Seismological evidence for differential rotation of the Earth's inner core

Song, Xiaodong; Richards, Paul G.

doi:10.1038/382221a0

Cited by 404 publications

(293 citation statements)

References 29 publications

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“…Both the rotation direction (eastward) and the rotation rate (0.6ø/yr) determined using the SPA data are consistent with the initial estimate of an eastward inner core rotation of about 1.0ø/yr froin the SSI-COL pathway [Song and Richards, 1996] and the more robust estimate of 0.3-1.1ø/yr from dense samples of the SSI-Alaska pathway [Song, submitted manuscript, 1999].…”

Section: Resultssupporting

confidence: 60%

Support for differential inner core superrotation from earthquakes in Alaska recorded at South Pole station

Song¹,

Li²

2000

J. Geophys. Res.

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Abstract. Observational evidence for differential inner core rotation has recently been reported. The most convincing evidence has been restricted to one ray path from earthquakes in the South Sandwich Islands to stations in Alaska. Here we show that the differential travel times between PKP(DF) and PKP(BC) for a new ray path from earthquakes in southern Alaska to the station at the South Pole have increased by about 0.6 s over 37 years, providing support for a differential inner core rotation. We apply a newly developed technique to invert simultaneously for the inner core structure and the rotation rate. The rotation rate determined from the new pathway is about 0.6ø/yr faster than the mantle, compatible with the estimates of the rate from earthquakes in the South Sandwich Islands to stations in Alaska. We also address some criticisms concerning the reported detection of the differential inner core rotation, including biases from event mislocations, biases from mantle heterogeneity, and uncertainty in the tilt of the inner core anisotropy axis. We show that although these potential biases would affect the determination of the rotation rate, they are unlikely to account for the observed time dependence in the BC-DF times. We found no evidence for a correlation between BC-DF residuals and event magnitudes as suggested previously.

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Section: Resultssupporting

confidence: 60%

Support for differential inner core superrotation from earthquakes in Alaska recorded at South Pole station

Song¹,

Li²

2000

J. Geophys. Res.

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“…The Earth's inner core enjoyed sudden public interest as "the planet within a planet" when seismologists found the first seismic evidence that it rotates faster than the mantle (Song and Richards, 1996;Su et al, 1996). The discovery was a timely one as some modern geodynamo simulations predicted this superrotation (Glatzmaier and Roberts, 1996), and earlier studies had already speculated that the inner core is likely rotating due to coupling through magnetic torques (e.g.…”

Section: Introductionmentioning

confidence: 95%

“…The early seismic evidence came from the observation that differential body wave travel times between phases that turn deep in the outer core, PKP(BC), and phases that penetrate into the inner core, PKP(DF), change with time. Song and Richards (1996) observed a 0.3s change over 30 years for differential times measured for paths from a source region in the South Sandwich Islands (SSI) in the Atlantic Ocean to global seismic network (GSN) station COL (College, Alaska). Using certain assumptions about the structure of the inner core, this change in time was converted into a 1 • /yr superrotation of the inner core.…”

Section: Introductionmentioning

confidence: 99%

“…The idea of the inner core behaving like a single crystal was quickly questioned when temporal variations were found for some paths (e.g. Song and Richards, 1996 for the path from the SSI source region to station COL) but not for others (e.g. Souriau, 1998a for the path from the Novaja Zemlya nuclear test site to GEOSCOPE station DRV, Dumont D'Urville in Antarctica).…”

Section: Introductionmentioning

confidence: 99%

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

Laske¹,

Masters²

2003

Earth's Core: Dynamics, Structure, Rotation

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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.

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“…Systematic changes in these travel times over the past 30 years have been attributed to rotation of the laterally varying velocity structure inside the inner core [Song and Richards, 1996;Suet al, 1996]. Estimates of the rotation rate depend on both the changes in the travel-time measurements and the amplitude of the lateral variations in seismic velocity.…”

Section: -8276/99/1999gl90027150500mentioning

confidence: 99%

A comparison of geodetic and seismic estimates of inner‐core rotation

Buffett¹,

Creager²

1999

Geophysical Research Letters

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Abstract.Seismological observations suggest the inner core has been rotating faster than the mantle during the past 30-50 years. Differential rotation can produce large gravitational torques on the mantle which, if not balanced by other torques, would be detectable as changes in the rotation rate of the mantle (Length of Day or LOD). We compare seismological estimates of inner core rotation with estimates derived by inverting LOD observations using models of gravitational coupling. Model predictions depend on several parameters, but for the expected range of parameter values, the seismological estimates of relative rotation are at least an order of magnitude larger than those predicted by the LOD data. Furthermore, the LOD data predict oscillation in the angular alignment of the inner core and mantle during the past 30-50 years, while the seismological data seem to require a component of steady rotation. We show that steady rotation of the inner core implies a steady torque on the mantle. This torque is not accelerating the rotation rate of the mantle, so it must be opposed by another torque. The required torque is consistent with a frictional electromagnetic stress on the base of the mantle due to a westward flow at the top of the core.

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Seismological evidence for differential rotation of the Earth's inner core

Cited by 404 publications

References 29 publications

Support for differential inner core superrotation from earthquakes in Alaska recorded at South Pole station

Support for differential inner core superrotation from earthquakes in Alaska recorded at South Pole station

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

A comparison of geodetic and seismic estimates of inner‐core rotation

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