Inner core tilt and polar motion

The position at Earth's surface where the rotation axis intersects the crust changes with time, and this displacement is commonly referred to as polar motion. The observed polar motion includes a secular drift, but also a decadal fluctuation that has sometimes been referred to as the Markowitz wobble (Figure 1, below). I have particpated in a recent study in which we have shown that these decadal fluctuations are caused primarily by large scale mass displacement at Earth's surface. But a contribution from the core is also likely.

Some of past work on this topic includes how a tilt of the flattenned (elliptical) shape of the inner core can cause polar motion. In a series of studies, I have developed models to calculate the amplitude of the polar motion that results when an equatorial torque tilts the inner core out of its alignment with the mantle.

I also investigated the effect of elastic deformation inside the inner core on the period of the inner core wobble, a normal mode of nutation of Earth similar to the Chandler wobble, but involving the inner core (Figure 2).

Figure 1: Trajectory of the axis of Earth's rotation with respect to a fixed point at the surface, viewed from above the North pole. m1 is in the direction of Greenwich. Units are milliarcseconds (mas); 30 mas corresponds approximately to 1 meter at the Earth's surface. Figure taken from Dumberry, GJI, 2008.

Figure 2: Geometry of the inner core wobble: the figure axis of the inner core is offset from the figure axis of the rest of Earth. As a function of time, the tilted inner core precesses in a prograde direction with a period of approximately 7.5 years. Figure taken from Dumberry, GJI, 2009.

Some of my papers on this topic