Site Response and Wave Propagation Effects in the Eastern United States

Fourier amplitude spectra from regional earthquakes in the eastern United States are used in a parametric inversion for source, path, and site effects. Five earthquakes are selected for analysis during the operation of the United States National Seismic Network (US), EarthScope’s USArray Transportable Array (TA), and additional permanent and temporary arrays to maximize station coverage. A global search algorithm is used to solve for site response from 0.1 to 15 Hz, corner frequency, geometrical spreading (r−γ), and frequency-dependent anelastic attenuation in the form Q(f)=Qofα, in which r is the hypocentral distance and f is the frequency. Tradeoff between moment and geometric spreading is handled by fixing the moment. The tradeoff between corner frequency and Q(f) is approached by selecting the value of corner frequency that minimizes an objective function defined over all stations. Values of site response and attenuation parameters show a strong spatial correlation with the physiographic provinces of the eastern United States. Site responses for the Atlantic Coastal Plain are consistent with previous work using spectral ratios relative to an average response at several reference sites, defined by strong resonance peaks whose frequencies correlate with the thickness of sediments. Site responses for the other physiographic provinces are markedly different from the coastal plain, with a lack of distinct resonance peaks and a broad moderate high at frequencies from 0.1 to 0.5 Hz. Similar to site response, Q(f) has a strong correlation with physiographic province, showing lower values on the coastal plain and higher values inland. Geometric spreading exponent, γ, decreases with increasing hypocenter distance from just above 1 at a few tens of kilometers to 0.9 at 500 km. The limited range in geometric spreading values is attributed to starting the Fourier transform window at the S-wave arrival for all distances and averaging over multiple wave types.