G. Laske and G. Masters
IGPP, Scripps Inst. of Oceanography, U.C. San Diego SIO-0225, La Jolla, CA 92093; ph. 619-534-8774; e-mail: glaske@ucsd.edu
Differential rotation of the inner core has been inferred by several body-wave studies with most agreeing that a superrotation may exist with a rate between 0.2 and 3 degress per year. The wide range of inferred rotation rate is caused by the sensitivity of such studies to local complexities in structure which have been demonstrated to exist. Free-oscillation "splitting functions" are insensitive to local structure and are therefore better candidates for estimating differential inner core rotation. We use a recently developed method for analyzing free oscillations which is insensitive to earthquake source, location and mechanism to constrain this differential rotation. In a prior study, we found that inner core differential rotation has been essentially yero over the last 20 years. We have revisited this issue, including additional earthquakes and modes in our analysis. The normal modes investigated here are also quite sensitive to mantle structure. It has therefore been suggested that our results may be influenced by the mantle corrections we apply in our analysis. We show that this is not the case but that fluctuations in rotation rates are mode specific. Possible causes for this are discussed in this poster. Our new best estimate for differential inner core rotation is a barely significant superrotation of 0.13 +/- 0.11 degrees per year, which is still consistent with the idea that the inner core is gravitationally locked to the mantle.