NEW GEOLOGIC MAPS IN THE MEMPHIS, TENNESSEE AREA HELP ASSESS THE POTENTIAL FOR EARTHQUAKE-INDUCED GROUND FAILURE

Historic earthquakes in the Mississippi Embayment region have liquefied unconsolidated, saturated deposits near, and possibly, in Memphis. Recently published geologic maps of six 7 1/2-minute quadrangles characterize near-surface deposits across the land surface and to depths of 10-30 m along several cross section lines. Thicknesses of stratigraphic units and depth to bedrock were interpreted from geophysical and drill hole logs (GWI Shelby County Subsurface Database). Seismologists used the geologic maps, sections, and seismologic properties (for example S-wave velocities and depth-to-water table) to assess the likelihood of earthquake-induced liquefaction in Memphis under different scenarios of ground-shaking.

Silty loess, 4.5-16 m thick, and local, thin, silty colluvium mantle the hilly upland adjacent to the Mississippi River. We infer these deposits, if at less than field saturation during strong earthquake shaking, are unlikely to liquefy because no paleoliquefaction features were seen in the loess and engineering laboratory studies support this inference. However, sandy alluvial deposits that underlie floodplains of the Mississippi River and the tributary Wolf River are more likely to liquefy. Structures founded on these deposits incur risk. We conclude this based on relict sand blow features on the floodplain of the Mississippi and sand dikes in banks (viewed in cross section) of the Wolf River. The dikes were probably feeder conduits of “sand blows” or paleoliquefaction features, formed during the large New Madrid, MO earthquakes of 1811-1812, or an earlier or later earthquake, such as the >6 magnitude quake centered 60 km northwest of Memphis near Marked Tree, AR in 1843. This work was funded by the National Cooperative Geologic Mapping and Earthquake Hazard Reduction Programs of the USGS.