J.-M. Kendall

Efficacy of the post-perovskite phase as an explanation for lowermost-mantle seismic properties

Constraining the chemical, rheological and electromagnetic properties of the lowermost mantle (D″) is important to understand the formation and dynamics of the Earths mantle and core. To explain the origin of the variety of characteristics of this layer observed with seismology, a number of theories have been proposed, including core–mantle interaction, the presence of remnants of subducted material and that D″ is the site of a mineral phase transformation. This final possibility has been rejuvenated by recent evidence for a phase change in MgSiO3 perovskite (thought to be the most prevalent phase in the lower mantle) at near core–mantle boundary temperature and pressure conditions. Here we explore the efficacy of this ‘post-perovskite’ phase to explain the seismic properties of the lowermost mantle through coupled ab initio and seismic modelling of perovskite and post-perovskite polymorphs of MgSiO3, performed at lowermost-mantle temperatures and pressures. We show that a post-perovskite model can explain the topography and location of the D″ discontinuity, apparent differences in compressional- and shear-wave models and the observation of a deeper, weaker discontinuity. Furthermore, our calculations show that the regional variations in lower-mantle shear-wave anisotropy are consistent with the proposed phase change in MgSiO3 perovskite.

Variations in late syn-rift melt alignment inferred from shear-wave splitting in crustal earthquakes beneath the Ethiopian rift

[1]xa0The northern Main Ethiopian rift (MER) marks the transition from continental rifting to incipient seafloor spreading. We constrain anisotropy of the upper-crust in the MER and its uplifted rift flanks using shear-wave splitting from 24 earthquakes located beneath 18 broadband stations. Along the axis of the MER the fast polarization direction is oriented between ∼N and ∼NNE, parallel to Quaternary-Recent faults, aligned cones and maximum horizontal stress. Delay times are highest (0.24 s) where independent seismic studies show evidence of shallow partial melt. We attribute anisotropy along the rift axis to aligned melt-filled micro-cracks and dikes. At stations flanking the rift, the fast polarization direction is oriented ∼NE and delay-times are smaller (0.04–0.14 s). The lower amount of anisotropy is consistent with reduced melt away from the rift axis. These results show melt-induced anisotropy persists into the crust, and magma injection localizes and accommodates strain just prior to continental break-up.

Shear wave splitting observations in the Archean Craton of western Superior

Shear wave splitting observations in the Archean Superior Province of the Canadian Shield show moderate to large delay times (1.1–2.1 s) with azimuths suggesting upper-mantle anisotropy is subparallel to the structural grain of the craton. Two regions with uniform anisotropy azimuth (62°±7° and 87°±3°) are separated by a transitional zone showing strong dependence of observed azimuth on source direction, suggesting that lateral structure is being observed. No splitting was observed at stations near the coast of Hudson Bay, continuing a pattern of weak splitting in surrounding Proterozoic orogenic belts. Current absolute plate motion is consistent with the direction of anisotropy, but would not explain the regional contrasts. Instead, the anisotropy appears to be related to Archean structure and tectonic history.

Evidence of midmantle anisotropy from shear wave splitting and the influence of shear-coupled P waves

[1]xa0Until recently, the midmantle region of the Earth (the transition zone and the uppermost lower mantle) has been considered isotropic. Recent work by Wookey et al. [2002] presented evidence of anisotropy in the midmantle region near the Tonga-Kermadec subduction zone, inferred from shear wave splitting in teleseismic S phases at Australian seismic stations. This data set is revisited to examine the possibility of contamination by shear-coupled P waves. Using reflectivity modeling, we explore the effects of these phases on the measurement of shear wave splitting. Contamination of such measurements is demonstrated for short epicentral distances (Δ ∼ 30°). Wave field decomposition is a simple technique which can be used to separate these phases from the main S wave arrival. The Tonga-Australia data set is reprocessed after wave field decomposition. Shear wave splitting ranging between 0.7 and 6.2 s is observed, a range comparable to that observed by Wookey et al. [2002]. As before, the polarization of the fast shear wave is observed to be predominantly horizontal. However, the magnitudes of splitting for several events at short epicentral distances are significantly reduced, suggesting some influence of shear-coupled P waves in the original analysis. The results are compared with possible models of mantle anisotropy, and the results can be best explained by significant anisotropy in the midmantle region. The best candidate location for this anisotropy is in the uppermost lower mantle, and scenarios involving the alignment of lower mantle or subducted materials by regional dynamic processes are suggested to account for this.

A local, crossing‐path study of attenuation and anisotropy of the inner core

[1]xa0We report results from studying a region under the north Pacific using 46 ray paths along east-west and north-south directions to study the nature of inner core anisotropy, and find an anisotropy signal in differential PKPBC–PKPDF at about the resolution of the data, with north-south paths faster than east-west, but no dependence of attenuation on wavespeed or depth in this part of the inner core. Inner core Q is 130−52+255 between 140–340 km depth. The observations also provide constraints on the degree of homogeneous meridional anisotropy possibly present in the core, to between 0.1 and 0.6% velocity differences in the fast and slow directions, significantly smaller than the 2–4% axi-symmetric anisotropy in the inner core. The small meridional component to anisotropy argues against significant contributions to anisotropy from low-order inner core convection or stresses imposed on the inner core by the outer core field.

Upper mantle anisotropy beneath the Seychelles microcontinent

[1] The Seychelles plateau is a prime example of a microcontinent, yet mechanisms for its creation and evolution are poorly understood. Recently acquired teleseismic data from a deployment of 26 stations on 18 islands in the Seychelles are analyzed to study upper mantle seismic anisotropy using SKS splitting results. Strong microseismic noise is attenuated using a polarization filter. Results show significant variation in time delays (δt = 0.4–2.4 s) and smooth variations in orientation (ϕ = 15°–69°, where ϕ is the polarization of the fast shear wave). The splitting results cannot be explained by simple asthenospheric flow associated with absolute plate motions. Recent work has suggested that anisotropy measurements for oceanic islands surrounding Africa can be explained by mantle flow due to plate motion in combination with density-driven flow associated with the African superswell. Such a mechanism explains our results only if there are lateral variations in the viscosity of the mantle. It has been suggested that the Seychelles are underlain by a mantle plume. Predictions of flow-induced anisotropy from plume-lithosphere interaction can explain our results with a plume possibly impinging beneath the plateau. Finally, we consider lithospheric anisotropy associated with rifting processes that formed the Seychelles. The large variation in the magnitude of shear wave splitting over short distances suggests a shallow source of anisotropy. Fast directions align parallel to an area of transform faulting in the Amirantes. Farther from this area the orientation of anisotropy aligns in similar directions as plate motions. This supports suggestions of transpressive deformation during the opening of the Mascarene basin.

Rapid Continental Breakup and Microcontinent Formation in the Western Indian Ocean

Two of the main factors that determine the nature of a rifted continental margin are rheology and magmatism during extension. Numerical models of lithospheric extension suggest that both factors vary with extension rate; yet until now extension rates of studied margins, as indicated by the rate of initial seafloor spreading, are mostly less than -30 mm/yr on each margin. This article presents the first geophysical results from the Seychelles-Laxmi Ridge conjugate pair of rifted margins which separated at -65 mm/yr. n nThe Seychelles, with its spectacular exposures of Precambrian granite, was the earliest scientifically recognized microcontinent and arguably remains the classic example of one [Wegener, 1924; Matthews and Davies, 1966]. However, it is still unknown whether microcontinents result from plumes, changes in plate-boundary forces, lithospheric heterogeneity, or a combination of these factors.

Predicting the seismic implications of salt anisotropy using numerical simulations of halite deformation

In the past, the potential for seismic anisotropy in salt structures and its effect on their seismic imaging has received little attention. We consider the plausibility of salt anisotropy through linked numerical studies of salt deformation and its seismic consequences. Numerical models are used to predict lattice preferred orientations (LPOs) in halite polycrystalline aggregates subjected to axial extension and simple shear. The elastic constants for the deformed polycrystalline aggregate are then calculated. Simple models representing a salt sill and the stem of a diapir are created using these elastic constants. Ray tracing is used to investigate the effects of halite LPO on the propagation of seismic waves. The results suggest that salt anisotropy can cause significant traveltime effects and could lead to significant errors in seismic interpretation in salt environments if this anisotropy is ignored. We also investigate potential amplitude variation with offset and azimuth (AVOA) for the reflection from the top and bottom of an anisotropic salt sill. Ray paths with a shear-wave leg within the salt display strong AVOA effects with a clear four-fold symmetry.

Testing the the Ability of Surface Arrays to Locate Microseismicity

Here we test the ability of surface sensors to locate microseismic activity using signals recorded whilst 18 perforation shots, with a variety of magnitudes, were trigged in 6 different wells beneath an array. As the perforation shots provide signals with known position and origin time the analysis of this dataset provides useful insights into the ability and accuracy of surface arrays for monitoring microseismic activity. We apply a migration style approach to locate the perforation shots. The imaging method is based on stacking all data consistent with an arrival from a particular time and point in the subsurface and is ideally suited for application to large arrays of surface sensors as it does not require any manual analysis of the data, such as picking arrival times. In all but one case the signals from the perforation shots are not visible in the raw or pre-processed data. However, clear source images are produced for 12 of the perforation shots. The results show that arrays of surface sensors are capable of imaging microseismic activity, even when the signals from events are not visible in individual traces. Factors which contribute to the successful imaging of the perforation shots could include the shot’s position and its relationship to the lithology, the presence of other noise sources, and the coupling of the perforation shot with the ground.

Fractured reservoir characterization using P-wave AVOA analysis of 3D OBC data

Fracture systems can provide significant hydrocarbon storage and directional permeability so in many reservoirs their identification and characterization can be exploited to improve production and increase economic potential. For example, such information can be used to optimize production from horizontal wells by guiding their drilling perpendicular to aligned fracturing, which will generally provide higher yields than wells drilled parallel to fractures. Additionally fracture characterization could help guide waterflood-assisted production and identify compartments with good flow properties. Identification of such fracture systems is possible in 1D using core samples and well logs but, to fully optimize production of fractured reservoirs, it is desirable to understand the 3D distribution of fractures including the mapping of “swarms.” These fractures are generally too small to be imaged using standard 3D seismic imaging techniques. However, they often occur in aligned sets and create anisotropy in seism...