Doctor of Philosophy
Atkinson, G. M.
This thesis examines the ground motion modeling parameter kappa (κ) in the stable continental region of eastern Canada. Kappa characterizes the decay of spectral amplitudes at high frequencies due to near-surface material de-amplification and is important in seismic hazard assessments. Kappa has significant economic and seismic safety implications for critical infrastructure such as nuclear power plants and dams.
To examine kappa in eastern Canada, a database of ground motions to analyze near-source ground motion characteristics is developed. The database consists of ground motion records from 3357 earthquakes of moment magnitude (M) ≥1.5 recorded within 150 km of 25 seismic stations in eastern Canada. κ, is determined using two different methods: a classical Fourier acceleration spectral technique pioneered by Anderson and Hough , applicable to M≥3.5 earthquakes; and Anderson and Humphrey’s  broadband method, applicable to M≥1.5 earthquakes. Sensitivity and error of κ are examined for each method and used to better understand biases introduced by assumptions. Correlations between κ and physical seismic station characteristics, such as site shear-wave velocity, are examined to better understand what drives kappa.
Kappa as determined using the classical method is on average 7ms on the horizontal component records and 0ms on the vertical for 9 seismic stations along the St. Lawrence River. Using the second method, kappa is determined for all 25 seismic stations and is on average -7ms on the horizontal, and -10ms on the vertical component. Negative kappa values are likely due to an issue inherent in the broadband inversion method where there is a trade-off of κ and corner frequency for small magnitude earthquakes. κ should be viewed as not being significantly different from zero when primary anelastic attenuation effects are modeled through regional whole-path crustal attenuation. Key findings regarding kappa include: (i) on average, kappa is not significantly different from zero on hard rock sites; (ii) kappa has high record-to-record variability both within and between sites; (iii) kappa is smaller on the vertical than the horizontal component; and (iv) kappa on rock sites does not appear to correlate with site-specific characteristic parameters (e.g., VS30, VSrock, and instrument housing).
Summary for Lay Audience
Analysis of earthquake ground motions is important for assessment and mitigation of earthquake damage to infrastructure. To examine ground motions in eastern Canada, a ground motion database is created from earthquakes recorded at seismic stations. Using this database, high frequency ground motion characteristics, in particular ground motion amplitude decay at high frequencies is examined using the parameter kappa (κ). Ground motions due to earthquakes are affected by earthquake source, propagation of waves to the surface, and near-surface materials, such as soil or rock, amplification and de-amplification of ground motion. Ground motion is measured as a function of time using a seismometer. Using the time series, the Fourier amplitude spectrum, which displays the ground motion recording as a function of frequency, can be useful to determine earthquake ground motion characteristics. From a near-surface material ground motion perspective, rock sites, in general, are considered beneficial with respect to structures, as they tend to have less amplification of damaging ground motion over most engineering frequencies (5-10Hz) ground motion on rock sites tends to exceed that for soil sites for the same magnitude and distance. Thus, ground motion attributes of rock sites are important for the evaluation of infrastructure that is sensitive to high-frequency motions, such as critical equipment in nuclear power plants.
The thesis is divided into three key sections. The first section describes the development of a ground motion database for 3357 earthquakes in eastern Canada for 25 seismic stations. The second section of this thesis measures κ using a common methodology modified to reflect ground motion modeling techniques. The last section of this thesis examines high frequency characteristics using a ground motion modeling approach combined with a broadband method, which uses low frequency ground motions to aid in defining high frequency ground motions. Potential errors that can be introduced when measuring high frequency characteristics of ground motions are discussed in sections two and three.
In summary, this thesis provides a ground motion database, in conjunction with empirically determined high frequency ground motion characteristic parameter, κ, values.
Palmer, Samantha M., "Examining κ, the high frequency spectral decay parameter, in Eastern Canada" (2022). Electronic Thesis and Dissertation Repository. 8680.
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