Deployment of temporary seismic stations after the 2011 Mineral, Virginia (USA), earthquake produced a well-recorded aftershock sequence. The majority of aftershocks are in a tabular cluster that delineates the previously unknown Quail fault zone. Quail fault zone aftershocks range from ~3 to 8 km in depth and are in a 1-km-thick zone striking ~036° and dipping ~50°SE, consistent with a 028°, 50°SE main-shock nodal plane having mostly reverse slip. This cluster extends ~10 km along strike. The Quail fault zone projects to the surface in gneiss of the Ordovician Chopawamsic Formation just southeast of the Ordovician–Silurian Ellisville Granodiorite pluton tail. The following three clusters of shallow (<3 km) aftershocks illuminate other faults. (1) An elongate cluster of early aftershocks, ~10 km east of the Quail fault zone, extends 8 km from Fredericks Hall, strikes ~035°–039°, and appears to be roughly vertical. The Fredericks Hall fault may be a strand or splay of the older Lakeside fault zone, which to the south spans a width of several kilometers. (2) A cluster of later aftershocks ~3 km northeast of Cuckoo delineates a fault near the eastern contact of the Ordovician Quantico Formation. (3) An elongate cluster of late aftershocks ~1 km northwest of the Quail fault zone aftershock cluster delineates the northwest fault (described herein), which is temporally distinct, dips more steeply, and has a more northeastward strike. Some aftershock-illuminated faults coincide with preexisting units or structures evident from radiometric anomalies, suggesting tectonic inheritance or reactivation.

The 23 August 2011 M w (moment magnitude) 5.7 ± 0.1, Mineral, Virginia, earthquake was the largest and most damaging in the central and eastern United States since the 1886 M w 6.8–7.0, Charleston, South Carolina, earthquake. Seismic data indicate that the earthquake rupture occurred on a southeast-dipping reverse fault and consisted of three subevents that progressed northeastward and updip. U.S. Geological Survey (USGS) “Did You Feel It?” intensity reports from across the eastern United States and southeastern Canada, rockfalls triggered at distances to 245 km, and regional groundwater-level changes are all consistent with efficient propagation of high-frequency seismic waves (~1 Hz and higher) in eastern North America due to low attenuation. Reported damage included cracked or shifted foundations and broken walls or chimneys, notably in unreinforced masonry, and indicated intensities up to VIII in the epicentral area based on USGS “Did You Feel It?” reports. The earthquake triggered the first automatic shutdown of a U.S. nuclear power plant, located ~23 km northeast of the main shock epicenter. Although shaking exceeded the plant’s design basis earthquake, the actual damage to safety-related structures, systems, and components was superficial. Damage to relatively tall masonry structures 130 km to the northeast in Washington, D.C., was consistent with source directivity, soft-soil ground-motion amplification, and anisotropic wave propagation with lower attenuation parallel to the northeast-trending Appalachian tectonic fabric. The earthquake and aftershocks occurred in crystalline rocks within Paleozoic thrust sheets of the Chopawamsic terrane. The main shock and majority of aftershocks delineated the newly named Quail fault zone in the subsurface, and shallow aftershocks defined outlying faults. The earthquake induced minor liquefaction sand boils, but notably there was no evidence of a surface fault rupture. Recurrence intervals, and evidence for larger earthquakes in the Quaternary in this area, remain important unknowns. This event, along with similar events during historical time, is a reminder that earthquakes of similar or larger magnitude pose a real hazard in eastern North America.

Observations made during geologic mapping prior to the moment magnitude, M w 5.8 2011 Virginia (USA) earthquake are important for understanding the event. Because many Paleozoic ductile faults in the Piedmont of Virginia show signs of brittle overprint, relict faults in the epicentral area represent potential seismogenic surfaces in the modern stress regime. Three major faults that reportedly dissect the early-middle Paleozoic bedrock in the epicentral area are reviewed here: the Shores fault of uncertain age, which has been depicted as internal to the Early Ordovician or Cambrian metaclastic Potomac terrane; the Late Ordovician Chopawamsic fault, which represents the Potomac-Chopawamsic terrane boundary; and the late Paleozoic Long Branch fault, which is internal to the Middle Ordovician Chopawamsic terrane. Our mapping reveals no evidence for the Shores fault, as previously depicted, in the epicentral area, and has led to revision of the position and surface trace of the Chopawamsic fault. Both these features are considered to have no connection to the 2011 event. Ductile strain features in a previously unrecognized zone related to the Long Branch fault are considered with a simple analysis of aftershocks along the brittle Quail fault that followed the 2011 Virginia earthquake. Internal to the Chopawamsic Formation, this Bend of River high-strain zone coincides in three dimensions with the aftershock-defined fault plane for the 2011 event. The spatial coincidence of the modern seismogenic surface (Quail fault) and Paleozoic metamorphic fabrics leads us to interpret that this zone of Paleozoic ductile strain, now located in the shallow crust, served as a guide to modern brittle intraplate rupture in 2011.

ABSTRACT The San Andreas Fault System — hereafter SAFS — has three major segments with notably different characteristics ( Figure 1 ). The Northwestern Segment extends from the Mendocino triple junction southeast for 720 km, with Salinian crystalline rocks on the west paradoxically abutting oceanic Franciscan sediments and volcanics on the east. The 275-km-long Central, or Big Bend, Segment has essentially identical crystalline rocks contiguous on either side. The 330-km-long onland part of the Southeastern Segment encompasses the Salton Trough, also has crystalline rocks on both sides, and has a high geothermal gradient typical of crustal spreading centers. This is the only segment of the SAFS that appears to be a crustal plate boundary. This study is based on a review and analysis of a formidable array of published data on the Big Bend Segment of the SAFS and of relevant geology to both its north and south (Appendix A, page 31). Its conclusion is that only modest strike-slip displacement, on the order of 15 to 50 km, has occurred in the Big Bend Segment. Areal geology displays abundant evidence of lateral shortening, including thrust and reverse faulting and mountain building whose vertical magnitude is comparable to horizontal displacements. The nature of the San Gabriel fault is also analyzed here because of past wide acceptance of it as an ancestral SAFS during Early Pliocene time, which produced a claimed right-lateral offset of about 48 km. The conclusion here is that both the proposal of a one-time connection of the San Gabriel fault with the SAFS and a 48-km displacement on the former are unwarranted. The conclusions of this paper stand in contrast to the multiple conflicting claims of major right-lateral offsets. Also, geologists have commonly expressed opinions of the timing and extent of displacement without acknowledging that they differed notably from earlier estimates.