Electronic Thesis and Dissertation Repository

Thesis Format

Monograph

Degree

Master of Science

Program

Geophysics

Supervisor

Molnar, Sheri

Abstract

Shear-wave velocity (VS) depth profiling and associated seismic site classifications were performed at 15 sites across Metro Vancouver, British Columbia using passive seismic and surface wave methods. Inversion model parameters are constrained at each site using nearby geodata in combination with developing regression models of shear-wave velocity with depth for three primary stratigraphic units. Statistical methods such as a Bayesian Information Criterion are applied post-inversion to evaluate models between and within varying parameterizations. Data evaluation metrics, including the use of microtremor horizontal-to-vertical spectral ratios (MHVSRs), are applied to identify two common deviations from the simple case of normally dispersive laterally homogeneous soils typically associated with surface-wave methods: lateral variations and velocity inversions. Lateral site variability is overcome by using the spatial variability in MHVSR peak frequency to sub-divide the site into quadrants for which quadrant-specific dispersion curve inversion provides reliable site classification for each quadrant. Velocity inversions are captured by performing inversions using partial fundamental mode dispersion curves. Partial inclusion of apparent-mode dispersion estimates is a reasonable compromise to modelling velocity inversions, providing a site classification between that of removing the apparent mode estimates (minimum velocity inversion modelling) and wrongfully treating apparent mode estimates as the fundamental mode (maximum velocity inversion modelling). These accessible approaches overcoming lateral site variability and apparent-mode dispersion estimates related to velocity inversions are proposed to obtain reliable seismic site classifications.

Summary for Lay Audience

Metro Vancouver lies approximately 250 km northeast of the Cascadia subduction zone, a 1000 km long fault with potential for a magnitude (M) 9.0+ megathrust earthquake. Earthquake preparedness in Metro Vancouver requires accurate assessments of seismic hazard (potential for ground shaking) at the neighbourhood scale. This is because ground shaking amplitude varies with distance and underlying site or ground conditions. Softer sediments, such as sand or peat, tend to amplify seismic waves travelling up to the surface, particularly when underlain by a stiff (bedrock) layer. The large variation in ground conditions throughout Metro Vancouver means earthquake ground shaking can change dramatically between the various municipalities. I investigate numerous non-invasive seismic field survey methods (surveys performed at the ground surface, without drilling boreholes) to obtain shear-wave velocity with depth in the subsurface. The shear-wave velocity of the upper 30 meters at a site is used to classify underlying site conditions and thereby seismic design ground motions according to the national building code. Passive seismic (microtremor, ambient vibration) and surface wave methods are non-invasive seismic methods that are relatively easy to perform at a given site, and less costly in terms of field work.

I investigate the feasibility of passive seismic and surface wave field survey methods at sites with relatively complex ground conditions. The combination of passive seismic and surface wave field surveying is needed at sites with lateral variability in their subsurface ground conditions to obtain more than one site classification to properly characterize the site in terms of seismic hazard. Sites in which alternating shear-wave velocity occurs with depth (velocity reversals or inversions) require additional data processing to obtain a reasonable site classification. This thesis proposes accessible approaches to acquiring and processing of passive seismic and surface wave measurements to ensure reliable seismic site classification. The obtained velocity-depth information at each site will be used to predict ground shaking from future earthquakes and aid in regional hazard mapping, thereby impacting building design, regional urban planning, and emergency preparedness.

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