The Loess Soil Problem beneath Memphis, Tennessee

The physical properties of any soil become important in considerations of the probability of liquefaction--a temporary quicksand condition--during an earthquake (for more on earthquake-induced liquefaction, please see: “The Disastrous Effects of Earthquake Soil Liquefaction” at: http://www.semp.us/biots/biot_330.html). Memphis, Tennessee is doubly cursed because of its location in the seismically-active New Madrid Seismic Zone (NMSZ) and the very low load-bearing strength of the loess soil underlying downtown and along the Wolf and Loosahatachie Rivers.

Loess—One of Three Types of Soil Particles

Three types of soil particles are sand, silt, and clay. Sand is the largest of the soil particles, and is usually composed of quartz or other silica minerals. Sand particles may be jagged and irregular, or smooth and nearly round, depending on degree of weathering. Because of their relatively large size, sand particles are separated by voids called “pore spaces” that permit easy air and water flow.

Silts are finer particles--intermediate in size between sands and clays--that are notably uniform in size and shape. Loess silt–or, simply, loess—is unusual in that the particles have rounded edges from being blown by the wind. When moist, loess particles form water films around the particles, which allows each particle to slide past another. High silt soils have very low load-bearing strength when moist, and can collapse or shift under high precipitation, especially on slopes.

Clay particles are quite small and are silicate minerals that are formed by crystallization and precipitation of products of both mineral weathering and dissolution, and sedimentary processes. Clay is discussed elsewhere (see “What Is Clay” at: http://www.semp.us/biots/biot_226.html; accessed February 25, 2006).

Loess in the United States’ Midwest

Loess is sometimes called “rock flour” because of its origin in glacial deposits, where ancient glacial activity ground rocks very fine. After drying, these deposits were highly susceptible to wind erosion. Downwind deposits became very deep, up to 300 feet deep in areas of China and up to 100 feet in the loess bluffs along the Mississippi and Missouri Rivers. Loess deposits are unconsolidated particles, are geologically unstable by nature, and will erode even without being disturbed by humans. Even well-managed loess farmland can experience dramatic erosion. The wind that blew the loess particles to the east of the Mississippi Embayment region to form the escarpment on which Memphis sits, came from the west. The tragic agricultural and social catastrophe known as the “Dust Bowl” (1930s) occurred in areas of loess soils.

Loess Liquefaction in Memphis

Historic earthquakes in the Mississippi Embayment region have liquefied unconsolidated, saturated deposits near, and possibly, in Memphis, according to recent research. (1) Only recently (2004) have geologic maps been published for Memphis that characterize near-surface deposits as deep as 100 feet, using geophysical and drill hole measurements. Seismologists then 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.

The geoscientists found loess (they call it “silty loess”) of between 20 and 50 feet covering the hilly upland adjacent to the Mississippi River. Because they could not locate evidence of liquefaction in this area (e.g., sand fissures, sand blows), they inferred that these “deposits, if at less than field saturation during strong earthquake shaking are unlikely to liquefy.”

Estimating Potential Loss: the HAZUS Methodology by FEMA

The Federal Emergency Management Agency (FEMA) has been concerned about the catastrophic effect of a potential earthquake in the New Madrid Seismic Zone with an epicenter near Memphis. As a result, FEMA officials have been working with the Central US Earthquake Consortium to better assess risk, using the HAZUS methodology, which is FEMA’s software program for estimating potential losses from disasters. (4)

This methodology identifies the number of buildings damaged, the number of casualties, the amount of damage to transportation systems, disruption to the electrical and water utilities, the number of people displaced from their homes, estimated cost of repairing projected damage and other effects. The purpose of applying this methodology is to prepare for a disaster in terms of land-use planning and facility-siting decisions (e.g., a map-based analysis of the potential intensity of ground shaking from a postulated earthquake that identifies those parts of the community that will experience the most violent shaking and the buildings at greatest risk of damage); prioritization of retrofit or abatement programs (e.g., an estimate of building damage that provides the basis for establishing programs to mitigate or strengthen buildings that may collapse in earthquakes by providing estimates of damages and casualties); regional, state, and local emergency response and contingency planning (e.g., estimates of casualties and of damage to buildings and utilities); medical and relief agency preparedness and response (e.g., estimates of casualties and homelessness); and assistance planning (e.g., an estimate of dollar losses that will help the state and Federal governments plan for assistance to jurisdictions and disaster victims). (4)

The application of HAZUS to Memphis during a New Madrid earthquake is sobering (please see: http://www.fema.gov/hazus/dl_madrid_eq.shtm for a PowerPoint presentation with maps; accessed February 25, 2006). The study involves a hypothetical 6.5 earthquake, and reports on six functional areas of city risk: health and medical services plan and implementation capability; transportation plan and implementation strategy; housing recovery strategy; emergency management assistance compact; and post-disaster building inspection capability.

Summary

Memphis, Tennessee is at risk for harm from an earthquake generated by the New Madrid Seismic Zone because of the nature of its underlying loess soil. Loess has very low load-bearing strength when moist, and can collapse or shift under high precipitation, especially on slopes. Liquefaction can be expected in certain areas along the Mississippi and Wolf Rivers in Memphis during an earthquake of a certain size, say greater than 6.5 magnitude on the Richter scale. The HAZUS methodology applied by FEMA to the Greater Memphis Area demonstrates catastrophic potential due to liquefaction, including major flooding from ruptured loess dams and landslides.

Sources:

1. David W. Moore, et al: “New Geologic Maps in the Memphis, Tennessee Area Help Assess the Potential for Earthquake-Induced Ground Failure. US Geologic Survey. November 7, 2004. Available online at: http://gsa.confex.com/gsa/2004AM/finalprogram/abstract_76517.htm; accessed February 25, 2006.

2. Roy B. Van Arsdale: “Earthquake Liquefaction Susceptibility Mapping in Memphis, Tennessee. Department of Earth Sciences, University of Memphis. April 2004. Available online at: http://gsa.confex.com/gsa/2004NC/finalprogram/abstract_71259.htm; accessed February 25, 2006.

3. See FEMA: HAZUS at: http://www.fema.gov/hazus/; accessed February 25, 2006.