Seismic investigations of core-mantle boundary structure and source properties of deep-focus earthquakes

This dissertation investigates geophysical problems concerning the structure of the Earth's interior and the physics of the earthquake source by stacking seismograms recorded at global and regional networks. The core-mantle boundary (CMB) region contains thermal and compositional heterogeneities at different length-scales, including thin layers at the base of the mantle with large P-wave velocity reductions (ULVZs). Core-reflected seismic phases PcP and ScP are sensitive to velocity and density changes within ULVZs, discontinuity sharpness, and CMB variability on short length-scales. Amplitudes of globally-recorded PcP and ScP reveal a sharp average CMB with no more than 10% velocity reductions in the mantle for P- and S-waves. The amplitudes do not support proposed 30% S-wave velocity reductions or core-mantle transition zones. No precursor arrivals to PcP and ScP are visible on regional network stacks at times and amplitudes predicted for ULVZ properties. This suggests ULUZs transition gradually from the lower mantle. Diminished core-grazing P-waves sampling the CNM in a localized region of the mid-Pacific provide evidence for short length-scale variation, possibly reflecting dynamical processes.; Time histories of moment release provide insight into rupture processes of deep earthquakes, whose physical mechanism remains unknown. We compare source time functions computed from stacks of teleseismic P-waves of 111 deep earthquakes with M_W ≥ 6.4 and depth ≥ 100 km. An abrupt change in character occurs at 550 km. Earthquakes deeper than 550 km have shorter durations and simpler time functions. Shallow events have longer durations, and the 350–550 km range averages more subevents. Radiated energy-to-moment ratios are lower than for large shallow earthquakes and maximum seismic efficiency decreases slightly with depth. Initiations and terminations are consistent with self-similar rupture, although large events tend to begin more rapidly. Aftershock productivity declines significantly 350–550 km, then rises below 550 km. Individual subduction zones generally demonstrate similar depth dependence, but differences suggest thermal or structural properties of slabs influence rupture. If transformational faulting of metastable olivine operates 350–550 km, these observations suggest a change in mechanism occurs around 550 km, possibly involving another metastable mineral.