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 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 <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 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 ~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 −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.

The M5.8 23 August 2011 Mineral, Virginia, earthquake was felt over nearly the entire eastern United States and was recorded by a wide array of seismic broadband instruments. The earthquake occurred ~200 km southeast of the boundary between two distinct geologic belts, the Piedmont and Blue Ridge terranes to the southeast and the Valley and Ridge Province to the northwest. At a dominant period of 3 s, coherent postcritical P-wave (i.e., direct longitudinal waves trapped in the crustal waveguide) arrivals persist to a much greater distance for propagation paths toward the northwest quadrant than toward other directions; this is probably related to the relatively high crustal thickness beneath and west of the Appalachian Mountains. The seismic surface-wave arrivals comprise two distinct classes: those with weakly dispersed Rayleigh waves and those with strongly dispersed Rayleigh waves. We attribute the character of Rayleigh wave arrivals in the first class to wave propagation through a predominantly crystalline crust (Blue Ridge Mountains and Piedmont terranes) with a relatively thin veneer of sedimentary rock, whereas the temporal extent of the Rayleigh wave arrivals in the second class are well explained as the effect of the thick sedimentary cover of the Valley and Ridge Province and adjacent Appalachian Plateau province to its northwest. Broadband surface-wave ground velocity is amplified along both north-northwest and northeast azimuths from the Mineral, Virginia, source. The former may arise from lateral focusing effects arising from locally thick sedimentary cover in the Appalachian Basin, and the latter may result from directivity effects due to a northeast rupture propagation along the finite fault plane.

The Aftershock Imaging with Dense Arrays (AIDA) project recorded 12 days of high-density seismic array data following the 23 August 2011 Mineral, Virginia (USA), earthquake. AIDA utilized short-period, vertical-component seismographs at 201 locations to record closely spaced data that would reduce spatial aliasing. Interstation correlation enabled a detection threshold between magnitude −1.5 and −2. A joint hypocenter and velocity inversion algorithm was applied to compressional and shear wave arrival times for 300 of the larger events. Traveltime misfits were minimized using a constant velocity of Vp = 6.2–6.25 and Vs = 3.61–3.63. Hypocenter location error estimates for this range of velocities are ~100 m. Little to no three-dimensional variation exists in the seismic velocity of the upper crust, consistent with the aftershock zone being within a single crystalline rock terrane. The hypocenter locations define a 1–2-km-wide cloud with a strike of ~029° and dip of ~53°E, consistent with the focal mechanism of the main shock. The cloud bends ~5° along strike and has a slightly shallower dip angle below ~6 km depth, indicating a broad, complex fault zone with a slightly concave shape. This study shows that seismic arrays comparable to those used in controlled-source seismology can be successfully applied to aftershock sequences, and that dense array data can produce high-resolution information about earthquake rupture zones.

This volume presents a comprehensive, worldwide history of seismological studies of the Earth's crust using controlled sources from 1850 to 2005. Essentially all major seismic projects on land and the most important oceanic projects are presented. The time period of 1850 to 1939 is presented as a general synthesis, and from 1940 onward the history and results are subdivided into a separate chapter for each decade, with the material ordered by geographical region. Each chapter highlights the major advances achieved during that decade in terms of data acquisition, processing technology, and interpretation methods. For all major seismic projects, we provide specific details regarding the field observations, interpreted crustal cross section, and key references. We conclude with global and continental scale maps of all field measurements and interpreted Moho contours. An accompanying DVD contains important out-of-print publications and an extensive collection of controlled-source data, location maps, and crustal cross sections.