New Insights about the Character of the PP Precursors at High Frequency

The energy on the seismogram before the arrival time of the seismic main PP wave is called PP precursor. Although the name of PP precursor corresponds to the term PP, the components do not only concern the wave reflection back from the surface. The early part of the PP precursors overlapped with the P or Pdiff wave coda, meanwhile the PKiKP wave arrived earlier than the PP wave when the distance is larger than about 100°, which results in a mixture with the PKiKP phase. The origin of the PP precursors was usually regarded as the discontinuities of the mantle, like 410 km and 660 km discontinuities. However, the arrival time and the slowness from seismic array beamforming of the precursors very close to the PP wave both disagree with this interpretation. Some middle mantle reflection layers (e.g., at 1000 km) may contribute to the unknown phase before the PP wave at the long period, but cannot explain the gradually increasing energy before the phase at high frequency. We have observed the PP precursors at high frequencies of 1–2 Hz at an epicentral distance between 95° and 115° from the earthquakes whose magnitude is larger than 7.0 Mw and the source depth is shallower than 100 km. The global PP precursors show that it originates not only from the off-great-circle scattering at some regional subduction slabs. The stacking result is compared to our Monte Carlo simulation of 3D scattering with a 1D spherically symmetric heterogeneity model, which has much potential to be improved. Single scattering of the middle and upper mantle (shallower than 1000 km depth) allows for the generation of the emerging PP precursor at high frequencies. The scattering process is similar to the one responsible for the generation of the Pdiff coda, which is generated by scattering in the whole mantle. As a consequence, the scattering pattern of the PP precursors is PP*P or P*PP, where the asterisk indicates the scattering, which explains that the slowness of some PP precursors is higher than the PP wave but sometimes similar to the P or Pdiff wave.