Electronic Thesis and Dissertation Repository

Thesis Format

Monograph

Degree

Master of Science

Program

Geophysics

Supervisor

Molnar, Sheri

Abstract

In Greater Vancouver, strong earthquake shaking has not been recorded to quantify expected nonlinear soil response. Earthquake horizontal to vertical spectral ratios (eHVSR) are evaluated at seismic stations located at the edges or deeper center of sedimentary basins in Mexico City, the Kanto basin and Anchorage with available weak-to-strong motion recordings that have similar linear site response to Greater Vancouver to constrain the expected nonlinear soil response for future large earthquakes affecting Greater Vancouver. The empirical results confirm that the very low fundamental frequency (f0 < 0.6 Hz) at deep Fraser River delta sites will be less affected by strong shakings and suggest that for Fraser River delta edge sites the amplification at the predominant frequency (f1) could increase up to 1.77 times with a strong event. This thesis’ empirical nonlinear site response research is being used to validate numerical site response analyses of Fraser River delta sites.

Summary for Lay Audience

The Cascadia subduction zone has the potential for a M 9.0 + megathrust earthquake, making the population of greater Vancouver, that is located at the northern end of the subduction zone, prone to strong shaking. The intensity at which an earthquake is felt in a particular location is affected by the materials through which the earthquake waves travel in the last few hundred meters, this phenomenon in spatial variation of earthquake shaking is known as local site effects. Seismic site characteristics, such as the fundamental frequency of the site (f0), the shear wave velocity (Vs) depth profile and the time average Vs of the top 30 m (Vs30), correlates well with observed site effects.

Under weak seismic loadings (low to moderate shaking intensities), soils behave in an elastic way, meaning soil properties do not change during the shaking. Under larger shear strains (strong shaking intensities), nonlinear soil behavior will occur, often manifesting as (de)amplification in shaking intensity with shifts in f0 to lower frequency (longer period) due to decreasing shear strength and increasing damping in the soil. Nonlinear soil response is often quantified by comparing strong main-shock earthquake recordings with weak after-shock recordings. Empirical studies have described nonlinear soil behavior occurring when peak ground accelerations (PGA) exceed 100 cm/s2. In Greater Vancouver, all the recorded earthquake motions are weak (e.g., maximum PGA of 53.8 cm/s2 during the 1976 magnitude 5.3 Pender Island earthquake) and there are no recorded strong ground motions of nonlinear soil response. To constrain the expected nonlinear soil response in Greater Vancouver, we evaluated site response from both weak and strong earthquake recordings at seismic stations in Mexico City, the Kanto basin in Japan, and Anchorage, Alaska, that have similar linear site response to Fraser River delta sites in Greater Vancouver, as well as similarities in f0, Vs depth profile and geological setting. The results of this thesis suggests that soil nonlinear response will be constrained to the near surface in the Fraser River delta and likely manifest as amplification at the predominant frequency (~1 Hz) for basin edge sites and as minimal to absent deamplification and shear strength reduction of the entire deep soil column (f0) for deeper basin sites.

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