The Earth's free oscillations and the differential rotation of the inner core

Abstract

Differential rotation of the inner core has been inferred by several body-wave studies with most agreeing that a superrotation may exist with a rate between 0.2 • and 3 • per year. The wide range of inferred rotation rate is caused by the sensitivity of such studies to local complexities in structure which have been demonstrated to exist. Freeoscillation "splitting functions" are insensitive to local structure and are therefore better candidates for estimating differential IC rotation more accurately. We use a recently developed method for analyzing free oscillations which is insensitive to earthquake source, location and mechanism to constrain this differential rotation. In a prior study, we found that inner core differential rotation has been essentially zero over the last 20 years. We revisit this issue, including additional earthquakes and modes in our analysis. Our best estimate is a barely significant superrotation of 0.13±0.11 • /yr, which is still consistent with the idea that the inner core is gravitationally locked to the mantle.

Deep Earth Structure – The Earth’s Cores

“…This relies on the identification of long wavelength, even degree heterogeneities inside the inner core, after correction for Earth rotation, ellipticity, and mantle heterogeneities. A later study (Laske and Masters, 2003), with more modes and more events, shows that the results are consistent whatever the mantle model used for corrections, and again emphasize the dependence of the result to the mode considered (Figure 19). Sharrock and Woodhouse (1998) analyzed seven modes for three time periods ranging from 1977 to 1996.…”

“…The splitting of the modes in multiplets gives information on the anisotropy, once the contributions of Earth rotation and ellipticity have been corrected (see, e.g., Tromp, 1995;Durek and Romanowicz, 1999;Laske and Masters, 2003). The splitting of the modes in multiplets gives information on the anisotropy, once the contributions of Earth rotation and ellipticity have been corrected (see, e.g., Tromp, 1995;Durek and Romanowicz, 1999;Laske and Masters, 2003).…”

“…Modified fromLaske G and Masters G (2003) The Earth's free oscillations and the differential rotation of the inner core. The results vary with modes and are often compatible with a null rotation.…”

Deep Earth Structure: The Earth’s Cores

“…They rely on the identification of long wavelength, even degree heterogeneities inside the inner core, after correction for mantle heterogeneities. A later study (Laske and Masters, 2003), with more modes and more events, shows that the results are consistent for different mantle corrections and again emphasizes the dependence of the result on the mode considered, with rotation rates either positive or negative ( Figure 18). This method has the potential to estimate accurately the rotation rate, but a good Earth coverage and the use of a large number of modes are necessary.…”

“…The splitting of the modes in multiplets gives information on the anisotropy, once the contributions of the Earth's rotation and ellipticity are corrected (e.g., Durek and Romanowicz, 1999;Dziewonski and Gilbert, 1971;Irving and Deuss, 2011a;Laske and Masters, 2003;Tromp, 1993). They avoid the problem of sparse sampling due to body waves.…”