seismicity of the reykjanes peninsula 1971 1976 abstract the reykjanes peninsula oblique rift a segment of the mid atlantic plate boundary connects the reykjanes ridge to the rift zones of iceland the axis of the deformation zone of the plate boundary can be traced as a rather narrow epicentral zone of earthquakes that is highly oblique to the spreading direction of the north america plate with respect to the eurasia plate the seismicity of the area is episodic active periods with a duration of about a decade are separated by more quiet periods of one to two decades our observation period spans a good part of an episode that began in 1967 and ended in 1975 nine earthquake swarms illuminate a 50 km long segment of the plate boundary the epicentral zone cuts obliquely across the fissure swarms of the volcanic systems of the peninsula geothermal areas are preferentially located at the intersection of the fissure swarms with the seismic zone outside the intersections with the seismic zone the fissure swarms show very low seismicity the tectonic characteristics gradually change along the plate boundary segment from being similar to those of a spreading segment in the western part to being more like a transform zone in the eastern part this is seen in the type of earthquake sequences maximum magnitude and type of faulting

the structure of seismogenic strike slip faults in the eastern part of the reykjanes peninsula oblique rift sw iceland abstract a considerable part of the seismic risk to the capital area of iceland is provided by a series of strike slip faults at the mid atlantic plate boundary within a distance range of 15 35 km about two thirds of the icelandic population lives within this distance range from the source areas of m 6 6 5 earthquakes the structure of the plate boundaries in sw iceland is relatively complex three branches meet in the hengill triple junction the oblique rift of the reykjanes peninsula the south iceland seismic zone a transform zone and the western volcanic zone a rift zone the reykjanes peninsula oblique rift has an over all trend of 70 highly oblique with respect to the spreading direction 101 in this region it contains both volcanic systems and seismogenic strike slip faults the fissure swarms of individual volcanic systems contain normal faults and fissures with a ne trend also quite oblique to the plate boundary the fissure swarms fade out towards the ne and sw as they extend into the plates on either side overprinting this pattern of volcano tectonic structures are sets of parallel northerly striking transcurrent faults that generate the largest earthquakes in this zone their surface expressions are en echelon fracture arrays and push up structures the sense of displacement is right lateral in the 15 km long section of the rift studied here we find evidence for at least six faults of this type the length of individual faults may exceed 12 km the distance between them varies from 1 to 5 km and together they form a bookshelf type fault system taking up the left lateral component of plate movements across the oblique rift zone

coseismic stress changes and crustal deformation on the reykjanes peninsula due to triggered earthquakes on 17 june 2000 1 a large mw 6 5 earthquake struck the south iceland seismic zone sisz on 17 june 2000 the 17 june main shock triggered increased seismicity over a large area and significant slip on at least three distinct faults on the reykjanes peninsula up to 87 km to the west of the event a second large mw 6 4 earthquake in the sisz occurred on 21 june 2000 about 17 km west of the 17 june main shock this event does not appear to have triggered as much activity on the reykjanes peninsula as the 17 june main shock although the epicenter was closer crustal deformation signals due to the june 2000 earthquakes on the reykjanes peninsula were observed with campaign and continuous gps and synthetic aperture radar interferometry with the largest coseismic deformation signal near lake kleifarvatn we model the faults using three uniform slip rectangular dislocations in an elastic half space best fit uniform slip models consistent with seismic and geodetic data indicate that all three faults trend n s and the motion on them was primarily right lateral strike slip our study suggests that the event near kleifarvatn had a significantly larger moment than seismic estimates indicating a component of aseismic slip on the fault lasting no more than several hours static coulomb failure stress change calculations indicate that the event at kleifarvatn increased the coulomb stress at the hypocenter of the nupshlidarhals event by 0 1 0 2 mpa as well as loading the hvalhnukur fault

high b values in the leaky segment of the tjornes fracture zone north of iceland are they evidence for shallow magmatic heat sources near the seamount holl in the tjornes fracture zone tfz north of iceland two anomalies of high seismic b values have been found in a depth shallower than 10 km according to the results of similar b value anomalies beneath volcanic edifices such as mt st helens a magma chamber was suspected in the region on first view the existence of a long living magma body between the island of kolbeinsey and holl seamount appears to be confirmed by reflection seismic surveys conducted in the area with shallow water depths of 400 m they show a clear deep reflector however the reflector is best imaged in a migrated record at 1500 m s i e the speed of sound in water and it can be modeled by single scattering from the bathymetry in the area which leads to the conclusion that the apparent reflection images the seafloor rather than a magma chamber the recent volcanism along manareyjar ridge with the last eruption in 1868 69 and the hydrothermalism east of grimsey island would be trivially explainable by a magma chamber in the subsurface however there is no evidence for a magma chamber in this more southern part of the grimsey lineament gl even in a volcanic area such as the gl as the leaky part of the tfz neither the existence of high b values nor an apparent deep reflector prove the existence of a magma chamber instead the seismic imaging of magma chambers in areas of intermediate to slow spreading ridge areas appears to be questionable

strain partitioning and stress rotation at the north anatolian fault zone from aftershock focal mechanisms of the 1999 izmit mw 7 4 earthquake summary we investigate aftershock focal mechanisms of the m w 7 4 izmit earthquake of 1999 august 17 on the western north anatolian fault zone nafz spatial clustering and the orientation of 446 fault plane solutions are analysed the izmit mainshock occurred as a right lateral slip on an ew trending near vertical fault plane aftershock clusters define four individual fault segments focal mechanisms surrounding the epicentres of the izmit and subsequent duzce mainshock m w 7 1 1999 november 12 indicate predominantly strike slip but also normal faulting aftershocks in the area between the izmit and duzce segments are mainly related to ew oriented normal faulting delineating a small pull apart structure beneath the easternmost sea of marmara alignments of aftershocks suggest branching of the nafz into three or more active segments that differ significantly in terms of their focal mechanism characteristics the distribution of aftershock focal mechanisms corresponds to fault segmentation of the nafz in the izmit duzce region produced by coseismic slip areas with large amounts of coseismic slip show aftershocks that are predominantly strike slip but low slip barriers show mostly normal faulting aftershocks stress tensor inversions of the aftershock focal mechanisms show rotations of the local stresses following the izmit mainshock in the izmit sapanca area the maximum horizontal compressive stress axis is horizontally rotated counter clockwise by 8 with respect to the coseismic and long term regional stress field towards the eastern end of the rupture karadere duzce area stresses are rotated clockwise we conclude that the izmit earthquake caused significant stress partitioning along the rupture the direction of stress rotation is related to the orientation of the individual fault segments along the nafz

deep downward fluid percolation driven by localized crust dilatation in iceland from a detailed seismic survey on a transform like plate setting in iceland we propose a spatial relationship between highly deformed crustal columns and areas of low vp vs ratio which are best explained by high fluid pressures at the base of the seismogenic crust combined with geological observations our seismic data suggest transient and local increases in crust permeability at specific highly dilating areas triggered by dynamical stresses from high magnitude regional earthquakes fluids are then trapped at supercritical conditions in the deep brittle crust which in turn increases the capacity of the rocks to deform through cracking and faulting

structural architecture of a highly oblique divergent plate boundary segment abstract the reykjanes peninsula in southwest iceland is a highly oblique spreading segment of the mid atlantic ridge oriented about 30 from the direction of absolute plate motion we present a complete and spatially accurate map of fractures for the reykjanes peninsula with a level of detail previously unattained our map reveals a variability in the pattern of normal oblique and strike slip faults and open fractures which reflects both temporal and spatial strain partitioning within the plate boundary zone fracture density varies across the length and width of the peninsula with density maxima at the ends and at the northern margin of the zone of volcanic activity fractures with similar strike cluster into distinct structural domains which can be related to patterns of faulting predicted for oblique extension and to their spatial distribution with respect to volcanic fissure swarms additional structural complexity on the reykjanes peninsula can be reconciled with magmatic periodicity and associated temporal strain partitioning implied by gps data as well as locally perturbed stress fields individual faults show variable slip histories indicating that they may be active during both magmatic and amagmatic periods associated with different strain fields

gas emissions and crustal deformation from the krýsuvik high temperature geothermal system iceland abstract the kr suvik volcanic system is located on the oblique spreading reykjanes peninsula sw iceland since early 2009 the region has been undergoing episodes of localized ground uplift and subsidence from april november 2013 we operated near real time monitoring of gas emissions in kr suvik using a multi component gas analyzer system multi gas collecting data on gas composition from a fumarole h2o co2 so2 h2s the dataset in this study comprises a near continuous gas composition time series the quantification of diffuse co2 gas flux analytical results for direct samples of dry gas seismic records and gps data gas emissions from the kr suvik geothermal system were examined and compared with crustal deformation and seismicity the gas emissions from the kr suvik system are h2o dominated with co2 as the most abundant dry gas species followed by smaller concentrations of h2s the average subsurface equilibrium temperature was calculated as 278 c this is consistent with previous observations made through sporadic spot sampling campaigns in addition the semi continuous multi gas dataset reveals higher variations in gas composition than previously reported by spot sampling the diffuse soil co2 flux is found to be variable between the three studied degassing areas in kr suvik ranging from 10 9 70 9 t day with the highest flux in hveradalir where the multi gas station is located the total flux is estimated as 101 t day comparison between multi gas and geophysical data shows that peaks of h2o rich emissions appears to follow crustal movements coinciding with the h2o rich peaks so2 is detected in minor amounts 0 6 ppmv allowing for calculations of h2o so2 co2 so2 and h2s so2 ratios this is the first time so2 has been detected in the kr suvik area the large variations in h2o co2 and h2o h2s ratios are considered to reflect variable degassing activity in the fumarole the activity of the fumarole appears less intense during intervals of low or no recorded seismic events the h2o co2 and h2o h2s ratios are lower presumably due to h2o condensation affecting the steam jet before reaching the multi gas inlet tube

fracture populations on the reykjanes peninsula iceland comparison with experimental clay models of oblique rifting 1 we have used high resolution scanned air photos and field measurements to analyze fracture population systematics for the reykjanes fissure swarm in order to determine the effect of rift obliquity on the evolution of fracture populations on the reykjanes peninsula sw iceland the peninsula is oriented approximately 30 oblique to the direction of plate motion data show significant differences between the strike length and degree of development of fractures along the margin of the fissure swarm and those in the center of the zone of active volcanism along the margin fractures are long highly segmented extension fractures and normal faults with offset on the order of several meters and a predominant strike 20 oblique to the trend of the plate boundary rift axis in the center of the zone of active volcanism fractures are generally shorter straighter fewer in number and strike approximately perpendicular to the direction of maximum horizontal extension and parallel to the trend of eruptive fissures right lateral oblique slip faults striking approximately perpendicular to the spreading direction are present in the center of the rift zone scaled experimental models of oblique extension predict a significant change in fracture strike at the rift margins due to the presence of a secondary stress field on the reykjanes peninsula this corresponds to the boundary between thicker and thinner brittle crust the models confirm that right lateral oblique or strike slip faults accommodate significant left lateral shear where deformation is distributed in a rift zone

variations in the frequency magnitude distribution with depth in two volcanic areas mount st helens washington and mt spurr alaska the frequency magnitude distribution of earthquakes characterized using the b value is examined as a function of space beneath mount st helens 1988 1996 and mt spurr 1991 1995 at mount st helens two volumes of anomalously high b b 1 3 can be observed at depths of 2 6 3 6 km below the crater floor and below 6 4 km these anomalies coincide with 1 the depth of vesiculation of ascending magma and 2 the suggested location of a magma chamber at mount st helens study of mt spurr reveals an area of high b value b 1 3 at a depth of about 2 3 4 5 km below the crater floor of the active vent crater peak we propose that the higher material heterogeneity in the vicinity of a magma chamber or conduit due to vesiculation of the ascending magma is the main cause of the increased b value at shallow depths alternatively interaction of magma with groundwater may have increased pore pressure and lowered the effective stress the deeper anomaly at mount st helens is likely caused by high thermal stress gradients in the vicinity of the magma chamber our results indicate that detailed mapping of the frequency magnitude distribution can be used as a tool to trace vesiculation and locate active magma chambers

contemporary horizontal velocity and strain rate fields of the pacific australian plate boundary zone through new zealand we have inverted velocity solutions from nine geodetic networks distributed across new zealand to derive present day continuous horizontal velocity and strain rate fields at the earth s surface throughout the country the nine networks contain a total of 362 global positioning system gps stations that have been observed at least twice and at least a year apart between 1991 and 1998 the model velocity field is expanded as bicubic spline interpolation functions defined within a curvilinear grid that covers the country and extends into the assumed rigid australian and pacific plates to west and east the inversion jointly minimizes the magnitudes of fitted strain rates and the misfit to the observed velocity data the spline technique allows high spatial resolution of strain rate variations especially in regions with spatially dense gps data expansion of the model velocity field on the surface of a sphere allows arbitrarily large areas to be studied previously known aspects of new zealand plate boundary deformation are highlighted with improved resolution including back arc extension in the taupo volcanic zone rotation of the hikurangi forearc away from this zone in the north island and a band of high shear strain rate under the southern alps to the southeast of the alpine fault our results reveal several new features including a region of enhanced shear straining apparently associated with strike slip faults in the southern north island and a band of contractional straining subparallel and well east of the alpine fault that is similar to features found in numerical and sandbox models of continental collision

evidence of instantaneous dynamic triggering during the seismic sequence of year 2000 in south iceland 1 we analyze the coseismic stress perturbation during the 17 june 2000 south iceland seismic sequence the main shock ms 6 6 was followed by three large events within a few tens of seconds 8 26 and 30 s located within 80 km the aim of this paper is to investigate short term fault interaction and instantaneous triggering this happens when a fault perturbed by a stress change fails before the end of the transient stress perturbation we compute the shear normal and coulomb stress changes as functions of time in a stratified elastic half space by using discrete wave number and reflectivity methods we calculate dynamic stresses caused by the main shock at the hypocenters of these three subsequent events our numerical results show that the onset of the last two events is slightly delayed with respect to the arrival time of the second positive peak of coulomb stress variation while the first event occurred after the first positive stress peak we have also analyzed the response of a spring slider system representing a fault governed by a rate and state dependent friction law perturbed by shear and normal stress variations caused by the main shock the fault response to the computed stress perturbations is always clock advanced we have found suitable constitutive parameters of the modeled fault that allow the instantaneous dynamic triggering of these three earthquakes if the initial sliding velocity is comparable with the tectonic loading velocity we obtained failure times close to the observed origin times for low values of the initial effective normal stress

determining rheology from deformation data the case of central italy 1 the study of geodynamics relies on an understanding of the strength of the lithosphere however our knowledge of kilometer scale rheology has generally been obtained from centimeter sized laboratory samples or from microstructural studies of naturally deformed rocks in this study we present a method that allows rheological examination at a larger scale utilizing forward numerical modeling we simulated lithospheric deformation as a function of heat flow and rheological parameters and computed several testable predictions including horizontal velocities stress directions and the tectonic regime to select the best solutions we compared the model predictions with experimental data we applied this method in italy and found that the rheology shows significant variations at small distances the strength ranged from 0 6 0 2 tn m within the apennines belt to 21 6 tn m in the external adriatic thrust these strength values correspond to an aseismic mantle in the upper plate and to a strong mantle within the adriatic lithosphere with respect to the internal thrust we found that strike slip or transpressive but not compressive earthquakes can occur along the deeper portion of the thrust the differences in the lithospheric strength are greater than our estimated uncertainties and occur across the adriatic subduction margin using the proposed method the lithospheric strength can be also determined when information at depth is scarce but sufficient surface data are available

strain accumulation along an oblique plate boundary the reykjanes peninsula southwest iceland strain accumulation along an oblique plate boundary the reykjanes peninsula southwest iceland

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

temporal and spatial variation of local stress fields before and after the 1992 eruptions of crater peak vent mount spurr volcano alaska we searched for changes in local stress field orientation at mount spurr volcano alaska between august 1991 and december 2001 this study focuses on the stress field orientation beneath crater peak vent the site of three eruptions in 1992 and beneath the summit of mount spurr local stress tensors were calculated by inverting subsets of 140 fault plane solutions for earthquakes beneath crater peak and 96 fault plane solutions for earthquakes beneath mount spurr we also calculated an upper crustal regional stress tensor by inverting fault plane solutions for 66 intraplate earthquakes located near mount spurr during 1991 2001 prior to the 1992 eruptions and for 11 months beginning with a posteruption seismic swarm the axis of maximum compressive stress beneath crater peak was subhorizontal and oriented n67 76 e approximately perpendicular to the regional axis of maximum compressive stress n43 w the strong temporal correlation between this horizontal stress field rotation change in position of the 1 3 axes relative to regional stress and magmatic activity indicates that the rotation was related to magmatic activity and we suggest that the crater peak stress field rotation resulted from pressurization of a network of dikes during the entire study period the stress field beneath the summit of mount spurr also differed from the regional stress tensor and was characterized by a vertical axis of maximum compressive stress we suggest that slip beneath mount spurr s summit occurs primarily on a major normal fault in response to a combination of gravitational loading hydrothermal circulation and magmatic processes beneath crater peak online material regional and local fault plane solutions

stress and the direction of slip on fault planes the shear stress direction on a fault plane depends only on four of the six components of the stress tensor assuming only that the slip direction marks the shear stress direction on any fault plane and that stress is homogeneous it is possible to estimate these four stress parameters from populations of fault planes with known slip directions as several workers have observed different formulations of the problem may yield varying best fitting stresses and estimates of uncertainty in the simplest case no assumptions are made regarding the orientations of fault planes relative to the stress tensor thus the technique allows for the possibility that the fault planes may be very weak here we present simple analytical and graphical descriptions of the field of admissible fault geometries relative to any four parameter stress model which can be used to illustrate the significance of various inverse strategies in particular this paper explores the effects of using two alternative measures of misfit between an observed fault datum and stress model 1 the pole rotation the angle between the observed and predicted slip direction on the observed fault plane and 2 the minimum rotation the smallest angle between the observed fault geometry and any fault geometry which is consistent with the model by allowing for variation of the fault plane as well as the slip vector the minimum rotation procedure generally achieves a more stable and presumably realistic estimate of the actual discrepancy between a fault observation and stress model than the pole rotation procedure in a test case using 17 earthquake focal mechanisms from the yuii region of eastern taiwan separate inversions based on the two misfit criteria yield different optimum stress models and uncertainty estimates additional constraints on the stress tensor such as the effect of friction can be superimposed on the ones used here

strain and stress fields along the gibraltar orogenic arc constraints on active geodynamics abstract the gibraltar orogenic arc in the western mediterranean represents a complex region of active deformation related to the oblique nubia eurasia convergence process to increase the knowledge on the ongoing active processes in this region we have used the most up to date and comprehensive geodetic crustal motion and stress fields to this end we analyzed both continuous and campaign mode gps data collected between 1999 00 and 2011 00 across the area and compiled a multidisciplinary dataset of well constrained stress indicators to be compared with the geodetic results the main results highlight the oblique nature of the nubia eurasia convergence which provides the largest component of the observed stress pattern and is responsible for a significant strain rate field along the gibraltar orogenic arc we discuss our findings with respect to available geological seismological and geophysical data in order to verify their coherency compared to more relevant geodynamical models proposed in literature according to previous studies we confirmed how much of the secondary stress pattern can be related to the gravitational potential energy field which may also be responsible for some 2d stress strain rate angular discrepancies observed in large areas of the betics in addition taking into account the sub orthogonal azimuthal relationship between the shmax and ehmin directions and the fast polarization directions we conjectured a deep dynamic process controlling both the crustal stress field and the surface deformation on large areas of the orogenic arc finally although the models proposed to explain the geodynamic pattern of the gibraltar orogenic arc are supported by a discrete number of geological and geophysical observations it is only the back arc extension and westward rollback model that is able to adequately account for the vast majority of the observations based on our findings and other evidences we retain that this process could still be active beneath the gibraltar orogenic arc

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

contemporary crustal deformation within the pamir plateau constrained by geodetic observations and focal mechanism solutions we used an updated data set of 192 gps derived surface velocities and 393 earthquake focal mechanisms mw 3 0 hypocenter depths 30 km to evaluate the spatial variations in the surface strain rate and crustal stress regime throughout the pamir plateau and its surrounding regions the strain rate field was estimated using the spline in tension approach that solves for the surface velocity in a rectangular grid and the stress field was predicted from focal mechanism solutions using the damped regional scale stress inversion drssi method of hardebeck and michael journal of geophysical research https doi org 10 1029 2005jb004144 2006 the results show that the crustal stress field around the pamir plateau is predominantly characterized by nnw sse compression and e w extension which is consistent with the principal orientations of the two dimensional surface strain rate tensor this agreement supports the notion that the pamir and southwestern tien shan are uniformly strained blocks in particular the fan shaped rotational pattern between text shmax and the strain rate from the western pamir to the tajik basin shows that the counterclockwise rotation of the text shmax orientation is associated with vertical deformation which is consistent with the idea of schurr et al tectonics 33 8 2014tc003576 2014 concerning the gravitational collapse and westward extrusion of the crust in the western pamir we propose that such a stress strain pattern dominated by nnw ese oriented compression and e w trending extension originated from a combination of the northward push of the indian continent and the southward subduction of the tien shan

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

quasi analytical treatment of spatially averaged radiation transfer in complex terrain 1 we provide a new quasi analytical method to compute the subgrid topographic influences on the shortwave radiation fluxes and the effective albedo in complex terrain as required for large scale meteorological land surface or climate models we investigate radiative transfer in complex terrain via the radiosity equation on isotropic gaussian random fields under controlled approximations we derive expressions for domain averaged fluxes of direct diffuse and terrain radiation and the sky view factor domain averaged quantities can be related to a type of level crossing probability of the random field which is approximated by long standing results developed for acoustic scattering at ocean boundaries this allows us to express all nonlocal horizon effects in terms of a local terrain parameter namely the mean square slope emerging integrals are computed numerically and fit formulas are given for practical purposes as an implication of our approach we provide an expression for the effective albedo of complex terrain in terms of the sun elevation angle mean square slope the area averaged surface albedo and the ratio of atmospheric direct beam to diffuse radiation for demonstration we compute the decrease of the effective albedo relative to the area averaged albedo in switzerland for idealized snow covered and clear sky conditions at noon in winter we find an average decrease of 5 8 and spatial patterns which originate from characteristics of the underlying relief limitations and possible generalizations of the method are discussed

subsidence at the geysers geothermal field n california from a comparison of gps and leveling surveys between 1994 and 1996 three gps surveys were conducted in the geysers region of northern california our aim was to constrain models of the stresses and strains induced by geothermal power production in that region each survey spanned the geysers geothermal field and consisted of typically 40 monuments these monuments had been previously employed in a series of first order leveling surveys during the 1970 s this earlier study had determined that the geysers region was subsiding with a maximum rate of 0 048 0 0055 m yr between 1973 and 1977 in order to be able to directly compare the leveling and gps surveys we transform them to the same reference frame using the geoid 96 geoid model for the period 1977 1996 we determine a maximum susbsidence rate of 0 047 0 002 m yr we then model this subsidence using a series of point sources of contraction and find their optimal configuration by applying the random cost method and the f test the minimum volume strain that we find consistent with this subsidence is approximately 5 10 4 such a strain cannot be explained by a thermoelastic mechanism but does seem to be consistent with poroelastic deformation and a quasi static reservoir bulk modulus k of 3 6 109 pa

andersonian wrench faulting in a regional stress field during the 2010 2011 canterbury new zealand earthquake sequence the initial mw7 1 darfield earthquake sequence was centred west of christchurch city in the south island of new zealand but aftershocks including a highly destructive mw6 3 event eventually extended eastwards across the city to the coast the mainshock gave rise to right lateral strike slip of up to 5 m along the segmented rupture trace of a subvertical fault trending 085 58 across the canterbury plains for c 30 km in agreement with teleseismic focal mechan isms near field data however suggest that the mainshock was composite initiating with reverse slip north of the surface rupture stress determinations for the central south island show maximum compressive stress s1 to be horizontal and oriented 115 58 the principal dextral rupture there fore lies at c 3 08 to regional s1 the classic andersonian orientation for a low displacement wrench fault an aftershock lineament trending c 1458 possibly represents a conjugate left lateral strike slip structure this stress field is also consistent with predominantly reverse slip reactivation of nne ne faults along the southern alps range front the main strike slip fault appears to have a low cumulative displacement and may represent either a fairly newly formed fault in the regional stress field or an existing subvertical fault that happens to be optimally oriented for frictional reactivation

stress tensor inversion using detailed microearthquake information and stability constraints application to olfus in southwest iceland using detailed microearthquake data we present a stress tensor inversion scheme with new methods for selecting the fault planes and allowing for errors in the focal mechanisms the nonuniqueness of earthquake focal mechanisms is accounted for in our inversion scheme through the introduction into the inversion of a range of well fitting focal mechanisms for each event the range of focal mechanisms significantly improves the quality of the estimated stress tensor relative localization of clusters of microearthquakes is used to obtain information about which nodal plane could be correct fault plane the clusters frequently fall on a common fault plane and if there are acceptable focal mechanisms where one nodal plane has orientation similar to the common plane we assume this is the correct fault plane for the event if there is no predefined fault plane we utilize a simple mohr coloumb failure criterion to obtain a physical choice of fault plane between the two nodal planes in the focal mechanism the nodal plane with highest relative instability is chosen as the fault plane differences between the instability and the standard slip angle criterion are investigated the new inversion scheme has been applied to microearthquake data from the olfus area in the vicinity of the southwest iceland triple junction we estimate an oblique strike slip state of stress maximum horizontal stress at n30 e and minimum horizontal stress at n60 w with significant normal faulting influence the instability fault selection criterion predicts very well the orientation of faults mapped by relative localization

an integrated global model of present day plate motions and plate boundary deformation summary in this paper we present a global model gsrm 1 of both horizontal velocities on the earth s surface and horizontal strain rates for almost all deforming plate boundary zones a model strain rate field is obtained jointly with a global velocity field in the process of solving for a global velocity gradient tensor field in our model we perform a least squares fit between model velocities and observed geodetic velocities as well as between model strain rates and observed geological strain rates model velocities and strain rates are interpolated over a spherical earth using bi cubic bessel splines we include 3000 geodetic velocities from 50 different mostly published studies geological strain rates are obtained for central asia only and they are inferred from quaternary fault slip rates for all areas where no geological information is included a priori constraints are placed on the style and direction but not magnitude of the model strain rate field these constraints are taken from a seismic strain rate field inferred from normalized focal mechanisms of shallow earthquakes we present a global solution of the second invariant of the model strain rate field as well as strain rate solutions for a few selected plate boundary zones generally the strain rate tensor field is consistent with geological and seismological data with the assumption of plate rigidity for all areas other than the plate boundary zones we also present relative angular velocities for most plate pairs we find that in general there is a good agreement between the present day plate motions we obtain and longterm plate motions but a few significant differences exist the rotation rates for the indian arabian and nubian plates relative to eurasia are 30 13 and 50 per cent slower than the nuvel1a estimate respectively and the rotation rate for the nazca plate relative to south america is 17 per cent slower on the other hand caribbean north america motion is 76 per cent faster than the nuvel 1a estimate while crustal blocks in the india eurasia collision zone move significantly and self consistently with respect to bounding plates only a very small motion is predicted between the nubian and somalian plates by integrating plate boundary zone deformation with the traditional modelling of angular velocities of rigid plates we have obtained a model that has already been proven valuable in for instance redefining a no net rotation model of surface motions and by confirming a global correlation between seismicity rates and tectonic moment rates along subduction zones and within zones of continental deformation

numerical models of faulting at oblique spreading centers obliquely spreading mid ocean ridges such as the reykjanes ridge display two distinct fault sets distinguishable by orientation and position on axis faults are oriented oblique to both the trend of the axis and the normal to the relative plate separation vector while faults on the flanks strike approximately parallel to the ridge axis numerical modeling techniques are used here to simulate the development of faulting on the reykjanes ridge stresses acting in a cross section through the lithosphere at a slow spreading ridge are investigated using the fast lagrangian analysis of continua flac explicit difference modeling software the predicted stresses from the cross sectional models are imposed as a condition in boundary element models of fracture propagation and linkage on axis fault simulations run under conditions similar to the reykjanes ridge successfully reproduce the mapped distribution of faults and predict the observed orientation of the axial volcanic ridges simulations of fractures away from the axis show the development of axis parallel faults by the interaction and linkage of fractures which have been rafted off axis also in accord with observations stresses modeled in cross section favor downdip displacement on faults dipping toward the ridge axis

forecasting the path of a laterally propagating dike an important aspect of eruption forecasting is predicting the path of propagating dikes we show how lateral dike propagation can be forecast using the minimum potential energy principle we compare theory to observed propagation paths of dikes originating at the bar arbunga volcano iceland in 2014 and 1996 by developing a probability distribution for the most likely propagation path the observed propagation paths agree well with the model prediction we nd that topography is very important for the model and our preferred forecasting model considers its in uence on the potential energy change of the crust and magma we tested the in uence of topography by running the model assuming no topography and found that the path of the 2014 dike could not be hindcasted the results suggest that lateral dike propagation is governed not only by deviatoric stresses but also by pressure gradients and gravitational potential energy furthermore the model predicts the formation of curved dikes around cone shaped structures without the assumption of a local deviatoric stress eld we suggest that a likely eruption site for a laterally propagating dike is in topographic lows the method presented here is simple and computationally feasible our results indicate that this kind of a model can be applied to mitigate volcanic hazards in regions where the tectonic setting promotes formation of laterally propagating vertical intrusive sheets

correlation of tremor activity with tidal stress in the northern cascadia subduction zone 1 we analyze hourly data from five tremor episodes in the northern cascadia subduction zone over the period 2003 2005 provided by the tremor activity monitoring system tams all five tremor episodes correspond to slow slip events observed by gps fourier decomposition is used to separate the hourly tremor counts for each episode into long period 0 2 2 cpd components the tidal component of the observations is compared with theoretical stress variations at depths of 20 30 and 40 km with 40 km being the depth of the interpreted subduction thrust interface the stress variations are predicted by a 2 d ocean tide loading model combined with estimates of stress variations from earth tides we find that the shear stress in the thrust direction and the compressive normal stress on shallow dipping surfaces correlates with the data significantly better than the confining stress over the range of depths investigated the relative amplitudes of tidal shear stress and compressive normal stress result in positive coulomb stress favoring slip peak tremor activity occurs at times of maximum tidal shear stress in the thrust direction which would assist slow slip and would suggest that tidal tremor and slip are colocated the response of the tremor to tidal shear stress is roughly proportional to the mean activity level controlled by tectonic conditions of stress and pore pressure a significant nontidal daily variation in tremor activity of unknown origin is identified

current stress and strain rate fields across the dead sea fault system constraints from seismological data and gps observations abstract the stress and strain rate fields characterizing the dead sea fault system are investigated by using seismological and geodetic observations in order to assess spatial variations in the regional stress field we compiled a multidisciplinary dataset of well constrained horizontal indicators by merging all available data reported in literature with the data obtained in this study through weighted stress inversions of focal plane solutions our findings indicate that the state of stress is characterized by the coexistence of a normal faulting stress regime with the primarily strike slip one according to the regional frame illustrated by previous geological and seismological observations an updated velocity field computed from new observations and earlier published data depicts the general left lateral motion of the dead sea fault system well in agreement with previous studies we detected some differences in the slip rate pattern between the northern and the southern sectors of the fault system the geodetic strain rate field highlights how much of the deformation is accommodated along the fault system itself in a narrow region the comparison between the stress and the strain rate directions reveals that both orientations are near parallel clearly indicating that present day crustal stress and ground deformation patterns are chiefly driven by the same tectonic processes

temporal stress changes associated with the 2008 may 29 mw 6 earthquake doublet in the western south iceland seismic zone on 2008 may 29 two magnitude m w similar to 6 earthquakes occurred on two adjacent n s faults in the western south iceland seismic zone the first main shock was followed approximately 3 s later by the rupture on a parallel fault about 5 km to the west an intense aftershock sequence was mostly confined to the western fault and an e w aligned zone extending west of the main shock region into the reykjanes oblique rift in this study a total of 325 well constrained focal mechanisms were obtained using data from the permanent icelandic sil seismic network and a temporary network promptly installed in the source region following the main shocks which allowed a high resolution stress inversion in short time intervals during the aftershock period more than 800 additional focal mechanisms for the time period 2001 2009 obtained from the permanent sil network were analysed to study stress changes associated with the main shocks results reveal a coseismic counter clockwise rotation of the maximum horizontal stress of 11 10 degrees 95 per cent confidence level in the main rupture region from previous fault models obtained by inversion of geodetic data we estimate a stress drop of about half of the background shear stress on the western fault with a stress drop of 8 10 mpa the pre event shear stress is estimated to 16 20 mpa the apparent weakness of the western fault may be caused by fault properties pore fluid pressure and the vicinity of the fault to the western rift zone but may also be due to the dynamic stress increase on the western fault by the rupture on the eastern fault further a coseismic change of the stress regime from normal faulting to strike slip faulting was observed at the northern end of the western fault this change could be caused by stress heterogeneities but may also be due to a southward shift in the location of the aftershocks as compared to prior events

constraints on fault slip rates of the southern california plate boundary from gps velocity and stress inversions summary we use global positioning system gps velocities and stress orientations inferred from seismicity to invert for the distribution of slip on faults in the southern california plate boundary region of particular interest is how long term slip rates are partitioned between the indio segment of the san andreas fault saf the san jacinto fault sjf and the san bernardino segment of the saf we use two new sets of constraints to address this problem the first is geodetic velocities from the southern california earthquake center s scec crustal motion map version 3 by shen et al which includes significantly more data than previous models the second is a regional model of stress field orientations at seismogenic depths as determined from earthquake focal mechanisms while gps data have been used in similar studies before this is the first application of stress field observations to this problem we construct a simplified model of the southern california fault system and estimate the interseismic surface velocities using a backslip approach with purely elastic strain accumulation following meade et al in addition we model the stress orientations at seismogenic depths assuming that crustal stress results from the loading of active faults the geodetically derived stressing rates are found to be aligned with the stress orientations from seismicity we therefore proceed to invert simultaneously gps and stress observations for slip rates of the faults in our network we find that the regional patterns of crustal deformation as imaged by both data sets can be explained by our model and that joint inversions lead to better constrained slip rates in our preferred model the sjf accommodates 15 mm yr 1 and the indio segment of the saf 23 mm yr 1 of right lateral motion accompanied by a low slip rate on the san bernardino segment of the saf anomalous fault segments such as around the 1992 m w 7 3 landers surface rupture can be detected there observed stresses deviate strongly from the long term loading as predicted by our simple model evaluation of model misfits together with information from palaeoseismology may provide further insights into the time dependence of strain accumulation along the san andreas system

an improved method for determining the regional stress tensor using earthquake focal mechanism data application to the san fernando earthquake sequence the orientations of fault planes and slip directions indicated by a population of earthquake focal mechanisms can be used to determine best fit regional principal stress directions and r 2 1 3 1 a measure of relative stress magnitudes under the assumption of uniform stress in the source region this analysis allows for the possibility that failure occurs on preexisting zones of weakness of any orientation in the inversion we perform a grid search of stress models to find the one which requires the smallest total rotation of all the fault planes that is needed to match the observed and predicted slip directions the method allows for errors in orientations of both the fault planes and slip directions we have an objective means for identifying the more likely of the two possible fault planes from each focal mechanism relative to a given stress model thus we do not face the problem of ambiguity of nodal planes which plagues other analyses of this kind by using a grid search of stress models rather than a linearization scheme we are able to perform a realistic error analysis and thus establish confidence limits for the preferred regional stresses the method can be used to investigate possible stress inhomogeneities during earthquake sequences by analyzing subsets of the data population the technique has been applied to 76 events from the san fernando earthquake sequence for which we have found best fit stresses plunge and azimuth 1 7 187 2 27 281 3 62 84 and r 0 65 the average misfit between this stress model and all the data is about 8 and all but eight of the aftershocks have misfits of less than 20 these values are considerably less than the uncertainty of the focal mechanism determinations therefore significant stress inhomogeneities are not required by the data our analysis does not support the suggestion of a change in stresses during the aftershock sequence as proposed by others on the basis of an apparent change in focal mechanisms

nucleation growth and linkage of faults in oblique rift zones results from experimental clay models and implications for maximum fault size we use scaled clay models to study the temporal evolution of fault populations in experiments of moderately oblique a5 608 and highly oblique a5 308 distributed extension where a is the angle between the rift axis and the direction of displacement faults nucleate at random heterogeneities enhancing nucleation of diffuse clusters of new faults in the highly oblique model clusters of displacement normal faults form parallel arrays leading to fault growth dominated by tip propagation and along strike linkage until maximum length is achieved subsequent growth of rift subparallel faults leads to a phase of growth characterized by linkage and formation of branching faults in the moderately oblique model clusters form in a stepping geometry leading to growth dominated by linkage faults nucleate and grow more rapidly and their growth is less restricted than in the highly oblique model our results have implications for the maximum size earthquake to be expected in an oblique rift setting

extension direction re orientation in the oceanic rift of iceland and comparison with continental rifts abstract in this study we report the results of a structural analysis carried out on fault systems that form the active rift segments of two areas of iceland namely 1 the northern part of the eastern rift segment northern volcanic zone nvz and 2 the western rift segment that include the western volcanic zone thingvellir area and the reykjanes peninsula in the n s trending nvz mainly krafla volcanic area the obtained extension directions trend around ese at thingvellir the extension directions trend ca nw se and are perpendicular to the main ne sw trending faults in the ene wsw trending tectono magmatic segments of the reykjanes peninsula our data show a marked difference with the gps data and spreading directions the extension directions trend roughly orthogonal to the strike of the boundary faults in all the investigated sectors extension shows an overall southward increase in divergence from the regional directions indicated by gps vectors this behavior has been compared with the continental main ethiopian rift of east africa where structural studies show a re orientation of the extension direction between the rift margins and the internal sectors resulting in a divergence between local and regional extension directions aimed at investigating the causes of these differences we performed a new series of analogue models by varying different parameters such as rift obliquity and width our findings suggest that rift width exerts a primary control on the boundary axis variation of the extension direction in particular significant differences in fault orientation and local direction of extension between the margins and the rift axis are observed in wider rift valleys e g continental rift of ethiopia conversely fault orientation and local extension direction are much more homogeneous in narrower rift valleys e g rifts of iceland where a variations in the direction of extension from the margins to the rift axis are not observed

triggered fault slip on june 17 2000 on the reykjanes peninsula sw iceland captured by radar interferometry 1 dynamically triggered seismicity followed shortly after a ms 6 6 earthquake in iceland on june 17 2000 smaller earthquakes occurred on the reykjanes peninsula up to 100 km from the mainshock rupture using interferometric analysis of synthetic aperture radar images insar we measure crustal deformation associated with three triggered deformation events the largest of these occurred at lake kleifarvatn 85 km west of the mainshock epicenter modeling of the insar data reveals strikeslip on a north striking fault with a geodetic moment of 6 2 1017 nm equivalent to magnitude mw 5 8 earthquake a seismological estimate of the moment is not yet available because the seismic signature of this event is partly hidden by the mainshock waveform the paucity of aftershocks on the triggered rupture plane suggests some aseismic slip there compatible with a thin seismogenic crust high heat flow hydrothermal alteration and the presence of fluids in the area

the baltic shield earthquakes abstract the baltic shield area is a low seismicity area with few seismic events exceeding m l 4 since 1979 dense regional seismic networks have been in operation and have allowed accurate location including focal depths and fault plane solutions of more than 200 earthquakes in the range m l 0 6 4 5 the location of the southern sweden seismicity shows that a long part of the protogine zone the border between the younger western and older eastern crustal rocks marks a change in seismic activity the focal depths of the earthquakes indicate three seismically different crustal layers upper crust 0 13 km in northern sweden 0 18 km in southern sweden middle crust down to 35 km and the seismically quiet lower crust the gutenberg b value is smaller for the middle crust than for the upper crust the fault plane solutions show that strike slip is dominant along the tornquist line the southwestern border of the baltic shield area normal faulting occurs in northern sweden reverse faulting is more common than normal faulting the stresses released by the earthquakes show a remarkable consistency considering the small sizes of the earthquakes the data are consistent with a n60w direction of the regional principal compression supporting the view that the baltic shield seismicity is related to plate tectonic processes rather than to uplift the spatial distribution of the baltic shield seismicity is consistent with a model in which the earthquakes are considered as sudden breakdowns of asperities of normally stably sliding faults aseismic sliding is estimated to be more than 20 000 times more extensive than seismic sliding the rate at which southern sweden presently deforms horizontally is estimated to be 1 mm yr

display and quantitative assessment of distributions of earthquake focal mechanisms summary this paper describes quantitative methods for evaluating the distribution of focal mechanism orientations of groups of earthquakes and shows how to display this distribution on a triangle diagram we present a 2 based statistical test for determining whether two sets of focal mechanisms are drawn from distinct populations we apply these methods to 3625 better determined mechanisms for shallow earthquakes in the harvard centroid moment tensor cmt catalogue we describe the distributions of mechanisms in the whole catalogue and for catalogue subsets in some specific tectonic environments in addition we explore the geographical locations of mechanisms with orientations that occur relatively infrequently that is mechanisms that are unlike thrust normal or strike slip mechanisms such mechanisms are relatively rare along mid ocean ridges and in oceanic subduction zones the majority of these unusual mechanisms occur along plate boundaries where crustal thickness is highly variable and in regions where the plate convergence direction becomes oblique and thus relative motion changes from convergence to transform motion

dating of seasonal snow firn accumulation layers using pollen analysis reliable chronologies in ice cores and snow pits from many alpine glaciers in latitudes between 60 n and 60 s are often difficult to establish owing to problems with annual layer counting problems arise from melting wind erosion and the negligible amount of precipitation in some seasons all of which tend to obscure the seasonal variations in 18 o and chemical concentrations that are typically used to date ice cores however alpine glaciers contain many species of pollen grains that peak at particular times of the year we used the peaks in betulaceae pinus artemisia and a combination of abies and picea pollen species to determine the four seasonal layers of a snow pit on belukha glacier in russia s altai mountains comparing the pollen dated profiles with wind and precipitation records allows us to determine where a seasonal layer is missing thus the pollen dating method described here may be a useful tool to measure the annual snow deposition on alpine glaciers even when some seasonal layers are eroded by wind or missing due to negligible precipitation

crustal stress field in southern california and its implications for fault mechanics we present a new high spatial resolution image of stress orientation in southern california based on the inversion of earthquake focal mechanisms we use this image to study the mechanics of faulting in the plate boundary region the stress field contains significant spatial heterogeneity which in some cases appears to be a result of the complexity of faulting and in other cases appears to be a cause temporal changes in the stress field are also observed primarily related to major earthquakes the observed 15 10 rotation of the stress axes due to the 1992 m7 3 landers mainshock implies that the deviatoric stress magnitude in the crust is low of the order of 10 mpa this suggests that active faults in southern california are weak the maximum principal stress axis near the san andreas fault is often at 50 to the fault strike indicating that the shear stress on the fault is comparable to the deviatoric stress the san andreas in southern california may therefore be a weak fault in a low strength crust

calculating horizontal stress orientations with full or partial knowledge of the tectonic stress tensor calculating horizontal stress orientations with full or partial knowledge of the tectonic stress tensor

seismic structure beneath the reykjanes peninsula southwest iceland inferred from array derived rayleigh wave dispersion abstract the aim is to obtain a site specific s wave structure beneath the reykjanes peninsula southwest iceland nine broadband stations of the reykjanet network are used to find rayleigh wave phase velocity dispersion in a relatively wide range of periods from 3 to 50 s the records analyzed were made in the years 2013 to 2015 and concern fourteen selected earthquakes whose epicentral distances range from tens of kilometers to almost ten thousand kilometers our approach to retrieving rayleigh phase velocity dispersion involves two partly independent methods allowing for array apertures larger than a wavelength 1 the zero crossing point method and 2 the phase plane method the two methods used here work with seismograms decomposed into quasi harmonic components and implicitly assume a single plane wave propagation the good match between the dispersion curves obtained by means of the two methods indicates that the assumption has been reasonably fulfilled the rayleigh wave phase velocity dispersion data are inverted into a horizontally layered isotropic s wave velocity model of the earth s crust and uppermost mantle by a modified method of the single parameter variation at shallow depths the derived model is rather similar to some previous models that were derived predominantly from the arrival times of body waves at depths exceeding about 20 km the dispersion data require low s wave velocities indicating a noticeable low velocity zone

crustal deformation at the oblique spreading reykjanes peninsula sw iceland gps measurements from 1993 to 1998 in 1993 and 1998 a 38 point gps network was surveyed on the reykjanes peninsula sw iceland according to the nuvel 1a plate motion model the spreading rate of the north american and eurasian plates in sw iceland is 18 9 0 5 mm yr toward n102 7 1 1 e highly oblique to the plate boundary instead of oblique spreading the measurements indicate left lateral shear strain accumulation parallel to the reykjanes peninsula seismic zone n76 e at the rate of about eyx 0 2 strain yr tensor shear strain subsidence is generally observed toward the seismic zone a local maximum subsidence of 60 mm was measured in the svartsengi geothermal area subsidence in this area has previously been detected with geodetic measurements and is considered a result of geothermal usage expansion was observed in the area of the hengill triple junction this is probably a result of magma accumulation beneath mount hromundartindur as previous seismic and geodetic measurements have indicated using a simple screw dislocation model we fit the majority of the data assuming a left lateral shear zone at depth along the seismic zone we estimate locking depth of 6 5 km and deep slip rate of 16 5 mm yr the maximum left lateral displacement predicted by the screw dislocation model 11 85 0 06 mm yr is consistent with the observed value of 11 9 0 5 mm yr if the hengill area is excluded little extension is observed across the peninsula the discrepancy between the nuvel 1a oblique spreading and the observed transcurrent motion is thought to be caused by lack of magma intrusion into the crust during this time period

revel a model for recent plate velocities from space geodesy 1 we present a new global model for recent plate velocities revel describing the relative velocities of 19 plates and continental blocks the model is derived from publicly available space geodetic primarily gps data for the period 1993 2000 we include an independent and rigorous estimate for gps velocity uncertainties to assess plate rigidity and propagate these uncertainties to the velocity estimates the velocity fields for north america eurasia and antarctica clearly show the effects of glacial isostatic adjustment and australia appears to depart from rigid plate behavior in a manner consistent with the mapped intraplate stress field two thirds of tested plate pairs agree with the nuvel 1a geologic 3 myr average velocities within uncertainties three plate pairs caribbean north america caribbean south america and north america pacific exhibit significant differences between the geodetic and geologic model that may reflect systematic errors in nuvel 1a due to the use of seafloor magnetic rate data that do not reflect the full plate rate because of tectonic complexities most other differences probably reflect real velocity changes over the last few million years several plate pairs arabia eurasia arabia nubia eurasia india move more slowly than the 3 myr nuvel 1a average perhaps reflecting long term deceleration associated with continental collision several other plate pairs including nazca pacific nazca south america and nubia south america are experiencing slowing that began 25 ma the beginning of the current phase of andean crustal shortening

kinematic models of plate boundary deformation in southwest iceland derived from gps observations 1 we use data from gps campaign and continuous measurements from 1992 to 2004 in sw iceland to map the surface velocity field from the reykjanes peninsula to the eastern volcanic zone we divide the time series into preseismic july 1992 to june 2000 and postseismic june 2000 to may 2004 time intervals and we estimate gps station velocities for each interval as well as coseismic offsets due to the june 2000 earthquake sequence in the south iceland seismic zone sisz in addition to the plate spreading the preseismic velocity field shows the effects of inflation at hengill and hekla volcanoes whereas the postseismic velocities show deformation following the june 2000 earthquakes we consider several kinematic models to explain the preseismic velocities our preferred model includes several dislocations and point sources in an elastic half space with left lateral slip along the plate boundary on the reykjanes peninsula and below the sisz opening across the reykjanes peninsula and the western and eastern rift zones the optimal model has a locking depth of about 8 km in the central and eastern part of the reykjanes peninsula with a deep slip rate of about 17 mm yr and an opening of about 9 mm yr this locking depth is in agreement with the thickness of the seismogenic crust on the reykjanes peninsula which appears to vary between 7 and 9 km for the sisz we obtain a deep slip rate of about 19 mm yr below 16 km depth which is considerably deeper than the earthquake hypocenter depths in the area

present day crustal stress field in greece inferred from regional scale damped inversion of earthquake focal mechanisms in this study we utilize regional and teleseismic earthquake moment tensor solutions in order to infer the contemporary crustal stress in the greek region we focus on crustal earthquakes and select only solutions with good waveform fits and well resolved nodal planes a data set of 1614 focal mechanisms is used as input to a regional scale damped stress inversion algorithm over a grid whose node spacing is 0 35 several resolution and sensitivity tests are performed in order to ascertain the robustness of our results our findings show that for most of the greek region the largest principal stress 1 is vertically oriented and that the minimum principal stress axis 3 are subhorizontal with a predominant n s orientation in the sw peloponnese the orientation of 3 axes rotates clockwise and in se aegean counterclockwise these results are in agreement with the generally accepted model that slab rollback combined with gravitational spreading of the aegean lithosphere are the main causes of the extension transitions between different faulting types in nw greece or in the aegean occur within narrow zones in the order of tens of kilometers a visual comparison of the principal horizontal stress axes and the principal strain axes derived from gps observations shows good agreement suggesting that the crust in the greek region behaves largely in an elastic manner

improved source modelling through combined use of insar and gps under consideration of correlated data errors application to the june 2000 kleifarvatn earthquake iceland summary simultaneous use of multiple independent data sets can improve constraints on earthquake source model parameters however the ways in which data sets have been combined in the past are manifold and usually qualitative in this paper we present a method to combine geodetic data in source model estimations which includes characterizing the data errors and estimating realistic model parameter uncertainties caused by these errors we demonstrate this method in a case study of the june 2000 kleifarvatn earthquake which occurred on reykjanes peninsula in iceland we begin by showing to what extent additional data can positively influence the source modelling results by combining both gps and descending orbit insar data which were used in two earlier studies of that event with insar data from an ascending orbit we estimate the data error covariances of the insar observations and base the data weights in our model parameter optimization on the corresponding data variance covariance matrix we also derive multiple sets of synthetic data errors from the estimated data covariances that we use to modify the original data to generate numerous data realizations from these data realizations we estimate the model parameter uncertainties we first model the kleifarvatn earthquake as a simple uniform slip fault and subsequently as a fault with variable slip and rake our fault model matches well with the field observations of coseismic surface ruptures and its near vertical dip 83 agrees with the regional faulting style as well as with aftershock locations the two published source models of the event on the other hand both differ from our model as well as differing for one another these studies which were based on the descending insar data alone the first study and on that same data and gps data the second study both neglect correlations in the insar data and do not report model parameter uncertainties therefore to compare these results with our model we simulate the earlier model estimation set ups and provide realistic estimates of the model parameters uncertainties for these cases we then discuss the significance of the difference between the existing fault models and demonstrate that both the inclusion of additional independent data as well as the covariance based data weights improve the model parameter estimation

the stress pattern of iceland moritz ziegler mojtaba rajabi oliver heidbach gylfi pall hersir kristjan agustsson sigurveig arnadottir arno zang

three dimensional imaging of the p and s wave velocity structure and earthquake locations beneath southwest iceland three dimensional imaging of the p and s wave velocity structure and earthquake locations beneath southwest iceland