On PP-P differential travel time measurements

This study investigates the stability of PP-P travel time measurements using a waveform crosscorrelation method on both broadband and long-period data. This study finds correspondence between 50% of the PP-P travel times read from broadband and long-period data, but also finds 50% of the measurements differ more than 1.0 second. The inconsistent measurements are demonstrated to be due to two causes. (1) Diffraction near the core-mantle boundary causes dispersion of the P phase which yields an underestimate of the PP-P travel time determined from long-period data. This effect is identified at epicentral distances as small as 88° and may amount to several seconds. (2) Interference of the PP phase with secondary arrivals causes distortion of the PP waveform and produces a non-systematic measurement error. This effect are most clearly seen on broadband seismograms and may even hamper an unambiguous broadband PP-P travel time measurement. Although strong high-frequency effects associated with interference are filtered out on the long-period data, the long-period PP-waveform may still be contaminated. We conclude that the accuracy of PP-P travel time measurements is on the order of 1 s for both broadband and long-period seismograms.