the Shape of Ground Motion Attenuation Curves

Abstract

The shape of the attenuation curve of the Fourier spectrum of acceleration is established for southeastern Canada from empirical data. The dataset is comprised of 1200 digital vertical-component seismograms, from 100 earthquakes recorded on 30 stations of the Eastern Canada Telemetered Network. The distance range of the observations is 10 to 1700 km; focal depths are from 5 to 30 km. Results indicate that the attenuation curve has three distinct sections. At distances less than 70 km, corresponding to attenuation of the direct wave, amplitudes decay slightly more rapidly than⅟R (R-1.1). Between 70 and 130 km, where the direct wave is joined by SmS, spectral amplitudes are approximately constant. Beyond 130 km, corresponding to the Lg phase, amplitudes decay at a rate that is consistent with R-0.5 and Q = 670 f0.33. The findings support the hypothesis of Burger et al. (1987), that postcritical reflections from the Moho discontinuity play an important role in determining the shape of the attenuation curve. However, the influence of SmS is subtle, allowing the shape to be approximated by simple functional forms. For example, the commonly assumed bilinear form (R-1, R-0.5, with frequency-dependent Q) fits well if the transition from direct wave to Lg spreading is placed at approximately 60 km. A linear form (R-1 over all distances, with constant Q = 2000) also provides a satisfactory fit overall. We find little evidence for the dependence of attenuation on focal depth or tectonic province.