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

• Published in: Geophysical Journal International
• Main Authors: Mäkinen, Anna M., Deuss, Arwen
• Format: Text
• Language: English
• Published: Oxford University Press 2011
• Subjects: Seismology; Sandwich Islands; South Sandwich Islands; Alaska; Aleutian Islands
• Online Access: http://gji.oxfordjournals.org/cgi/content/short/187/1/355; https://doi.org/10.1111/j.1365-246X.2011.05146.x

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°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 new method 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°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°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 ...