High-frequency precursors to P wave arrivals in New Zealand: Implications for slab structure

This report revisits the very early high-frequency slab phases from earthquakes in the Kermadec slab (between -25 degrees S and -37 degrees S) that arrive as a precursor to the P wave onset at stations in New Zealand. The analysis of short-period digital records for station SNZO (South Karori New Zealand) for the time period between 1980 and 1986 involved 4 times as many data as in previous studies. We confirm most of the earlier observations, in particular the relationship between source region and observed waveform. Numerical waveform modeling using two-dimensional slab models confirm that the high-frequency precursor can be explained by P wave propagation in a thin (8-10 km) high-velocity layer (HVL). The sensitivity of the high-frequency phase to source position in the slab is investigated, and we demonstrate that the slab phase does not survive interruptions or strong lateral variations of the HVL. Assuming the shape of the slab from tomographic images and continuity of the HVL, we infer that the large negative travel time residuals can be explained if P wave propagation is 4 to 5% faster than in the ambient mantle for depths to 300 km and about 2% faster for depths between 300 and 600 km. Rather than resorting to regional differences in structure and composition of the HVL, we argue that specific source-receiver configurations and the shape of the slab explain the observed regional variation in the character of the high-frequency slab phases.