Dynamic origins of seismic wavespeed variation in [formula omitted

Dan J Bower,Michael Gurnis,Daoyuan Sun

doi:10.1016/j.pepi.2012.10.004

Abstract

The D″ discontinuity is defined by a seismic velocity increase of 1–3% about 250km above the core–mantle boundary (CMB), and is mainly detected beneath locations of inferred paleosubduction. A phase change origin for the interface can explain a triplicated arrival observed in seismic waveform data and is supported by the recent discovery of a post-perovskite phase transition. We investigate the interaction of slabs, plumes, and the phase change within D″ in 2-D compressible convection calculations, and predict waveform complexity in synthetic seismic data. The dynamic models produce significant thermal and phase heterogeneity in D″ over small distances and reveal a variety of behaviors including: (1) distinct pPv blocks separated by upwellings, (2) notches at the top of a pPv layer caused by plume heads, (3) regions of Pv embedded within a pPv layer due to upwellings. Advected isotherms produce complicated thermal structure that enable multiple crossings of the phase boundary. Perturbations to S, SdS, and ScS arrivals (distances <84 degrees) are linked to the evolutionary stage of slabs and plumes, and can be used to determine phase boundary height and velocity increase, volumetric wavespeed anomaly beneath the discontinuity, and possibly the lengthscale of slab folding near the CMB. Resolving fine-scale structure beneath the interface requires additional seismic phases (e.g., Sd, SKS) and larger distances (>80 degrees).

Highlights

The D discontinuity is characterized by a seismic velocity increase of 1– 3% approximately 250 kilometers above the core-mantle boundary (CMB)
Overview of D slab dynamics We describe typical slab dynamics using model S2 (Fig. 2) as all models exhibit similar behavior
The pPv region has constant thickness except where a few thin stationary plumes emanate from the lower thermal boundary layer and the phase boundary is perturbed to higher pressure

Summary

Introduction

The D discontinuity is characterized by a seismic velocity increase of 1– 3% approximately 250 kilometers above the core-mantle boundary (CMB) (see review by Wysession et al, 1998). Seismic waveform modeling detects a velocity jump beneath locations of inferred paleosubduction, including Alaska, the Caribbean, Central America, India, and Siberia. The discontinuity can explain a triplicated arrival SdS (PdP) between S (P) and ScS (PcP) observed in waveform data (Lay and Helmberger, 1983). Between epicentral distances 65–83 degrees, SdS is a composite arrival from a discontinuity reflection Sbc (Pbc) and a ray turning below the interface Scd (Pcd). The horizontally-polarized S-wave (SH) triplicated arrival is often analyzed at ∼ 10 s period. Shorter periods (∼ 1 s) can detect the P-wave triplication (PdP) but data stacking is required to suppress noise

Results

Discussion

Conclusion