Shear-wave velocity structure and attenuation derived from aftershock data of the 2011 Mineral, Virginia, earthquake
Shear-wave velocity structure and attenuation derived from aftershock data of the 2011 Mineral, Virginia, earthquake (in The 2011 Mineral, Virginia, earthquake, and its significance for seismic hazards in eastern North America, J. Wright Horton (editor), Martin C. Chapman (editor) and Russell A. Green (editor))
Special Paper - Geological Society of America (2015) 509: 81-93
A dense seismic array was deployed at a 2 km spacing to record the aftershocks of the M (sub w) (moment magnitude) 5.8 Mineral, Virginia (USA), earthquake in 2011. The three-component seismometers, installed on a 60-km-long profile, recorded 40 aftershocks over 9 days of deployment. Based on manual picking of P-wave (primary, compressional) and S-wave (secondary, shear) arrival times of 15 aftershocks, we find that the P-wave propagates with a velocity of 6.15 km/s through the upper crust, and the direct S-wave travels with a velocity of 3.66 km/s within the first 20 km (Vs (sub <20km) ) and decreases slightly to 3.54 km/s (Vs (sub >20km) ) for distances >20 km. Hence, the aftershock data show a Vp/Vs ratio of 1.68 within the first 20 km of hypocentral distance, and a ratio of 1.73 for distances >20 km. We attribute the small decrease in Vs with increased distance to the complex geologic setting: the recording array was deployed across the geologic boundary between the Quantico Formation and the Ta River Metamorphic Suite. Near-source attenuation of S-waves (amplitude decay with hypocentral distance R) was measured using approximately 1200 digital seismograms (north-south and east-west components) from 40 aftershocks. The decay of amplitude was extracted using a nonlinear least-squares regression for different frequency bands: 1-2, 2-4, 4-8, and 8-16 Hz. For 1-2 Hz the decay can be described as a function of distance (R) as R (super -0.8) , for 2-4 Hz as R (super -0.9) , for 4-8 Hz as R (super -1.05) , and for 8-16 Hz as R (super -1.15) . The decay exponents, or b values, increase approximately 9%-15% from a lower to the next higher analyzed frequency band. These values are valid to a distance of as much as approximately 45 km from the aftershocks.