Science

Potential Earthquake Dangers at the PGDP

Common Terminology

When scientists discuss the potential danger of an earthquake, they frequently use the following terms:

  1. Seismic - Of, subject to, or caused by an earthquake or earth vibration. Earthshaking: an issue of seismic proportions and ramifications.
  2. Seismicity – the frequency or magnitude earthquake activity in a particular area
  3. Earthquake Magnitude – a mathematical measure of the strength of an earthquake based on the maximum size of the seismic waves produced. The magnitude of an earthquake is normal expressed in terms of a number associated with the Richter scale.

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  4. Seismic Risk – the probability of an earthquake of a particular magnitude
  5. Seismic Hazard – the probability of damage of a particular magnitude

Sometimes geologists or seismologists (scientists who study earthquakes) use the terms seismic risk and seismic hazards interchangeably, which can add to people’s confusion. Perhaps one way to distinguish these two terms is to think of the old children’s story about the Three Little Pigs and the Big Bad Wolf. If you remember, the story tells about three little pigs, one who built a house out of straw, one that build a house out of wood, and one that built a house out of bricks. It also tells the story about how a Big Bad Wolf would come to each of their houses saying “Little Pig, Little Pig, let me in” to which the pig would say, “Not by the hair of my chinning chin chin” to which the Big Bad Wolf would reply – “Then I’ll huff and I’ll puff and I’ll blow your house in.” If you remember the story, the Wolf was successful in blowing the houses in of the first two pigs but not the third – the one made out of bricks.


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At this point you might be asking yourself, what does this have to do with earthquakes – well, the point of this is that in comparison to seismic risk and seismic hazard – we would say the magnitude of the Wolf’s blowing is analogous to seismic risk while the impact of the Wolf’s blowing is analogous to seismic hazard. Thus in our case, the seismic hazard is related to what type of precautions have been taken (or design modifications made) to protect against a particular seismic risk. Thus in many cases, just because there is a significant seismic risk does not necessarily mean there is a significant seismic hazard. While it may certainly cost a lot more money to build a structure with the same seismic hazard in an earthquake zone than one not in an earthquake zone, that does not mean that it is impossible to do so. Thus we find as an example, that California has several nuclear power plants – http://www.energy.ca.gov/nuclear/california.html.

PGDP Seismic Sources

Three seismic sources have the potential to affect the PGDP (see Figure 1 below). These include the New Madrid Seismic Zone (centered near the juncture of Kentucky , Missouri, and Tennessee); the Wabash Valley Seismic Zone (in southeast Illinois and southwest Indiana); and background seismicity. While it should be emphasized that there is currently no definitive evidence to support a conclusion that the geology or existing faults present at the PGDP site are directly part of the New Madrid Seismic Zone, the new Madrid Seismic Zone presents the most prominent seismic risk to the PGDP.

Four or five major earthquakes are believed to have occurred in the New Madrid Seismic Zone in late 1811 and early 1812 (see Figure 2). The most significant earthquakes during this period (December 16, 1811, January 23 and February 7, 1812) are estimated to have had a magnitude between M7.0-7.5 (Hough et al. 2000; Hough and Martin 2002). The biggest quake since 1811-1812 was a 6.6-magnitude quake on October 31, 1895, with an epicenter at Charleston, Missouri. The quake damaged virtually all buildings in Charleston, creating sand volcanoes by the city, cracked a pier on the Cairo Rail Bridge and toppled chimneys in St. Louis, Missouri, Memphis, Tennessee, Gadsden, Alabama and Evansville, Indiana (USGS, 2009). For more information about the New Madrid Earthquakes see: here.

More than 4000 earthquakes have been recorded in the region since 1974 (see Figure 3). While this may seem like a lot, the majority of the quakes have had a magnitude of less than M3 and do not have an impact on seismic safety. There have only been a few earthquakes with a magnitude larger than 5.0 in the region since 1895.

Seismic Concerns

Because of the proximity of the PGDP to the New Madrid Seismic Zone, some citizens have raised concerns with regard to the potential safety or integrity of the existing buildings, burial grounds, or landfills at the PGDP. These concerns have also been raised with regard to the safety and integrity of any potential new structures such as a CERCLA landfill or future facilities that might be built at the site after the PGDP is shutdown and demolished. In addressing these concerns, the following facts need to be considered:

  1. Various faults have been identified in the geology of western Kentucky, including the PGDP site. Faults, by themselves however, do not necessarily pose a significant seismic risk. The actual seismic risk at a particular site will be dependent upon several additional factors such as the expected magnitude of an earthquake at or near that site and the probability of such a magnitude occurring.
  2. Kentucky State regulations prohibit the construction of any landfill “within 200 feet of a fault that displays Holocene displacement (i.e. the displacement has occurred in the last 10,000 years). After several geologic studies (both regional and site specific fault and paleoliquefaction studies) no evidence of Holocene displacement has been detected at the PGDP or the surrounding area. Thus, there does not appear to be any seismic regulatory barrier to the construction of a hazardous waste landfill at the PGDP site.
  3. Prior to the construction of any future landfill, several additional analyses will have to be performed in order to determine what level of seismic risk to expect at the design location. These include:

    • Determining the magnitude of the expected level of seismic activity at the bedrock – this depends on an analysis of regional historic seismic activity and a specified earthquake probability. 40 CFR Chapter 1 (258.14) requires that any landfill containment system be designed to withstand the peak ground acceleration (PGA) at bedrock “depicted on a seismic hazard (sic) map, with a 90 percent or greater probability that the acceleration will not be exceed in 250 years, or the maximum expected horizontal acceleration based on a site-specific seismic risk assessment.”

      The phrase “90 percent or greater probability that the acceleration will not be exceed in 250 years” effectively means that the landfill containment system must be designed to withstands the bedrock Peak Ground Acceleration (PGA) of the strongest earthquake ground motion that will affect the landfill in 2500 years. (This can be expressed in a variety of ways, and is roughly equivalent to a 2% Probability of Exceedence in 50 years).

      Note: several different seismic analyses have been done for the PGDP site, based on both probabilistic and deterministic methods. Depending upon the assumptions used in the analyses, results ranging from 0.5 g to 0.7 have been determined and published.

    • Determining the magnitude of the expected level of seismic activity at the surface – this depends on an analysis of the soils between the bedrock and the surface. In theory, this could either lead to and increase (amplification) or decrease (de-amplification) of the impact of the PGA at the surface – where the particular structure is to be built. A preliminary analysis of the conditions at the southern end of the site have indicated that the expected motion at the surface will actually be de-amplified by 33%. Thus, if one were to use a conservative estimate of the PGA to be 0.7, the associate ground motion at the surface would be expected to be 0.67*0.7 = 0.48 g.
    • Determining the frequency of the expected level of seismic activity at the surface – once this is determined, engineers then check to make sure that the frequency does not match the design frequency of the structure (which could cause the structure to resonate leading to an increase in the shaking) or the frequency of the foundation soils (which could lead to liquefaction of the soils) – if either of these conditions are found to exist then either the soils associated with the foundation are modified, or the structure is designed in such a way to avoid this critical frequency.
  4. Before any final decision is made with regard to the placement and design of any future landfill, the CERCLA design criteria require that the public be involved in the decision making process. To date, the DOE has held two public informational meetings about current proposals to construct a hazardous waste landfill at the PGDP. The first set of meetings was held on November 18 and 20, 2008 and the second meeting was held on March 24, 2009. DOE is currently planning to hold a third meeting about this structure this fall.

For additional information about seismic issues at the PGDP or the potential CERCLA cell please consult: this site.


Figure 1. Tectonic Map of Midwest Region


Figure 2. Map of New Madrid seismicity. Epicenters of earthquakes since 1974 shown by red dots. Approximate locations of large historical earthquakes shown by black crosses.


Figure 3. Seismicity between 1974 and 2005 in the central United States (Wang and Woolery, 2008).

"Historic Earthquakes: Near Charleston, Mississippi County, Missouri". United States Geological Survey. http://earthquake.usgs.gov/regional/states/events/1895_10_31.php. . Retrieved 2009-09-23