Detection of an Ultra Low Velocity Zone beneath Central Mexico with PcP Waveform Modeling

PcP phases from a Guatemalan earthquake recorded at Nevada stations of the International seismic network (IM) display evidence of anomalous seismic structure at the core-mantle boundary (CMB); in particular, preand post-cursor phases to PcP and ScP. The data is examined for evidence of an ultra-low velocity zone (ULVZ) by identifying and modeling precursors to PcP. Precursory arrivals to PcP may be generated by the interaction of PcP with a thin layer above the core-mantle boundary (CMB). One-dimensional modeling demonstrates that standard earth models of the core-mantle boundary, which lack ULVZ structure, cannot produce the observed PcP data. An ULVZ model with thickness of 7.5 km, velocity reductions of 10% and 20% for P velocity and S velocity, respectively, and 20% density increase provides the best fit to the observed PcP waveforms. Combining these results with other nearby studies of the CMB region indicate that ultra-low velocity zones are an intermittent and short wavelength feature of the CMB.