Examining κ, the high frequency spectral decay parameter, in Eastern Canada

Samantha M. Palmer, The University of Western Ontario

Abstract

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 [1984], applicable to M≥3.5 earthquakes; and Anderson and Humphrey’s [1991] 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., V_S30, V_Srock, and instrument housing).