page_title: Inner core super-rotation
section_title: Seismic observations
hierarchical_section_title: Inner core super-rotation / Seismic observations
caption_attribution_description: English: A random epicenter is chosen with the recording of the waves in approximately Alaska. The PKP(BC) does not travel through the inner core, while the PKP(DF) does.
context_page_description: Inner core super-rotation is a theorized eastward rotation of the inner core of Earth relative to its mantle, for a net rotation rate that is faster than Earth as a whole. A 1995 model of Earth's dynamo predicted super-rotations of up to 3 degrees per year; the following year, this prediction was supported by observed discrepancies in the time that p-waves take to travel through the inner and outer core.
  Seismic observations have made use of a direction dependence of the speed of seismic waves in the inner core, as well as spatial variations in the speed. Other estimates come from free oscillations of Earth. The results are inconsistent and the existence of a super-rotation is still controversial, but it is probably less than 0.1 degrees per year.
  When geodynamo models take into account gravitational coupling between the inner core and mantle, it lowers the predicted super-rotation to as little as 1 degree per million years. For the inner core to rotate despite gravitational coupling, it must be able to change shape, which places constraints on its viscosity.
context_section_description: The main observational constraints on inner core rotation come from seismology. When an earthquake occurs, two kinds of seismic wave travel down through the Earth: those with ground motion in the direction the wave propagates (p-waves) and those with transverse motion (s-waves). S-waves do not travel through the outer core because they involve shear stress, a type of deformation that cannot occur in a liquid. In seismic notation, a p-wave is represented by the letter P when traveling through the crust and mantle and by the letter K when traveling through the outer core. A wave that travels through the mantle, core and mantle again before reaching the surface is represented by PKP. For geometric reasons, two branches of PKP are distinguished: PKP(AB) through the upper part of the outer core, and PKP(BC) through the lower part. A wave passing through the inner core is referred to as PKP(DF). (Alternate names for these phases are PKP1, PKP2 and PKIKP.) Seismic waves can travel multiple paths from an earthquake to a given sensor.
  PKP(BC) and PKP(DF) waves have similar paths in the mantle, so any difference in the overall travel time is mainly due to the difference in wave speeds between the outer and inner core. Song and Richards looked at how this difference changed over time. Waves traveling from south to north (emitted by earthquakes in the South Sandwich Islands and received at Fairbanks, Alaska) had a differential that changed by 0.4 seconds between 1967 and 1995. By contrast, waves traveling near the equatorial plane (e.g., between Tonga and Germany) showed no change.
  One of the criticisms of the early estimates of super-rotation was that uncertainties about the hypocenters of the earthquakes, particularly those in the earlier records, caused errors in the measurement of travel times. This error can be reduced by using data for doublet earthquakes. These are earthquakes that have very similar waveforms, indicating that the earthquakes were very close to each other (within about a kilometer). Using doublet data from the South Sandwich Islands, a study in 2015 arrived at a new estimate of 0.41° per year.
caption: Schematic of PKP(BC) and PKP(DF) waves
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