FAULT REACTIVATION, SEISMIC HAZARD, AND BACKGROUND NOISE EAST OF THE ROCKY MOUNTAINS

One reason to study ancient basement faults that have undergone Phanerozoic reactivation is that their reactivation could indicate the fault poses a seismic hazard. In much of the central and eastern U.S. (CEUS), sparse, scattered, background seismicity indicates a low level of ubiquitous present-day seismic faulting. This seismicity is treated straightforwardly in hazard computations and the geologic study of its faults would add little. How much should a fault be reactivated to distinguish itself from the background seismic faulting? C.S. Mueller (USGS) has calculated rates of CEUS background seismicity as part of updating the 1997 USGS national seismic-hazard maps. Global analogues to the CEUS have had rare historical earthquakes at least as large as moment magnitude M 7. Rare, large earthquakes dominate the long-term occurrence and accumulation of seismic slip. Mueller’s rates allow calculation of likely recurrence intervals (t) for CEUS M 7 earthquakes. Published regressions estimate the slip (s) typically produced by such an earthquake. Geologic maps of eight CEUS states show similar areal densities (d) of mapped faults 50 km long, or 50-km sections of longer faults, about the typical length of an M 7 rupture zone. Together, t, s, d, and their standard deviations allow estimates of the average, long-term fault slip rate (V) attributable mainly to rare, M 7, background earthquakes. Thus, a fault rises above the CEUS background if its slip rate significantly exceeds V. The pertinent slip must have occurred after a fault reached its likely present level of activity. Thus, pertinent slip on Appalachian or Ouachita faults would be post-orogenic, and pertinent slip in Mesozoic or Neoproterozoic-Cambrian rifts and passive margins would be post-rifting. At the 0.95 confidence level, this analysis suggests that faults in the CEUS central craton should slip faster than 2.3 m/m.y. to justify study for hazard assessments, or faster than 3.8 m/m.y. in the continental rim of Phanerozoic rifts, passive margins, and orogens. Faults offsetting oldest Quaternary strata less than 6 m on the Atlantic seaboard may represent only background seismicity, as may offsets of the basal Cambrian unconformity on the stable platform of less than 1.2 km.