Time-Dependent Inner Core Structures Examined by Repeating Earthquakes in the Southwest Pacific Subduction Zones

Abstract:

Time-dependent inner core structure is interpreted as differential rotation of the Earth’s inner core. This inference is made on the basis of variations deviated from an isotropic and homogeneous inner core structure and the amount of velocity perturbations progressively evolving as a function of calendar time. Most compelling evidences for inner core rotation come from the inner core structures beneath Colombia and Venezuela, characterized by strong anisotropy and lateral variation, for the South Sandwich Islands earthquakes recorded by College (COL) and other seismic stations in Alaska. Repeating earthquakes with highly similar waveforms can minimize the potential artifacts due to inter-event separation and unknown short-scale mantle heterogeneities, and can acquire robust measurement of time shift due to temporal change of inner core structures. Moderate repeating earthquake sequences (RES) in the Tonga–Kermadec–Vanuatu in the southwest Pacific subduction zones are studied over a 20-year time window between 1990 and 2009. I select 13 RES consisting of two or three events with time separation of 2 – 14.4 years and analyze the PKiKP–PKPdf, PKPbc–PKPdf, and PKPab–PKPdf phase pairs recorded by the European, African, and central Asian stations sampling the eastern hemisphere of the inner core. I measure the double differential time of the phase pairs using waveform cross-correlation. Majority of the double differential time measurements within ±50 millisecond can largely be explained by the time shift due to inter-event distance on the order of hundreds of meters or less and null change of the PKPdf phase. These observations indicate inner core structures in the eastern hemisphere are uniform and probably insensitive to motion of the inner core.