Inner‐core fine‐scale structure from scattered waves recorded by LASA

Abstract

Recent observations of inner‐core scattering (ICS) waves provide evidence that the outermost 300 km of the inner‐core has strong heterogeneities with a length scale of a few kilometers. These waves follow a path similar to that of the inner‐core–reflected waves PKiKP and were originally observed in data from 16 events in the distance range 58° to 73° recorded by the Large Aperture Seismic Array (LASA). Here we present additional observations of the ICS waves from a total of 78 events recorded by LASA at distances from 18° to 98°. We use a modified version of the Generic Array Processing software package to identify ICS waves on the basis of travel time, back azimuth, ray parameter, amplitude, and coherence. There are 44 events that produce clear ICS waves. We then perform forward modeling of the observed ICS waves using a Monte Carlo seismic phonon method that allows for multiple scattering along the raypath. Most of the ICS waves appear without a visible PKiKP phase, initially grow in time, and have a spindle‐shaped envelope. The duration, risetime, and decay rates of the observed ICS waves can be best explained by small‐scale volumetric heterogeneities in the outermost few hundred kilometers of the inner core. The average Qc value for the 44 events is ∼600. Most clear ICS waves are found for raypaths sampling the Pacific Ocean and Asia, and relatively few observations are from the Atlantic Ocean, roughly consistent with the recently observed hemispheric difference in the inner‐core structure.