thus occur essentially anywhere in intraplate regions neither infrastructure nor populations may be prepared for earthquakes human induced earthquakes that cause nuisance are rare but in some cases may be a significant problem e g in the hydrocarbon producing areas of oklahoma usa as the size of projects and density of populations increase the potential nuisance of induced earthquakes is also increasing and effective management strategies are needed

post eruptive gravity changes from 1990 to 1996 at krafla volcano iceland the 1975 1984 krafla rifting episode was a major lava and dyke producing event associated with the release of extensional strain accumulated over more than 200 years at the divergent plate boundary in north iceland the present work provides a unique example of gravity decreases and increases sustained over a long period following a major eruptive episode at a rift volcano after height correction persistent net gravity decreases over the source of observed mogi type deflation occur with gravity increases occurring further away from this centre of deformation gravity decreases are interpreted in terms of drainage from a shallow magma chamber the net gravity decreases require that at least 4 1010 kg of magma must have been drained during the 6 year observation period assuming a density of 2700 kg m 3 this magma would occupy 1 5 107 m3 and by analogy with results obtained for kilauea this implies a magma chamber volume change of 4 1 106 m3 this is consistent with the chamber volume change deduced from ground deformation data assuming a poisson s ratio of 0 25 and a mogi source net gravity increases are more spatially extensive and are most likely caused by migration of the steam water interface and or closure of micro fractures in lavas above the magma chamber during post eruptive cooling and contraction we present a model for the krafla magma chamber in which a cooling contracting and draining magma body causes subsidence at the surface these results contrast with observations from the askja caldera iceland where post eruptive deflation has been shown to be accompanied by negligible net gravity changes above the mogi type source in the caldera long term post eruptive deflation and magma drainage have not been observed at subduction related volcanoes this may be a function of a difference in magma viscosity

major tectonic rotation along an oceanic transform zone northern iceland evidence from field and paleomagnetic investigations abstract we provide structural and paleomagnetic evidence for major distributed rotational shear adjacent to and including the active husavik flatey transform fault in the southern part of the tjornes fracture zone of north iceland in the flateyjarskagi peninsula immediately south of the fault dike and bedding trends gradually change over about 11 km in approaching the transform fault main trace in map view this pattern is seen as a progressive clockwise curvature of structural elements that equals or in some cases exceeds 90 we present paleomagnetic analyses that verify large clockwise rotations of the same order as the curvature of structural elements observed we compare our field and paleomagnetic observations with results from careful analog modeling that demonstrate how regional curvature of structures results from distributed strain that dominates the early stages of transform shear deformation prior to the appearance of a transform fault system comprised of discrete shears our paleomagnetic results were obtained from one relatively undisturbed reference site and three sites of deformed rocks in flateyjarskagi and two in tjornes peninsula these results indicate large vertical axis rotations and the progressive deflection of dike trends throughout northern flateyjarskagi also serves as a separate quantitative estimate of vertical axis rotation the clockwise vertical axis rotation estimates of 71 97 from dikes are less than those deduced from paleomagnetism but the order of magnitude is correct and we attribute the differences as mainly reflecting imperfect bed attitude adjustments used in analyses the problem is typical for deformed rocks from high latitude sites with steeply inclined paleomagnetic vectors we consider our field measurements of dikes as more accurate indicators of true tectonic rotation this regional deformation of miocene rocks occurred early in the evolution of the tfz transform probably beginning ca 7 ma when an older rift zone present in western north iceland jumped to its present nvz location in northeast iceland the early deformations involved shear strain distributed over a region about 20 km broad and produced the curved structures this phase was followed by the sequential development of more narrowly distributed discrete shears generating the initial husavik flatey transform fault

glacial isostatic adjustment and the viscosity structure underlying the vatnajokull ice cap iceland we examine the dependence of glacial isostatic adjustment gia due to changes in the vatnajokull ice cap iceland on the underlying viscosity structure iceland offers a unique case study for gia research with a thinner elastic lithosphere underlain by a low viscosity zone or asthenosphere as opposed to regions such as fennoscandia or north america described by a thicker lithosphere while not necessarily featuring an asthenosphere

decadal modulation of repeating slow slip event activity in the southwestern ryukyu arc possibly driven by rifting episodes at the okinawa trough we studied 38 slow slip events sses in 1997 2016 beneath the iriomote island southwestern ryukyu arc japan using continuous global navigation satellite system gnss data these sses occur biannually on the same fault patch at a depth of 30 km on the subducting philippine sea plate slab with average moment magnitudes mw of 6 6 here we show that the slip accumulation rate cumulative slip lapse time of these sses fluctuated over a decadal time scale the rate increased twice around 2002 and 2013 concurrently with earthquake swarms in the okinawa trough this suggests that episodic activations of the back arc spreading at the okinawa trough caused extra southward movement of the block south of the trough and accelerated convergence at the ryukyu trench

mining induced crustal deformation in northwest germany modelling the rheological structure of the lithosphere abstract large scale coal mining in germany causes deloading of the crust by the removal of weight the crust responds by regional uplift while locally in the mining areas proper the surface subsides by sagging above extracted coal seams and removed groundwater in this study only the regional uplift is addressed the deloading consists of the total annual mass of coal and ground water removal since the 19th century for all significant mines the uplift data are from repeated levellings between about 1950 and 1980 reduced to mean annual rates of elevation change these observations are interpreted as the visco elastic response of the crust and uppermost mantle but alone they cannot discriminate layering from a homogeneous viscous half space or an elastic layer above such a half space if the seismically determined layered structure of upper and lower crust lithosphere mantle and asthenosphere is assumed to constrain the rheological structure a plausible model can be found by inversion only the lower crustal and asthenospheric effective maxwell viscosities are subjected to inversion a model consistent with the observations has the following properties 15 km of elastic upper crust 15 20 km thick lower crust of surprisingly low viscosity of 6 10 17 0 3 pas 30 50 km visco elastic mantle lithosphere with an assumed average viscosity of about 10 22 pas a visco elastic asthenosphere assumed bottomless with 5 10 19 0 6 pas the low viscosity values are consistent with active rifting and above average heat flow in the region the very low effective viscosity of the lower crust in this model it could be even lower if the thickness is smaller may be explained by elevated stresses quartz rheology and water content as suggested by xenoliths from the lower crust below the eifel region as a by product the tectonic subsidence in the lower rhine embayment filled with tertiary and quaternary sediments can be better defined by taking the anthropogenic effect into account

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

the westward drift of the lithosphere a rotational drag net westward rotation of the lithosphere relative to the underlying mantle is a controversial phenomenon fi rst attributed to tidal effects and later to the dynamics of mantle convection in spite of a number of independent geological and geophysical arguments for westward tectonic drift this phenomenon has received little recent attention we suggest that this differential rotation is a combined effect of three processes 1 tidal torques act on the lithosphere generating a westerly directed torque decelerating earth s spin 2 the downwelling of the denser material toward the bottom of the mantle and in the core slightly decreases the moment of inertia and speeds up earth s rotation only partly counterbalancing the tidal drag 3 thin 3 30 km layers of very low viscosity hydrate channels occur in the asthenosphere it is suggested that shear heating and the mechanical fatigue self perpetuate one or more channels of this kind which provide the necessary decoupling zone of the lithosphere

the kverkfjoll fissure swarm and the eastern boundary of the northern volcanic rift zone iceland rift zones at the divergent plate boundary in iceland consist of central volcanoes with swarms of fractures and fissures extending away from them fissure swarms can display different characteristics in accordance with their locations within the 50 km wide rift zones to better discern the characteristics of fissure swarms we mapped tectonic fractures and volcanic fissures within the kverkfjoll volcanic system which is located in the easternmost part of the northern volcanic rift zone nvz to do this we used aerial photographs and satellite images we find that rifting structures such as tectonic fractures holocene volcanic fissures and hyaloclastite ridges are unevenly distributed in the easternmost part of the nvz the kverkfjoll fissure swarm extends 60 km north of the kverkfjoll central volcano holocene volcanic fissures are only found within 20 km from the volcano the fjallgar ar area extending north of the kverkfjoll fissure swarm is characterized by narrow hyaloclastite ridges indicating subglacial volcanism we suggest that the lack of fractures and holocene volcanic fissures there indicates decreasing activity towards the north in the easternmost part of the nvz due to increasing distance from the long term spreading axis we argue that arcuate hyaloclastite ridges at the eastern boundary of the northern volcanic rift zone are mainly formed during deglaciations when three conditions may occur firstly eruption rate increases due to decompression of the mantle secondly the high tensile stresses accumulated during glaciations due to lack of magma supply may be relieved as magma supply increases during deglaciations thirdly faulting may occur during unloading due to differential movements between the thinner and younger northern volcanic rift zone crust and the thicker and older crust to the east of it

plate motion at the ridge transform boundary of the south cleft segment of the juan de fuca ridge from gps acoustic data 1 we measure the present day plate velocity of the juan de fuca ridge 25 km off axis to be 63 6 3 6 mm a at s67 2 e 7 9 degrees 1 relative to the pacific plate pa this velocity was derived from gps acoustic gpsa measurements in 2000 2001 2002 and 2003 that observed the position of a seafloor array 44 43 n 130 03 w 2900 m depth with a repeatability of 4 6 mm three transient events at the juan de fuca ridge and blanco transform account for 10 of this motion in viscoelastic modeling suggesting that the observed gpsa pa velocity is due primarily to steady state plate dynamics subtracting the modeled transient motion gives a velocity of 57 3 3 9 mm a at s72 9 e 12 1 degrees 1 which is consistent at the 95 confidence level with the velocity calculated from the wilson 1993 0 0 78 ma euler pole therefore this site is interpreted to be in a region of continuous full rate plate motion a robust result of this study which holds with and without correcting for transient motions these results provide direct geodetic evidence that spreading occurs predominantly within 25 km of the axis at this intermediate spreading rate ridge previously reported geodetic monitoring across the 1 km wide axial valley from 1994 1999 and 2000 2003 shows no significant extension chadwell et al 1999 hildebrand et al 1999 chadwick and stapp 2002 w w chadwick personal communication 2006 and seismic monitoring shows no activity this suggests the crust between 0 5 and 25 km off axis accommodates 26 mm of aseismic deformation each year through some combination of near axis fault motion and elastic strain accumulation

nonlinear strain buildup and the earthquake cycle on the san andreas fault two contrasting models of the earthquake deformation cycle on strike slip faults predict significant temporal declines in shear strain rate near the fault accompanied by a progressive broadening of the zone of deformation adjacent to it in the thin lithosphere model transient deformation results from flow in the asthenosphere due to stress relaxation following faulting through most or all of the lithosphere for an earth model with a thick elastic lithosphere plate thickness depth of seismic slip transient motions are due to postearthquake aseisrnic slip below the coseismic fault plane data from the san andreas fault indicate a long term temporal decrease in strain rate that persists for at least 30 years and may extend through the entire earthquake cycle observations support a cycle long rate decrease and a temporal spreading of the deformation profile only if movement cycles on the northern and southern locked sections of the fault are basically similar if so the usually lower strain rates and broader deformation zone currently observed on the southern san andreas represent a later evolutionary stage of the northern locked section where a great earthquake is a more recent occurrence although the data allow some extreme models to be discarded no sufficiently strong constraints exist to decide between the thin and thick lithosphere models regardless of the appropriate model the geodetic observations themselves indicate that strain buildup is sufficiently nonlinear to cause significant departures from recurrence estimates based on linear strain accumulation and the time predictable model

evidence from the oman ophiolite for sudden stress changes during melt injection at oceanic spreading centres the system of dikes in the uppermost mantle below oceanic spreading centres bears witness to the processes involved in the supply of melt to the ridge such systems cannot be studied on currently active ridges but can be seen in ophiolites in particular the oman ophiolite allows us to examine segments of a fast spreading ridge1 here we present a statistical analysis of the orientation of intrusions in the uppermost mantle section of the oman ophiolite spreading centres which points to the existence of two systems dikes whose azimuth is parallel to the sheeted dikes which we thus consider as related to the extensional lithospheric stress field and sills roughly parallel to the moho2 branching contemporaneous dikes and sills are frequently observed we suggest that these relations can be explained by repeated sudden changes of the dominant stress field from lithospheric to asthenospheric such changes could result from the episodic relaxation of lithospheric stress in response to tension fracturing of the lithospheric lid related to sudden melt surges this episodic behaviour is recorded in the sheeted dike complex with each dike each about one metre wide representing one of these fractures

locking depth and slip rate of the husavik flatey fault north iceland derived from continuous gps data 2006 2010 summary located at the northern shore of iceland the tjornes fracture zone tfz is a 120 km offset in the mid atlantic ridge that connects the offshore kolbeinsey ridge to the on land northern volcanic zone this transform zone is seismically one of the most active areas in iceland exposing the population to a significant risk however the kinematics of the mostly offshore area with its complex tectonics have not been adequately resolved and the seismic potential of the two main transform structures within the tfz the grimsey oblique rift gor and the husavik flatey fault hff in particular is not well known in summer 2006 we expanded the number of continuous gps cgps stations in the area from 4 to 14 the resulting gps velocities after four years of data collection show that the tfz accommodates the full plate motion as it is predicted by the morvel plate motion model in addition envisat interferograms reveal a transient uplift signal at the nearby theistareykir central volcano with a maximum line of sight uplift of 3 cm between summers of 2007 and 2008 we use a combination of an interseismic backslip and a mogi model in a homogeneous elastic half space to describe the kinematics within the tfz with a non linear optimization approach we fit the gps observations and estimate the key model parameters and their uncertainties which are among others the locking depth the partition of the transform motion between the two transform structures within the tfz and the slip rate on the hff we find a shallow locking depth of 6 3 1 7 1 2 km and transform motion that is accommodated 34 3 per cent by the hff and 66 3 per cent by the gor resulting in a slip velocity of 6 6 0 6 mm yr 1 for the hff assuming steady accumulation since the last two large m6 5 earthquakes in 1872 the seismic potential of the fault is equivalent to a mw6 8 0 1 event

the stress shadow induced by the 1975 1984 krafla rifting episode 1 it has been posited that the 1975 1984 krafla rifting episode in northern iceland was responsible for a significant drop in the rate of earthquakes along the husavik flatey fault hff a transform fault that had previously been the source of several magnitude 6 7 earthquakes this compelling case of the existence of a stress shadow has never been studied in detail and the implications of such a stress shadow remain an open question according to rate state models intense stress shadows cause tens of years of low seismicity rate followed by a faster recovery phase of rate increase here we compare the long term predictions from a coulomb stress model of the rifting episode with seismological observations from the sil catalog 1995 2011 in northern iceland in the analyzed time frame we find that the rift induced stress shadow coincides with the eastern half of the fault where the observed seismicity rates are found to be significantly lower than expected given the historical earthquake activity there we also find that the seismicity rates on the central part of the hff increased significantly in the last 17 years with the seismicity progressively recovering from west to east our observations confirm that rate state theory successfully describes the long term seismic rate variation during the reloading phase of a fault invested by a negative coulomb stress coincident with this recovery we find that the b value of the frequency magnitude distribution changed significantly over time we conclude that the rift induced stress shadow not only decreased the seismic rate on the eastern part of the hff but also temporarily modified how the system releases seismic energy with more large magnitude events in proportion to small ones this behavior is currently being overturned as rift induced locking is now being compensated by tectonic forcing

twenty five years of geodetic measurements along the tadjoura asal rift system djibouti east africa since most of tadjoura asal rift system sits on dry land in the afar depression near the triple junction between the arabia somalia and nubia plates it is an ideal natural laboratory for studying rifting processes we analyze these processes in light of a time series of geodetic measurements from 1978 through a network of about 30 gps sites covers the republic of djibouti additional points were also measured in yemen and ethiopia stations lying in the danakil block have almost the same velocity as arabian plate indicating that opening near the southern tip of the red sea is almost totally accommodated in the afar depression inside djibouti the asal ghoubbet rift system accommodates 16 1 mm yr of opening perpendicular to the rift axis and exhibits a pronounced asymmetry with essentially null deformation on its southwestern side and significant deformation on its northeastern side this rate slightly higher than the large scale arabia somalia motion 13 1 mm yr suggests transient variations associated with relaxation processes following the asal ghoubbet seismovolcanic sequence of 1978 inside the rift the deformation pattern exhibits a clear two dimensional pattern along the rift axis the rate decreases to the northwest suggesting propagation in the same direction perpendicular to the rift axis the focus of the opening is clearly shifted to the northeast relative to the topographic rift axis in the petit rift a rift in rift structure containing most of the active faults and the seismicity vertical motions measured by differential leveling show the same asymmetric pattern with a bulge of the northeastern shoulder although the inner floor of the rift is subsiding with respect to the shoulders all sites within the rift system show uplift at rates varying from 0 to 10 mm yr with respect to a far field reference outside the rift

benthic foraminifera from a recent shallow water hydrothermal environment in the aeolian arc tyrrhenian sea benthic foraminiferal assemblages from sediments around active or recently active hydrothermal vents in the aeolian arc have been investigated in four areas with different hydrothermal mineralization and seafloor exhalative activity at depths between 60 and 300 m active hydrothermal spots were distinguished from the surrounding sediments by direct observations by a remotely operated vehicle rov and subsequent seafloor sampling samples were taken at different water depths and are characterized by a number of mineralogies reflecting the changing temperature of the hydrothermal fluids overall benthic foraminifera are absent from sites with high temperature hydrothermal emissions and their distribution at sites of low temperature emissions is very patchy depending on the physical and or chemical conditions reflecting differences in temperature and composition of the emitted fluids in the secca del capo area which is free from recent emission activity the benthic foraminiferal population is indicative of a normal marine environment with globocassidulina subglobosa cassidulina crassa and gavelinopsis lobatulus in the basiluzzo secca dei pesci and panarea areas which are characterized by different degrees of hydrothermal activity three distinct assemblages have been recognized the first assemblage is dominated by ammodiscus catinus glomospira charoides and glomospira gordialis and indicates hydrothermal influence the second assemblage with lobatula lobatula asterigerinata mamilla rosalina globularis and quinqueloculina spp as main constituents indicates a seagrass environment influenced by diffuse emissions the third assemblage with abundant a catinus ammoglobigerina globigeriniformis g gordialis and haplophragmoides canariensis suggests low ph conditions resulting from acidic hydrothermal emissions leading to the dissolution of calcium carbonate tests d 2005 elsevier b v all rights reserved

strain accumulation across the asal ghoubbet rift djibouti east africa the asal ghoubbet rift in the republic of djibouti is a subaerial continuation of the gulf of aden ridge system many geological and tectonic arguments indicate that it represents one of the most typical spreading segments of the world rift system geodetic measurements regularly conducted from 1979 to 1983 across this area show an accumulation of extension in the inner floor of the rift valley with a rate of about 60 mm yr perpendicular to the normal faults that bound the rift periodic leveling surveys across major faults in the rift show vertical creep movements of about 5 to 10 mm yr although the observed deformations could be contamined by postseismic relaxation after a rifting episode in november 1978 the regularity and the magnitude of these movements during the last two years argue for a steady state mechanism of deformation of the rift valley the deformation is uniformly distributed in time and in space inside the inner floor of the rift the necking of the lithosphere in the central part of the rift and the high temperature gradient imply a ductile rheology in the inner floor which is stretched in a steady state process

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

post rifting stress relaxation at the divergent plate boundary in northeast iceland interaction of the elastic lithosphere with the underlying anelastic asthenosphere causes strain to propagate along the earth s surface in a diffusion like manner following tectonism at plate boundaries this process transfers stress between adjacent tectonic segments and influences the temporal tectonic pattern along a plate boundary observations of such strain transients have been rare and have hitherto been confined to strike slip and underthrusting plate boundaries1 here we report the observation of a strain transient at the divergent spreading plate boundary in iceland a global positioning system survey undertaken a decade after an episode of dyke intrusion accompanying several metres of crustal spreading reveals a spatially varying strain field with the expected diffusion pulse shape and an amplitude three times greater than the 5 7 cm that would be expected from the average spreading rate2 a simple one dimensional model with a thin elastic layer overlying a viscous layer fits the data well and yields a stress diffusivity of 1 1 a 0 3 m2s 1 combined with struc tural information from magnetotelluric measurements this implies a viscosity of 0 3 2 1019 pa s a value comparable to that derived for iceland from post glacial rebound23 but low compared with estimates for mantle viscosity obtained elsewhere3

tectonic volcanic and hydrothermal features of a submarine portion of the aeolian arc tyrrhenian sea abstract multibeam side scan sonar remotely operated vehicle and bottom sampling data have been acquired over a sector of the aeolian arc to assess its volcano tectonic characteristics and hydrothermal mineralization potential in this paper we report a description of the areas where major tectonic volcanic and hydrothermal activity has been observed there areas are the basiluzzo area the secca dei pesci area the southwestern panarea platform the secca del capo highs the offshore area between salina and lipari and the eastern slope of vulcano the study region lies at the junction of two major e w and ne sw trending tectonic lineaments through which the extensional faulting of the calabrian and sicilian margins leading to the formation of the tyrrhenian back arc sea took place the recent tectonics of the study area dominated by ne sw and e w trending extensional structures appears to be still controlled by the processes which have originated the tyrrhenian back arc opening many newly discovered recent volcanic features have been observed e g 1 three pairs of volcanic cones aligned along ne sw tectonic lineaments in the offshore between salina and lipari 2 a field of fresh tube and pillow lavas in the eastern offshore of vulcano 3 outcrops of fresh volcanic rocks in the panarea platform in particular around basiluzzo islet suggesting recent volcanic activity two types of hydrothermal mineralization were sampled i e sediment hosted sulphide deposits and fe rich crusts and sediments the polymetallic sulphide deposits were sampled in a depression south of basiluzzo islet for which area a kuroko type hydrothermal system reaching high temperature black ore deposition is suggested in the other sectors of the study area however a lower temperature hydrothermal circulation is thought to have originated the fe rich tetsusekiei type samples

the deep structure of the eolian arc filicudi panarea vulcano sector in light of gravity magnetic and volcanological data abstract the magnetic and gravimetric methods of investigation have been applied to the central sector of the eolian arc combined with geovolcanological data for the purpose of defining the structural framework in which the volcanism developed the processing of the magnetic and gravimetric data involved 3d modeling of the volcanic bodies and the application of a depth separation filter in order to subtract from the measured field the effect caused by the subaerial and submarine volcanic edifices and by all surface inhomogeneities magnetic and gravimetric 2 5d modeling on selected profiles has also been done in a joint interpretation of the magnetic and gravimetric data constrained by volcanological knowledge the interpretation of the final maps and profiles has demonstrated the existence of extensional tectonics resulting in the development of horst and graben volcanism occurs in the graben areas in particular the salina lipari vulcano magmatic axis has developed in a very deep articulated graben of lithospheric importance and completely filled by volcanic deposits and intrusive bodies the origin of this structure and of the associated igneous activity is suggested to be related to trans tensional tectonics along a plio pleistocene regional strike slip fault system

on the rifting dynamics of plate divergence and magma accumulation at oceanic ridge axes rifting dynamics at spreading axes is governed by two processes the large scale plate divergence and the local magma accumulation in the crust mantle transition layer both evolve simultaneously a model is developed particularly for the situation in iceland where a well studied rifting episode occurred in the krafla volcanic system 1975 1984 both the divergence and the buoyant rise of magma create tensile deviatoric stress in the axial region but while divergence generates an altogether extensional stress field uprising of buoyant melt produces tension only near the axis but compression of the sides the buoyant rise is driven by the differential pressure gradient in rock and melt the processes are studied with a two dimensional finite element routine presently thermal effects are neglected the model parameters are density difference size of the buoyant body externally applied stress field mechanical properties of rock and melt relatively small amounts of divergence and small increases of buoyancy are shown to generate axial tension which can overcome the tensile strength axial tension produced by buoyant bodies can even overcome lateral compression observed long intervals of quiescence require either large rock strength quasi continuous stress relaxation small buoyant bodies and or a compressive deviatoric stress normal to the axis during much of the time between rifting episodes buoyant rise and injection of melt must be important in generating compression

post glacial rebound and asthenosphere viscosity in iceland during the weichselian glaciation iceland was covered with an ice cap which caused downward flexure of the earth s surface the post glacial rebound in iceland was very rapid being completed in about 1 000 years the length of this time interval constrains the maximum value of asthenosphere viscosity in iceland to be 1 times 10 sup 19 pa s or less further clarification of the ice retreat and uplift history may reveal lower viscosity current changes in the mass balance of icelandic glaciers must lead to measurable elevation changes considering this low viscosity expected current elevation changes around the vatnajoekull ice cap are of the order of 1 cm per year due to mass balance change in this century

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

gps applications for geodynamics and earthquake studies geodetic measurements obtained with the global positioning system gps are increasingly more widely applied in geophysical studies in this paper we review the changes to the technology of gps geodesy over the last five years that are responsible for this increased applicability we survey geophysical investigations employing gps to measure coseismic postseismic and interseismic deformation plate motion and crustal deformation at plate boundaries volcano deformation and the deformation associated with glacial isostatic adjustment and its application to sea level studies we emphasize the use of gps determinations for the modeling of this wide variety of geophysical phenomena we also discuss the recent advent of permanent gps networks for regional geophysical studies as well as the possible future of gps surveying in light of the recent advances

extension across a divergent plate boundary the eastern volcanic rift zone south iceland 1967 1994 observed with gps and electronic distance measurements the average spreading rate in south iceland 19 mm yr is distributed over two parallel rift zones we measured a global positioning system gps network of 42 stations in the eastern zone in 1994 this network consists of stations measured with gps in 1986 and 1992 stations in a 60 km long distance profile measured several times since 1967 and a few new stations the 1994 gps data were processed using the bernese software version 3 5 and the average position uncertainties are about 3 mm in horizontal components and about 8 mm in the vertical component for baselines up to 100 km comparison with results of former gps campaigns gives a uniform extension steady in time perpendicular to the spreading axis a strain rate of 0 12 0 01 strain yr is observed across the 100 km wide network or an extension of about 12 mm yr minor deformation is observed in direction parallel to the spreading axis observations along the distance profile which lies across the rift zone gave a significant contraction during the period 1967 1977 but gave an extension during 1977 1994 this extension is about 138 47 mm and is mainly accommodated in the western part of the profile the observed extension across the rift zone during 1986 1994 can be simulated with a simple model of infinitely long dike intrusions into an elastic layer overlying a viscous layer this model is not able to simulate observed contraction along the distance profile 1967 1977 the observed irregularities along the western part of the distance profile coincide in space and time with volcanic and tectonic unrest near the hekla volcano the disturbances are probably caused by some common underlying process leading to crustal deformation eruptions and earthquakes

joint estimation of afterslip rate and postseismic relaxation following the 1989 loma prieta earthquake global positioning system gps data from campaigns carried out over the 5 years following the 1989 loma prieta earthquake and leveling data measured in 1990 and 1992 define the postseismic velocity field around the loma prieta rupture zone subtraction of a background velocity field yields a residual velocity pattern which we interpret as the product of two physical processes 1 slow afterslip along distinct planes in the upper cru st and 2 viscoelastic relaxation of the lower crust and upper mantle biirgmann et al 1997 previously derived an afterslip model involving uniform afterslip on two optimally determined planes in cluding oblique reverse slip on the coseismic rupture and reverse slip on a shallow thrust fault to the northeast of the san andreas fault we further consider models of distributed slip on these two fault planes plus a viscoelastic relaxation pattern which depends on a suitable cose ismic rupture model and crust and mantle viscosities several fault models from the literature were considered for the 1989 coseismic rupture with nearly identical impact on the results simultaneous maximum likelihood inversion of the gps and leveling data for afterslip distri bution and viscosity yields the following results 1 a good fit to the data is obtained by smooth afterslip distributions without any viscoelastic relaxation being required 2 tangible broad scale viscoelastic relaxation of the lower crust and upper mantle are present in this data set at 97 confidence and a lower crustal viscosity of 1029 pa s is obtained however the viscosity of both the lower crust and upper mantle are poorly constrained 3 for a given misfit 20 less integrated afterslip and smoother afterslip distributions result when viscoelas tic relaxation is included 4 maximum slip rates on the slip distributed models are 3 5 crn yr the dominant patches estimated on the two planes fill in the entire depth range 4 13 km without significant overlap and deeper afterslip is not required the afterslip distribution on the coseismic rupture plane is strongly dominated by reverse slip immediately southeast of the main center of coseismic reverse slip

postrifting anelastic deformation around the spreading plate boundary north iceland 1 modeling of the 1987 1992 deformation field using a viscoelastic earth structure a third global positioning system gps survey of a regional network surrounding the krafla volcanic system north iceland was conducted in 1992 following a major crustal spreading episode which began in this system in 1975 differencing the 1992 results with those from 1987 and 1990 reveals a regional deformation field with a maximum rift normal expansion rate of 4 5 cm year near the rift decreasing to 3 cm year at large distances the time averaged spreading rate in north iceland 1 8 cm year cannot account for this deformation the vertical deformation field reveals regional uplift throughout the network area at its maximum closest to the rift and decreasing with distance three different models are applied to study the postdike injection ground deformation 1 stress redistribution in an elastic layer over a viscoelastic half space 2 stress redistribution in an elastic viscous layered medium and 3 continued opening at depth on the dike plane in an elastic half space using model 1 the effects of historical episodes in the region are subtracted from the observed displacement fields and the remaining motion is modeled as relaxation following the recent krafla rifting episode the best fit model involves a half space viscosity of 1 1 x 10 18 pa s a relaxation time of 1 7 years and an elastic layer thickness for northeast iceland of 10 km the vertical field indicates that the krafla dike complex rifted the entire elastic layer using model 2 the motion 1987 1990 and 1990 1992 can be simulated adequately given the survey errors but the 1987 1992 deformation is poorly fitted suggesting that a more realistic geophysical model is required using model 3 a range of dikes will fit the deformation field