Three-dimensional structure of the western Los Angeles and Ventura basins, and implications for regional earthquake hazards.

摘要

This dissertation investigates the geometry, kinematics, slip history, and earthquake potential of active faults in the western Los Angeles Basin, and defines the three-dimensional velocity structure of the Ventura basin to improve assessments of strong ground motions that will result from future earthquakes. Chapter 1 considers the Palos Verdes fault, a structurally complex oblique-reverse fault composed of three segments, each with a distinct geometry and displacement history. Analyses of offset stratigraphic markers constrain a post-Miocene strike-slip rate of 3.0 +/- 0.3 mm/yr and a total oblique slip rate of 4.0 +/- 0.3 mm/yr, and fault area to magnitude relations suggest that the fault is capable of generating earthquakes ranging from M w 6.6-6.9 for single segment ruptures up to Mw 7.3 for multi-segment earthquakes. Chapter 2 investigates the Compton fault, which is the largest thrust fault underlying the Los Angeles basin. Using the observed geometry of the Compton - Los Alamitos fold trend and other structures, we develop two kinematically viable structural geometries for the Compton fault, and consider alternatives for the interaction of the Compton fault with other structures. Mapping of growth strata on the backlimb of the Compton fault indicate that the fault is composed of two segments with distinct slip histories and rates, and suggest an increase in slip rate at about 0.8 Ma. Ruptures of the Compton fault and adjacent fault segments show potential earthquake magnitudes of up to Mw 7.1-7.4. Chapter 3 presents an analysis of the structural geometry of the Ventura basin and the seismic velocity (Vp) characteristics of the sedimentary basin fill. The basin geometry is constrained by mapping the top basement surface and considering stratigraphic offsets across major basin-bounding faults. The velocity structure of the basin is described by a simple power law function of depth. The velocity model is then used to characterize the ground motions associated with four historical earthquakes, and the results demonstrate the effect of basin structure on the amplification of ground motions. This basin model will be used for future numerical wave propagation simulations to assess the impact of basin resonance and rupture directivity on coseismic ground motions.