New insights into the differential rotation of the inner core

Time-dependent travel times of seismic waves traversing the inner core (IC) from repeating earthquakes provided compelling evidence for an IC differential motion. In this dissertation, we conducted a systematic search for strong repeating earthquakes in the last three decades to examine the global and regional patterns of the temporal changes of the IC. We performed extensive analyses on the quality of the repeating earthquakes and quantified the error (σic) of travel time measurements from all possible sources except the IC temporal changes. We set 2σic as a threshold for judging whether an IC temporal change is significant. No significant temporal changes were found in most parts of the IC, but large temporal changes (over 3σic) were observed beneath four regions in northern hemisphere (North Atlantic, Northeast Pacific, Russian Far East/Sea of Okhotsk, and Europe/North Africa), besides the well-known Central America anomaly in previous studies. Most large temporal changes were associated with time lapses of over 6 years and smaller distances, possibly resulting from the rotation shifting the laterally varying top 300 km of the IC. We also observed enigmatic very large abrupt temporal changes (as short as 44 days), which may be related to disturbances caused by the great Sumatra earthquakes. A new path from Chile to Central Asia, sampling North Atlantic, suggests a small-scale and steep lateral velocity gradient of the IC and a slow eastward IC rotation of 0.051°/year from 1995 to 2014. The rotation rate seems to be non-steady, at the rate of ~0.1 °/year before 2006 and nearly zero after 2006. The non-steady pattern is also consistent with our observations from the SSI to Central Asia path, which samples a different patch of the IC. We also investigated whether the variations at the IC boundary (ICB) is an alternative explanation of the time-varying IC waves (as claimed by some previous studies), by further analyzing the reflected (PKiKP) waves from the waveform doublets. We used non-IC reference phase (mainly SKP), which eliminated ambiguity where the temporal changes come from. We found that the temporal changes have always started at refracted PKIKP and the travel time changes correlate better with PKIKP, rather than the PKiKP. Changes in reflected PKiKP can be easily contaminated by the strong and time-varying PKIKP coda and therefore are not reliable indicators for changes of the ICB. We conclude that the temporal changes come mostly (if not all) from the interior of the IC, rather than the changes of ICB, which is also supported by previous seismic observations. The differential rotation of the IC shifting its heterogeneous uppermost structures is still the simplest and most reasonable explanation for the origin of the time-varying IC waves. A rotation rate of about 0.05-0.1° per year with possible decadal fluctuation can reconcile all temporal change observations from body waves, IC scattering, and normal mode data.U of I OnlyAuthor requested U of Illinois access only (OA after 2yrs) in Vireo ETD syste