STRUCTURAL SETTING OF THE 2011 M5.8 VIRGINIA EARTHQUAKE FROM SEISMIC REFLECTION DATA (Invited Presentation)

The 23 August, 2011, M5.8 Virginia earthquake occurred beneath the Piedmont province of central Virginia at a depth of about 6 km, with the focal mechanism and most of the aftershocks defining a N~30E striking fault plane dipping about 45 degrees to the southeast. Projecting the 2011 earthquake hypocenter southwest onto the reprocessed Interstate 64 seismic reflection profile shows that the earthquake, and most of the earlier events in the central Virginia seismic zone, occurred within the allochthonous, thin-skinned thrust sheets that override the east edge of Proterozoic Grenville crust. The hypocenter is beneath surface exposures of the Chopawamsic terrane, which is an island-arc terrane that was accreted to the continent during the Paleozoic. However, the Chopawamsic terrane is interpreted to form a thin (4.5 km or less) thrust sheet, which if true implies that the hypocenter lies within the underlying, relatively non-reflective Cambrian Candler(?) Formation. The 6 km depth of the mainshock hypocenter and the aftershocks appear to form a fault plane that extends upward to shallow depths, presumably cutting through the subhorizontal thrust fault at the base of the Chopawamsic terrane. Two implications of this geometry are that older faults at the surface within the thrust sheet may be detached from Paleozoic faults at hypocentral depths, and that the fault that ruptured in the 2011 earthquake is younger than the early and middle Paleozoic terranes. Likely times of formation are during the transpressive Alleghanian Orogeny or the Mesozoic rifting event. The hypocenter also lies above a thick (~2 km) sequence of highly reflective strata likely composed of the Catoctin and Lynchburg Formations deposited during the Neoproterozoic Iapetan rifting event and thrust westward in the Paleozoic. The highly-reflective strata do not appear to be continuous reflectors, which allows for the possibility that they are cut by faults that penetrate to mid-crustal levels. However, the scattering of earthquakes within the central Virginia seismic zone suggests that there is not a single major thrust fault that forms the locus of seismicity in the area. Rather, the seismicity appears to be distributed throughout the allochthonous thrust sheets.