Creat membership Creat membership
Sign in

Forgot password?

Confirm
  • Forgot password?
    Sign Up
  • Confirm
    Sign In
home > search

Now showing items 1 - 12 of 12

  • Mid-mantle anisotropy in subduction zones and deep water transport

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

    Download Collect
  • 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.
    Download Collect
  • [Springer Theses] Plate Deformation from Cradle to Grave ||

    Nowacki, Andy  

    Download Collect
  • Deformation of the lowermost mantle from seismic anisotropy

    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.
    Download Collect
  • 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.
    Download Collect
  • 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.
    Download Collect
  • Evidence for cross rift structural controls on deformation and seismicity at a continental rift caldera

    Lloyd, Ryan   Biggs, Juliet   Wilks, Matthew   Nowacki, Andy   Kendall, J-Michael   Ayele, Atalay   Lewi, Elias   Eysteinsson, Hjalmar  

    In continental rifts structural heterogeneities, such as pre-existing faults and foliations, are thought to influence shallow crustal processes, particularly the formation of rift faults, magma reservoirs and surface volcanism. We focus on the Corbetti caldera, in the southern central Main Ethiopian Rift. We measure the surface deformation between 22nd June 2007 and 25th March 2009 using ALOS and ENVISAT SAR interferograms and observe a semi-circular pattern of deformation bounded by a sharp linear feature cross-cutting the caldera, coincident with the caldera long axis. The signal reverses in sign but is not seasonal: from June to December 2007 the region south of this structure moves upwards 3 cm relative to the north, while from December 2007 until November 2008 it subsides by 2 cm. Comparison of data taken from two different satellite look directions show that the displacement is primarily vertical. We discuss potential mechanisms and conclude that this deformation is associated with pressure changes within a shallow (<1 km) fault-bounded hydrothermal reservoir prior to the onset of a phase of calderawide uplift. Analysis of the distribution of post-caldera vents and cones inside the caldera shows their locations are statistically consistent with this fault structure, indicating that the fault has also controlled the migration of magma from a reservoir to the surface over tens of thousands of years. Spatial patterns of seismicity are consistent with a cross-rift structure that extents outside the caldera and to a depth of similar to 30 km, and patterns of seismic anisotropy suggests stress partitioning occurs across the structure. We discuss the possible nature of this structure, and conclude that it is most likely associated with the GobaBonga lineament, which cross-cuts and pre-dates the current rift. Our observations show that pre-rift structures play an important role in magma transport and shallow hydrothermal processes, and therefore they should not be neglected when discussing these processes. (C) 2018 The Authors. Published by Elsevier B.V.
    Download Collect
  • The limits of ray theory when measuring shear wave splitting in the lowermost mantle with ScS waves

    Nowacki, Andy   Wookey, James  

    Download Collect
  • Characterising hydrothermal fluid pathways beneath Aluto volcano, Main Ethiopian Rift, using shear wave splitting

    Nowacki, Andy   Wilks, Matthew   Kendall, J.-Michael   Biggs, Juliet   Ayele, Atalay  

    Download Collect
  • Automated seismic waveform location using Multichannel Coherency Migration (MCM)-II. Application to induced and volcano-tectonic seismicity

    Shi, Peidong   Nowacki, Andy   Rost, Sebastian   Angus, Doug  

    Locating microseismic events is essential for many areas of seismology including volcano and earthquake monitoring and reservoir engineering. Due to the large number of microseismic events in these settings, an automated seismic location method is required to perform real time seismic monitoring. The measurement environment requires a precise and noise-resistant event location method for seismic monitoring. In this paper, we apply Multichannel Coherency Migration (MCM) to automatically locate microseismic events of induced and volcano-tectonic seismicity using sparse and irregular monitoring arrays. Compared to other migration-based methods, in spite of the often sparse and irregular distribution of the monitoring arrays, the MCM can show better location performance and obtain more consistent location results with the catalogue obtained by manual picking. Our MCM method successfully locates many triggered volcano-tectonic events with local magnitude smaller than 0, which demonstrates its applicability on locating very small earthquakes. Our synthetic event location example at a carbon capture and storage site shows that continuous and coherent drilling noise in industrial settings will pose great challenges for source imaging. However, automatic quality control techniques including filtering in the frequency domain and weighting are used to automatically select high-quality data, and can thus effectively reduce the effects of continuous drilling noise and improve source imaging quality. The location performance of the MCM method for synthetic and real microseismic data sets demonstrates that the MCM method can perform as a reliable and automatic seismic waveform analysis tool to locate microseismic events.
    Download Collect
  • Automated seismic waveform location using Multichannel Coherency Migration (MCM)–II. Application to induced and volcano-tectonic seismicity

    Shi, Peidong   Nowacki, Andy   Rost, Sebastian   Angus, Doug  

    Download Collect
  • New advances in using seismic anisotropy, mineral physics and geodynamics to understand deformation in the lowermost mantle RID E-1106-2011 RID G-6113-2011

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

    The D '' region, which lies in the lowermost few hundred kilometres of the mantle, is a central cog in the Earth's heat engine, influencing convection in the underlying core and overlying mantle. In recent years dense seismic networks have revealed a wealth of information about the seismic properties of this region, which are distinct from those of the mantle above. Here we review observations of seismic anisotropy in this region. In the past it has been assumed that the region exhibits a simple form of transverse isotropy with a vertical symmetry axis (VTI anisotropy). We summarise new methodologies for characterising a more general style of anisotropy using observations from a range of azimuths. The observations can be then used to constrain the mineralogy of the region and its style of deformation by a lattice preferred orientation (LPO) of the constituent minerals. Of specific interest is the recent discovery of the stability of the post-perovskite phase in this region, which might explain many enigmatic properties of D ''. Mantle flow models based on density models derived from global tomographic seismic velocity models can be used to test plausible mineralogies, such as post-perovskite, and their deformation mechanisms. Here we show how linked predictions from mineral physics, geodynamical modelling and seismic observations can be used to better constrain the dynamics, mineralogy and physical properties of the lowermost mantle. (C) 2011 Elsevier Ltd. All rights reserved.
    Download Collect
1

Contact

If you have any feedback, Please follow the official account to submit feedback.

Turn on your phone and scan

Submit Feedback