A regional seismic reflection line (I‐64) across the Virginia Piedmont has provided a stacked section suitable for an integrated interpretation of geophysical data in the region. A highly reflective upper crust, an allochthonous Blue Ridge Province, underlying thrust sheets including the Blue Ridge master decollement, and a basal decollement at a depth of about 9 km (3 s) are confirmed on the seismic data. Immediately east of the Blue Ridge Province, Appalachian structures plunge to as much as 12 km (4 s) depth. The Evington Group, Hardware terrane, and Chopawamsic metavolcanic rocks (Carolina terrane) crop out in the Piedmont Province, and numerous eastward dipping reflections originate from these rocks in the subsurface. These eastward dipping reflectors overlie a gently west dipping (10°–15°), highly reflective zone that varies in depth from 1.5 s (4.5 km) beneath the Goochland terrane to 4 s (12 km) beneath the rocks of the Evington Group. Some of the overlying eastward dipping reflections apparently root in this zone. The zone may include decollement surfaces along which the overlying rocks were transported. Relatively few reflections originate from within autochthonous Grenville basement at the western end of the profile. The Goochland granulite terrane is interpreted to be a westward thrust nappe structure that has overridden a portion of the Chopawamsic metavolcanic rocks. A broad zone of east dipping (20°–45°) reflections bounds the Goochland terrane on the east. These reflections may originate from deformation zones and continue to Moho depths. They appear to be correlative with similar events seen on other Appalachian lines. The pervasiveness of the zone of east dipping events on other seismic reflection lines and the continuity of the adjacent Piedmont gravity high suggest continuity of crustal features along the length of the Appalachians. A major conclusion of this study is that crustal thinning is responsible for the main components of the gravity field in Virginia, that is, the Appalachian gravity gradient and the Piedmont gravity high. The crust thins from about 52 km beneath the Appalachian mountains to about 35 km beneath Richmond, Virginia, and then rethickens by up to 10 km beneath the zone of east dipping reflections (mylonites?) east of Richmond. The I‐64 seismic data also contain a sequence of reflections at about 9–12 s, indicative of lower crustal layering; the base of this zone of reflections coincides almost exactly with the Mohorovicic discontinuity interpreted from earlier refraction work. The layering extends about 70 km west from Richmond, Virginia, and is interpreted as a lower crustal transition zone that is believed to persist across most of Virginia.

Most natural crustal earthquakes fall into one of two categories:(1) those associated with the dynamics of plate tectonics, or (2) those associated with the elements of the hydrologic cycle. This paper presents a comprehensive listing of published examples of hydroseismicity, a hypothesis that attributes most intraplate and near-intraplate earthquakes to the dynamics of the hydrologic cycle, which includes hurricanes and typhoons. Results from 30 worldwide studies of earthquake-rainfall correlations published during the past 22 years are referenced. These investigations were conducted in both intraplate and plate marginal environments on five continents. Collectively, they provide strong support for the hydroseismicity hypothesis as a viable explanation via pore-fluid-pressure diffusion for the occurrence of many earthquakes, regardless of the host tectonic regime. Signatures of pore-fluid-pressure diffusion in the Earth's crust are ubiquitous. Slow earthquakes result from crack development driven by pore-fluid-pressure diffusion. These earthquakes, also called silent earthquakes, take days, weeks, or even months to release energy instead of seconds or minutes as in normal earthquakes. Typhoons can trigger slow earthquakes in some areas. Hurricanes are believed to have triggered earthquakes in the eastern United States. Their explanation is provided for the most part by Biot's theory for wave propagation and pore-fluid-pressure diffusion in poroelastic media.

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Interpretive reprocessing of seismic reflection data has elucidated Paleozoic and Grenvillian structures in the southern Appalachians. The seismic data include a 7500‐km² grid of ADCOH, Seisdata, and COCORP reflection profiles that traverse the Blue Ridge and Inner Picdmont geologic provinces of North Carolina, South Carolina, and Georgia. Surface geology and potential field data were used to constrain the interpretation. The reprocessed seismic reflection data have delineated the internal and external geometry of the crystalline Blue Ridge‐Inner Piedmont allochthon, including the locations of the Blue Ridge master décollement, Haycsville fault, and Brevard fault zone. On the basis of the reprocessed data, all of the major faults within the allochthonous upper crust sole in the Blue Ridge master décollement. Reflections extending to the southeast from beneath the surface location of the Hayesville fault to the Blue Ridge thrust might be the seismic signature of a high strain zone. This implies that internal deformation of the Blue Ridge allochthon associated with the Alleghanian orogeny might have occurred farther to the west than has been previously documented from field studies. Relative amplitude seismic data enabled the discrimination between Blue Ridge‐Inner Piedmont crystalline rocks and underlying lower Paleozoic shelf strata, thereby delineating the Blue Ridge thrust. The interpreted geometry constrains the top of the shelf sequence beneath the Blue Ridge to depths of less than 3 km. This relatively shallow depth of the shelf strata together with the presence of duplex structures and bright spots that are imaged within the sequence might imply favorable conditions for hydrocarbon exploration beneath the Blue Ridge. Midcrustal reflections from within the upper‐to‐Iower crust are interpreted to originate from preserved Grenvillian structures that were reactivated at the basement surface during Late Proterozoic‐Early Cambrian extension. Reflection continuity is occasionally disrupted by interpreted post‐Grenvillian, pre‐Early Cambrian low‐density intrusions. Topography at the basement surface, possibly caused by the intrusions, is interpreted to have controlled the formation of some of the structures within the overlying allochthon, including Blue Ridge and Brevard fault zone ramps. Correlation of seismic time‐structure contour maps with available gravity data and two‐dimensional gravity modeling suggest that anomalies in the gravity field can be attributed to low‐density sources within the autochthonous crust. Discontinuous reflection packages from depths of 36–42 km are interpreted to originate from the Mohoroviĉiĉ discontinuity. The reflectors trend about N15°E with a true dip of approximately 15°NW.

I explore the hypothesis that most intraplate earthquakes and their aftershock sequences are triggered by pore-fluid pressure increases. As proposed in this paper, data from the magnitude 5.7 Virginia earthquake of 23 August 2011 show that this is a two-step process. (1) First, from areas where there is greater than normal meteoric recharge, pore-fluid pressure diffusion by means of Biot slow waves transfers more pore-fluid pressure towards a future hypocentre. Here the cumulation of Biot slow waves produces a steady increase in pore-fluid overpressure until a main shock is triggered. (2) Then, aftershocks occur in the zone reaching from the depth of the main shock to a depth of a few kilometres below the land surface, preferring to localize in a weaker, pervasive anisotropic crustal fabric, in response to locally increased permeability and pore-fluid pressure transients caused by the main shock. The primary corrosive agent responsible for reducing the strength of silicate minerals in this upper crustal zone is water, so that quartz-rich crust tends to have lower values of Poisson's ratio. I show here that increases in pore-fluid overpressure from normal groundwater recharge can start crack dilation leading to fracturing and the creation of new permeability. Previous chemical analyses across the Central Virginia Piedmont that hosted the 2011 Virginia shock show high upper crustal quartz content. This proposed two-step model for a main shock-aftershock sequence explains why intraplate earthquakes are rarely correlated with recognizable brittle faults at the Earth's surface.Supplementary material: A biography of John Costain is available at https://dx.doi.org/10.6084/m9.figshare.c.2854324.v3

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Reflection seismic data recorded on the Atlantic Coastal Plain to examine suspected faulting of the shallow (200 m) basement were contaminated by severe ground roll noise with an apparent velocity of 587 m/sec, 25 Hz dominant frequency, and 23 m apparent wavelength. A receiver interval of 5 m and a total spread length of 285 m were used to obtain 24‐fold coverage. Although the noise problem was severe, overlapping source and/or receiver arrays on the surface were not used for the recording geometry in order to avoid a decrease in resolution caused by the smoothing effect of overlapping subsurface coverage. Instead, the large‐amplitude surface waves were attenuated by a process called Vibroseis® whitening (VSW). VSW is based on the application of time‐varying amplitude scaling before Vibroseis crosscorrelation. A conventional method of automatic digital gain control was found to be effective for this purpose. This scaling results in a signal‐to‐noise ratio improvement equal to the gain expected from crosscorrelation. The scaling window length and the length of sweep are the only parameters required to define the signal‐to‐noise improvement for a given swept‐frequency band. A shorter scaling window and longer sweep length give better results because of the higher gain obtained by crosscorrelation. An increase in the quality of Atlantic Coastal Plain data clearly showed that VSW processing resulted in high resolution on stacked sections and made it possible to map shallow basement reflections at 0.2 sec, revealing faults with up to 10 m offset. Reflections from the sedimentary section above basement also became distinct.