Anisotropy of heterogeneity scale lengths in the lower mantle from PKIKP precursors

Abstract

The anisotropy of heterogeneity scale lengths in the lower mantle is investigated by modelling its effect on the high‐frequency precursors of PKIKP scattered by the heterogeneities. Although models having either an isotropic or an anisotropic distribution of scale lengths can fit the observed coda shapes of short‐period precursors, the frequency content of broad‐band PKIKP precursors favours a dominantly isotropic distribution of scale lengths. Precursor coda shapes are consistent with 1 per cent fluctuations in P velocity in the wavenumber band 0.05–0.5 km−1 extending to 1000 km above the core–mantle boundary, and with a D″ region open to circulation throughout the lower mantle. The level of excitation of PKIKP precursors observed in the frequency band 0.02–2 Hz requires a power spectrum of heterogeneity that is nearly white or slowly increasing with wavenumber. Anisotropy of scale lengths may exist in a D″ layer having larger horizontal than vertical scale lengths and produce little or no detectable effects on PKIKP precursors for P‐velocity perturbations as high as 3 per cent when averaged over a vertical scale of several kilometres, and much higher when averaged over scales of hundreds of metres or less.

Distribution of Fine-Scale Mantle Heterogeneity from Observations of Pdiff Coda

“…The best fit was found for the model containing scatterers distributed throughout the mantle. Cormier (1999) modeled the Hedlin et al (1997) observations with a more complete single-scattering theory and found that about 1% velocity variations must extend at least 1000 km above the CMB to explain the time dependence in the precursor amplitudes.…”

Strong seismic scatterers near the core–mantle boundary north of the Pacific Anomaly

“…Particular advantages of using PKP precursors are that they are sensitive to heterogeneity at a scale as small as $10 km, and are not contaminated by the coda of other phases due to their earlier arrival times. There have been successful applications of PKP precursors studies on global scales (e.g., Hedlin et al, 1997;Cormier, 1999;Margerin and Nolet, 2003b;Mancinelli and Shearer, 2013), as well as regional scales (e.g., Vidale and Hedlin, 1998;Wen and Helmberger, 1998;Niu and Wen, 2001;Miller and Niu, 2008;Thomas et al, 2009;Frost et al, 2013). In regional studies that incorporate well-sited arrays of seismographs, greater information about the anomalies, such as their boundary locations and sharpness, may be resolvable.…”

Estimation of Fault Strength: Reconstruction of Stress Before the 1995 Kobe Earthquake

“…The apparent onset of anisotropy with the velocity jump at the top of Dµ suggests a link between the two, which could arise if there is a chemically distinct layer (1), a transition to a sheared zone of enhanced velocity heterogeneity (29), a change in iron partitioning associated with pressureinduced change in spin state (30), or a major phase change (31,32). Improved azimuthal constraints are required to distinguish among these possibilities and that of MgO above, but we now have unequivocal evidence that extensive regions of Dµ anisotropy contain more complex structure than simple VTI, with high-resolution seismic analysis required to detect and map the structure.…”