surface stresses associated with arrested dykes in rift zones many theoretical models predict that arrested dykes may generate major grabens at rift zone surfaces arrested dyke tips in eroded rift zones however are normally not associated with major grabens or normal faults that could be generated by dyke induced stresses ahead of the tips and normal faults and grabens tend to be less common in those parts of eroded rift zones where dykes are comparatively abundant similarly there are feeder dykes as well as dykes arrested a few metres below the surface that do not generate faults or grabens at the surface here i propose that this discrepancy between theoretical models and field observations may be explained by the mechanical layering of the crust numerical models presented here show that abrupt changes in young s moduli layers with high dyke normal compressive stresses stress barriers and weak horizontal contacts have large effects on the dyke induced stress fields for the models considered the surface tensile stresses induced by arrested dykes are normally too small to lead to significant fault or graben formation at the rift zone surface the only significant dyke induced surface tensile stresses 2 mpa in these models are for a dyke tip arrested at 1 km depth below the surface of a rift zone with a weak contact at 400 m depth and subject to extension that tensile stress however peaks above the ends of the weak horizontal contact which in the model considered occur at distances of 4 km to either side of the dyke and shows no simple relation to the depth to the dyke tip thus for a layered crust with weak contacts straightforward inversion of surface geodetic data to infer dyke geometries may result in unreliable results

effective tension shear relationships in extensional fissure swarms axial rift zone of northeastern iceland abstract the geometry of fracture systems in selected areas of the active krafla fissure swarm mid atlantic ridge northeastern iceland is analysed based on geodetic analysis of the present day topography at the top of holocene basaltic lava flows which fill the axial rift zone the deformation of this initially horizontal surface can be reconstructed extensional deformation is localised at all scales and block tilting though present remains minor using simple models of the surface expression of normal faults the geometrical characteristics of the topographic features related to active deformation during tectonic volcanic events are quantitatively analysed at crustal depths of about 1 km normal faults are present and have an average 70 dip comparison with the dip data of older normal faults observed in the uplifted and eroded shoulders of the rift zone at palaeodepths of 1 2 km indicates that this dip determination is valid comparisons between the local case study and structural analyses of active fissure swarms on a larger scale suggest that normal faulting plays a major role in the middle section of the thin newly formed brittle crust of the rift zone in the axial oceanic rift zone of ne iceland the extensional deformation in the upper crust is dominated by horizontal tension and shear of normal sense their relative importance depending on depth absolute tension dominates in the uppermost several hundred metres of the crust resulting in the development of fissure swarms effective tension plays an important role at a deeper level 2 5 km because of the presence of magmatic fluid pressure from magma chambers which feed dyke injections at crustal depths of about 1 km normal shear prevails along fault planes which dip 60 75 this importance of normal shear at moderate depth between upper and lower crustal levels where tension prevails is pointed out within the extensional context of rifting these variations of tectonic behaviour with depth are controlled by both the lithostatic pressure and the effective tension induced by the presence of magmatic fluid pressure

localisation of normal faults in multilayer sequences abstract existing conceptual growth models for faults in layered sequences suggest that faults first localise in strong and brittle layers and are later linked in weak and ductile layers we use the discrete element method for modelling the growth of a normal fault in a brittle ductile multilayer sequence the modelling reveals that faults in brittle ductile sequences at low confining pressure and high strength contrast localise first as mode i fractures in the brittle layers low amplitude monoclinal folding prior to failure is accommodated by ductile flow in the weak layers the initially vertically segmented fault arrays are later linked via shallow dipping faults in the weak layers faults localise therefore as geometrically and kinematically coherent arrays of fault segments in which abandoned fault tips or splays are a product of the strain localisation process and do not necessarily indicate linkage of initially isolated faults the modelling suggests that fault tip lines in layered sequences are more advanced in the strong layers compared with weak layers where the difference in propagation distance is most likely related to strength and or ductility contrast layer dependent variations in fault localisation rates generate fringed rather than smooth fault tip lines in multilayers

relay zone geometry and displacement transfer between normal faults recorded in coal mine plans abstract overlap lengths separations and throw gradients were measured on 132 relay zones recorded on coal mine plans throws on the relay bounding fault traces are usually 2 m and individual structures are recorded on only one seam throw gradients associated with relay zones are not always higher than on single faults but asymmetry of throw profiles is diagnostic of relay zones bed geometries around larger faults in opencast mines are used to assess the displacement accommodated by shear in the vertical plane normal to the faults and displacement transfer accommodated by shear in the fault parallel plane three dimensional structure is defined for two relay zones each recorded on five seam plans these relay zones are effectively holes through the fault surfaces and overlap occurs between salients or lobes of the parent fault surfaces lobes initially terminated at tip lines but as the faults grew gradually rejoined the main fault surfaces along branch lines this type of relay zone originates by bifurcation of a single fault surface at a locally retarded tip line and is an almost inevitable result of a tip line irregularity

high resolution fault displacement mapping from three dimensional seismic data evidence for dip linkage during fault growth abstract detailed mapping using high resolution three dimensional seismic data has revealed a number of sub horizontal anomalies in the distribution of vertical displacement throw on the planes of growth faults in the gulf of mexico recognition of these anomalies is highly sensitive to the interval at which the fault displacements are sampled because they represent local decreases in throw which are confined to small discrete parts of the fault planes the distribution of the anomalies is inconsistent with model displacement fields of quasi elliptical concentric contours and is therefore incompatible with models of fault growth by uniform slip distribution and radial tip line propagation an alternative model is proposed whereby the evolution of a fault plane is established by the propagation and linkage of precursor fault segments in the dip direction dip linkage overlap and linkage of fault tips in the dip direction results in relay structures that are sub parallel to fault strike and therefore displacement minima that are sub horizontal on normal and thrust faults however since they are orthogonal to the main slip direction on the fault these structures have a low preservation potential and are therefore unlikely to be well resolved on cross sectional seismic profiles

modelling the processes of cliff top erosion and deposition under extreme storm waves at exposed sites on the deep water coasts of the british isles cliff top storm deposits ctsds occur on cliffs at a variety of elevations above sea level and may occasionally reach up to 50 m above sea level time series field mapping of ctsds has demonstrated their formation over the historical period and their continued modification during major past storms this paper seeks to clarify the morphogenetic context of ctsds model the wave conditions and forces encountered at the cliff face and cliff top platform and propose mechanisms to link wave processes to cliff face quarrying and landwards cliff top transport of quarried blocks to deposition zones at the rear of the cliff top platform we report on wave tank experiments using scaled cliff and wave conditions from a cliff in the shetland islands to focus on three situations incident waves lower than the cliff edge height at the same height as the cliff edge height and higher than the cliff edge height the modelling suggests that steep waves of 10 m and above impacting on a 15 m high cliff will result in impact pressures sufficient to promote crack propagation block detachment and lifting of large blocks large but not necessarily steep waves of the same height as the cliff edge produce sufficient impact pressures and water flow over the cliff edge and platform to entrain blocks transport and deposit them on the cliff top where cliff top height is below wave crest elevation green water bore flow occurs sufficient to force rotation or lifting of blocks out of cliff top and cliff top platform sockets high flow velocities rapidly accelerate and transport blocks inland until the flow attenuation results in deposition of blocks at the limit of run up the model results provide a good explanatory framework to account for the quarrying of the upper part of the cliff face and cliff top platform under storm wave conditions and provide an insight into the exceptional velocities experienced over the cliff top platform under bore flow conditions the modelling results show that extreme storm waves are capable of quarrying transporting and depositing large blocks at altitude and significant distances inland and so present serious questions about the use of such deposits as diagnostic of palaeo tsunami

mathematical modeling of basal coastal cliff erosion in uniform strata a theoretical approach abstract this article utilizes the general assumption that cliff erosion is dependent on the strength of wave assailing forces and the resistance of the cliff material it considers the possibility of understanding and describing coastal cliff erosion processes particularly basal erosion through a mathematical analysis of one of the clearest manifestations of its action i e the development of the basal cliff notch through mechanical wave action it analyzes profile change through time assuming a cliff composed of uniform strata deterioration of the cliff is expressed by means of the erosion function which in this study is related to the cliff erosion rate an equation termed the belov davies williams bdw equation has been formulated for a moving surface and this is proposed as a mathematical model of cliff profile change explicit solutions of the bdw equation are found for both steady state and time dependent erosion rates in both cases it is assumed that wave erosion intensity decreases

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

evolution of vertical faults at an extensional plate boundary southwest iceland vertical faults having both opening and vertical displacements are common in southwest iceland and hypotheses vary regarding whether they propagated to the surface from below or from the surface downward we address this issue through a study of vertical faults and associated surface fracture zones in regions of both oblique and normal spreading in southwest iceland individual fracture segments are commonly rotated out of the general trend of the fracture zone suggesting oblique motion along subsurface normal faults these faults commonly breach the upper hinge lines of narrow monoclinal folds that flank many fault traces on the hanging wall side based on these field observations and the results of numerical models we propose that 60 758 dipping normal faults in the subsurface propagated to the surface from below vertical fractures formed at the upper tips of the faults at depths of between 250 and 500 m 25 50 of the fault length in response to stress concentrations along the tip line model results indicate that narrow monoclinal folds develop at the surface above these vertical fractures which subsequently breach the monoclines along the upper hinge line forming vertical fault scarps and open fissures at the surface if vertical fractures utilize pre existing cooling joints in basalt to connect directly to the surface the hanging wall is able to pull apart from the footwall without the development of a surface monocline along the fault trace q 2003 elsevier ltd all rights reserved

tectonics and magma dynamics coupling in a dyke swarm of iceland abstract to study the dynamics of dyke and fissure swarms in iceland we analyse the distribution of the dykes across the tertiary alftafjordur dyke swarm and that of open fissures and faults across the active krafla fissure swarm within these narrow 5 to 10 km wide and long length width ratio 10 swarms deformation is accommodated both by tectonics and magmatism the dyke swarm built during the westward regional tilting of the host rock lava pile the populations of dykes suggest that a characteristic thickness at about 3 m is involved during dyking dykes show a quasi periodic distribution across the swarm and form clusters spaced by about 2 5 km the analysis of the fissure distribution in the krafla volcanic system reveals a cluster spacing of about half of that of the dyke clusters we discuss the potential origins of these features as a result of magma dynamics host rock properties and tectonics clustering possibly reflects the intrinsic mechanical response to stretching of the upper levels of the brittle crust we propose a three dimensional model of dyke swarms in which magma likely migrates laterally within the upper crust from igneous centres where high magmatic supply rates locally perturb the regional stress field

normal fault growth in layered basaltic rocks the role of strain rate in fault evolution abstract conceptual models for the evolution of dilatant faults in volcanic rift settings involve a step wise growth pattern involving upward propagation of subsurface faults and surface monocline formation which are breached by subvertical open faults immature discontinuous normal faults are considered representative of the early stages of mature linked faults that accommodate extensional strains we consider the evolution of surface breaking normal faults using a comparison of the distribution and geometry of normal faults from two volcanic rift zones the koa e fault system hawai i and the krafla fissure swarm ne iceland field mapping highlights similarities to current predicted geometries but also prominent differences that are not reconciled by current models variable deformation styles record magma supply changes within the rift zones which drive local strain rate gradients building on existing studies we present a conceptual model of fault growth that accounts for spatial and temporal changes in strain rate within the deforming regions we propose that faults in separate rift systems may not advance through the same stages of evolution and that faults within individual rift systems can show differing growth patterns variations in surface strains may be indicative of subsurface magmatic system changes with important implications for our understanding of volcano tectonic coupling

patterns and rates of erosion produced by high energy wave processes on hard rock headlands the grind of the navir shetland scotland abstract the grind of the navir is an ignimbrite headland on the exposed atlantic coast of the shetland islands scotland during storms offshore wave heights exceed 20 m and deep water close inshore allows high energy waves to impact on these cliffs the cliff top at 15 m above sea level is awash with wave water when wave heights exceed 8 m a condition met in storms in most years detailed mapping ground photography and patterns of lichen colonisation and growth allow the processes patterns and rates of erosion to be assessed on different parts of the headland over the past 100 years recent fractures in the rock of the cliff face and top indicate that wave impact forces exceed the 1 5 kpa tensile strength of the rock and fresh sockets low on the cliff face record the removal of fracture bounded blocks that are lost to seaward on the upper part of the cliff face upward moving water flow is capable of removing blocks from both vertical faces and stepped overhangs clefts in the cliff face are extended inwards and upwards by crack propagation and block removal leading to the development of slot caves the movement of waves across the cliff top in fast moving bores quarries blocks from the cliff face close to the edge and from rock steps on the cliff top promoting further rock fracturing socket sizes and block characteristics indicate that blocks of 1 m 3 are rotated from sockets on the cliff top platform and carried landwards for up to 60 m to be deposited in a series of boulder ridges on the cliff top multiple boulder movements during a storm in january 2005 generated impact marks and orientated striations that allow the pattern of water movement to be reconstructed across considerable areas of cliff top the average rate of erosion at the grind is estimated at 1 3 6 0 mm yr on the cliff face and 5 mm yr on the cliff top although erosion rates vary in both time and space although block movement on the cliff top occurs in most years erosion of rock by block removal is most active during major storms nevertheless some parts of the cliff top and face remain blackened by lichen and so have experienced little recent erosion over the last 70 years or so although erosion occurs at the base of the cliffs in recent decades it has been concentrated on upper part of the cliff face and top cliff top erosion is most intense where stepped geos act as channels for waves to access the cliff top the most active features on the cliff top are the boulder ridges where substantial changes can be traced to storms over the past century unlike conventional models of cliff erosion where erosion is concentrated by wave attack at the waterline wave impacts on this headland may occur on any part of cliff face and top and appear particularly effective on the upper cliff and cliff top platform

geometry kinematics and deformation rates along the active normal fault system in the southern apennines implications for fault growth throw rates throws and kinematic data have been collected from a localised system of active normal faults in the southern apennines italy to assess how its growth history differs from that for a distributed fault system in the central apennines both show evidence for fault interaction in that 1 faults located centrally along strike have higher throws and throw rates than distal faults and 2 cumulative throw and throw rate profiles summed across strike show central maxima with values decreasing to zero at fault system tips however although throw rates increased through time in the central apennines we are unable to resolve such changes if they exist for the southern apennines specifically throw rates derived from offsets of 18 ka geomorphic surfaces are consistent with total throws in the southern apennines when extrapolated back through time but overestimate total throws in the central apennines by a factor of 2 4 suggesting an increase in throw rate through time for the latter we discuss why some fault systems appear to have throw rates that are constant through time whilst others accelerate during growth interaction

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

field characteristics of laterally emplaced dikes anatomy of an exhumed miocene dike swarm in reydarfjordur eastern iceland abstract many dikes of the tertiary breiddalur dike swarm in reydarfjordur have a great lateral extent but are vertically upward and or downward discontinuous field relationships indicate that they are not the feeders of the lava pile they intrude their characteristics can be best interpreted as them constituting the fossil expression of lateral magma emplacement analogous to that occurring in the actively spreading krafla volcanic center of the neovolcanic zone average dike thickness increases clearly with depth and the thickness versus depth relationship is different from that described for other tertiary dike swarms in eastern iceland it is suggested that dikes represent the extent of finite spreading which at higher levels in the crust is expressed by fissures and other extensional structures

terraces and gilbert type deltas in crater lakes in ismenius lacus and memnonia mars paleolacustrine environments have been widely recognized from landform associations on the surface of mars in contrast to paleolakes in more open areas putative crater lakes commonly display well developed sedimentary landforms especially wave formed terraces and gilbert type deltas this study focuses on terraces and deltas of western memnonia and ismenius lacus as on earth these landforms provide important indications of water dynamics and sedimentary processes the putative martian paleolakes must have been active for thousands of years to have permitted the water movement and wave formation necessary to form one or more terraces fan shaped landforms at the mouths of channels entering crater lake areas may indicate various channel water interaction of these gilbert type deltas are particularly informative their presence indicates multiple flooding events whereby the inlet channels conveyed coarse grained sediment to the crater lake both terraces and deltaic progradation indicate that the crater lakes of memnonia and ismenius lacus were active for long time periods at least for thousands of years during the post heavy bombardment period of the later martian history the unequivocal presence of standing bodies of water of relatively long durations indicates that climatic settings very different from those prevailing today must have occurred at least locally and temporarily during parts of the martian planetary history

from surface fault traces to a fault growth model the vogar fissure swarm of the reykjanes peninsula southwest iceland abstract the vogar fissure swarm is one of four en echelon fracture swarms that connect the reykjanes ridge to the south iceland seismic zone and the western volcanic zone occurring in an area of flat topography this fissure swarm is clearly visible at the surface where it can be seen to affect recent postglacial lavas using remote sensing methods to identify and measure all the faults and fractures in the swarm combined with additional field observations and measurements we measured 478 individual fractures 33 of them being faults and 67 being fissures the fracture lengths show roughly log normal distributions most of the individual fractures belong to 68 main composite fractures seven of which are longer than 2500 m and correspond to the main fault scarps of the fissure swarm we showed that these main faults are distributed along five equally spaced zones 500 m apart and a few kilometers long we drawn 71 across strike profiles to characterize the shape of the fault scarps and 5 along strike profiles to characterize the evolution of vertical throw along the main faults each fault consists of a coalescence of individual segments of approximately equal length fault throws are never larger than 10 m and are smallest at the junctions between individual segments analyses of along strike throw profiles allowed us to determine the early stages of growth after coalescence the earliest stage is characterized by an increase in the throw of the central parts of segments this is followed by a second stage during which the throw increases at the junctions between segments progressively erasing these small throw zones

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

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

interaction and linkage of extension fractures and normal faults examples from the rift zone of iceland abstract field and photogeological studies were made of 90 zones of interacting fracture segments along the rift zone of iceland each zone consists of a pair of extension fractures or a pair of normal faults with lengths from metres to kilometres these zones evolve from an underlapping stage through an overlapping stage the most common configuration and finally to a linkage stage of all the zones only 7 are underlapping whereas 93 are overlapping with hook shaped fracture pairs the length width ratios of the overlapping zones have a mean value of 3 5 the preferred geometry of the overlapping zones depends upon the initial configuration of the interacting fractures length overstep and the development of the interaction in the overlapping zones most fracture pairs show moderate shear strike slip components related to local variations in the extension opening directions vertical displacements on normal faults decrease as the overstep and length of overlapping zones increase both in turn are proportional to the total lengths of the faults forming the pair the geometrical and kinematic features of overlapping spreading centres at mid ocean ridges show close similarities to those reported here these similarities indicate that the architecture and evolution of overlapping zones are scale independent

monitoring resistivity changes prior to earthquakes in parkfield california with telluric arrays a telluric array has been designed to monitor long period changes of resistivity in parkfield prior to the next characteristic earthquake there the array straddles the san andreas fault and utilizes dipoles which range from 5 to 18 km long a method of data analysis using robust processing in the time domain and singular value decomposition yields daily estimates of telluric coefficients which are stable to within 1 the array has detected several fluctuations above the noise level of the measurements most of these fluctuations are uncorrelated with bursts of seismic activity changes in water levels anomalous strain and creep or magnetic storms however the largest fluctuation in 2 years coincided with a tensor strain anomaly and both of these preceded a m 3 7 earthquake by approximately 1 month this is the largest earthquake to occur in parkfield since the array was installed

electrical resistivity changes not observed with the 28 september 2004 m6 0 parkfield earthquake on the san andreas fault california 1 an 18 a long experiment to capture changes in electrical resistivity prior to during and after a m6 earthquake on the san andreas fault at parkfield california culminated with an event of that magnitude on 28 september 2004 resistivity was monitored with a telluric array giving precisions in measured parameters of 0 2 or less no coseismic resistivity changes greater than 5 could have occurred or they would have been detected by the array and any precursory changes would have been much smaller because maximum possible precursory strains were 4000 times smaller than the coseismic strain given that the earthquake ruptured directly beneath the telluric array this experiment places a bound on the amount resistivity might change at the hypocentral location changes measured with instruments farther from earthquakes can only be smaller it is still possible that a measurable change with a larger earthquake or other fault geometries are capable of generating larger signals but our results show clearly that future experiments should be designed with precisions of much better than 0 1 if they are to be successful

estimating cliff retreat in southern california considering sea level rise using a sand balance approach abstract a sand balance coastal profile model for estimating cliff and shoreline retreats considering sea level rise is discussed the model specifically designed for cliffed coasts fronted by sandy beaches conditionally permits beach and cliff retreat to occur independently and includes subaerial cliff erosion and external beach sand sources the model accommodates complex nearshore and inland topography high volume beaches and variable cliff composition and is suitable where local sand balance is thought to be a primary controller of coastal evolution over decade century time scales designating an upper active beach boundary and beach cliff intersection divides the coastal profile into active beach and cliff sections separated by a back beach buffer the buffer acts as a sand reservoir and delays marine driven cliff erosion resulting in lower estimated cliff retreat compared to previous models neglecting protective beaches the model was applied on 21 km of cliffs in marine corps base camp pendleton california considering sea level rise ranging from 0 5 to 2 m over 100 yrs using 207 profiles sand budget deficits estimated from historical data and sand inputs from terrestrial erosion estimated from a time series of lidar data modeled mean and maximum scenario cliff retreats ranged from 4 87 m and 21 179 m respectively and provide order of magnitude estimates but are reliant on model assumptions and do not include potential coastal changes unrelated to local sand balance the results underscore the influence of protective beaches on cliff retreat

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

recurrence models and parkfield california earthquakes main shocks of the earthquake sequences that occurred on the parkfield section of the san andreas fault in central california in 1922 1934 and 1966 are characterized by southeast rupture expansion over the same 20 to 30 km long section of the fault whereas the seismic moments for the 1922 and 1934 events are identical to within a precision of 10 the seismic moment for 1966 is 20 greater than for the earlier events to within a precision of 20 the parkfield area seismicity in general seems well described by a recurring moderate size characteristic earthquake repeating the same epicenter magnitude seismic moment rupture area and southeast direction of rupture expansion an unexplained 10 year advance of the 1934 event is the only discrepancy in the hypothesis that the parkfield earthquakes in 1857 1881 1901 1922 1934 and 1966 represent a strictly periodic process assuming the strictly periodic model and the absence since 1966 of the perturbations hypothesized for the 1922 to 1934 period the next characteristic parkfield earthquake should occur between 1983 and 1993

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

tectonics of the thingvellir fissure swarm sw iceland abstract the thingvellir fissure swarm dissects 9000 year old pahoehoe lava and contains about 100 fractures of average orientation n29 3e the average length of fractures is 620 m the minimum being 57 m and the maximum 7 7 km the maximum width and throw on a single fracture are 68 m and 40 m respectively most fractures are vertical at the surface and must be the result of an absolute tensile stress the geometry and arrangement of the fractures indicate that they have grown by coalescence of initially offset small fractures it is concluded that most fractures attain depths of the order of several hundred meters or less but that the largest faults attain depths of many kilometers comparison with the vogar fissure swarm on the reykjanes peninsula suggests that the thingvellir swarm may have the greater rate of dilation the total maximum postglacial dilation of the vogar swarm is only 15 m whereas the corresponding figure for the thingvellir swarm is about 100 m

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

magnetotelluric evidence for crustal suture zones bounding the southern great valley california a geoelectric section inferred from a regional magnetotelluric study across the coast ranges the great valley and the sierra nevada reveals significant variations in electrical resistivity zones of lower resistivity interpreted at depths from 10 km to at least 30 km lie near mapped geologic boundaries between the coast ranges and the great valley and beneath the eastern side of the great valley the former boundary is inferred by others to separate the subduction complex of the coast ranges from the mafic basement of the great valley the lower resistivities are most likely associated with metasediments trapped between the coast ranges ophiolite and the former oceanic crust beneath the great valley the latter boundary is problematic but may be evidence for a deep metasedimentary section trapped between the ophiolites beneath the great valley and the granitic rocks of the sierra nevada the lack of change in the magnetotelluric phase across the great valley indicates that a suture zone marked by lower resistivities is unlikely to be present beneath the valley however this does not preclude the existence of a resistive suture zone

throw rate variations within linkage zones during the growth of normal faults case studies from the western volcanic zone iceland abstract this work investigates how throw rates vary within fault bends and sites of fault linkage during the process of normal fault growth in the western volcanic zone iceland through detailed field mapping and field measurements of fault throws normal faults are mapped and along strike throw profiles are constructed in order to understand how the throw rates relate with the local fault geometry along faults at different stages of linkage the results show that throw rates increase within linkage zones and propagating fault bends independently from the stage of maturity of the fault bend this implies that 1 the relationship between the local fault geometry and the along strike distribution of throw rate is driven by the deeper part of the fault where established fault bends start propagating to the surface 2 faults grow first by linkage and coalescence of separate faults and then by accumulation of slip on the resultant fault in agreement with models of fault growth by linkage and coalescence 3 incipient fault bends can produce uncertainty associated with palaeoseismological results if fault bends remain unrecognised moreover this work demonstrates that existing models showing increased co seismic and throw rates within fault bends and sites of fault linkage found in continental extensional settings are valid in a geodynamic context of a mid oceanic rifts

fault and graben growth along active magmatic divergent plate boundaries in iceland and ethiopia recent studies highlight the importance of annual scale dike induced rifting episodes in developing normal faults and graben along the active axis of magmatic divergent plate boundaries mdpb however the longer term 102 105 years role of diking on the cumulative surface deformation and evolution of mdpb is not yet well understood to better understand the longer term normal faults and graben along the axis of mdpb we analyze fissure swarms in iceland and ethiopia we first focus on the simplest case of immature fissure swarms with single dike fed eruptive fissures these consist of a 1 km wide graben bordered by normal faults with displacement up to a few meters consistent with theoretical models and geodetic data a similar structural pattern is found with asymmetric and multiple graben within wider mature fissure swarms formed by several dike fed eruptive fissures we then consider the lateral termination of normal faults along these grabens to detect their upward or downward propagation most faults terminate as open fractures on flat surface suggesting downward fault propagation this is consistent with recent experiments showing dike induced normal faults propagating downward from the surface however some normal faults also terminate as open fractures on monoclines which resemble fault propagation folds this suggests upward propagation of reactivated buried faults promoted by diking these results suggest that fault growth and graben development as well as the longer term evolution of the axis of mdpb may be explained only through dike emplacement and that any amagmatic faulting is not necessary

sensitivity of the telluric monitoring array in parkfield california to changes of resistivity parkfield is the site of characteristic earthquakes which repeat every 22 plus minus 5 years so that the next one is due between 1983 and 1993 the authors have installed a telluric monitoring array to detect possible changes of resistivity prior to the next characteristic earthquake however the array detects only changes in the telluric response and an independent estimate of the geoelectrical structure is needed in order to locate the perturbed regions and to determine the magnitude of the change in resistivity they have developed a three dimensional electrical model for parkfield based on 12 shallow 1 500 m schlumberger soundings and 31 deep soundings using their array the model is complex but shows that rock adjacent to and southwest of the san andreas fault is conductive less than 10 ohm m to depths of at least 10 km the authors infer that these low resistivities are due to brecciation from intense shearing along the fault they have also used this geoelectric structure to predict the sensitivity of their telluric array to changes in resistivity along the fault changes in resistivity beneath the array at depths from 1 5 to 6km produce the largest effects at the surface 1 change formore a perturbation of 1 at depth changes of resistivity from depths of 6 12 km yielded lower sensitivities but are still detectable because the top of the seismogenic zone in parkfield is at 5 km the array should be sensitive to changes of resistivity prior to the next earthquake less

hydrofracturing stress measurements in the iceland research drilling project drill hole at reydarfjordur iceland two independent suites of hydrofracturing stress measurements were conducted in the top 600 m of the iceland research drilling project deep hole at reydarfjordur east iceland as indicated by the continuously extracted drill core the tested section consists of tertiary subaerial tholeiitic lava flows cut by many basaltic dikes the density of the basalt was used to estimate the vertical stress v in megapascals at depth d in meters v 0 027d in the 33 to 270 m depth range the horizontal principal stresses hmax and hmin as calculated from hydrofracturing results increase gradually with depth but at a lower rate than the vertical stress hmin 2 1 0 016d hmax 3 3 0 016d in the 300 to 550 m depth range a major perturbation occurs with both horizontal stress magnitudes rapidly rising between 300 and 400 m and then uniformly decreasing between 400 and 550 m to a level apparently predictable by the relationships obtained in the top 270 m conditions favoring normal faulting v hmax hmin were established in the 540 to 580 m range and are conjectured to persist at greater depths since the vertical stress gradient is higher than that of hmax and hmin this would be in accord with the predominant fault type in east iceland the relative low horizontal stress values and the small difference between them are supportive of ocean floor spreading theories and contradict near surface high compressive stresses previously determined by hast 1973 in east iceland the large increase in horizontal stress in the range of 300 to 500 m depth is not understood a correlation appears to exist between the zone of almost unintruded thick lava flows 200 500 m and that of high stresses an intuitive model suggesting locked in stresses in this zone is proposed the hydrofracturing determined overall mean direction of hmax is n50 e while a more selective sampling of results yields n40 e the measured stress directions appear to be reasonably compatible with the local structure such as dike direction nne and lava dip wsw significantly hmax direction is subparallel to the n20 e trend of the nearest fissure swarms in the axial rift zone this almost coincidence of directions like the measured stress magnitudes is also in agreement with the model of spreading lithospheric plates being pulled apart

crustal spreading due to dikes and faults in southwest iceland abstract a 30 km long profile extending from 3 million year old pleistocene rocks to the holocene rocks of the rift zone in southwest iceland is dissected by 443 dikes and 156 normal faults nearly all the dikes are basaltic with an average strike of n26 e which is subparallel to the trend of fractures in the nearby rift zone their average dip is 69 and average thickness 1 37 m both values much lower than averages for regional tertiary dike swarms in iceland many dikes are seen to change into sills the great majority of the faults are pure dip slip with an average throw of 10 2 m their average strike n37 e is different from the average strike of the dikes in this area and the fractures in the nearby rift zone their average dip is 75 2 which is somewhat steeper than in the tertiary areas but the dips are less in the interbedded sedimentary layers than in the lava flows growth faults are very rare in the area and most of the faults appear to have completed their growth in less than 10 000 years in subsections crustal dilation due to dikes is 0 4 3 7 that due to faults is 0 6 5 3 and that due to both dikes and faults is 1 04 6 44 in these subsections average fault spacing is one fault per 170 563 m spacing decreases toward the active rift zone it is concluded that in typical fissure swarms of the rift zone the dilation at the surface is almost entirely due to extensional fractures and gaping vertical normal faults in the depth range of a couple of hundred metres to perhaps 1 km the non vertical faults contribute most to the dilation at deeper crustal levels however dilation due to dikes becomes dominant

field examples and numerical modelling of oversteps and bends along normal faults in cross section abstract oversteps and bends along cross sections through small scale numerical modelling distinct element analysis has been carried out to simulate the patterns of deformation at oversteps and bends area increase pull apart development and localised stress concentration occur at extensional oversteps and bends stress concentration and a tendency for compaction are typical of contractional oversteps and bends such results are compatible with the structures observed in the field a sensitivity analysis of the modelling has been carried out to investigate the effects of variations in mechanical parameters on deformation although models with weaker properties are more deformable the overall geometry of deformation varies little with variations in mechanical parameters

changes in complex resistivity during creep in granite a sample of westerly granite was deformed under constant stress conditions a pore pressure of 5 mpa a confining pressure of 10 mpa and an axial load of 170 mpa pore volume changes were determined by measuring the volume of pore fluid 0 01m kclaq injected into the sample after 6 days of creep characterized by accelerating volumetric stain the sample failed along a macroscopic fault measurements of complex resistivity over the frequency range 0 001 300 hz taken at various times during creep showed a gradual increase in both conductivity and permittivity when analysed in terms of standard induced polarization ip techniques the changing complex resistivity resulted in systematic changes in such parameters as percent frequency effect and chargeability these results suggest that it may be possible to monitor the development of dilatancy in the source region of an impending earthquake through standard ip techniques

rock coasts with particular emphasis on shore platforms rock coasts are one of the most common elements of the world s littoral zone and they are often important sources of sediment for estuaries and beaches economic development growing populations and the potential effects of rising sea level provide a practical need to understand the dynamics of these coasts although there have been significant advances in our understanding of rocky coastal systems further progress has been hindered by a lack of researchers and by the often imperceptible changes that generally occur within human time scales this paper reviews rock coast processes and landforms in a variety of morphogenic environments one of our most fundamental challenges is to determine the degree to which rock coasts are contemporary rather than inherited features from the quaternary when changes in sea level and climate were responsible for marked variations in the nature intensity and elevational distribution of the marine and subaerial processes that sculpture rock coasts

formation and growth of normal faults at the divergent plate boundary in iceland the divergent plate boundary in iceland is characterized by 40 80 km long and 5 10 km wide swarms of tension fractures 102 m long and normal faults 103 m long the upper part of the crust is mainly composed of lava flows with abundant columnar joints that are mostly perpendicular to the lava contacts the lava flows are horizontal at the surface of the rift zone but become tilted at the rate of 1 for every 150 m depth in the crust at the surface of the rift zone the joints are vertical and parallel to the vertical principal stress because of tilting of the lava pile the columnar joints become oblique to this stress hence becoming potential shear fractures and form echelon sets at greater depths in the crust theoretical considerations suggest that normal faults start to nucleate on sets of en echelon columnar joints and or large scale tension fractures at crustal depths of 0 5 1 5 km the width depth must be the smallest controlling dimension of many faults nevertheless there is a positive linear relation r 0 91 between maximum throw and length of the holocene faults if the faults grow as self similar structures the throw length relationship can be explained by a similar relation between fault length and width

temporal variations in the electrical resistivity of the earth s crust an electrical resistivity monitoring experiment was conducted on a portion of the san andreas fault near hollister california in 1979 and early 1980 receiving dipoles up to 20 km away from a 140 a 2 km transmitting dipole were used to monitor apparent resistivities with accuracies as high as 0 1 for some of the sites several of the transmitter receivers clearly show long term changes in resistivity greater than the data errors annual variations at a site outside the fault zone exceed 10 and sites in the fault zone show month to month changes of a few percent there are also significant variations in the vicinity of the transmitter which would be expected to produce correlated changes at other sites but this was not observed in the data there were no earthquakes of magnitudes greater than 3 0 during this period of high accuracy monitoring so that no conclusions can be drawn about changes in resistivity associated with major strain events future experiments to relate resistivity changes to earthquakes must not only have high sensitivity but must also be designed to distinguish such an event from normal background fluctuations that we have measured unless the resistivity variations associated with the event are of the order of several percent their detection will be a truly formidable task

the measurement of pebble impacts and wave action on shore platforms and beaches the swash force transducer swashometer abstract an instrument has been designed that can measure pebble impacts and wave forces in a harsh high energy turbulent near shore environment the instrument weighs some 50 kg and stands 60 cm in height and has a pebble sensor head that is interchangeable in that the size and shape can be representative of beach pebbles e g discoid spherical blade or rod like impacts on the sensor are recorded on a computer and the instrument can be programmed to run for varying time periods two characteristic force versus time traces representing a different wave activity are presented the instrument performed well throughout several tidal cycles and under a variety of energy conditions

infrastructure and mechanics of volcanic systems in iceland the divergent plate boundary in iceland consists of more than two dozen systems where most of the volcano tectonic activity takes place at the surface the volcanic systems are characterised by 5 20 km wide and 40 100 km long swarms of tension fractures 102 m long normal faults 103 m long and volcanic fissures the holocene fissure swarms are confined to less than 10 000 year old basaltic lava flows mostly pahoehoe occurring near the centre of the active rift zone in any particular swarm the number of tension fractures exceeds that of normal faults all tension fractures and normal faults are vertical at the surface indicating that the surface parts were generated by an absolute tension in addition to a fissure swarm many volcanic systems have a central volcano some of which have developed collapse calderas in the late tertiary and pleistocene lava pile of iceland extinct volcanic systems are represented by local sheet swarms and regional dyke swarms the sheet swarms are normally circular or slightly elliptical several kilometres in radius and are confined to the extinct central volcanoes many swarms are associated with large plutons exposed at 1 2 km depth beneath the initial top of the rift zone and presumably the uppermost parts of extinct crustal magma chambers and in short traverses up to 90 of the rock may consist of sheets the sheets have a very variable strike dip on average 45 65 mostly towards the centre of the associated volcano and have an average thickness of about 1 m the regional dykes occur outside central volcanoes in swarms that are commonly 50 km long and 5 10 km wide in several kilometre long traverses commonly 1 5 of the rock consist of dykes but occasionally as much as 15 28 most regional dykes are subparallel and subvertical the average dyke thickness in the pleistocene swarms is less than 2 m but 4 6 m in the tertiary swarms while active the volcanic systems in the rift zone are supplied with magma from reservoirs located at the depth of 8 12 km at the boundary between the crust and upper mantle the reservoirs are partially molten with totally molten top regions and of cross sectional areas similar to those of the volcanic systems that they feed some active volcanic systems especially those that develop central volcanoes high temperature areas and calderas have in addition to the deep seated magma reservoirs shallow crustal magma chambers located at 1 3 km depth which in turn are fed by the deeper magma reservoirs it is proposed that the regional dyke swarms are supplied with magma largely from the deep seated reservoirs whereas the local sheet swarms are mainly fed from the associated crustal magma chambers because the volume of a crustal chamber is less than that of its deeper source reservoir a single magma flow dyke intrusion from the reservoir may trigger tens of magma flows sheet intrusions from the chamber which is one explanation for the enormous number of sheets associated with many central volcanoes the sheets follow the stress trajectories of the local stress fields around the source chambers whereas the regional stress field associated with the divergent plate movements controls the emplacement of the regional dykes it is suggested that many dykes develop as self affine some as self similar structures when applied to the krafla volcanic system in northern iceland model calculations suggest that a magma flow from the krafla reservoir with regional dyke formation should occur on average once every several hundred years and that tens of sheets might be injected from the shallow krafla chamber and triggered by a single magma flow from the krafla reservoir these results are in broad agreement with the available data

propagation of normal faults to the surface in basalt koae fault system hawaii normal fault scarps of the koae fault system on kilauea volcano consistently display locally breached monoclines underlain by prominent cavities deep gaping fissures on the footwall finer fissures on the hanging wall and buckles at the scarp base elastic analyses reveal that this assemblage forms as a fault propagates up towards the surface rather than down from it models of a planar blind normal fault with a dip exceeding 60 yield a monocline with a tensile stress concentration at the surface where gaping fissures occur a stronger subsurface tensile stress concentration near the blind fault tip where cavities occur and a compressive stress concentration at the surface where buckles occur the footwall fissures grow down from the tensile stress concentration at the surface and link with a fault as its scarp grows in contrast the cavities initiate at depth near the fault tip and propagate with it up towards the surface the hanging wall fissures apparently open in response to slip on late forming blind antithetic faults near the surface stoped blocks derived from footwall fissure walls help prop the footwall fissures open as a normal fault breaches the surface the fissures cavities and scarp rubble provide highly conductive hydraulic pathways

earthquake deformation cycle on the san andreas fault near parkfield california six moderate earthquakes are thought to have ruptured the parkfield segment of the san andreas fault since 1857 the similar characteristics of the three seismically recorded events together with the quasi regular 22 year recurrence interval have led to the forecast of a similar event in 1988 5 years in this study the potential for the hypothesized earthquake is assessed by estimating the time required for interseismic straining to recover the strain energy released by the most recent june 1966 parkfield earthquake for a simple model of the earthquake cycle this interval is proportional to the ratio of the 1966 seismic moment to the interseismic moment deficit rate baseline lengths measured before and after the 1966 earthquake are inverted to estimate the magnitude and distribution of the combined coseismic and transient postseismic slip although the geodetic data determine only broad spatial averages of the fault slip they do require that slip at depth exceeded the surface offsets measured in 1966 inversions with extreme prior models are calculated to assess the range of seismic moment and moment deficit rates consistent with the available geodetic data inversions assuming only that the slip distributions are to some degree smooth yield strain energy recovery intervals of 14 25 years extreme values range from as little as 6 years to as much as 29 years in comparison historic recurrence intervals of parkfield earthquakes range from 12 to 32 years these results are consistent with the expectation of a magnitude 5 6 earthquake near parkfield by 1993

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

propagation of brittle failure triggered by magma in iceland abstract the architectures of normal faults at a divergent plate boundary in iceland are examined by combining surface fault observations with cross sectional studies at different structural levels to constrain a model of failure propagation the structures of holocene faults defining graben are analyzed to characterize the upper most parts of ruptures the shapes of faults resulting from growth and interaction of separate segments are used to better understand failure propagation inferred to occur in the intervening stages of displacement accumulation and lateral propagation pleistocene faults in volcanic sequences exhumed from 800 to 1000 m are analyzed to characterize deeper portions of failure that occurred beneath the central rift zone tertiary dikes exhumed from depths of 1300 1500 m are studied to infer how magma controls the failure initiation field studies in combination with a literature review indicate that the planar ruptures are likely to initiate at depth as magma filled vertical fractures and lengthen upward and laterally as failures propagate to higher crustal levels they are likely to change into inclined normal faults at near surface levels faults link with cooling joints and dilational fractures propagating downward from the surface it is suggested that the inferred stages of fault propagation are characteristic for normal faults developed at spreading ridges

overlapping faults intrabasin highs and the growth of normal faults normal fault systems bounding extensional basins are typically adjoined by a series of subbasins separated by intrabasin highs the strata within these basins form syndepositional anticlines and synclines whose axes are transverse to the strike of the main bounding fault one possible explanation for these intrabasin highs is that they result from persistent along strike deficits in fault displacement such deficits are incompatible with scaling relationships observed between fault displacement and length based on large populations of faults we present data from active normal faults within the basin and range province and from inactive normal faults of the newark basin of eastern north america demonstrating a clear correlation between the along strike position of overlapping splay faults and the location of intrabasin highs as well as syndepositional transverse folds summed displacements for all faults within an intrabasin high are comparable to the displacements on faults bounding flanking subbasins o

propagation of blind normal faults to the surface in basaltic sequences insights from 2d discrete element modelling abstract a discrete element model is used to investigate the progressive deformation of a thin basaltic cover overlying a pre existing blind normal fault as it propagates to the surface the cover materials representing basalt are homogeneous strong and display elastic brittle material behaviour cover deformation is seen to evolve through a series of distinct stages initial displacement on the underlying fault produces a very gentle monoclinal flexure with continued displacement open fractures develop at the monocline surface and propagate downwards whilst the deeper fault propagates upwards simultaneously a series of fractures in the future hanging wall of the main fault develop in the upper part of the cover the monoclinal flexure is then cut by these structures producing a surficial fault and fracture bounded wedge finally a prominent surface fracture and the upward propagating fault link cutting the entire cover sequence this fault is dilatant in the upper c 100 m of the cover has a significant surface aperture and forms a prominent fault scarp many of the key model results are strikingly similar to those seen in natural settings and emphasise that the occurrence of dilatant faults open fractures and cavities caves in extensional settings is not necessarily restricted to the very shallow section but can extend to several hundred metres depth therefore the results have implications for permeability and fluid flow in such settings comparison is also made with a weak cover experiment using granular materials with no cohesion or tensile strength similar to the dry sand used in many analogue modelling studies

geometry formation and development of tectonic fractures on the reykjanes peninsula southwest iceland abstract a study was made of 903 fractures most of which are of postglacial age on the reykjanes peninsula some 120 fractures belonging to the vogar fissure swarm he completely within the same postglacial lava flow and were studied in great detail for the vogar fractures 75 are pure tension fractures and 25 are normal faults the average length of the vogar fractures is 611 m the average width is 0 6 m the average spacing measured perpendicular to the average strike n54e of fractures is 456 m the estimated average depth is of the order of several hundreds of meters and the average throw of the faults is 2 3 m all major fractures on the peninsula appear to have formed by coalescence of smaller fractures as a result of absolute tensile stress the fractures group into four fissure swarms each being a part of a volcanic system which it is proposed is fed by an elongated partially molten magma reservoir located at a depth of about 8 km beneath the peninsula it is suggested that a slight magmatic pressure increase in these reservoirs and resulting uplift and tensile stress at the surface is the main cause of postglacial fracture formation

spatial and temporal variations in growth rates along active normal fault systems an example from the lazio abruzzo apennines central italy abstract the geometry kinematics and rates of active extension in lazio abruzzo italian apennines have been measured in order to gain a better understanding of the spatial and temporal variations in fault growth rates and seismic hazards associated with active normal fault systems we present fault map traces throws throw rates and slip directions for 17 parallel en echelon or end on active normal faults whose 20 40 km lengths combine to form a soft linked fault array ca 155 km in length and ca 55 km across strike throw rates derived from observations of faulted late glacial features and holocene soils show that both maximum throw rates and throw rate gradients are greater on centrally located faults along the strike of the array total throws and throw gradients show similar spatial variations but with weaker relationships with distance along strike when summed across strike throw rates are increasingly high towards the centre of the array relative to summed throws we interpret the above to suggest that throw rates have changed in the recent past ca 0 7 ma from spatially random fault growth rates initiating at 2 5 3 3 ma to growth rates that are greater on centrally located faults we interpret this as evidence for fault interaction producing throw rate variations that drive throw profile readjustment on these crustal scale soft linked faults the results are used to discuss seismic hazards in the region which are quantified in a second paper in this issue

surface effects of triggered fault slip on reykjanes peninsula sw iceland three large earthquakes mw 4 5 were triggered within 5 min 85 km west of a mw 6 5 earthquake in the south iceland seismic zone sisz we report on surface effects of these triggered earthquakes which include fresh rupture widespread rockfall disrupted rockslides and block slides field data confirm that the earthquakes occurred along n striking right lateral strike slip faults field data also support the conclusion from modeling of insar data that deformation from the second triggered event was more significant than for the other two a major hydrological effect was the draining of water through an open fissure on a lake bed lowering the lake level by greater than 4 m field relationships suggest that a component of aseismic slip could have been facilitated by water draining into the fault zone d 2003 elsevier science b v all rights reserved