Time-dependent inner core structures examined using repeating earthquakes in subduction zones of the southwest Pacific

Abstract

This study examines time-dependent inner core structures using waveforms and double differential times of the PKP(bc-df) and PKP(ab-df) phases measured from repeating earthquakes in the southwest Pacific subduction zones. Repeating earthquakes can eliminate potential artefacts of interevent distance and improve the measurement precision of temporal changes in PKPdf phases due to differential rotation of the Earth's inner core. PKPdf waves from the southwest Pacific primarily sample the eastern hemisphere of the inner core along equatorial paths. Time separation of repeating earthquakes ranges from 4 to 14.4 yr. Most observed double differential times of PKP(bc-df) and PKP(ab-df) are within ±70 ms, with no systematic changes as a function of time separation or calendar time. Null temporal changes of the PKPdf wave could indicate a smooth regional-scale lateral velocity gradient in the eastern hemisphere of the inner core. Uncertainties in the data prohibit statistically meaningful estimates of the lateral velocity gradient, temporal trend, inner core differential rotation rate, or decadal oscillations. Synthetic seismograms are used to test the effects of several possible artefacts and to quantify the magnitudes of velocity perturbations relative to previous estimates. These artefacts are quantitatively assessed to determine their expected effects on the measurements.