ALBERT

Description: Seismicity in the northeastern U. S. can often be correlated to preexisting structural features. This correlation is found over a wide range of scales and distinct types of correlation characterize the opposite ends of this range. Firstly, patterns of seismicity measured in tens or hundreds of kms are recognized in maps of recent epicenters from regional networks and in maps of earlier macroseismic epicenters. These patterns are characterized by epicenter bands of resolvable widths, but, within these bands, individual ruptures are often oriented at large angles to the strike of the bands. Several of these seismicity patterns can be spatially correlated to structural features in the crystalline layer of the upper crust which is the source of seismicity. These features are recognized as major lithologic boundaries or suture zones, characterized by ductile deformation without evidence for a subsequent brittle phase. Hence, these large scale features are not preexisting weaknesses reactivated in the current regime. Stress concentration derived from lithologic and rheologic contrast along the boundaries may be an important mechanism in these cases. Secondly, high resolution of aftershock zones in areas where the seismogenic layer outcrops has allowed detailed correlation between ruptures and brittle features. Two cases, one in the Grenville terrane of the craton and the other in the Manhattan Prong along the crystalline core of the Appalachians, show similar characteristics. The ruptures are small, 1.5 and 0.5 kms respectively, but their geometry is well defined by the aftershocks. Both these ruptures are associated with zones of brittle fractures that can be followed at the surface for more than 10 kms. These fracture zones are primarily recognized as topographic lineaments, concentrations of slikensided joints and sets of small en echelon faults. The aftershocks zones are parallel to the individual faults in these en echelon fault sets. The total accumulated horizontal displacement across these fracture zones is small (less than 50 to 100 meters) when compared to the rates of moment release and to the inferred pre-Cenozoic age of these fracture zones. The maximum magnitude from these fracture zones is probably controlled by the size of individual faults in the en echelon pattern, rather than by the much larger dimension of the entire fracture zone.