A P-WAVE VELOCITY MODEL OF EARTHS CORE

Present Earth core models derived from the retrieval of global Earth s tructure are based on absolute travel times, mostly from the Internati onal Seismological Centre (ISC), and/or free-oscillation eigenfrequenc ies, Many core phase data are left out of these constructions, e.g,, P KP differential travel times, amplitude ratios, and waveforms, This st udy is an attempt to utilize this additional information to construct a model of core P wave velocity which is consistent with the different types of core phase data available, In conjunction with our. waveform modeling we used 150 differential time measurements and 87 amplitude ratio measurements, which were the highest-quality observations chosen from a large population of Global Digital Seismograph Network (GDSN) records, As a, result of fitting these various data sets, a one-dimens ional P wave velocity model of the core, PREM2, is proposed, This mode l, mollified from the Preliminary Reference Earth Model (PREM) (Dziewo nski and Anderson, 1981), skews a better fit to the combined data set than any of the existing: core models, Major features of the model inc lude a sharp velocity discontinuity at the inner core boundary (ICB), with a large jump (0.78 km/s), and a low velocity gradient at the base of the fluid col e. The velocity is nearly constant over the lower 10 0 km of the outer core, The model features a depth-dependent Q(alpha) structure in the inner cole such that a constant t() for the inner co re fits the amplitude ratios and waveforms of short-period waves moder ately well, This means the top of the inner core is more attenuating t han the deeper part of the inner core. In addition, the P velocity in the lowermost mantle is reduced from that of PREM as a baseline adjust ment for the observed separations of the DF and AB branches of PKP at large distances.