Estimate of inner core rotation rate from United Kingdom regional seismic network data and consequences for inner core dynamical behaviour

We analyse over 12 500 records of southwest Pacific earthquakes recorded by the UK network for PKPBC and PKPDF phases to seek constraints on the inner core’s rotation rate. Careful analysis and strict rejection criteria yield 655 PKPBC–PKPDF differential travel time residuals computed with respect to the AK135 reference model, densely sampling a small region of the inner core over a time period of 15 years. The data images both lateral and radial velocity heterogeneity in the inner core located geographically beneath the north Pacific between radii of 870 and 1080 km. The dominant feature is a longitudinal wave speed gradient centred at about 180° longitude. In combination with different earthquake catalogues and inner core anisotropy models, we explore different techniques to use this gradient to constrain the rotation rate. Our estimates range from 0.45±0.25 to 0.74±0.29°/yr relative to the mantle, with two of four estimates including zero rotation at the 95% (2σ) confidence level, indicating that rotation is marginally detectable. As an alternative to monotonic rotation, we find weak evidence for rotational oscillation of the inner core on time scales of about 280 days. In consequence, these observations suggest that: (1) viscous coupling between the outer and inner core simulated using viscous hyperdiffusivity is not appropriate for modelling inner core rotation; (2) temperature gradients between the interior and exterior of the tangent cylinder in the core are also small; (3) gravitational torques coupling the inner core to the mantle are large, on the order of 2×1021 Nm; and (4) the inner core’s viscosity is high, 3.9×1019 Pas.