Simulations of Pre-critical PKiKP Coda Waves with Implications for Small-Wavelength Heterogeneity in Earth's Inner Core
Abstract
Evidence for fine-scale ( 1-10 km) heterogeneity in Earth's inner core was first reported nearly 20 years ago by Vidale and Earle (2000) using data recorded between 1969 and 1975 at the now-defunct Large Aperture Seismic Array in Montana. Those authors observed long, emergent, spindle-shape coda following pre-critical PKiKP waves at distances of 58º - 73º, and successfully used single-scattering ray theory to model their observations with 1.2% root-mean-square (RMS) velocity variations randomly distributed within the upper few hundred kilometers of the inner core with a dominant scale-length of 2 km. Several later studies reported similar observations at small-aperture arrays of the International Monitoring System and likewise interpreted them in terms of volumetric heterogeneity within the inner core. Leyton and Koper (2007) used a ray theoretical, single-scattering approach to evaluate alternative origins for the PKiKP coda waves and found that volumetric heterogeneity in the lower mantle, topography on core-mantle boundary (CMB), and topography on the inner core boundary (ICB) could not match the observations. Peng et al. (2008) used the seismic phonon method of Shearer and Earle (2004) and further showed that multiple scattering in the lowermost mantle could not match the PKiKP coda observations. Here we use an updated version of the phonon code, which accounts for topography on the CMB and ICB as well as volumetric heterogeneity throughout the solid Earth, to explore alternative origins for PKiKP coda waves. We do not obtain any energy following PKiKP from mantle-only scatterer models. For simulating scattered energy from the ICB, we use two kinds of models: a thin-layer model with volumetric heterogeneity in just the top 10 km of the inner core and a rough interface model. To generate significant energy after PKiKP, we need at least 2% root-mean-square (RMS) velocity variation with 2 km scale length for the thin layer model, or about 1 km RMS ICB topography fluctuation with correlation size about 3 km. However, in both of these cases the synthetic coda envelopes do not grow in time and distance as they do in the observations. Thus, volumetric heterogeneity within the outer inner core is still our preferred explanation for emergent, spindle-shaped PKiKP coda at pre-critical distances.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFMDI43B0026P
- Keywords:
-
- 1507 Core processes;
- GEOMAGNETISM AND PALEOMAGNETISMDE: 1510 Dynamo: theories and simulations;
- GEOMAGNETISM AND PALEOMAGNETISMDE: 3924 High-pressure behavior;
- MINERAL PHYSICSDE: 7207 Core;
- SEISMOLOGY