| Earthquakes that generate similar (correlated) waveforms at common stations can be used to detect inner core motion and to relocate earthquakes in high precision. In the first chapter, I have discovered several high-quality waveform doublets in the South Sandwich Islands (SSI) region. Their waveform records show an apparent and consistent temporal change of inner core travel times at up to 57 stations in and near Alaska, as well as a dissimilarity of PKP(DF) coda. Using waveform doublets avoids the artifact of earthquake mislocations and the contamination of small-scale heterogeneities. The results in this chapter confirm the original claim of seismological evidence for inner core super-rotation.; In the second chapter, I extend the search for high-quality repeating earthquakes to the Aleutian Islands (AI) region, the Kuril Islands (KI) region, the Tonga-Fiji-Solomon Islands (TFSI) region, and an earthquake nest in Bucaramanga, Colombia. As the results, I have found more than 100 high-quality earthquake doublets. Observation from one SSI doublet recorded at INK shows an inner core travel-time change of ∼ 0.1 sec over ∼ 6 years, confirming the inner core differential motion occurring beneath Central America. The observations from one Al doublet recorded at BOSA, and from one KI doublet recorded at BDFB, show an inner core travel-time change of ∼ 0.1 sec over ∼ 7 years and ∼ 6 years respectively, providing additional evidence for the temporal change of inner core properties beneath Central Asia and Canada respectively. On the other hand, observations from one TFSI doublet recorded at PTGA, and from one Bucaramanga doublet recorded at WRAB and CHTO, show no temporal change of inner core travel times for the three corresponding ray paths, of which the path in the inner core is nearly parallel to the equatorial plane. Such pattern of observations showing both presence and absence of inner core travel-time change may be explained by the geometry and relative directions of ray path, lateral velocity gradient, and particle motion due to inner core super-rotation.; For the Bucaramanga nest earthquakes, in the third chapter I have also achieved teleseismic high-precision relocation of the nest by applying a double-difference (DD) algorithm to phase picks from EHB bulletin of Engdahl et al. [1998] and waveform cross-correlation (WCC) measurements. The DD relocation of all Bucaramanga nest events reveals that about 700 events (∼ 70% of all events in the past 40 years) focus on a small fault-like volume that has dimensions of 20 km x 10 km x 10 km. A map view of the relocated seismicity shows an elongated structure striking at ∼ 135°. An on-fault view indicates an approximately 10 km thick band of seismicity, tilted at ∼ 40° to the SE. Further improvement of the pick-based DD relocation using WCC measurements on a set of 33 similar events images a complex near-vertical weak zone with a width of ∼ 5 km. Focal mechanisms of the nest earthquakes are highly variable. It may be that an intrinsic cause of the repeated (nest-type) activity, is indicated by the variability of focal mechanisms. However, the P axes generally align with the orientation of the relative plate movement between Nazca and Caribbean. I have proposed a slab-slab interaction model, in which the nest may represent a contact zone where two subducted slabs, belonging to the Nazca plate and the Caribbean plate respectively, collide and slide past each other. |
