Using P2KP Diffracted Phases and High Frequency Synthetic Seismograms to Interrogate Core Mantle Boundary Structure

Abstract:

Despite observations using a variety of different seismological phases, the structure of Earth’s Core-Mantle Boundary (CMB) region remains poorly understood. In the lowermost mantle, Large Low Velocity Provinces have been robustly observed with seismic tomography, and Ultra-Low Velocity Zones have been observed in many waveform studies. Furthermore, the accumulation of high velocity subducted slabs and the presence of mineralogical phase changes may also complicate this region. The upper Outer Core, while believed to be more homogeneous than the lower mantle, is similarly poorly understood with potentially fast or slow layers at its top being hypothesised as a regional or global feature. Further complications arise from CMB topography of largely unknown magnitude. Disentangling the effects of these complexities on seismic observations remains challenging, resulting in a variety of interpretations.

Short period P2KP (also known as PKKP) phases are readily observed at distances beyond their ray theoretical cut-off. These observations correspond to the P2KPdiff phase, where at least part of the wave diffracts along the CMB. This diffracted path leads to P2KPdiff having high sensitivity to CMB structure, akin to the properties of Pdiff. Travel time anomalies of both P2KP and P2KPdiff have been previously underutilised yet have the potential to increase sampling of the CMB which is severely limited by source-station geometry.

We examine systematic offsets in P2KP – P2KPdiff residual travel times observed at stations in the USA from deep, high magnitude earthquakes in the western Pacific. We correct the observed travel times with ray theoretical predictions for 3D mantle models and compare these observations to high frequency synthetic seismograms computed using AxiSEM for a range of heterogeneities in both the lower mantle and outer core. Initial results suggest that the residual travel times can be affected by structures in both the lowermost mantle and the outer core, but that there may be ways to distinguish between the effects of these two regions.