| I report the results of three different research projects united in the use of short-period, vertical component recordings from regional seismograph networks to image structures at the top and bottom of the Hawaiian hotspot plume. In Chapters 2 and 3, I study seismic structure at the base of the mantle using recordings of circum-Pacific earthquakes made in California and Hawaii. Velocity layering in the deep mantle can produce additional, lower amplitude arrivals preceding and following primary core-reflected phases. In Chapter 2 I exploit precursors to PcP to map a 0.5–0.6% increase in P-wave velocity (vP) that occurs 190 km above the core-mantle boundary (CMB) beneath the central and eastern equatorial Pacific. The reflector occurs in regions of both mantle downwelling and upwelling. In Chapter 3, I use ScP and three small additional arrivals to constrain velocity and density properties of ultralow-velocity zones (ULVZs) beneath the southern Pacific near Tonga-Fiji. Models that fit data stacks are consistent with ULVZs dominated by partially molten mantle and with thickness <20 km. The property best constrained by data is the density (ρ) increase relative to ambient mantle which I find must be less than 10%.; In chapter 4, I use scattered teleseismic P to S waves to map relative seismic scattering strength in the Hawaiian volcanic edifice. Strong evidence exists for modulation of seismogenesis by crustal heterogeneity related to scattering. For example, post-mainshock seismicity rates increase most where scattering is highest. The two most likely sources of relative scattering strength variations are: (1) compositional heterogeneity, which would mostly be caused by the presence of olivine rich magma cumulates, and (2) differences in the size, density, and/or orientations of crustal cracks and faults. Whether scattering heterogeneity predates deformation or develops as the island's structure evolves is a question of provenance that is difficult to answer (see also Revenaugh [2000]). Covariance between scattering potential and spatially diffuse seismicity seems to indicate that processes and/or structures dominant at the depth of most abundant seismicity have the greatest effect on scattering. |
