Two methods of determining the apparent source spectrum of ground motion are compared for small to moderate earthquakes in the Charlevoix Seismic Zone. The first of these methods uses empirical Green's functions (EGF), in which the spectrum of a target mainshock event is divided by the spectrum of a smaller event in the same location. The EGF approach removes the path, site, and instrument effects from the recorded motion by dividing them out, leaving only the source spectrum of the mainshock. The second method estimates the Fourier spectrum of acceleration near the source directly, by dividing the recorded mainshock spectra by the transfer functions representing the effects of instrument response, attenuation (path effects), and site effects.
The results of this study indicate agreement between the direct method and the EGF method. The EGF method is more limited in frequency range by the signal-to-noise ratios than is the direct method. In this study the frequency range was 0.5–10 Hz for the direct method, compared with 1.2–10 Hz for the EGF method.
The apparent source spectra are generally well represented by a simple Brune point source model of the radiation. Mainshock events had moment magnitudes ranging from 2.4 to 3.7, with Brune model stress drops in the range from 1 to 80 bars.