Precursory arrivals to PP

ISC (International Seismological Centre) traveltime data have been investigated to reveal the source of precursory arrivals to the PP phase observed at epicentral distances around 100°. The data include more than 25 000 traveltimes reported from 1971 to 1985. Most of the lead times of the precursory arrivals are found to be less than 75 s, and the frequency distribution of the lead times indicates that most of the precursory arrivals can be interpreted as scattering waves generated in the crust and the uppermost mantle. However, several peaks can be found in the frequency distribution, especially for impulsive onsets. These may correspond to certain types of deterministic waves such as P-waves reflected from the bottom sides of the velocity discontinuities in the upper mantle or those reflected asymmetrically in the crust or the uppermost mantle. Many of the impulsive onsets included in these peaks have paths travelling through the Tibetan region, suggesting the possible velocity discontinuity around 190 km associated with the low-velocity layer in this tectonically active region.