twin active spreading ridges in the north fiji basin southwest pacific recent work on the north fiji basin axial spreading ridge and on the active propagating ridge located west of the fiji islands has established the existence of two active spreading ridges that have been functioning synchronously in the basin since at least 1 or 1 5 ma this hypothesis is tested and the kinematics of the twin active ridges are refined further on the basis of analysis of bathymetric and magnetic data

comment on nature of the recent vertical ground movements inferred from high precision leveling data in an intraplate setting ne ardenne belgium by a demoulin and a collignon 01 using yearly leveling surveys performed from 1993 to 1998 in the ardenne as well as historic leveling results from 1948 and 1974 demoulin and collignon 2000 hereinafter referred to as d c observed that the total vertical ground movement less than 1 cm over a 20 to 30year period is barely higher than the yearly displacements measured to a few millimeters per year they eliminated various sources of errors from their measurements and subsequently interpreted the resulting short term oscillating crustal displacements as true tectonic motions within the upper crust accompanying the long term deformation of uplifting areas in intraplate settings their justification is that the leveling discontinuities coincide with faults they assume to be active in their conclusions d c suggested that such motions and their variations could be used to monitor potential seismogenic faults and even to foretell earthquakes 2 unfortunately it is our opinion that the data analysis of d c is inadequate and does not support the subsequent interpretation specifically we interpret the apparent oscillations in the leveling data as the expected expression of the noisy character of leveling differences with amplitudes ranges of 0 5 to 1 cm if this premise is accepted then there is also no evidence to suggest that the boundary faults in the study area are active we thus feel that the conclusions of d c are premature until their observations can be corroborated by improved observations or by other independent measurements in this note we present a careful statistical analysis to support our position

nature of the recent vertical ground movements inferred from high precision leveling data in an intraplate setting ne ardenne belgium yearly high precision leveling surveys have been performed from 1993 to 1998 along a 48 km long local network centered on gileppe lake at the foot of the ardenne massif belgium institut geographique militaire and institut geographique national igm ign levelings of 1948 and 1974 yielded additional information the data were corrected for collimation and thermal errors other sources of error earth tides atmospheric refraction rod miscalibration were estimated and corrections were made when necessary random errors of individual surveys range from 0 15 to 0 74 mm km the recorded elevation changes indicate the existence of tilted segments some 5 to 10 km in length with yearly tilt values amounting up to 2 rads fault movements of a few mm yr are also demonstrated the segment limits are spatially stable from one year to the next but very frequent sense reversals of the vertical motions are observed this explains why the total movement over a 20 to 30 year period is barely higher than the yearly displacements various nontectonic causes of vertical ground movement e g monument instability varying water loading of the lake sediment compaction etc have been considered yielding little satisfactory explanation of the patterns of observed elevation changes we propose that the recorded motions which we call reversible fault creep and reversible tilt respectively are a true tectonic signal reflecting the tectonic noise which accompanies the long term deformation of uplifting areas in intraplate settings such movements are characteristic of fault segments which may occasionally generate earthquakes

mantle phase changes and deep earthquake faulting in subducting lithosphere inclined zones of earthquakes are the primary expression of lithosphere subduction a distinct deep population of subduction zone earthquakes occurs at depths of 350 to 690 kilometers at those depths ordinary brittle fracture and frictional sliding the faulting processes of shallow earthquakes are not expected a fresh understanding of these deep earthquakes comes from developments in several areas of experimental and theoretical geophysics including the discovery and characterization of transformational faulting a shear instability connected with localized phase transformations under nonhydrostatic stress these developments support the hypothesis that deep earthquakes represent transformational faulting in a wedge of olivine rich peridotite that is likely to persist metastably in coldest plate interiors to depths as great as 690 km predictions based on this deep structure of mantle phase changes are consistent with the global depth distribution of deep earthquakes the maximum depths of earthquakes in individual subductions zones and key source characteristics of deep events

a new self organizing mechanism for deep focus earthquakes the mechanism of deep focus earthquakes has been a puzzle since their discovery almost 70 years ago1 2 because brittle fracture and frictional sliding at depths in excess of 100 200 km would require unrealistic rock strengths3 4 rock strength does increase with pressure but a few hundred mpa equivalent to 10 20 km depth suffices to inhibit most fracture and elevated temperature activates ductile mechanisms that operate at stresses less than the fracture stength a range of mechanisms has been proposed for deep earthquakes including plastic instabilities5 7 shear induced melting8 11 and instabilities accompanying recrystallization12 13 or polymorphic phase transformation14 23 each of these proposed mechanisms has exhibited certain inherent weaknesses see kirby22 for review and discussion here we report experimental observations of high pressure faulting of metastable mg2geo4 olivine as it undergoes incipient transformation to a spinel 24 25 structure we present a model in which this faulting arises from the generation propagation and linking up of spinel filled anticracks26 when applied to the olivine spinel transformation in the earth s mantle the anticrack model27 satisfactorily accounts for the similarities and differences between shallow and deep earthquakes and removes the problems associated with frictional sliding

rupture characteristics of the deep bolivian earthquake of 9 june 1994 and the mechanism of deep focus earthquakes the m w 8 3 deep 636 kilometers bolivian earthquake of 9 june 1994 was the largest deep focus earthquake ever recorded seismic data from permanent stations plus portable instruments in south america show that rupture occurred on a horizontal plane and extended at least 30 by 50 kilometers rupture proceeded at 1 to 3 kilometers per second along the down dip azimuth of the slab and penetrated through more than a third of the slab thickness this extent is more than three times that expected for a metastable wedge of olivine at the core of the slab and thus appears to be incompatible with an origin by transformational faulting these large events may instead represent slip on preserved zones of weakness established in oceanic lithosphere at the earth s surface

systematic analysis of deep seismicity 200 centroid moment tensor solutions for earthquakes between 1977 and 1980 summary a systematic study of deep seismicity is performed using waveform data from the gdsn between 1977 and 1980 and the centroid moment tensor method of dziewonski et al two hundred new cmt solutions are found for events deeper than 100km their moments obtained using the same algorithm range between 7 x and 1 4 x lo2 dyne cm a factor of 20 000 a statistical analysis of the cmt parameters provides important conclusions among these the shift in origin time for deep events is found to be almost independent of the seismic moment providing an indication of the complexity of deep earthquakes the deviation from the double couple model appears to be correlated to the state of stress of the subducting slabs mechanisms showing down dip compression are generally dominated by the eigenvalue corresponding to the compression axis and down dip tension correlates with the predominance of the tensional eigenvalue comparison with moment tensors obtained by other authors using different waveform data shows good agreement between different methods comparison with published focal plane mechanisms confirms the precision and reliability of the cmt method examples of cmt solutions used to identify patterns of seismicity associated with deep fault planes are shown

geomorphology and ecology unifying themes for complex systems in biogeomorphology the interaction of geomorphic and ecologic landscape components has been largely conceptualized as independent in one direction geomorphic processes and landforms shape the distribution of biota conversely in the other direction biota modify geomorphic processes and landforms increasingly the interactions between geomorphic and ecological components are more circular and developmentally intertwined in this paper i integrate these two independent perspectives within the framework of complexity theory i outline four themes that characterize complex systems in biogeomorphology multiple causality and the concept of recursivity the influence of organisms that function as ecosystem engineers the expression of an ecological topology and ecological memory implicit in all of these themes is the recognition that biogeomorphic systems are open and path dependent they may exhibit a range of assembly states from self reinforcing stability domains to more transient configurations of organisms and environment

intermediate and deep earthquakes in spain recent improvements in the seismological networks on the ibero maghrebian region have permitted estimation of hypocentral location and focal mechanisms for earthquakes which occurred at south spain alboran sea and northern morocco of deep and intermediate depth with magnitudes between 3 5 and 4 5 intermediate depth shocks range from 60 to 100 km with greater concentration located between granada and malaga fault plane solutions of 5 intermediate shocks have been determined they present a vertical plane in ne sw or e w direction seismic moments of about 1015 nm and dimensions of about 1 km have been determined from digital records of spanish stations p wave forms are complex this may be explained by the crustal structure near the station discontinuities in the upper mantle and inhomogeneities near the source deep activity at about 650 km has only 3 shocks since 1954 1954 1973 1990 shocks are located at a very small region fault plane solutions show a consistent direction of the pressure axis dipping 45 in e direction for the 1990 shock seismic moment is 1016 nm and dimensions 2 6 km thep waves are of simpler form with a single pulse the intermediate and deep activities are not connected and no activity has been detected between 100 and 650 km the intermediate shocks may be explained in terms of a recent subduction from africa under iberia in se direction the very deep activity must be related to a sunk detached block of lithospheric material still sufficiently cold and rigid to generate earthquakes

imaging of subducted lithosphere beneath south america tomographic images are produced for the deep structure of the andean subduction zone beneath western south america the data used in the imaging are the delay times of p pp and pwp phases from relocated teleseismic earthquakes in the region regionally structural features larger than about 150 km are resolved by the data presentations of layer anomaly maps and cross sections reveal 1 the nazca slab is probably continuous laterally and at depth over most regions 2 the offset between the north and south deep earthquake zones containing the 1994 deep bolivia main shock and its aftershocks can be modelled by a northwest striking and steeply northeast dipping slab structure and 3 the nazca slab clearly penetrates the lower mantle beneath central south america but is partly deflected in the southern deep zone

seafloor spreading in the north fiji basin southwest pacific abstract the seapso leg 3 cruise december 1985 was carried out in the north fiji basin aboard the ifremer r v jean charcot the main purpose of the cruise was to carry out a geophysical and geological study of the central part of the basin between 16 s and 22 s to this end bathymetric profiles with the help of the seabeam multichannel echosounder single channel seismic reflection profiles and magnetic and gravimetric profiles were made moreover geological samples were taken by dredging and piston coring as well as water samples with the help of a multiprobe one of the aims of the cruise was to explore the present day spreading axis of the north fiji basin we were able to show that accretion the rate of which is about 7 cm yr in the north fiji basin has been constrained in a n s direction for 3 ma very recently between 0 6 and 0 7 ma ago the accretion process changed resulting in the creation of a ridge ridge trench triple junction centered at 173 30 e and 16 40 s water samples taken in the vicinity of this triple junction show significant anomalies of methane and manganese which are probably linked to hydrothermal events in its morphological characteristics transversal as well as longitudinal the ridge of the north fiji basin shows striking similarities to the east pacific rise it presents the same differences in level the same sizes of structures and the same particular features overlapping spreading centers offsets etc another aim of the seapso cruise was to study the complex deformation area located immediately to the west of the fiji islands which we interpret as a distensive type deformation in a strike slip system

the deep seismicity of the tyrrhenian sea summary l he study reappraises the deep seismicity of the tyrrhenian sea careful examination of the quality of reported hypocentres shows that the earthquakes define a zone dipping nw about 200 km along strike 50 km thick and reaching a depth of about 500 km the zone is slightly concave to the nw at a depth of 300 km but contrary to many previous reports is not tightly concave nor are there significant spatial gaps in the seismicity which is effectively continuous with depth seismicity is however concentrated in the depth interval 250 300 km where the dip of the seismic zone changes from 70 above 250 km to a more gentle dip of 45 at greater depths seven fault plane solutions are available for the largest earthquakes in this depth interval all of them consistent with a p axis down the dip of the seismic zone and all of them requiring movement on faults out of the plane of the subducting slab two deep earthquakes near naples lie well outside the main zone of activity for one of which a fault plane solution is available that has ap axis not aligned with the dip of the seismic zone the tightly concave slabgeometry favoured by other reports is supported mainly by the location of these events near naples which we think may represent deformation in a separate probably shallower dipping piece of subducted lithosphere the lack of shallow seismicity and particularly of thrust faulting earthquakes at the surface projection of the benioff zone suggests that active subduction has ceased estimates of the convergence rate responsible for subduction in the last 10 myr far exceed the present convergence rate of africa and eurasia suggesting that the subduction was related instead to the stretching and thinning of the crust in the tyrrhenian sea

the depths of the deepest deep earthquakes the maximum depth of seismic activity is a fundamental observation providing a constraint on models of mantle dynamics although most recent investigations of mantle seismicity and dynamics state that seismicity extends to about 700 km focal depths of 720 km have been reported for large earthquakes as well as depths greater than 800 km for small events we have examined focal depths of events in several catalogs and conclude that 1 the deepest reliable focal depths are at about 670 680 km and events with focal depths beneath 600 km occur in several widely separated geographic regions 2 observations supporting focal depths exceeding 680 km are usually few poor or inconsistent 3 the reduction of seismic activity beneath about 650 km is quite abrupt however we cannot determine whether activity stops completely at 670 680 km or the maximum size of events decreases gradually beneath 650 km with magnitudes of 6 5 7 0 occasionally occurring at 650 km magnitudes of 5 5 6 0 at 680 km 4 0 4 5 at 695 km etc several models can explain the absence of seismic activity in the lower mantle two models are consistent with the abrupt termination of activity at about 650 680 km in widely separated areas a barrier which resists penetration by the subducting lithosphere or a phase transition which modifies the properties of the subducted material as it goes into the lower mantle

displacement of surface monuments vertical motion several measurements of vertical ground motion at pinon flat observatory california indicate the overall weakness and instability of the earth s weathered surface with respect to the underlying rock cumulative long period motions of order 0 5 mm per year dominate these records though smaller elastic deformations caused by precipitation loading atmospheric loading and tidal strains are evident at higher frequencies all of these help to characterize the near surface material the long period records suggest that near surface weathering is the dominant influence on monument motion at least at this site on crystalline rock in a semi arid environment rainfall loading gives an average vertical modulus of 2 6 gpa for the material in the uppermost 26 m of the ground compared with 88 gpa for granite under moderate confining stress atmospheric loading gives similar results but indicates the ground is permeable to airflow at periods longer than a few hours earth tide records show poisson s ratio to be 0 09 in contrast to the normative range of 0 2 0 25 establishing that horizontal strains couple only weakly into vertical ones so that vertical strains near the surface are a poor measure of areal strain the form of the ground surface displacement power spectrum indicates that analyses of geodetic surveys would be improved with the inclusion of a monument positioning error budget that increases with time because of ground instability and the generally small rate of crustal tectonic motions deeply emplaced monuments will be needed for observational programs designed to detect short term changes in crustal deformation over baselengths of order 10 km and less

morphology of the valu fa spreading ridge in the southern lau basin multibeam echosounder surveys and seismic reflection profiling were conducted in the southern lau back arc basin sw pacific the bathymetric survey covered a 130 km long section of the active spreading ridge 3 cm yr half spreading rate the n s trending valu fa ridge vfr while seismic profiles concentrated on a 35 km long ridge section analysis of these data shows a morphologically segmented ridge which is underlain by a continuous magma chamber the ridge shows similarities but also differences to the segmentation model developed for mid ocean ridges analogous to this model the section of the vfr studied displays third and fourth order segmentation the amplitudes of the along axis undulations however are a factor of 5 to 10 greater than at mid ocean ridges additionally the vfr has numerous small volcanic cones along the axis which have previously only been observed at much slower spreading ridges in cross section the vfr has a very steep narrow triangular shape which is thought to result from the high viscosity of its andesitic lavas the axial magma chamber appears to vary in width 1 4 km and location relative to the axis beneath different morphological segments perhaps the most significant deduction from our combined data is that the magma chamber is widest beneath the most prominent axial discontinuity a small overlapping spreading center osc this is counter to observations from mid ocean ridges off axis traces of ridge segments allow us to reconstruct the evolution of this osc for the past 220 kyr we conclude that this back arc spreading axis shows a segmentation pattern which is similar to mid ocean ridges but it is influenced and modified by the geochemistry of its lavas and its proximity to the island arc

s p conversion from the transition zone beneath tonga and the nature of the 670 km discontinuity summary using more than 300 recordings of deep tonga earthquakes from the warramunga seismic array central australia we have searched for short period s p conversions from the 670 km discontinuity arriving in the p wave codas the array seismograms were stacked and plotted in sections to optimize observation of s670p yielding the following results for the northern part of tonga 17 3 18 4 s s670p phases are observed with apparent conversion depths between about 660 and 680 km for the central part 20 2 22 3 s s670p is not clearly observed for the southern part 23 1 25 7 s s670p is observed for conversion depths between about 660 and 700 km intermittent occurrence of s p conversion may result from different characteristic source radiation patterns among the three sections overall uncertainties in conversion depths resulting from unmodelled lateral velocity variations and hypocentral location errors are perhaps a few tens of kilometres the s p rays must travel either within or very close to the subducted slab because of the source receiver slab dip geometry so we conclude that large 50 km deformations of the 670 km discontinuity do not occur beneath tonga our observations appear to be inconsistent with a chemical discontinuity at 670 km but are easily explained if the 670 km discontinuity is a strongly pressure dependent and perhaps moderately endothermic phase change in both the slab and surrounding mantle

global variability in subduction thrust zone forearc systems deviations of slip vector azimuths of interplate thrust earthquakes from expected plate convergence directions at oblique subduction zones provide kinematic information about the deformation of forearcs and indirect evidence on the dynamics of the plate boundary a global survey of slip vectors at major trenches of the world reveals a large variability in the kinematic response of forearcs to shear produced by oblique convergence the variability in forearc deformation inferred from slip vector deflections is suggested to be caused by variations in forearc rheology rather than in the stresses acting on subduction zone thrust faults estimated apparent macroscopic rheologies range from elastic to perfectly plastic or viscous forearc rheologies inferred from slip vectors do not correlate with age of the subducting lithosphere but continental forearcs or old arcs appear to deform less than oceanic or young arcs the inferred absence of forearc deformation at continental arcs from this study is counter to inferences drawn from compiled geologic information on forearc faults correlations of the apparent forearc rheology with backarc spreading convergence rate slab dip arc curvature and downdip length of the thrust contact are poor however great subduction zone earthquakes occur where forearcs are apparently more elastic i e less deformed by oblique convergence which suggests that the mechanical properties of forearcs rather than stress magnitude on thrust faults control both the kinematic behavior of forearcs and where great subduction zone earthquakes occur

role of ridge jumps and ridge propagation in the tectonic evolution of the lau back arc basin southwest pacific gloria sidescan imagery of the central lau basin reveals a complex neotectonic framework in which the central lau spreading center is actively propagating southward at the expense of the eastern lau spreading center the latter is located about 75 km closer to the active tofua arc and the transfer of spreading motion across this distance is achieved via a relay spreading ridge segment new magnetic and bathymetric interpretations suggest that the majority of the central and southern basin was formed by the eastern lau spreading center between 5 ma and the present day the processes of arcward ridge migration ridge propagation and basinward arc migration may reflect the complex interplay of the subduction zone and regional tectonics in driving back arc spreading and may be a common feature of this tectonic environment

reply to comment by t camelbeeck et al on nature of the recent vertical ground movements inferred from high precision leveling data in an intraplate setting ne ardenne belgium

intraplate seismicity of the pacific basin 1913 1988 we establish here a comprehensive database of intraplate seismicity in the pacific basin relocation and analysis of 894 earthquakes yield 403 reliable intraplate earthquakes during 1913 1988 these numbers do not include earthquake swarms which account for another 838 events most of the remainder 304 events are actually plate boundary earthquakes that have been erroneously located in intraplate regions a significant number occur in recent years when location capabilities should have guarded against this situation relocations involve a careful linear inversion of p and s arrivals accompanied by a monte carlo statistical analysis we have also attentively removed the high number of clerical errors and nuclear tests that exist in epicenter bulletins

localized polymorphic phase transformations in high pressure faults and applications to the physical mechanism of deep earthquakes earthquake mechanisms based on frictional instabilities are widely accepted for relatively shallow earthquakes such mechanisms in unmodified form are highly unlikely for deep earthquakes because of the expected high frictional shear stresses at depths of 300 700 km several alternative mechanisms have been proposed here i discuss a new type of shear instability observed in experiments on ordinary polycrystalline ice and polycrystalline tremolite by my colleagues and me these shear instabilities lead to rapid stress drops and sound emissions and have the following characteristics 1 a single sharp shear zone forms in the maximum shear stress orientation 2 confining pressure does not suppress the instability and under some conditions raising confining pressure pc lowers the differential stress at which it occurs 3 stress drops can occur repeatedly after reloading at about the same differential stress 4 the mean stress vs temperature conditions at which the instabilities occur in ice and tremolite place them in the fields of thermodynamic stability of ice ii and diopside talc respectively although i have not as yet confirmed the existence of these high pressure phases in the shear zones transformation in the shear zone is implicated because such shear instabilities do not occur in similar materials in which polymorphic phase changes do not occur reconstructive polymorphic phase changes localized along shear zones may be a general phenomenon in crystalline materials stressed non hydrostatically near transition boundaries moreover by analogy the reconstructive phase transformations that are thought to occur in deeply subducted lithosphere may lead to localized shear instabilities manifested by deep earthquakes and produce the observed double couple first motion distribution finally if this is the earthquake source mechanism in deeply subducted lithosphere then earthquakes should shut off when all of the polymorphic phase changes have occurred this may explain the maximum depth of earthquakes at about 680 km

deep earthquakes in the southwest pacific a tectonic interpretation this paper examines the spatial distribution of deep earthquakes in the tonga fiji new hebrides region of the southwest pacific our interpretation emphasizes the complex cenozoic tectonics of the pacific indo australian plate boundary as a primary control on the distribution and deformation of subducted lithosphere most deep earthquakes in the interarc region are associated with the contorted pacific plate lithosphere subducted at the tonga trench however anomalous groups of deep earthquakes located west of the tonga zone are unrelated to the present plate configuration tectonic reconstructions of the region to 8 m y b p provide circumstantial evidence that 1 the anomalous events west of the tonga zone occur in two pieces of detached lithosphere subducted at the vitiaz and proto new hebrides trenches during the late miocene 2 the flattening of the inclined seismic zone in northernmost tonga is related to the rapid opening of the lau basin since 4 m y b p and 3 the sharp westward curvature of the tonga seismic zone in this area coincides with a preexisting bend in the late miocene vitiaz arc the sharpness of its present curvature appears to be a secondary effect of shear flow in the lower mantle and compression between detached vitiaz and attached tonga lithosphere thus much of the contortion of the subducted lithosphere beneath tonga fiji may be produced by local tectonic interactions rather than collision of the slab with an impenetrable boundary in the midmantle

effects of relative plate motion on the deep structure and penetration depth of slabs below the izu bonin and mariana island arcs an increasing number of seismological studies indicate that slabs of subducted lithosphere penetrate the earth s lower mantle below some island arcs but are deflected or rather laid down in the transition zone below others recent numerical simulations of mantle flow also advocate a hybrid form of mantle convection with intermittent layering we present a multi disciplinary analysis of slab morphology and mantle dynamics in which we account explicitly for the history of subduction below specific island arcs in an attempt to understand what controls lateral variations in slab morphology and penetration depth central in our discussion are the izu bonin and mariana subduction zones we argue that the differences in the tectonic evolution of these subduction zones in particular the amount and rate of trench migration can explain why the slab of subducted oceanic lithosphere seems to be at least temporarily stagnant in the earth s transition zone below the izu bonin arc but penetrates into the lower mantle below the mariana arc we briefly speculate on the applicability of our model of the temporal and spatial evolution of slab morphology to other subduction zones although further investigation is necessary our tentative model shows the potential for interpreting seismic images of slab structure by accounting for the plate tectonic history of the subduction zones involved we therefore hope that the ideas outlined here will stimulate and direct new research initiatives

deep seismicity and modes of deformation in tonga subduction zone the geometry of deep 400 km earthquakes in the subducting western edge of the pacific plate when related to the local distribution of seismic activity reveals a complex pattern of planes of shear failure enabling the modes of deformation by which the descending plate shortens and thickens to be identified the deepest part of the slab is displaced laterally suggesting that it fails to penetrate the discontinuity at 670 km

isolated deep earthquakes and the fate of subduction in the mantle the locations of earthquakes with depth in the mantle provide direct evidence of the trajectory and state of stress of subducting lithosphere the occurrence over the last decade of isolated large deep earthquakes outside the main wadati benioff zones in the kurile izu bonin and chile deep seismic areas in addition to the observed flat distributions of deep seismicity beneath the banda sea and fiji plateau areas and the large number of deep earthquakes located away from the main slab in the tonga arc supports recent tomographic and other seismological studies leading to a reconsideration of slabs as simple planar structures sinking undeformed into the lower mantle the focal geometries of these isolated earthquakes which often exhibit near vertical compressional stresses are distinct from adjacent events in the main wadati benioff zone they are also different in orientation from events in the deflecting toe of the slab and from the stress orientation expected if the deflected slab acts as a stress guide these isolated mantle earthquakes are interpreted to take place in subducted lithosphere that has deflected to a horizontal posture at the base of the upper mantle conversely the deep seismicity in the adjacent wadati benioff zone is associated with the deflection of the subducting lithosphere

restoration of floodplain topography by sand splay complex formation in response to intentional levee breaches lower cosumnes river california restoration of sustainable geomorphic processes that create floodplain topography through development of sand splay complexes at intentional breaches is one method to promote variability in physical structure needed for habitat restoration the topography of splay complexes provides a range of floodplain elevations that creates local variability in i inundation duration and frequency and depth to ground water that influence riparian vegetation establishment and ii flow depth and velocity that create refuge for fish two intentional levee breaches along the lowland cosumnes river central valley ca were evaluated during water years 1999 and 2000 in order to document changes in morphology and relief associated with deposition of sandsplay complexes during the study period annual peak flow recurrence intervals ranged from 1 to 3 years and water flowed through the breaches for a minimum of 55 days during water year 1999 and 53 days during water year 2000 at the two study sites rapid vertical accretion and scour occurred within the first several years after intentionally breaching the levee at the accidental forest floodplain constructed in 1995 and at the corps breach floodplain constructed in 1997 splay complexes are organized into a variety of landforms including lateral levees and lobes separated by new floodplain channels maximum deposition measured on the splay surface is 0 36 m year while maximum scour in channels is 0 27 m year juxtaposition of floodplain splay deposition and adjacent channel scour creates relief ranging from 1 6 to 0 25 m that decreases with distance from the breach and that becomes more pronounced over time as higher magnitude floods scour channels in the old floodplain sediment and deposit new sand and silt onto the surface of the splay the ratio of splay complex height to depth of formative flow is estimated as 0 4 progradation of main and secondary splay channels takes place by down floodplain sand transport 25 m year maximum large wood recruited onto the floodplain through the breach promotes local scour and deposition that enhances topographic variability at one of the study sites initial grading of a low setback berm prior to opening the breach forced a change in floodplain flow direction and the geometry of the splay complex additionally progradation of the complex is arrested by an excavated pond that creates a sediment trap we present a conceptual model that describes the importance of floods in constructing and modifying sand splay complexes that create floodplain topography the potential habitat variability created as floodplain topography evolves is the linkage between physical and ecological processes that are critical for restoration d 2002 elsevier science b v all rights reserved

structure and evolution of lithospheric slab beneath the sunda arc indonesia tomographic imaging reveals seismic anomalies beneath the sunda island arc indonesia that suggest that the lithospheric slab penetrates to a depth of at least 1500 kilometers the sunda slab forms the eastern end of a deep anomaly associated with the past subduction of the plate underlying the mesozoic tethys ocean in accord with previous studies the lithospheric slab was imaged as a continuous feature from the surface to the lower mantle below java with a local deflection where the slab continues into the lower mantle the deep slab seems to be detached from the upper mantle slab beneath sumatra this complex slab structure is related to the tertiary evolution of southeastern asia and the indian ocean region

tomographic inversion of p and pp data for aspherical mantle structure below the northwest pacific region summary to investigate the morphology of subducted slab in the mantle below northwest pacific island arcs we inverted traveltime residuals for aspherical variations in p wave propagation velocity relative to the radially symmetric iasp91 reference model the tomographic method used is based on a step wise linearization of the inversion problem first we relocated isc international seismological centre hypocentres with re identified p and pp phase data using the iasp91 traveltime tables the variance of p residuals relative to iasp91 traveltimes was 17 per cent less than the variance of p data reported by the isc relative to the jeffreys bullen j b traveltime tables second we performed a linearized lsqr inversion for earth structure and source relocation with the p and pp residuals obtained from the first step using iasp91 as the reference model for seismic velocities the incorporation of the depth phase pp in the tomographic inversions has two major advantages 1 the pp data provide constraints on focal depth and thus reduce the trade off between source relocation and structure and 2 the pp ray paths improve the sampling of earth structure in the shallow mantle and transition zone we used more than 2 times 106 and about 1 times 105p and pp wave traveltime residuals respectively from about 40 000 earthquakes with epicentres in the study region that were recorded at one or more of the 2300 globally distributed seismological stations considered in this study we assessed the spatial resolution in the tomographic images with checker board type sensitivity tests these tests reveal high resolution of upper mantle and transition zone structure particularly below the central part of our study region structure with wavelengths of the order of 100 km is resolved below japan whereas structure with wavelengths of the order of 300 km is well resolved below the kuril izu bonin and ryukyu arcs small scale structure is poorly resolved in depth below the northern part of the kuril kamchatka arc and below the izu bonin and mariana arcs this limits the interpretation of slab structure and mantle flow from tomographic images alone with this limitation in mind we conclude from the tomographic images that subducted slab deflects in the mantle transition zone below the geographical area encompassed by the kuril basin the japan sea and the northern part of the philippine sea this is in good agreement with the results of other recently published tomographic studies the occurrence of earthquakes several hundred kilometres off the inclined wadati benioff seismic zones and inferences about 660 km discontinuity topography in contrast slab like structures of high p wave velocity are imaged in the lower mantle below the deepest earthquakes of the northern kuril kamchatka and mariana seismic zones this is indicative of local slab penetration of the lower mantle from tomographic images we cannot discern between compositionally or thermally induced variations in seismic velocity however with regard to the nature of the boundary between upper and lower mantle our observations argue against either compositional mantle layering with large contrasts in intrinsic density or phase changes with steep clapeyron slopes

deep earthquakes beneath the fiji basin sw pacific earth s most intense deep seismicity in stagnant slabs abstract previous work has suggested that many of the deep earthquakes beneath the fiji basin occur in slab material that has been detached and foundered to the bottom of the transition zone or has been laid down by trench migration in a similar recumbent position since nowhere else in the earth do so many earthquakes occur in slabs stagnated in the transition zone these earthquakes merit closer study accordingly we have assembled from historical and modern data a comprehensive catalogue of the relocated hypocenters and focal mechanisms of well located deep events in the geographic area between the bottoms of the main vanuatu and tonga wadati benioff zones two regions of deep seismogenesis are recognized there i 163 deep shocks have occurred north of 15 s in the vityaz group from 1949 to 1996 these seismological observations and the absence of other features characteristic of active subduction suggest that the vityaz group represents deep failure in a detached slab that has foundered to a horizontal orientation near the bottom of the transition zone ii another group of nearly 50 outboard deep shocks occur between about 450 and 660 km depth west of the complexly buckled and offset western edge of the tonga wadati benioff zone their geometry is in the form of two or possibly three small circle arcs that roughly parallel the inferred motion of tonga trench migration earthquakes in the southernmost of these arcs occur in a recumbent high seismic wavespeed slab anomaly that connects both to the main inclined tonga anomaly to the east and a lower mantle anomaly to the west van der hilst r 1995 complex morphology of subducted lithosphere in the mantle beneath the tonga trench nature vol 374 pp 154 157 both groups show complexity in their focal mechanisms the major question raised by these observations is the cause of this apparent temporary arrest in the descent of the tonga slab into the lower mantle we approach these questions by considering the effects of buoyant metastable peridotite in cold slab material that was detached and rapidly foundered or was buckled segmented and laid out in the transition zone

quaternary tectonics in the northern ardennes belgium abstract a number of nw se or nnw sse trending radial faults are recognized on the northern slope of the ardenne massif they have been active since the oligocene as depicted by deformations of planation surfaces and other morphological features the study of river terraces demonstrates their quaternary activity with vertical movements ranging from 4 to 10 m during the late quaternary this fault activity is related to the opening of the lower rhine embayment

calibration of a superconducting gravimeter by comparison with an absolute gravimeter fg5 in boulder this paper reports the results of a calibration of a superconducting gravimeter using simultaneous measurements of earth tides with an absolute gravimeter nine consecutive days of observations were made with both the c024 superconducting gravimeter and the fg5 202 absolute gravimeter at the noaa table mountain gravity observatory near boulder colorado the precision of the calibration factor is better than 0 1 the calibration factor obtained in this fashion agrees well with that obtained from a moving platform this experiment provides a noise estimate for both the superconducting and absolute gravimeter in addition the local air pressure admittances of both instruments compare well with a value close to 0 35 gal mbar

microearthquakes and tectonics in an active back arc basin the lau basin abstract an ocean bottom seismograph obs array was deployed for 20 22 days in late 1984 to investigate the precise locations of microearthquakes and their tectonic implications for active back arc opening in the northern lau basin using p and s wave arrival times from four or more obss the hypocenters of 300 shallow earthquakes were located with a high confidence level the magnitudes of most obs located earthquakes were estimated to be less than four in the northern half of the survey area a narrow linear zone of microearthquakes trending nnw sse has been identified the northern part of the narrow seismic zone is within a central axial depression at the southern end of the peggy ridge further south the trend of the seismic zone becomes more n s the narrow seismic zone seems to be composed of at least six seismic segments offset by short aseismic zones most of the seismic segments trend nnw sse suggesting a system of left stepping en echelon spreading ridges where the spreading ridge segment is seismically inactive and the transform fault is active the spreading ridges appear to strike n s or nnw sse but the direction of the back arc opening is considered to be nw se no hypocenters were located with a high level of precision in the area south of latitude 18 s except a small isolated zone of shallow earthquakes at the southeastern part of the survey area we suggest that the shallow earthquakes in this isolated seismic zone were intraplate events in the tonga platelet this platelet is separated from the major indo australian plate by the back arc opening system in the lau basin

subduction zone earthquakes and stress in slabs the pattern of seismicity as a function of depth in the world and the orientation of stress axes of deep and intermediate earthquakes are explained using viscous fluid models of subducting slabs with a barrier in the mantle at 670 km 670 km is the depth of a seismic discontinuity and also the depth below which earthquakes do not occur the barrier in the models can be a viscosity increase of an order of magnitude or more or a chemical discontinuity where vertical velocity is zero longn versus depth wheren is the number of earthquakes shows 1 a linear decrease to about 250 300 km depth 2 a minimum near that depth and 3 an increase thereafter stress magnitude in a subducting slab versus depth for a wide variety of models shows the same pattern since there is some experimental evidence thatn is proportional toe where is a constant and is the stress magnitude the agreement is encouraging in addition the models predict down dip compression in the slab at depths below 400 km this has been observed in earlier studies of earthquake stress axes and we have confirmed it via a survey of events occurring since 1977 which have been analysed by moment tensor inversion at intermediate depths the models predict an approximate but not precise state of down dip tension when the slab is dipping the observations do not show an unambiguous state of down dip tension at intermediate depths but in the majority of regions the state of stress is decidedly closer to down dip tension than it is to down dip compression chemical discontinuities above 670 km or phase transitions with an elevation of the boundary in the slab predict when incorporated into the models stress peaks which are not mirrored in the profile of seismicity versus depth models with an asthenosphere and mesosphere of appropriate viscosity can not only explain the state of stress observed in double benioff zones but also yield stress magnitude profiles consistent with observed seismicity models where a nonlinear rheology is used are qualitatively consistent with the linear models

frequency moment distribution of deep earthquakes implications for the seismogenic zone at the bottom of slabs abstract we present a systematic investigation of the variation with depth of the frequency of earthquake occurrence vs seismic moment based on 16 years of harvard centroid moment tensor cmt solutions we analyze depth variations of earthquake size distribution in terms of variations in the absolute value of the slope of the regression of the logarithm of the population vs seismic moment a quantity known as the parameter the shallowest earthquakes 0 50 km depth exhibit a well defined and robust size distribution regime characterized by a discontinuous increase in with increasing moment others have shown that this increase probably represents the effects of a physical limit in the dimensions of the area of seismogenic slip of shallow earthquake sources the population of deep earthquakes in the depth interval 500 600 km shows two markedly different distributions the deep earthquakes in the tonga region feature an initially high value 0 92 at small moments and a lower value 0 41 at high moments in contrast the size distribution of non tonga deep events shows the reverse of those changes 0 41 at low moment and 1 17 at higher moment to help explain these observations we propose a model of deep seismogenesis that assumes three dimensional earthquake source regions that vary principally in their transverse dimensions the two segment behavior in the tonga region and other subduction zones is thought to represent in part constraints owing to the threshold of completeness of the cmt catalog and to its short time interval of sampling we interpret the differences between tonga and other deep wadati benioff zones as being a consequence of tonga s markedly higher subduction rate and hence its colder thermal structure and presumably thicker region of seismogenesis we interpret the critical moments at which values change in terms of variations in the transverse thickness of deep seismogenic zones and estimate that it is about 11 km for the tonga region and about 3 km for other zones at depths of 500 600 km these results are generally consistent with deep earthquakes being restricted to wedge shaped regions of peridotite persisting metastably to as deep as 700 km in old rapidly descending and hence cold slabs failure is thought to occur in metastable peridotite by transformational faulting great deep earthquakes present special challenges to any theory of deep earthquakes based on slab thermal structure for example a continuing question is how such large events can fit in a thermally controlled seismogenic zone that is diminishing in its transverse dimensions with increasing depth the very concept of a scale invariant earthquake size distribution may be inappropriate for these rare events and the unusual settings in which they are found

modelling the flooding capacity of a polish carpathian river a comparison of constrained and free channel conditions abstract the gravel bed biala river polish carpathians was heavily affected by channelization and channel incision in the twentieth century not only were these impacts detrimental to the ecological state of the river but they also adversely modified the conditions of floodwater retention and flood wave passage therefore a few years ago an erodible corridor was delimited in two sections of the biala to enable restoration of the river in these sections short channelized reaches located in the vicinity of bridges alternate with longer unmanaged channel reaches which either avoided channelization or in which the channel has widened after the channelization scheme ceased to be maintained effects of these alternating channel morphologies on the conditions for flood flows were investigated in a study of 10 pairs of neighbouring river cross sections with constrained and freely developed morphology discharges of particular recurrence intervals were determined for each cross section using an empirical formula the morphology of the cross sections together with data about channel slope and roughness of particular parts of the cross sections were used as input data to the hydraulic modelling performed with the one dimensional steady flow hec ras software the results indicated that freely developed cross sections usually with multithread morphology are typified by significantly lower water depth but larger width and cross sectional flow area at particular discharges than single thread channelized cross sections they also exhibit significantly lower average flow velocity unit stream power and bed shear stress the pattern of differences in the hydraulic parameters of flood flows apparent between the two types of river cross sections varies with the discharges of different frequency and the contrasts in hydraulic parameters between unmanaged and channelized cross sections are most pronounced at low frequency high magnitude floods however because of the deep incision of the river both cross section types are typified by a similar low potential for the retention of floodwater in floodplain areas the study indicated that even though river restoration has only begun here it already brings beneficial effects for flood risk management reducing flow energy and shear forces exerted on the bed and banks of the channel in unmanaged river reaches only within wide unmanaged channel reaches can the flows of low frequency high magnitude floods be conveyed with relatively low shear forces exerted on the channel boundary in contrast in channelized reaches flow velocity and shear forces are substantially higher inevitably causing bank erosion and channel incision

t waves from the great 1994 bolivian deep earthquake in relation to channeling of s wave energy up the slab we report the observation of t phases recorded at a number of pacific island sites following the great bolivian deep earthquake of june 9 1994 by studying their arrival times at the various stations we unravel the process of conversion from seismic to acoustic energy at the south american coastline we find that the maxima of amplitude of the phase are incompatible with propagation along the great circle from source to receiver but rather require the generation of the acoustic wave at a common point at the arica bight for all stations in the south pacific and even farther south for stations in hawaii and the bonin islands the timing of these conversions corresponds to the arrival of regional s waves at the conversion points the high frequency nature of the t phase requires the channeling of the s wave from the seismic source to conversion point through low attenuation material which in turns adds supportive evidence for the mechanical continuity of the downgoing slab under that section of south america despite the observed gap in seismicity between 300 and 600 km

nature rehabilitation by floodplain excavation the hydraulic effect of 16 years of sedimentation and vegetation succession along the waal river nl abstract the ewijkse plaat is a floodplain along the waal river nl in 1988 the floodplain was excavated as part of a program for enlargement of the discharge capacity and was assigned as a nature rehabilitation area this paper describes the combined geomorphological and vegetation evolution of the floodplain until 16 years after the initial excavation using elevation data and data on vegetation structure derived from detailed aerial stereographic imagery the impact of these processes on flow velocity and water surface elevation was evaluated by using a hydraulic model within 16 years the excavated amount of sediment was redeposited in the area the dominant geomorphological process after excavation was vertical accretion of the floodplain which resulted in the formation of natural levees the amount of sedimentation was correlated to the across floodplain flow r 2 0 89 in the research period 41 of the sedimentation took place during two single major flood events the creation of pioneer stages by excavation promoted softwood forest establishment which influenced the sedimentation pattern significantly the landscape evolved toward structure rich vegetation nine years after excavation the initial hydraulic gain was lost by the combined effect of sedimentation and vegetation succession implications for river and nature management are discussed

indication of the uplift of the ardenne in long term gravity variations in membach belgium summary we report on the results of 7 yr of collocated gravity observations made with an fg5 absolute ag gravimeter and a gwr c series superconducting gravimeter sg located at the membach geophysical station in eastern belgium the sg gravity residuals track changes in gravity periodically observed by the ag at the microgal level further in the sg residual signal we distinguish a quasi seasonal term that can be mostly explained by variations in local water storage effects in the ag time series we observe a small trend in the gravity of 0 6 0 1 gal yr 1 perhaps indicating that the membach station is being displaced upwards by about 3 0 mm yr 1 an uplift of the region is confirmed by global positioning system gps measurements performed 3 km away we are able to explain the features in the gravity time series in terms of water storage variability post glacial rebound and tectonic activity

magnetic anomalies and tectonic fabric of marginal basins north of new zealand detailed airborne magnetic studies conducted over the region of the s w pacific marginal basins extending from the solomon islands to new zealand suggest that three major phases of basin formation and island arc development have occurred in this region development of the tasman sea took place during the late cretaceous paleocene development of the basins to the east of the tasman sea occurred predominantly during the oligocene as well as during the upper miocene to recent the south fiji basin consisting of the kupe and minerva abyssal plains is marked by the presence of possibly two rrr triple junction spreading centers that were active between the times of anomalies 13 to 7 36 25 5 m y the kupe abyssal plain shows the presence of residual magnetic anomalies 7 to 13 of the eastern limb of the proposed spreading center the western limb appears to have been subducted beneath the present site of the three kings rise this seafloor spreading phase calculated half spreading rate of 35 mm yr was coincident with the overthrusting phase of the new caledonia ultramafic rocks during that period active volcanism along the then continuous solomons new hebrides fiji lau island arc was taking place magnetic anomalies from 1 to 4 0 8 m y b p are seen to extend along a clearly defined lineation pattern over the north fiji basin analysis of the magnetic anomalies suggests that during the upper miocene at about the time of magnetic anomaly 4 8 m y the solomons lau island arc commenced breaking up through the development of an active spreading center calculated half spreading rate of 35 mm yr located between the new hebrides and fiji portions of the island arc portions of the northeastern sector of the minerva abyssal plain were subducted beneath the north fiji basin during this period between 0 8 m y b p at the time of about anomaly 3 to 2 4 5 3 5 m y the development of the north fiji basin extended into the lau basin and havre trough thus separating the tonga and lau ridges as a result a distinct series of active spreading centers developed southward along the axis of the havre trough lau basin the havre trough is marked by present seismic and submarine volcanic activity residual magnetic anomalies 1 2 0 1 8 m y b p form a symmetrical pattern around the axis of the trough extending south from the lau basin to the active taupo white island volcanic zone of new zealand phases of development of the marginal basins described in this paper may be correlated with phases of changes in the direction and rate of pacific plate motion

a reassessment of the deep fiji earthquake of 26 may 1932 abstract the 1932 deep fiji earthquake is studied from the point of view of comparing it to the recent large events in the transition zone spectral analysis and body wave modeling suggest a moment of 3 4 10 27 dyn cm within a multiplicative or divisive factor of 1 5 which makes it comparable to but not significantly larger than the great 1994 earthquake 770 km to the north its seismic moment remains well below that of the largest deep shocks 1994 in bolivia or 1970 in colombia relocation efforts show that the earthquake took place on the front edge of the wadati benioff zone in an area where the latter features a complex geometry involving significant warping the mechanism of the earthquake involves down dip compression and can be readily explained in terms of the prevailing large scale stresses in the slabs

the depth of the deepest historical earthquakes we usep ands times listed in the international seismological summary to relocate 23 historical earthquakes 1927 1963 reported as occurring at or below 670 km in all cases our relocated hypocenters are shallower than the starting depths furthermore all events converge to 691 km or less with a precision estimated at 10 km this study upholds the results of stark and frohlich who had usedpp p times of post wwssn earthquakes to constrain reliable hypocentral depths to no greater than 684 km in particular we reject rothe s claim that a 1963 event in the vicinity of new guinea occurred at a depth of more than 780 km

seismic evidence for continuity of the deep slab beneath central and eastern peru seismograms of peru brazil deep focus earthquakes recorded on the nearby carnegie broadband station at cuzco peru cus are characterized by an anomalous large amplitude p wave arrival that follows about 1 5 s after direct p the anomalous arrival is interpreted to be an underside wide angle reflection from the upper surface of the descending nazca plate somewhere in the depth range 150 400 km and in a region where the slab is wholly aseismic particle motion analysis of the cus seismograms shows that the direct p wave has arrival angles as predicted from simple ray tracing the large impulsive anomalous phase termed pr also arrives as a p wave but is of opposite polarity and about 1 5 times the amplitude of the direct p arrival the back azimuth of pr is about 20 clockwise from the path of direct p and its angle of emergence is significantly larger forward modeling calculations indicate that the seismic observations are consistent with a model in which pr originates as a p wave that travels up inside the slab from the source and is then reflected at wide angle from the steeply dipping upper plate boundary results from modeling lead to the following conclusions 1 the descending nazca plate exists in and is probably continuous through the aseismic region between 150 and 500 km depth 2 the plate is very steeply dipping 70 below depths of about 150 or 200 km and 3 the velocity discontinuity at the reflection point must be relatively sharp probably less than about 10 km in thickness

phase transition buoyancy contributions to stresses in subducting lithosphere the sequence of phase transitions undergone by minerals with increasing depth in earth s mantle is perturbed within subducting lithospheric slabs by their thermal structure such perturbation of equilibrium phase relations gives rise to relative buoyancy contrasts between slab and mantle that contribute to the state of stress within the slab while other factors contribute to overall slab stresses thermal and phase transition effects largely control the structure of the stress field within the slab the resulting maximum in down dip compressive stress within the slab corresponds to the observed peak in depth distribution of deep seismicity furthermore metastable persistence of lower pressure phases within the cold slab should give rise to localized shear stresses whose distribution corresponds to observed features of subduction zone seismicity these observations are independent of the variety of failure mechanisms proposed for deep seismogenesis

patterns of deep seismicity reflect buoyancy stresses due to phase transitions thermal perturbation of mantle phase relations in subduction zones gives rise to significant buoyancy anomalies finite element modeling of stresses arising from these anomalies reveals transition from principal tension to compression near 400 km depth down dip compression over 400 690 km peaking at 550 km and transition to rapidly fading tension below 690 km such features even when complicated by olivine metastability are consistent with observed patterns of deep seismicity that such a simple model neglecting all effects other than buoyancy anomalies due to temperature and to thermal perturbation of olivine phase relations successfully generates so many observed features of deep seismicity suggests that these buoyancy anomalies are significant contributors to the stress field in subducting slabs it also suggests that the depth distribution of deep seismicity may largely reflect the state of stress in the slab rather than simply a particular mechanism of stress release

the human role in changing river channels abstract direct consequences of the human role where human activity affects river channels through engineering works including channelization dam construction diversion and culverting have been long recognised marsh 1864 thomas 1956 the less obvious indirect effects of point and reach changes occurring downstream and throughout the basin however are much more recently appreciated dating from key contributions by strahler strahler a n 1956 the nature of induced erosion and aggradation in w l thomas ed man s role in changing the face of the earth university of chicago press chicago 621 638 wolman wolman m g 1967 a cycle of sedimentation and erosion in urban river channels geografiska annaler 49a 385 95 schumm schumm s a 1969 river metamorphosis proceedings american society of civil engineers journal hydraulics division 95 255 73 and graf graf w l 1977 the rate law in fluvial geomorphology american journal of science 277 178 191 these are complemented by effects of alterations of land use such as deforestation intensive agriculture and incidence of fire with the most extreme effects produced by building activity and urbanisation changing river channels are most evident in the channel cross section where changes of size shape and composition are now well established with up to tenfold increases or decreases illustrated by results from more than 200 world studies in addition the overall channel planform the network and the ecology have changed specific terms have become associated with changing river channels including enlargement shrinkage and metamorphosis although the scope of adjustment has been established it has not always been possible to predict what will happen in a particular location because of complex response and contingency the ways in which changes in cross section relate to reach and network changes are less clear despite investigations showing the distribution of changes along segmented channels when considering the human role in relation to changing river channels at least five challenges persist first because prediction of the nature and amount of likely change at a particular location is not certain and because the contrasting responses of humid and arid systems needs to be considered modelling is required to reduce uncertainty as was first emphasised by burkham burkham d e 1981 uncertainties resulting from changes in river form american society civil engineers proceedings journal hydraulics division 107 593 610 second feedback effects incorporated within the relationship between changes at channel reach and network scales can have considerable implications especially because changes now evident may have occurred or have been initiated under different environmental conditions third consideration of global climate change is imperative when considering channel sensitivity and responses to threshold conditions fourth channel design involving geomorphology should now be an integral part of restoration procedures this requires fifthly greater awareness of different cultures as a basis for understanding constraints imposed by legislative frameworks better understanding of the ways in which the perception of the human role in changing river channels varies with culture as well as varying over time should enhance application of design for river channel landscapes

formation of a metastable olivine wedge in a descending slab we present a thermal model of a descending slab in which the transformation of olivine to spinel is controlled by pressure and temperature dependent reaction kinetics two different formulations of the kinetics are considered with the main discriminant being the temperature range over which olivine converts to spinel at pressures of about 15 gpa about 500 515 c and 560 650 c we use a finite element method to solve the coupled heat conduction perpendicular and parallel to the dip of the slab and kinetics equations and we include the latent heat of the phase transformation latent heat release together with heat conduction parallel to the dip of the slab reduces significantly the length of the metastable olivine wedge and results in a very thin 5 km two phase region we employ the thermal parameter sin is the velocity of the descending slab is the age of the slab and is the dip of the slab to the horizontal to interpret the results for the length of the metastable wedge for values of the thermal parameter smaller than about 4000 and 7000 km depending on the model of the kinetics no metastable olivine wedge exists the critical value of 7000 km for the thermal parameter corresponds to the kinetics model with the lowest transition temperature range the length of the metastable olivine wedge is also found to be very sensitive to the model of the kinetics and to the effects of adiabatic heating if the occurrence of deep earthquakes is related to the transformation of metastable olivine to spinel then data on earthquake depth versus thermal parameter require that the onset of the reaction takes place at temperatures of about 550 575 c in this case the slab thermal parameter must be larger than 10 000 km for the metastable olivine wedge to extend down to 660 km depth but deep earthquakes occur near 660 km depth in slabs with thermal parameter as small as about 5000 km south america for example either some deep earthquakes are unrelated to olivine metastability or our knowledge of olivine spinel reaction kinetics is incomplete

spatial hydrogeomorphological influences on sediment and nutrient deposition in riparian zones observations from the garonne river france abstract this paper investigates the influence of geomorphological setting on riparian zone sedimentation within a reach of the river garonne france during three major floods the sampling design was stratified to reflect landforms constructed by fluvial processes e g floodplain lateral benches islands side channels and point bars observed sedimentation varied significantly with flood event planform context landform type and associated vegetation cover and in some cases with sample location within the landform lowest sedimentation was associated with the flood with the smallest peak discharge peak sediment concentration and sediment load sites under natural riparian vegetation experienced higher sedimentation than poplar plantations sites on concave outer banks received less sedimentation than those on convex inner banks sedimentation on floodplain sites and higher benches was lower than on low benches point bars and side channels there was considerable interdependence among these patterns reflecting the underlying geomorphological forms and processes meandering rivers tend to evolve through erosion of concave banks and deposition on convex banks point bar features tend to be built along convex banks whilst concave banks are eroded into higher floodplain and bench features as a result concave banks tend to be bordered by higher riparian margins that are less frequently flooded than convex banks where river margins are developed for agriculture the higher less frequently flooded sites are preferentially selected analyses of the quantity calibre nutrient and carbon content of the deposited sediment reveal further significant relationships which reflect the geomorphological structure of the riparian zone sediment particle size coarsens in locations with higher amounts of sedimentation the quantities of total organic carbon toc total organic nitrogen ton and total phosphorus tp all increase as the quantity of deposited sediment increases the concentration of toc and ton also increased significantly with an increase in the percentage of silt plus clay in the deposited sediments based upon the above observations a conceptual model is proposed which considers the spatial pattern in riparian zone sedimentation according to riparian morphology and flood magnitude the implications of channel incision for the functioning of the model are also discussed

historical geomorphic analysis 1932 2011 of a by passed river reach in process based restoration perspectives the old rhine downstream of the kembs diversion dam france germany the old rhine downstream of the kembs diversion dam is one of the largest by passed river reaches in the world 50 km it offers a unique opportunity to study the morphological effects of by passing and address physical and ecological restoration approaches in regulated rivers we conduct a space time analysis of channel adjustment over a period of 80 years 1932 to 2011 we examine planform changes from aerial photographs erosional and depositional patterns from vertical profiles sediment sizes within the active channel and the new established floodplain and we date riparian vegetation encroachment results show that the old rhine exhibited rapid response to the completion of the by passing scheme in the 1950s with a 26 narrowing in median active channel width between 1956 and 2008 from vegetation encroachment on dewatered channel margins mostly groyne fields the narrowing was accompanied by overbank fine sediment deposition 1 5 cm y 1 aggradation since 1950 as well as slight bed degradation 0 7 cm y 1 since 1950 we found no downstream propagation of active channel narrowing over time nor propagation of bed degradation the channel was already significantly adjusted prior to the diversion scheme following the nineteenth century river straightening and groyne construction by passing dewatering mainly provided new pioneer habitat for synchronous vegetation establishment and promoted channel stability by decreasing sediment transport owing to peak flow reduction the morphological budget calculated over the past 20 years estimated a downstream output for coarse sediments at 16 000 m3 y 1 with 80 originating from bed degradation and 20 from bank erosion without significant inputs from upstream the present day morphodynamics remain sensitive to changes because of dynamic bed armouring 2 1 this retrospective analysis permits us to discuss management strategies for altered rivers recommended activities are gravel reintroduction with particle sizes finer than the armoured river bed to enhance sediment transport and promote form dynamism under current flood magnitudes this should be coupled with river widening to allow for complex depositional patterns that benefit ecological habitats these actions should not be implemented without prior analysis of local sediment transfer processes based on in situ experiments and modelling

fluvial geomorphology and river engineering future roles utilizing a fluvial hydrosystems framework abstract river engineering is coming under increasing public scrutiny given failures to prevent flood hazards and economic and environmental concerns this paper reviews the contribution that fluvial geomorphology can make in the future to river engineering in particular it highlights the need for fluvial geomorphology to be an integral part in engineering projects that is to be integral to the planning implementation and post project appraisal stages of engineering projects it should be proactive rather than reactive areas in which geomorphologists will increasingly be able to complement engineers in river management include risk and environmental impact assessment floodplain planning river audits determination of instream flow needs river restoration and design of ecologically acceptable channels and structures there are four key contributions that fluvial geomorphology can make to the engineering profession with regard to river and floodplain management 1 to promote recognition of lateral vertical and downstream connectivity in the fluvial system and the inter relationships between river planform profile and cross section 2 to stress the importance of understanding fluvial history and chronology over a range of time scales and recognizing the significance of both palaeo and active landforms and deposits as indicators of levels of landscape stability 3 to highlight the sensitivity of geomorphic systems to environmental disturbances and change especially when close to geomorphic thresholds and the dynamics of the natural systems and 4 to demonstrate the importance of landforms and processes in controlling and defining fluvial biotopes and to thus promote ecologically acceptable engineering challenges facing fluvial geomorphology include gaining full acceptance by the engineering profession widespread utilization of new technologies including gps gis image analysis of satellite and airborne remote sensing data computer based hydraulic modeling and geophysical techniques dovetailing engineering approaches to the study of river channels which emphasize reach scale flow resistance shear stresses and material strength with catchment scale geomorphic approaches empirical predictions bed and bank processes landform evolution and magnitude frequency concepts producing accepted river channel typologies fundamental research aimed at producing more reliable deterministic equations for prediction of bed and bank stability and bedload transport and collaboration with aquatic biologists to determine the role and importance of geomorphologically and hydraulically defined habitats

repeated absolute gravity measurements for monitoring slow intraplate vertical deformation in western europe in continental plate interiors ground surface movements are at the limit of the noise level and close to or below the accuracy of current geodetic techniques absolute gravity measurements are valuable to quantify slow vertical movements as this instrument is drift free and unlike gps independent of the terrestrial reference frame repeated absolute gravity ag measurements have been performed in oostende belgian coastline and at 8 stations along a southwest northeast profile across the belgian ardennes and the roer valley graben germany in order to estimate the tectonic deformations in the area the ag measurements repeated once or twice a year can resolve elusive gravity changes with a precision better than 3 7 nm s yr 95 confidence interval after 11 years even in difficult conditions after 8 15 years depending on the station we find that the gravity rates of change lie in the 3 1 8 1 nm s yr interval and result from a combination of anthropogenic climatic tectonic and glacial isostatic adjustment gia effects after correcting for the gia the inferred gravity rates and consequently the vertical land movements reduce to zero within the uncertainty level at all stations except julich due to man induced subsidence and sohier possibly an artifact due to the shortness of the time series at that station

subducting slabs stagnant in the mantle transition zone the p wave velocity structure beneath the western pacific is found from the international seismological center first arrival data special attention was paid to the deep structure beneath the wadati benioff zone we discretized the whole mantle into blocks with finer blocks in the region of interest to obtain the velocities of all the blocks this way of discretization minimizes a problem with tomographic studies of regional scale difficulty in making corrections for the effects outside the region of interest our solution is iterative with the alternate step of the relocation of earthquakes using the whole mantle model of inoue et al 1990 as a starting model a first order smoothness constraint was imposed to suppress the possible fluctuation of the solution around the initial model the essential result depends little on whether the reference spherical model is smooth or discontinuous near 400 and 670 km depths we examined the resolution by calculating the resolving kernels for selected blocks and by reconstructing the checkerboard test patterns of velocity perturbation and the test structures of subducting lithosphere the resolution is depth dependent but in general good enough to see the slab configuration beneath the southern kurile japan izu bonin arcs and the java arc it is relatively poor beneath the northern kurile and mariana arcs the seismic image of subducting slab beneath the southern kurile to bonin arcs bends to subhorizontal near the leading edge of the wadati benioff zone and extends continentward over a distance of more than 1000 km the subhorizontal portion of the slab connects a high velocity blob to the bottom that reaches a depth of at least 800 km across the 670 km discontinuity under the japan arc although the image of the java slab directly penetrates the 670 km discontinuity it then bends to a shallow dip with a considerable spread reaching a depth of about 1200 km these results suggest that descending slabs of lithosphere in the western pacific tend to be stagnant in the transition zone under a subtle control of the 670 km discontinuity although stagnant slab materials eventually descend into the lower mantle they no longer maintain their original configuration below the 670 km discontinuity

environment friendly reduction of flood risk and infrastructure damage in a mountain river case study of the czarny dunajec abstract migration of a mountain river channel may cause erosional risk to infrastructure or settlements on the valley floor following a flood of 2010 a cutbank in one of the bends of the main channel of the czarny dunajec polish carpathians approached a local road by 50 m to arrest the erosion of the laterally migrating channel water authorities planned construction of a ditch cutting the forested neck of the bend reinforcement of the ditch banks and damming the main channel with a boulder groyne in order to avoid channelization of the highly valued multithread river reach that would deteriorate its ecological status and cause increased flood risk to downstream reaches an alternative approach to prevent bank erosion was proposed the new scheme applied in 2011 included opening of the inlets to inactive side braids located by the neck of the bend of the main channel this solution reestablished the flow in the steeper low flow channels allowing us to expect a cutoff and abandonment of the main channel during subsequent floods gravelly deflectors were constructed directly below the inlets to the reactivated side channels to divert the flow into the channels and prevent the water from entering the main channel hydraulic measurements performed before and after the implementation of the scheme confirmed that it enabled shifting the main water current with the highest average velocity and bed shear stress from the braid closest to the road to the most distant braid similar surveys of fish and benthic macroinvertebrate communities indicated that flow reactivation in the side channels was beneficial for these groups of river biota increasing their abundance and taxonomic richness in the reach not only was the implemented solution significantly less expensive but it also enhanced ecological functions of the multithread channel and the variability of physical habitat conditions and maintained the role of the reach as a wood debris trap however avulsion of the main channel in the river bend immediately upstream during the flood in may 2014 again caused erosional risk to the road although at another location this indicates that with the highly unstable multithread channel pattern of the czarny dunajec the best practice of river maintenance in a relatively unmanaged valley reach would be allowing free channel migration within the floodplain area and reinforcing where necessary the boundary between the erodible river corridor and the managed terrace

the deep seismicity of the tyrrhenian sea the deep seismicity of the tyrrhenian sea is analysed using data from a new instrumental catalogue of the seismicity of the italian area we use algorithms for the determination of absolute and relative hypocentral locations and for the evaluation of the geometry and coherence of the state of stress within the subducting slab the structure of the benioff zone although simpler than previously indicated reveals anomalous traits both in the seismicity distribution and in the stress geometry confirming that standard subduction models cannot be applied in the tyrrhenian region the velocity anomaly and the location of few isolated events indicate that the subducted slabs extend to the north along the apenninic chain approximately to the latitude of the irpinia region in central italy 42 n

diffuse back arc deformation in the southwestern pacific earthquake distribution and focal mechanisms from the lau and north fiji back arc basins indicate a diffuse and shear dominated system of deformation at odds with traditional plate tectonic models of lithospheric accretion in back arc basins the entire back arc region between the tonga and new hebrides arcs is more realistically modelled as a giant internally deformed pull apart basin formed along a left step in the transform boundary between the pacific and indo australian plates

populus nigra l establishment and fluvial landform construction biogeomorphic dynamics within a channelized river populations of the riparian pioneer species populus nigra l which establish on alluvial bars within river channels modulate sediment dynamics and fluvial landforms dense cohorts of p nigra have colonized gravel point bars along the channelized river garonne france during the last 20 years and have enhanced the vertical lateral and longitudinal development of the bars for this period the geomorphic characteristics of two wooded point bars on this laterally stable river are closely linked to the spatial distribution and intensity of establishment and resistance of different cohorts of p nigra furthermore p nigra colonization dynamics were controlled by engineer effects of this same species this relationship is illustrated by a significant correlation between key geomorphic and biological variables measured in situ and characterized with a set of four aerial photographs taken between 2000 and 2010 the development of wooded point bars which are discrete biogeomorphic units over the studied period appear to result from a specific biogeomorphic positive feedback of matter aggregation and vegetation establishment related to sediment trapping and stabilization by pioneer engineer plants we propose a conceptual model of biogeomorphic unit construction for channelized lateral stable rivers we consider the resultant biogeomorphic units as functional from an ecological point of view because p nigra enhances at the cohort scale i its own inherent capacity to resist hydrogeomorphic disturbances and ii its resilience capacity as a result of successful colonization especially downstream of mature poplar stands

cenozoic tectonics on the hautes fagnes plateau belgium abstract morphological features under which deformations of oligocene surfaces are included demonstrate the varied tectonic activity of the hautes fagnes plateau during cenozoic times beyond the general uplift of the region a local movement raised the baraque michel massif by about 100 m above the nearby crest of la vecquee moreover several zones of radial nw se faulting are active on the plateau they are related to the opening of the lower rhine embayment and represent the ultimate sw extension of the fault system associated with this opening

earthquakes associated with diffuse zones of deformation in the oceanic lithosphere some examples abstract we review the results of source parameters of earthquakes associated with the eastern end of the azores gibraltar plate boundary the davie ridge near madagascar and the horizon bank in the north fiji basin to characterize the nature of present day tectonics in diffuse zones of deformation in the oceanic lithosphere in comparison with typical plate boundaries diffuse zones of deformation are in general characterized by complex morphological expressions and scattered seismicity of up to several hundred kilometers width although the average rate of displacement across these regions is not resolvable by current models of global plate reconstruction the deformation is not truly intraplate in nature because strain is concentrated and often distinct tectonic boundaries can be identified earthquakes with seismic moments as large as 8 10 20 nm have occurred in bands of scattered seismicity that are associated with diffuse zones of deformation in the oceanic lithosphere in two cases the zones of diffuse deformation continue into the continental lithosphere east of gibraltar and north of the davie ridge however the largest earthquakes occur in the oceanic part of these two zones in contrast to previous reports the depth of earthquakes is quite shallow in regions where a mixture of focal mechanisms occur the consistent parameter is usually the orientation of the p or the t axes instead of that of the nodal planes or slip vectors thus as in the continental lithosphere zones of diffuse deformation in the oceanic lithosphere seem to be better described by a regional compression or extension field rather than by displacement on discrete faults

geological and geophysical evidence for large palaeo earthquakes with surface faulting in the roer graben northwest europe from the analysis of geological geodetic and geophysical data we provide clear evidence of seismogenic faults capable of producing large earthquakes in intraplate europe previous studies paulissen vandenberghe gullentops 1985 van den berg et al 1994 geluk et al 1994 have yielded some constraints on the rate of crustal deformation along the roer valley a graben structure crossing the netherlands belgium and germany and have allowed us to address the fundamental questions can intraplate earthquakes rupture the surface in this part of stable continental europe and if so what is their return period detailed palaeoseismic investigations have been carried out in belgium along a 10 km long fault scarp which is the morphological expression of the feldbiss fault the southwestern border fault of the roer graben camelbeeck meghraoui 1996 the scarp is multiple and the frontal fault scarp offsets young deposits and alluvial terraces field investigations using geological and geomorphological methodologies combined with geophysical prospecting provide evidence of holocene seismic surface faulting from 14c dating it is suggested that the last earthquake along the fault scarp occurred between 610 ad and 890 ad levelling profiles across the scarp suggest that it produced a vertical coseismic displacement of 0 5 1 m along the scarp if we suppose that the last surface faulting earthquake ruptured the whole seismogenic layer 17 km thickness over a minimum length 10 km corresponding to the length of the bree scarp with an average slip of 0 6 m its seismic moment was at least 3 1 1018 n m mw 6 3 for an average rigidity 3 1010 pa by estimating the offset of the main terrace of the maas river by slippage along the feldbiss fault we calculate the average late pleistocene vertical deformation rate as 0 08 0 04 mm yr 1 palaeoseismic information combining the trench and geomorphic observations suggests the occurrence of two surface faulting earthquakes during the last 20 kyr a third dates before 28 35 kyr bp then if the time distribution of earthquakes is uniform a return period of 12 5 ka and a vertical deformation rate of 0 06 0 04 mm yr 1 are inferred

horizontal shear flow in the mantle beneath the tonga arc in the tonga subduction zone a systematic shear deformation in which deep material moves relatively to the south is superimposed on the mode of down dip shortening which is characteristic of the region we interpret this observation together with a tectonic reconstruction in terms of the interaction between the subducting lithospheric slab and a horizontal shear flow in the mantle