Very Slow Rotation of Earth's Inner Core From 1971 to 1974

Abstract

The rate that Earth's inner core rotates relative to the mantle and crust has been debated for decades. Nonrotational processes, including internal deformation and flow in the outer core, have also been proposed to explain observed seismic changes. The observed changes thus far have been so inconsistent and weak as to hamper convincing interpretation. Here, we examine waves backscattered from within the inner core, which can more robustly evaluate rotation, from two nuclear tests 3 years apart in Novaya Zemlya, Russia. We have extended our previous analysis of these explosions using precise station corrections and the full Large Aperture Seismic Array, thus revealing how the time shifts depend on slowness and lag time and halving our rotation rate estimate. Our derived 0.07°/year inner core superrotation rate from 1971 to 1974 is more robust and slower than most previous estimates and may require interesting reinterpretations of localized signals previously interpreted as inner core rotation.

Temporal Changes of the Inner Core From Globally Distributed Repeating Earthquakes

“…the recent Transportable Array (TA) stations in Alaska may allow detailed mapping of inner core and lower mantle structures along the path from SSI to Alaska to test further whether the different rotation rates can be reconciled. On the other hand, it is interesting to note that the rotation rate from the small-scale velocity gradient of our new path is consistent with the recent estimate from small-scale inner core scatters (0.07 ± 0.02°/year) by Vidale (2019).…”

“…This explanation is consistent with, indeed predicted by, some geodynamo theories in that the inner core is driven to rotate by electromagnetic coupling (Gubbins, 1981; Glatzmaier & Roberts, 1995). Depending on the underlying assumptions and the data types, estimates of the rotation rate vary greatly, from 1 degree per year (Song & Richards, 1996), a fraction of degree per year (Creager, 1997; Song, 2000; Vidale et al, 2000; Zhang et al, 2005), to values of a few times smaller (Vidale, 2019) or even one degree per million years (Waszek et al, 2011). The initial estimate was based on a model of tilted inner‐core anisotropy axis from the Earth's rotation axis (Su & Dziewonski, 1995), which was later suggested to be problematic (Dziewonski & Su, 1998).…”

“…Creager (1997) observed a significant lateral velocity gradient in the inner core for the SSI to Alaska path and proposed that the temporal change of the DF travel times results from a shift of laterally varying inner core structure from the inner core rotation. The interpretation is well accepted and has been used to infer the rotation rate in many subsequent studies (Song, 2000;Vidale, 2019;Vidale et al, 2000;Zhang et al, 2005). Patterns of inner core scattering waves were used to infer slower rotation rates from time shifts of inner core scatters (Vidale, 2019;Vidale et al, 2000).…”

“…The interpretation is well accepted and has been used to infer the rotation rate in many subsequent studies (Song, 2000;Vidale, 2019;Vidale et al, 2000;Zhang et al, 2005). Patterns of inner core scattering waves were used to infer slower rotation rates from time shifts of inner core scatters (Vidale, 2019;Vidale et al, 2000). Studies using the Earth's normal mode suggest barely significant rotation of the inner core (Laske & Masters, 2003).…”

On the relation of the lunar recession and the length-of-the-day

“…The more recent estimates from seismic observations suggest a low value of about 0.1 or less than 0.2 degree per year, the core turning slightly faster than the mantle towards the East, see Deuss (2014) and Souriau and Calvet (2015) for reviews. A recent re-analysis of observations by the full Large Aperture Seismic Array of seismic waves back scattered by the inner core for two soviet nuclear tests of 1971 and 1974 show time shifts supporting a differential rotation of 0.07 degree/yr, much lower than current estimates (Vidale 2019). Tsuboi and Butler (2020) have used antipodal earthquakes and stations to analyse the time shifts of wave propagation near the core.…”