Shear-wave velocity structure and attenuation derived from aftershock data of the 2011 Mineral, Virginia, earthquake (original) (raw)

2015, Geological Society of America

A dense seismic array was deployed at a 2 km spacing to record the aftershocks of the M w (moment magnitude) 5.8 Mineral, Virginia (USA), earthquake in 2011. The three-component seismometers, installed on a 60-km-long profi le, 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 fi nd 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 fi rst 20 km (Vs <20km ) and decreases slightly to 3.54 km/s (Vs >20km ) for distances >20 km. Hence, the aftershock data show a Vp/Vs ratio of 1.68 within the fi rst 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 ~1200 digital seismograms (northsouth 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 -0.8 , for 2-4 Hz as R -0.9 , for 4-8 Hz as R -1.05 , and for 8-16 Hz as R -1.15 . The decay exponents, or b values, increase ~9%-15% from a lower to the next higher analyzed frequency band. These values are valid to a distance of as much as ~45 km from the aftershocks.