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  • Journal article
    Collins GS, Lynch E, McAdam R, Davison TMet al., 2017,

    A numerical assessment of simple airblast models of impact airbursts

    , Meteoritics & Planetary Science, Vol: 52, Pages: 1542-1560, ISSN: 1086-9379

    Asteroids and comets 10–100 m in size that collide with Earth disrupt dramatically in the atmosphere with an explosive transfer of energy, caused by extreme air drag. Such airbursts produce a strong blastwave that radiates from the meteoroid's trajectory and can cause damage on the surface. An established technique for predicting airburst blastwave damage is to treat the airburst as a static source of energy and to extrapolate empirical results of nuclear explosion tests using an energy-based scaling approach. Here we compare this approach to two more complex models using the iSALE shock physics code. We consider a moving-source airburst model where the meteoroid's energy is partitioned as two-thirds internal energy and one-third kinetic energy at the burst altitude, and a model in which energy is deposited into the atmosphere along the meteoroid's trajectory based on the pancake model of meteoroid disruption. To justify use of the pancake model, we show that it provides a good fit to the inferred energy release of the 2013 Chelyabinsk fireball. Predicted overpressures from all three models are broadly consistent at radial distances from ground zero that exceed three times the burst height. At smaller radial distances, the moving-source model predicts overpressures two times greater than the static-source model, whereas the cylindrical line-source model based on the pancake model predicts overpressures two times lower than the static-source model. Given other uncertainties associated with airblast damage predictions, the static-source approach provides an adequate approximation of the azimuthally averaged airblast for probabilistic hazard assessment.

  • Journal article
    Morgan JV, 2016,

    The formation of peak rings in large impact craters

    , Science, Vol: 354, Pages: 878-882, ISSN: 0036-8075

    Large impacts provide a mechanism for resurfacing planets through mixing near-surface rocks with deeper material. Central peaks are formed from the dynamic uplift of rocks during crater formation. As crater size increases, central peaks transition to peak rings. Without samples, debate surrounds the mechanics of peak-ring formation and their depth of origin. Chicxulub is the only known impact structure on Earth with an unequivocal peak ring, but it is buried and only accessible through drilling. Expedition 364 sampled the Chicxulub peak ring, which we found was formed from uplifted, fractured, shocked, felsic basement rocks. The peak-ring rocks are cross-cut by dikes and shear zones and have an unusually low density and seismic velocity. Large impacts therefore generate vertical fluxes and increase porosity in planetary crust.

  • Journal article
    D'Arcy M, Whittaker AC, Roda Boluda DC, 2016,

    Measuring alluvial fan sensitivity to past climate changes using a self-similarity approach to grain size fining, Death Valley, California

    , Sedimentology, Vol: 64, Pages: 388-424, ISSN: 1365-3091

    The effects of climate change on eroding landscapes and the terrestrial sedimentary record are poorly understood. Using mountain catchment–alluvial fan systems as simple analogues for larger landscapes, a wide range of theoretical studies, numerical models and physical experiments have hypothesised that a change in precipitation rate could leave a characteristic signal in alluvial fan sediment flux, grain size and down-system fining rate. However, this hypothesis remains largely untested in real landscapes. This study measures grain-size fining rates from apex to toe on two alluvial fan systems in northern Death Valley, California, USA, which each have well-exposed modern and ca 70 ka surfaces, and where the long-term tectonic boundary conditions can be constrained. Between them, these surfaces capture a well-constrained temporal gradient in climate. A grain-size fining model is adapted, based on self-similarity and selective deposition, for application to these alluvial fans. This model is then integrated with cosmogenic nuclide constraints on catchment erosion rates, and observed grain-size fining data from two catchment-fan systems, to estimate the change in sediment flux from canyon to alluvial fan that occurred between mid-glacial and modern interglacial conditions. In a fan system with negligible sediment recycling, an approximately 30% decrease in precipitation rate led to a 20% decrease in sediment flux and a clear increase in the down-fan rate of fining, supporting existing landscape evolution models. Consequently, this study shows that small mountain catchments and their alluvial fan stratigraphy can be highly sensitive to orbital climate changes over <105 year timescales. However, in the second fan system it is observed that this sensitivity is completely lost when sediment is remobilised and recycled over a time period longer than the duration of the climatic perturbation. These analyses offer a new approach to quantitatively reconstructing the

  • Journal article
    Summersgill, Kontoe S, Potts D, 2016,

    On the use of nonlocal regularisation in slope stability problems

    , Computers and Geotechnics, Vol: 82, Pages: 187-200, ISSN: 1873-7633

    The present study examines the use of nonlocal regularisation in a coupled consolidation problem of an excavated slope in a strain softening material. The considered boundary value problem allows for a thorough evaluation of the nonlocal regularisation approach, as it does not entail any kinematic restraint on the slip surface development. The examined nonlocal strain softening constitutive model requires the specification of one additional parameter, the defined length DL, which essentially modifies the rate of softening. In addition, the optional radius of influence parameter, RI, can be specified to reduce the number of local strains referenced in the nonlocal strain calculation and thus increase the efficiency of the analysis. The nonlocal strain softening constitutive model reduces significantly the mesh dependency of cut slope analyses for a range of mesh layouts and element sizes in comparison to the conventional local strain softening approach. The nonlocal analyses are not entirely mesh independent, but the predicted time to failure and horizontal displacement over time are much more consistent compared to analyses that employ the local strain softening constitutive model. Further investigation, computing the Factor of Safety of various mesh arrangements showed that for drained conditions the nonlocal regularisation eliminates the mesh dependence shown for the same analyses by the conventional local strain softening model. The impact of the two nonlocal parameters, DL and RI, on the numerical predictions is also parametrically examined. The parameter DL modifies the softening rate of the soil and therefore its selection should be based on simulating a realistic softening rate for the examined soil material. The RI parameter was found to reduce significantly the computational cost at the expense of affecting the development of the secondary slip surfaces. Overall though, the results and the critical slip surface were similar for all considered values of RI.

  • Journal article
    Allen H, Jackson CA-L, Fraser AJ, 2016,

    Gravity-driven deformation of a youthful saline giant: the interplay between gliding and spreading in the Messinian Basins of the Eastern Mediterranean

    , Petroleum Geoscience, Vol: 22, Pages: 340-356, ISSN: 1354-0793

    The triggers and drivers for salt-related deformation on continental margins are intensely debated, reflecting uncertainties regarding the diagnostic value of certain structural styles, in addition to the fundamental mechanics associated with the two principal mechanisms (gliding and spreading). Determining the triggers and drivers for salt-related deformation is important because they provide insights into continent-scale geodynamic processes, the regional kinematics of gravity-driven deformation, and sediment dispersal and hydrocarbon prospectivity. The processes associated with and timing of deformation of Messinian salt in the offshore eastern Mediterranean are uncertain, thus is our understanding of the geodynamic evolution of this tectonically complex region. We here use an extensive 2D and 3D seismic reflection dataset to test models for the salt-tectonic development of Messinian salt. We contend that gliding and spreading were not mutually exclusive, but likely overlapped through time and space, showing a close relationship local and far-field tectonics (gliding), as well as differential overburden loading (spreading). We also argue that intrasalt strain and seismic-stratigraphic patterns can be explained by a model invoking a single, post-Messinian period of salt-related deformation, rather than a more complex model involving two separate, non-coaxial deformation events occurring during and after salt deposition.

  • Journal article
    Albalushi A, Neumaier M, Fraser AJ, Jackson CA-Let al., 2016,

    The impact of the Messinian Salinity Crisis on the petroleum system of the Eastern Mediterranean: a critical assessment using 2D-petroleum system modelling

    , Petroleum Geoscience, Vol: 22, Pages: 357-379, ISSN: 1354-0793

    The offshore Levant Basin demonstrates one of the most phenomenal natural examples of a working petroleum system associated with a relatively rapid unloading and loading cycle caused by the the Messinian Salinity Crisis (MSC). In this study, 2D basin and petroleum systems modelling suggests that the geologically instantaneous water unloading of c. 2070 m and subsequent rapid salt deposition and refill impacts the subsurface pore pressure and temperature in the underlying sediments. The pressure drop is modelled to be instantaneous, whereas the impact on temperature is more of a transient response. This has important consequences for the shallow sub-Messinian biogenic petroleum system, which is assumed to have experienced fluid brecciation associated with massive fluid escape events. Deeper Oligo-Miocene sediments are far less affected, thus indicating a "preservation window" for biogenic gas accumulations, which hosts the recent discoveries (Tamar, Leviathan, Aphrodite). Hydrocarbon accumulations of a "bubble point oil" composition are modelled to have experienced cap expansion during the drawdown, with the pressure drop being the primary control. This study suggests that seal-limited traps are expected to have undergone a catastrophic seal failure whereas the impact of the MSC is modelled to be less destructive for size-limited and particularly charge-limited traps.

  • Conference paper
    Pedone G, Tsiampousi A, Cotecchia F, Zdravkovic Let al., 2016,

    Effects of soil-vegetation-atmosphere interaction on the stability of a clay slope: a case study

    , 3rd European Conference on Unsaturated Soils, Publisher: EDP Sciences, ISSN: 2267-1242

    Deep and slow landslide processes are frequently observed in clay slopes located along the Southern Apennines (Italy). A case study representative of these processes, named Pisciolo case study, is discussed in the paper. The geo-hydro-mechanical characteristics of the materials involved in the instability phenomena are initially discussed. Pluviometric, piezometric, inclinometric and GPS monitoring data are subsequently presented, suggesting that rainfall infiltration constitutes the main factor inducing slope movements. The connection between formation of landslide bodies and slope-atmosphere interaction has been demonstrated through a hydro-mechanical finite element analysis, whose results are finally reported in the work. This analysis has been conducted employing a constitutive model that is capable of simulating both saturated and unsaturated soil behaviour, as well as a boundary condition able to simulate the effects of the soil-vegetation-atmosphere interaction.

  • Conference paper
    Tsiampousi A, Zdravkovic L, Potts DM, 2016,

    Soil-atmosphere interaction in unsaturated cut slopes

    , 3rd European Conference on Unsaturated Soils, Publisher: EDP Sciences, ISSN: 2267-1242

    Interaction between atmosphere and soil has only recently attracted significant interest. Soil-atmosphereinteraction takes place under dynamic climatic conditions, which vary throughout the year and are expected to sufferconsiderable alterations due to climate change. However, Geotechnical Analysis has traditionally been limited tosimplistic approaches, where winter and summer pore water pressure profiles are prescribed. Geotechnical Structures,such as cut slopes, are known to be prone to large irreversible displacements under the combined effect of wateruptake by a complex vegetation root system and precipitation. If such processes take place in an unsaturated materialthe complexity of the problem renders the use of numerical analysis essential. In this paper soil-atmosphereinteraction in cut slopes is studied using advanced, fully coupled partially saturated finite element analyses. The effectof rainfall and evapotranspiration is modelled through sophisticated boundary conditions, applying actualmeteorological data on a monthly basis. Stages of low and high water demand vegetation are considered for a periodof several years, before simulating the effect of vegetation removal. The analysis results are presented with regard tothe serviceability and stability of the cut slope.

  • Journal article
    Tsaparli V, Kontoe S, Taborda D, Potts DMet al., 2016,

    Vertical ground motion and its effects on liquefaction resistance of fully saturated sand deposits

    , Proceedings of the Royal Society A: Mathematical, Physical & Engineering Sciences, Vol: 472, ISSN: 1471-2946

    Soil liquefaction has been extensively investigated over the years with the aim to understand its fundamental mechanism and successfully remediate it. Despite the multi-directional nature of earthquakes, the vertical seismic component is largely neglected, as it is traditionally considered to be of much lower amplitude than the components in the horizontal plane. The 2010–2011 Canterbury earthquake sequence in New Zealand is a prime example that vertical accelerations can be of significant magnitude, with peak amplitudes well exceeding their horizontal counterparts. As research on this topic is very limited, there is an emerging need for a more thorough investigation of the vertical motion and its effect on soil liquefaction. As such, throughout this study, uni- and bidirectional finite-element analyses are carried out focusing on the influence of the input vertical motion on sand liquefaction. The effects of the frequency content of the input motion, of the depth of the deposit and of the hydraulic regime, using variable permeability, are investigated and exhaustively discussed. The results indicate that the usual assumption of linear elastic response when compressional waves propagate in a fully saturated sand deposit does not always hold true. Most importantly post-liquefaction settlements appear to be increased when the vertical component is included in the analysis.

  • Journal article
    Phillips T, Jackson CA-L, Bell RE, Duffy OD, Fossen Het al., 2016,

    Reactivation of intrabasement structures during rifting: a case study from offshore southern Norway

    , Journal of Structural Geology, Vol: 91, Pages: 54-73, ISSN: 0191-8141

    Pre-existing structures within crystalline basement may exert a significant influence over the evolution of rifts. However, the exact manner in which these structures reactivate and thus their degree of influence over the overlying rift is poorly understood. Using borehole-constrained 2D and 3D seismic reflection data from offshore Southern Norway we identify and constrain the three-dimensional geometry of a series of enigmatic intrabasement reflections. Through 1D waveform modelling and 3D mapping of these reflection packages, we correlate them to the onshore Caledonian thrust belt and Devonian shear zones. Based on the seismic-stratigraphic architecture of the post-basement succession we identify several phases of reactivation of the intrabasement structures associated with multiple tectonicevents. Reactivation preferentially occurs along relatively thick (c. 1km), relatively steeply dipping (c. 30°) structures, with three main styles of interactions observed between them and overlying faults: (i) faults exploiting intrabasement weaknesses represented by intra-shear zone mylonites; (ii) faults that initiate within the hangingwall of the shear zones, inheriting their orientation and merging with said structure at depth; or (iii) faults that initiate independently from and cross-cut intrabasement structures. We demonstrate that large-scale discrete shear zones act as a long-lived structural template for fault initiation during multiple phases of rifting.

  • Journal article
    Abolghasemi M, Piggott MD, Spinneken J, Vire A, Cotter CJ, Crammond Set al., 2016,

    Simulating tidal turbines with multi-scale mesh optimisation techniques

    , Journal of Fluids and Structures, Vol: 66, Pages: 69-90, ISSN: 1095-8622

    Embedding tidal turbines within simulations of realistic large-scale tidal flows is a highly multi-scale problem that poses significant computational challenges. Here this problem is tackled using actuator disc momentum (ADM) theory and Reynolds-averaged Navier-Stokes (RANS) with, for the first time, dynamically adaptive mesh optimisation techniques. Both k-ω and k-ω SST RANS models have been developed within the Fluidity framework, an adaptive mesh CFD solver, and the model is validated against two sets of experimental flume test results. A brief comparison against a similar OpenFOAM model is presented to portray the benefits of the finite element discretisation scheme employed in the Fluidity ADM model. This model has been developed with the aim that it will be seamlessly combined with larger numerical models simulating tidal flows in realistic domains. This is where the mesh optimisation capability is a major advantage as it enables the mesh to be refined dynamically in time and only in the locations required, thus making optimal use of limited computational resources.

  • Journal article
    Stafford PJ, Sullivan TJ, Pennucci D, 2016,

    Empirical correlation between inelastic and elastic spectral displacement demands

    , Earthquake Spectra, Vol: 32, Pages: 1419-1448, ISSN: 8755-2930

    Inelastic spectral displacement demand is arguably one of the most effective, simplified means of relating earthquake intensity to building damage. However, seismic hazard assessment is typically conducted using empirical ground-motion prediction equations (GMPEs) that only provide indications of elastic spectral response quantities, which an engineer subsequently relates to inelastic demands using empirical relationships such as the equal-displacement rule. An alternative approach is to utilize relationships for the inelastic spectral displacement demand directly within the seismic hazard assessment process. Such empirical relationships are developed in this work, as a function of magnitude, distance, building period and yield strength coefficient, for four different hysteretic models that are representative of a wide range of possible structural typologies found in practice. The new relationships are likely to be particularly useful for performance-based seismic design and assessment.

  • Journal article
    Reeve MT, Jackson CA-L, Bell RE, Magee C, Bastow IDet al., 2016,

    The Stratigraphic Record of Pre-breakup Geodynamics: Evidence from the Barrow Delta, offshore Northwest Australia

    , Tectonics, Vol: 35, Pages: 1935-1968, ISSN: 1944-9194

    The structural and stratigraphic evolution of rift basins and passive margins has been widely studied, with many analyses demonstrating that delta systems can provide important records of post-rift geodynamic processes. However, the apparent lack of ancient syn-breakup delta systems and the paucity of seismic imaging across continent-ocean boundaries means the transition from continental rifting to oceanic spreading remains poorly understood. The Early Cretaceous Barrow Group of the North Carnarvon Basin, offshore NW Australia was a major deltaic system that formed during the latter stages of continental rifting, and represents a rich sedimentary archive, documenting uplift, subsidence and erosion of the margin. We use a regional database of 2D and 3D seismic and well data to constrain the internal architecture of the Barrow Group. Our results highlight three major depocentres: the Exmouth and Barrow sub-basins, and southern Exmouth Plateau. Over-compaction of pre-Cretaceous sedimentary rocks in the South Carnarvon Basin, and pervasive reworking of Permian and Triassic palynomorphs in the offshore Barrow Group, suggests that the onshore South Carnarvon Basin originally contained a thicker sedimentary succession, which was uplifted and eroded prior to breakup. Backstripping of sedimentary successions encountered in wells in the Exmouth Plateau depocentre indicate anomalously rapid tectonic subsidence (≤0.24 mm yr-1) accommodated Barrow Group deposition, despite evidence for minimal, contemporaneous upper crustal extension. Our results suggest that classic models of uniform extension cannot account for the observations of uplift and subsidence in the North Carnarvon Basin, and may indicate a period of depth-dependent extension or dynamic topography preceding breakup.

  • Conference paper
    Skiada E, Kontoe S, Stafford P, Potts DMet al., 2016,

    Canyon Depth Effect on Surface Ground Motion

    , 1st International Conference on Natural Hazards & Infrastructure

    Topographic effects are rarely accounted for in seismic design codes, despite their potential to significantly modify surfaceground motions. This paper investigates the influence of a canyon’s slope height on the surface ground motion through aparametric time-domain Finite Element (FE) study. A two-dimensional plane-strain model of an idealised canyon isconsidered for vertically propagating SV waves, using wavelets as input excitation. The model consists of two step-likeslopes with slope height (H), in a homogeneous linear elastic soil layer overlying rigid bedrock. The analysis results showthat the distribution of topographic aggravation at the ground surface varies significantly with normalized canyon depthover the input wavelength (H/λ) and it does not necessarily reach a maximum at a specific H/λ ratio, as has been suggestedin previous studies. The validity of this conclusion is investigated for different depths to bedrock and soil layer properties.

  • Journal article
    Schmidt L, Fouxon I, Krug D, van Reeuwijk M, Holzner Met al., 2016,

    Clustering of particles in turbulence due to phoresis

    , Physical Review E, Vol: 93, ISSN: 1539-3755

    We demonstrate that diffusiophoretic, thermophoretic, and chemotactic phenomena in turbulence lead to clustering of particles on multifractal sets that can be described using one single framework, valid when the particle size is much smaller than the smallest length scale of turbulence l0. To quantify the clustering, we derive positive pair correlations and fractal dimensions that hold for scales smaller than l0. For scales larger than l0 the pair-correlation function is predicted to show a stretched exponential decay towards 1. In the case of inhomogeneous turbulence we find that the fractal dimension depends on the direction of inhomogeneity. By performing experiments with particles in a turbulent gravity current we demonstrate clustering induced by salinity gradients in conformity to the theory. The particle size in the experiment is comparable to l0, outside the strict validity region of the theory, suggesting that the theoretical predictions transfer to this practically relevant regime. This clustering mechanism may provide the key to the understanding of a multitude of processes such as formation of marine snow in the ocean and population dynamics of chemotactic bacteria.

  • Journal article
    Adam A, Pavlidis D, Percival J, Salinas P, Xie Z, Fang F, Pain C, Muggeridge A, Jackson Met al., 2016,

    Higher-order conservative interpolation between control-volume meshes: Application to advection and multiphase flow problems with dynamic mesh adaptivity

    , Journal of Computational Physics, Vol: 321, Pages: 512-531, ISSN: 1090-2716

    A general, higher-order, conservative and bounded interpolation for the dynamic and adaptive meshing of control-volume fields dual to continuous and discontinuous finite element representations is presented. Existing techniques such as node-wise interpolation are not conservative and do not readily generalise to discontinuous fields, whilst conservative methods such as Grandy interpolation are often too diffusive. The new method uses control-volume Galerkin projection to interpolate between control-volume fields. Bounded solutions are ensured by using a post-interpolation diffusive correction. Example applications of the method to interface capturing during advection and also to the modelling of multiphase porous media flow are presented to demonstrate the generality and robustness of the approach.

  • Journal article
    Massart BYG, Jackson MD, Hampson GJ, Johnson HD, Legler B, Jackson CA-Let al., 2016,

    Effective flow properties of heterolithic, cross-bedded tidal sandstones: Part 1. Surface-based modeling

    , AAPG Bulletin, Vol: 100, Pages: 697-721, ISSN: 0149-1423

    Tidal heterolithic sandstones are commonly characterized by millimeter- to centimeter-scale intercalations of mudstone and sandstone. Consequently, their effective flow properties are poorly predicted by (1) data that do not sample a representative volume or (2) models that fail to capture the complex three-dimensional architecture of sandstone and mudstone layers. We present a modeling approach in which surfaces are used to represent all geologic heterogeneities that control the spatial distribution of reservoir rock properties (surface-based modeling). The workflow uses template surfaces to represent heterogeneities classified by geometry instead of length scale. The topology of the template surfaces is described mathematically by a small number of geometric input parameters, and models are constructed stochastically. The methodology has been applied to generate generic, three-dimensional minimodels (9 m3 volume) of cross-bedded heterolithic sandstones representing trough and tabular cross-bedding with differing proportions of sandstone and mudstone, using conditioning data from two outcrop analogs from a tide-dominated deltaic deposit. The minimodels capture the cross-stratified architectures observed in outcrop and are suitable for flow simulation, allowing computation of effective permeability values for use in larger-scale models. We show that mudstone drapes in cross-bedded heterolithic sandstones significantly reduce effective permeability and also impart permeability anisotropy in the horizontal as well as vertical flow directions. The workflow can be used with subsurface data, supplemented by outcrop analog observations, to generate effective permeability values to be derived for use in larger-scale reservoir models. The methodology could be applied to the characterization and modeling of heterogeneities in other types of sandstone reservoirs.

  • Journal article
    Sosa Gonzalez V, Bierman PR, Fernandes NF, Rood DHet al., 2016,

    Long-term background denudation rates of southern and southeastern Brazilian watersheds estimated with cosmogenic 10Be

    , Geomorphology, Vol: 268, Pages: 54-63, ISSN: 1872-695X

    In comparison to humid temperate regions of the Northern Hemisphere, less is known about the long-term (millennial scale) background rates of erosion in Southern Hemisphere tropical watersheds. In order to better understand the rate at which watersheds in southern and southeastern Brazil erode, and the relationship of that erosion to climate and landscape characteristics, we made new measurements of in situ produced 10Be in river sediments and we compiled all extant measurements from this part of the country.New data from 14 watersheds in the states of Santa Catarina (n = 7) and Rio de Janeiro (n = 7) show that erosion rates vary there from 13 to 90 m/My (mean = 32 m/My; median = 23 m/My) and that the difference between erosion rates of basins we sampled in the two states is not significant. Sampled basin area ranges between 3 and 14,987 km2, mean basin elevation between 235 and 1606 m, and mean basin slope between 11 and 29°. Basins sampled in Rio de Janeiro, including three that drain the Serra do Mar escarpment, have an average basin slope of 19°, whereas the average slope for the Santa Catarina basins is 14°. Mean basin slope (R2 = 0.73) and annual precipitation (R2 = 0.57) are most strongly correlated with erosion in the basins we studied. At three sites where we sampled river sand and cobbles, the 10Be concentration in river sand was greater than in the cobbles, suggesting that these grain sizes are sourced from different parts of the landscape.Compiling all cosmogenic 10Be-derived erosion rates previously published for southern and southeastern Brazil watersheds to date (n = 76) with our 14 sampled basins, we find that regional erosion rates (though low) are higher than those of watersheds also located on other passive margins including Namibia and the southeastern North America. Brazilian basins erode at a pace similar to escarpments in southeastern North America. Erosion rates in southern and southeastern Brazil are directly and positively relat

  • Journal article
    Bora SS, Scherbaum F, Kuehn N, Stafford PJet al., 2016,

    On the relationship between Fourier and response spectra: Implications for the adjustment of empirical ground-motion prediction equations (GMPEs)

    , Bulletin of the Seismological Society of America, Vol: 106, ISSN: 1943-3573

    The functional form of empirical response spectral ground-motion predictionequations (GMPEs) is often derived using concepts borrowed from Fourier spectralmodeling of ground motion. As these GMPEs are subsequently calibrated with empiricalobservations, this may not appear to pose any major problems in the prediction ofground motion for a particular earthquake scenario. However, the assumption that Fourierspectral concepts persist for response spectra can lead to undesirable consequenceswhen it comes to the adjustment of response spectral GMPEs to represent conditions notcovered in the original empirical data set. In this context, a couple of important questionsarise, for example, what are the distinctions and/or similarities between Fourier andresponse spectra of ground motions? And, if they are different, then what is the mechanismresponsible for such differences and how do adjustments that are made to Fourieramplitude spectrum (FAS) manifest in response spectra? The present article explores therelationship between the Fourier and response spectrum of ground motion by usingrandom vibration theory (RVT). With a simple Brune (1970, 1971) source model, RVTgeneratedacceleration spectra for a fixed magnitude and distance scenario are used. TheRVT analyses reveal that the scaling of low oscillator-frequency response spectral ordinatescan be treated as being equivalent to the scaling of the corresponding Fourierspectral ordinates. However, the high oscillator-frequency response spectral ordinatesare controlled by a rather wide band of Fourier spectral ordinates. In fact, the peakground acceleration, counter to the popular perception that it is a reflection of the highfrequencycharacteristics of ground motion, is controlled by the entire Fourier spectrumof ground motion. Additionally, this article demonstrates how an adjustment made toFAS is similar or different to the same adjustment made to response spectral ordinates.For this purpose, two cases: adjustments to the stres

  • Journal article
    Maguire R, Ritsema J, van Keken PE, Fichtner A, Goes Set al., 2016,

    P- and S-wave delays caused by thermal plumes

    , Geophysical Journal International, Vol: 206, Pages: 1169-1178, ISSN: 0956-540X

    Many studies have sought to seismically image plumes rising from the deep mantle in order to settle the debate about their presence and role in mantle dynamics, yet the predicted seismic signature of realistic plumes remains poorly understood. By combining numerical simulations of flow, mineral-physics constraints on the relationships between thermal anomalies and wave speeds, and spectral-element method based computations of seismograms, we estimate the delay times of teleseismic S and P waves caused by thermal plumes. Wave front healing is incomplete for seismic periods ranging from 10 s (relevant in traveltime tomography) to 40 s (relevant in waveform tomography). We estimate P-wave delays to be immeasurably small (<0.3 s). S-wave delays are larger than 0.4 s even for S waves crossing the conduits of the thinnest thermal plumes in our geodynamic models. At longer periods (>20 s), measurements of instantaneous phase misfit may be more useful in resolving narrow plume conduits. To detect S-wave delays of 0.4–0.8 s and the diagnostic frequency dependence imparted by plumes, it is key to minimize the influence of the heterogeneous crust and upper mantle. We argue that seismic imaging of plumes will advance significantly if data from wide-aperture ocean-bottom networks were available since, compared to continents, the oceanic crust and upper mantle are relatively simple.

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