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  • Conference paper
    Rimoy SP, Jardine RJ, 2015,

    Axial capacity ageing trends of piles driven in silica sands

    , Pages: 637-642

    Field studies of piles driven in silica sands have shown axial capacity increases (set-up) over the days to months that follow driving, long after installation induced pore pressures have dissipated. Three main hypotheses have been proposed regarding the governing mechanisms and tested to a limited extent through relatively modest databases of pile load tests. However, a secure understanding of the governing mechanisms is required before projecting ageing predictions for other cases and considering how ageing trends might be affected by, for example, scale, pile type, groundwater regime or submerged offshore conditions. This paper reports an extended database study aimed at assessing the distinct ageing contributions of base and shaft loads, the potential influence of compression or tension axial loading directions, the effects of prior loading history, the influence of groundwater type as well as the effects of pile diameter and material. The analysis is informed by parallel highly instrumented model tests that were designed to investigate and identify the fundamental ageing mechanisms. The findings have important implications for pile design and re-assessment.
  • Journal article
    Stafford PJ, 2015,

    Variability and uncertainty in empirical ground-motion prediction for probabilistic hazard and risk analyses

    , Geotechnical, Geological and Earthquake Engineering, Vol: 39, Pages: 97-128, ISSN: 1573-6059

    © The Author(s) 2015. The terms aleatory variability and epistemic uncertainty mean different things to people who routinely use them within the fields of seismic hazard and risk analysis. This state is not helped by the repetition of loosely framed generic definitions that actually inaccurate. The present paper takes a closer look at the components of total uncertainty that contribute to ground-motion modelling in hazard and risk applications. The sources and nature of uncertainty are discussed and it is shown that the common approach to deciding what should be included within hazard and risk integrals and what should be pushed into logic tree formulations warrants reconsideration. In addition, it is shown that current approaches to the generation of random fields of ground motions for spatial risk analyses are incorrect and a more appropriate framework is presented.
  • Journal article
    Jacobs CT, Avdis A, Mouradian SL, Piggott MDet al., 2015,

    Integrating research data management into geographical information systems

    , CEUR Workshop Proceedings, Vol: 1529, Pages: 7-17, ISSN: 1613-0073

    Ocean modelling requires the production of high-fidelity com-putational meshes upon which to solve the equations of motion. The production of such meshes by hand is often infeasible, considering the complexity of the bathymetry and coastlines. The use of Geographical Information Systems (GIS) is therefore a key component to discretising the region of interest and producing a mesh appropriate to resolve the dynamics. However, all data abociated with the production of a mesh must be provided in order to contribute to the overall recomputability of the subsequent simulation. This work presents the integration of re-search data management in QMesh, a tool for generating meshes using GIS. The tool uses the PyRDM library to provide a quick and easy way for scientists to publish meshes, and all data required to regenerate them, to persistent online repositories. These repositories are abigned unique identifiers to enable proper citation of the meshes in journal articles.
  • Journal article
    Agrusta R, van Hunen J, Goes S, 2014,

    The effect of metastable pyroxene on the slab dynamics

    , Geophysical Research Letters, Vol: 41, Pages: 8800-8808, ISSN: 1944-8007

    Seismic studies show that some subducting slabs penetrate straight into the lower mantle, whereas others seem to flatten near the base of the mantle transition zone. Slab stagnation is often attributed to an increase in viscosity and phase transformations in the olivine system. However, recent mineral physics studies showed that due to extremely low transformational diffusion rates, low-density metastable pyroxene may persist into the transition zone in cool slabs. Here we use a dynamically fully self-consistent subduction model to investigate the influence of metastable pyroxene on the dynamics of subducting oceanic lithosphere. Our results show that metastable pyroxene affects slab buoyancy at least as much as olivine metastability. However, unlike metastable olivine, which can inhibit slab penetration in the lower mantle only for cold, old, and fast slabs, metastable pyroxene is likely to also affect sinking of relatively young and slow slabs.
  • Journal article
    Stephenson SN, Roberts GG, Hoggard MJ, Whittaker ACet al., 2014,

    A Cenozoic Uplift History of Mexico and its Surroundings From Longitudinal River Profiles

    , Geochemistry, Geophysics, Geosystems, Vol: 15, Pages: 4734-4758, ISSN: 1525-2027

    Geodynamic models of mantle convection predict that Mexico and western North America share a history of dynamic support. We calculate admittance between gravity and topography, which indicates that the elastic thickness of the plate in Mexico is 11 km and in western North America it is 12 km. Admittance at wavelengths > 500 km in these regions suggests that topography is partly supported by subcrustal processes. These results corroborate estimates of residual topography from isostatic calculations and suggest that the amount of North American topography supported by the mantle may exceed 1 km. The Cenozoic history of magmatism, sedimentary flux, thermochronometric denudation estimates, and uplifted marine terraces imply that North American lithosphere was uplifted and eroded during the last 30 Ma. We jointly invert 533 Mexican and North American longitudinal river profiles to reconstruct a continent-scale rock uplift rate history. Uplift rate is permitted to vary in space and time. Erosional parameters are calibrated using incision rate data in southwest Mexico and the Colorado Plateau. Calculated rock uplift rates were 0.15–0.2 mm/yr between 25 and10 Ma. Central Mexico experienced the highest uplift rates. Central and southern Mexico continued to uplift at 0.1 mm/yr until recent times. This uplift history is corroborated by independent constraints. We predict clastic flux to the Gulf of Mexico and compare it to independent estimates. We tentatively suggest that the loop between uplift, erosion, and deposition can be closed here. Mexico's staged uplift history suggests that its dynamic support has changed during the last 30 Ma.
  • Journal article
    Reeve MT, Bell RE, Duffy OB, Jackson CA-L, Sansom Eet al., 2014,

    The Development of Non-Colinear Fault Systems: What Can We Learn From 3D Seismic Reflection Data?

    , Journal of Structural Geology, Vol: 70, Pages: 141-155, ISSN: 0191-8141

    Many rift zones exhibit a range of fault orientations, rather than simple colinear faults that strike orthogonal to the least principal stress. The formation of non-colinear fault sets has implications in assessing rift zone kinematics, as well as determining the palaeo-stress state in extensional basins. Using high-resolution borehole-constrained 3D seismic reflection data we deduce the likely mechanisms responsible for the formation of a population of non-colinear faults in the Måløy Slope area of the northern North Sea. Three basement-displacing fault populations exist on the Måløy Slope; (i) large (>1 km throw), N-S-striking faults, and smaller (<250 m throw) (ii) N-S- and (iii) NE-SW-striking faults, all of which initiated in the Middle Jurassic. Coeval growth of these fault populations, and the apparent correlation between the NE-SW faults and a NE-SW-trending gravity and magnetic anomaly high suggests that variation in basement properties may have represented a NE-SW trending zone of weakness which was subsequently exploited during Jurassic extension. The results of our study have critical implications for understanding the larger-scale kinematic evolution of the North Sea rift, arguing that major rotations in the extension direction are not required to generate multiple fault sets locally or across the rift.
  • Journal article
    Kimura S, Holland PR, Jenkins A, Piggott Met al., 2014,

    The Effect of Meltwater Plumes on the Melting of a Vertical Glacier Face

    , JOURNAL OF PHYSICAL OCEANOGRAPHY, Vol: 44, Pages: 3099-3117, ISSN: 0022-3670
  • Journal article
    McPhillips D, Bierman PR, Rood DH, 2014,

    Millennial-scale record of landslides in the Andes consistent with earthquake trigger

    , NATURE GEOSCIENCE, Vol: 7, Pages: 925-930, ISSN: 1752-0894
  • Journal article
    Holgate NE, Hampson GJ, Jackson CA-L, Petersen SAet al., 2014,

    Constraining uncertainty in interpretation of seismically imaged clinoforms in deltaic reservoirs, Troll field, Norwegian North Sea: Insights from forward seismic models of outcrop analogs

    , AAPG BULLETIN, Vol: 98, Pages: 2629-2663, ISSN: 0149-1423
  • Journal article
    Bland PA, Collins GS, Davison TM, Abreu NM, Ciesla FJ, Muxworthy AR, Moore Jet al., 2014,

    Pressure-temperature evolution of primordial solar system solids during impact-induced compaction

    , Nature Communications, Vol: 5, Pages: 1-13, ISSN: 2041-1723

    Prior to becoming chondritic meteorites, primordial solids were a poorly consolidated mix of mm-scale igneous inclusions (chondrules) and high-porosity sub-μm dust (matrix). We used high-resolution numerical simulations to track the effect of impact-induced compaction on these materials. Here we show that impact velocities as low as 1.5 km s−1 were capable of heating the matrix to >1,000 K, with pressure–temperature varying by >10 GPa and >1,000 K over ~100 μm. Chondrules were unaffected, acting as heat-sinks: matrix temperature excursions were brief. As impact-induced compaction was a primary and ubiquitous process, our new understanding of its effects requires that key aspects of the chondrite record be re-evaluated: palaeomagnetism, petrography and variability in shock level across meteorite groups. Our data suggest a lithification mechanism for meteorites, and provide a ‘speed limit’ constraint on major compressive impacts that is inconsistent with recent models of solar system orbital architecture that require an early, rapid phase of main-belt collisional evolution.
  • Journal article
    Collins GS, 2014,

    Numerical simulations of impact crater formation with dilatancy

    , Journal of Geophysical Research: Planets, Vol: 119, Pages: 2600-2619, ISSN: 2169-9097

    Impact‐induced fracturing creates porosity that is responsible for many aspects of the geophysical signature of an impact crater. This paper describes a simple model of dilatancy—the creation of porosity in a shearing geological material—and its implementation in the iSALE shock physics code. The model is used to investigate impact‐induced dilatancy during simple and complex crater formation on Earth. Simulations of simple crater formation produce porosity distributions consistent with observations. Dilatancy model parameters appropriate for low‐quality rock masses give the best agreement with observation; more strongly dilatant behavior would require substantial postimpact porosity reduction. The tendency for rock to dilate less when shearing under high pressure is an important property of the model. Pressure suppresses impact‐induced dilatancy: in the shock wave, at depth beneath the crater floor, and in the convergent subcrater flow that forms the central uplift. Consequently, subsurface porosity distribution is a strong function of crater size, which is reflected in the inferred gravity anomaly. The Bouguer gravity anomaly for simulated craters smaller than 25 km is a broad low with a magnitude proportional to the crater radius; larger craters exhibit a central gravity high within a suppressed gravity low. Lower crustal pressures on the Moon relative to Earth imply that impact‐induced dilatancy is more effective on the Moon than Earth for the same size impact in an initially nonporous target. This difference may be mitigated by the presence of porosity in the lunar crust.
  • Journal article
    Fourel L, Goes S, Morra G, 2014,

    The role of elasticity in slab bending

    , Geochemistry, Geophysics, Geosystems, Vol: 15, Pages: 4507-4525, ISSN: 1525-2027

    Previous studies showed that plate rheology exerts a dominant control on the shape and velocity of subducting plates. Here, we perform a systematic investigation of the role of elasticity in slab bending, using fully dynamic 2-D models where an elastic, viscoelastic, or viscoelastoplastic plate subducts freely into a purely viscous mantle. We derive a scaling relationship between the bending radius of viscoelastic slabs and the Deborah number, De, which is the ratio of Maxwell time over deformation time. We show that De controls the ratio of elastically stored energy over viscously dissipated energy and find that at inline image, substantially less energy is required to bend a viscoelastic slab to the same shape as a purely viscous slab with the same intrinsic viscosity. Elastically stored energy at higher De favors retreating modes of subduction via unbending, while trench advance only occurs for some cases with inline image. We estimate the apparent Deborah numbers of natural subduction zones and find values ranging from inline image to > 1, where most zones have low inline image, but a few young plates have De > 0.1. Slabs with inline image either have very low viscosities or they may be yielding, in which case our De estimates may be underestimated by up to an order of magnitude, potentially pointing towards a significant role of elasticity in inline image of the subduction zones. In support of such a role of elasticity in subduction, we find that increasing De correlates with increasing proportion of larger seismic events in both instrumental and historic catalogues.
  • Journal article
    Han B, Zdravkovic L, Kontoe S, 2014,

    Stability investigation of the Generalised-α time integration method for dynamic coupled consolidation analysis

    , Computers and Geotechnics, Vol: 64, Pages: 83-95, ISSN: 1873-7633

    In this paper, the stability of the Generalised-α time integration method (the CH method) for a fully coupled solid-pore fluid formulation is analytically investigated for the first time and the corresponding theoretical stability conditions are proposed based on a rigorous mathematical derivation process. The proposed stability conditions simplify to the existing ones of the CH method for the one-phase formulation when the solid–fluid coupling is ignored. Furthermore, by degrading the CH method to the Newmark method, the stability conditions are in agreement with the ones proposed in previous stability investigations on coupled formulation for the Newmark method. The analytically derived stability conditions are validated with finite element (FE) analyses considering a range of loading conditions and for various soil permeability values, showing that the numerical results are in agreement with the theoretical investigation. Then, the stability characteristics of the CH method are explored beyond the limits of the theoretical investigation, assuming elasto-plastic soil behaviour which is prescribed with a bounding surface plasticity constitutive model. Since the CH method is a generalisation of a number of other time integration methods, the derived stability conditions are relevant for most of the commonly utilised time integration methods for the two-phase coupled formulation.
  • Journal article
    Jackson CA-L, Rodriguez CR, Rotevatn A, Bell REet al., 2014,

    Geological and geophysical expression of a primary salt weld; an example from the Santos Basin, Brazil

    , Interpretation, Vol: 2, Pages: SM77-SM89

    Primary salt welds form at the base of minibasins in response to complete evacuation of autochthonous salt. Analytical and numerical models suggest it is difficult to completely remove salt from a weld by viscous flow alone, which is especially true in multilayered evaporites, within which flow is likely heterogeneous due to lithologically controlled viscosity variations. Welds are of importance in the hydrocarbon industry because they may provide a hydrodynamic seal and trap hydrocarbons or may allow transmission of fluids from source to reservoir rocks. Few papers document the subsurface expression of welds, principally because of they have not been penetrated or because associated data are proprietary. We use 3D seismic and borehole data from the Santos Basin, offshore Brazil to characterise the geological and geophysical expression of a primary weld associated with the flow of Aptian salt. Seismic data suggest that, locally, presalt and postsalt rocks are in contact at the base of an Upper Cretaceous minibasin, implying that several apparent welds, separated by low-relief salt pillows, are present. However, borehole data indicate that 22 m of anhydrite, carbonate and sandstone are present in one of the welds, indicating that this and other welds may be incomplete. Our study shows that seismic data may be unable to discriminate between a complete and incomplete weld, and we suggest that, during the subsurface analysis of welds, the term ‘apparent weld’ is used until borehole data unequivocally proves the absence of salt. Furthermore, we speculate that preferential expulsion of halite and potash salt from the autochthonous layer during viscous flow and welding resulted in the formation of an incomplete weld, which, when compared to the initial autochthonous layer, is volumetrically enriched in non-evaporite lithologies and relatively viscous evaporite lithologies (anhydrite). The composition and stratigraphy of the autochthonous layer may thus dictate
  • Journal article
    Davison TM, Cielsa FJ, Collins GS, Elbeshausen Det al., 2014,

    The effect of impact obliquity on shock heating in planetesimal collisions

    , Meteoritics & Planetary Science, Vol: 49, Pages: 2252-2265, ISSN: 1086-9379
  • Journal article
    Bell RE, Jackson CA-L, Whipp PS, Clements Bet al., 2014,

    Strain migration during multiphase extension: observations from the northern North Sea

    , Tectonics, Vol: 43, Pages: 1936-1963

    Many rifts develop through multiphase extension; it can be difficult, however, to determine how strain is distributed during reactivation because structural and stratigraphic evidence associated with earlier rifting is often deeply buried. Using 2D and 3D seismic reflection and borehole data from the northern North Sea, we examine the style, magnitude and timing of reactivation of a pre-existing, Permian-Triassic (Rift Phase 1) fault array during a subsequent period of Middle Jurassic-to-Early Cretaceous (Rift Phase 2) extension. We show that Rift Phase 2 led to the formation of new N-S-striking faults close to the North Viking Graben, but did not initially reactivate pre-existing Rift Phase 1 structures on the Horda Platform. We suggest that, at the beginning of Rift Phase 2, strain was focused in a zone of thermally weakened lithosphere associated with the Middle Jurassic North Sea thermal dome, rather than reactivating extant faults. Diachronous reactivation of the Permian-Triassic fault network did eventually occur, with those faults located closer to the Middle Jurassic to Early Cretaceous rift-axis reactivating earlier than those toward the eastern margin. This diachroneity may have been related to flexural down-bending as strain became focused within the North Viking Graben and/or the shifting of the locus of rifting from the North Sea to the proto-North Atlantic. Our study shows that the geometry and evolution of multiphase rifts is not only controlled by the orientation of the underlying fault network, but also by the thermal and rheological evolution of the lithosphere and variations in the regional stress field.
  • Journal article
    Fogwill CJ, Turney CSM, Golledge NR, Rood DH, Hippe K, Wacker L, Wieler R, Rainsley EB, Jones RSet al., 2014,

    Drivers of abrupt Holocene shifts in West Antarctic ice stream direction determined from combined ice sheet modelling and geologic signatures

    , Antarctic Science, Vol: 26, Pages: 674-686, ISSN: 1365-2079

    Determining the millennial-scale behaviour of marine-based sectors of the West Antarctic Ice Sheet (WAIS) is critical to improve predictions of the future contribution of Antarctica to sea level rise. Here high-resolution ice sheet modelling was combined with new terrestrial geological constraints (in situ14C and 10Be analysis) to reconstruct the evolution of two major ice streams entering the Weddell Sea over 20 000 years. The results demonstrate how marked differences in ice flux at the marine margin of the expanded Antarctic ice sheet led to a major reorganization of ice streams in the Weddell Sea during the last deglaciation, resulting in the eastward migration of the Institute Ice Stream, triggering a significant regional change in ice sheet mass balance during the early to mid Holocene. The findings highlight how spatial variability in ice flow can cause marked changes in the pattern, flux and flow direction of ice streams on millennial timescales in this marine ice sheet setting. Given that this sector of the WAIS is assumed to be sensitive to ocean-forced instability and may be influenced by predicted twenty-first century ocean warming, our ability to model and predict abrupt and extensive ice stream diversions is key to a realistic assessment of future ice sheet sensitivity.
  • Journal article
    Nichols KK, Bierman PR, Rood DH, 2014,

    <sup>10</sup>Be constrains the sediment sources and sediment yields to the Great Barrier Reef from the tropical Barron River catchment, Queensland, Australia

    , Geomorphology, Vol: 224, Pages: 102-110, ISSN: 0169-555X

    Estimates of long-term, background sediment generation rates place current and future sediment fluxes to the Great Barrier Reef in context. Without reliable estimates of sediment generation rates and without identification of the sources of sediment delivered to the reef prior to European settlement (c. 1850), determining the necessity and effectiveness of contemporary landscape management efforts is difficult. Here, using the ~2100-km2 Barron River catchment in Queensland, Australia, as a test case, we use in situ-produced 10Be to derive sediment generation rate estimates and use in situ and meteoric 10Be to identify the source of that sediment, which enters the Coral Sea near Cairns. Previous model-based calculations suggested that background sediment yields were up to an order of magnitude lower than contemporary sediment yields. In contrast, in situ 10Be data indicate that background (43 t km-2 y-1) and contemporary sediment yields (~45 t km-2 y-1) for the Barron River are similar. These data suggest that the reef became established in a sediment flux similar to what it receives today. Since western agricultural practices increased erosion rates, large amounts of sediment mobilized from hillslopes during the last century are probably stored in Queensland catchments and will eventually be transported to the coast, most likely in flows triggered by rare but powerful tropical cyclones that were more common before European settlement and may increase in strength as climate change warms the south Pacific Ocean. In situ and meteoric 10Be concentrations of Coral Sea beach sand near Cairns are similar to those in rivers on the Atherton Tablelands, suggesting that most sediment is derived from the extensive, low-gradient uplands rather than the steep, more rapidly eroding but beach proximal escarpment. © 2014 .
  • Journal article
    Kontoe S, Avgerinos V, Potts DM, 2014,

    Numerical validation of four analytical solutions and their use for equivalent-linear seismic analysis of circular tunnels

    , Soil Dynamics and Earthquake Engineering, Vol: 66, Pages: 206-219

    The first part of this paper presents an extensive validation of four analytical solutions for the seismic design of circular tunnels. The validation is performed with a quasi-static Finite Element (FE) model which conforms to the assumptions of the analytical solutions. Analyses are performed for a wide range of flexibility ratios, slippage conditions at soil-lining interface, assuming both drained and undrained behaviour. Based on the numerical predictions the relative merits of the considered analytical solutions are discussed and recommendations are given for their use in design. The second part of this paper explores the use of equivalent linear soil properties in analytical solutions as an approximate way of simulating nonlinearity. The results of equivalent linear site response analyses are used as an input for the analytical solutions. The comparison of the analytical predictions with nonlinear numerical analysis results is very satisfactory. The results of this study suggest that analytical solutions can be used for preliminary design using equivalent linear properties and the corresponding compatible strain as an approximate way of accounting for nonlinear soil response.
  • Journal article
    Hill J, Collins GS, Avdis A, Kramer SC, Piggott MDet al., 2014,

    How does multiscale modelling and inclusion of realistic palaeobathymetry affect numerical simulation of the Storegga Slide tsunami?

    , Ocean Modelling, Vol: 83, Pages: 11-25, ISSN: 1463-5003

    The ∼8.15 ka Storegga submarine slide was a large (∼3000 km3), tsunamigenic slide off the coast of Norway. The resulting tsunami had run-up heights of around 10–20 m on the Norwegian coast, over 12 m in Shetland, 3–6 m on the Scottish mainland coast and reached as far as Greenland. Accurate numerical simulations of Storegga require high spatial resolution near the coasts, particularly near tsunami run-up observations, and also in the slide region. However, as the computational domain must span the whole of the Norwegian-Greenland sea, employing uniformly high spatial resolution is computationally prohibitive. To overcome this problem, we present a multiscale numerical model of the Storegga slide-generated tsunami where spatial resolution varies from 500 m to 50 km across the entire Norwegian-Greenland sea domain to optimally resolve the slide region, important coastlines and bathymetric changes. We compare results from our multiscale model to previous results using constant-resolution models and show that accounting for changes in bathymetry since 8.15 ka, neglected in previous numerical studies of the Storegga slide-tsunami, improves the agreement between the model and inferred run-up heights in specific locations, especially in the Shetlands, where maximum run-up height increased from 8 m (modern bathymetry) to 13 m (palaeobathymetry). By tracking the Storegga tsunami as far south as the southern North sea, we also found that wave heights were high enough to inundate Doggerland, an island in the southern North Sea prior to sea level rise over the last 8 ka.

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