APPLICATION OF SEISMIC REFLECTION IN THE NEW MADRID SEISMIC ZONE

Earthquakes of the New Madrid seismic zone occur primarily along faults within the late Precambrian to early Cambrian Reelfoot rift of the northwestern Mississippi embayment. Reelfoot rift faults were intermittently reactivated during the Phanerozoic culminating in middle Cretaceous uplift, formation of a major unconformity, and Late Cretaceous through early Tertiary subsidence of the Mississippi embayment. The upper Mississippi embayment consists of up to 1 km of flat-lying Late Cretaceous, Tertiary, and Quaternary sediments that overlie seismogenic Reelfoot rift faults.

A major objective of seismic hazard assessment is to determine the location, geometry, and displacement history of Quaternary faults. Because there are no faults exposed at the surface in the New Madrid seismic zone, investigators have obtained petroleum seismic reflection lines and acquired high resolution P and S-wave reflection data. The deep industry reflection lines allow regional mapping of faults to near seismogenic depths and the shallow high-resolution reflection data images faults within overlying Tertiary and, in some cases, Quaternary sediments. Shallow reflection has also been very helpful in citing trenches in paleoseismic studies.

Seismic reflection remains essential in the regional mapping of faults and determination of fault displacement history in seismic hazard assessment within the New Madrid seismic zone. However, a significant consequence of years of reflection research is a change in the interpretation of topography within the Mississippi River valley. Recent conventional interpretation of the Mississippi River valley has been that the topographic highs and lows are solely a consequence of the complex history of the Mississippi River as it drained the Pleistocene ice sheets. Seismic reflection conducted across the Benton Hills in southeastern Missouri, Crowley's Ridge in northeastern Arkansas, and the eastern Mississippi River bluffs in western Tennessee (among others) have revealed faults bounding these landforms. These observations suggest that we should view Mississippi River valley landforms as primarily tectonic in origin and that landforms of different ages may reflect local shifts in seismic/tectonic activity through time.