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Geophysics seminar: John Vidale (USC) "Slow inner core motion - 20 years of tracking a shifty target."

02/12/2020 - 1:00pm
Munk Conference Room
Event Description: 

Talk Abstract:

The Earth's solid inner core, which is surrounded by the liquid outer core, is not attached to the overlying mantle. It is only inhibited from differentially spinning by gravitational, magnetic, and viscous coupling through the outer core. The rate that Earth's inner core rotates relative to the mantle and crust has been debated for decades. Nonrotational processes, including internal deformation and flow in the outer core, have also been proposed to explain observed seismic changes. The observed changes thus far have been so inconsistent and weak as to hamper convincing interpretation. 
I’ll first extensively review the history, then examine waves backscattered from within the inner core, which can more robustly evaluate rotation, from two nuclear tests 3 years apart in Novaya Zemlya, Russia. We have extended our previous analysis of these explosions using precise station corrections and the full Large Aperture Seismic Array and an additional pair of US explosions in the Aleutians, thus revealing how the time shifts depend on slowness and lag time and halving our already turgid rotation rate estimate. Our derived ~0.07°/year inner core super-rotation rate from 1969 to 1974 is more robust, more tightly constrained, and slower than most previous estimates and may require interesting reinterpretations of localized signals generally interpreted as inner core rotation.
Below is a link to a recent National Geographic article on the topic, which adds a little color to the controversies.
For more information on this event, contact: 
Peter Shearer
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