Detection of Pdiff Postcursors caused by the Hawaii Mega-ULVZ

Core-diffracted shear waves (Sdiff) have been used to study the large-scale deep anomalies called Large Low Velocity Provinces (LLVP) and small-scale Ultra Low Velocity Zones (ULVZ) at the base of the mantle. With an estimated radius of 450km, a thickness of 20km, and a 20% shear wave velocity (Vs) reduction, the Hawaii mega-ULVZ is one of the best studied small-scale anomalies in the lower mantle, but there are virtually no direct observational constraints on its compressional wave velocity (Vp) structure. Combined observations of Vs and Vp are insightful for distinguishing between several proposed origins for ULVZs, particularly the partial melt hypothesis, which is expected to produce a Vs:Vp drop of 3:1.

In this study, we use array seismology stacking methods to study the relatively high frequency wavefield (2-10 seconds of period) recorded in the central USA from an event in Papua New-Guinea in March 2010. We wereable to detect a post-cursor to the core-diffracted compressional (Pdiff) wave on multiple sub-arrays and to estimate the P-wave anomaly required in the basal layer of the Hawaii mega-ULVZ to explain the observed delay times. Using a 2D raytracer, we find that for a 450km radius anomaly, a reduction of 35% in both Vp and Vs is needed to explain the delay times of the high frequency postcursors for the core-diffracted P and S waves for this one event.

As shown by Li et al. (2022) for Sdiff, we observe a frequency dependence of the delay times for the Sdiff and Pdiff postcursors. The Pdiff postcursors seemingly merge with the main phase when low pass filtered under 8-10 seconds of period, which could mean that the ULVZ has a layered internal structure. If this is the case, differences in wavelength and sensitivity kernels for Pdiff and Sdiff could help decipher the internal structure of the Hawaii mega-ULVZ. Moreover, the Pdiff postcursors could be caused by a deeper melt-rich layer that could explain the strong anomalies required to explain the delay times, which would be indistinguishable from the liquid outer core for Sdiff. We explore other events with sensitivity to the Hawaiian ULVZ to find Pdiff postcursors that provide more constraints on the drop in Vp, its internal structure, and the potential deep melt-rich layer.