J.-Michael Kendall

Seismic anisotropy in the upper mantle 2. Predictions for current plate boundary flow models

[1]xa0The anisotropic seismic structure due to flow-induced mineral alignment is investigated for a series of models designed to simulate deformation in the upper mantle within a few hundred kilometers of a plate boundary. The orientation distributions of olivine:enstatite aggregates evolve along streamlines of each flow model, based on each grains plastic response to the local stress/strain field. The effective elastic tensor for these textured aggregates provides predictions of P wave anisotropy and shear wave splitting throughout the model space. P and S travel time delay patterns and fast shear wave polarization angles are found to vary significantly with incidence angle for a given model. Comparison of predicted fast P direction for our method versus a finite-strain based estimate shows that agreement is acceptable for much of the model space, but notable differences occur in regions up to several tens of kilometers in size. Two-dimensional models of spreading center flow are presented for slow and fast rates and for several cases in which the ridge migrates over the deeper mantle. The effect of flow in the third dimension is addressed in a few calculations. For one comparison of flow in the mantle wedge at a subduction zone, the introduction of trench parallel flow causes significant changes in the predicted patterns of P wave anisotropy (magnitude, more than orientation) and SKS splitting.

Sensitivity of teleseismic body waves to mineral texture and melt in the mantle beneath a mid-ocean ridge

Seismic energy propagating through the mantle beneath an oceanic spreading centre develops a signature due both to the subaxial deformation field and to the presence of melt in the upwelling zone. Deformation of peridotite during mantle flow results in strong preferred orientation of olivine and significant seismic anisotropy in the upper 100 km of the mantle. Linked numerical models of flow, texture development and seismic velocity structure predict that regions of high anisotropy will characterize the subaxial region, particularly at slow–spreading mid–ocean ridges. In addition to mineral texture effects, the presence of basaltic melt can cause travel–time anomalies, the nature of which depend on the geometry, orientation and concentration of the melt. In order to illustrate the resolution of subaxial structure that future seismic experiments can hope to achieve, we investigate the teleseismic signature of a series of spreading centre models in which the mantle viscosity and melt geometry are varied. The P–wave travel times are not very sensitive to the geometry and orientation of melt inclusions, whether distributed in tubules or thin ellipsoidal inclusions. Travel time delays of 0.1–0.4 s are predicted for the melt distribution models tested. The P–wave effects of mineral texture dominate in the combined melt–plus–texture models. Thus, buoyancy–enhanced upwelling at a slow spreading ridge is characterized by 0.7–1.0 s early P–wave arrival times in a narrow axial region, while the models of plate–driven–only flow predicts smaller advances (less than 0.5 s) over a broader region. In general S–wave travel times are more sensitive to the melt and show more obvious differences between melt present as tubules as opposed to thin disks, especially if a preferred disk orientation exists. Mineral texture and the preferred alignment of melt inclusions will both produce shear–wave splitting, our models predict as much as 4 s splitting in some cases. Keywords: melt in the mantle, seismic travel–time anomalies, seafloor seismic arrays, seismic anisotropy, melt seismic signature, seismic heterogeneity

Seismic anisotropy of the upper mantle 1. Factors that affect mineral texture and effective elastic properties

[1]xa0Flow-induced mineral alignment in the Earths mantle affects the nature of seismic wave propagation. Since measurements of seismic travel time and shear wave splitting are a key means by which the structure of the upper mantle can be imaged, it is important to understand the factors that contribute to variability in elastic properties. Seismic anisotropy associated with lattice-preferred orientations of minerals in peridotite is the focus of this study. One way to better understand anisotropy in a convecting upper mantle is to simulate texture evolution based on certain assumptions. Simulations of the development of olivine and orthopyroxene alignment along streamlines of a mantle flow field illustrate how continuously varying strain conditions affect the resulting orientation distribution. There are various uncertainties in rock texture simulations, and the aim here is to investigate how much model assumptions may influence the results. A model of upper mantle flow in the vicinity of an oceanic spreading center is used to illustrate these points. First we assess how sensitive results are on assumptions of different polycrystal plasticity models, specifically lower bounds and viscoplastic self-consistent approaches. We also investigate how recrystallization during deformation might affect the texture that develops along a streamline. The effects of grain growth and nucleation produce, as expected, textures that are different from the deformation-only case. However, the basic P wave structure of the predicted anisotropy is similar between models for streamlines in a slow-spreading, passive flow model as is commonly used for simulating flow near this type of plate boundary. Shear wave splitting patterns are more complex and differ somewhat more between the models in the off-axis region. We also compare predictions for texture development of olivine-only aggregates to that of mixed composition with 70% olivine and 30% orthopyroxene. Although we consider only one aspect of such polyphase deformation, i.e., plastic deformation on specified crystal slip systems, our results are consistent with field observations of the orientation distributions in ophiolitic peridotites. Finally, we determine how simplifying assumptions about the symmetry of the elastic anisotropy can bias interpretations of seismic travel time and shear wave splitting. Whereas a hexagonal approximation can lead to underestimates of the degree of anisotropy, an orthorhombic approximation is found to closely match the results predicted for a general elastic tensor corresponding to an orientation distribution whose symmetry is as low as monoclinic or triclinic.

Estimating anisotropy parameters and traveltimes in the τ-p domain

The presence of anisotropy influences many aspects of seismic wave propagation and has therefore implications for conventional processing schemes. To estimate the anisotropy, we need both forward modelling and inversion tools. Exact forward modelling in anisotropic media is generally done by raytracing. However, we present a new and fast method, using the τ‐p transform, to calculate exact P and SV reflection moveout curves in stratified, laterally homogeneous, anisotropic media which requires no ray tracing. Results are exact even if the SV‐waves display cusps. In addition, we show how the same method can be used for parameter estimation.Since inversion for anisotropic parameters is very nonunique, we develop expressions requiring only a reduced number of parameters. Nevertheless, predictions using these expressions are more accurate than Taylor series expansions and are also able to handle cusps in the SV traveltime curves. In addition, layer stripping is a linear process. Therefore, both effective (aver...

Investigating the heterogeneity of the D” region beneath the northern Pacific using a seismic array

[1]xa0Seismic array recordings are used to study the heterogeneity of a 15° × 25° region of the lowermost mantle beneath the northern Pacific. We investigate P waves from northwestern Pacific events, 68° to 82° from the Yellowknife array in northern Canada. Anomalous arrivals (PdP) are observed 2–13.5 s after P with a slowness 0.4–1.2 s/deg smaller than P, suggesting that they are reflections from a D″ discontinuity. We use vespagrams (slant stacks) and f-k analyses to determine travel times and slowness vectors respectively. The f-k technique simultaneously estimates both the horizontal slowness and backazimuth of arrivals at a receiver, with better resolution than vespagrams. Travel time analysis reveals a mean discontinuity height of 241 km above the core-mantle boundary in the region studied here. However, there appears a systematic variation in this thickness which ranges from 211 to 336 km. The f-k analyses also reveal variations between P and PdP backazimuths, further implying the existence of lateral variations in D″ in this area. This site is thought to be a transition region from an area of mantle downwelling to a region of upwelling; thus the variations in heterogeneity in this region may be related to its proximity to a site of paleoslab accumulation.

Approximate separation of pure-mode and converted waves in 3-C reflection seismics by τ-p transform

The use of multicomponent receivers allows one to record the complete elastic wavefield. This is desirable since knowledge of both P- and S-wave characteristics yields better insights into subsurface lithologic and structural rock properties than P-wave knowledge alone. It is often assumed in multicomponent processing that the vertical z-component contains principally pure-mode P-wave arrivals and that the inline horizontal x-component consists mainly of P-SV convertedwave energy. This assumption actually becomes worse with increasing offset. Contaminating energy on either component should ideally be removed to prevent degeneration of stack quality. Often, the x-component is more contaminated than the z-component because P-wave incidence angles (from vertical) are larger than those of P-S waves. This renders processing of the x-component more challenging. Removal of contaminating energy on either component may lead to sharper images. One way of removing such undesired energy is by means of wavefield separation. We propose a simple, approximate wavefield-separation scheme in the τ -p domain to better