Discriminating Between Causes of D″ Anisotropy Using Reflections and Splitting Measurements for a Single Path

Abstract

Knowledge of deep mantle deformation is based on seismic anisotropy: the variation of seismic wave speed and polarization with direction. Measuring this directional dependency requires azimuthal seismic coverage at D″ depth-the bottom few hundred kilometers of the mantle-which is often a limit in retrieving the style of anisotropy. Shear wave splitting is the standard technique for probing mantle anisotropy, and recently, reflections from the D″ region have been used to infer azimuthal anisotropy. Here we combine observations and modeling of D″ reflections with shear wave splitting along a given raypath direction in order to constrain mineralogy and dynamics of the lower mantle. From our modeling, a clear distinction between different anisotropic media is possible by using both types of observations together but only one directional path. We focus on the lowermost mantle beneath the central Atlantic Ocean by using South-Central American earthquakes recorded in Morocco. We find complex azimuthal and distance variation for both polarities of D″ reflections and shear wave splitting parameters, which rules out a simple style of anisotropy-such as vertical transverse isotropy-for the region. Our preferred model consists of a phase transition from a randomly oriented bridgmanite to lattice-preferred orientation fabric in postperovskite, developed in a tilted plane sheared along a roughly SW-NE deformation direction.

Deformation of NaCoF<sub>3</sub> perovskite and post-perovskite up to 30 GPa and 1013 K: implications for plastic deformation and transformation mechanism

“…Structural parameters of NaCoF 3 were obtained from Shirako et al (2012), and cell parameters and B factors were further refined using the data. Crystallite size and microstrains were refined using the Popa line-broadening sizestrain model (Popa, 1998).…”

A potential post-perovskite province in D″ beneath the Eastern Pacific: evidence from new analysis of discrepant SKS–SKKS shear-wave splitting

“…Indeed, SPO and LPO may yet prove to be complementary mechanisms, depending on the length scale of deformation within D with respect to the seismic wavelengths used. Recent forward modelling efforts (Ford et al, 2015;Creasy et al, 2017;Pisconti et al, 2019) have improved our constraints on D anisotropy, although most candidate mechanisms produce plausible results. Further expansion of these methods to remove the reliance on single-crystal elastic tensors, along with improving our observational constraints through the integration of ScS, SKS and SKKS shear-wave splitting data with reflected PdP and SdS polarities (Creasy et al, 2019) will allow to greatly improve our understanding of D anisotropy.…”

Mapping lithological boundaries in mines with array seismology and in situ acoustic emission monitoring

“…In particular, cluster C1 of seismic activity, as reported in Philipp et al (2015), is confined to the edge of the salt rock and does not extend into the host rock. In this study, we do not attempt to discriminate between different causes for the observed reflections based on amplitude, since amplitudes of seismic waves depend on many factors, such as source directivity, radiation pattern, and anisotropy which leads to magnification or reduction of the reflection coefficients in certain travelling directions (e.g., Pisconti, et al, 2019). Source directivity and radiation pattern might also be a cause for the lack of clear out of plane signals in several events of our dataset, which leaves us with only 17 out of 52 events that show the additional arrival.…”