Abstract:
High amplitude, high frequency, and laterally coherent seismic phases from the core-mantle boundary (CMB) are observed on data from Peaceful Nuclear Explosions in Siberia. These arrivals are observed at 2600-4000 km offset with travel times and move-outs consistent with CMB reflections (PcP) and are readily correlated because of the small station interval of 10 km. The duration and complexity of the arrivals are inconsistent with a simple reflection from a single CMB interface. They require the combination of PcP and a complex precursor phase (PdP) from the top of an ultralow-velocity zone (ULVZ) despite the fact that earthquake recordings indicate that there is no ULVZ present beneath this part of Siberia. Lateral variations in the waveform imply that the thickness and physical properties of the ULVZ change with a wavelength of 150-200 km and an amplitude of up to 8 km. We find at least one location where there is no ULVZ present. At another well-sampled location, the precursor has higher frequency content than the primary arrival. Reflectivity modeling of these PcP/PdP phases shows that kilometer-scale layering may be required in the ULVZ to explain the seismic waveform and the frequency split. The modeling shows very low velocity in the ULVZ with reductions of 25 and 40% for P and S waves, respectively. This indicates higher percentages of partial melt in the ULVZ than hitherto interpreted. Our observations indicate that the ULVZ may be present over wide areas of the Earth with a thickness below the resolution limit of seismological data with a lower frequency content.
