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Now showing items 1 - 16 of 27

  • Impact-Seismic Investigations of the InSight Mission

    Daubar, Ingrid   Lognonne, Philippe   Teanby, Nicholas A.   Miljkovic, Katarina   Stevanovic, Jennifer   Vaubaillon, Jeremie   Kenda, Balthasar   Kawamura, Taichi   Clinton, John   Lucas, Antoine   Drilleau, Melanie   Yana, Charles   Collins, Gareth S.   Banfield, Don   Golombek, Matthew   Kedar, Sharon   Schmerr, Nicholas   Garcia, Raphael   Rodriguez, Sebastien   Gudkova, Tamara   May, Stephane   Banks, Maria   Maki, Justin   Sansom, Eleanor   Karakostas, Foivos   Panning, Mark   Fuji, Nobuaki   Wookey, James   van Driel, Martin   Lemmon, Mark   Ansan, Veronique   Bose, Maren   Stahler, Simon   Kanamori, Hiroo   Richardson, James   Smrekar, Suzanne   Banerdt, W. Bruce  

    Impact investigations will be an important aspect of the InSight mission. One of the scientific goals of the mission is a measurement of the current impact rate at Mars. Impacts will additionally inform the major goal of investigating the interior structure of Mars.In this paper, we review the current state of knowledge about seismic signals from impacts on the Earth, Moon, and laboratory experiments. We describe the generalized physical models that can be used to explain these signals. Adiscussion of the appropriate source time function for impacts is presented, along with spectral characteristics including the cutoff frequency and its dependence on impact momentum. Estimates of the seismic efficiency (ratio between seismic and impact energies) vary widely. Our preferred value for the seismic efficiency at Mars is which we recommend using until we can measure it during the InSight mission, when seismic moments are not used directly. Effects of the material properties at the impact point and at the seismometer location are considered. We also discuss the processes by which airbursts and acoustic waves emanate from bolides, and the feasibility of detecting such signals.We then consider the case of impacts on Mars. A review is given of the current knowledge of present-day cratering on Mars: the current impact rate, characteristics of those impactors such as velocity and directions, and the morphologies of the craters those impactors create. Several methods of scaling crater size to impact energy are presented. The Martian atmosphere, although thin, will cause fragmentation of impactors, with implications for the resulting seismic signals.We also benchmark several different seismic modeling codes to be used in analysis of impact detections, and those codes are used to explore the seismic amplitude of impact-induced signals as a function of distance from the impact site. We predict a measurement of the current impact flux will be possible within the timeframe of the prime mission (one Mars year) with the detection of approximate to a few to several tens of impacts. However, the error bars on these predictions are large.Specific to the InSight mission, we list discriminators of seismic signals from impacts that will be used to distinguish them from marsquakes. We describe the role of the InSight Impacts Science Theme Group during mission operations, including a plan for possible night-time meteor imaging. The impacts detected by these methods during the InSight mission will be used to improve interior structure models, measure the seismic efficiency, and calculate the size frequency distribution of current impacts.
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  • Seismic anisotropy and mantle flow below subducting slabs

    Walpole, Jack   Wookey, James   Kendall, J-Michael   Masters, T-Guy  

    Subduction is integral to mantle convection and plate tectonics, yet the role of the subslab mantle in this process is poorly understood. Some propose that decoupling from the slab permits widespread trench parallel flow in the subslab mantle, although the geodynamical feasibility of this has been questioned. Here, we use the source-side shear wave splitting technique to probe anisotropy beneath subducting slabs, enabling us to test petrofabric models and constrain the geometry of mantle fow. Our global dataset contains 6369 high quality measurements - spanning similar to 40,000 km of subduction zone trenches - over the complete range of available source depths (4 to 687 km) - and a large range of angles in the slab reference frame. We find that anisotropy in the subslab mantle is well characterised by tilted transverse isotropy with a slow-symmetry-axis pointing normal to the plane of the slab. This appears incompatible with purely trench-parallel flow models. On the other hand it is compatible with the idea that the asthenosphere is tilted and entrained during subduction. Trench parallel measurements are most commonly associated with shallow events (source depth <50km) - suggesting a separate region of anisotropy in the lithospheric slab. This may correspond to the shape preferred orientation of cracks, fractures, and faults opened by slab bending. Meanwhile the deepest events probe the upper lower mantle where splitting is found to be consistent with deformed bridgmanite. Crown Copyright (C) 2017 Published by Elsevier B.V. All rights reserved.
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  • Impact-Seismic Investigations of the InSight Mission

    Daubar, Ingrid   Lognonné, Philippe   Teanby, Nicholas A.   Miljkovic, Katarina   Stevanović, Jennifer   Vaubaillon, Jeremie   Kenda, Balthasar   Kawamura, Taichi   Clinton, John   Lucas, Antoine   Drilleau, Melanie   Yana, Charles   Collins, Gareth S.   Banfield, Don   Golombek, Matthew   Kedar, Sharon   Schmerr, Nicholas   Garcia, Raphael   Rodriguez, Sebastien   Gudkova, Tamara   May, Stephane   Banks, Maria   Maki, Justin   Sansom, Eleanor   Karakostas, Foivos   Panning, Mark   Fuji, Nobuaki   Wookey, James   van Driel, Martin   Lemmon, Mark   Ansan, Veronique   Böse, Maren   Stähler, Simon   Kanamori, Hiroo   Richardson, James   Smrekar, Suzanne   Banerdt, W. Bruce  

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  • Constraining lowermost mantle anisotropy with body waves:a synthetic modelling study

    Creasy, Neala   Pisconti, Angelo   Long, Maureen D.   Thomas, Christine   Wookey, James  

    Different mechanisms have been proposed as explanations for seismic anisotropy at the base of the mantle, including crystallographic preferred orientation of various minerals (bridgmanite, post-perovskite and ferropericlase) and shape preferred orientation of elastically distinct materials such as partial melt. Investigations of the mechanism for D '' anisotropy usually yield ambiguous results, as seismic observations rarely (if ever) uniquely constrain a mechanism or orientation and usually rely on significant assumptions to infer flow patterns in the deep mantle. Observations of shear wave splitting and polarities of SdS and PdP reflections off the D '' discontinuity are among our best tools for probing D '' anisotropy; however, currently available data sets cannot constrain one unique scenario among those suggested by the mineral physics literature. In this work, we determine via a forward modelling approach what combinations of body wave phases (e.g. SKS, SKKS and ScS) are required to uniquely constrain a mechanism for D '' anisotropy. We test nine models based on single-crystal and polycrystalline elastic tensors provided by mineral physics studies. Our modelling predicts fast shear wave splitting directions for SKS, SKKS and ScS phases, as well as polarities of P- and S-wave reflections off the D '' interface, for a range of propagation directions, via solution of the Christoffel equation. We run tests using randomly selected synthetic data sets based on a given starting model, controlling the total number of measurements, the azimuthal distribution, and the type of seismic phases. For each synthetic data set, we search over all possible elastic tensors and orientations to determine which are consistent with the synthetic data. Overall, we find it difficult to uniquely constrain the mechanism for anisotropy with a typical number of seismic anisotropy measurements (based on currently available studies) with only one measurement technique (SKS, SKKS, ScS or reflection polarities). However, data sets that include SKS, SKKS and ScS measurements or a combination of shear wave splitting and reflection polarity measurements increase the probability of uniquely constraining the starting model and its orientation. Based on these findings, we identify specific regions (i.e. North America, northwestern Pacific and Australia) of the lowermost mantle with sufficient ray path coverage for a combination of measurement techniques.
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  • Mantle anisotropy beneath the Earth's mid-ocean ridges

    Nowacki, Andy   Kendall, J. -Michael   Wookey, James  

    Observations of seismic anisotropy at oceanic spreading centres offer insights into mid-ocean ridge processes and the formation of new plates. Here, remote observations of seismic anisotropy beneath mid-ocean ridges are made using measurements of source-side shear wave splitting. Over 100 high-quality measurements are made using earthquakes that occur near mid-ocean ridges and transform faults, but are observed at teleseismic distances. In general, for off-axis ridge events, the polarisation of fast shear waves, phi '', is approximately parallel to the spreading direction. Nearer the ridge (similar to 50 km), phi '' becomes more scattered and is often ridge-parallel. Delay times, fit, tend to increase from <1 s near the ridge axis to similar to 3 s further away. Slow-spreading regions (Gakkel and Southwest Indian Ridges) show smaller amounts of splitting than faster spreading centres. At transform zones, the pattern is more complex. Coverage beneath the East Pacific Rise is especially good, and we observe a systematic increase in delay times in S wave splitting measurements compared to previous SKS splitting observations made at ocean-bottom seismometers. One compatible explanation is the presence of horizontally-aligned, connected layers of melt at depth; this is also compatible with other observations of the 'LAB' discontinuity and surface-wave derived measurements of radial anisotropy. (C) 2011 Elsevier B.V. All rights reserved.
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  • Constraining lowermost mantle anisotropy with body waves: A synthetic modeling study

    Creasy, Neala   Pisconti, Angelo   Long, Maureen D   Thomas, Christine   Wookey, James  

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  • MSAT-A new toolkit for the analysis of elastic and seismic anisotropy

    Walker, Andrew M.   Wookey, James  

    The design and content of MSAT, a new Matlab toolkit for the study and analysis of seismic and elastic anisotropy, is described. Along with a brief introduction to the basic theory of anisotropic elasticity and a guide to the functions provided by the toolldt, three example applications are discussed. First, the toolkit is used to analyse the effect of pressure on the elasticity of the monoclinic upper mantle mineral diopside. Second, the degree to which a model of elasticity in the lowermost mantle can be approximated by transverse isotropy is examined. Finally backazimuthal variation in the effective shear wave splitting caused by two a nisotropic layers where the lower layer is dipping is calculated. MSAT can be freely reused for any purpose and the implementation of these and other examples are distributed with the source code. (C) 2012 Elsevier Ltd. All rights reserved.
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  • Anisotropy as cause for polarity reversals of D '' reflections RID G-6113-2011

    Thomas, Christine   Wookey, James   Brodholt, John   Fieseler, Thomas  

    Recordings of seismic events that sample the deep mantle can test different hypotheses of mantle processes and composition. Seismic reflections from structures in the D '' region the bottom 200-400 km of the Earth's mantle can provide information on the velocity contrasts in this region. By studying the waveforms and polarities of the D '' reflections in P and S-waves, we can potentially distinguish between different explanations for the observed structures, such as phase transitions, mineral texture or thermal anomalies. Here we use source-receiver combinations that contain reflections from D '' in two different regions that are both characterised by fast seismic velocities in tomographic models. Beneath the Caribbean a positive S-velocity contrast but negative P-wave velocity contrast across the D '' reflector has been reported previously, consistent with a model of a phase change in MgSiO(3). In the second fast velocity region (Eurasia) we detect positive P- and S-wave velocity contrasts in two orthogonal paths crossing in the lowermost mantle indicating a different scenario for D ''. A path that crosses this region in 45 to the other two great circle paths shows evidence for a negative P velocity contrast. One explanation to reconcile observations in both regions is a phase transition from perovskite to post-perovskite with a fraction of 12% preferred crystal alignment in the post-perovskite phase. Depending on the travel direction of the waves with respect to the flow direction in the lower mantle, positive or negative velocity jumps can be expected. Other anisotropic models are considered but cannot fully explain the range of observations we find in the data. (C) 2011 Elsevier B.V. All rights reserved.
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  • New views of the Earth's interior RID G-6113-2011

    Wookey, James  

    The Earth beneath our feet is a complex place. James Wookey reports on a meeting that took a broad multidisciplinary approach to understanding its composition, structure and dynamics."New Views of the Earth's Interior", a meeting jointly supported by the British Geophysical Association and the Mineralogical Society of Great Britain and Ireland, was held in London, in February 2009. Speakers and participants covered a broad range of Earth and environmental sciences and included geochemists, mineral physicists, geodynamicists and seismologists. Topics covered aspects of the Earth's interior from the crust to the inner core, and from the earliest history to its contemporary state, and perspectives were provided from theoretical, experimental and observational viewpoints. The discussion showed that while enormous progress has been made, still much remains to be resolved, and the future of understanding the structure and processes of the Earth's interior lies in a multidisciplinary approach.
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  • Evaluating post-perovskite as a cause of D '' anisotropy in regions of palaeosubduction

    Nowacki, Andy   Walker, Andrew M.   Wookey, James   Kendall, J. -Michael  

    Seismic anisotropy in the Earth's lowermost mantle (D-'') is often attributed to the alignment of MgSiO3 post-perovskite (ppv) by the movement of dislocations in response to mantle flow. However, ppv's plastic yield surface is not known; nor do we know if this is the main deformation mechanism. We make use of a heterogeneous, generally anisotropic model of elasticity in D-'' derived from a 3-D model of mantle flow, which is obtained by inversion of geophysical observables. Unlike previous approaches, completely general, 3-D flow and full anisotropy are permitted, yielding more information to compare with observations than has been possible before. We model observations of anisotropy in D-'' by calculating the shear wave splitting predicted in ScS waves for a series of models of ppv plasticity. We find that observations in regions of the lowermost mantle beneath subduction zones are best fit by a model which accommodates slip on (010). Our results show that, within one standard deviation, slip on (010)-or a mechanism giving the same style of anisotropy-explains D-'' anisotropy beneath these regions.
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  • D '' anisotropy beneath Southeast Asia RID G-6113-2011

    Thomas, Christine   Wookey, James   Simpson, Matthew  

    Earthquakes from the Banda Sea and Flores Sea region recorded at the Kyrgyz Network in the Tien Shan region show evidence for anisotropy in the lowermost mantle beneath South East Asia. Shear-wave splitting is measured for the ScS phase after a receiver-side correction is applied. Individual results show scatter in measured time lag and fast direction, but stacking these gives a stable estimate of 0.8 +/- 0.04 s and 43 +/- 4 degrees. This equates to a fast-shear wave with a polarisation deviation from horizontal ( at the bounce point) of around 9 degrees. The simplest model to explain is a transversely isotropic medium with a slight dip to the east, and an inferred anisotropy of similar to 0.25%. This is compatible with previous observations of V(SH) > V(SV) for this region. Scenarios which might explain the observed anisotropy include sheared melt pockets or aligned post-perovskite crystals associated with palaeosubduction of the Pacific plate and Indian plate.
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  • Stratigraphic filtering and source penetration depth RID G-6113-2011

    van der Baan, Mirko   Wookey, James   Smit, Dirk  

    Seismic exploration underneath highly heterogeneous layers such as basalt flows is possible by lowering the principal source frequency. Unfortunately this also reduces resolution. Wave-localization theory is a multiple scattering theory that can be used to study stratigraphic filtering in chaotic lithologies. It predicts the apparent attenuation due to scattering of a plane wave traversing a layer with high velocity fluctuations. It can therefore predict the optimum principle source frequency in the trade-off between loss of resolution and increased penetration depth. We show how this can be done with the help of a few statistical parameters derived from a well-log analysis; namely, the average background velocity, the expected standard deviation in the velocity fluctuations, the typical scale length of the heterogeneities and the thickness of the basalt layer. In the likely situation that no local well logs exist, a multitude of scenarios can easily be examined at low cost.
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  • Inner-core shear-wave anisotropy and texture from an observation of PKJKP waves

    Wookey, James   Helffrich, George  

    Since the discovery of the Earth's core a century ago(1), and the subsequent discovery(2) of a solid inner core (postulated to have formed by the freezing of iron(3)) seismologists have striven to understand this most remote part of the deep Earth. The most direct evidence for a solid inner core would be the observation of shear-mode body waves that traverse it, but these phases are extremely difficult to observe. Two reported observations in short-period data(4,5) have proved controversial(6). Arguably more successful have been studies of longer-period data(6,7), but such averaging limits the usefulness of the observations to reported sightings. We present two observations of an inner-core shear-wave phase at higher frequencies in stacked data from the Japanese High-Sensitivity Array, Hi-Net(8). From an analysis of timing, amplitude and waveform of the 'PKJKP' phase we derive constraints on inner-core compressional-wave velocity and shear attenuation at about 0.3 Hz which differ from standard isotropic core models(9). We can explain waveform features and can partially reconcile the otherwise large differences between core wavespeed and attenuation models that our observations apparently suggest if we invoke shear-wave anisotropy in the inner core. A simple model of an inner core composed of hexagonal close-packed iron with its c axis aligned perpendicular to the rotation axis(10) yields anisotropy that is compatible with both the shear-wave anisotropy that we observe and the well-established 3 per cent compressional-wave anisotropy.
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  • Deformation of the lowermost mantle from seismic anisotropy RID E-1106-2011 RID G-6113-2011

    Nowacki, Andy   Wookey, James   Kendall, J-Michael  

    The lowermost part of the Earth's mantle-known as D ''-shows significant seismic anisotropy, the variation of seismic wave speed with direction(1-5). This is probably due to deformation-induced alignment of MgSiO(3)-post-perovskite (ppv), which is believed to be the main mineral phase present in the region. If this is the case, then previous measurements of D '' anisotropy, which are generally made in one direction only, are insufficient to distinguish candidate mechanisms of slip in ppv because the mineral is orthorhombic. Here we measure anisotropy in D '' beneath North and Central America, where material from subducting oceanic slabs impinges(6) on the core-mantle boundary, using shallow as well as deep earthquakes to increase the azimuthal coverage in D ''. We make more than 700 individual measurements of shear wave splitting in D '' in three regions from two different azimuths in each case. We show that the previously assumed(2,3,7) case of vertical transverse isotropy (where wave speed shows no azimuthal variation) is not possible, and that more complicated mechanisms must be involved. We test the fit of different MgSiO(3)-ppv deformation mechanisms to our results and find that shear on (001) is most consistent with observations and the expected shear above the core-mantle boundary beneath subduction zones. With new models of mantle flow, or improved experimental determination of the dominant ppv slip systems, this method will allow us to map deformation at the core-mantle boundary and link processes in D '', such as plume initiation, to the rest of the mantle.
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  • The 24th January 2016 Hawassa earthquake:Implications for seismic hazard in the Main Ethiopian Rift

    Wilks, Matthew   Ayele, Atalay   Kendall, J. -Michael   Wookey, James  

    Earthquakes of low to intermediate magnitudes are a commonly observed feature of continental rifting and particularly in regions of Quaternary to Recent volcanism such as in the Main Ethiopian Rift (MER). Although the seismic hazard is estimated to be less in the Hawassa region of the MER than further north and south, a significant earthquake occurred on the 24th January 2016 in the Hawassa caldera basin and close to the Corbetti volcanic complex. The event was felt up to 100 km away and caused structural damage and public anxiety in the city of Hawassa itself. In this paper we first refine the earthquake's location using data from global network and Ethiopian network stations. The resulting location is at 7.0404 degrees N, 38.3478 degrees E and at 4.55 km depth, which suggests that the event occurred on structures associated with the caldera collapse of the Hawassa caldera in the early Pleistocene and not through volcano-tectonic processes at Corbetti. We calculate local and moment magnitudes, which are magnitude scales more appropriate at regional hypocentral distances than (mb) at four stations. This is done using a local scale (attenuation term) previously determined for the MER and spectral analysis for ML and Mw respectively and gives magnitude estimates of 4.68 and 4.29. The event indicates predominantly normal slip on a N-S striking fault structure, which suggests that slip continues to occur on Wonji faults that have exploited weaknesses inherited from the preceding caldera collapse. These results and two previous earthquakes in the Hawassa caldera of M > 5 highlight that earthquakes continue to pose a risk to structures within the caldera basin. With this in mind, it is suggested that enhanced monitoring and public outreach should be considered. (C) 2016 Elsevier Ltd. All rights reserved.
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  • Local Magnitude Discrepancies for Near-Event Receivers:Implications for the UK Traffic-Light Scheme

    Butcher, Antony   Luckett, Richard   Verdon, James P.   Kendall, J. -Michael   Baptie, Brian   Wookey, James  

    Local seismic magnitudes provide a practical and efficient scale for the implementation of regulation designed to manage the risk of induced seismicity, such as Traffic-Light Schemes (TLS). We demonstrate that significant magnitude discrepancies (up to a unit higher) occur between seismic events recorded on nearby stations (<5 km) compared with those at greater distances. This is due to the influence of sedimentary layers, which are generally lower in velocity and more attenuating than the underlying crystalline basement rocks, and requires a change in the attenuation term of the M-L scale. This has a significant impact on the United Kingdom's (U.K.) hydraulic fracturing TLS, whose red light is set at M-L 0.5. Because the nominal detectability of the U.K. network is M-L 2, this scheme will require the deployment of monitoring stations in close proximity to well sites. Using data collected from mining events near New Ollerton, Nottinghamshire, we illustrate the effects that proximity has on travel path velocities and attenuation, then perform a damped least-squares inversion to determine appropriate constants within the ML scale. We show that the attenuation term needs to increase from 0.00183 to 0.0514 and demonstrate that this higher value is representative of a ray path within a slower more attenuating sedimentary layer compared with the continental crust. We therefore recommend that the magnitude scale M-L =3D log(A) + 1.17 log(r) + 0.0514r - 3.0 should be used when local monitoring networks are within 5 km of the event epicenters.
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