Velocity structure near the inner core boundary from waveform modeling

Abstract

A velocity model of the inner core boundary (ICB) region is developed from broadband wave‐form modeling. The data consist of long period and short period record sections gathered from the U.S. arrays, WWSSN, and LRSM. Deep events beneath Indonesia were used; these sampled the ICB structure beneath the North Pacific. The Cagniard‐de Hoop technique was used to generate the synthetics, which allowed considerable flexibility in forward modeling these individual PKP phases. Our preferred model was developed from modifications of the Preliminary Reference Earth Model (PREM). It contains a relatively low velocity gradient above the ICB, a sharp jump at the ICB of about 0.78 km/s, and a relatively large gradient underneath. These features are constrained largely by the differential travel times and amplitude ratios of PKIKP and PKiKP of the short period section. Further modification of PREM in the fluid core was required to explain the long‐period observations associated with PKP waveforms. Our modeling results suggest a low‐velocity gradient at the bottom (roughly 400 km) of the fluid core. A low Q zone at the top of the inner core is required to fit the relative amplitudes of PKIKP phases similar to earlier studies. This model indicates possible inhomogeneity above the ICB, as suggested by other authors.