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.

Geophysical observations of earth structure, kinematics, and dynamics have served as a core driver in the development of our current understanding of how the Earth evolves. They have provided essential insights that inform our ability to mitigate its hazards and effectively utilize its critical resources. Since the 1960s, when geophysical measurements played a central role in establishing the plate tectonics paradigm, geophysical techniques have become an increasingly sophisticated mainstay in our scientific tool kit for addressing a wide range of scientific and societal needs. From detailing the complex structural and compositional heterogeneities of the Earth's lithosphere and mantle to monitoring the tectonic processes that shape the Earth's surface and deep interior, from finding and monitoring the extraction of critical natural resources that are increasingly rare to real time warning systems that can provide life-saving alerts of tsunami and seismic shaking, geophysics continues to play an increasingly important role in our lives. The myriad ways in which geophysics has revolutionized our understanding of our planet, from core to ionosphere, are too vast to properly represent in any single review. Presented here are selected highlights from the myriad geophysical investigations of the solid Earth over the past 50 years. In an attempt to set some defensible boundaries, and with some consideration for the patience of the reader, I made some relatively arbitrary choices on field boundaries. Gravity, for example, has seen a dramatic resurgence due in part to advances such as satellite gravimetry (e.g., GRACE [Gravity Recovery and Climate Experiment]), but I chose to defer that to the realm of geodesy, along with GPS and InSAR (interferometric synthetic aperture radar). Slow earthquakes and episodic slip and tremor are also clearly important new phenomena for geophysical study, but to my mind they are more appropriately considered as tectonic developments. However, I trust the selected examples provided are representative of the impressive past impact and the exceptional future promise of the field of geophysics as a whole.

Abstract The tectonic evolution of the Uralide orogen began during the Late Palaeozoic as the continental margin of Baltica entered an east-dipping (today's coordinates) subduction zone beneath the Magnitogorsk and Tagil island arcs. The subsequent arc–continent collision resulted in the development and emplacement of an accretionary complex over the continental margin, the development and deformation of a foreland basin, and the extrusion of high-pressure rocks along the arc–continent suture. There is mounting evidence that, at about the same time as arc–continent collision was occurring along this margin of Baltica, eastward-directed subcontinental subduction of the Uralian oceanic crust was also taking place beneath the Kazakhstan plate. This subcontinental subduction is thought to have resulted in the formation of a continental volcanic arc. The final closure of the Uralian ocean basin and the start of collision between the Baltica and Kazakhstan plates occurred during the Late Carboniferous. This continent-continent collision resulted in development of the Late Carboniferous to Early Triassic western foreland fold and thrust belt and foreland basin of the Uralides. The foreland fold and thrust belt displays a large amount of basement involvement, extensive reactivation of pre-existing faults, and a small amount of shortening. At the same time, widespread strike-slip faulting accompanied by melt generation and granitoid emplacement took place in the interior part of the Uralides, leading to the transfer of material laterally along the strike of the orogen. The final crustal structure of the Uralides that resulted from the combination of all of these tectonic events is bivergent, with a crustal root reaching c . 53 km depth.