A New Seismic Phase to Detect Mid-Mantle Scatterers

Abstract

Small-scale mid-mantle heterogeneities (<100 km) are revealed by various short-period seismic waves with different sampling paths. Here, using dense stations in China, we present the observations of a new seismic phase that arrives as a PP precursor. This arrival represents a P-wave scattered at mid-mantle heterogeneities as PdpP (“d” indicates the depth of heterogeneity) and appears in two forms: (a) a wave train with increasing slowness from P/Pdiff to PP slowness; (b) a single arrival with unique slowness between Pdiff and PP slowness. Waveform modeling results suggest that both types of PdpP signals can be simultaneously generated by lower mantle scatterers with scales larger than 20 km, situated close to the source region. Furthermore, focusing on the observations of PdpP from Fiji-Tonga earthquakes, we locate a series of scatterers at depth of 800–1,350 km that may be related to the Pacific slab subducting along the Tonga-Kermadec trench.