Relict Paleozoic faults in the epicentral area of the 23 August 2011 central Virginia earthquake; assessing the relationship between preexisting strain and modern seismicity
Relict Paleozoic faults in the epicentral area of the 23 August 2011 central Virginia earthquake; assessing the relationship between preexisting strain and modern seismicity (in The 2011 Mineral, Virginia, earthquake, and its significance for seismic hazards in eastern North America, J. Wright Horton (editor), Martin C. Chapman (editor) and Russell A. Green (editor))
Special Paper - Geological Society of America (2015) 509: 331-343
Observations made during geologic mapping prior to the moment magnitude, M (sub 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.