accelerated stress buildup on the southern san andreas fault and surrounding regions caused by mojave desert earthquakes a sequence of four mw 6 earthquakes including the 1992 mw 5 7 3 landers and mw 5 7 1 hector mine earthquakes occurred in the mojave desert in the 1990s in close proximity to the southern san andreas fault inducing stress changes on several of its segments we calculate that coseismic slip combined with postseismic relaxation of viscous lower crust and or upper mantle has led to a coulomb stress increase of 2 3 3 5 bar on the san bernardino mountain segment of the southern san andreas fault between 1992 and 2001 with a projected increase of 3 6 4 9 bar by the year 2020 in comparison the calculated coseismic stress increase is 1 8 bar for this segment this accelerated buildup of stresses is predicted to bring the san bernardino mountain segment which last ruptured more than 190 yr ago closer to a potentially major rupture meanwhile we project a net stress decrease of as much as 23 5 bar between 1992 and 2020 for the western coachella valley segment if the fault is governed by low effective friction or an increase of 1 5 bar if the fault is governed by high effective friction coulomb stresses are calculated to decrease on the mojave segment by as much as 21 bar between 1992 and 2020 accelerated stress buildup is also predicted to occur on parts of the san jacinto elsinore and calico faults the pattern of the observed post landers aftershock clustering and the calculated coulomb stress buildup on the calico fault is similar to that noted in the hector mine region prior to the 1999 mw 5 7 1 earthquake these results imply that the stress changes caused by an earthquake may still play a role in triggering future quakes in neighboring crust many years later through viscoelastic processes

geodetically constrained models of viscoelastic stress transfer and earthquake triggering along the north anatolian fault over the past 80 years 8 mw 6 7 strike slip earthquakes west of 40 longitude have ruptured the north anatolian fault naf from east to west the series began with the 1939 erzincan earthquake in eastern turkey and the most recent 1999 mw 7 4 izmit earthquake extended the pattern of ruptures into the sea of marmara in western turkey the mean time between seismic events in this westward progression is 8 5 11 years 67 confidence interval much greater than the timescale of seismic wave propagation seconds to minutes the delayed triggering of these earthquakes may be explained by the propagation of earthquake generated diffusive viscoelastic fronts within the upper mantle that slowly increase the coulomb failure stress change cfs at adjacent hypocenters here we develop three dimensional stress transfer models with an elastic upper crust coupled to a viscoelastic burgers rheology mantle both the maxwell m 4 1018 1 1019 pa s and kelvin k 1 1018 1 1019 pa s viscosities are constrained by studies of geodetic observations before and after the 1999 izmit earthquake we combine this geodetically constrained rheological model with the observed sequence of large earthquakes since 1939 to calculate the time evolution of cfs changes along the north anatolian fault due to viscoelastic stress transfer apparent threshold values of mean cfs at which the earthquakes in the eight decade sequence occur are between 0 02 to 3 15 mpa and may exceed the magnitude of static cfs values by as much as 177 by 2023 we infer that the mean time dependent stress change along the northern naf strand in the marmara sea near istanbul which may have previously ruptured in 1766 may reach the mean apparent time dependent stress thresholds of the previous naf earthquakes

grain size trends associated with net sediment transport patterns an example from the belgian continental shelf grain size trends in relation to net sediment transport pathways are examined using some commonly used grain size parameters the results of the trend analysis for 15 types of the trends are compared with a known net sediment transport pathways established on the basis of the sandbank hydro and sediment dynamics i e general water and sediment movement patterns and bedform asymmetry it is shown that 1 the results for grain size trends associated with a worsening in sorting along the transport pathways have little similarity to transport pathways 2 the results for grain size trends associated with an improvement in sorting along the transport pathways have relatively high degree of similarity to the identified pathways and 3 the residual pattern derived on the basis of a combined grain size trend used elsewhere gao and collins 1992 appears to be most similar and suitable for defining transport pathways

post seismic and interseismic fault creep i model description we present a model of localized aseismic fault creep during the full interseismic period including both transient and steady fault creep in response to a sequence of imposed coseismic slip events and tectonic loading we consider the behaviour of models with linear viscous non linear viscous rate dependent friction and rate and state dependent friction fault rheologies both the transient post seismic creep and the pattern of steady interseismic creep rates surrounding asperities depend on recent coseismic slip and fault rheologies in these models post seismic fault creep is manifest as pulses of elevated creep rates that propagate from the coseismic slip these pulses feature sharper fronts and are longer lived in models with rate state friction compared to other models with small characteristic slip distances in rate state friction models interseismic creep is similar to that in models with rate dependent friction faults except for the earliest periods of post seismic creep our model can be used to constrain fault rheologies from geodetic observations in cases where the coseismic slip history is relatively well known when only considering surface deformation over a short period of time there are strong trade offs between fault rheology and the details of the imposed coseismic slip geodetic observations over longer times following an earthquake will reduce these trade offs while simultaneous modelling of interseismic and post seismic observations provide the strongest constraints on fault rheologies

coseismic displacements from the hector mine california earthquake results from survey mode global positioning system measurements we describe the collection and processing of global positioning system gps data from 77 locations around the hector mine earthquake which we use to estimate coseismic displacements related to this shock the existence of pre event gps data some collected to monitor postseismic displacements from the 1992 landers earthquake and some to establish survey control in the meizoseismal area provided a relatively dense coverage close to the rupture zone the data available were collected mostly within the 2 years prior to the 1999 earthquake we reobserved many points within a few days after the shock and all within 6 months after we include corrections for interseismic motion to provide the best value possible for coseismic motion caused by this earthquake the displacements in general display the pattern expected for a strike slip fault though a few show significant vertical motion the maximum horizontal displacement observed was 2 m one station between fault ruptures showed little horizontal motion but significant uplift

seismic remote sensing for the earthquake source process and near source strong shaking a case study of the october 16 1999 hector mine earthquake a new methodology to automatically determine the earthquake source process and near source strong ground motions using regional and local distance data was used to study the october 16 1999 hector mine earthquake mw7 1 broadband displacement data were inverted for the seismic moment tensor and to resolve the fault plane ambiguity independent of aftershock location and surface faulting information the identified nnw striking fault plane was used to invert for the distribution of fault slip over a planar surface the slip model was used to predict near fault peak ground velocity which was found to agree well with the trinet shakemap and also with near fault observations that were not available at the time of the initial analysis our method if automated is capable of providing near source strong ground motion information within 30 minutes of the earthquake origin time even in cases where there are few near fault recording instruments

stress modulation on the san andreas fault by interseismic fault system interactions during the interseismic phase of the earthquake cycle between large earthquakes stress on faults evolves in response to elastic strain accumulation driven by tectonic plate motions because earthquake cycle processes induce non local stress changes the interseismic stress accumulation rate on one fault is influenced by the behavior of all nearby faults using a geodetically constrained block model we show that the total interseismic elastic strain field generated by fault interactions within southern california may increase stressing rates on the mojave and san bernardino sections of the san andreas fault within the big bend region by as much as 38 relative to estimates from isolated san andreas models assuming steady fault system behavior since the c e 1857 fort tejon earthquake shear stress accumulated on these sections due only to interaction with faults other than the san andreas reaches 1 mpa 3 times larger than the coseismic and postseismic stress changes induced by recent southern california earthquakes stress increases along big bend sections coincide with the greatest earthquake frequency inferred from a 1500 yr long paleoseismic record and may affect earthquake recurrence intervals within geometrically complex fault systems including the sections of the san andreas fault closest to metropolitan los angeles

morphology and maintenance of steep dunes near dune asymmetry transitional areas on the shallow shelf beibu gulf northwest south china sea abstract symmetrical dunes can be found in areas where dune asymmetry changes sense but in areas where dunes are not expected to be moving the development and maintenance of symmetrical dunes are poorly understood in the southeastern beibu gulf we have identified five typical dune asymmetry transitions at the depths of shallower than 50 m basing on the latest bathymetric data those regions are separated into two classes according to their different shapes and locations sand transport directions are deduced by dune migrations between surveys simulated regional currents and sand size trends the two classes of the regions are found to experience opposing net sand transports from the north and south around the asymmetry transitions the dunes have impressive steep sections relatively symmetrical shapes and almost straight crests they have a general height length h l relation h 0 0575 l1 0836 r2 0 834 which is steeper than dunes in other seas some of the dunes have extraordinary large heights exceeding a quarter of water depth which is attributed to the preserved sand body bidirectional sand accretion erosion in troughs and absence of significant erosion on crests the steepness and straight crests of dunes adapt to the nearly symmetrical or weak asymmetrical reversing currents which induce flow separations and reversals on both flanks of the dunes it also suggests that dunes develop differently under sufficient and limited sand supply dunes will gradually grow in size when being continually supplied with sand however if sand supply is restricted dunes may separate or turn into more isolated individuals to sustain the h l relation the h l relation of dunes can be maintained during either growth or separation overall the nearly symmetrical or weak asymmetrical reversing currents maintain the steep shapes of dunes while variations in sand supply affect their sizes to work out the detailed transports of sediment future observations of flow structures and related models of such two sided steep dunes are still required

the 1995 kobe japan earthquake a long delayed aftershock of the offshore 1944 tonankai and 1946 nankaido earthquakes the 1995 kobe earthquake produced predominantly right lateral slip along the northeast southwest trending nojima fault approximately 200 km landward of the nankai trough southwest japan the last major earthquakes along the nankai trough the m 8 0 1944 tonanakai and m 8 2 1946 nankaido earthquakes involved about 4 m of interplate slip each releasing about 100 years of accumulated stress from the northwest southeast convergence of the philippine sea phs plate by calculating the change in coulomb failure stress on the nojima fault due to 1944 46 to 1995 postseismic relaxation of a thin low viscosity channel beneath southwest japan we show that stress diffusion from the 1940s interplate earthquakes increased the likelihood of future failure on the nojima fault by slowly but progressively unloading the normal stress on the fault over a 50 yr period

anomalous radiocarbon ages for foraminifera shells the causes for discordant radiocarbon results on multiple species of planktonic foraminifera from high sedimentation rate marine sediments are investigated we have documented two causes for these anomalous results one is the addition of secondary radiocarbon for which we have to date only one firm example it involves an opal rich sediment the other is the incorporation of reworked material again we have to date only one firm example it involves a rapidly deposited ocean margin sediment however we have three other examples where reworking is the most likely explanation on the basis of this study it is our conclusion that where precise radiocarbon dating of high deposition rate marine sediment is required a prerequisite is to demonstrate that concordant ages can be obtained on pairs of fragile and robust planktic shells for sediment rich in opal it is advisable to check for secondary calcite by comparing ages obtained on acid leached samples with those on unleached samples

residual sediment transport paths on a tidal sand bank a comparison between the modified mclaren model and bedform analysis abstract in order to evaluate the validity of the modified mclaren model for sand transport on a tidal sand bank 42 grab samples taken from the northern part of the kwintebank were decalcified sieved and analyzed for statistical parameters the areal distribution of mean grain size standard deviation and skewness were defined and used to determine the residual transport directions the grain size trends taken into consideration were that the sediment becomes finer better sorted and more negatively skewed along the transport pathways f b and the sediment becomes coarser better sorted and more positively skewed along the transport pathways c b these were compared with the transport paths inferred from bedform asymmetries obtained by side scan sonar records there were major differences between the sediment transport paths resulting from the two techniques when the f b trend and small dune asymmetries are compared only 5 out of 24 cases showed agreement 30 whereas in 9 cases the trends were 180 30 different there was even less agreement between the c b trend and the asymmetry of the small dunes only 1 out of 24 cases agreed 30 and almost half of the cases were 90 30 different the sediment transport directions deduced from the morphological analysis of the large dunes all point in the direction of the peak flood current the large dunes seem to be more suitable indicators of the sediment transport paths than the small ones however they conflict with the vectors resulting from the f b trend only one out of 20 cases agrees 30 while in 13 cases the vectors are 180 30 different and in three cases 90 30 neither do the transport directions from the c b trend agree with those deduced from the large dunes only one case agrees 30 while in four out of 20 cases the vectors point in opposite directions in four cases the vectors are 90 30 different the asymmetric cross section of the sand bank suggests sediment transport in a northwest direction it does not seem probable however that sediment transport occurs by migration of the whole sand bank the transport directions inferred from the bank morphology are quite different from the f b trend in 12 out of 42 cases the vectors agree 30 in four cases the vectors point in opposite directions and in 15 cases they are 90 30 different there is slightly more agreement between the c b trend and the transport directions deduced from the sand bank 16 out of 42 cases agree 30 in nine cases the vectors point in opposite directions and three cases are 90 30 different from this study it seems that the residual sediment transport directions can best be deduced from the morphological characteristics of the large dunes the departures between the results of the sediment trend analysis according to the modified mclaren model and the morphology of the subaqueous bedforms are partly explained by the location of the study area near the edge of the sand bank probably better results of the sediment trend analysis would be obtained if samples over the whole length of the sand bank were analyzed and compared perhaps a smaller spacing between the samples would improve the results as well and maybe the samples should not have been decalcified possibly the different transport patterns are attributed to different sediment transport processes which makes a comparison virtually impossible

poroelastic triggering in the 9 22 january 2008 nima gaize tibet earthquake sequence a sequence of three earthquakes of ms 6 4 5 9 and 5 4 occurred in southern central tibet on 9 16 and 22 january 2008 interferometric satellite aperture radar insar observations indicate that the two largest events occurred on two northeast trending northwest dipping normal faults separated by 9 5 km with slip reaching 1 2 m on the main fault and 0 75 m on the second we develop a three dimensional finite element model to calculate the coulomb stress change due to poroelastic deformation in the crust after the first event using a uniform permeability of 8 10 15 m 2 for faults and the host rock leads to negligible temporal pore pressure change in the two weeks after the first event however assuming higher permeability along faults 10 14 to 10 11 m 2 the second fault channels the flow of pore fluids upward from the deeper region where the pore pressure increase was the largest immediately after the first event in this case the poroelastic coulomb stress on the second fault increased suddenly by 1 10 5 pa after the first earthquake and continued to increase by as much as 70 in the subsequent week furthermore the distribution of coulomb stress change on the second fault correlates precisely with the slip distribution inferred for the second event these findings suggest that the first earthquake not only triggered the second event but the coulomb stress change it induced on the second fault plane controlled the extent of the second rupture

changes in static stress on southern california faults after the 1992 landers earthquake the magnitude 7 5 landers earthquake of 28 june 1992 was the largest earthquake to strike california in 40 years the slip that occurs in such an earthquake would be expected to induce large changes in the static stress on neighbouring faults these changes in stress should in turn affect the likelihood of future earthquakes stress changes that load faults towards failure have been cited as the cause of small1 5 moderate6 and large7 earthquakes conversely those that relax neighbouring faults have been related to a decrease in seismicity5 here we use an elastic half space model8 to estimate the stress changes produced by the landers earthquake on selected southern california faults including the san andreas we find that the estimated stress changes are consistent with the triggering of four out of the five aftershocks with magnitude greater than 4 5 and that the largest changes 1 10 bar occurring on part of the san bernardino segment of the san andreas fault may have decreased the time to the next magnitude 8 earthquake by about 14 years

tidal and residual currents around a norfolk sandbank currents on either side of well bank were recorded for 41 days in may june 1981 in order to distinguish locally tidally generated asymmetries pertinent to maintenance of the bank from larger scale and wind driven flows semi diurnal currents were dominant nearly rectilinear and slightly inclined to the bank axis fourth diurnal currents were consistent with previously observed and computed fourth diurnal tides for the area overall amplitudes varied as the square of the semi diurnal amplitude hence they appear to be generated by tides but only a small part locally residual currents average over a tidal cycle were always in the sense of clock wise circulation around the bank consistent with simple models and giving strong evidence of local generation by tides this circulation combines with the semidiurnal currents to maintain the bank the slow increase of the residuals with the semi diurnal amplitude is interpreted as an effect of strong semi diurnal currents a persistent component onto the bank 7 m above the bottom at one position may indicate distortion of the frictional layer by the sloping bottom or by bank curvature

potential gain and loss of sand by some sand banks in the southern bight of the north sea abstract a detailed echo sounding and side scan sonar survey has shown that sand banks in the southern bight of the north sea have rounded heads in the approximate upstream direction and tapered tails in the approximate downstream direction of net regional sand transport thus the plan view of sand banks can be used as an indicator of the approximate direction of net sand transport sand wave aprons occur in front of the heads and downstream of the tails of the sand banks the asymmetry of the sand waves implies that sand is being added to sand wave aprons at the heads and to the upstream flanks of these sand banks but is shed from the downstream sand wave aprons there is no evidence for a closed sand circulation on the banks

secondary aftershocks and their importance for aftershock forecasting the potential locations of aftershocks which can be large and damaging are often forecast by calculating where the mainshock increased stress we find however that the mainshock induced stress field is often rapidly altered by aftershock induced stresses we find that the percentage of aftershocks that are secondary aftershocks or aftershocks triggered by previous aftershocks increases with time after the mainshock if we only consider aftershock sequences in which all aftershocks are smaller than the mainshock the percentage of aftershocks that are secondary also increases with mainshock magnitude using the california earthquake catalog and monte carlo trials we estimate that on average more than 50 of aftershocks produced 8 or more days after m 5 mainshocks and more than 50 of all aftershocks produced by m 7 mainshocks that have aftershock sequences lasting at least 15 days are triggered by previous aftershocks these results suggest that previous aftershock times and locations may be important predictors for new aftershocks we find that for four large aftershock sequences in california an updated forecast method using previous aftershock data and neglecting mainshock induced stress changes can outperform forecasts made by calculating the static coulomb stress change induced solely by the mainshock manuscript received 20 november 2002

fault plane solutions for the 1984 morgan hill california earthquake sequence evidence for the state of stress on the calaveras fault fault plane solutions for 946 aftershocks of the april 24 1984 morgan hill california m6 2 earthquake reveal a pattern of complex faulting within a 10 km wide zone surrounding the calaveras fault the fault plane solutions fall into three groups strike slip mechanisms located along the calaveras fault with north northwest striking dextral slip planes nearly parallel to the fault strike slip mechanisms located northeast of the calaveras fault with north striking dextral slip planes and reverse mechanisms located southwest of the calaveras fault with northeast or southwest dipping slip planes the average azimuth of p axes for aftershocks located on the calaveras fault is n10 1 e in contrast the average azimuths of p axes for aftershocks northeast and southwest of the calaveras fault are n49 7 e and n37 3 e respectively by assuming that the earthquakes occur on preexisting cracks in response to a uniformly oriented regional stress field we are able to infer from the observed average slip directions an orientation for 1 the axis of maximum horizontal compression 63 80 from the local strike of the calaveras fault such a high angle orientation of 1 is incompatible with the classical andersonian model of strike slip faulting but is consistent with an alternate model in which a weak calaveras fault is driven by plate motion slightly convergent with respect to the san andreas fault system in central california if this inferred high angle uniform stress field is combined with the changes in the static stress field calculated with elastic dislocation theory for the 1984 morgan hill main shock the resulting stress field predicts most of the observed spatial pattern of the aftershocks and their fault plane solutions the orientations of the inferred aftershock slip planes are generally consstent with the fracture orientations predicted by the coulomb failure criterion areas with predicted increases in the coulomb failure function of only a few tenths of a megapascal correspond to areas of intense aftershock activity the invariance of focal mechanisms in the vicinity of the south end of the main shock rupture before and after the main shock suggests that the magnitude of compressive stress in this area is at least 10 mpa in addition to explaining a preponderance of the seismic observations of the aftershock sequence the presence of a predominantly fault normal compressive regional stress field accounts for the contemporary development of folds reverse faults and topographic relief observed throughout the central california coast ranges

large earthquake triggering clustering and the synchronization of faults large earthquakes are sometimes observed to trigger other large earthquakes on nearby faults the magnitudes of the calculated coulomb stress transfers presumed to cause the triggering are 10 2 10 3 of the earthquake stress drops the earthquake stress drops and the triggering delay times are similarly small with respect to the natural recurrence time of the earthquakes this requires that both faults be simultaneously very close to the ends of their seismic cycles paleoseismological data show that for the same regions prior earthquakes have occurred in clusters of ruptures of several faults separated by long quiescent periods both observations suggest that synchronization is occurring between faults theory and observations indicate that synchronization can occur between nearby faults with positive stress coupling and intrinsic slip velocities within an entrainment threshold in the south iceland seismic zone the central nevada seismic belt and the eastern california shear zone several synchronous clusters that apparently act independently can be recognized this behavior is the 3d equivalent of the phase locking that results in the seismic cycle of individual faults being dominated by large characteristic earthquakes and for synchronization of fault segments along a given fault rupture patterns of repeated individual earthquakes or earthquake clusters are not identical in either the 2d or 3d cases the state of this system which exhibits strong indications of synchrony without exact repetition may be called fuzzy synchrony

mantle flow beneath a continental strike slip fault postseismic deformation after the 1999 hector mine earthquake two recent large earthquakes in the mojave desert california the magnitude 7 3 1992 landers and magnitude 7 1 1999 hector mine earthquakes have each been followed by elevated crustal strain rates over periods of months and years geodetic data collected after the hector mine earthquake exhibit a temporally decaying horizontal velocity field and a quadrant uplift pattern opposite to that expected for localized shear beneath the earthquake rupture we interpret the origin of this accelerated crustal deformation to be vigorous flow in the upper mantle in response to the stress changes generated by the earthquake our results suggest that transient flow in the upper mantle is a fundamental component of the earthquake cycle and that the lower crust is a coherent stress guide coupling the upper crust with the upper mantle

late quaternary rate of slip along the san jacinto fault zone near anza southern california the anza section of the san jacinto fault southern california has not experienced a groundbreaking earthquake since at least 1918 and probably since before 1899 leading previous workers to designate this 20 km long fault segment as a seismic gap study of displaced pleistocene and holocene alluvial fan and fluvial deposits across the san jacinto fault near anza california dated using 14c and soil development age control indicates a minimum right lateral slip rate of 9 2 2 mm yr since 9 5 ka and a slip rate of 11 9 5 mm yr since 14 ka 12 9 5 mm yr since 17 ka and 13 10 6 mm yr since 50 ka these estimates agree with a previously determined 700 ka to present minimum slip rate of 10 2 mm yr based on an estimated average slip rate of 12 mm yr from the longer term slip rate estimates about 0 8 m of potential slip has accumulated if the 1918 earthquake broke the anza segment over 1 1 m of potential slip has accumulated if the last earthquake occurred prior to 1899 these data attest to the potential for an earthquake m6 5 for this section of the fault

dynamics diffusion and geomorphological significance of tidal residual eddies many shallow tidal areas show the occurrence of ebb and flood surplusses organized in cell like structures or tidal residual eddies they seem to arise from a nonlinear transfer of vorticity from the oscillating tidal field to the mean field the irregular structure of the bottom topography or the coastline acting as a catalyst to produce the necessary vorticity gradients the dynamical characteristics of the eddies assist our understanding of the geomorphology of the sea bed

evolution of a classic sand ridge field maryland sector north american inner shelf the ridge and swale topography of the middle atlantic bight is best developed on the delaware maryland inner shelf here sand ridges can be seen in all stages of formation several aspects of the ridge field are pertinent to the problem of ridge genesis the first is ridge morphology there is a systematic morphologic change from shoreface ridges through nearshore ridges to offshore ridges which reflects the changing hydraulic regime as successively more seaward ridges are examined maximum side slope decreases the ratio of maximum seaward slope to maximum landward slope decreases and the cross sectional area increases these changes in ridge morphology with depth and distance from shore appear to be equivalent to the morphologic changes experienced by a single ridge during the course of the holocene transgression a second aspect is the change in bottom sediment characteristics that accompanies these large scale morphologic changes megaripples sand waves and mud lenses appear in the troughs between nearshore and offshore ridges these changes indicate that the storm flows which maintain ridges are less frequently experienced in the deeper sector and that the role of high frequency wave surge becomes less important relative to the role of the mean flow component in shaping the sea floor a third aspect is the systematic relationship of grain size to topography grain size is 90 out of phase with topography so that the coarsest sand lies between the axis of the landward trough and the ridge crest while the finest sand lies between the ridge crest and the axis of the seaward trough this relationship is characteristic of large scale bedforms finally flow was measured and transport calculated on the same ridge during a one month period november 1976 threshold was exceeded only during storm events mean transport was southerly and a little seaward with respect to both the ridge crest and the shoreline these flow measurements are in conformity with the pattern of smaller bedforms a 43 year time series of bathymetric change for this ridge reveals a systematic pattern of landward flank erosion seaward flank deposition and seaward crest migration sand ridges are considered the consequence of constructive feedback between an initial topography and the resulting distribution of bottom shear stress the relationship between grain size and topography supports this model but does not account directly for the oblique angle of the ridge with respect to the coastline this feature may be due to a more rapid alongshore migration rate of the inshore edge of the ridge than the offshore edge and the relationship between this migration rate and the rate of shoreface retreat

triggering effect of m 4 5 earthquakes on the earthquake cycle of repeating events at parkfield california stress perturbations influence earthquake recurrence and are of funda mental importance to understanding the earthquake cycle and determining earthquake hazard the large population of repeating earthquakes on the san andreas fault at parkfield california provides a unique opportunity to examine the response of the repeating events to the occurrence of moderate earthquakes using 187 m 0 4 to 1 7 repeating earthquake sequences from the high resolution seismic network cat alog we find that the time to recurrence of repeating events subsequent to nearby m 4 5 earthquakes is shortened suggesting triggering by major events the triggering effect is found to be most evident within a distance of 5 km corresponding to static coseismic stress changes of 0 6 26 6 kpa and decays with distance we also find coherently reduced recurrence intervals from 1993 to 1998 this enduring recurrence acceleration over several years reflects accelerated fault slip and thus loading rates during the early 1990s online material figures of event chronologies of res estimates of dt and coseismic static stress changes

delayed triggering of the 1999 hector mine earthquake by viscoelastic stress transfer stress changes in the crust due to an earthquake can hasten the failure of neighbouring faults and induce earthquake sequences in some cases1 2 3 4 5 the 1999 hector mine earthquake in southern california magnitude 7 1 occurred only 20 km from and 7 years after the 1992 landers earthquake magnitude 7 3 this suggests that the hector mine earthquake was triggered in some fashion by the earlier event but uncertainties in the slip distribution and rock friction properties associated with the landers earthquake have led to widely varying estimates of both the magnitude and sign of the resulting stress change that would be induced at the location of the hector mine hypocentre with estimates varying from 1 4 bar ref 6 to 0 5 bar ref 7 more importantly coseismic stress changes alone cannot satisfactorily explain the delay of 7 years between the two events here we present the results of a three dimensional viscoelastic model that simulates stress transfer from the ductile lower crust and upper mantle to the brittle upper crust in the 7 years following the landers earthquake using viscoelastic parameters that can reproduce the observed horizontal surface deformation following the landers earthquake our calculations suggest that lower crustal or upper mantle flow can lead to postseismic stress increases of up to 1 2 bar at the location of the hector mine hypocentre during this time period contributing to the eventual occurrence of the 1999 hector mine earthquake these results attest to the importance of considering viscoelastic processes in the assessment of seismic hazard8 9 10 11

use of swath bathymetry in the investigation of sand dune geometry and migration around a near shore banner tidal sandbank banner tidal sandbanks in the bristol channel have been repeatedly surveyed with a multibeam sonar to study the geometry and migration patterns of superimposed dunes the data presented in this paper constitute one of the first studies concerned with sediment transport around a banner sandbank helwick sands in the bristol channel using repeated swathbathymetry the data reveal that the dunes maintain their shapes over a period of 11 months and that they migrate in opposite directions on the alternate sides of the bank curiously dunes connect over the crest of the bank despite opposing sediment transport directions on the flanks dune height increases with water depth as found in similar environments we suggest how the morphology of the dunes results from the complex interaction between surface waves and tidal currents that occurs within the proximity of the headland 1995 furthermore repeated full bathymetric coverage of sub tidal sedimentary structures using swath sonar technology will provide information of their dynamics and threedimensional characteristics which are of interest for the understanding of coastal and shelf sedimentary transport this paper describes the morphology and dynamical characteristics of sand dunes at the eastern end of the helwick sandbank lying near a headland in the bristol channel fig 1 the analysis is based on two sets of swathbathymetry data collected on 26 september 2001 and 20 august 2002 the main objective of this paper is to demonstrate the potential for tracking dune migration and dune geometry changes from repeated bathymetric multibeam surveys this paper is therefore intended to a describe the morphological characteristics of the dunes and their spatiotemporal evolution b estimate dune migration rate and from that infer sediment transport flux and c relate these data to the work elsewhere of sediment transport around sandbanks and their maintenance in describing the sand dunes in this paper we follow the terminology developed by ashley 1990

segmentation of the northern san jacinto fault zone southern california accurate forecasting of large earthquakes in the san jacinto fault zone depends on the correct determination of fault segmentation we have searched for discontinuities in the fault zone using as data geological maps of the fault traces relocated earthquake hypocenters focal mechanisms slip rates and historic large earthquake rupture zones we identify nine principal discontinuities in the northern san jacinto fault zone these are characterized as structural discontinuities bends steps branches intersections geodetic discontinuities slip rate and lithologic discontinuities most of the discontinuities have more than one defining characteristic we also analyzed the local strain field orientation along the fault zone significant lateral discontinuities in the strain field orientation are coincident with many of the structural and geodetic discontinuities we identify segment lengths defined from the data vary from about 7 to 35 km this range of segment lengths is consistent with the lengths of large historical earthquake ruptures however evidence from small earthquake hypocenter patterns suggests the possibility that in some areas large earthquakes may rupture multiple adjacent segments in these cases maximum magnitudes may be larger than those observed historically because of the short historic record and the sparse paleoseismic record probabilistic estimates of earthquake recurrence for this fault zone have depended heavily on assumed characteristic lengths of individual rupture segments ours and previous delineations of segment lengths have varied considerably indicating that assumed segment length is a major source of uncertainty in the recurrence calculations this source of uncertainty should be taken into account in calculations of the reliability of the probabilistic hazard estimates

mechanisms underlying the regional morphological differences between the northern and southern radial sand ridges along the jiangsu coast china abstract radial sand ridges rsrs spread in a fan shaped pattern over the seabed of the southern yellow sea along the jiangsu coast china with pronounced differences between the northern and southern channel shoals indicated as ncs and scs respectively a depth averaged nested numerical model is employed to analyze the tidal hydrodynamics and sediment dynamics of the rsrs model results show considerable regional differences in terms of tidal current vector ellipses eight shaped ellipses are typical in the ncs where rotational tidal waves dominate while the scs are characterized by egg shaped ellipses because of the dominance of progressive tidal waves sand ridges tend to be elongated and straight in the ncs while short and dincontinuous in the scs numerical lagrange particles released near the ridges in the ncs tend to travel across the crest of the ridges rather than move around them as observed in the scs consistently the patterns of sediment convergence and divergence indicate that suspended sediments converge over the whole xiaoyinsha ridge located in the ncs while converge to the two sides of the hetunsha ridge located in the scs model results also highlight the importance of initial sediment compositions in determining patterns of the net sediment fluxes the suspended sediment concentration ssc time series during one tidal cycle may show single or double peaks depending on the grain size and the related lag effect bed load transport is much smaller than suspended load so sediment compositions should be carefully considered when simulating the ssc field and the hydrodynamic of the morphological evolution of the rsrs at the regional scale overall this study suggests that the ncs and scs may be treated as two relatively independent geomorphological systems characterized by different tidal flows and sediment compositions

controls of shear zone rheology and tectonic loading on postseismic creep 1 postseismic deformation is well documented in geodetic data collected in the aftermath of large earthquakes in the postseismic time interval gps is most sensitive to creep in the lower crust or upper mantle activated by earthquake generated stress perturbations in these regions deformation may be localized on an aseismic frictional surface or on a ductile shear zone these two hypotheses imply specific rheologies and therefore time dependence of postseismic creep hence postseismic creep constitutes a potential probe into the rheology of aseismic regions of the lithosphere i present a simple shear zone model of postseismic creep in which the rheology of the creeping element can be varied in the absence of tectonic loading during the postseismic time interval the displacement history of the shear zone obeying a power law rheology with stress exponent n follows an analytical relaxation curve parameterized by 1 n for a frictional surface postseismic creep follows the same relaxation law in the limit 1 n 0 a rough estimate of the apparent stress exponent can be obtained from continuous gps records application to data collected after the 1994 sanriku earthquake yields 1 n 0 1 which is consistent with dislocation creep mechanisms however the records of two other subduction zone events the 2001 peru event and the 1997 kronotski earthquake and a continental strike slip earthquake the 1999 izmit earthquake require negative 1 n rather than characterizing the shear zone rheology these negative exponents indicate that reloading of the shear zone by tectonic forces is important numerical simulations of postseismic deformation with nonnegligible reloading produce curves that are well fit by the generalized relaxation law with 1 n 0 although the actual stress exponent of the rheology is positive while this prevents rheology from being tightly constrained by the studied gps records it indicates that reloading is important in the postseismic time interval in other words the stress perturbation induced by an earthquake is comparable to the stress supported by ductile shear zones in the interseismic period

three dimensional finite element modeling of stress evolution around the xiaojiang fault system in the southeastern tibetan plateau during the past 500 years abstract the left lateral strike slip xiaojiang fault system is a main boundary between the southeastern tibetan plateau and the south china block and accumulates several large m 7 earthquakes in the past 500 years and a dozen of moderate 5 7 earthquake ruptures occurred in the past 500 years are simulated with split node technique in seismogenic upper crust we investigate the coulomb failure stress changes mainly on vertical planes parallel to the different strands of the xiaojiang fault system and at the epicenters of large m 7 earthquakes in the past 500 years and of moderate 5 7 earthquakes that occurred in the past 500 years both from their coseismic and postseismic effects among which the postseismic effect results in the stressing rate being obviously accelerated near epicenters of the large m 7 earthquakes after their occurrence thus the percentage of positive coulomb failure stress changes of 0 01 mpa at the epicenters increases from 37 to 50 for the large m 7 earthquakes and from 43 to 64 for the moderate 5

a theory of the ka band radar imaging mechanism of a submerged wreck and associated bed forms in the southern north sea the ka band radar imaging mechanism of the submerged wreck sand ribbon of the motor vessel m v birkenfels in the southern north sea is investigated by applying the quasi specular scattering theory and considering the capillary as well as the gravity wave ranges of the wave energy density spectrum for the imaging of wrecks and other oceanographic and meteorological phenomena at the sea surface it is assumed that quasi specular scattering becomes dominant at higher radar frequencies like ka and x band and wind speeds 7 8 m s 1 multibeam echo sounder images of the birkenfels wreck and associated sand ribbons as well as other available environmental in situ data have been analyzed the formation of sand ribbons at the sea bed and the manifestation of its radar signatures at the water surface are caused by an elliptical vortex or helical flow cell triggered by unidirectional tidal current flow interacting with the wreck the difference between simulated and measured normalized radar cross section nrcs modulation as a function of the space variable is less than 31 6 results are presented for nrcs simulations dependent on position for different effective incidence angles unidirectional current speeds wind speeds and relaxation rates the calculated current gradient or strain rate of the imaging theory has the same order of magnitude as those obtained for marine sand waves this implies that the responsible hydrodynamic interaction mechanism is able to produce radar signatures of submerged wrecks sand ribbons and make them visible at the sea surface

delayed seismicity rate changes controlled by static stress transfer on 15 june 2010 a m w5 7 earthquake occurred near ocotillo california in the yuha desert this event was the largest aftershock of the 4 april 2010 m w7 2 el mayor cucapah emc earthquake in this region the emc mainshock and subsequent ocotillo aftershock provide an opportunity to test the coulomb failure hypothesis cfs we explore the spatiotemporal correlation between seismicity rate changes and regions of positive and negative cfs change imparted by the ocotillo event based on simple cfs calculations we divide the yuha desert into three subregions one triggering zone and two stress shadow zones we find the nominal triggering zone displays immediate triggering one stress shadowed region experiences immediate quiescence and the other nominal stress shadow undergoes an immediate rate increase followed by a delayed shutdown we quantitatively model the spatiotemporal variation of earthquake rates by combining calculations of cfs change with the rate state earthquake rate formulation of dieterich 1994 assuming that each subregion contains a mixture of nucleation sources that experienced a cfs change of differing signs our modeling reproduces the observations including the observed delay in the stress shadow effect in the third region following the ocotillo aftershock the delayed shadow effect occurs because of intrinsic differences in the amplitude of the rate response to positive and negative stress changes and the time constants for return to background rates for the two populations we find that rate state models of time dependent earthquake rates are in good agreement with the observed rates and thus explain the complex spatiotemporal patterns of seismicity

reply to comment by l d stott on anomalous radiocarbon ages for foraminifera shells a correction to the western tropical pacific md9821 81 record

measurements of the reynolds stress components close to a marine sand bank abstract soulsby r l 1981 measurements of the reynolds stress components close to a marine sand bank mar geol 42 35 47 observations of turbulence have been made at a height of 65 cm above the sea bed in water 42 m deep on the edge of the skerries sand bank off southwest england the observations extended over a 4 h period from the time of maximum flood velocity to slack water electromagnetic current meters were used to measure the streamwise u cross stream v and vertical w velocity components while simultaneously a conventional current meter measured the speed and direction 5 m below the water surface ratios of u2 1 2 v2 1 2 and w2 1 2 to shear velocity agreed with accepted values for laboratory and atmospheric flows near to the surface the current was found to flow across the bank while near the bed it ran nearly parallel to the isobaths consequently the bottom current was directed 480 clockwise of the surface current the azimuth of the reynolds stress uw vw near the bed lay between these directions and was directed about 60 anticlockwise of the near bed current this balanced the cross stream as well as the streamwise shear a lateral shear stress uv associated with the bank was observed taken together with previous measurements of the velocity distribution over the bank this yielded a value for lateral eddy viscosity of 1 2 105 cm2 s 1 the scale over which this acts is of the order 1 km and the value is appreciably smaller than reported values deduced for larger scales the veering of the current acts to lessen the bottom current convergence associated with coriolis and curvature effects and may contribute to lateral sorting by preferentially carrying finer sediment toward the crest of the bank the lateral eddy viscosity coefficient may be useful in assessing the diffusion of sediment on or off the bank

viscoelastic flow in the lower crust after the 1992 landers california earthquake space geodesy showed that broad scale postseismic deformation occurred after the 1992 landers earthquake three dimensional modeling shows that afterslip can only explain one horizontal component of the postseismic deformation whereas viscoelastic flow can explain the horizontal and near vertical displacements the viscosity of a weak about 10 km thick layer in the lower crust beneath the rupture zone that controls the rebound is about 10 18 pascal seconds the viscoelastic behavior of the lower crust may help to explain the extensional structures observed in the basin and range province and it may be used for the analysis of earthquake hazard

tide and wave dynamics on a sand bank from the deep shelf of the western channel approaches abstract the sedimentary response of deep shelf sediments to the interplay of tides and waves based on the example of the celtic sea sand banks is described grab samples and 1500 km of multibeam and side scan data have been collected from the 300 km2 densely surveyed kaiser bank area the bank is 140 170 m deep 30 m high and 60 km long oriented perpendicular to the shelf edge and nearly parallel to the major axis of the tidal ellipse surficial sediments of the bank consist of medium to gravely biolithoclastic sands they are swept by tidal currents that reach 0 9 m s 1 m above the seabed and under the occasional influence of waves the mobile sands are commonly bedformed and rest on a highly backscattering lag the flanks below 140 m mainly exhibit transverse tidal dunes and sand ribbons whereas the top of the bank mostly displays discontinuous sand patches and wave ripples the sand patches are interpreted as the remnants of tidal bedforms reworked by waves calculations of the threshold of motion of four modal grain sizes under various conditions at the seabed show that tidal bedforms are active during spring tides sometimes associated with waves whereas only the largest annual waves may explain the observed wave ripples as in active tidal banks there is a reversal of tidal bedload transport from one bank side to the other the sedimentation rate is very low on the kaiser bank in addition there is a loss of sediment at both bank extremities and the tidal bedload cross bank transfer is very small probably controlled by shelf residual currents and long term drift of sediment resuspended by waves however the present day action of waves at the bank top is less intense than it was at lower sea levels during the holocene as evidenced by seismic studies

implications of deformation following the 2002 denali alaska earthquake for postseismic relaxation processes and lithospheric rheology 1 during the first 2 years following the 2002 mw 7 9 denali alaska strike slip earthquake a large array of global positioning system gps receivers recorded rapid postseismic surface motions extending at least 300 km from the rupture and at rates of more than 100 mm yr in the near field here we use three dimensional 3 d viscoelastic finite element models to infer the mechanisms responsible for these postseismic observations we consider afterslip both from an inversion of gps displacements and from stress driven forward models poroelastic rebound and viscoelastic flow in the lower crust and upper mantle several conclusions can be drawn 1 no single mechanism can explain the postseismic observations 2 significant postseismic flow below a depth of 60 km is required to explain observed far field motions best explained by a weak upper mantle with a depth dependent effective viscosity that ranges from 1019 pa s at the moho 50 km depth to 3 4 1018 pa s at 100 km depth 3 shallow afterslip within the upper crust occurs adjacent to and beneath the regions of largest coseismic slip 4 there is a contribution from deformation in the middle and lower crust from either lower crustal flow or stress driven slip afterslip is preferred over broad viscoelastic flow owing to the existence of seismic velocity discontinuities across the fault at depth though our modeling does not favor either mechanism if the process is viscoelastic relaxation the viscosity is a factor of 3 greater than the inferred mantle viscosity 5 poroelastic rebound probably contributed to the observed postseismic deformation in the immediate vicinity of the denali totschunda junction these conclusions lead us to infer an alaskan mechanical lithosphere that is about 60 km thick overlying a weak asthenosphere and a denali fault that cuts through the entire lithosphere with shear accommodated by faulting in the top 20 km and time dependent aseismic shear below

tidal and storm influenced sedimentation from a cambrian transgressive passive margin sequence the basal cambrian clastic sediments in northwest scotland record a transgressive sequence of barrier island to tidal shelf quartz arenites eriboll sandstone lower member and pipe rock overlain by a storm dominated interval of mixed clastic carbonate sediments fucoid beds two shallowing events interrupt this sequence the lower near the middle of the pipe rock and the upper at the top of the fucoid beds which led to the deposition of the quartz arenites of the salterella grit variations in the proportion of storm deposited facies and in the thickness of tidally generated structures suggests that tidal currents progressively increased during deposition of the quartz arenites and then decreased until deposition of the mixed clastic carbonate deposits these features suggest amplification of tides as the shelf approached a resonant width during transgression further transgression produced a wide non resonant shelf where tides were frictionally damped and storm processes were dominant similar sequences of the same age in western newfoundland eastern greenland and spitsbergen may also have been produced by the shelf bordering the north american craton responding to tidal amplification followed by frictional damping

giant sandwaves in the hola glacial trough off vesteralen north norway abstract seabed data acquired from the hola glacial trough off vesteralen north norway reveal the morphology of spectacular sandwave fields and large sandwaves in great detail the sandwave fields are among the deepest known occurring in water depths of 200 260 m multibeam bathymetry and backscatter shallow seismic sediment samples and video data collected by the mareano programme and bottom current modelling results have been used to interpret morphology transport and distribution of the seabed sediments sand supply to the hola trough was initially from wave and current reworking of glacial and glaciomarine deposits during the eustatic sea level lowstand and sea level rise after the last glaciation more recently deposited bioclastic sand dominates the upper part of the sand unit which locally reaches 10 m thickness sand predominantly accumulates behind ridges where current strength decreases and in topographic lows straight crested and barchan type sandwaves up to 3 km long and 7 m high are interpreted to form by tidal and geostrophic currents with bottom current speeds reaching 0 7 0 8 m s 1 a large cluster of coral reefs occur immediately east of the barchan type sandwaves where modelling shows bottom current velocities up to c 1 m s 1 sandwaves occur in four major sandwave fields strong variation in the nature of sandwaves over short distances e g migration direction between neighbouring sandwave fields point to important local variations in current regime and sedimentary processes the present work demonstrates how local topography and interaction of several current systems may produce complex depositional patterns on a local scale detailed multibeam mapping and video inspection of open shelf troughs in other narrow shelf settings worldwide may reveal similar sandwave fields

a stratigraphic investigation of the celtic sea megaridges based on seismic and core data from the irish uk sectors the celtic sea contains the world s largest continental shelf sediment ridges these megaridges were initially interpreted as tidal features formed during post glacial marine transgression but glacigenic sediments have been recovered from their flanks we examine the stratigraphy of the megaridges using new decimetric resolution geophysical data correlated to sediment cores to test hypothetical tidal vs glacial modes of formation the megaridges comprise three main units 1 a superficial fining upward drape that extends across the shelf above an unconformity underlying this drape is 2 the melville formation mfm which comprises the upper bulk of the megaridges sometimes displaying dipping internal acoustic reflections and consisting of medium to coarse sand and shell fragments characteristics consistent with either a tidal or glacifluvial origin the mfm unconformably overlies 3 the upper little sole formation ulsfm previously interpreted to be of late pliocene to early pleistocene age but here shown to correlate to late pleistocene glacigenic sediments forming a precursor topography the superficial drape is interpreted as a product of prolonged wave energy as tidal currents diminished during the final stages of post glacial marine transgression we argue that the stratigraphy constrains the age of the mfm to between 24 3 and 14 ka bp based on published dates coeval with deglaciation and a modelled period of megatidal conditions during post glacial marine transgression stratigraphically and sedimentologically the megaridges could represent preserved glacifluvial features but we suggest that they comprise post glacial tidal deposits mfm mantling a partially eroded glacial topography ulsfm the observed stratigraphy suggests that ice extended to the continental shelf edge

modes 2d method of detecting strike shift shear zones one of the goals of using the global positioning system gps and other geodetic survey techniques in tectonics has been to detect boundaries such as faults or shear zones between rigid or mildly deforming crustal masses the calculation of infinitesimal strains and rotations with gps data has been widely used to detect shear zones but it has been largely unsuccessful because infinitesimal strain and rotation although useful in many other ways is non diagnostic of shear zones or faults our approach is to work with components of deformation not strain and to design specifically a diagnostic method of detecting shear zones this paper introduces the first part of our method the detection of two dimensional strike shift shear zones modes 2d the modes 2d method has three elements 1 determination of the orientation of a suspected strike shift shear zone by analyzing components of a deformation tensor derived from a data set of displacements in an arbitrary coordinate system 2 resolution of the deformation tensor into the coordinate system parallel and normal to the detected shear zone and 3 exploration of the resolved data set for evidence for a belt of inhomogeneous deformation which is an essential characteristic of a shear zone the operation of modes 2d is illustrated herein with a theoretical survey network across an ideal shear zone developed with a buried dislocation fault and with a survey network afforded by the crossing of the kaynasli viaduct by the 1999 duzce bolu earthquake rupture in turkey

short term dynamics and maintenance processes of headland associated sandbanks shambles bank english channel uk abstract the short term over a spring tidal cycle dynamics of a headland associated sandbank shambles bank english channel are investigated by means of field measurements synchronous data using an acoustic doppler current profiler adcp and repeated side scan sonar imagery and two dimensional 2d hydrodynamic and sediment transport models the dynamics of the bank are described in terms of along and cross bank velocity components sand transport pathways and bedform asymmetries the results showed the occurrence of a net bedload convergent zone along the crest of the bank as a result of anti clockwise veering of sediment transport towards the crest this transport pattern is suggested to be the result of the dynamic interplay between two distinct tidally related processes acting over the sandbank during each phase of the tidal cycle a the formation and different stages of a transient tidal eddy that drives bedload movement during the flood phase of the tide and b the bottom friction induced by the presence of the sandbank that governs the bedload transport dynamics during the ebb phase of the tidal cycle

mechanisms for linear sandbank formation and maintenance in relation to dynamical oceanographic observations abstract theories associated with flow in the vicinity of headlands and sandbanks together with proposed mechanisms for sandbank formation and maintenance are reviewed to exemplify water and sand movement around a sandbank on a tidally dominated shelf tidal currents in the vicinity of a headland associated linear sandbank are described these data are integrated with sedimentological information on bedforms and grain size distributions the sandbank under detailed investigation the scarweather sands is located along the northern coastline of the bristol channel uk which is an area susceptible to high energy inputs the maximum spring tidal range here is around 10m and a 6000km fetch is available for storm wave approach from the atlantic ocean but the area is 460km from the shelf break detailed observations are presented from 11 long term current meter stations located within 10km of the sandbank currents are described in terms of residual water circulation shallow water harmonics and tidal ellipse characteristics bedload transport paths are inferred from the distribution of bedforms grain size and m2 m4 tidal current interactions transport vectors are predicted from near bed current meter observations combined with empirically derived formulae it is demonstrated that water circulation and sand transport paths form part of a clockwise eddy system around the sandbank mechanisms controlling the location and the characteristics of this particular sandbank are shown to be explicable in terms of a combination of models for sandbank formation and maintenance the results of the present study are thought to be applicable directly to headland associated sandbanks elsewhere it is shown on the basis of theoretical considerations that the pattern of residual water movement and possibly sand transport around this sandbank could be extended to continental shelves on a global basis

coulomb stress changes from earthquakes to underground excavation failures abstract evidence from shallow tunnels and mines indicates that in certain cases deformation and failure are not confined to the vicinity of the excavation front as is widely believed this situation has recently been described as lateral strain propagation from a source to remote parts of the excavation through low strength rocks a similar situation is observed in deep excavations mainly expressed by rock bursts and micro earthquakes we suggest that slow propagation or seismic radiation of deformation away from the excavation front can be explained on the basis of the theory of coulomb failure stress changes in analogy to faulting in large earthquakes local increase of stresses due to the excavation or failure in a certain point leads to a slight increase of stresses around this point if adjacent points are already at a critical level for failure this slight increase triggers a laterally domino style propagating new failure yet in contrast to earthquakes triggered deformation in mining can in principle be predicted and counteracted

sandbanks sandwaves and megaripples on spitsbergenbanken barents sea abstract recently acquired multibeam echosounder data from the shallowest part 26 53 m depth of spitsbergenbanken in the western barents sea reveal a variety of bedforms including megaripples sandwaves and sandbanks the bedforms exhibit varying degrees of superimposition and differ in their age of formation and present depositional regime being either active or moribund these are the first observations of co occurring current induced bedforms in the western barents sea and provide evidence of a high energy environment in the study area the bedforms indicate both sediment erosion and transport and confirm that there is enough sand available in this area to maintain them such conditions are not known to be common in the western barents sea and reflect the unique oceanographic and benthic environment of spitsbergenbanken

environmental setting morphology and volumetric evolution of the middelkerke bank southern north sea abstract the middelkerke bank a tidal sandbank located in the southern north sea on the belgian continental platform has been the object of a multidisciplinary and international research project the morphological and hydrodynamical setting of the study area is presented a complete side scan sonar coverage of the bank allowed a detailed analysis of the morphology of the bank and of its superimposed bedforms large dunes cover the flanks and summit of the middelkerke bank they have a constant orientation a height ranging from 0 5 to 5 m a wavelength from 75 to 150 m and in most cases an asymmetrical profile the slopes of both lee and stoss flanks are very low small and medium dune fields occur on the entire bank and in the adjacent swales they are 2 d and present a wavelength ranging from 1 to 15 m flood and ebb oriented small dunes cover respectively the western and eastern flank of the bank bathymetric surveying of the middelkerke bank was carried out to assess the importance of volumetric variations of the bank mass caused by seasonal factors results show that changes in the bank volume can be quite important the bank s volume decreases in periods of heavy weather after which the process of sand uppiling acting during long periods of fair weather conditions causes the bank to restore itself volumetric variations of the total bank volume in a winter summer period can range along certain sections of the bank from 21 to 26 despite these large seasonal volumetric variations the bank shows a long term dynamic equilibrium as proven by the obtained evolution trend

evolution of stress in southern california for the past 200 years from coseismic postseismic and interseismic stress changes summary the seismicity of southern california results from stresses that arise from the relative motion of the pacific and north american plates being accommodated along the san andreas fault saf system and the eastern california shear zone ecsz here we calculate how the stress field in southern california has evolved over the past two centuries due to interseismic loading as inferred from current gps observations of surface velocities from redistributions of static stress induced by large m w 6 5 earthquakes since the 1812 wrightwood quake and postseismic viscoelastic relaxation associated with these events that serves to transfer coseismic stresses from the deep warm lower crust and upper mantle to the overlying seismogenic upper crust we calculate coulomb stress changes on vertical strike slip faults striking parallel to the saf and at the hypocenters on the rupture planes of all m w 6 events over the past two centuries our results suggest that the 1857 m w 8 2 fort tejon earthquake by far the largest event to have occurred in the region over the past two centuries had a profound influence on the state of stress in southern california during the 19th century inducing significant stress increases to the north parkfield region and adjoining creeping saf and south southern saf and san jacinto fault and stress relief across the southern ecsz these stress changes were then greatly magnified by postseismic relaxation through the early part of the 20th century slow interseismic build up of stress further loads all major strike slip faults and works to reload the areas of the ecsz where stress was relieved by the 1857 quake our calculations suggest that only 56 of hypocenters were pushed closer to failure by preceding coseismic stress changes suggesting that the occurrence of large earthquakes is not strongly determined by coseismic coulomb stress changes this percentage rises to 70 when postseismic stress changes are also considered our calculations demonstrate the importance of postseismic viscoelastic relaxation in the redistribution of stress following large earthquakes we find however that postseismic processes associated with events more than about a decade old are near completion and thus do not significantly influence the regional velocity field presently observed in southern california

stress loading from viscous flow in the lower crust and triggering of aftershocks following the 1994 northridge california earthquake following the m w 6 7 northridge earthquake significant postseismic displacements were resolved with gps using a three dimensional viscoelastic model we suggest that this deformation is mainly driven by viscous flow in the lower crust such flow can transfer stress to the upper crust and load the rupture zone of the main shock at a decaying rate most aftershocks within the rupture zone especially those that occurred after the first several weeks of the main shock may have been triggered by continuous stress loading from viscous flow the long term decay time of aftershocks about 2 years approximately matches the decay of viscoelastic loading and thus is controlled by the viscosity of the lower crust our model provides a physical interpretation of the observed correlation between aftershock decay rate and surface heat flow

time dependent changes in failure stress following thrust earthquakes two dimensional 2 d viscoelastic finite element models were used to calculate the time dependent changes in coulomb failure stresses following thrust earthquakes due to respective effects of relaxation of viscous lower crust or upper mantle and postseismic creep on the main fault or its downdip extension results suggest that thrust earthquakes cause a coseismic increase in coulomb stress along antithetic lobes normal to the slip plane following a quake creep processes that reduce stresses in a ductile lower crust or upper mantle are calculated to cause a transfer of stress to the upper crust under certain conditions transfer of stress may lead to a further buildup of high coulomb stress along the base of the upper crust potentially shortening the time to failure of other faults in the region the conditions under which an antithetic lobe of high coulomb stress are favored to expand at the base of the upper crust postseismically within a few decades include the following the lower crust or upper mantle has an effective viscosity not greater than 1019 pa s the thrust fault has a moderate dip angle 40 50 the brittle ductile transition is deep enough to provide a corridor at the base of the upper crust for expansion and the crust has a low apparent coefficient of friction 0 2 postseismic increases in coulomb stress within the upper crust may also be caused by aseismic creep on the fault stress changes due to this mechanism are maximized with a high apparent coefficient of friction analysis of experimentally determined non newtonian flow laws suggests that wet granitic quartz and feldspar aggregates may have a viscosity of the order of 1019 pa s the calculated rate of stress transfer from a viscous lower crust or upper mantle to the upper crust becomes faster with increasing values of the power law exponent and the presence of a regional compressive strain rate results of this 2 d analysis suggest a potentially important role of viscous flow in controlling time dependent postseismic stress changes that warrant further investigation using 3 d viscoelastic analysis

stress triggering of the 1999 hector mine earthquake by transient deformation following the 1992 landers earthquake the m 7 3 june 28 1992 landers and m 7 1 october 16 1999 hector mine earthquakes california both right lateral strike slip events on nnw trending subvertical faults occurred in close proximity in space and time in a region where recurrence times for surface rupturing earthquakes are thousands of years this sug gests a causal role for the landers earthquake in triggering the hector mine earth quake previous modeling of the static stress change associated with the landers earthquake shows that the area of peak hector mine slip lies where the coulomb failure stress promoting right lateral strike slip failure was high but the nucleation point of the hector mine rupture was neutrally to weakly promoted depending on the assumed coefficient of friction possible explanations that could account for the 7 year delay between the two ruptures include background tectonic stressing dissi pation of fluid pressure gradients rate and state dependent friction effects and post landers viscoelastic relaxation of the lower crust and upper mantle by employing a viscoelastic model calibrated by geodetic data collected during the time period between the landers and hector mine events we calculate that postseismic relaxa tion produced a transient increase in coulomb failure stress of about 0 7 bars on the impending hector mine rupture surface the increase is greatest over the broad surface that includes the 1999 nucleation point and the site of peak slip further north since stress changes of magnitude greater than or equal to 0 1 bar are associated with documented causal fault interactions elsewhere viscoelastic relaxation likely con tributed to the triggering of the hector mine earthquake this interpretation relies on the assumption that the faults occupying the central mojave desert i e both the landers and hector mine rupturing faults were critically stressed just prior to the landers earthquake

the 1992 landers earthquake sequence seismological observations the m w 6 1 7 3 6 2 1992 landers earthquakes began on april 23 with the m w6 1 1992 joshua tree preshock and form the most substantial earthquake sequence to occur in california in the last 40 years this sequence ruptured almost 100 km of both surficial and concealed faults and caused aftershocks over an area 100 km wide by 180 km long the faulting was predominantly strike slip and three main events in the sequence had unilateral rupture to the north away from the san andreas fault the m w6 1 joshua tree preshock at 33 n58 and 116 w19 on 0451 ut april 23 was preceded by a tightly clustered foreshock sequence m 4 6 beginning 2 hours before the mainshock and followed by a large aftershock sequence with more than 6000 aftershocks the aftershocks extended along a northerly trend from about 10 km north of the san andreas fault northwest of indio to the east striking pinto mountain fault the m w7 3 landers mainshock occurred at 34 n13 and 116 w26 at 1158 ut june 28 1992 and was preceded for 12 hours by 25 small m 3 earthquakes at the mainshock epicenter the distribution of more than 20 000 aftershocks analyzed in this study and short period focal mechanisms illuminate a complex sequence of faulting the aftershocks extend 60 km to the north of the mainshock epicenter along a system of at least five different surficial faults and 40 km to the south crossing the pinto mountain fault through the joshua tree aftershock zone towards the san andreas fault near indio the rupture initiated in the depth range of 3 6 km similar to previous m 5 earthquakes in the region although the maximum depth of aftershocks is about 15 km the mainshock focal mechanism showed right lateral strike slip faulting with a strike of n10 w on an almost vertical fault the rupture formed an arclike zone well defined by both surficial faulting and aftershocks with more westerly faulting to the north this change in strike is accomplished by jumping across dilational jogs connecting surficial faults with strikes rotated progressively to the west a 20 km long linear cluster of aftershocks occurred 10 20 km north of barstow or 30 40 km north of the end of the mainshock rupture the most prominent off fault aftershock cluster occurred 30 km to the west of the landers mainshock the largest aftershock was within this cluster the m w6 2 big bear aftershock occurring at 34 n10 and 116 w49 at 1505 ut june 28 it exhibited left lateral strike slip faulting on a northeast striking and steeply dipping plane the big bear aftershocks form a linear trend extending 20 km to the northeast with a scattered distribution to the north the landers mainshock occurred near the southernmost extent of the eastern california shear zone an 80 km wide more than 400 km long zone of deformation this zone extends into the death valley region and accommodates about 10 to 20 of the plate motion between the pacific and north american plates the joshua tree preshock its aftershocks and landers aftershocks form a previously missing link that connects the eastern california shear zone to the southern san andreas fault

on the formation of sand banks of finite extent we consider a model where the fluid depth depends on both horizontal coordinates quasi steady depth uniform non divergent fluid flow is governed by inertial pressure and bottom frictional forces sand transport is proportional to the cube of the instantaneous current but augmented by a down slope component and by wind wave action and sand is conserved it is found that low parallel banks grow fastest so that in an extensive spatially uniform sea previous calculations for linear banks are appropriate the inclination of banks to the tidal currents can be interpreted in terms of similarly inclined deposition bands resulting from vorticity generation and advection in flow over a small isolated hump a small bump can evolve to an equilibrium bank typically after an initial rapid extension across the tidal currents provided that sand is sufficiently restricted and particularly if some wind wave action prevents growth up to the sea surface sand banks are likely to be in a late stage of evolution when the main change is a slow lengthening as the net current and transport along the bank side slows and turns around the bank end with net deposition the equilibrium is apparently stable except when there is an overall bed slope in the direction of the tidal currents or when sand is abundant

irregular recurrence of large earthquakes along the san andreas fault evidence from trees old trees growing along the san andreas fault near wrightwood california record in their annual ring width patterns the effects of a major earthquake in the fall or winter of 1812 to 1813 paleoseismic data and historical information indicate that this event was the san juan capistrano earthquake of 8 december 1812 with a magnitude of 7 5 the discovery that at least 12 kilometers of the mojave segment of the san andreas fault ruptured in 1812 only 44 years before the great january 1857 rupture demonstrates that intervals between large earthquakes on this part of the fault are highly variable this variability increases the uncertainty of forecasting destructive earthquakes on the basis of past behavior and accentuates the need for a more fundamental knowledge of san andreas fault dynamics

the relationship of sediment transport rates and paths to sandbanks in a tidally dominated area off the coast of east anglia u k sand transport measurements of bedload and suspended load in the sizewell dunwich banks area east anglia have shown that the suspended mode is dominant the depth integrated spring tidal residual is 5 66 g cm 1 sec 1 although the neap rate is only one fifth of this the calculated bedload transport rates also vary from 0 012 to 0 040 g cm 1 sec 1 correlating with changing meteorological conditions in order to predict the bedload sediment circulation pattern from midwater current meter measurements five sediment transport equations were calibrated using fluorescent dyed sand yalin s relationship gave the best estimates the bed shear stress was determined by extrapolating the velocity profile as a power law relationship with an exponent equal to 0 1 from midwater down to 2 m and as a lognormal profile from 2 m to the sea bed roughness length values appropriate to the substrate were used although bedload transport residuals are mainly to the south the banks trend northwards from the coast and have also elongated in this direction this is thought to be in response to the dominance of the suspended sediment transport it is suggested that a tidal residual eddy mechanism is responsible for the banks maintenance similar to the process operating in start bay devon the well documented westward movement of the banks is likely to be related to wave processes

a more precise chronology of earthquakes produced by the san andreas fault in southern california improved methods of radiocarbon analysis have enabled us to date more precisely the earthquake ruptures of the san andreas fault that are recorded in the sediments at pallett creek previous dates of these events had 95 confidence errors of 50 100 calendar years new error limits are less than 23 calendar years for all but two of the dated events this greater precision is due to larger sample size longer counting time lower background noise levels more precise conversion of radiocarbon ages to calendric dates and better stratigraphic constraints and statistical techniques the new date ranges with one exception fall within the broader ranges estimated previously but our estimate of the average interval between the latest 10 episodes of faulting is now about 132 years variability about the mean interval is much greater than was suspected previously five of the nine intervals are shorter than a century three of the remaining four intervals are about two to three centuries long despite the wide range of these intervals a pattern in the occurrence of large earthquakes at pallett creek is apparent in the new data the past 10 earthquakes occur in four clusters each of which consists of two or three events earthquakes within the clusters are separated by periods of several decades but the clusters are separated by dormant periods of two to three centuries this pattern may reflect important mechanical aspects of the fault s behavior if this pattern continues into the future the current period of dormancy will probably be greater than two centuries this would mean that the section of the fault represented by the pallett creek site is currently in the middle of one of its longer periods of repose between clusters and sections of the fault farther to the southeast are much more likely to produce the next great earthquake in california the greater precision of dates now available for large earthquakes recorded at the pallett creek site enables speculative correlation of events between paleoseismic sites along the southern half of the san andreas fault a history of great earthquakes with overlapping rupture zones along the mojave section of the fault remains one of the more attractive possibilities

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

mobility of continental mantle evidence from postseismic geodetic observations following the 1992 landers earthquake the crust around the rupture zone of the 1992 landers earthquake has continued to deform in the years following the earthquake at rates 3 times greater than pre earthquake rates we use a combination of global positioning system gps and synthetic aperture radar insar data collected during a 3 year epoch following the earthquake in order to investigate postseismic mechanisms responsible for the high transient velocities in order to maximize the potential signal from viscoelastic relaxation we evaluate and model postseismic relaxation following the first few months of documented accelerated deformation the combination of gps and insar data allows us to establish viscoelastic relaxation of the lower crust and upper mantle as the dominant postseismic process and to discriminate among possible viscoelastic models the data particularly require the presence of a highly ductile uppermost mantle beneath the central mojave domain with temperature between the wet and dry basalt solidus this is consistent with independent seismic and geochemical inferences of a regionally warm uppermost mantle further consideration of seismic velocity variations in conjunction with faulting patterns within the mojave desert suggests that the primary faulting characteristics of the mojave desert namely the pervasive late cenozoic deformation within the eastern california shear zone versus the near absence of faults in the western mojave domain are controlled by the rheology of the uppermost mantle

forecasting the evolution of seismicity in southern california animations built on earthquake stress transfer 1 we develop a forecast model to reproduce the distribution of main shocks aftershocks and surrounding seismicity observed during 1986 2003 in a 300 310 km area centered on the 1992 m 7 3 landers earthquake to parse the catalog into frames with equal numbers of aftershocks we animate seismicity in log time increments that lengthen after each main shock this reveals aftershock zone migration expansion and densification we implement a rate state algorithm that incorporates the static stress transferred by each m 6 shock and then evolves coulomb stress changes amplify the background seismicity so small stress changes produce large changes in seismicity rate in areas of high background seismicity similarly seismicity rate declines in the stress shadows are evident only in areas with previously high seismicity rates thus a key constituent of the model is the background seismicity rate which we smooth from 1981 to 1986 seismicity the mean correlation coefficient between observed and predicted m 1 4 shocks the minimum magnitude of completeness is 0 52 for 1986 2003 and 0 63 for 1992 2003 a control standard aftershock model yields 0 54 and 0 52 for the same periods four m 6 0 shocks struck during the test period three are located at sites where the expected seismicity rate falls above the 92 percentile and one is located above the 75 percentile the model thus reproduces much but certainly not all of the observed spatial and temporal seismicity from which we infer that the decaying effect of stress transferred by successive main shocks influences seismicity for decades finally we offer a m 5 earthquake forecast for 2005 2015 assigning probabilities to 324 10 10 km cells

large scale sediment bodies and superimposed bedforms on the continental shelf close to the strait of gibraltar interplay of complex oceanographic conditions and physiographic constraints a previously unknown field of large scale sedimentary bodies has been mapped and studied on the continental shelf off the cape trafalgar near the strait of gibraltar with particular emphasis on the relationship between large scale sediment bodies and the superimposed bedforms this study is based on a grid of 975 km of high resolution seismic profiles collected at water depths ranging between 15 and 60 m high variability of large scale sedimentary bodies is attributed to the complex interaction of hydrodynamic agents the most prominent sedimentary features are sand banks and ridges that indicate long term southwest and southward directed sediment transport patterns possibly due to the interplay of two dominant current systems flowing southward and westward these sediment bodies evolve laterally to distinct external geometries such as sand shoals in shallow water and sand sheets in the vicinity of larger sand banks that indicate moderate current velocities in addition pre existing physiography is considered to play a role in the generation of certain sediment bodies developed over inclined surfaces or confined laterally by elevations relationships between superimposed bedforms mostly very large dunes and underlying sediment bodies vary across the study area most superimposed bedforms occur over the complex mosaic of sediment banks and sheets suggesting the interaction of several high energy currents with different directions such as tidal and or wind driven currents copyright 2010 john wiley sons ltd

spatial and temporal distribution of slip for the 1992 landers california earthquake we have determined a source rupture model for the 1992 landers earthquake m w 7 2 compatible with multiple data sets spanning a frequency range from zero to 0 5 hz geodetic survey displacements near field and regional strong motions broadband teleseismic waveforms and surface offset measurements have been used explicitly to constrain both the spatial and temporal slip variations along the model fault surface our fault parameterization involves a variable slip multiple segment finite fault model which treats the diverse data sets in a self consistent manner allowing them to be inverted both independently and in unison the high quality data available for the landers earthquake provide an unprecedented opportunity for direct comparison of rupture models determined from independent data sets that sample both a wide frequency range and a diverse spatial station orientation with respect to the earthquake slip and radiation pattern in all models consistent features include the following 1 similar overall dislocation patterns and amplitudes with seismic moments of 7 to 8 10 26 dyne cm seismic potency of 2 3 to 2 7 km 3 2 very heterogeneous unilateral strike slip distributed over a fault length of 65 km and over a width of at least 15 km though slip is limited to shallower regions in some areas 3 a total rupture duration of 24 sec and an average rupture velocity of 2 7 km sec and 4 substantial variations of slip with depth relative to measured surface offsets the extended rupture length and duration of the landers earthquake also allowed imaging of the propagating rupture front with better resolution than for those of prior shorter duration strike slip events our imaging allows visualization of the rupture evolution including local differences in slip durations and variations in rupture velocity rupture velocity decreases markedly at shallow depths as well as near regions of slip transfer from one fault segment to the next as rupture propagates northwestward along the multiply segmented fault length the rupture front slows as it reaches the northern limit of the johnson valley landers faults where slip is transferred to the southern homestead valley fault an abrupt acceleration is apparent following the transfer this process is repeated and is more pronounced as slip is again passed from the northern homestead valley fault to the emerson fault although the largest surface offsets were observed at the northern end of the rupture our modeling indicates that substantial rupture was also relatively shallow less than 10 km in this region

morphology and sedimentation of sand bodies in the tidal shelf sea of eastern taiwan strait abstract changyun ridge located in the central eastern part of the taiwan strait consists of sand bodies the eastern changyun sand ridge 65 km long and 13 km wide a sand shoal 53 km long and 22 km wide and a sand sheet 60 km long 40 km wide and 22 m in height the eastern changyun sand ridge and the sand sheet are oriented parallel to the coastline but the sand shoal is perpendicular to the coast the eastern changyun sand ridge is oriented oblique to the narrow reciprocal tidal ellipses the sand shoal and sand sheet are normal and oblique to the broad rotary tidal ellipses respectively surface sediments of the changyun ridge are composed of very fine to coarse sands in a progressive seaward decrease in grain size the configuration and spatial distribution of these three sand bodies with corresponding decrease in tidal speed and in grain size together suggest that the sand bodies developed sequentially from the near shore to the offshore area presumably in response to westward shifting of the north flowing tidal currents sand waves occur widespreadly on the sand shoal and seldom appear on the eastern changyun sand ridge occurrences of sand waves suggest that the sand shoal is considered to be active and the sand ridge to be moribund chirp sonar profiles reveal the internal structures of the sand shoal showing a relatively flat basal reflector underlying the sand body the thickness of the sand accumulation is about 22 25 m and it is firstly determined by the depth difference between the modern sea bed and the basal reflector to better describe morphology and document dimensions of these sand bodies the presence of several short inclined reflectors dipping northeast and down lapping the basal flat reflector indicate that sands are most likely transported from south to north for the sand shoal the sand bodies of the changyun ridge rest upon a relative flat basal reflector suggesting a mainly hydrodynamically controlled origin the variations of the hydrodynamic conditions mainly tidal currents on the central eastern taiwan strait shelf seem to be the primary control on sedimentary facies shift morphological changes and sand ridge growth for the changyun ridge the changyun ridge is ongoing to receive sands from the south in a transitional development stage as long as the sand supply continues and over a longer time period the sand bodies eventually will develop into three individual linear sand ridges separated by swales oriented parallel to the peak currents

tide induced bedload transport pathways in a multiple sand ridge system offshore of hainan island in the beibu gulf northwest south china sea modern sand transport pathways in a multiple sand ridge system are elusive and have rarely been studied in recent years we report herein a field with four en echelon linear sand ridges offshore of hainan island in the beibu gulf and describe the distribution and morphology of these sand ridges in detail for the first time dune crest comparisons and seismic profiles are also interpreted to assess sand transport over the dunes based on a delft 3d model regional tidal currents and tide induced bedload transport in the multiple sand ridge system were simulated to provide insights into the related bedload transport paths the results show that bedload transport and residual flows are mostly directed to the south on the east side of the sand ridges and to the north on the west side and these differences coincide with dune asymmetries and migrations cross ridge transport is weak and mostly converges on the crests of sand ridges from the two flanks the bedload transport is unbalanced on either flank of the sand ridges thus leading to the asymmetry and crest kinks of the sand ridges a distinct correlation is not observed between the net cross ridge sand transport and sand ridge asymmetry in this system cross swale transport is significant between the neighboring sand ridges and helps construct the bedload transport circulation in the swales sand accretion also occurs in the swales and benefits from the southwestward bedload transport from the north end of sand ridge 3 distinct evidence has not been found for closed bedload transport circulation around sand ridges in this multiple sand ridge system although limited clockwise current vortices develop on several sand ridges additional field observations and simulations are required to characterize the suspended load transport in a multiple sand ridge system and the effects of local sand supply conditions on sand transport also need further evaluation copyright c 2018 john wiley sons ltd

seafloor response to flow in a southern hemisphere sand ridge field argentine inner shelf abstract the inner continental shelf adjacent to buenos aires province argentina is characterized by a series of sand ridges averaging 4 7 m in relief and spaced about 2 7 km apart the ridges trend north south forming a 20 to 35 southward opening angle with the northeast southwest trending coast although the adjacent coast is of depositional strand plain origin it is presently undergoing erosional retreat and the formation of the ridges appears to be part of the retreat process seafloor grain size distributions and bedform arrays are responses to the same velocity field that has formed the ridges along transects normal to the ridges grain size variation is 90 out of phase with the topography the coarsest sediment is not at ridge crests or trough axes but instead occurs on the landward slope finest sands are on the seaward slope side scan sonar reveals bedforms aligned obliquely to both ridges and the coast these are inferred to be sand waves the grain size and bedform patterns suggest that the formative flows are northward currents with an offshore flow component near the bottom although flows observed during the study period were dominantly north to south the grain size gradients and bedform orientations are equivalent to those characteristic of the atlantic shelf of north america with a north south inversion the comparison suggests that inner shelf transport directions in response to wind events can be predicted from the regional geometry and climate of the shelf

bedforms of the canadian eastern seaboard a comparison with global occurrences this paper presents a compilation of sedimentary bedforms identified from acoustic and seismic records collected over the southeastern canadian continental shelf in order to make the compilation it was found necessary to firstly review and synthesise the various terminologies used to describe the bedforms found and thereafter to relate the subsequent terminology to any diagnostic generic characteristic identifiable from the records sand waves are considered to be those forms that are composite in nature and are generated solely by the action of superimposed megaripples these latter features are identifiable adopting the classification of dalrymple et al 1978 which proves adequate for the acoustic differentiation of 2 d and 3 d types though we observed a greater coherence in bedform spacing of the 2 d types than reported hitherto sand ridges were adequately defined and recognised using the description of swift et al 1978 with respect to storm generated ridges and that of off 1963 and others with respect to tidal current ridges such ridges appear not to be uniquely described by ratios of their metrics the bedforms on the southeastern canadian continental shelf are abundant and wide spread virtually all types of bedforms recognisable on the u s shelf are present to the north the majority of these bedforms appear to be storm generated and are considered intermittently active many of the sand waves s s have no measureable relief though retaining their distinctive spatial geometry both active and moribund sand ridges were found of these ridges both tidal and storm generated types were identified the storm generated ridges were further broken down into shoreface connected ridges and offshore ridges a metrical analysis of the various bedforms show that a differentiation into stable bed phases can be made using height length ratios provided the bedforms are active and fully developed there is some overlap between megaripples and small sand waves and indeed between 2 d and 3 d megaripples yet the distinction is good between sand waves and sand ridges even moribund types sand ribbons in general can be confused with moribund sand waves and megaripples a detailed morphological examination is required to distinguish each feature data from sand ridges of the eastern canadian shelf combined with data from similar features found globally shows that ridge heights tend to cluster into three distinctive modes these modes are at heights of 7 13 and 27 m the first mode is correlated with storm generated ridges the second with tidal current ridges and the third possibly with submerged barrier islands

comment on anomalous radiocarbon ages for foraminifera shells by w broecker et al a correction to the western tropical pacific md9821 81 record 1 the effort to obtain high resolution paleoceanographic data often leads to continental margin settings where the flux of clastic sediments produces higher sediment accumulation rates however on continental margins the possibility for down slope transport of reworked sediment is a concern this is particularly true in tectonically active settings such as the western pacific which includes the maritime islands of the philippines and the adjacent archipelago the continental margins of the western tropical pacific are strategically located within the pacific warm pool and detailed paleoceanographic information from this region is a valuable asset in efforts to improve our understanding of what factors contribute to large and sometimes abrupt changes in global climate the acquisition of data sets such as sea surface temperatures from the tropical pacific warm pool through the last glacial cycle has indeed added measurably to a growing array of data from lowlatitude locations certainly there is much left to be learned from the high deposition rate records located in continental margin settings however over the past 2 years there have been concerns that one of the high deposition rate sequences in the western tropical pacific r v marion dufresne core md98 2181 is composed of down slope transport of reworked material or slumping broecker et al 2006 because this site has provided one of the highest resolution data sets of sea surface temperature and salinity information for the pacific warm water pool through the last glacial cycle stott et al 2002 2004 it is important to assess the stratigraphic integrity of the sequence and to identify where there may be problems with the record 2 the md98 2181 core was collected from a depth of 2114 m on the continental slope south of mindanao island during the last glacial and holocene sediments accumulated at this site at rates of between 50 and 80 cm kyr making this an ideal location to collect high resolution paleoceanographic information from the pacific warm pool stott et al 2004 we have worked on the md98 2181 core for the past eight years conducting stable isotope and mg ca measurements on planktonic and benthic foraminifers in an effort to document the timing and the magnitude of sea surface temperature and salinity changes in the western tropical pacific warm pool through the last glacial cycle stott et al 2002 2004 in doing so we have obtained c ages for 42 planktonic foraminifer samples from the holocene and late glacial portions of the core table 1 and figure 1 the majority of our md98 2181 samples were taken from u channel subcores these subcores were sectioned continuously at 2 cm increments in stott s laboratory and the samples were washed in buffered di water over a 63 mm sieve to remove fine clays the planktonic globigeriniodes ruber used for stable isotope and mg ca measurements and also for c determinations was picked from the 150 mm fraction whereas globigerinoides sacculifer was picked from the 250 mm fraction in some instances other more robust planktonic species have also been c dated including pulleniatina obliquiloculata and neogloboquadrina dutertrei which were picked from the 250 mm fractions m rincon who has been stott s laboratory manager for the past 15 years picked each sample used in these studies the g ruber do stratigraphy for md98 2181 is shown in figure 2 3 the late glacial to recent do stratigraphy from this core matches other high resolution sequences from this region with the exception of a 20 cm interval between 941 and 961 cm the samples from the 941 961 cm interval exhibit do values that are clearly higher than adjacent intervals immediately above and below figure 2 the interval centered at 950 cm would have been deposited during the last deglaciation and the apparent timing of the anomalously high do values would put it close to the time of the younger dryas for this reason it was not initially clear whether these values represented the presence of reworked glacial age material or if the values were indicative of a brief return to glacial like conditions during the deglaciation the intervals directly below 961 cm do not exhibit such high do values as those at 950 cm and the isotope stratigraphy appears to closely match that of other high resolution records from the western tropical pacific region stott et al 2004 4 in 2003 w broecker approached stott about a collaboration that involved c dating of planktonic and paleoceanography vol 22 pa1211 doi 10 1029 2006pa001379 2007 click here for full article

postseismic deformation associated with the 1992 m ω 7 3 landers earthquake southern california following the 1992 m 7 3 landers earthquake a linear array of 10 geodetic monuments at roughly 5 km spacing was established across the emerson fault segment of the landers rupture the array trends perpendicular to the local strike of the fault segment and extends about 30 km on either side of it the array was surveyed by global positioning system 0 034 0 048 0 381 1 27 1 88 2 60 and 3 42 years after the landers earthquake to measure both the spatial and temporal character of the postearthquake relaxation the temporal behavior is described roughly by a short term decay time 84 23 days exponential relaxation superimposed upon an apparently linear trend because the linear trend represents motions much more rapid than the observed preseismic motions we attribute that trend to a slower decay time greater than 5 years postseismic relaxation the curvature of which cannot be resolved in the short run 3 4 years of postseismic data about 100 mm of right lateral displacement and 50 mm of fault normal displacement accumulated across the geodetic array in the 3 4 year interval covered by the postseismic surveys those displacements are attributed to postseismic right lateral slip in the depth interval 10 to 30 km on the downward extension of the rupture trace the right lateral slip amounted to about 1 m directly beneath the geodetic array and the fault normal displacement is apparently primarily a consequence of the curvature of the rupture these conclusions are based upon dislocation models fit to the observed deformation however no dislocation model was found with rms residuals as small as the expected observational error

low sea level tidal origin of celtic sea sand banks evidence from numerical modelling of m2 tidal streams abstract numerical modelling of m 2 tidal streams in the celtic sea with a sea level lowered by 100 m indicates tidal currents about twice as strong as those at present such currents would have been sufficient to generate and maintain the linear tidal sand banks now found in that region as moribund bedforms furthermore an indicated clockwise rotation of the major axes of the tidal ellipses at a sea level lowered by 100 m with respect to the present day is in the correct sense to further the comparison with modern examples of actively maintained linear tidal sand banks

consequences of stress changes following the 1891 nobi earthquake japan abstract great earthquakes along the philippine sea interplate boundary central honshu usually rupture a large portion of the nankai trough and suruga trough however the 1944 tonankai and 1946 nankaido earthquakes the most recent great philippine sea events did not rupture the suruga trough mogi proposed that strain release from the 1891 nobi earthquake is responsible for the apparently premature northeast termination of the 1944 rupture zone using a recently derived fault model of the 1891 earthquake combined with a suitable rheological model of central honshu we quantify both the inland and offshore consequences of the nobi earthquake north of the median tectonic line the co seismic plus postseismic stress changes of the nobi earthquake promoted the failure of faults which subsequently ruptured in the decades following 1891 in contrast along the philippine sea interplate boundary the co seismic plus postseismic strain release of the nobi earthquake during the decades following 1891 has delayed the anticipated tokai earthquake by at least 20 yr

sand transport under the combined influence of waves and tidal currents an assessment of available formulae abstract various formulae are used to predict the transport of sand under the combined influence of waves and currents these approaches include the use of the unidirectional formulae of einstein 1950 frijlink 1952 yalin 1963 engelund and hansen 1967 sternberg 1972 ackers and white 1973 and gadd et al 1978 modified for the presence of waves in accordance with the techniques described by bijker 1967 and swart 1976 and those of bagnold 1963 madsen and grant 1976 and vincent et al 1981 predicted rates are compared with measurements based upon fluorescent sand tracer studies in an area of high tidal current and wave energy northern bristol channel u k in general predicted rates are lower than those measured and only the formulae of madsen and grant 1976 and the modified formula of sternberg 1972 appear to provide realistic estimates of transport under wave and tidally induced currents in comparison with the field measurements

compressional structures on mars abstract wrinkle ridges accommodate very low amounts of shortening strain around immense volcanic constructs and within impact basins on mars they originate from stresses distributed uniformly throughout the brittle lithosphere and are consistently located in stratified deposits including lava flows and sediment most recent models interpret wrinkle ridges as the surface manifestation of folding above underlying blind thrusts that accommodate similarly low strain and likely penetrate tens of kilometers into the brittle crust alternative models suggest shortening accommodated by some wrinkle ridges is confined to only the upper few kilometers of the crust the interpretation of the geometry of blind thrusts on mars appears to be quite varied and remains controversial although some models suggest they may ultimately flatten into the brittle ductile transition in the middle to lower crust small scale crenulations superposed on ridges are interpreted as produced by high level back thrusts nucleating at

fault slip rates effects of elastic heterogeneity on geodetic data and the strength of the lower crust in the salton trough region southern california 1 in southernmost california the salton trough sedimentary basin lies between the subparallel san jacinto and san andreas faults that together with the elsinore fault accommodate 80 of the 50 mm yr of pacific north american relative motion the slip rates on the san andreas and san jacinto faults have been a matter of recent debate and one source of uncertainty has been the influence of crustal strength heterogeneity due to elastically weak basin sediments on geodetic data to evaluate these effects we have modeled regional kinematics with elastic and viscoelastic finite element models that incorporate laterally and vertically varying crustal material properties we find that in general the effects of the sedimentary basin on surface deformation are relatively small and that to explain the geodetic velocity data the slip rate on the san andreas must be higher than that on the san jacinto fault consistent with traditional geologic estimates we also conclude that a relatively strong lower crust of viscosity 5 x 10 19 pa s exists in this region

modeling of short period surface wave propagation in southern california abstract for the 1992 landers mainshock the lateral heterogeneity of the lithosphere combined with an extended earthquake source may account for the complexity of the strong motion observations that cannot be easily interpreted by a simplified point source model in a laterally homogeneous medium both the effect of an extended source and that of lateral heterogeneity are examined using a forward modeling technique that combines distributed nonuniform slip on a fault plane in a 3d medium surface wave synthetics are constructed using surface wave gaussian beam method which allows for a mild degree of lateral heterogeneity we analyze the amplitude and waveform data of the landers strong motion recordings in southern california in order to identify the path effects and the effect of an extended source the overall source parameters of the landers earthquake are obtained by comparing synthetics with recordings at near source stations which lie within relatively homogeneous path structures this procedure yields a unilateral rupture propagating northwesterly over two distinct rupture segments with somewhat different focal mechanisms the surface wave phase velocity map is modified to fit the data recorded at larger hypocentral distances the final improved model generates a very good agreement between observations and synthetics in the presence of lateral heterogeneity the focusing defocusing and multipath interference are found to contribute significantly to the strong motion shaking in a 3d basin

the role of non uniform roughness in the formation of sand ribbons abstract sand ribbons typically are shallow water sedimentary features characterized as thin elongate bands of sand overlying a coarser lag sediment which is exposed between them the ribbons are oriented along the direction of the dominant current and are formed by a secondary circulation pattern that includes a small component of the boundary shear stress directed toward the center of the sand ribbons a perturbation expansion coupled with flume experiments indicates that this circulation pattern is linked to differences in the bottom roughness between the sand ribbons and the adjacent coarser strips the non uniform turbulence field arising from this roughness distribution drives a secondary helical flow the expansion indicates that there is a preferred ratio of sand ribbon spacing to water depth 4 1 for unstratified channel flow conditions the secondary flow pattern forces the downstream flow to readjust so that the boundary shear stress over the smooth sand strips is considerably smaller than the mean stress this serves to retard the movement of sand through such a system

water and sediment movement in the vicinity of linear sandbanks the norfolk banks southern north sea geophysical surveys current meter measurements and fluorescent sand tracer experiments were undertaken in the vicinity of the broken bank the data are used to establish localised and regional patterns of water and sediment movement the results obtained confirm mechanisms proposed previously for sandbank growth with particular reference to the north sea these include the clockwise circulation of water and sand with convergence at the crestline secondary helical water circulations do not appear to be a dominant process influencing sandbank maintenance however such circulation may still be important in the initial formation of linear sandbanks and or at some stages in the bank development on a regional basis residual currents bedform asymmetries and tracer dispersion indicate an offshore sediment transport component this observation supports the hypothesis of other investigators formed on the basis of geomorphological observations and numerical model predictions concerning the movement of sand under extreme conditions moreover these transport pathways may persist under low wave and tidally dominated conditions as opposed to being restricted to storm activity

comparisons of measured and predicted sediment transport rates in tidal currents abstract this paper describes bedload transport measurements in a large scale sedimentary system on the british continental shelf steady flow sediment transport theories are applied to oscillatory tidal currents and various equations are compared with measured transport rates from tracer dispersion experiments the effects of waves on both measured and predicted rates are briefly considered and the predicted directions of sediment movement compared with those inferred from sedimentological data

sand waves and sediment transport around the end of a tidal sand bank two detailed surveys have been made of the north western end of the haisborough sand off the norfolk coast using echosounder 3 5 khz reflection profiler and side scan sonar asymmetrical sand waves indicate north westerly sand movement on the southern side and south easterly sand movement on the steeper northern side of the bank secondary superimposed megaripples which are probably better indicators of sediment movement give evidence of a cross bank component between the north westerly and south easterly facing sand waves on the tip of the bank there is a zone of symmetrical sand waves these are usually taken to indicate zero net transport but in this case the oblique orientation of megaripples in their troughs indicates transport parallel to the sand wave crests this suggests the route by which sand travels around the end of the bank to form a roughly closed circulation sediment textural parameters support the notion that sand is winnowed from the foot of the bank on both sides and is transported to the middle with an overall net transport from the south to the north analysis of charts dating back to 1886 shows that the bank is stable within the error limits of position fixing though that could allow more than 0 25 km shift to the north east in 100 years to pass undetected a box model is drawn up for the estimated sediment fluxes around the end of the bank and implications for residence times and circulation rates are drawn from it