ALBERT

Description: We used aftershocks of the 2011 Mineral, Virginia, earthquake to study geometrical spreading at hypocentral distances less than 60 km in the central Virginia seismic zone. Sixty-nine aftershocks, occurring from 25 August 2011 through 24 December 2011, provided the data. We used the coda-normalization method to estimate the attenuation coefficient associated with geometrical spreading. We filtered the time-domain signals in several octave-wide frequency bands and examined attenuation of peak S -wave amplitude in the 1.0–30.0 Hz frequency range. Amplitude was assumed to decrease as a function of hypocenter distance R according to R – . The coefficient of attenuation was examined for the three-component S -wave amplitudes, with corrections for SH and SV radiation patterns. We observed no systematic frequency dependence of . The coefficient of attenuation for the radial and transverse components, assuming infinite quality factor Q , derived as a weighted mean over the entire range of frequencies (1–30 Hz), are both 1.51±0.05. The weighted mean value of the attenuation coefficient on the vertical component over the same range of frequencies is 1.45±0.05, slightly less than for the horizontal components. We corrected the data assuming three Q models. The estimated geometrical spreading coefficients are in the 1.30–1.46 range, depending on the assumed Q model and component, which is only slightly less than the estimates of determined assuming infinite Q . The estimated attenuation coefficients differ significantly from the value of 1.0 expected for a whole space. The results for the horizontal components are in agreement with previous full-wavefield modeling. However, the observed vertical-component attenuation is substantially less than that predicted by the synthetics. The depths of the earthquakes are less than 8 km, so these results may not be representative of geometrical spreading in parts of eastern North America where earthquakes occur at greater depths. Online Material: Table of earthquake hypocenters and focal mechanisms.