Occurring only 13 months apart, the moment magnitude, M w 3.4 Germantown, Maryland (16 July 2010), and M w 5.8 Mineral, Virginia (23 August 2011), earthquakes rocked the U.S. national capital region, drawing renewed attention to the occurrence of intraplate seismicity in the Mid-Atlantic region of the eastern United States. We model the Coulomb stress transferred by these earthquakes to fault zones in the Mid-Atlantic region that were active during the Cenozoic. In most cases, the Mineral earthquake brought these preexisting Cenozoic faults further from failure. This unloading, like all changes in stress located more than 30 km from the epicenter, was very small (~1–3 mbar) and therefore unlikely to affect the occurrence of earthquakes at the regional scale. Between 30 and 15 km away, however, the maximum Coulomb failure stress change ranged one to three orders of magnitude greater (~0.05–6 bar), levels on par with the stress changes that triggered historical earthquakes in California. The Mineral earthquake generated an increase in Coulomb failure stress of ~0.5 bar 10 km from the rupture, ~5 bar 5 km from the rupture, and ~20 bar at the edge of the rupture on receiver faults oriented like the mainshock. The geographic location of the aftershock-defined Northwest fault in the epicentral region may be explained by Coulomb stress transfer from the mainshock. Elastic dislocation modeling of surface deformation caused by the Mineral earthquake indicates a maximum of ~7 cm of permanent vertical surface displacement directly above the center of the rupture.

Abstract This field trip provides an overview of both the geology underlying the city of Philadelphia and the hydrology of the Schuylkill River watershed. Philadelphia is located at the contact between two major East Coast physiographic provinces, the New Jersey Coastal Plain and the Piedmont. The Piedmont rocks are examined during this field trip. In this area, the Piedmont exhibits low to moderate relief and is underlain by folded and faulted sedimentary and metasedimentary rocks of early Paleozoic age. These rocks are interpreted to represent the collision of a magmatic arc with a continental landmass and adjacent forearc basin sediments during the early Paleozoic; they are intensely deformed and metamorphosed to the amphibolite and granulite facies. The Piedmont rocks appear in the type section of the Wissahickon Formation, which will be the geologic focus of this field trip. The hydrology of the Schuylkill River and one of its major tributaries, Wissahickon Creek, will also be discussed during the trip.

Abstract This walking tour will consider local geology, touch on local history, and focus on the building stones used in downtown Dayton. Building stones and construction materials used along Main Street are the main interest. Special attention will be paid to “Dayton’s own,” the Dayton limestone—a stone considered by State Geologist Edward Orton in the second half of the nineteenth century ( Orton, 1870 , 1893 ) as one of Ohio’s finest building stones. The Dayton Formation (Dayton limestone) was used extensively as a building stone in the Dayton area (and farther afield) during the nineteenth and early years of the twentieth centuries during the growth of Dayton. Perhaps the zenith of the Dayton limestone building-stone industry is characterized by the Old Courthouse (1850), an important building in the Greek-Revival architectural-style that saw the use of Dayton limestone not only for the exterior of the building but also, unusually, and perhaps with a little too much enthusiasm, for slabs of limestone for the roof. This building has much local historical significance— both Presidents Abraham Lincoln and John F. Kennedy addressed the public from its steps. Dayton limestone was used for the commemorative stone from the State of Ohio, installed in 1850, inside the Washington Memorial, Washington, D.C., and for part of the “Ohio House” built for the International Exhibition at Fairmount Park, Philadelphia, Pennsylvania, commemorating the centennial of the signing of the Declaration of Independence in 1876. Use of the stone is also documented in Cincinnati, Columbus, and Chicago. Recent developments along East Monument Avenue and Patterson Boulevard— RiverScape and the Patterson Boulevard canal walk—as well as some of the buildings, will be discussed. The “Great Dayton Flood” of 1913 probably resulted in excess of four hundred deaths along the Great Miami River valley and its watershed. The Miami Conservancy District oversees the flood-prevention scheme that developed after the 1913 flood; their headquarters are housed in a building that overlooks the Great Miami River in downtown Dayton. Flood-prevention modifications to the Great Miami River can be seen adjacent to downtown.