Master of Science
Molnar, Sheri E.
Site characterization is a crucial component in assessing seismic hazard, typically involving in situ shear-wave velocity (VS) depth profiling, and measurement of site amplification including site period. These methods are ideal for soil sites and less reliable in more complex geologic settings including rock sites. A multi-method approach to earthquake site characterization is tested at 25 seismograph stations across Eastern Canada. It is typically assumed these stations are installed on hard rock. We seek to identify which site characterization methods are most suitable at rock sites as well as to confirm the hard rock assumption. Active-source refraction and surface wave array techniques consistently provide velocity measurements at rock sites; passive-source array testing is less consistent but most suitable to constraining the rock Vs. Bayesian inversion of surface wave dispersion curves provides Vs probability distributions and importantly includes uncertainty in the rock Vs. We succeed in estimating rock Vs at 16 stations, among those we measure rock Vs at only 7 stations. The majority of sites are classified as hard rock but span hard-to-soft rock classification due to variance in Vs. A multi-method site characterization approach is also tested at stiff glaciated upland sites across Vancouver, British Columbia. From the 10 sites investigated, we determine an average Vs of 438 m/s for the glaciated sediments beneath Vancouver which vary in thickness from 20 m to over 100 m from north to south.
Summary for Lay Audience
There is earthquake hazard present in both Eastern and Western Canada. Eastern Canada is not near a plate boundary but has had historical large events that are possible in future. Western Canada is nearby an active plate boundary with lots of earthquakes, and southwestern British Columbia has the highest seismic hazard in Canada. Earthquakes cannot be predicted, but we can estimate the resulting ground shaking. The geology of the near surface impacts the amount of shaking that occurs in the event of an earthquake. Stiffer materials (e.g. rock) have a higher stiffness (velocity) than material that is softer (e.g. sand) and typically have lower intensity shaking than soft soil sites.
In Canada, networks of earthquake recording instruments (seismometers) are deployed in areas susceptible to earthquakes and near high consequence infrastructure (e.g. nuclear power plants) to monitor events. The earthquake recordings are used to develop models to predict expected shaking at the surface. Seismometers of the Canadian National Seismograph Network are installed on rock surfaces, assumed as hard rock. No testing is done at the seismometer location to verify this hard rock assumption. If the assumed stiffness of the rock is incorrect, then the model we develop to predict earthquake shaking will be incorrect. We perform non-invasive seismic testing at 25 seismic stations across Eastern Canada to verify this hard rock assumption. We also test a variety of non-invasive seismic methods to determine which method is most useful in confirming the rock’s stiffness. These non-invasive seismic methods are best suited to soft ground, so we also test their applicability to measure stiffness at 10 stiff glaciated sites in Vancouver.
Ladak, Sameer, "Earthquake site characterization of rock sites in Eastern Canada and stiff ground sites in Vancouver, British Columbia" (2020). Electronic Thesis and Dissertation Repository. 6972.