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Intraplate earthquakes within the eastern United States represent brittle faulting in the upper crystalline crust at shallow to moderate depths. The premise—that intraplate seismicity in crystalline crust occurs along postorogenic brittle faults formed during extensional events—permits us to distinguish three large intraplate-earthquake domains in the eastern United States. These domains are: the Appalachian accreted terranes; the Grenvillian accreted terranes; and the midcontinent accreted terranes. These crystalline domains are separated by terrane boundaries, where metamorphic zones, formed during terrane accretion, seal off older brittle faults in an older terrane from any direct connection with newer brittle faults in a newer accreted terrane. Strain within each domain accumulates on preexisting fault zones formed during previous postorogenic extensional events. Although older faults in adjacent regions may be sealed off within independent terranes, extensional events that postdate accretion may generate younger brittle faults that cross the sealed boundaries. Thus, breaches created by younger migrating hotspots, failed rifts, and/or impact structures may provide local connecting zones across sealed boundaries.

Detailed fracture studies within the Southern Appalachian accreted terranes, Triassic rift basins therein, and overlapping Cretaceous–Cenozoic passive-margin strata of the southern Atlantic Coastal Plain show that recurrent Mesozoic–Cenozoic brittle faulting: (1) is postorogenic and occurs along fracture sets formed during failed-to-successful Mesozoic rifting during the breakup of Pangea and opening of the Atlantic Ocean; (2) is not related to brittle reactivation of orogenic ductile shear zones or metamorphic fabrics; (3) is largely confined to accreted crystalline terranes in the Appalachians separated by metamorphosed zones, referred to previously, which are herein called sealed boundaries; (4) has younger, shorter fracture sets that are confined by older fault zones; and (5) formed in a temporal sequence of fracture sets that have a hierarchical ordering and scaling of recurrently active fault zones, with older, linked sets forming longer, more through-going fault zones, which bound large polygonal crustal blocks within accreted terranes.

The second intraplate-earthquake domain is the Mesoproterozoic Grenville basement. Crystalline terranes of the Grenville orogen (ca. 1.2–0.9 Ga) were accreted during formation of the Rodinian supercontinent. The Rigolet phase (ca. 1.02–0.9 Ga) of the Grenville orogeny was characterized by a shift from contraction to NW-SE extension with development of core complexes (e.g., Adirondacks). Protracted cooling, continued NW-SE-extension, and passage of a hotspot (ca. 750–600 Ma) preceded the breakup of Rodinia and opening of the Iapetus Ocean at ca. 565 Ma. Thus, the Grenville orogen also contains a sequence of postorogenic brittle fracture sets, which subsequently formed recurrently active brittle fault zones. The third intraplate-earthquake domain contains the Archean–Mesoproterozoic orogenic belts in the midcontinent region, which are cut by brittle fracture sets related to: failed rifting events (e.g., ca. 1.1 Ga Midcontinent rift; Cambrian Southern Oklahoma aulacogen, Reelfoot rift, and Rough Creek graben); successful Triassic rifting and opening of the Gulf of Mexico; and Mid-Cretaceous passage of the Bermuda hotspot. Regional fracture sets formed during each of these postorogenic events and the temporal sequence of fracture sets determine the hierarchical ordering and scaling of recurrently active fault zones. Fault-plane solutions in major seismic zones within these three domains (Charleston, South Carolina, and central Virginia; east Tennessee and Giles County, Virginia; New Madrid and Wabash—Arkansas, Illinois, Kentucky, Missouri, Tennessee) are consistent with their occurrence along reactivated older fault zones.

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