University of Western Ontario - Electronic Thesis and Dissertation Repository

Degree

Doctor of Philosophy

Program

Geophysics

Supervisor

Dr. Gail M. Atkinson

Abstract

In this study, ground motions from earthquakes in North America with moment magnitude (M) 3.0 to 6.0 were investigated to reveal regional differences in ground motion amplitudes.

First, we examined several attenuation forms to evaluate their ability to describe the decay of response spectral amplitudes in different regions across North America. Linear, bi-linear and tri-linear regression forms with different combinations of geometric spreading coefficients were tested to assess their ability to describe spectral amplitude decay from 0.33 to 10 Hz in the distance range from 10 to 400 km. We found that the linear model has steeper slope in the west (~1.3) than in the east (~1), and may not extend well over large distances. The bi-linear form offers a good compromise between simplicity and the ability to model amplitude decay appropriately at both near and regional distances. Although tri-linear models are a better fit to the data in some regions (Eastern North America), they may have no practical advantage over simpler models.

In the second step, a simple and robust approach to estimate moment magnitude (M) for events of M<6 was presented from regional seismographic observations (vertical component), recorded in the distance range from 100 to 400 km. M can be estimated from 1-Hz response spectral amplitudes for events in North America with an uncertainty (standard deviation of residuals) of <0.2 units in most regions, using the simple relationships provided herein.

Finally, regional differences in amplitudes across North America were investigated. Ratios of log-averaged response spectral amplitudes in each region to those in Southern California were examined to identify the effects of magnitude, frequency and distance on regional differences. For the horizontal component, differences due to site effects were considered by correcting all amplitudes to equivalent values for B/C boundary site conditions. It was assumed that site effects on the vertical component are negligible. We find that most of the observed amplitude differences appear to be attributable to complex frequency-dependent differences in regional attenuation. For example, the change in the slope of apparent geometric spreading due to the transition from direct body waves to surface waves occurs at closer distances in Eastern Canada/Northeastern United States than other regions, and may be frequency-dependent. Regional differences in ground-motion amplitudes across North America show a clear frequency and distance dependence. This dependency is especially significant between ground motions in Eastern North America versus Southern California.

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