control of rupture by fault geometry during the 1966 parkfield earthquake a reanalysis of the available data for the 1966 parkfleld california earthquake mr 5 suggests that although the ground breakage and aftershocks extended about 40 km along the san andreas fault the initial dynamic rupture was only 20 to 25 km in length the foreshocks and the point of initiation of the main event locate at a small bend in the mapped trace of the fault detailed analysis of the p wave first motions from these events at the gold hill station 20 km southeast indicates that the bend in the fault extends to depth and apparently represents a physical discontinuity on the fault plane other evidence suggests that this discontinuity plays an important part in the recurrence of similar magnitude 5 to 6 earthquakes at parkfield analysis of the strong motion records suggests that the rupture stopped at another discontinuity in the fault plane an en echeion offset near gold hill that lies at the boundary on the san andreas fault between the zone of aseismic slip and the locked zone on which the great 1857 earthquake occurred foreshocks to the 1857 earthquake occurred in this area sieh 1978 and the epicenter of the main shock may have coincided with the offset zone if it did a detailed study of the geological and geophysical character of the region might be rewarding in terms of understanding how and why great earthquakes initiate where they do

local earthquake tomography of central costa rica transition from seamount to ridge subduction summary the structure and seismicity of the subduction zone of central costa rica have been investigated with local earthquake tomography down to ca 50 km depth seismic traveltime data sets of three on and offshore seismic networks were combined for a simultaneous inversion of hypocentre locations 3 d structure of p wave velocity and v p v s ratio using about 2000 highquality events the seismicity and slab geometry as well as v p and v p v s show significant lateral variation along the subduction zone corresponding to the changes of the incoming plate which consists of serpentinized oceanic lithosphere in the northwest a seamount province in the centre and the subducting cocos ridge in the southeast of the investigation area three prominent features can be identified in the v p and v p v s tomograms a high velocity zone with a perturbation of 4 10 per cent representing the subducting slab a low velocity zone 10 20 per cent in the forearc crust probably caused by deformation fluid release and hydration and a low velocity zone below the volcanic arc related to upwelling fluids and magma unlike previously suggested the dip of the subducting slab does not decrease to the south instead an average steepening of the plate interface from 30 to 45 is observed from north to south and a transition from a plane to a step shaped plate interface this is connected with a change in the deformation style of the overriding plate where roughly planar partly conjugated clusters of seismicity of regionally varying dip are observed it can be shown that the central costa rica deformation belt represents a deep crustal transition zone extending from the surface down to 40 km depth this transition zone indicates the lateral termination of the active part of the volcanic chain and seems to be related to the changing structure of the incoming plate as well

the causes of piping in a set of abandoned agricultural terraces in southeast spain this study aims to identify some of the factors contributing to the piping process in abandoned terraces in seven sites located in the murcia region southeast spain the local landscape has the appearance of badland geomorphology possessing deep valleys which until the 1970s were terraced and cultivated the terraces have now been abandoned to the piping process the research focuses on the relationship between the size and position of pipes and the physical and geochemistry properties of the local highly dispersive marl lithology and the extent to which the reworking of the materials for terrace cultivation has exacerbated piping erosion morphologically the terracing in the stream beds has resulted in a series of gentle benches separated by steeper risers with variable drop riser height a series of flat infiltration surfaces result and because the resculpting produces a reduced bulk density infill with enhanced throughflow infiltrating water is forced along steep hydraulic gradients to a low outfall at the base of the riser paralleling studies of land levelling and terrace abandonment in similar materials elsewhere piping in the study area is shown to be closely related to the physical and chemical characteristics of the materials its sodicity and dispersive character considerable differences in structure texture mineralogical composition and material s surface crust and subsoil geochemical properties emerged from site signature plots and these results suggest that agricultural processes affect the dispersive character of the crust materials presumably because agricultural activity increases the buffering effect of organic acids on the dispersed clay this is supported by the observation that on the one site that was levelled but not used for agriculture a stable organically distinct crust like the abandoned agricultural sites has not formed the changes to the material s aggregate stability and bulk density properties when terracing occurs is examined but insufficient evidence is available from the data collected to unpick the causes of aggregate stability changes in the study plots the way in which differences between crust and subsoil enhance infiltration is discussed and the role of the terrace drop in influencing hydraulic gradients and pipe development is illustrated a possible index to describe the relationship between evacuated material volumes and the controlling parameters discussed above is tested it is concluded that terracing which is intended as a conservation practice has in dispersive materials actually enhanced piping leading to the abandonment of farming

on the dense bubble clouds and near bottom turbulence in the surf zone 1 measurements of high frequency 100 khz acoustical backscatter in the surf zone have identified the presence of a wall formed by dense assemblages of bubbles generated by breaking gravity waves and persisting for periods up to 30 min these bubbles are advected by alongshore and cross shore rip currents and are controlled by the local wave field turbulence dissolution and buoyancy during the summer of 2000 the location of the bubble wall south of scripps pier california was tracked for a 6 week period distinct categories of bubble wall defined by the backscatter intensity data are observed distinguished by the presence or absence of rip current activity the results suggest that presence or absence of long period waves 10 20 s may play a role in controlling the bubble wall type during periods without rip currents the location of the bubble wall is controlled by the incoming wave field simultaneous turbulence dissipation measurements 0 4 m above the seafloor show clear dependence on the local water depth beneath incoming waves the acoustical backscatter data are interpreted using in situ bubble and turbulence measurements in conjunction with a simple model based on bubble sorting by buoyancy dissolution and turbulence

imaging crustal structure in south central costa rica with receiver functions an array of broadband seismometers transecting the talamanca range in southern costa rica was operated from 2005 until 2007 in combination with data from a short period network near quepos in central costa rica this data is analyzed by the receiver function method to image the crustal structure in south central costa rica two strong positive signals are seen in the migrated images interpreted as the moho at around 35 km depth and an intra crustal discontinuity 15 km depth a relatively flat crustal and moho interface underneath the north east flank of the talamanca range can be followed for a lateral distance of about 50 km parallel to the trench with only slight changes in the overall geometry closer to the coast the topography of the discontinuities shows several features most notably a deeper moho underneath the talamanca mountain range and volcanic arc under the highest part of the mountain ranges the moho reaches a depth of about 50 km which indicates that the mountain ranges are approximately isostatically compensated local deviations from the crustal thickness expected for isostatic equilibrium occur under the active volcanic arc and in south costa rica in the transition region between the active volcanic arc and the talamanca range both the moho and intracrustal discontinuity appear distorted possibly related to the southern edge of the active volcanic zone and deformation within the southern part of the central costa rica deformed belt near the volcanoes irazu and turrialba a shallow converter occurs correlating with a low velocity low density body seen in tomography and gravimetry applying a grid search for the crustal interface depth and vp vs ratio cannot constrain vp vs values well but points to generally low values 1 7 in the upper crust this is consistent with quartz rich rocks forming the mountain range

subduction zones an introduction to comparative subductology abstract the subduction model for trench arc back arc systems was deduced more or less as a logical consequence of sea floor spreading many features in these systems such as thrust type inter plate earthquakes and deep earthquakes along wadati benioff zones are readily explained by the subduction model but other features such as extensional spreading and high heat flow in back arc regions and arc volcanism are difficult to explain an approach to solve these problems may be to recognize the existence of the two basically different modes of subduction one chilean type causing a compressional stress regime in the arc and back arc regions the other mariana type causing a tensional stress regime implications of the two types of the mode of subduction for some tectonic problems such as vertical movement and sediment accretion at trenches are discussed with special reference to the results of recent dsdp active margin drilling in discussing the possible causes of the two modes it is suggested that the large scale thermal and stress regimes in the back arc areas may not be the direct products of simple subduction but of some additional factors involving much greater energy such as mantle flow associated with the motion of major plates finally the possible great importance of various types of collision accretion and erosion at subduction zones is emphasized

controls of soil pipe frequency in upland blanket peat 1 soil pipes were surveyed in 160 british blanket peat catchments using consistent application of ground penetrating radar soil pipes were found in all catchments the mean frequency of piping was 69 per kilometer of surveyed transect land management moorland gripping appears to exert the most important control on hillslope pipe frequency in blanket peats management practice in peatlands may therefore induce more rapid subsurface erosion carbon loss and landform denudation via enhanced preferential flow topographic position is also important with topslopes having greater pipe frequencies than footslopes followed by midslopes with lowest frequencies slope gradient however is not a significant factor in controlling blanket peat pipe frequency i propose that peat structural properties inherited from the way a blanket peat develops on a hillslope strongly control pipe network development this is manifested in the way slope position appears to control pipe frequency aspect appears not to influence pipe frequency in blanket peats except that it does play a weak role in catchments with annual precipitation 1500 mm here southwesterly facing slopes tend to have more frequent piping

arc parallel shear deformation and escape flow in the mantle wedge of the central america subduction zone evidence from p wave anisotropy he p wave anisotropy of the crust and uppermost mantle has been determined in two separate areas of the continental margin of central america covering offshore and onshore parts of southern nicaragua and northern and central costa rica our study is based on traveltime data from local earthquakes recorded with three combined onshore and offshore seismic networks that had been deployed in three half year intervals between 2002 and 2006 the p wave traveltime data have been tomographically inverted in terms of an elliptical anisotropic p wave velocity distribution in the upper crust beneath costa rica the seismic anisotropy is of the order of 2 5 and locally strongly variable in orientation corresponding to the complicated tectonic structure in contrast the upper mantle beneath both study regions shows stronger anisotropy of the order of 5 10 and a more coherent orientation of the axis of fast p wave velocity that varies systematically on regional scales the upper mantle of the incoming oceanic plate is characterized by a trench normal orientation of the fast p wave velocity axis presumably corresponding to mineral alignment lattice preferred orientation lpo in transport direction this pattern is corroded in the uppermost part of the subduction zone possibly by the influence of bending related trench parallel faults and serpentinization that can overprint or annihilate the lpo anisotropy the upper mantle of the overriding plate and mantle wedge shows a clear trench parallel orientation of the fast p wave velocity axis the anisotropy is stronger in nicaragua than in southern central costa rica factor of 2 orders of magnitude the development of a stronger anisotropy in the nicaragua could be driven by a change in the stress regime from compressional in southern costa rica to transpressional in nicaragua corresponding to a change from near orthogonal subduction in the se to oblique subduction and slab retreat in the nw from a comparison with s sks wave gps and geochemical observations follows that the most likely explanation for the observed pattern of p wave anisotropy in the mantle wedge is lpo caused by a trench parallel shear deformation and or nw oriented escape flow originating in the compressional zone near the cocos ridge collision area

historic surface slip along the san andreas fault near parkfield california the parkfield earthquake prediction experiment is focusing close attention on the 44 km long section of the san andreas fault that last ruptured seismically in 1966 ms 6 0 the 20 km long central segment of the 1966 parkfield rupture extending from the mainshock epicenter at middle mountain southeastward to gold hill forms a 1 to 2 km salient northeastward away from the dominant n40 w strike following the 1966 earthquake afterslip aseismic slip has been nearly constant moderate parkfield earthquakes have recurred on average every 21 years since 1857 when a great earthquake m 8 ruptured at least as far north as the southern parkfield segment many measurements of slip have been made near parkfield since 1966 nevertheless much of the history of surface slip remained uncertain especially the total amount associated with the 1966 event in 1985 we measured accumulated slip on the four oldest cultural features offset by the fault along the 1966 parkfield rupture segment we interpret net slip on each feature as a sum of event slip sum of coseismic and rapid preseismic and postseismic slip from parkfield earthquakes and steady interseismic slip as measured over the last 20 years on nearby alinement arrays creep meters and trilateration lines we assumed for each site that event slip was identical for the 1922 1934 and 1966 parkfield events and that long term average rate of interseismic slip was constant between all events two fences on the southern segment southeast of gold hill indicate event slip of 13 and 15 cm and interseismic slip rate of 0 36 and 0 30 cm yr since 1959 and 1908 respectively at these sites redundant independent data support our assumption that both event and interseismic slip occur uniformly on the central segment near parkfield both the 1934 and the 1966 ruptures offset a bridge built in 1932 interseismic slip rate near the bridge has been about 1 1 cm yr since 1966 thus we deduce an average event slip of 31 cm for the 1934 and 1966 earthquakes on a parallel fault trace 1 km to the southwest slip was about 8 cm in 1966 thus total event slip summed across the entire fault zone near parkfield was nearly 40 cm on middle mountain 4 km north of the 1966 mainshock epicenter an offset fence indicates 17 cm of slip in 1966 and a 2 26 cm yr interseismic slip rate since circa 1946 thus the central segment of the 1966 rupture is characterized by much larger event slip 40 cm than both distal segments 15 cm this amount of surface slip per event is about twice what had been previously assumed larger 1966 surface slip in the central part of the rupture is geodetically compatible with a coseismic slip of 65 10 cm slip on a narrow buried asperity between middle mountain and gold hill that has been inferred from the depth distribution of early aftershocks assuming our characteristic surface slip model one can further deduce a deficit in slip since the great 1857 earthquake taking the long term slip rate as 3 3 cm yr the surface slip deficit is 3 0 2 m south of gold hill but only 0 3 0 3 m northward from parkfield

nonlinear teleseismic tomography at long valley caldera using three dimensional minimum travel time ray tracing the authors explore the impact of three dimensional minimum travel time ray tracing on nonlinear teleseismic inversion this problem has particular significance when trying to image strongly contrasting low velocity bodies such as magma chambers because strongly refracted and or diffracted rays may precede the direct p wave arrival traditionally used in straight ray seismic tomography they use a simplex based ray tracer to compute the three dimensional minimum travel time ray paths and employ an interative technique to cope with nonlinearity results from synthetic data show that their algorithm results in better model reconstructions compared with traditional straight ray inversions the authors reexamine the teleseismic data collected at long valley caldera by the u s geological survey the most prominent feature of their result is a 25 30 low velocity zone centered at 11 5 km depth beneath the northwestern quandrant of the caldera beneath this at a depth of 24 5 km is a more diffuse 15 low velocity zone in general the low velocities tend to deepen to the south and east the authors interpret the shallow feature to be the residual long valley caldera magma chamber while the deeper feature may represent basaltic magmas ponded in the midcrust the deeper position of the prominent low velocity region in comparison to earliermore tomographic images is a result of using three dimensional rays rather than straight rays in the ray tracing the magnitude of the low velocity anomaly is a factor of approximately 3 times larger than earlier models from linear arrival time inversions and is consistent with models based on observations of ray bending at sites within the caldera these results imply the presence of anywhere from 7 to 100 partial melt beneath the caldera 40 refs 1 fig 1 tab less

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

the seismic source parameters of the 1991 costa rica aftershock sequence evidence for a transcurrent plate boundary the april 22 1991 valle de la estrella costa rica earthquake ms 7 6 was a back arc thrusting event associated with the underthrusting of the caribbean plate beneath central america a network of three passcal type portable instruments was deployed to monitor the aftershock activity in southern costa rica 2 to 6 weeks after the main shock the waveforms recorded on three component midperiod seismometers were used to recover source information for 15 small aftershocks magnitudes between 3 2 and 4 4 with a linear moment tensor inversion method we conducted several tests to investigate the effects of unknown structure and event mislocation on source parameter recovery the longer period waveforms in general are less sensitive to the effects of the structural details so that the essential source information can be successfully extracted from the waveform data the earlier part of the seismic wave forms has proven to be the most important carrier of the source information a gross crustal model can be used to describe the structure for the source study the small changes in the waveform character resulting from the mislocation of the events or inexact green s functions generated from the oversimplified crustal model do not prohibit us from the recovery of the source orientation at local distances in contrast the determination of the focal depth is subject to uncertainty because of the lack of detailed structural information our focal mechanisms are generally in good agreement with p wave first motion fault plane solutions determined from a local short period network the aftershocks show a clear spatial segmentation based on focal mechanism type most aftershocks near or southeast of the main shock were thrusting events with focal mechanisms similar to the main shock in contrast a cluster of aftershocks northwest of the main shock showed dominantly left lateral strike slip motion on a northeasterly striking nodal plane this suggests that a diffuse deformation zone exists in central costa rica and is characterized by left lateral strike slip motion this diffuse transcurrent deformation zone coincides with several geologic and geophysical features and perhaps is a result of the slower subduction rate of the buoyant cocos ridge than its adjacent segments along the middle america trench mat the diffuse transcurrent boundary may intersect with the north panama deformed belt npdb near limon costa rica and is very likely a plate boundary for the proposed panama block

overland flow velocity and roughness properties in peatlands overland flow is an important component of peatland hydrology hydrological models of peatlands are being developed that require estimates of flow velocity and its controls however surprisingly little is known about overland flow velocities in peatlands some peatlands have also been drained using open ditches and these need to be incorporated into flow models this paper presents field data on the velocity of overland flow and drain flow in upland peatlands the relationships between flow velocity vegetation cover slope and water depth are explored sphagnum provided a significantly greater effective hydraulic roughness to overland flow than peatland grasses in all cases a significant break in process occurred for flows with water depths of around 1 cm so that there were two components of the roughness curve this is consistent with partial submergence theory for very shallow flows where resistance increases with depth as the soil surface first becomes fully submerged while each surface cover type should be considered separately the results also suggest that a first order estimate of darcy weisbach roughness and mean velocity can be based on a single parameter for each surface cover this paper presents an empirical overland flow velocity forecasting model that can be applied to peatlands the model combines the partially submerged component for flows with water depths below 1 cm with the fully submerged component for flows with depths up to 5 cm which are representative of the depths of flows that occur across peatlands

soil pipe contribution to steady subsurface stormflow flow through a saturated idealized hillslope with a single soil pipe was simulated using a finite difference solution to the equation for three dimensional darcian flow in saturated heterogeneous media the proportions of hillslope discharge originating from flow through the soil matrix and from flow through a soil pipe were determined considering such factors as the radius depth and length of the pipe pipe spacing and the length and slope of the hillslope results demonstrate that soil pipes can contribute a significant amount and in many instances the majority of total subsurface stormflow

a model for the horizontal directionality of breaking wave noise in the surf zone a model is presented for the horizontal directionality of the noise generated by individual breaking waves in the surf zone the model is based on the interaction between sound radiated by ringing bubbles created in a breaking wave crest and the population of acoustically quiescent bubbles left on the seaward side of the wave in the shallow water waveguide the effect of the quiescent bubbles is to absorb sound from all but the very ends of the breaking crest resulting in the formation of acoustic hot spots the model calculations are in good agreement with observations of the horizontal directionality of noise from individual breaking waves in the surf zone

seismic tomography and earthquake locations in the nicaraguan and costa rican upper mantle the central american subduction zone exhibits large variations in geochemistry downgoing plate roughness and dip and volcano locations over a short distance along the arc results from joint inversions for vp vp vs and hypocenters from the tomography under costa rica and nicaragua tucan experiment give insight into its geometry and structure in both costa rica and nicaragua the intermediate depth seismic zone is a single layer no more than 10 to 20 km thick tomographic images show that throughout nicaragua and costa rica the slowest mantle p wave velocities appear below and behind the volcanic front indicating likely zones of highest temperature extending 80 to 120 km depth a sheet of high vp vs thought to be caused by melt is imaged directly beneath the nicaraguan volcanoes whereas a weaker broader anomaly is imaged beneath the costa rican volcanoes potentially indicating a greater extent of melting beneath nicaragua within the downgoing plate anomalously low velocities occur at least 20 30 km below wadati benioff zone seismicity to depths of 140 km beneath nicaragua and to 60 km depth beneath costa rica they indicate 10 20 serpentinized upper mantle of the downgoing plate beneath nicaragua similar to that inferred from refraction seaward of the trench but continuing to subarc depths this unusually hydrated lithosphere may introduce more water into the nicaraguan mantle initiating increased amount of melting and fluid flux to the arc

contrasting flow pathways within tropical forest slopes of ultisol soils there are very few experimental studies identifying hydrological pathways within rain forest slopes such knowledge is however necessary to understand why forest disturbance affects rainfall riverflow response and nutrient migration this study examines flow pathways within lowland rain forest slopes comprising udults of the ultisol soil order experimentation was conducted on four se asian hillslope units each 5 5 m in plan in the bukit timah catchment singapore island and in the w8s5 catchment sabah borneo island the flow pathways were identified by artificial tracer experiments we evaluated how well hydrometric calculations based on tensiometry and permeametry measurements predicted the tracer patterns the tracer work indicated much faster subsurface flows at bukit timah than w8s5 for the storms studied some explanation of the greater subsurface waterflows at bukit timah in comparison to w8s5 is afforded by the less steep moisture release curves which maintain hydraulic conductivity as the soil dries vertical flow of the tracer through the upper 1 m of soil predominated 90 per cent of percolation in the bukit timah slopes in some contrast a major component approximately 60 per cent of the tracer percolation was directed laterally within the w8s5 slopes the flow vectors calculated using the hydrometric methods did however grossly under estimate the degree of lateral deflection of waterflow generated at w8s5 and to a lesser extent over estimated it at bukit timah in part these errors may relate to the inability of traditional hydrometric techniques to fully characterize the effect of the large and small natural soil pipes present within both catchments in conclusion the study indicates that marked variations in flow vectors exist within the udult great group of se asian soils and hydrometric calculations may be poor predictors of these dominant pathways copyright 2005 john wiley sons ltd

sediment and particulate carbon removal by pipe erosion increase over time in blanket peatlands as a consequence of land drainage land drainage is common in peatlands artificially drained blanket peat catchments have been shown to have a significantly greater soil pipe density than intact catchments this paper investigates the role of surface land drains in the enhancement of soil piping in blanket peats the density of piping was found to significantly increase in a linear fashion with the age of the drainage thirty five years after drains were cut slopes would be expected to have twice the density of soil piping than would an undrained blanket peat catchment the rate of pipe erosion increases exponentially over time so that particulate carbon loss from subsurface pipes is greatest where drains are oldest

eroding blanket peat catchments global and local implications of upland organic sediment budgets globally peatlands account for circa 50 of terrestrial carbon storage containing as much carbon as is present in the atmosphere the uplands of the uk have an extensive cover of blanket peat but much of it is actively eroding this paper presents a detailed organic sediment budget for a blanket peat catchment in the north pennines and comparative data from a catchment in the southern pennines the catchments have total sediment yields organic and mineral of 44 and 267 t km 2 a 1 and organic sediment yields 31 and 195 t km 2 a 1 respectively they represent two extremes of a spectrum of eroded peat catchments it is demonstrated that the lower sediment yields in the north pennines are associated with extensive natural revegetation of the catchment and consequent reductions in slope channel linkage construction of a carbon budget for the north pennine catchment demonstrates that particulate carbon losses associated with the fluvial suspended sediment load are the largest single carbon loss from the system the system is currently close to carbon neutral but much higher carbon losses associated with actively eroding systems such as the south pennine site would make these systems a major carbon source the possibility that enhanced summer temperatures and winter storminess will accelerate erosion of upland mires means there is a risk that physical degradation of peatlands could become a significant positive feedback on global warming mitigation of these potential global effects will depend on local management informed by a clear understanding of peatland sediment dynamics the sediment budget data here suggest that in gullied peatlands revegetation of gully floors is an effective control on sediment flux so that techniques such as gully blocking are likely to be effective approaches to erosion control

seismicity of the caribbean nazca boundary constraints on microplate tectonics of the panama region there are at least three kinds of models that have been used to explain plate motions and boundaries in the panama region these models suggest that the major caribbean nazca boundary is 1 a thrust fault along which the panama deformed belt has developed 2 a left lateral strike slip fault along the southern margin of panama or 3 a diffuse array of mostly thrust faults which cause internal deformation to occur throughout panama in this paper we use joint epicenter location methods to obtain improved locations for earthquakes that occurred in this region between 1964 and 1985 and we use waveform modeling to obtain more precise depths for five events hypocenters of 13 events beneath the panama deformed belt suggest that overthrusting of panama upon the caribbean plate causes active deformation which supports model 1 but does not rule out models 2 and 3 the nature of seismic faulting along the panama deformed belt is complex and supports the premise that slow north south convergence loads the cold thick caribbean lithosphere and causes it to fail seismically epicenters of 64 events along the panama fracture zone indicate that this north south trending rightlateral transform fault is splayed north of 6 n overall the seismicity throughout suggests that the panama arc can be treated as a microplate in accordance with models 1 or 2 similarly a possible second microplate may exist south of panama but if so it does not affect regional plate motions significantly the strongest evidence for model 3 occurs in the easternmost and westernmost portions of panama where the diffuse seismicity prevents us from resolving specific microplate triple junction boundaries

complex effective depth of the ocean bottom the effective depth approximation is extended to allow for energy loss on reflection at an attenuating low shear speed solid ocean bottom the resulting complex effective depth leads to a novel and efficient algorithm for finding the exact complex eigenvalues for the normal modes in isovelocity water over an attenuating fluid or solid bottom while avoiding numerical searches the method gives rapid normal mode solutions for shallow water propagation including the effects of leaky modes

effect of random sea surface and bottom roughness on propagation in shallow water the potential difficulties associated with modeling acoustic propagation in shallow water environments are well documented larger scale deterministic features combine with random fluctuations in the water column sediment sea surface and water sediment interface to produce an extremely complicated propagation regime the extent to which each of these factors needs to be included in realistic propagation modeling remains to be quantified toward this end results from a series of detailed simulations generated using the parabolic equation method are presented beginning with a deterministic downward refracting sound speed profile and a known sloping bottom realizations of the random features are sequentially added to the simulation wind driven surface gravity waves and power law bottom roughness are considered the individual and cumulative effects of these scattering mechanisms on the acoustic wavefront are quantified successive interactions with the random interfaces are studied a modal decomposit

monitoring near shore shingle transport under waves using a passive acoustic technique passive acoustic techniques have been used to measure shingle gravel sediment transport in very shallow water near the wave breaking zone on a beach the experiments were conducted at 1 1 scale in the large wave flume grose wellen kanal gwk at hannover germany the frequency spectrum induced by shingle mobilized under breaking waves can be distinguished from other ambient noise and is found to be independent of water depth and wave conditions the inverse relationship between centroid frequency and representative grain size is shown to remain valid in shallow water wave conditions individual phases of onshore and offshore transport can be identified analysis of the acoustic frequency spectrum provides insight into the mechanics of phase resolved shingle transport

factors controlling the spatial distribution of soil piping erosion on loess derived soils a case study from central belgium abstract collapsible loess derived soils are prone to soil piping erosion where enlargement of macropores may lead to a subsurface pipe network and eventually to soil collapse and gully development this study aims at understanding the main factors controlling spatial patterns of piping in loess derived soils under a temperate climate to map the spatial distribution of piping and identify the environmental controls on its distribution a regional survey was carried out in a 236 km2 study area in the flemish ardennes belgium orthophotos taken at optimal field conditions winter were analyzed to detect piping in open landscapes and ground thruthing was systematically done through field surveys in total 137 parcels having 560 collapsed pipes were mapped dimensions of the sinkholes and local slope gradient were measured in the field and topographical variables were derived from lidar data land use plays an important role as 97 of the sites with piping are found under pasture the probability of piping increases rapidly on hillslopes with gradients exceeding 8 and with a concave profile and plan curvature enhancing subsurface flow concentration the zones with soil profiles on shallow loess over a relatively thin layer of homogeneous blue massive clays aalbeke member are most prone to piping soil characteristics are of less importance to explain piping occurrence furthermore the topographical threshold line indicating the critical slope gradient for a given contributing drainage area was determined this threshold line negative power relation is similar to the threshold line for shallow gully initiation

structure of the costa rica convergent margin offshore nicoya peninsula we present the results of a seismic refraction survey conducted offshore costa rica near the nicoya peninsula a dip profile and three strike profiles were carried out over 22 ocean bottom hydrophones and seismographs and were also recorded by land receivers these data are used to construct a crustal structure model of the convergent margin from 20 km seaward of the middle america trench onto the nicoya peninsula the best constrained portion of our model is the velocity at the top of the margin wedge immediately below the slope apron velocities increase from 3 5 to 4 2 to 4 6 km s at distances of 10 20 and 30 50 km landward of the trench these velocities are higher than observed within margin wedges at other well studied convergent margins but lower than the velocities within the adjacent nicoya complex which are 5 5 km s at similar depths below the surface we interpret the margin wedge velocities as indicating that material similar to the nicoya complex extends seaward to near the lower slope but that fracturing alteration or accretion processes have lowered the velocity of the margin wedge with respect to the nicoya complex the seismic refraction data cannot constrain the exact thickness or velocity of a possible low velocity zone lvz overlying the subducting plate however geologically reasonable structures are only produced with a lvz 400 m thick velocities in the upper part of oceanic layer 2 are 3 5 4 0 km s within the subducting slab these velocities are unusually low for oceanic crust of this age and may correlate with a proposed highly permeable zone at the top of the subducting crust the top of the subducted slab is well resolved and deepens from 5 km depth at the trench to 15 16 km depth at the nicoya peninsula coastline the dip angle of the subducting plate increases from 6 to 13 at a distance of 30 km from the trench interplate seismicity appears to become common 55 km from the trench where the plate boundary is at 14 km depth

seismicity and tectonics of the subducted cocos plate we have examined teleseismic earthquake locations reported by the international seismological centre isc for the middle america region and selected 220 as the most reliable these hypocenters and other data are used to delineate the deep structure of the subducted cocos plate the results indicate that the subducted plate consists of three major segments segment i extends from the panama fracture zone to the nicoya peninsula the structure of this segment is poorly defined segment ii is the largest and best defined segment this segment consists of two parts iia and iib part iia extends from the nicoya peninsula to western guatemala and is very well defined and continuous in structure its strike follows the curvature of the trench and dips at about 60 part iib extends from western guatemala to orizaba mexico the dip of this part of the segment decreases slightly toward the northwest and its strike is more northward than that of the trench segment iii extends from orizaba to the rivera fracture zone and is not well defined due to a lack of earthquake activity beneath about 100 km its orientation differs markedly from segment ii and strikes somewhat more westward than the trench between parts iia and iib of segment ii the subducted plate seems to be continuous bending smoothly to accommodate the change in geometry local network data from costa rica suggest there may be a tear between segments i and ii between segments ii and iii there is a gap in the hypocenters which makes it difficult to define the boundary the change in geometry between these two segments indicates that there may be a tear and two strike slip focal mechanisms in the region support this conclusion we find no convincing evidence supporting the existence of segments smaller than the three described above if there is smaller scale segmentation in the shallow part of the subducting plate the plate must still maintain enough continuity to appear continuous at greater depths there is no evidence for any major tear in the subducted plate associated directly with either the tehuantepec ridge or the orozco fracture zone the shallow subduction at the northwestern end of segment ii may be related to the bouyancy of the tehuantepec ridge the cocos ridge is probably directly responsible for the change in geometry between segments i and ii and may even be slowing or stopping subduction in segment i the structure of the subducted plate in segment ii and the changes in the character of volcanism along the arc can be related to the relative motion of the north american and caribbean plates the present geometry of part iib of segment ii is more consistent with the probable configuration of the trench about 7 ma ago than with the present configuration indicating that the north american plate is overriding the subduction zone appendices 2 3 and 4 are available with entire article on microfiche order from american geophysical union 2000 florida avenue n w washington dc 20009 document b84 009 2 50

controls on the spatial distribution of natural pipe outlets in heavily degraded blanket peat abstract natural soil pipes are recognised as a common geomorphological feature in many peatlands and they can discharge large quantities of water and sediment however little is known about their morphological characteristics in heavily degraded peat systems this paper presents a survey of pipe outlets in which the frequency and extent of natural soil pipes are measured across a heavily gullied blanket peat catchment in the peak district of northern england over a stream length of 7 71 km we determined the occurrence and size of 346 pipe outlets and found a mean frequency of 22 8 km 1 gully bank topographic position was an important control on the size and depth of pipe outlets aspect had a large influence on pipe outlet frequency with southwest and west facing gully banks hosting more than 43 of identified pipe outlets pipe outlets on streambanks with signs of headward retreat were significantly larger and closer to the peat surface compared to pipe outlets that issued onto uniform streambank edges we suggest that larger pipe frequencies are observed on gully banks that are more susceptible to desiccation cracking and propose that future peatland restoration works could prioritise mitigating against pipe formation by revegetating and reprofiling south and west facing gully banks

an example of rapid gully initiation and extension by subsurface erosion coastal san mateo county california abstract gully networks developed on coastal hillslopes in san mateo county california extended nearly 300 from 1941 to 1978 primarily by subsurface erosion in one drainage subsurface erosion expanded the gully network by 80 m nearly doubling the gully length from 1982 to 1987 intense gullying is restricted to areas underlain by members of the pliocene purisima formation a unit rich in unstable volcanic lithic fragments which weather to produce expansive smectite clays gully development proceeds through a sequence of lateral eluviation of clays near the base of the a horizon piping development of an open conduit in which erosion proceeds by turbulent concentrated flow tunneling and finally collapse of the tunnel roof exposing an open gully measurements made in one gully system during a 25 yr storm showed that the subsurface drainage network carried four times as much flow as the surface drainage net and had average suspended sediment concentrations an order of magnitude higher

three dimensional imaging of steeply dipping structure near the san andreas fault parkfield california shot gathers from the parkfield california deep crustal seismic reflection line recorded in 1977 by cocorp reveal coherent events having horizontal to reverse moveouts these events were migrated using a multioffset three dimensional kirchhoff summation method this method is a ray equation back projection inversion of the acoustic wave field which is valid under the born wkbj and far field assumptions migration of full wave acoustic synthetics having the same limitations in geometric coverage as the cocorp survey demonstrates the utility of the imaging process the images obtained from back projection of the survey data suggest that the gold hill fault carries ultramafic rocks from the surface to 3 km depth at a dip greater than 45 degrees where it joins the san andreas fault which may cut through more homogeneous materials at shallow depths to the southwest a 2 km tertiary sedimentary section appears to terminate against a near vertical fault the zone between this fault and the san andreas may be floored at 3 km by flat lying ultramafics lateral velocity inhomogeneities are not accounted for in the migration but in this case do not seriously hinder the reconstruction of reflectors

gully head retreat rates in the semi arid highlands of northern ethiopia due to overgrazing and agricultural intensification gully erosion severely affects sub saharan countries however insufficient quantitative studies exist for this part of the world this paper presents data on gully head retreat rates in northern ethiopia and relates these rates to gully and environmental characteristics the monitoring of headcuts over one rainy season 2010 revealed that present day retreat rates are low with average annual linear rl areal ra and volumetric ve retreat rates of 0 34 m y 1 1 70 m2 y 1 and 5 2 m3 y 1 respectively these results express the positive effects of recent soil and water conservation practices on gully stabilization significantly higher values of rl up to 1 93 m y 1 occurred in the vertisol areas affected by soil piping when considering the medium to long time scale 1 47 years using archival terrestrial and aerial photographs headcut retreat rates proved to be significantly higher than those in the short term the averages for rl ra and ve are 3 8 m y 1 31 5 m2 y 1 and 47 7 m3 y 1 respectively retreat rates are up to 10 times higher after road construction for the medium to long term headcut retreat rates were positively related to the catchment area a a power relationship that best describes the relation between ve and a is ve 0 53 a0 31 r2 0 27 n 18 compared to other areas worldwide regressive erosion has been rapid in ethiopia as a result of the degraded and steep landscape combined with erosive rains and the occurrence of vertisols in vertisols headcut retreat is controlled by soil piping because no adequate techniques exist to control gully initiation and development in vertisols alternative techniques should be developed that deactivate soil pipes

microearthquake imaging of the parkfield asperity microearthquake data from a downhole seismometer network on the san andreas fault appear to outline two aseismic asperities that may correspond to the locations of the foreshocks and main shocks of the parkfield characteristic earthquakes the source parameters of the microearthquakes show that a few of the earthquakes have significantly higher stress drops than most furthermore the magnitude frequency statistics suggest that at local magnitude 0 6 the cumulative number of small events begins to fall off the usual gutenberg richter b 1 relation in which the number of events increases exponentially with decreasing magnitude the downhole seismometer data establish a baseline from which the evolution of the earthquake process at parkfield can be monitored and suggest that different mechanical conditions than those that lead to the typical gutenberg richter relation may be operating for the smallest of parkfield microearthquakes

the april 22 1991 valle de la estrella costa rica mw 7 7 earthquake and its tectonic implications a broadband seismic study the rapture process of a large back arc thrusting earthquake the april 22 1991 valle de la estrella costa rica mw 7 7 earthquake is investigated using broadband body waves and long period surface waves we find the source process to be relatively simple with the source models separately obtained from body and surface waves being very consistent the event occurred on a shallow southwest dipping rapture plane on which most energy is released updip of the hypocentral location 10 20 km deep high frequency radiation appears to have been released over a relatively small source area our preferred model has a focal mechanism with strike 102 10 dip 17 14 and rake 63 17 a seismic moment of 3 8 1 5 1020 n m and a total rupture duration of 40 6 s the earthquake appears to be associated with the north panama deformed belt npdb a thrust and fold complex that has accommodated the oroclinal deformation of panama this event along with previous large events north of panama in 1882 and 1916 indicates that there is substantial convergence along the npdb marking the npdb as a probable emerging plate boundary it remains difficult to gauge the earthquake hazard in the region because of the tectonic complexity

subduction zone structure and magmatic processes beneath costa rica constrained by local earthquake tomography and petrological modelling summary a high quality data set of 3790 earthquakes were simultaneously inverted for hypocentre locations and 3 d p wave velocities in costa rica tests with synthetic data and resolution estimates derived from the resolution matrix indicate that the velocity model is well constrained in central costa rica to a depth of 70 km northwestern and southeastern costa rica are less well resolved owing to a lack of seismic stations and seismicity maximum h2o content and seismic wave speeds of mid ocean ridge basalt and harzburgite were calculated for metamorphic phase transformations relevant to subduction both the 3 d p wave velocity structure and petrological modelling indicate the existence of low velocity hydrous oceanic crust in the subducting cocos plate beneath central costa rica intermediate depth seismicity correlates well with the predicted locations of hydrous metamorphic rocks suggesting that dehydration plays a key role in generating intermediate depth earthquakes beneath costa rica wadati benioff zone seismicity beneath central costa rica shows a remarkable decrease in maximum depth toward southeastern costa rica the presence of asthenosphere beneath southeastern costa rica which entered through a proposed slab window may explain the shallowing of seismicity due to increased temperatures and associated shallowing of dehydration of the slab tomographic images further constrain the existence of deeply subducted seamounts beneath central costa rica large low p wave velocity areas within the lower crust are imaged beneath the southeasternmost volcanoes in central costa rica these low velocities may represent anomalously hot material or even melt associated with active volcanism in central costa rica tomographic images and petrological modelling indicate the existence of a shallow possibly hydrated mantle wedge beneath central costa rica

three dimensional velocity structure in northern california coast ranges from inversion of local earthquake arrival times abstract the three dimensional p wave velocity structure in the upper crust in the northern california coast ranges from santa rosa to ukiah is obtained by simultaneous inversion of local earthquake and refraction travel time data for velocity and hypocentral parameters in this region where there are lateral heterogeneities the three dimensional inversion provides important results not found in two dimensional refraction studies the medium is parameterized with velocities at the intersections gridpoints of a nonuniform three dimensional grid travel times are calculated with approximate ray tracing along arcuate paths which are reasonable for lateral distances less than 45 km inversions with 5 to 10 and 2 to 3 km gridspacing achieved 80 per cent decreases in data variance the inversion with 2 to 3 km gridspacing reduced the rms residual level to approximately the level of timing error anomalously low velocities to at least 6 km depth are found along the maacama and healdsburg rodgers creek hrc fault zones the low velocity region is widest at the en echelon junction of the maacama and hrc fault zones low velocities are associated with the clear lake basin where there is a thick sequence of volcanics interbedded with sedimentary deposits there is a high velocity body located southeast of the geysers between the maacama and collayomi fault zones which may be associated with igneous intrusives or high grade metamorphism related to an underlying inferred partial melt body modeled in previous teleseismic travel time studies there is some indication in the vapor dominated geothermal reservoir of lower velocities relative to surrounding water saturated rock as has been shown in some laboratory experiments hypocenter locations obtained in the inversion show little variation from hypocenters obtained through inversion for one dimensional velocity model and station corrections for the inversion with 2 to 3 km gridspacing however some groups of events become more tightly clustered

tomographic inversion of local earthquake data from the hengill grensdalur central volcano complex iceland parameters is generally low however a quantitative analysis of resolution provides an unambiguous assessment of well resolved volumes within the better resolved regions of the model the averaging volumes are 1 2 km and 2 4 km in vertical and horizontal extent respectively results of tomographic inversion image three distinct bodies of anomalously high velocity two of these extend from near the surface to a depth of about 3 km these high velocity volumes are located directly beneath the surface expressions of the extinct grensdalur volcano and the extinct husmuli basalt shield the third high velocity structure occurs in the depth range of 3 4 km but does not extend to the surface these three high velocity bodies are interpreted to be solidified magmatic intrusions relatively low velocities underlay limited portions of the trace of the present accretionary axis and a low velocity body is imaged in the roots of the active hengill volcano the volume of lower velocities located beneath the surface expression of the hengill volcano is interpreted to be a region of partial melt

first results from a new seismographic network in costa rica central america abstract the occurrence of past large earthquakes in space and time has been useful in estimating locations and repeat times for similar future events kelleher et al 1973 and mccann et al 1979 the lack of major energy release in and near costa rica during the last 30 years i e seismic slip deficiency as shown by mcnally and minster 1981 and a recent increase in earthquake activity motivated the organization and installation of a permanent seismographic network the network became operational in early 1984 with financial support from the office of foreign disaster assistance ofda usaid the costa rica network will provide the data to help understand the nature of the tectonic interaction of the cocos and caribbean plates and to evaluate the potential for present and future earthquake hazards in light of the historical earthquake activity

three dimensional velocity structure of the upper mantle beneath costa rica from a teleseismic tomography study summary a tomographic study has been carried out in costa rica utilizing teleseismic events recorded by the national seismic network ovsicori una the data consist of 130 earthquakes which occurred at epicentral distances of 23d 100d p phases and 110d 180d pkp phases 1378 iasp91 traveltime residuals were computed and inverted using an iterative linearized inversion procedure which incorporates 3 d minimum traveltime ray tracing the 3 d velocity structure of the upper mantle as derived from teleseismic tomography shows a northeastward dipping high velocity body beneath northern costa rica interpreted as the subducting cocos plate although less evident a similar pattern of lateral heterogeneities characterizes the deep structure of central costa rica there is no evidence for deep high velocity bodies beneath southern costa rica the 3 d reconstruction of the subducting cocos plate shows a steeply dipping slab subvertical beneath northern costa rica here the oceanic lithosphere is older and denser than in central costa rica where the slab dips at about 60d southern costa rica does not show a dipping slab this is in accordance with the subduction collision of the thick and buoyant cocos ridge the geometry of the slab the maximum velocity anomalies and the sudden change in the local seismicity depth pattern suggest the existence of a tear in the subducted plate located between the northern and central costa rican subduction zones the strain field in the overriding plate indicates a strong structural control induced by the along trench variations in subduction northern costa rica shows slight tensile deformation while southern costa rica displays moderate to strong compressive deformation the maximum horizontal gradient of the strain field is located in central costa rica where a sinistral transcurrent fault system crosses the country from the pacific to the atlantic coast this system is likely to be generated by the along trench variations in subduction

sediment yield and delivery in the blanket peat moorlands of the southern pennines in contrast to much previous research on blanket peat moorland which has concentrated upon studies of the form and causes of gully erosion this paper attempts to investigate sediment transport and to estimate both short term and long term sediment yields in such terrain the research was conducted on wessenden head moor to the west of huddersfield yorkshire where automatic stream sampling continued over a period of two years use of corrected rating curves ferguson 1988 provided a mean estimate of sediment yield over this period of 55 t km 2 yr 1 in addition an estimate of longer term sediment yield was derived from four reservoir sediment surveys in the wessenden valley total yield was 203 69 t km 2 yr 1 including an organic fraction of 38 82 t km 2 yr 1 stream sampling at three sites on shiny brook including headwaters and the outflow to the reservoir suggested that there is great temporal and spatial variability in mineral and organic inputs to the reservoirs although not excessive in gravimetric terms the low density of peat means that there is a serious erosion problem estimates of erosion rates for the peat gully network at shiny brook appear to confirm earlier evidence concerning the relatively recent occurrence of this erosion within the last two centuries

gully erosion impacts factors and control gully erosion attracts increasing attention from scientists as reflected by two recent international meetings poesen and valentin eds catena 50 2 4 87 564 li et al 2004 gully erosion under global change sichuan science technology press chengu china 354 pp this growing interest is associated with the increasing concern over off site impacts caused by soil erosion at larger spatial scales than the cultivated plots the objective of this paper is to review recent studies on impacts factors and control of gully erosion and update the review on dgully erosion and environmental change importance and research needst poesen et al 2003 catena 50 2 4 91 134 for the farmers the development of gullies leads to a loss of crop yields and available land as well as an increase of workload i e labour necessary to cultivate the land gullies can also change the mosaic patterns between fallow and cultivated fields enhancing hillslope erosion in a feedback loop in addition gullies tend to enhance drainage and accelerate aridification processes in the semi arid zones fingerprinting the origin of sediments within catchments to determine the relative contributions of potential sediment sources has become essential to identify sources of potential pollution and to develop management strategies to combat soil erosion in this respect tracers such as carbon nitrogen the nuclear bomb derived radionuclide 137 cs magnetics and the strontium isotopic ratio are increasingly used to fingerprint sediment recent studies conducted in australia china ethiopia and usa showed that the major part of the sediment in reservoirs might have come from gully erosion gullies not only occur in marly badlands and mountainous or hilly regions but also more globally in soils subjected to soil crusting such as loess european belt chinese loess plateau north america and sandy soils sahelian zone north east thailand or in soils prone to piping and tunnelling such as dispersive soils most of the time the gullying processes are triggered by

piping and pipeflow in a deep peat catchment abstract natural pipes are common in many upland blanket peats yet little is known about pipe network morphology or pipeflow processes most information on soil piping comes from the shallow peaty podzols of the welsh uplands where monitoring suggests that pipes may be important contributors to streamflow this paper presents information on piping and pipeflow from a deep upland blanket peat catchment in the pennine hills of northern england pipe outlets are found throughout the soil profile ranging from the underlying substrate at 3 m depth to pipes which are within a few centimetres of the surface mean pipe diameters range from 3 to 70 cm some pipes are over 150 m long slopes in the catchment are less steep than those usually associated with soil piping continuous flow records were obtained from 15 gauging sites on 8 separate pipes the pipeflow response from deep blanket peat was found to be different to that reported in the shallow peaty podzols of the welsh uplands the distinction between ephemeral and perennial pipe types does not appear to be useful within the deep pennine blanket peat response times from all of the pipes are short even from pipes deep within the peat at the same time pipes have a prolonged recession limb such that they maintain low flow for longer periods than most other runoff production processes within the catchment pipeflow contributes around 10 of the streamflow volume but can at times contribute up to 30 soil pipes may therefore be far more important in some upland peat catchments than previous work has hitherto suggested

quaternary uplift astride the aseismic cocos ridge pacific coast costa rica the pacific coast of costa rica lies within the central american forearc and magmatic arc region that was created by northeastward subduction of the cocos plate beneath the caribbean plate at the middle america trench from the peninsula de nicoya south eastward toward the peninsula de osa and the peninsula de burica on the panamanian border the middle america trench loses its physiographic expression where it intersects the aseismic cocos ridge interaction between subduction of the buoyant aseismic cocos ridge and the overriding caribbean plate is invoked to explain the variation in rates of vertical crustal uplift along a coastal transect from nicoya to burica the pliocene and pleistocene stratigraphic record and holocene marine terraces and beach ridge complexes indicate that maximum rates of crustal uplift have occurred on the peninsula de osa immediately landward of the aseismic cocos ridge crustal uplift rates decrease northwest toward the peninsula de nicoya and to a lesser extent southwest toward the peninsula de burica the late quaternary stratigraphy on the peninsula de osa is subdivided into two major chronostratigraphic sequences from groupings of radiocarbon dates crustal uplift rates calculated from these sequences systematically decrease from 6 5 to 2 1 m ka north east across the peninsula deformation of the peninsula is modeled as uplifted and down to the northeast tilted fault blocks with an angular rotation rate of 0 03 to 0 06 per thousand years although less well constrained crustal uplift rates on the peninsula de nicoya 200 km to the northwest of the peninsula de osa vary from 1 m ka for pliocene and pleistocene sediments to 2 5 m ka for holocene marine terraces in the quepos region 100 km to the northwest of the peninsula de osa calculated uplift rates derived from incision of late quaternary fluvial terraces range from 0 5 to 3 0 m ka on the peninsula de burica only 60 km to the southwest of the peninsula de osa calculated uplift rates range from 4 7 m ka for a late holocene marine terrace to 1 2 m ka for post late pliocene deep sea sediments the variations in calculated uplift rates on the peninsula de osa constrain a dynamic model for subduction of the cocos ridge and the resulting uplift of the overriding caribbean plate deflection of the caribbean plate is modeled using various effective elastic thicknesses as the response of an elastic plate to the buoyant force of the subducted cocos ridge because the shape of the subducted end of the cocos ridge is unknown two scenarios are evaluated 1 a radially symmetric ridge with a slope similar to the slope of the flanks of the ridge and 2 a ridge where the subducted end was truncated by the panama fracture zone the best fit model utilizes a truncated ridge that has been subducted during the past 0 5 m y 50 km beneath the overriding caribbean plate which has an effective elastic thickness of 5 km the model predicts that the highest uplift rate should be 3 7 m ka and occur on the southwest coast of the peninsula de osa the rate of uplift slows considerably to the northeast and indicates that the peninsula de osa is tilting to the northeast which agrees with observations in that region the predicted uplift rate attributed to aseismic ridge subduction also decreases along the coast both north and south of the peninsula de osa resulting in little uplift that can be attributed to cocos ridge subduction in the northwestern portions of the peninsula de nicoya

shallow focus seismicity composite focal mechanism and tectonics of the valle central of costa rica abstract a network of seismographs operating in the valle central of costa rica has recorded many small earthquakes near the cities of cartago and san jose this seismicity is similar in many ways to the shallow focus intraplate seismicity of central america to the north the earthquakes occur within tens of kilometers of quaternary volcanic centers at shallow focal depths the earthquakes occur predominantly on strike slip faults with the nodal plane that would correspond to a left lateral fault striking approximately east northeast and the nodal plane that would correspond to a right lateral fault striking approximately north northwest the shocks have a tendency to occur in seismic swarms the region of highest seismicity in our study was located southwest of cartago about 10 km from the meizoseismal zones of destructive earthquakes of 1841 and 1910 in detail the recently recorded small earthquakes seem to have occurred on different faults or fault segments than the 1910 earthquakes the tendency for shallow focus intraplate earthquakes to occur within kilometers of earthquakes that occurred several decades earlier has been noted elsewhere in central america the occurrence of shocks on distinct faults within the overall region of high activity appears similar to the occurrence of earthquakes on different fault strands in managua nicaragua we discuss the valle central seismicity in light of hypotheses proposed for the shocks farther north in central america our data can be interpreted in terms of the hypothesis that shallow focus intraplate earthquakes in central america concentrate on zones of strike slip faults that pass through offsets of the volcanic chain our data can also be interpreted in terms of the hypothesis that the earthquakes occur as the response of minor faults to high regional stresses throughout the region surrounding the volcanic chain both hypotheses leave some characteristics of the seismicity unexplained although these characteristics are not crucial evidence against the hypotheses a third hypothesis that the valle central source regions are different than intraplate source regions northward in central america and are occurring in a developing transform plate boundary between the caribbean and nazca plates is plausible on the basis of the regional plate tectonic environment but it is not strongly supported by the local geology of the valle central or by the characteristics of seismicity

the geometry of the wadati benioff zone under southern central america and its tectonic significance results from a high resolution local seismographic network abstract we present here a detailed geometry of the wadati benioff zone under costa rica obtained from seismicity recorded by a dense local seismographic network jointly operated by the costa rica volcanological and seismological observatory national university and the charles f richter seismological laboratory university of california santa cruz underneath the nicaragua costa rica border the wadati benioff zone smoothly contorts from steep to shallow dip angles nw to se but does not show evidence of a brittle tear as postulated by others however further to the se ne of puerto caldera costa rica the wadati benioff zone does show a segmentation the quesada sharp contortion at intermediate depths h 70km nw of this sharp contortion the deepest portion of the seismically active slab dips at about 80 and reaches maximum depths ranging from 200 km near the nicaragua costa rica border to 135 km under ciudad quesada to the se the deeper portion of the wadati benioff zone dips at about 60 and the seismicity does not extend below depths ranging from 125 km behind the volcanic arc to 50 km east of quepos in southern costa rica east of 83 55 w we find no evidence of the wadati benioff zone deeper than 50 km the obtained geometry and other known tectonic features related to the subduction of the cocos plate under the caribbean plate along the southern terminus of the middle america trench nicaragua and costa rica correlate well with along trench variations in age of the subducted cocos plate some of these tectonic features are 1 the shallowing of middle america trench bathymetry from nw to se 2 variations in the energy release within the subducted slab 3 differences in coupling between cocos and caribbean plates 4 the termination of the central america volcanic chain in central costa rica 5 distinct stress field variations on the overriding caribbean plate the subduction of the cocos ridge under southern costa rica is partially responsible for some of these features shallowing of the middle america trench in southern costa rica and for the high uplift rates of the outer arc however as the presence of the panama fracture zone limits the subducted extension of the cocos ridge to less than 100 km from the trench we propose that the overall geometry of the wadati benioff zone is controlled by abrupt along trench changes in the age of the subducted cocos plate

hydrological connectivity of soil pipes determined by ground penetrating radar tracer detection soil pipes are common and important features of many catchments particularly in semi arid and humid areas and can contribute a large proportion of runoff to river systems they may also signi cantly in uence catchment sediment and solute yield however there are often problems in nding and de ning soil pipe networks which are located deep below the surface ground penetrating radar gpr has been used for non destructive identi cation and mapping of soil pipes in blanket peat catchments while gpr can identify subsurface cavities it cannot alone determine hydrological connectivity between one cavity and another this paper presents results from an experiment to test the ability of gpr to establish hydrological connectivity between pipes through use of a tracer solution sodium chloride was injected into pipe cavities previously detected by the radar the gpr was placed downslope of the injection points and positioned on the ground directly above detected soil pipes the resultant radargrams showed signi cant changes in re ectance from some cavities and no change from others pipe waters were sampled in order to check the radar results changes in electrical conductivity of the pipe water could be detected by the gpr without data post processing when background levels were increased by more than approximately twofold it was thus possible to rapidly determine hydrological connectivity of soil pipes within dense pipe networks across hillslopes without ground disturbance it was also possible to remotely measure travel times through pipe systems the passing of the salt wave below the gpr produced an easily detectable signal on the radargram which required no post processing the technique should allow remote sensing of water sources and sinks for soil pipes below the surface the improved understanding of owpath connectivity will be important for understanding water delivery solutional and particulate denudation and hydrological and geomorphological model development copyright 2004 john wiley sons ltd

earthquake locations and three dimensional crustal structure in the coyote lake area central california previous work on the simultaneous inversion method has been improved and extended to incorporate iterative solution for earthquake locations and laterally heterogeneous structure approximate ray tracing and parameter separation are important elements of the improved method application of the method to p wave arrival time data recorded by stations of the u s geological survey central california network yields a three dimensional model for the velocity structure of the upper crust in an area encompassing the rupture zone of the coyote lake earthquake of august 1979 very strong correlations between the velocity model and the geology and gravity and magnetic anomalies are observed improved estimates of the locations of earthquakes in the study area are also determined the relocation of explosions indicates epicentral accuracies of the order of a kilometer or better based on the revised hypocentral locations it is concluded that the san andreas fault is vertical in this area with no actual offset between the epicenters and the fault trace in contrast the calaveras has two or more active fault surfaces one nearly vertical and another dipping 75 to the northeast

seismic measurements of the internal properties of fault zones the internal properties within and adjacent to fault zones are reviewed principally on the basis of laboratory borehole and seismic refraction and reflection data the deformation of rocks by faulting ranges from intragrain microcracking to severe alteration saturated microcracked and mildly fractured rocks do not exhibit a significant reduction in velocity but from borehole measurements densely fractured rocks do show significantly reduced velocities the amount of reduction generally proportional to the fracture density highly fractured rock and thick fault gouge along the creeping portion of the san andreas fault are evidenced by a pronounced seismic low velocity zone lvz which is either very thin or absent along locked portions of the fault thus there is a correlation between fault slip behavior and seismic velocity structure within the fault zone high pore pressure within the pronounced lvz may be conductive to fault creep deep seismic reflection data indicate that crustal faults sometimes extend through the entire crust models of these data and geologic evidence are consistent with a composition of deep faults consisting of highly foliated seismically anisotropic mylonites

subduction of the eastern panama basin and seismotectonics of northwestern south america spreading along the cocos nazca plate boundary since the breakup of the farallon plate in the miocene has resulted in the formation of the panama basin and a complex interaction of plates in and near northwestern south america current plate boundaries have been defined and segments of subducted lithosphere identified through selection of hypocentral locations of earthquakes considering only well located events and through focal mechanism determinations the existence of relict plate boundaries bathymetric features and the panamanian isthmus has affected the subduction process of the nazca plate beneath south america and determined the present day configuration of the subducting lithospheric plate there is no single triple junction separating the caribbean south american and nazca plates instead the panamanian isthmus and surrounding areas are accommodating east west compression and a lesser degree of north south compression along a series of thrust faults striking nw to ne and the andean ranges of ecuador colombia and venezeula are moving as a block nne relative to the rest of the south american plates along a system of faults following the front of the eastern cordillera the subducted portions of the panama basin and old farallon plate have become segmented into three pieces recognized in this study from north to south they are 1 a bucaramanga segment continuous with the caribbean seafloor northwest of colombia 2 a cauca segment continuous with oceanic crust nazca plate currently being subducted beneath south america at the colombia ecuador trench and 3 an ecuador segment at the northern end of the subducted lithospheric plate which is dipping at a small angle to the east beneath northern peru the segmentation of the subducted plate can be explained by the buoyancy of bathymetric features which have been partially subducted

lithospheric structure of the costa rican isthmus effects of subduction zone magmatism on an oceanic plateau we present the results of a multidisciplinary geophysical study conducted to investigate the lithospheric structure of the costa rican isthmus the physical properties of the lithosphere are resolved by three dimensional 3 d simultaneous inversion of velocity anomalies and hypocenter parameters using local earthquakes and 2 d forward modeling of onshore and offshore seismic refraction and gravity data according to our results the northern half of the costa rican isthmus is constituted by a 40 km thick crust with a 6 to 7 km thick oceanic crust subducting under it the uppermost level of the basement and most of the marginal wedge show intermediate velocities and high densities in good agreement with those described for flood basalts the midlevel shows velocities and densities representative of oceanic crust the bottommost level 20 40 km shows high velocities and densities typical of mafic rocks and the upper mantle displays anomalously low densities and velocities intracrustal heterogeneities at intermediate wavelengths are indicated by prominent velocity anomalies these results are consistent with a basement beneath the costa rican isthmus being part of the caribbean plateau originated at 85 90 ma with the onset of the galapagos hotspot the upper level corresponds to the flood basalts extruded during this phase and it includes most of the marginal wedge the second level represents the preexisting oceanic crust the mafic lower crust intracrustal heterogeneities and anomalous upper mantle are interpreted to be built up by underplating intrusion and crystallization of basaltic melts formed under the influence of subducting lithosphere dehydration

sound generation and air entrainment by breaking waves in the surf zone this paper presents the results of acoustic and optical measurements of individual breaking waves in the surf zone two hydrophones horizontally separated and deployed in 2 m of water off la jolla shores beach california were used to measure the source spectrum of breaking surf and characterize propagation through the surf zone over length scales of order 10 m the acoustic data show an acoustically active region within a wave which propagates shoreward the production of sound within the active region is associated with the formation of bubble plumes above 500 hz the sound is consistent with the radiation from individual bubble oscillations pictures were taken of the bubble plumes formed beneath the breaking surf providing estimates of the plume size and bubble size distribution and void fraction within a plume the density of bubbles scales as a 2 5 for a 1 mm where a is bubble radius and total void fractions of 0 3 0 4 were measured theoretical calculations show that ra

seismological studies at parkfield i simultaneous inversion for velocity structure and hypocenters using cubic b splines parameterization a model for the 3d p and s wave velocity structure at parkfield has been developed by the systematic inversion of arrival times for microearthquakes and surface vibroseis sources recorded by nearby calnet stations and by the local borehole installed 10 station high resolution seismic network there a modification of the linear b splines model parameterization method of thurber 1983 to a cubic b splines formulation through tests inverting synthetic data sets for the parkfield configuration is found to produce smooth velocity distributions that still retain some small scale features application of the method to 953 p wave and 288 s wave arrivals from 111 microearthquakes and 8 vibroseis locations throughout the study area yields a velocity structure having variable resolution a low velocity zone appears to coincide with the creeping segment of the san andreas fault zone northwest of the 1966 parkfield earthquake epicenter an increase of about 15 per cent in seismic velocities is seen from the northeast to southwest across this segment of the fault zone the higher velocities to the southwest apparently correspond to the granitic basement the contrast across the fault is subdued to the southeast along the 1966 rupture zone a high vp vs ratio of 1 9 is seen in a 2 km wide volume near the location of the 1966 main shock hypocenter presumably the nucleation zone for the expected m 6 parkfield earthquake

water table variability and runoff generation in an eroded peatland south pennines uk hydrological monitoring in an eroded south pennine peatland shows that persistent and frequent water table drawdowns occur at gully edge locations defining a deeper and thicker acrotelm than is observed in intact peatlands an erosional acrotelm antecedent water table elevation is a key control on the hydrological response to precipitation events in particular runoff percent the timing of peak discharges and maximum water table elevations significant discharge is generated whilst water table elevations are relatively low at gully edge locations and this has a strong influence on flow pathways four characteristics of runoff response are recognised i the rapid development of macropore pipe flow at the start of the storm ii peat rewetting water table elevation increase and continued macropore pipe flow iii maximum water table elevations and peak stream discharge with throughflow occurring within the erosional acrotelm and rapid flow through the subsurface macropore pipe network iv rapidly declining water table elevations and stream flow following the cessation of rainfall gully edge peats provide a key linkage between the hillslope hydrological system and channel flow so that their influence on the hydrological functioning of the peatlands is disproportionate to their aerial extent within the catchment future climate change may lead to further degradation of the bogs and a reinforcement of the importance of erosion gullies to runoff generation and water quality