This dissertation investigates the processes of active mountain building and their implications for regional earthquake hazards in the Longmenshan fold-and-thrust belt, China, and the Western Transverse Ranges of California. Chapters two and three examine the styles of active faulting in the Longmenshan and the mechanism of uplift of the adjacent Tibetan Plateau. We construct cross-sections extending through the Sichuan basin and into the Longmenshan and show that shortening and structural relief correlate with topography. We conclude that the mountain belt is tectonically active and that crustal shortening is sufficient to produce and maintain the Longmenshan topography without calling on alternatively proposed uplift mechanisms such as lower crustal inflation. During the course of this research, the 2008 M w7.9 Wenchuan earthquake occurred, further documenting the active nature of this fold-and-thrust belt. We construct a 3D fault model illustrating the earthquakes complex source geometry, which involved slip on two imbricate faults and breached a significant lateral segment boundary. To assess the activity and shortening rates elsewhere in the region, we modeled the thrust belt as a critical taper wedge and examine the implications of these findings for seismic hazard assessment. Appendix A1 provides a basic velocity model for the Sichuan basin and Longmen Shan, in the form of several 1D velocity-depth curves.;Chapter four examines the potential for similar multi-segment thrust fault earthquakes in the Western Transverse Ranges, California. Our study focuses on the Ventura Avenue anticline, a rapidly uplifting (> 5mm/yr) structure where there is disagreement about the nature of the underlying Ventura fault and its seismic hazards. We provide new data and construct a 3D model of the system showing that the Ventura fault extends to seismogenic depth beneath the anticline, and interpret the anticline as a fault-propagation fold. High-resolution seismic data suggest that the fault deforms late Pleistocene and younger strata, implying that the fault system is active. We show that the Ventura faults links a series of onshore and offshore faults at depth and along strike, that if ruptured together could generate Mw7.3-7.6 earthquakes. Such multi-segment earthquakes pose a considerable seismic hazard to surrounding communities in southern California.
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