EXPLORING THE EASTERN U.S. FOR EARTHQUAKES OUTSIDE OF THE MAJOR SEISMIC ZONES

Geologic studies of past earthquakes in the central and eastern U.S. (CEUS) have largely focused on areas of historical seismicity, like the New Madrid, Central Virginia and Charleston seismic zones. These studies primarily search for and analyze near-surface faulting expressed as scarps and structural uplifts, or they discern the history of strong shaking from features such as seismogenic paleoliquefaction, sackungs or landslides. However, recent earthquakes like those near Sparta, North Carolina, or Dover, Delaware, show that CEUS earthquakes are not limited to known seismic zones and that they often have subdued or no surface deformation. Scarps tens of kms or more in length from large Holocene earthquakes have not been found in the CEUS outside of the Cheraw and Meers faults and the New Madrid seismic zone, suggesting that any other Holocene earthquakes produced only subtle surface deformation. These observations emphasize the potential value of exploring large areas of the CEUS for evidence of past strong ground shaking such as paleoliquefaction, landsliding or damage to fragile geologic features like cave formations; or to analyze features that exclude strong ground shaking such as precariously balanced rocks or undisturbed soft sediments. Reconnaissance for these features may be especially effective in the CEUS, where an earthquake of a given size produces a much larger area of strong ground shaking than in the western U.S. due to less attenuation of seismic waves with distance. Furthermore, sediment-covered areas such as the Atlantic Coastal Plain sediments or glacial deposits can substantially amplify ground shaking and often have near-surface soft sediments prone to liquefaction. The lack of caves in the mid-Atlantic portions of the Atlantic Coastal Plain province, the relatively unconsolidated sediments in upper parts of the Atlantic Coastal Plain, the prevalence of glacial erratics and tors in the northeastern U.S. and Appalachian areas, and the sparsity of lakes in the southeastern U.S. indicate that different methods of detecting past ground shaking will be more effective in specific areas. Here I review some of these methods and the opportunities they provide to constrain earthquake histories in different regions of the CEUS.