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Multidecadal variation of the Earth’s inner-core rotation

DOI:10.1038/s41561-022-01112-z
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Yi Yang
Yi Yang
Yi Yang
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Xiaodong Song
Xiaodong Song
Xiaodong Song
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Abstract and Figures

Differential rotation of Earth’s inner core relative to the mantle is thought to occur under the effects of the geodynamo on core dynamics and gravitational core–mantle coupling. This rotation has been inferred from temporal changes between repeated seismic waves that should traverse the same path through the inner core. Here we analyse repeated seismic waves from the early 1990s and show that all of the paths that previously showed significant temporal changes have exhibited little change over the past decade. This globally consistent pattern suggests that inner-core rotation has recently paused. We compared this recent pattern to the Alaskan seismic records of South Sandwich Islands doublets going back to 1964 and it seems to be associated with a gradual turning-back of the inner core as a part of an approximately seven-decade oscillation, with another turning point in the early 1970s. This multidecadal periodicity coincides with changes in several other geophysical observations, especially the length of day and magnetic field. These observations provide evidence for dynamic interactions between the Earth’s layers, from the deepest interior to the surface, potentially due to gravitational coupling and the exchange of angular momentum from the core and mantle to the surface. Multidecadal oscillation of the Earth’s inner core, coinciding with length of day and magnetic field variations, is experiencing a pause and reversing, according to analysis of repeating seismic waves traversing the inner core since the 1960s.
Seismic raypaths used in the study There are eight different paths from repeating earthquakes (stars) to seismic stations or arrays (triangles), including: (1) SSI to station AAK; (2) SSI to Yellowknife array (YKA); (3) SSI to ILAR array; (4) SSI to station INK; (5) Peru–Chile trench (PCT) to Kazakhstan network (KZ); (6) Aleutian Islands (AI) to station BOSA; (7) Java trench (JVT) to station PLAL; and (8) Kurile Islands (KI) to station PLCA. The thickened lines indicate the ray segments in the inner core (IC); their colours, red and blue, denote the DF rays from the CD and BC distance ranges, respectively. The two insets show the travel time curves and raypaths of different PKP branches, including the DF branch traversing the IC, the BC and AB branches traversing the outer core (OC) and the CD branch reflecting at the ICB.
Seismic raypaths used in the study There are eight different paths from repeating earthquakes (stars) to seismic stations or arrays (triangles), including: (1) SSI to station AAK; (2) SSI to Yellowknife array (YKA); (3) SSI to ILAR array; (4) SSI to station INK; (5) Peru–Chile trench (PCT) to Kazakhstan network (KZ); (6) Aleutian Islands (AI) to station BOSA; (7) Java trench (JVT) to station PLAL; and (8) Kurile Islands (KI) to station PLCA. The thickened lines indicate the ray segments in the inner core (IC); their colours, red and blue, denote the DF rays from the CD and BC distance ranges, respectively. The two insets show the travel time curves and raypaths of different PKP branches, including the DF branch traversing the IC, the BC and AB branches traversing the outer core (OC) and the CD branch reflecting at the ICB.
… 
Examples highlighting a pattern change in the inner-core temporal changes in the recent decade a,b, Waveforms from an SSI multiplet along Paths 2 and 3 showing the pattern change in S. The last two digits of the event years and time lapse (in years) are labelled on the left. For Path 2 at CD distance range (a), the normalized waveforms are aligned by cross-correlation of the entire window. For Path 3 at BC distance range (b), they are aligned with BC, and the weak inner-core waves and the background noise preceding the dashed line (onset of BC) are amplified. c, Waveforms from a PCT multiplet along Path 5 showing the pattern change in DF relative arrival times (ddt). It is plotted in the same way as b. The horizontal 2-s segment is the DF time window used to measure the ddt.
Examples highlighting a pattern change in the inner-core temporal changes in the recent decade a,b, Waveforms from an SSI multiplet along Paths 2 and 3 showing the pattern change in S. The last two digits of the event years and time lapse (in years) are labelled on the left. For Path 2 at CD distance range (a), the normalized waveforms are aligned by cross-correlation of the entire window. For Path 3 at BC distance range (b), they are aligned with BC, and the weak inner-core waves and the background noise preceding the dashed line (onset of BC) are amplified. c, Waveforms from a PCT multiplet along Path 5 showing the pattern change in DF relative arrival times (ddt). It is plotted in the same way as b. The horizontal 2-s segment is the DF time window used to measure the ddt.
… 
S of the inner-core waves from two subsets of doublets In one subset (left), both events of a doublet are before 2009; in the other subset (right), both events are after 2009. Each line segment plots a doublet from the origin time of the earlier event and the reference S of 0.95 (the dashed horizontal line) to the origin time of the later event and the corresponding S horizontally and vertically, respectively. Each error bar represents an S measurement ± its measurement error (σs; see Methods). The shaded vertical bar indicates the 95% CI for the time of the turning point for the pattern change from the ddt data (see main text).
S of the inner-core waves from two subsets of doublets In one subset (left), both events of a doublet are before 2009; in the other subset (right), both events are after 2009. Each line segment plots a doublet from the origin time of the earlier event and the reference S of 0.95 (the dashed horizontal line) to the origin time of the later event and the corresponding S horizontally and vertically, respectively. Each error bar represents an S measurement ± its measurement error (σs; see Methods). The shaded vertical bar indicates the 95% CI for the time of the turning point for the pattern change from the ddt data (see main text).
… 
Global temporal change of DF’s travel time (dt) and the comparison with the ΔLOD Note that the dt is proportional to the amount of the inner-core rotation (or its angular position); here, the amount of rotation is estimated from our recent determination of the averaged rotation rate (0.127° per year) for the time period of 1991–2010³⁵, but the position of 0° is arbitrarily set at the year 2000. a, The best-fitting curve and 95% CI for the six global paths at BC distance and the region. Each segment denotes a scaled ddt measurement, plotted from the point on the curve at the origin time of the earlier event (green point) to a point offset by the scaled ddt (divided by the path-dependent factor; Extended Data Fig. 4) at the origin time of the later event. The inset shows the histograms (with an increment of 1.0) of the ddt measurements and residuals normalized by the corresponding measurement uncertainty (σt) and the dashed line shows a standard normal distribution with the same sample size as the indication of the uncertainty level (Methods). b, Same as a but for the path of the lower-quality SSI doublets to the COL station in Alaska. c, Comparisons between the trends of dt in a and b and the Earth’s −ΔLOD. The 65-year component −ΔLOD (grey, dashed) is extracted from the yearly averages⁴⁰ (grey, solid) and shifted to the later time by 3.7 years for the optimal correlation.
Global temporal change of DF’s travel time (dt) and the comparison with the ΔLOD Note that the dt is proportional to the amount of the inner-core rotation (or its angular position); here, the amount of rotation is estimated from our recent determination of the averaged rotation rate (0.127° per year) for the time period of 1991–2010³⁵, but the position of 0° is arbitrarily set at the year 2000. a, The best-fitting curve and 95% CI for the six global paths at BC distance and the region. Each segment denotes a scaled ddt measurement, plotted from the point on the curve at the origin time of the earlier event (green point) to a point offset by the scaled ddt (divided by the path-dependent factor; Extended Data Fig. 4) at the origin time of the later event. The inset shows the histograms (with an increment of 1.0) of the ddt measurements and residuals normalized by the corresponding measurement uncertainty (σt) and the dashed line shows a standard normal distribution with the same sample size as the indication of the uncertainty level (Methods). b, Same as a but for the path of the lower-quality SSI doublets to the COL station in Alaska. c, Comparisons between the trends of dt in a and b and the Earth’s −ΔLOD. The 65-year component −ΔLOD (grey, dashed) is extracted from the yearly averages⁴⁰ (grey, solid) and shifted to the later time by 3.7 years for the optimal correlation.
… 
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Nature Geoscience | Volume 16 | February 2023 | 182–187 182
nature geoscience
Article
https://doi.org/10.1038/s41561-022-01112-z
Multidecadal variation of the Earth’s
inner-core rotation
Yi Yang    & Xiaodong Song   
Dierential rotation of Earth’s inner core relative to the mantle is thought
to occur under the eects of the geodynamo on core dynamics and
gravitational core–mantle coupling. This rotation has been inferred from
temporal changes between repeated seismic waves that should traverse the
same path through the inner core. Here we analyse repeated seismic waves
from the early 1990s and show that all of the paths that previously showed
signicant temporal changes have exhibited little change over the past
decade. This globally consistent pattern suggests that dierential inner-core
rotation has recently paused. We compared this recent pattern to the Alaskan
seismic records of South Sandwich Islands doublets going back to 1964
and it seems to be associated with a gradual turning-back of the inner core
relative to the mantle as a part of an approximately seven-decade oscillation,
with another turning point in the early 1970s. This multidecadal periodicity
coincides with changes in several other geophysical observations, especially
the length of day and magnetic eld. These observations provide evidence
for dynamic interactions between the Earth’s layers, from the deepest
interior to the surface, potentially due to gravitational coupling and the
exchange of angular momentum from the core and mantle to the surface.
Earth’s inner core gradually grows from the cooling and solidification
of a liquid outer core to the present radius of ~1,220 km. The chemical
buoyancy and latent heat released from the iron phase transition at the
inner-core boundary (ICB) power the outer-core convection and the geo-
dynamo that generate the Earth’s magnetic field
1
. The electromagnetic
(EM) torque from the geodynamo tends to rotate the inner core in the
liquid outer core2–4. Seismic observations suggest a highly heterogeneous
inner core, with fine-scale scatters
5
, regional velocity variations
6–8
and
long-wavelength to hemispherical structures9–11. It is also believed that
the gravitational coupling between the heterogeneous mantle and inner
core slows down the relative rotation or turns it into an oscillation12–16.
Evidence for the differential rotation of the Earth’s inner core was
first reported from temporal changes of the seismic waves traversing it
(PKIKP, commonly referred as DF; Fig. 1) over years or decades from the
South Sandwich Islands (SSI) to the College seismic station (code COL;
Fig. 1) in Alaska17. The inner-core temporal changes were subsequently
confirmed, particularly from collocated nuclear blasts
18
or earthquake
waveform doublets
19,20
, which are repeating earthquakes with nearly
identical waveforms at common receivers21. The temporal changes have
also been interpreted as localized growth or melting at the ICB, with-
out the necessity of invoking the inner-core differential rotation22–24.
However, more recent studies25–27 suggest that the temporal changes
are related to the interior rather than surficial heterogeneities of the
inner core, still favouring the interpretation of the differential rotation.
Using high-quality global doublets and lower-quality SSI doublets,
here, we show surprising observations that indicate the inner core has
nearly ceased its rotation in the recent decade and may be experienc-
ing a turning-back in a multidecadal oscillation, with another turning
point in the early 1970s.
Global observations of the inner-core temporal
changes
We systematically investigated the temporal changes of inner-core PKP
waves (Fig. 1) along all known paths from previous studies of earthquake
Received: 21 December 2021
Accepted: 5 December 2022
Published online: 23 January 2023
Check for updates
SinoProbe Laboratory and Institute of Theoretical and Applied Geophysics, School of Earth and Space Sciences, Peking University, Beijing, China.
e-mail: xiao.d.song@gmail.com
Content courtesy of Springer Nature, terms of use apply. Rights reserved
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