Global seismic body-wave observations of temporal variations in the Earth's inner core, and implications for its differential rotation

Anna M. Maekinen*, Arwen Deuss

*Corresponding author for this work

Research output: Contribution to journalArticleAcademicpeer-review

Abstract

Differential rotation of the Earth's inner core has been predicted in some geodynamo models, and seismic studies over the past 15 yr have resolved rotation rates up to 1 degrees yr(-1). Most previous seismic body-wave studies have focussed on South Sandwich Islands events recorded at station COL in Alaska. Here, we present a globally extended study into temporal variations in the inner core over some 25 yr, using PKPbc-PKPdf traveltime residuals. To test for differential rotation of the inner core, displacement of inner-core heterogeneities over time is sought. We introduce a newmethod of space-flattening to remove the effect of spatial variations on the time variations; this allows for the use of both polar, semi-equatorial and equatorial geometries. First, we reanalyse polar paths from South Sandwich Islands events to stations COL and INK in North America. These stations yield a differential rotation of the inner core at a rate of 0.12-0.38 degrees yr(-1) in an eastward direction, in agreement with previous studies. However, station DAWY, which has a very similar path through the inner core as COL, yields at best a westward differential rotation of the inner core. Thus DAWY results are incompatible with the COL/INK inferred rotation. Secondly, earthquakes in the Aleutian Islands region, observed at BOSA and LBTB in southern Africa, exhibit temporal variations that are incompatible with the South Sandwich Islands-COL/INK inferred rotation rate. Thirdly, Kuril Islands events, recorded in South America at station BDF, yield inconclusive results. Finally, our final piece of evidence for the irreconcilability of differential inner-core rotation with global data comes from using earthquakes in the Vanuatu region, recorded at BCAO/BGCA in Central Africa, an equatorial geometry. These residuals resolve a westward inner-core rotation at a rate of 0.14 degrees yr(-1), over the same time period that South Sandwich Islands events indicate an eastward rotation. As any rigid-body rotation should yield the same direction and rate independent of where the inner core is sampled, our results allow us to reject previously reported inner-core differential rotation rates of up to 0.1-0.5 degrees yr(-1). Instead, our results suggest that structure in either the inner or the outer core is varying with time, over relatively short timescales and in ways that cannot be explained by, and do not support, a differentially rotating inner core.

Original languageEnglish
Pages (from-to)355-370
Number of pages16
JournalGeophysical Journal International
Volume187
Issue number1
DOIs
Publication statusPublished - Oct 2011

Keywords

  • Magnetic field
  • Core
  • outer core and inner core
  • Body waves
  • Seismic tomography
  • Wave propagation
  • PKIKP TRAVEL-TIMES
  • SUPER-ROTATION
  • FORM DOUBLETS
  • PKP(BC)-PKP(DF) TIMES
  • TELESEISMIC DOUBLETS
  • MANTLE HETEROGENEITY
  • FREE OSCILLATIONS
  • ANISOTROPY
  • SCALE
  • BENEATH

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