PALEOSEISMIC INVESTIGATION OF THE GRAND RIVER TECTONIC ZONE, NORTH-CENTRAL MISSOURI

Although the primary sources of large (M > 7) earthquakes affecting mid Missouri are typically considered within the New Madrid Seismic Zone (NMSZ), other potentially seismogenic structures warrant investigation. One such structure is the Grand River Tectonic Zone (GRTZ), which strikes NW-SE from the northern NMSZ. Along the GRTZ, where the lower Grand River Valley intersects the Missouri River in north-central Missouri, observations suggest Holocene liquefaction. Geomorphic mapping facilitated by LiDAR data has identified multiple Holocene terraces, including an extensive early Holocene surface herein called the “Triplett terrace”. Soil character mapping from overhead imagery shows areas throughout the Triplett terrace having mottled, chaotic characteristics, suggesting irregular features in the terrace surface. Soil test pits and soil borings document that these features may result from subsurface liquefaction. The southeast edge of the terrace also shows possible evidence of lateral spreading, and cut banks along the lower Grand River also expose clastic dikes in Holocene alluvium. Furthermore, the Triplett terrace depicts hints of deformation, with an overall northward tilt opposing the regional flow direction of the Grand River. There is also a subtle linear ridge southwest of the village of Triplett, MO, which may reflect localized warping of the Triplett surface. Seismic imaging across this features indicates a step in the sediment-bedrock interface below the hinge. A lesser degree of apparent warping of a lower, younger terrace also occurs along the same trend. Along the Grand River, possible clastic dikes are also observed in recent alluvium. Taken together, these observations suggests that this site experienced strong shaking from a proximal source during the Holocene.