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-p eriod data. This study finds correspondence between 50% of the PP-P tr avel times read from broadband and long-period data, but also finds 50 % of the measurements differ more than 1.0 second. The inconsistent me asurements are demonstrated to be due to two causes. (1) Diffraction n ear the core-mantle boundary causes dispersion of the P phase which yi elds an underestimate of the PP-P travel time determined from long-per iod data. This effect is identified at epicentral distances as small a s 88 degrees and may amount to several seconds. (2) Interference of th e PP phase with secondary arrivals causes distortion of the PP wavefor m and produces a non-systematic measurement error. This effect are mos t clearly seen on broadband seismograms and may even hamper an unambig uous broadband PP-P travel time measurement. Although strong high-freq uency effects associated with interference are filtered out on the lon g-period data, the long-period PP-waveform may still be contaminated. We conclude that the accuracy of PP-P travel time measurements is on t he order of 1 s for both broadband and long-period seismograms.