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

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

field observations on the morphodynamic evolution of a low energy rip current system abstract a field experiment was conducted at palm beach nsw australia to investigate morphodynamic relationships operating within a rip current system during a near complete sequence of beach state evolution under decreasing energy conditions observations and measurements were obtained from nearshore surveys multiple dye releases oblique photographs and the deployment of various combinations of nine ducted impellor flow meters and five pressure sensors over a seven day period in rip and feeder channels the dataset provides a unique opportunity to assess and quantify aspects of the accretionary model of wright and short mar geol 56 1984 93 118 for intermediate beaches the morphology of the main rip channel exhibited distinct sequential adjustments with a narrowing of channel width an increase in channel depth a reduction in the cross sectional area available for rip flow and an increase in morphological relief rip flow velocity increased during the observed evolution and was also modulated by the tide experiencing maximums at low tide and minimums at high tide morphologic and kinematic adjustments of the rip system were most pronounced during the transition to a transverse bar and rip state the existence of a dominant feeder in a twin feeder and rip system contributed to the migration and establishment of a rip head over the seaward slope of the opposite longshore bar net erosion in the rip channel was balanced by net deposition in the feeder channels and bar crests and qualitative observations suggest that this rip head bar both receives transported sediments from the rip channel and contributes sediments to the ongoing accretion of the beach system a direct relationship was found to exist between rip morphology and flow with rip velocity increasing as cross sectional rip channel area decreases and these results are incorporated in a conceptual modification of the model of wright and short the results of this study are restricted to a subset of commonly occurring beaches but show that accretionary beach state evolution on intermediate beaches is indeed characterised by an increase in rip current velocity the nature of which can be linked to morphologic control in addition the rip head bar is identified as a potentially key component of low energy rip system evolution

zircon age data from a greenstone of the archaean yilgarn block australia mid proterozoic heating or uplift a new age determination has been made on the greenstones of the agnew jones creek area western australia by use of the zircon u pb dating procedure late stage granophyre from the differentiated kathleen valley doleritic sill provided zircons which are otherwise unobtainable from mafic rock suites an upper intercept age of 2 795 38 ma extends the known length of the otherwise tightly established late archaean tectonic cycle at agnew from 250 ma to 320 ma a lower intercept of age 1 345 153 ma is interpreted to record either a hitherto unknown very low temperature heating or a hitherto unknown tectonic uplift and cover removal in the mid proterozoic the existance of a postulated ancient unconformity of craton size dimensions may be being confirmed and dated apart from some inconclusive olivine and pyroxene rb sr isotopic data from 190km to the west the only evidence known that could support either possibility is found outside the yilgarn block itself minor perturbations to a simple two stage discordia pattern imply further small amounts of lead loss at later times studies of this nature could eventually provide information on the later history of stabilized cratons

form and dimensions of dykes in eastern iceland abstract a study was made of about 700 dykes in eastern iceland the majority of these belong to three swarms about 73 dip within 10 of the vertical most strike between 10 and 40 ne the strike of the dykes within the southernmost swarm alftafjordur changes along its trace from almost n at the north end to nne ssw southward along the swarm the average thickness of the dykes is about 4 1 m and the thickness does not change notably along the alftafjordur swarm the thinner dykes tend to have smaller grains than the thicker dykes of five dykes followed laterally the longest is over 22 km the thickness of individual dykes changes irregularly along their length and the dyke is often offset where its thickness changes many dykes appear to be completely discontinuous but some parts are connected by veins where the dykes end in a vertical section most of them simply taper away only about 10 5 of the dykes occupy faults the mechanical and thermal effects of the dykes on the country rock are small many of the dykes appear to be non feeders i e dykes that never reached the surface to feed lava flows using the length width ratio the depth of origin of three dykes has been estimated the maximum depth of origin of these three dykes is 7 5 km 9 km and 10 km below the original surface

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

a trend surface technique for discrimination of surf zone morphology rip current channels a major problem facing coastal nearshore studies is the conversion of survey data to precise two and three dimensional representations of complex surf zone topography in addition to inherent problems in providing accurate survey data most signi cant topographical boundaries are often delineated in a subjective manner a trend surface technique is hereby presented which provides an objective rigorous and replicable method of identifying morphological rip system channel boundaries in the surf zone using an extensive eld dataset obtained over two eld experiments including an almost complete cycle of low energy intermediate beach state evolution this technique is shown to produce accurate planimetric dimensions for a range of topographic beach state morphologies previously absent or highly descriptive in the literature formal morphometric de nitions of geometric dimensions for rip morphologies including rip channel width relief depth and cross sectional areas are provided several new aspects of rip current morphology are discussed from results of the approach the approach described here is also intended to illustrate the potential for future trend surface applications to surf zone and other geomorphological studies copyright 2003 john wiley sons ltd

rip current type spacing and persistence narrabeen beach australia abstract long term daily rip observations on narrabeen beach australia were made together with daily measurements of wave height period direction surf zone width and beach state a total of 3513 rips were observed on 270 days over a 19 month period results show that rips accompany moderate to high waves and are associated with intermediate beach types rip spacing follows wave conditions increasing in spacing size and intensity as waves rise and conversely as they fall rips are a function of both the prevailing and antecedent wave conditions and the rate and direction of change in wave conditions rips are classified into three types erosion mega and accretion erosion rips are initiated by rising seas in the transverse bar and rip and higher beach states they accompany general beach erosion they are widely spaced narrabeen ys 300 500 m 100 200 m increasing in size and intensity with the waves until the fully dissipative ripless state is reached they are both temporally and spatially highly variable persisting in one location for only hours to a day or so except where topographically controlled they are a major mechanism for the seaward transport of sediment and under extreme conditions may extend over a kilometer seaward of the breaker zone mega rips are large scale 1 km topographically controlled erosion rips that persist when nearshore and or embayment topography prevents the development of the fully dissipative state by inducing wave refraction and persist longshore gradients in surf zone dynamics that in turn drive the rip circulation accretion rips prevail during stable or falling wave conditions usually following erosion rip formation they are associated with general beach accretion are more closely spaced narrabeen ys 170 250 m 100 m less intense and become increasingly topographically arrested by crescentic bars and rhythmic bar beach morphology of the longshore bar trough rhythmic bar and beach and transverse bar and rip beach states they are relatively stable both spatially and temporally and may persist in one location for days to weeks if wave conditions remain favorable they are fair weather rips which may disappear by infilling in the low tide terrace beach state or be destroyed by rising seas rips generally are absent from the reflective low tide terrace and fully dissipative beach states

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

dynamics of a high energy dissipative surf zone vright l d guza r t and short a d 1982 dynamics of a high energy dissipative surf zone mar geol 45 41 62 pressure and horizontal current u v time series were measured at different positions across the inner 150 m of a wide 500 m surf zone of a microtidal high wave energy beach incident waves had average heights of 3 4 m with maxima of 5 m and periods of 12 to 15 sec bores of broken waves diminished in height at a nearly constant rate as they progressed across the surf zone the ratio of bore height h to local water depth h was everywhere less than 1 for even the highest bores and was on the order of 0 40 for the significant bores at incident wave frequencies rip circulation was weak or absent but a moderate longshore current was present shore normal flows were vertically segregated with strong net onshore flows prevailing just below the surface accompanied by weaker net seaward flows near the bed spectra of water surface oscillations as determined from pressure u and v reveal that most of the energy in the inner surf zone was at infragravity frequencies periods greater than 30 sec shoreward decay of wave energy at incident wave frequencies was accompanied by shoreward growth of infragravity energy near the beach the infragravity oscillations had heights on the order of 1 m cross spectra show that the infragravity oscillations were standing in the shore normal direction from the relative magnitudes of infragravity versus incident wave currents it is inferred that the surf beat may be an order of magnitude more important than incident waves to the transport of sediment in the inner surf zone

morphodynamics of a large scale rip current system at muriwai beach new zealand abstract field measurements of rip spacing nearshore morphology water surface elevation and eulerian and lagrangian flows were made in order to investigate the morphodynamics of a large scale rip current system at muriwai beach new zealand muriwai is a high energy meso tidal beach characterised by modal breaker wave heights of 2 5 m and incident wave periods of 10 15 s the monitored rip system was characterised by a 400 m long 75 m wide longshore feeder channel and a 150 m wide rip neck channel oriented obliquely to the shore and extending over a distance of almost 400 m during the experiment the beach evolved from a longshore bar trough and rip state to a transverse bar and rip configuration mean eulerian flow velocities obtained from ducted flowmeters deployed on the margin of the feeder channel and rip neck were on the order of 1 m s 1 and instantaneous flows were commonly in excess of 2 m s 1 mean lagrangian surface flow velocities extending from the base of the feeder through to the rip head were obtained by tracking rip floaters and were on the order of 0 7 m s 1 with maximums in the rip neck region of 1 4 m s 1 a distinct tidal modulation of rip current flow existed with maximum velocities occurring at low tide and minimum velocities at high tide comparison with other rip studies suggests that although the magnitude of the morphodynamic and hydrodynamic processes occurring within large scale rip systems is extreme the behaviour of these rip systems is very similar to low energy rips with much smaller spatial scales there is evidence to suggest that distinct morphodynamic scaling relationships exist between these environments

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

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

dynamics of volcanic systems in iceland example of tectonism and volcanism at juxtaposed hot spot and mid ocean ridge systems volcanic systems are swarms of tectonic fractures and basalt volcanoes formed as a result of plate pull as the plates are pulled apart associated with the mid ocean ridges and the magma dynamics of the iceland mantle plume most systems are 40 150 km long 5 20 km wide and develop a central volcano they supply magma to all eruptions in iceland data obtained in the last few years have greatly improved our knowledge of their volcanotectonic environment as a result the geometry of the plume is better constrained and the crust previously considered thin 10 km is now modeled as thick 20 40 km depending on the location of the volcanic systems their activity either decreases or increases faulting in the two main seismic zones from this we can infer that emplacement of the feeder dike to the largest historical eruption in iceland that of laki in 1783 increased shear stress in the south iceland seismic zone and almost certainly triggered the largest m 7 1 in 1784 historical earthquake in ic

waves currents sediment flux and morphological response in a barred nearshore system abstract a shore normal array of seven bi directional electromagnetic flowmeters and nine surface piercing continuous resistance wave staffs were deployed across a multiple barred nearshore at wendake beach georgian bay canada and monitored for a complete storm cycle time integrated estimates of total itvf and net invf sediment volume flux together with bed elevation changes were determined using depth of activity rods the three bars ranging in height from 0 10 to 0 40 m accreted during the storm 0 03 m and the troughs were scoured 0 05 m sediment reactivation depths reached 0 14 m and 12 of the nearshore control volume was mobilized however the invf value for the storm was less than 1 of the control volume revealing a near balance in sediment volume in the bar system landward migration of the inner crescentic and second sinuous bars occurred in association with an alongshore migration of the bar form itself the outermost straight shore parallel bar remained fixed in location the surf zone was highly dissipative throughout the storm 3 8 10 2 192 10 2 and the wave spectrum was dominated by energy at the incident frequency spectral peaks at frequencies of the first harmonic and at one quarter that of the incident wave were associated with secondary wave generation just prior to breaking and a standing edge wave respectively the former spectral peak was within the 95 confidence band for the spectrum while the latter contributed not more than 10 to the total energy in the surface elevation spectrum even near the shoreline during the storm wave height exceeded 2 m h s and periods reached 5 s t p k orbital velocities exceeded 0 5 m s 1 u rm s and were above the threshold of motion for the medium to fine sands throughout the storm shore parallel flows in excess of 0 4 m s 1 were recorded with maxima in the troughs and minima just landward of the bar crest the rate and direction of sediment flux is best explained by the interaction of antecedent bed slopes with spatial gradients in the mean and asymmetry of the shore normal velocity field these hydrodynamic parameters represent steady flows superimposed on the dominantly oscillatory motion and assumed a characteristic spatial pattern from the storm peak through the decay period increases spatially in the magnitudes of both the mean flows and flow asymmetries cause an increasing net transport potential erosion decreases in these values spatially cause a decreasing net transport potential and thus deposition these transport potentials are increased or decreased through the gravity potential induced by the local bed slope shore parallel flow was important in explaining sediment flux and morphological change where orbital velocities mean flows and flow asymmetries were at a minimum

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

flow characteristics at the rip current neck under low energy conditions abstract this paper presents the flow signature of the rip current neck as determined by fourteen near bottom velocity records with a comparison made to 63 recordings in the surf zone the field experiments including wave monitoring and morphological mapping were conducted under relatively calm dissipative wave conditions tsig 3 6 s hsig 0 3 1 1 m at herzliya beach israel on the mediterranean shore the current data were analysed for shore normal and resultant components the processing of the time series in the form of instantaneous and smoothed velocities provided spectral energy orbital and current velocity specific current discharge and directional distribution data the unique characteristic of the neck environment was found to be its continuous unidirectional seaward flow due to the channeling effect this steady offshore drift constituted the fastest flow in the surf zone with a typically narrow 35 94 directional range through spectral analysis of the unidirectional return flow the morphodynamic process signature at the rip neck was defined by the relative contribution from the incident waves 78 the subharmonic edge waves 14 and the infragravity oscillations 8 the pulsatory jet like rip model with concentration of energy at low frequencies is not corroborated by this study the studied environment of the rip and bar pattern represented an inherited beach morphology at an initial accretional although not yet reflective stage

the longshore flow component in low energy rip channels the mediterranean israel abstract we examined in the field the near bed longshore v flow component in rip channels at the herzliyya beach israel on the southeastern mediterranean field experiments conducted under relatively calm wave conditions t sig 3 6 s h sig 0 3 1 1 m included current and wave monitoring and morphological mapping time series of instantaneous and smoothed shore parallel current velocities and of the orbital components were analysed including statistical and spectral processing the peak instantaneous velocities of waves and currents combined were in excess of the threshold motion of the local sand taken as 0 3 m s some velocities even exceeded the threshold for sheet flow transport 0 9 m s the feeder and the neck demonstrated the highest longshore flow components whereas the rip head typically showed the lowest longshore velocities the unsteadiness of the net longshore current was very high feeders typically showed a mean unidirectional longshore current however directional fluctuations gradually increased towards the rip head the data suggest a clear spectral structure most energy 46 67 was in the gravity range with 13 35 subharmonic and minor 13 16 in the infragravity band systematic cross shore spatial trends of the energy spectra components were linked to bar morphology and to exposure to incident waves

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

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

sources of crustal deformation associated with the krafla iceland eruption of september 1984 a decade long plate boundary rifting episode in northern iceland ended with the september 1984 fissure eruption of krafla volcano we apply a nonlinear inversion method to geodetic data collected before and after the eruption to infer the location geometry and strengths of deformation sources associated with the eruption the net outflow of magma from a 3 km deep magma chamber beneath the krafla caldera was 30 120 106 m a similar volume of magma 50 70 106 m was emplaced in a 1 meter wide 9 km long dike extending from the surface to 7 km depth furthermore at least 110 106 m of magma erupted accordingly a surplus of magma must have been expelled from a second reservoir the location of which although uncertain is likely to lie at depths greater than 5 km beneath krafla volcano it would be difficult to detect this deeper source because of the narrow aperture of the geodetic networks

discrimination of geophysical phenomena in satellite radar interferograms various geophysical phenomena are recorded in the interference patterns formed by differencing two synthetic aperture radar sar images the fringes generated by the topographic relief can be removed using a digital elevation model dem the remaining fringes map the change in satellite to ground range which occurred between the acquisition times of the two images by comparing different pairs of images spanning different intervals of time it is possible to discriminate between geophysical signal and interferometric artifact here we apply this pair wise logic to the area around the 1992 landers california earthquake using sar images acquired by the ers 1 satellite the range varies with time as a constant step impulsive or sloping function examples of each type include a dem error an aftershock two atmospheric perturbations and postseismic afterslip respectively their signatures are identified and separated using the pair wise logic

nearshore water motion and mean flows in a multiple parallel bar system abstract measurements of surface waves and nearshore water motion were made with wave staffs and electromagnetic currentmeters across the straight outer bars of a multiple parallel bar system at wasaga beach georgian bay ontario three sets of experiments were carried out between 1983 and 1985 and this paper examines the characteristics of spatial and temporal variations in incident waves and mean flows incident wave spectra during storms are typical of fetch limited conditions with a prominent peak in the wind wave frequency and a sharp cut off towards lower frequencies there is little evidence of significant energy at infragravity frequencies mean flows perpendicular to the shoreline during storms measured at 0 1 m above the bed are offshore with speeds of 0 04 0 10 m s 1 during moderate storms and 0 25 m s 1 during intense fall storms longshore currents are relatively weak and their direction closely follows changes in the incident wind angle about shore perpendicular there is no evidence of rip cell development and offshore return flow occurs as an undertow uniformly alongshore forming a simple two dimensional net circulation pattern the presence of the undertow very close to the bed is an important control on the direction of suspended sediment transport and thus in maintaining the equilibrium of the nearshore bars

experimental study of nearshore dynamics on a barred beach with rip channels 1 wave and current measurements are presented from a set of laboratory experiments performed on a fixed barred beach with periodically spaced rip channels using a range of incident wave conditions the data demonstrate that the presence of gaps in otherwise longshore uniform bars dominates the nearshore circulation system for the incident wave conditions considered for example nonzero cross shore flow and the presence of longshore pressure gradients both resulting from the presence of rip channels are not restricted to the immediate vicinity of the channels but instead are found to span almost the entire length of the longshore bars in addition the combination of breaker type and location is the dominant driving mechanism of the nearshore flow and both are found to be strongly influenced by the variable bathymetry and the presence of a strong rip current the depth averaged currents are calculated from the measured velocities assuming conservation of mass across the measurement grid the terms in both the cross shore and longshore momentum balances are calculated and their relative magnitudes are quantified the cross shore balance is shown to be dominated by the cross shore pressure and radiation stress gradients in general agreement with previous results however the rip current is shown to influence the wave breaking and the wave induced setup in the rip channel analysis of the longshore balance shows that the overall strength of the longshore feeder currents is governed by a balance between longshore pressure gradient forcing and an opposing radiation stress gradient in addition the longshore feeder currents show maxima in the bar trough region providing experimental evidence that longshore pressure gradients can shift longshore current maxima shoreward from the bar crest finally since the longshore radiation stress gradient in the surf zone is governed by the amount of wave dissipation on the rip current there exists a positive feedback mechanism whereby a strong rip current can weaken the radiation stress gradient opposing the feeder currents and lead to even stronger feeder currents and rips

morphodynamics of a bar trough surf zone abstract a field study was made of the distinguishing morphodynamic processes operating in a surf zone which perennially exhibits accentuated bar trough topography the longshore bar trough and rhytmic bar and beach states as described by wright and short 1984 characteristic features of the morphology include a shallow bar with a steep shoreward face a deep trough and a steep beach face this morphology which is favored by moderate breaker heights and small tidal ranges strongly controls the coupled suite of hydrodynamic processes in contrast to fully dissipative surf zones the bar trough surf zone is not at all saturated and oscillations at incident wave frequency remain dominant from the break point to the subaerial beach the degree of incident wave groupiness does not change appreciably across the surf zone infragravity standing waves which in dissipative surf zones dominate the inshore energy remain energetically secondary and occur at higher frequencies in the bar trough surf zone analyses of the field data combined with numerical simulations of leaky mode and edge wave nodal antinodal positions over observed surf zone profiles indicate that the frequencies which prevail are favored by the resonant condition of antinodes over the bar and nodes in the trough standing waves which would have nodes over the bar are suppressed sediment resuspension in the surf zone appears to be largely attributable to the incident waves which are the main source of bed shear stress in addition the extra near bottom eddy viscosity provided by the reformed non breaking waves traversing the trough significantly affects the vertical velocity profile of the longshore current whereas the bar is highly mobile in terms of onshore offshore migration rates the beach face and inner regions of the trough are remarkably stable over time

ripex observations of a rip current system rip current kinematics and beach morphodynamics were measured for 44 days at sand city monterey bay ca using 15 instruments composed of co located velocity and pressure sensors acoustic doppler current profilers and kinematic gps surveys the morphology consisted of a low tide terrace with incised quasi periodic rip channels representative of transverse bars offshore 17 m depth significant wave height and peak period ranged 0 20 3 0 m and 5 20 s the mean wave direction was consistently near 08 resulting in rip channel morphology which evolved in response to the changing wave characteristics an inverse relationship between sediment accreting on the transverse bar and eroding in the rip channel was found the spatial distribution of sediment is reflected in the background rip current flow field the mean velocity magnitudes within the rip channel transverse bars increased offshore onshore with decreasing tidal elevations and increased with increasing sea swell energy eulerian averaged flows were predominantly shoreward on the transverse bars and seaward within the rip channel throughout the experiment resulting in a persistent cellular circulation except during low wave energy the rip current spacing to the rip channel width was less than or equal to two which suggests that the rip currents are influenced by each other and that no two dimensional bar return flow should be present the vertical velocity profile on the bar indicated that the flow was predominantly shoreward the flow field within the surf zone was depth uniform except for significant shear occurring near the surface owing to stokes drift the wave induced transport hypothesis is evaluated d 2005 elsevier b v all rights reserved

transient rip currents and nearshore circulation on a swell dominated beach 1 eulerian and lagrangian measurements of nearshore circulation on a longshore uniform beach with perpendicularly incident swell waves are described both the eulerian and lagrangian data show a variable rotational part of the current field that is distinct from but coexists with that due to infragravity waves a well defined feature of the flow field includes transient rip currents which occur in varying locations and are not topographically controlled drifter trajectories show that the rips behave like a shallow plane jet consisting of a constrained neck region that flows into a spreading head region there is continuous modulation of the flow within the rip neck estimates of the length scale across the rip neck using two particle statistics are 20 30 m well defined discrete vortex features are observed at the initiation point of one of the necks and in one of the rip heads estimated dispersion values suggest that mixing is greatly enhanced outside the surf zone by the intermittent presence of the rip heads the dispersion calculated from both inside and outside the surf zone appears to show scale dependence with a power law close to 4 3 in a similar way to oceanic turbulence at much larger scales

long period interferograms reveal 1992 1998 steady rate of deformation at krafla volcano north iceland we formed interferograms of ers sar scenes covering the area of krafla n iceland with time span values of up to six years 1992 1998 our data reveals a steady deformation rate at krafla and within its fissure swarm with values reaching 2 1 cm y in the ground to satellite direction at the volcano the area affected by deformation extends 20 km both north and south of the volcano the best fit dislocation model consists of sills to the north and south of the volcano and a magma chamber located below the volcano all of them undergoing contraction

equilibrium slopes and cross shore velocity asymmetries in a storm dominated barred nearshore system abstract the near bed 0 10 m elevation horizontal velocities were measured across a two barred lacustrine shoreface during a storm and reveal a spatially coherent but temporally variable pattern in the low order moments of the velocity field large cross shore velocity asymmetries indicated 1 1 spatially extensive and temporally persistent offshore mean flows up to 0 18 m s 1 across the landward slope of the linear outer bar reflecting an undertow driven by a wave induced setup 2 2 an onshore directed skewness opposing the mean flow of wind wave origin and persistent under propagating bores in the inner bar system long after this asymmetry had disappeared from the outer bar extensive sediment reactivation with near zero net sediment flux over the whole of the upper shoreface indicated a sediment prism in a steady state volumetrically over the storm cycle uniform depths of sediment reactivation with zero bed elevation change over the storm cycle revealed local slopes in a steady state in the outer linear bar however time integration of the near bed velocity field at this location indicated an offshore net flux of water while sedimentary structures indicated bedform migration and thus at least bedload almost uniformly onshore when ripples and megaripples were present the sediment transport giving rise to the local equilibrium cannot be explained therefore on the basis of the mean properties of the near bed velocity vector but requires knowledge of the time varying instantaneous sediment transport vector onshore migration of the inner bar correlated well with the large and persistent onshore skewness of the velocity distribution in the inner surf zone associated with translatory surf bores throughout the storm decay period low frequency oscillations were only significant close to the shoreline but may have influenced the development of a sinuous three dimensional inner bar form as it migrated onshore

rip current review rip currents are shore normal narrow seaward flowing currents that originate within surf zone extend seaward of the breaking region rip head and can obtain relatively high velocities within the last decade there have been a significant number of laboratory and field observations within rip current systems an overview of rip current kinematics based on these observations and the scientific advances obtained from these efforts are synthesized rip current flows are partitioned into mean infragravity very low frequency vorticity and tidal contributions and it is found that each contributes significantly to the total data from the laboratory and the field suggest that the rip current strength increases with increasing wave energy and decreasing water depths the maximum mean current occurs inside the surf zone where the maximum forcing is present owing to the dissipation of waves

vertical and horizontal structure in cross shore flows an example of undertow and wave set up on a barred beach greenwood b and osborne p d 1990 vertical and horizontal structure in cross shore flows an example of undertow and wave set up on a barred beach coastal eng 14 543 580 eulerian measurements of the horizontal cross shore velocity field in the lowermost meter of the water column in association with measurements of waves and the mean elevation of the water surface across a nontidal low relief barred surf zone reveal a spatially coherent patterns of the timeaveraged first second and third moments of the velocity field which vary temporally in direct response to the incident waves b large asymmetries in the flow field with offshore directed mean flows upto 0 20 m s a and onshore directed velocity skewness the third moment normalized by the standard deviation upto 0 60 c mean flows decrease and oscillatory flows measured by the standard deviation of the velocity field increase towards the water surface d a fourth order polynomial provides the best fit to the cross shore set up reflecting the influence of variable energy dissipation due to topographic controls on wave breaking e significant linear correlations exist between the cross shore mean velocities and the maximum set up of the still water surface near to the shoreline upto 82 of the observed variability in the former can be accounted for by linear regressions f set up is highly correlated with the incident wave height 86 of the observed variability in the former can be accounted for by a linear regression g gradients in the mean water surface are significantly less than those predicted by a linear function of beach slope as originally proposed by bowen et al even using an appropriate breaking criterion 0 4 0 6 for the spilling breakers recorded cross shore circulation over the low relief barred nearshore slope is predominantly two dimensional over the outer bar in this case study the near bed mean flow is an undertow responding to the balance between the wave generated depth dependent momentum flux directed onshore the stress induced by the onshore mass transport under waves and wind and the hydraulic pressure gradient induced by set up of the mean water level

magmatic fissures and their systems in iceland a tectonomagmatic model 1 volcanic systems in iceland comprise a central volcano linked to magmatic fissures whose subsurface architecture remains enigmatic the surface dimensions and trends of active fissures show their spatial relation with fault systems structure of the fremri namur and hengill volcanic systems was analyzed the linear vent arrays marking recent fissures are tightly bound by vertical segmented faults while normal faults accommodating extension are common on the flanks of the fissure swarm evidence of normal frictional slip on the fissure bounded faults suggests that failure occurred in response to subsidence with subsequent dilation this is supported by inward tilting of strata adjacent to faults compatible with a downsag phase we propose that magmatic fissures have vertical feeders with lateral offshoots extending along the rift zone their inflation deflation during an eruptive cycle causes subsidence such magmatically generated faults can be subsequently modified by tectonic extension

sequential stage development of crescentic bars hahoterim beach southeastern mediterranean abstract sequential bar development involving in order multiple bars ridge and runnels transverse oblique bars and meandering or crescentic shore attached or detached bars of both outer and nested inner systems were delineated at hahoterim beach northern israel the concomitant wave data including wave directions energy spectrum significant wave height and length of the calm period showed strong correlation with the bar stages there is a notable lack of correlation between bar stages and incident wave periods which were quite variable even on a daily basis two bar developmental sequences were delineated one shore normal and the other initially oblique but gradually rotating to shore normal in the mature stage threshold conditions for the initial stage development of shore normal crescentic systems were defined as h1 3 1 0 m peak spectral density of 1 m2 hz and swell directions within 7 from the normal the outer crescentic systems formed within 1 to 2 weeks the nested inner crescentic bar system formed under different wave conditions development of inner micro cresentic bars required 3 to 7 weeks of calm waves h1 3 0 3 m except for a short initiating pulse of wave energy from waves of about i m in height the double crescentic pattern indicates the optimal evolutiona y stage of crescentic systems the residence time of bar types was determined by the frequency of crossing of these threshold conditions the outer bars and the mega rip channels composed the most stable inshore elements out of phase relationships between inner and outer bar systems resulted from the lag in response of the outer bars behind changes in wave direction among the inner crescentic bars and shore rhythms phase correlation was the was the rule regional aerial photography and wave data indicate that the bar wave relationships at hahoterim axe representative of the eastern flank of the nile including most of the israeli coast although the bar types described from other coasts are similar to those mapped at hahoterim and the threshold wave conditions for initiation of bar stage development appear to be similar the frequency of occurrence of specific bar types appears to be different at hahoterim

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

flow characteristics along the rip current system under low energy conditions abstract this paper aims to determine the flow signature of the rip current under low energy conditions based on current meters at herzliyya beach israel on the southeastern mediterranean shore field experiments conducted under relatively calm wave conditions t sig 3 6 s h sig 0 3 1 1 m included current and wave monitoring with morphological mapping current data were analysed for shore normal and resultant components and the time series of instantaneous and smoothed velocities provided current and orbital wave velocities statistical and spectral processing yielded energy distributions specific current discharge and directional data velocity and velocity time asymmetry increased from the feeders to the neck and subsequently slowed down towards the rip head the directional range of the flow was narrow in channeled feeders and even better concentrated at narrow necks with higher velocities the flows at the rip head were typically divergent systematic spatial change of energy spectra was observed along the rip system the subharmonic frequency band dominated the feeders and the inner surf embayments and decreased gradually toward the rip neck with a simultaneous gradual increase in incident wave frequency the infragravity band frequency was minor everywhere velocity spectra at oblique bars with sheltered areas differed from spectral changes at transverse bars with an exposed inshore the velocity at the rip neck was above the threshold for grain movement whereas at the head it was usually below the threshold textural parameters of the beach sand could not be systematically related to the current velocity i e no environmental groupiness was evident along the rip system

mean currents and sediment transport in a rip channel measurements of hydrodynamics and suspended sediment transport were carried out along a cross shore transect in a rip channel a tidally induced threshold for rip flow initiation was found this threshold depended upon the degree of wave energy dissipation and could be identified through a critical value of the ratio of significant wave height to water depth s in the rip neck with normally incident waves rip current velocities were well correlated with velocities predicted from a simple model involving the onshore mass transport in asymmetric breaking waves crossing the bar hydrodynamics and sediment transport were tidally modulated with strong offshore directed rip currents and associated offshore sediment transport due to the mean flow at low tide at high tide when the rip was inactive the tendency was for weak onshore directed mean flows and or oscillatory incident waves transporting smaller amounts of sediment landward the net result was a gradual lowering of the topographic relief along the rip channel with net deposition seaward of the rip outside the surf zone transport was mainly landward due to oscillatory incident waves

post glacial rebound of iceland during the holocene abstract the geodynamic context of iceland provides the rare opportunity to analyse the combined effects of a ridge and a hotspot on deformation processes from a digital elevation model field analysis and a compilation of previous work we propose a synthesis of vertical motions of iceland during the holocene with a focus on the post glacial rebound we determined two ancient marine limits one above and one below the present sea level estimated at 10 ka 300 years and 8150 350 years bp respectively we calculated an uplift phase of 40 170 m with a rate of 2 1 9 2 cm a 1 between 10 ka 300 years and 8150 350 years bp corresponding to the post glacial rebound of iceland following the weichselian glaciation spatial variations of the rebound are mainly related to the local glacial dynamics ice load and deglaciation history rather than the geodynamic context however the relaxation time deduced from uplift data is 4167 years in west iceland and 2000 years in ssw iceland we estimated viscosity from relaxation time ranging from 2 1 10 19 pa s to 3 2 10 19 pa s the significant difference in the relaxation time is due to local variation of the lithospheric thickness as a result of rifting rather than because of a variation of the viscosity

short term changes in the morphodynamic states of beaches and surf zones an empirical predictive model a time series of 6 5 years of daily observations of beach states and wave characteristics from southeastern australia was analyzed with the aim of determining the degree to which time varying beach state can be explained and predicted the six commonly occurring beach states dissipative longshore bar trough rhythmic bar and beach transverse bar and rip low tide terrace and reflective discussed earlier were related by means of discrete discriminant analyses to the parameter hb wst where hb is breaker height ws is the fall velocity of the beach sand and t is peak breaker period using a total of 1545 cases two values were used in the analyses the immediate value occurring on the day the particular state was observed and a weighted mean value expressing recently antecedent conditions the immediate value made a negligible contribution to explaining day to day beach state observations however the antecedent conditions showed a strong relationship and provide a successful means of prediction by examining cases where the time derivatives of both state and were near zero it was possible to define the equilibrium conditions associated with each state directions of change erosional or accretionary are predicted in terms of departures of from the equilibrium value appropriate to the beach state prevailing at the time change begins

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