Quaternary structural evolution and seismic hazards of the onshore Ventura basin, southern California, USA

Abstract

Fault interactions cause variations in patterns of deformation and stress distribution, which control the location and magnitude of earthquakes and can also have a significant impact on the evolution of the landscape. Consequently, a better understanding of how patterns of deformation vary in time and space and the timescales over which fault interactions occur is of critical importance to a sizeable amount of the global population who live within earthquake-prone regions. Devastating earthquakes can occur on well-studied faults, or faults that are either blind or unknown prior to an earthquake. Furthermore, there are several recent examples where large-magnitude earthquakes are attributed to synchronous ruptures of multiple faults with complex subsurface geometry and kinematics. For example, the 2016 Mw 7.8 Kaikoura earthquake is thought to have ruptured up to twelve major faults, at least two of which were unknown before the event occurred. The observation that such complex earthquake ruptures can occur brings into focus the need to use geologic and geomorphic field data to accurately characterize parameters such as subsurface fault geometry and fault slip rates. Additional information on tectonic activity and patterns of deformation can also be acquired from a quantitative analysis of landscape morphology. However, even in well-studied areas such as southern California, fundamental data such as fault slip rates or subsurface fault geometry are often poorly understood, which undermines a complete analysis of seismic hazards. This thesis integrates a multi-disciplinary approach incorporating geomorphic mapping, multiple cosmogenic isotope techniques to establish dates and rates of Earth surface processes, landscape topographic analysis, structural geology, and static Coulomb stress modelling. I investigate the degree to which patterns of deformation over multiple earthquakes cycles are variable in time and space and examine how potential variability in deformation controls the morphology of the landscape and impacts our interpretation of seismic hazards. I focus on the Ventura basin, southern California, USA, which is an ideal location to address such questions because despite an abundance of active reverse and thrust faults in proximity to major population centres, the subsurface geometry and slip rates for several key faults are not well quantified and there are few data on how the numerous active faults have shaped the evolution of the landscape. III First, I investigate evidence for a proposed blind fault in the Ventura basin, the Southern San Cayetano fault (SSCF), and the potential role of the SSCF in stress transfer between the Ventura fault and the San Cayetano fault in potential large-magnitude (Mw 7.5–8.0) multi-fault earthquakes. I examine late Quaternary alluvial fans and river terraces using field mapping, high-resolution lidar topographic data, 10Be surface exposure dating, and subsurface well data to provide evidence for a young, active SSCF. I calculate a Holocene reverse slip rate of 1.3 +0.5/-0.3 mm yr-1 and suggest that displacement rates for the SSCF have not varied significantly since the onset of activity on the SSCF around ~58 ka. I hypothesize that the SSCF could potentially act as a rupture pathway between the Ventura and San Cayetano faults in large-magnitude, multi-fault earthquakes in southern California. In the second part of the thesis, I examine the subsurface geometry of the SSCF and the potential structural connectivity and stress interactions between the SSCF and neighbouring faults. I present a series of structural cross sections along strike of the SSCF and a 3D fault model for the SSCF. These results provide evidence for a low-angle SSCF that dips ~15° north and connects with the western San Cayetano fault (WSCF) around 1.5–3.5 km depth. I incorporate the 3D fault model for the SSCF in static Coulomb stress modelling and find that triggered seismicity may occur on the SSCF and the WSCF because of ruptures on the eastern section of the San Cayetano fault. However, my results indicate that the role of static Coulomb stress transfer in the potential occurrence of multi-fault earthquakes in the Ventura basin is critically dependent on fault model adopted for the deep structure of faults. The results demonstrate that an accurate characterization of three-dimensional subsurface fault geometry is important for reducing uncertainties when assessing future patterns of regional seismicity and highlight the importance of integrating field observations, surface data, and subsurface data to create realistic fault inputs when modelling static Coulomb stress transfer. In the last section of the thesis, I investigate how fault evolution has controlled patterns of topographic relief development, channel morphology, and erosion in the Ventura basin. I employ cosmogenic isotope isochron burial dating of an important, yet poorly dated, Quaternary strain marker, the Saugus Formation, to reduce uncertainties in the assessment of rates of tectonic processes. My results confirm that the Saugus Formation increases in age from west to east along the axis of the Ventura basin with ages for the top of the exposed Saugus Formation of 0.38 +.017/-0.23 Ma at Ventura and 2.49 +0.25/-0.29 Ma in the eastern Ventura basin. The burial ages for the base of shallow marine sands, which underlie the Saugus Formation throughout the basin are 0.55 +0.80/-0.10 Ma at Ventura and 3.30 +0.30/-0.42 Ma in the eastern Ventura basin. Burial ages for the Saugus Formation in conjunction with published fault offsets suggest long-term slip rates of 7.1 +/- 1.0 mm yr-1 for the San Cayetano fault since ~1.54 Ma. In addition, I calculate 10Be-derived catchment-averaged erosion rates and compare erosion rates with fault displacement rates and the results of a morphometric landscape analysis. The comparison indicates a transient landscape response to tectonic forcing in the Ventura basin, where the erosion signal in fault hanging walls is not yet fully adjusted to various tectonic perturbations over the last ~1.5 Ma. These data demonstrate that on the local scale with uniform climate, such as the hanging wall of the San Cayetano, Ventura, and Southern San Cayetano faults, tectonic perturbations are the main drivers in patterns of topographic relief developments and highest stream gradients for periods up to 106 years. Overall, my results demonstrate that patterns of deformation can demonstrate significant spatial variability on timescales between 103 to 106 years. I find that fault interactions and the migration of deformation exert significant control on rates of fault activity and landscape morphology, and that patterns of deformation must be accurately modelled for a robust analysis of seismic hazards.