Cambridge, United Kingdom
Cambridge, United Kingdom

Time filter

Source Type

Motaghi K.,International Institute of Earthquake Engineering and Seismology | Tatar M.,International Institute of Earthquake Engineering and Seismology | Priestley K.,Bullard Laboratories
Journal of Seismology | Year: 2012

For the first time, we present the variation of crust-mantle boundary beneath the northeast Iran continental collision zone which is genetically part of the Alpine-Himalayan orogeny and beneath Central Iran which is a less-deformed tectonic block. The boundary was imaged by stacking teleseismic P-S converted phases and shows a strong variation of Moho from 27.5 km under Central Iran to 55.5 km beneath the Binalud foreland basin. The thickest crust is not located beneath the high topography of the Kopeh Dagh and Binalud mountain ranges suggesting that these mountain ranges are not supported by a crustal root. The simple gravity modeling of the Bouguer anomaly supports this idea. © 2011 Springer Science+Business Media B.V.

Boait F.C.,Bullard Laboratories | White N.J.,Bullard Laboratories | Bickle M.J.,Bullard Laboratories | Chadwick R.A.,British Geological Survey | And 3 more authors.
Journal of Geophysical Research: Solid Earth | Year: 2012

Time-lapse, three-dimensional (3D) seismic surveys have imaged an accumulation of injected CO2 adjacent to the Sleipner field in the North Sea basin. The changing pattern of reflectivity suggests that CO 2 accumulates within a series of interbedded sandstones and mudstones beneath a thick caprock of mudstone. Nine reflective horizons within the reservoir have been mapped on six surveys acquired between 1999 and 2008. These horizons have roughly elliptical planforms with eccentricities ranging between two and four. In the top half of the reservoir, horizon areas grow linearly with time. In the bottom half, horizon areas initially grow linearly for about eight years and then progressively shrink. The central portions of deeper reflective horizons dim with time. Amplitude analysis of horizons above, within, and below the reservoir show that this dimming is not solely caused by acoustic attenuation. Instead, it is partly attributable to CO2 migration and/or CO2 dissemination, which reduce the impedance contrast between sandstone and mudstone layers. Growth characteristics and permeability constraints suggest that each horizon grows by lateral spreading of a gravity current. This model is corroborated by the temporal pattern of horizon velocity pushdown beneath the reservoir. Horizon shrinkage may occur if the distal edge of a CO2-filled layer penetrates the overlying mudstone, if the buoyant plume draws CO2 upward, or if the effective permeability of deeper mudstone layers increases once interstitial brine has been expelled. Topographic control is evident at later times and produces elliptical planforms, especially toward the top of the reservoir. Our results show that quantitative mapping and analysis of time-lapse seismic surveys yield fluid dynamical insights which are testable, shedding light on the general problem of CO 2 sequestration. Copyright 2012 by the American Geophysical Union.

Elliott J.R.,University of Oxford | Nissen E.K.,Bullard Laboratories | England P.C.,University of Oxford | Jackson J.A.,Bullard Laboratories | And 4 more authors.
Journal of Geophysical Research: Solid Earth | Year: 2012

The 3rd September 2010 Mw 7.1 Darfield and 21st February 2011 Mw 6.3 Christchurch (New Zealand) earthquakes occurred on previously unknown faults. We use InSAR ground displacements, SAR amplitude offsets, field mapping, aerial photographs, satellite optical imagery, a LiDAR DEM and teleseismic body-wave modeling to constrain the pattern of faulting in these earthquakes. The InSAR measurements reveal slip on multiple strike-slip segments and secondary reverse faults associated with the Darfield main shock. Fault orientations are consistent with those expected from the GPS-derived strain field. The InSAR line-of-sight displacement field indicates the main fault rupture is about 45 km long, and is confined largely to the upper 10 km of the crust. Slip on the individual fault segments of up to 8 m at 4 km depth indicate stress drops of 6-10 MPa. In each event, rupture initiated on a reverse fault segment, before continuing onto a strike-slip segment. The non-double couple seismological moment tensors for each event are matched well by the sum of double couple equivalent moment tensors for fault slip determined by InSAR. The slip distributions derived from InSAR observations of both the Darfield and Christchurch events show a 15-km-long gap in fault slip south-west of Christchurch, which may present a continuing seismic hazard if a further unknown fault structure of significant size should exist there. Copyright 2012 by the American Geophysical Union.

Czarnota K.,Bullard Laboratories | Czarnota K.,Geoscience Australia | Hoggard M.J.,Bullard Laboratories | White N.,Bullard Laboratories | Winterbourne J.,BP Exploration Operating Co.
Geochemistry, Geophysics, Geosystems | Year: 2013

Despite its importance, the spatial and temporal pattern of dynamic topography generated by mantle convective circulation is poorly known. We present accurate estimates of dynamic topography from oceanic basins and continental margins surrounding Australia. Our starting point is measurement of residual depth anomalies on the oldest oceanic floor adjacent to the continental shelf. These anomalies were determined from a combined dataset of ~200 seismic reflection and wide-angle images of well-sedimented oceanic crust. They have amplitudes of between -1 km and +0.5 km, and their spatial variation is broadly consistent with long-wavelength free-air gravity and shallow seismic tomographic anomalies. Along the Northwest Shelf, a regional depth anomaly of -300 to -700 m intersects the adjacent continental shelf. The temporal evolution of this anomaly was determined by analyzing the stratigraphic architecture of an extensive carbonate platform, which fringes the shelf and records a dramatic switch from progradation to aggradation during Neogene times. Three-dimensional seismic mapping calibrated by boreholes was used to calculate water-loaded subsidence histories at rollover points of clinoforms along the shelf. At 9 ± 3 Ma, the rate of subsidence increases from 5 to up 75 m Myr -1, generating a subsidence anomaly of -300 to -700 m. The amplitude of this anomaly varies along the shelf and cannot be generated by glacio-eustatic sea-level variation. Instead, we propose that a regional subsidence episode, which affects both the proximal shelf and the distal oceanic basin, was generated by convective drawdown. By combining our results with other published estimates of uplift and subsidence, a map of Australia, which shows the spatial and temporal pattern of dynamic topography is presented. Most, but not all, of Australia's epeirogeny can be attributed to rapid northward motion of the Australian plate over a pre-existing pattern of convective circulation. © 2013 American Geophysical Union. All Rights Reserved.

Hartley R.A.,Bullard Laboratories | Roberts G.G.,Bullard Laboratories | White N.,Bullard Laboratories | Richardson C.,BP Institute
Nature Geoscience | Year: 2011

Sedimentary basins in the North Atlantic Ocean preserve a record of intermittent uplift during Cenozoic times1. These variations in elevation are thought to result from temperature changes within the underlying Icelandic mantle plume2. When parts of the European continental shelf were episodically lifted above sea level, new landscapes were carved by erosion, but these landscapes then subsided and were buried beneath marine sediments3. Here, we use three-dimensional seismic data to reconstruct one of these ancient landscapes that formed off the northwest coast of Europe during the Palaeocene-Eocene Thermal Maximum. We identify a drainage network within the landscape and, by modelling the profiles of individual rivers within this network, we reconstruct the history of surface uplift. We show that the landscape was lifted above sea level in a series of three discrete steps of 200-400 m each. After about 1 million years of subaerial exposure, this landscape was reburied. We use the magnitude and duration of uplift to constrain the temperature and velocity of a mantle-plume anomaly that drove landscape formation. We conclude that pulses of hot, chemically depleted, mantle material spread out radially beneath the lithospheric plate at velocities of ∼35 cm yr-1. © 2011 Macmillan Publishers Limited. All rights reserved.

Lythgoe K.H.,Bullard Laboratories | Deuss A.,Bullard Laboratories | Rudge J.F.,Bullard Laboratories | Neufeld J.A.,Bullard Laboratories | And 2 more authors.
Earth and Planetary Science Letters | Year: 2014

The structure of Earth's deep inner core has important implications for core evolution, since it is thought to be related to the early stages of core formation. Previous studies have suggested that there exists an innermost inner core with distinct anisotropy relative to the rest of the inner core. Using an extensive new data set of handpicked absolute travel time observations of the inner core phase PKIKP, we find that the data are best explained by variations in anisotropy between two hemispheres and do not require an innermost inner core. We demonstrate that observations of an innermost inner core are an artifact from averaging over lateral anisotropy variations. More significantly we show that hemispherical variations in anisotropy, previously only imaged in the upper inner core, continue to its centre. The eastern region has 0.5-1.5% anisotropy, whereas the western region has 3.5-8.8% anisotropy increasing with depth, with a slow direction at 57-61° to the Earth's rotation axis at all depths. Such anisotropy is consistent with models of aligned hcp or bcc iron aggregates. © 2013 Elsevier B.V.

Jenkins J.,Bullard Laboratories | Cottaar S.,Bullard Laboratories | White R.S.,Bullard Laboratories | Deuss A.,University Utrecht
Earth and Planetary Science Letters | Year: 2016

The presence of a mantle plume beneath Iceland has long been hypothesised to explain its high volumes of crustal volcanism. Practical constraints in seismic tomography mean that thin, slow velocity anomalies representative of a mantle plume signature are difficult to image. However it is possible to infer the presence of temperature anomalies at depth from the effect they have on phase transitions in surrounding mantle material. Phase changes in the olivine component of mantle rocks are thought to be responsible for global mantle seismic discontinuities at 410 and 660 km depth, though exact depths are dependent on surrounding temperature conditions. This study uses P to S seismic wave conversions at mantle discontinuities to investigate variation in topography allowing inference of temperature anomalies within the transition zone. We employ a large data set from a wide range of seismic stations across the North Atlantic region and a dense network in Iceland, including over 100 stations run by the University of Cambridge. Data are used to create over 6000 receiver functions. These are converted from time to depth including 3D corrections for variations in crustal thickness and upper mantle velocity heterogeneities, and then stacked based on common conversion points. We find that both the 410 and 660 km discontinuities are depressed under Iceland compared to normal depths in the surrounding region. The depression of 30 km observed on the 410 km discontinuity could be artificially deepened by un-modelled slow anomalies in the correcting velocity model. Adding a slow velocity conduit of -1.44% reduces the depression to 18 km; in this scenario both the velocity reduction and discontinuity topography reflect a temperature anomaly of 210 K. We find that much larger velocity reductions would be required to remove all depression on the 660 km discontinuity, and therefore correlated discontinuity depressions appear to be a robust feature of the data. While it is not possible to definitively rule out the possibility of uncorrected velocity anomalies causing the observed correlated topography we show that this is unlikely. Instead our preferred interpretation is that the 660 km discontinuity is controlled by a garnet phase transition described by a positive Clapeyron slope, such that depression of the 660 is representative of a hot anomaly at depth. © 2015 The Authors.

Elliott J.R.,University of Oxford | Copley A.C.,Bullard Laboratories | Holley R.,CGG NPA Kent | Scharer K.,U.S. Geological Survey | Parsons B.,University of Oxford
Journal of Geophysical Research: Solid Earth | Year: 2013

We use interferometric synthetic aperture radar (InSAR), body wave seismology, satellite imagery, and field observations to constrain the fault parameters of the Mw 7.1 2011 Van (Eastern Turkey) reverse-slip earthquake, in the Turkish-Iranian plateau. Distributed slip models from elastic dislocation modeling of the InSAR surface displacements from ENVISAT and COSMO-SkyMed interferograms indicate up to 9 m of reverse and oblique slip on a pair of en echelon NW 40°-54°dipping fault planes which have surface extensions projecting to just 10 km north of the city of Van. The slip remained buried and is relatively deep, with a centroid depth of 14 km, and the rupture reaching only within 8-9 km of the surface, consistent with the lack of significant ground rupture. The up-dip extension of this modeled WSW striking fault plane coincides with field observations of weak ground deformation seen on the western of the two fault segments and has a dip consistent with that seen at the surface in fault gouge exposed in Quaternary sediments. No significant coseismic slip is found in the upper 8 km of the crust above the main slip patches, except for a small region on the eastern segment potentially resulting from the Mw 5.9 aftershock on the same day. We perform extensive resolution tests on the data to confirm the robustness of the observed slip deficit in the shallow crust. We resolve a steep gradient in displacement at the point where the planes of the two fault segments ends are inferred to abut at depth, possibly exerting some structural control on rupture extent © 2013. American Geophysical Union. All Rights Reserved.

Walker R.T.,University of Oxford | Ramsey L.A.,BG Group | Jackson J.,Bullard Laboratories
Geological Magazine | Year: 2011

We describe the geomorphology of a large (∼10000 km2) internally draining region within the Zagros fold-and-thrust belt of Fars province, Iran. A series of wind gaps through fold crests and a continuous line of low-slope pixels in digital elevation models indicate the presence of an older, and now abandoned, through-going river system. We suggest, from the presence of the wind gaps, that the original through-going river system was abandoned as a direct result of fold growth. At present, through-going drainage in Fars is restricted to only two major rivers, the Kul and the Mand, which bound the margins of the internally drained region. The low gradients of the Kul and the Mand rivers are similar to those in topographic profiles drawn along the course of the abandoned drainage. The Mand and Kul rivers may be defeated in the future, causing an expansion of the internally drained region, and resulting in a profound change in the distribution of sediment and surface elevations within the Zagros. The internally draining part of the Zagros resembles the Central Iranian Plateau both in its geomorphology and in the apparently slow rates of deformation within it. We speculate that the development of internally drained basins and distribution of shortening within the range may be causally linked. The geomorphology that we describe might, therefore, record a stage in the southward expansion of the non-deforming and topographically high Central Iranian Plateau. © 2011 Cambridge University Press.

England P.,University of Oxford | Howell A.,Bullard Laboratories | Jackson J.,Bullard Laboratories | Synolakis C.,Technical University of Crete | Synolakis C.,University of Southern California
Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences | Year: 2015

The dominant uncertainties in assessing tsunami hazard in the Eastern Mediterranean are attached to the location of the sources. Reliable historical reports exist for five tsunamis associated with earthquakes at the Hellenic plate boundary, including two that caused widespread devastation. Because most of the relative motion across this boundary is aseismic, however, the modern record of seismicity provides little or no information about the faults that are likely to generate such earthquakes. Independent geological and geophysical observations of two large historical to prehistorical earthquakes, in Crete and Rhodes, lead to a coherent framework in which large to great earthquakes occurred not on the subduction boundary, but on reverse faults within the overlying crust. We apply this framework to the less complete evidence from the remainder of the Hellenic plate boundary zone, identifying candidate sources for future tsunamigenic earthquakes. Each such source poses a significant hazard to the North African coast of the Eastern Mediterranean. Because modern rates of seismicity are irrelevant to slip on the tsunamigenic faults, and because historical and geological data are too sparse, there is no reliable basis for a probabilistic assessment of this hazard, and a precautionary approach seems advisable. Copyright © 2015 The Author(s) Published by the Royal Society.

Loading Bullard Laboratories collaborators
Loading Bullard Laboratories collaborators