ALBERT

Description: We studied the response spectral amplitudes and peak ground acceleration (PGA) of earthquakes with moment magnitude ( M ) from 3.0 to 6.0 to investigate regional differences in ground-motion amplitudes across North America on a nonparametric basis, for a common National Earthquake Hazards Reduction Program B/C site condition (e.g., shear-wave velocity of 760 m/s in the top 30 m of the soil column). We use southern California as the reference region because it has the most plentiful data against which to compare other regions. Amplitudes of response spectra in eastern Canada/northeastern United States become increasingly larger than those in southern California as distance, frequency, and magnitude increases. In the central United States, ground-motion amplitudes are larger than those in southern California at distances more than 100 km (and lower at distances less than 100 km), over all frequency and magnitude ranges studied. Amplitudes for crustal events in the Pacific Northwest/British Columbia are lower than those in southern California at frequencies ≥1 Hz, over all magnitude and distance ranges (but have similar amplitudes below 1 Hz). Amplitudes in northern California are similar to those in southern California at low frequencies (below 3.3 Hz) but have lower amplitudes at high frequencies (PGA) over all distances and magnitudes. We conclude that regional amplitude differences are largely attributable to complex frequency-dependent differences in regional attenuation. For example, the slope of apparent geometric spreading at distances less than 100 km is significantly steeper (more negative) in eastern Canada/northeastern United States than other regions and may be frequency dependent. The regional attributes of the attenuation comparisons appear to be the same for both vertical and horizontal components, at least for B/C site conditions. Online Material: Figures of region-dependent ground-motion amplitudes, apparent geometric spreading, statistical tests of the ground motions, and figures and table of ratios of ground motions relative to those from southern California.