APPROACHES TO AND STATUS OF PALEOSEISMOLOGY FROM AN INTRAPLATE SEISMIC ZONE WITHIN A LARGE ALLUVIAL VALLEY, THE NEW MADRID SEISMIC ZONE OF THE CENTRAL UNITED STATES

The New Madrid seismic zone is an intraplate right-lateral strike-slip fault system mostly within the Mississippi Alluvial Valley. An historic earthquake sequence in 1811-1812 had estimated moment magnitudes of 7-8. Four prehistoric earthquakes or earthquake sequences have been dated A.D. 1450 ± 150, 900 ± 100, 300 ± 200, and B.C. 1100 ± 1500 using paleoliquefaction features, particularly those associated with native American artifacts. The two most recent prehistoric events were probably also earthquake sequences with approximately the same magnitude as the historic sequence.

Surface deformation in an alluvial setting provides examples of stream response to gradient changes that can be used to date past earthquakes. Stream responses include changes in channel morphology, deviations in the channel path from the regional gradient, changes in the direction of flow, anomalous longitudinal profiles, and aggradation or incision of the channel. Floodplain uplift or depression determines the flooding frequency and thus thickness and style of vertical accretion or drowning of a channel to form a lake. Vegetation experiences trauma and mortality due to ground failure during the earthquake and hydrologic changes after the earthquake. Identification and dating these physical and biologic responses allows source areas to be identified and seismic events dated.

Seven fault segments are recognized. Surface faulting has been recognized at three of these segments, Reelfoot fault, North New Madrid fault, and Bootheel fault. The Reelfoot fault is a compressive stepover along the strike-slip fault and has up to 11 m of surface relief deforming abandoned and active Mississippi River channels. The North New Madrid fault has only strike-slip motion and is recognized by modern microseismicity, geomorphic anomalies, and sand cataclasis. The Bootheel fault, not delineated with modern microseismicity, is associated with extensive liquefaction and truncated and offset channels. The fault has dominantly strike-slip motion with a vertical component. Other recognized surface deformation is relatively low relief folding at Big Lake/Manila high and Lake St. Francis/Marked Tree high, along the subsurface Blytheville arch. Deformation at each fault segment does not occur during each earthquake event, indicating that earthquake sources have varied throughout the Holocene.