Low Velocity Zones Near the Core Mantle Boundary From PKIKP Precursors

Abstract:

The core mantle boundary (CMB) is one of the most significant boundary layers in the Earth, where the silicate mantle meets the liquid iron-nickel outer core and thus is crucial in understanding the thermal, chemical and dynamic processes in the Earth. Besides, the 200 - 300 km region above this boundary, known as D”, exhibits strong heterogeneities of different scales, from thousands to tens of km. A variety of studies have been conducted to study this region, especially the small-scale structures. Among them, PKP precursors provide valuable complementary information on such small-scale heterogeneities due to their special ray geometries. In this study, we collected seismograms from earthquakes in South America recorded by stations in Australia. After band passing, in the short period seismograms, we observed clear PKP precursor phases with large amplitudes comparable to those of PKIKP. Additionally, these precursor phases arrive about 7-10 s before PKIKP. Using array processing methods, we calculated the slowness of these precursors and found the slowness is from 3.2 to 3.4. Furthermore, the precursors have a phase shift of π/2 compared to PKIKP phase. Our preliminary results indicate that the observed precursors may originate from small scatterers near the core mantle boundary in the southeast side of Australia, and the P velocities of the scatterers may have more than 10% reduction. More synthetic tests will be carried out to further constrain the geometry and velocity variations of the scatterers.