Investigating Evidence of Holocene Liquefaction in the Lower Grand River Valley, Missouri, USA: A Possible New Seismic Hazard Source Zone for Central Missouri

Novel seismic hazards within the US Midcontinent can be challenging to identify due to the reliance of those assessments on prior observed earthquakes and seismicity. Although the New Madrid Seismic Zone (NMSZ) has received most of the attention, other tectonic features warrant consideration. One such structure is the Grand River Tectonic Zone (GRTZ) is a structural feature that trends northwestward from the northern NMSZ. Along the NW end of the GRTZ, where the lower Grand River Valley intersects the Missouri River, observations suggest a history of liquefaction in the Holocene, despite the area being historically seismically quiescent. Soil character mapping from aerial / satellite imagery shows areas throughout the Triplett terrace that have a mottled, chaotic character, suggesting a prevalence of irregular sand deposits, likely sand blows. Cut banks along the lower Grand River expose clastic dikes in Holocene alluvium. Geomorphic mapping of the late Holocene Triplett terrace (~10 m above the modern Grand River) identifies a disturbed drainage system, where multiple drainages arc northward opposing the regional gradient, and much of the terrace appears to be tilted to the north, opposite the flow direction of the Grand River. Additionally, there is a linear ridge southwest of the village of Triplett, MO, that appears to coincide with the disruptions in the terrace, suggesting it may be a relict fault scarp. This combination of evidence of liquefaction and structural disruption suggests that this site experienced strong shaking from a proximal source during the Holocene.

To confirm the nature and extent of these features, ongoing field studies include multiple tasks: a detailed survey of cut banks along the lower Grand River to map the extent of clastic dikes and other liquefaction features, detailed soil descriptions throughout the mapped presumed sand blows, and field observations to describe the structure of the linear ridge near Triplett. These results provide initial constraints on the timing, nature, and extent of paleoseismicity within the lower Grand River valley.