STAFFORD FAULT SYSTEM: INVESTIGATING PALEOSEISMICITY IN VIRGINIA

The23 August 2011 5.8 magnitude earthquake near Mineral, Virginia and its aftershocks occurred in the Piedmont between several regionally mapped northeast-trending Mesozoic and/or Paleozoic faults (Spotsylvania fault zone (SFZ), Long Branch fault, and Chopawamsic fault). The Stafford fault system is parallel to the southeast-dipping Long Branch fault, and the SFZ trends N to the Stafford fault system; some of these faults are en echelon. Movement history of the Stafford fault system is much better documented than for the exposed Piedmont faults and consists of a series of compressional, high-angle, NE-trending, NW-dipping en echelon reverse faults that thrust Piedmont crystalline rocks over unconsolidated Cretaceous sediments and Cenozoic coastal plain stratigraphic units. The Stafford fault system has four discrete high-angle reverse fault zones, Dumfries, Fall Hill, Hazel Run, and Brooke. The Fall Hill aligns directly with the SFZ which is possibly a Mesozoic splay. The Stafford fault system fault zones preserve thicker sections on their down-thrown blocks and thinned or truncated sections on their up-thrown blocks with individual fault displacements ranging from 15 to 60 m. These faults show evidence of growth-fault movement, with older strata being offset more (e.g., top basement up to 60 m, top Paleocene up to 30 m, Pliocene and Pleistocene beds documented <1 m (interpreted up to 7m) across the high-angle reverse faults. These structural relationships demonstrate prolonged Cretaceous to Pleistocene reactivation of older fault systems. The SW -trending reach of the Potomac River from Washington D.C. to Quantico, VA., and the straight reach of the Potomac south of Quantico to Aquia Creek appear to be geomorphic expressions of the Stafford fault system. However, Holocene slip history of the Stafford fault system has not been studied and the Piedmont faults have very little coastal plain or alluvial cover from which event dates can be derived. Study of the M5.8 earthquake near Mineral , Virginia show that the strongest shaking was in the NE-SW direction and modeling and studies of others fault zones have shown that seismic energy can propagate along fault zones and transfer among faults if they are connected at depth. Clearly, the Stafford fault system offers one of the best paleoseismic records of the region.