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

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

impact of in situ stress and fault reactivation on seal integrity in the east irish sea basin uk abstract despite having been affected by several stages of exhumation during the cretaceous and cenozoic the contemporary stress state of the east irish sea basin eisb is poorly characterised as the basin is mature in terms of exploitation of hydrocarbons future exploration beyond the conventional sherwood sandstone group reservoir triassic necessitates a greater understanding of the in situ stress field while proposed natural gas storage and carbon sequestration schemes also require detailed stress field information using petroleum well data the in situ stress field of the eisb has been characterised to assess the mechanical seal integrity a strike slip stress regime most likely prevails in the basin meaning the maximum horizontal stress shmax is the greatest of the principal stresses interpretation of stress orientation data suggests that shmax is oriented 152 12 consistent with mean stress orientations across the wider region associated with plate boundary forces some degree of structural control appears to influence the orientation of shmax with orientations locally aligned sub parallel to major permo triassic basin bounding faults fault reactivation risk is evaluated through modelling the pore pressure increase required to induce failure on pre existing faults vertical faults striking 30 from shmax are optimally oriented to become reactivated under elevated pore pressure conditions for any project relying on an element of fault seal for the containment of buoyant fluids at the average reservoir depth of 800 m pore pressure increase should be less than 3 3 mpa to avoid reactivating pre existing optimally oriented faults higher pressure increases would be required to initiate reactivation of faults with other orientations vertical faults striking perpendicular to shmax are least likely to become reactivated and in the absence of halite seal integrity would instead be limited by caprock strength and capillary entry pressure major faults affecting the basin have been analysed for their slip tendency ratio of shear to normal stress which provides an indication of their susceptibility to become reactivated although the analysis is limited due to lack of an accurate 3d representation of the fault network the results suggest that many of the fault orientations observed in the eisb exhibit high slip tendencies including n s striking faults to the north and west of the east deemster fault where the shmax orientation is nw se faults striking perpendicular to shmax such as the lagman fault are least likely to become reactivated due to higher normal stresses that inhibit frictional sliding while faults striking parallel or very close to shmax also exhibit low slip tendency as they are not subjected to significant shear stresses

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

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

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

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

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 analysis and interpretation of stratification cycles beds in carbonate successions often form regular patterns either there is a cyclic repetition of beds with different thicknesses or bundles of beds occur separated from each other by some distinct layer any such group will be called a stratification cycle cycles of this type are from 0 5 m to several meters in thickness and they seem to be present throughout the geological column the analysis of sedimentary cycles involves three stages 1 regional mapping is necessary to understand the facies of the sediment establishing the lateral persistence of bedding eliminates the possibility that bedding is caused by diagenetic processes 2 petrographic examination and in particular analyses of primary structures are used to reconstruct the sedimentation conditions during cycle formation such conditions are extremely important for judging the time content of individual beds 3 the quantitative analysis of vertical sections using power spectral methods can find superimposed cycles and enables one to judge their degree of regularity the latter approach is illustrated using lower carboniferous limestone sections from nw ireland as an example spectral analysis finds a stratification cycle of 1 to 3 m in thickness each cycle consists of between five to seven limestone beds upper jurassic limestone marl successions from southern germany and late cretaceous and early tertiary successions from italy show a very similar cyclic pattern the scale of the cycles and their regularity suggest that they are ultimately caused by variations in the earth s orbit possibly by climatic cycles as was postulated by milankovitch however the exact mechanism which links sedimentation with orbital variations is not known

contemporary deformation and stressing rates in southern alaska summary the subduction of the pacific plate beneath north america induces broad scale stressing of the alaskan crust that has led to the development of the highest mountains in north america the highest slip rates along some of the longest strike slip faults on earth and widespread seismicity that includes the 1964 m9 2 alaska earthquake the second largest ever recorded these features are a consequence of deformation associated with three primary processes interseismic loading due to relative plate motions large earthquakes and postseismic processes how these mechanisms contribute to the evolution of stress in the alaskan crust is not well understood here we use observed contemporary surface velocities to constrain 2 d and 3 d viscoelastic numerical models of relative pacific north american plate motions coseismic slip associated with the 1964 m9 2 megathrust event and strike slip earthquakes on the transform boundary in 1949 m8 1 1958 m7 8 and 1972 m7 6 the four largest events prior to the 2002 m7 9 denali quake viscoelastic relaxation following these events and afterslip to gain insight into how these processes are shaping alaska today results suggest that interseismic deformation and on going post seismic deformation following the 1964 earthquake both contribute significantly to the gps measured contemporary velocity field viscoelastic relaxation associated with a mantle with a viscosity of 10 19 pa s is required to explain southerly directed velocities that are observed in the cook inlet region to well north of the denali fault results also suggest that subduction of the pacific plate leads to a broad zone of deformation with high stressing rates concentrated in a band that lies several hundred kilometres from the plate boundary coeval with the inboard location of the maximum locking depth of the megathrust interseismic deformation and stressing rates remain high further inland across the yakutat microplate where flat subduction extends the width of the locked plate interface calculations show that post seismic relaxation following the large strike slip events serves to reload these rupture surfaces while relieving stress on the eastern denali fault post seismic relaxation following the 1964 earthquake combined with coseismic stress changes promoted the triggering of the 2002 denali quake calculations also suggest that over the past 50 years high stress has accumulated on part of the thrust interface to the west and east of the 1964 rupture surface and along the queen charlotte fault to the south of the 1949 rupture surface

hemipelagic shelf sedimentation and climatic cycles the basal jurassic blue lias of south britain abstract the sedimentology cyclicity and hiatuses of the blue lias formation basal jurassic were investigated at five localities in south britain most carbonate mud which is the dominant carbonate component has been neomorphosed to microspar however carbonate mud was apparently supplied by coccoliths in zooplankton faecal pellets suggesting hemipelagic sedimentation clay is assumed to have been supplied by rivers the three sediment types involved reflect cyclic changes in bottom water oxygenation levels combined with changes in the clay to carbonate mud ratio walsh power spectra have been applied for the first time to measured sections in order to investigate the control of cyclicity this application is analogous to palaeoclimatological studies of pleistocene oxygen isotope ice volume records the spectra show that 1 or 2 sedimentary cycles of constant thickness occur in four out of five sections each of which represents several million years the maximum possible duration of these cycles is too short for their generation by sea level changes their regularity stability and order of duration tens of thousands of years are consistent with the hypothesis of sedimentary cyclicity controlled by climatic change itself forced by orbital changes in insolation milankovitch theory the duration of both cycles recognised is less than 93 000 years 93 ka and may record changes in orbital precession 21 ka and obliquity 41 ka assuming that one ammonite zone represents 1 ma comparison of the longest possible with the implied durations of the cycles suggests that the sections are at best 20 40 complete loss of sediment may have occurred during winter storms or hurricanes a model relating the sedimentary cycles to changes in the volume of runoff is proposed

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

phase gradient approach to stacking interferograms the phase gradient approach is used to construct averages and differences of interferograms without phase unwrapping our objectives for change detection are to increase fringe clarity and decrease errors due to tropospheric and ionospheric delay by averaging many interferograms the standard approach requires phase unwrapping scaling the phase according to the ratio of the perpendicular baseline and finally forming the average or difference however unique phase unwrapping is usually not possible since the phase gradient due to topography is proportional to the perpendicular baseline phase unwrapping is unnecessary prior to averaging or differencing phase unwrapping may be needed to interpret the results but it is delayed until all of the largest topographic signals are removed we demonstrate the method by averaging and differencing six interferograms having a suite of perpendicular baselines ranging from 18 to 406 m cross spectral analysis of the difference between two tandem interferograms provides estimates of spatial resolution which are used to design prestack filters a wide range of perpendicular baselines provides the best topographic recovery in terms of accuracy and coverage outside of mountainous areas the topography has a relative accuracy of better than 2 m residual interferograms single interferogram minus stack have tilts across the unwrapped phase that are typically 50 mm in both range and azimuth reflecting both orbit error and atmospheric delay smaller scale waves with amplitudes of 15 mm are interpreted as atmospheric lee waves a few global positioning system gps control points within a frame could increase the precision to 20 mm for a single interferogram further improvements may be achieved by stacking residual interferograms

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

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

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

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

radar interferometry and its application to changes in the earth s surface geophysical applications of radar interferometry to measure changes in the earth s surface have exploded in the early 1990s this new geodetic technique calculates the interference pattern caused by the difference in phase between two images acquired by a spaceborne synthetic aperture radar at two distinct times the resulting interferogram is a contour map of the change in distance between the ground and the radar instrument these maps provide an unsurpassed spatial sampling density 100 pixels km 2 a competitive precision 1 cm and a useful observation cadence 1 pass month 1 they record movements in the crust perturbations in the atmosphere dielectric modifications in the soil and relief in the topography they are also sensitive to technical effects such as relative variations in the radar s trajectory or variations in its frequency standard we describe how all these phenomena contribute to an interferogram then a practical summary explains the techniques for calculating and manipulating interferograms from various radar instruments including the four satellites currently in orbit ers 1 ers 2 jers 1 and radarsat the next chapter suggests some guidelines for interpreting an interferogram as a geophysical measurement respecting the limits of the technique assessing its uncertainty recognizing artifacts and discriminating different types of signal we then review the geophysical applications published to date most of which study deformation related to earthquakes volcanoes and glaciers using ers 1 data we also show examples of monitoring natural hazards and environmental alterations related to landslides subsidence and agriculture in addition we consider subtler geophysical signals such as postseismic relaxation tidal loading of coastal areas and interseismic strain accumulation we conclude with our perspectives on the future of radar interferometry the objective of the review is for the reader to develop the physical understanding necessary to calculate an interferogram and the geophysical intuition necessary to interpret it

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

widening of the krafla fissure swarm during the 1975 1981 volcano tectonic episode frequent distance measurements across the krafla fissure swarm north iceland recorded the extension accompanying the sequence of rifting events which started in december 1975 and lasted for 6 years an 80 to 90 km long section of the fissure swarm extended during this sequence of rifting and volcanic events maximum widening of about 8 m occurred 10 to 12 km north of leirhnjukur which is located above the center of the krafla magma reservoir from that location the amount of widening decreased north wards and is estimated to exceed 2 m where the seismicity indicated the northern termination of the present rifting off the north coast about 70 km north of leirhnjukur the amount of widening also decreased southwards and approached zero at 15 to 20 km south of leirhnjukur the ground deformation associated with these rifting events can be summarised as a narrow strip 1 to 2 km wide along the fissure swarm is heavily fractured with numerous open cracks parallel to the fissure swarm this fractured strip has subsided 2 to 3 m relative to its flanks the flanks of the fractured zone have been uplifted relative to regions farther away the uplift is not well constrained but tilt observations at several locations indicate about 1 m uplift the flanks of the rift zone have contracted perpendicular to the fissure swarm the maximum contractional strain exceeds 300 mm per km the amount of areal expansion windening of the fissure swarm times the length of the fractured zone associated with these rifting events is estimated to be about 0 30 km2 for individual events the area of expansion has been roungly proportional to the volume of subsidence above the krafla magma reservoir if the width of a new dike is equal widening of surface fissures the ratio of the subsidence volume to the area of expansion for the best observed events indicated a height of a new dike system as 2 4 to 2 8 km this ratio is significantly less for events of large lava production but even during these events the majority of magma leaving the krafla reservoir was apparently emplaced in subsurface fissures

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

three dimensional mechanical models for the june 2000 earthquake sequence in the south iceland seismic zone abstract a sequence of earthquakes including two m w 6 5 events occurred in june 2000 in an atypical transform zone located in the southwest of iceland the inter co and post seismic time intervals of the associated deformation cycle have been well recorded by previous geodetic studies including gps and insar using a numerical code based on a finite element method we analyze the influence of structural and rheologic heterogeneities on the earthquake deformation cycle within the south iceland seismic zone sisz the co seismic deformation is especially sensitive to the increase of lithospheric stiffness with depth as shown by a joint inversion of the geodetic data the estimated distribution of co seismic slip for the june 21 mainshock is deeper when estimated in a realistic layered lithospheric model than in a homogenous half space the geometry of the rheologic layers also plays an important role during the post seismic interval when several inter related processes poro elastic effects around the fault plane afterslip in it and or visco elastic relaxation in the lower crust and or upper mantle appear to occur on overlapping time and length scales we also consider the seismicity both in its contribution to stress transfer and in its tendency to migrate from east to west at least six processes may be involved in changing the stress distribution 1 propagation of seismic waves 2 changes of static stress caused by major co seismic slip 3 cascading seismicity 4 fluctuations in hydrological conditions 5 ductile flow of subcrustal rocks and 6 inter seismic strain accumulation all six of these phenomena occurred in the sisz before during and after the june 2000 earthquake sequence by using three dimensional finite element models with realistic geometric and rheologic configurations to match the observations we test the influence of the increase of rigidity with depth and the variable crustal thickness on processes 2 3 5 and 6 inter seismic strain 6 accumulating in an elastic upper crust that thickens eastward can produce an asymmetric stress distribution this process may explain the tendency of subsequent earthquakes to migrate from east to west across the south iceland seismic zone within a single sequence yet it cannot explain their timing or the location of the first event our modeling with realistic geometric and rheologic configurations suggests that the june 17 earthquake triggered the june 21 event by a combination of several time dependent post seismic processes

interseismic strain accumulation spin up cycle invariance and irregular rupture sequences using models of infinite length strike slip faults in an elastic layer above linear viscoelastic regions we investigate interseismic deformation in the models we investigate interseismic strain accumulation on mature faults is the result of the cumulative effects of all previous ruptures and is independent of the fault loading conditions the time for a fault to spin up to a mature state depends on the rheologies and the fault loading conditions after the model has spun up the temporal variation of shear stresses is determined by the fault slip rate and model rheologies the change in stress during spin up depends on the slip rate rheologies and fault loading conditions but is independent of the magnitude of the initial stress over enough cycles such that the cumulative deformation is block like the average mature interseismic velocities are equal to the interseismic velocities of an elastic model with the same geometry and distribution of shear moduli in a model that has spun up with the fault rupturing periodically the cumulative deformation is block like at the end of each seismic cycle and the interseismic deformation is cycle invariant i e the same in all cycles when the fault ruptures nonperiodically the fault spins up to a mature state that is the same as if the fault had ruptured periodically with the mean slip rate when the fault slip rate within each cycle varies the interseismic deformation evolves toward the cycle invariant deformation determined by the most recent fault slip rate around a fault whose slip rate has been faster slower than average interseismic velocities are larger smaller than the cycle invariant velocities and increase decrease from cycle to cycle

the signature of depth dependent viscosity structure in post seismic deformation summary geodetic observations of post seismic ground motion reflect the integrated effects of several relaxation mechanisms to evaluate the viscous relaxation component accurately the crustal viscosity structure beneath a region must be estimated in this study using a 3 d finite element model we describe the viscoelastic relaxation that follows an instantaneous strike slip faulting event in a crustal layer whose viscosity decreases exponentially with depth at any surface observation point the depth dependent viscosity ddv model displacement history closely fits the history predicted for a uniform viscosity unv model the difference can be minimized to obtain the unv viscosity u that best fits the ddv model displacement at that location the differences in displacement histories between ddv and best fit unv models are minor but depend on distance from fault the viscosity gradient and the fault configuration in the near field the ddv model prediction can be well approximated by the unv model regardless of the viscosity gradient and fault configuration on the other hand in the far field small differences between ddv model and best fit unv model become apparent the displacements are controlled by greater viscosities in later phases and the differences are greater for ddv models with greater viscosity gradient the more important result we obtain is that apparent viscosity u decreases with distance from the fault and its rate of decrease is directly diagnostic of the vertical viscosity gradient this result points to a practical method of analysing post seismic ground displacement data for the variation of viscosity in the ductile crust the apparent crustal viscosity should be determined at a series of points at increasing distance from the fault the variation of apparent viscosity with distance can then be used to directly infer the vertical gradient of viscosity in the crust

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

three dimensional viscoelastic finite element model for postseismic deformation of the great 1960 chile earthquake 2 5 10 19 pa s for the continental upper mantle the model well explains the gps observations numerical tests suggest that the continental mantle viscosity value is reasonably well constrained the model shows the prolonged postseismic seaward motion of the inland area to be a unique feature of earthquakes with very long rupture along strike and large coseismic fault slip for short rupture and small coseismic slip the motion will stop very quickly after the earthquake explaining why this phenomenon is not more commonly observed with an oceanic mantle viscosity of 10 20 pa s the model also provides an explanation for tide gauge constrained postseismic uplift 200 km from the trench that had previously been explained using a model of prolonged afterslip of a deep segment of the chile subduction fault index terms 1236 geodesy and gravity rheology of the lithosphere and mantle 8160 1242 geodesy and gravity seismic deformations 7205 3230 mathematical geophysics numerical solutions 7209 seismology earthquake dynamics and mechanics 7260 seismology theory and modeling keywords postseismic deformation viscoelastic stress relaxation gps finite element modeling chile subduction zone upper mantle viscosity

evidence of power law flow in the mojave desert mantle studies of the earth s response to large earthquakes can be viewed as large rock deformation experiments in which sudden stress changes induce viscous flow in the lower crust and upper mantle that lead to observable postseismic surface deformation laboratory experiments suggest that viscous flow of deforming hot lithospheric rocks is characterized by a power law in which strain rate is proportional to stress raised to a power n refs 2 3 most geodynamic models of flow in the lower crust and upper mantle however resort to newtonian linear stress strain rate relations here we show that a power law model of viscous flow in the mantle with n 3 5 successfully explains the spatial and temporal evolution of transient surface deformation following the 1992 landers and 1999 hector mine earthquakes in southern california a power law rheology implies that viscosity varies spatially with stress causing localization of strain and varies temporally as stress evolves rendering newtonian models untenable our findings are consistent with laboratory derived flow law parameters for hot and wet olivine the most abundant mineral in the upper mantle and support the contention that at least beneath the mojave desert the upper mantle is weaker than the lower crust

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

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

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

eocene magnetic stratigraphy at gubbio italy and its implications for paleogene geochronology the sequence of polarity zones established by alvarez and others for the cretaceous and paleocene in the scaglia formation in the bottaccione gorge at gubbio has now been extended through the uppermost paleocene into the upper eocene globigerinatheka semiinvoluta zone anomaly 18 the relationships between the standard paleogene foraminiferal zones and the sea floor anomalies are defined with the exception of the mid eocene zones of morozovella lehneri orbulinoides beckmanni and the base of the truncorota loides rohri zone which are obscured by inadequate preservation and a fault gap the base of the eocene lies above anomaly 25 the lower to middle eocene contact base lutetian lies in the upper part of anomaly 22 the base of the bartonian lies below the base of anomaly 18 and the base of the priabonian upper eocene lies within anomaly 18 radiometric ages appear to diverge systematically from a geochronology based on steady sea floor spreading rates suggesting that spreading may have been as much as a factor of 3 slower in the paleocene and the late eocene than in the early and middle eocene

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

kilometer scale kaiser effect identified in krafla volcano iceland the krafla rifting episode in 1975 1984 consisted of around 20 inflation deflation events within the krafla caldera where magma accumulated during inflation periods and was intruded into the transecting fissure swarm during brief periods of deflation we reanalyze geodetic and seismic data from the rifting episode and perform a time dependent inversion of a leveling time series for a spherical point source in an elastic half space using the volume change as a proxy for stress shows that during inflation periods the seismicity rate remains low until the maximum inflation of previous cycles is exceeded thus exhibiting the kaiser effect our observations demonstrate that this phenomenon commonly observed in small scale experiments is also produced in kilometer scale volcanic deformation this behavior sheds new light on the relationship between deformation and seismicity of a deforming volcano as a consequence of the kaiser effect a volcano may inflate rapidly without significant changes in seismicity rate

paleogene magnetic stratigraphy in umbrian pelagic carbonate rocks the contessa sections gubbio the umbrian sequence of pelagic carbonate rocks provides an opportunity for precise correlation between paleogene biostratigraphy and geomagnetic polarity history the red to pink paleocene to middle eocene scaglia rossa limestone the varicolored upper eocene scaglia variegata limestone and the gray green oligocene scaglia cinerea marlstone form a 250 m thick continuous exposure in the contessa valley near gubbio italy magnetostratigraphic lithostratigraphic and biostratigraphic investigations in three sections covering the entire paleogene have confirmed and dated the geomagnetic reversal sequence for most of this period the ferromagnetic mineral in the scaglia cinerea is magnetite and af demagnetization to peak fields of 20 mt is sufficient to define the characteristic remanent magnetization the scaglia variegata and pink scaglia rossa samples contain an additional hematite component which is very pronounced in dark red paleocene scaglia rossa samples changes of magnetic mineralogy take place in these limestones during heating especially above 500 c however thermal demagnetization is effective in isolating the characteristic remanence vectors which form almost antipodal clusters of directions representing normal and reversed polarities the directions are rotated counterclockwise partly due to post oligocene tectonism paleontological zonations of planktic foraminifera and calcareous nannofossils were hampered by poor preservation but the major epoch and stage 29 as in the gubbio bottaccione section paleocene eocene just above anomaly 25 eocene oligocene between anomalies 13 and 15 oligocene miocene just below anomaly 6c these correlations require slight modifications to previous conclusions on paleogene sea floor spreading rates

glacio isostatic deformation around the vatnajokull ice cap iceland induced by recent climate warming gps observations and finite element modeling 1 glaciers in iceland began retreating around 1890 and since then the vatnajokull ice cap has lost over 400 km 3 of ice the associated unloading of the crust induces a glacio isostatic respo

effects of topography on the interpretation of the deformation field of prominent volcanoes application to etna we have investigated the effects of topography on the surface deformation field of volcanoes our study provides limits to the use of classical half space models considering axisymmetrical volcanoes we show that interpreting ground surface displacements with half space models can lead to erroneous estimations of the shape of the deformation source when the average slope of the flanks of a volcano exceeds 20 tilting in the summit area is reversed to that expected for a flat surface thus neglecting topography may lead to misinterpreting an inflation of the source as a deflation comparisons of mogi s model with a three dimensional model shows that ignoring topography may lead to an overestimate of the source volume change by as much as 50 for a slope of 30 this comparison also shows that the depths calculated by using mogi s solution for prominent volcanoes should be considered as depths from the summit of the edifices finally we illustrate these topographic effects by analyzing the deformation field measured by radar interferometry at mount etna during its 1991 1993 eruption a three dimensional modeling calculation shows that the flattening of the deflation field near the volcano s summit is probably a topographic effect

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