ALBERT

Description: A high-resolution aeromagnetic survey, flown over the northern part of the New Madrid seismic zone in the Mississippi embayment, reveals linear features that generally parallel active seismic zones. This parallelism suggests that the linear magnetic features are related to faults. Modeling of these anomalies indicates that the associated magnetic sources are shallow, steeply dipping (〉80°) prism-like bodies. Their tops at depths of about 1 km are considerably shallower than the depth of crystalline basement (roughly 3 km). The bodies are typically 2 km wide. A plausible explanation for these bodies is that the magnetization within the sequence of generally nonmagnetic sedimentary rocks has been enhanced within and adjacent to fault zones. Such a magnetic enhancement could arise in several ways, including the emplacement of igneous intrusions, the authigenic growth of pyrrhotite, or the conversion of pyrite to magnetite. Whatever the cause of the magnetization contrast, the apparent relation between linear magnetic features and faults may lead to permissible stress models that accommodate the fault pattern inferred from the magnetic field.

Description: The deterministic approach to seismic hazard evaluation utilizes all available geologic/geophysical information to map the structure and nature of the crust in three dimensions that may relate to earthquake activity. However, information on the crystalline crust of the eastern United States from direct observations, drilling and sparse crustal seismic studies is limited. In contrast, regional gravity and magnetic anomaly data exist over the entire eastern United States and are available in a digital grid to facilitate processing and analysis. Although these data have serious limitations for detailed interpretation, they can be used to estimate the strength of the crust and the lithosphere and to map and characterize (1) zones of weakness such as paleorifts, sutures, and faults; (2) regions of potential stress amplifications such as plutons and irregularities in fault zones; and (3) basement terranes of generally consistent structural pattern that may delimit coherent regional seismic zones. Free-air, Bouguer, and isostatic gravity anomalies have different applications in the characterization of the crust for seismogenic purposes and complement magnetic anomaly maps which focus on upper crustal features. In concert, these data have provided the insight to interpret the host structures that together with related seismic and geoscience data, suggest causative mechanisms of the New Madrid seismic zone and other seismogenic regions of the eastern United States. As a result, we conclude that interpretations of geopotential anomalies are an essential ingredient in seismotectonic studies in the eastern United States, but they are only one of several tools required in the concerted effort of assessing seismic hazards. The presence of anomalies with a particular set of attributes neither confirms nor denies the possible spatial relationship to seismicity.