Lexington, KY (January 4, 2006) – “Our research shows that the earthquake ground motions for building codes and engineering designs in western Kentucky are overly conservative,” according to Zhenming Wang, a seismologist and civil engineer at the Kentucky Geological Survey (KGS) of the University of Kentucky in Lexington, Ky. Dr. Wang was invited to present the conclusions at the 2005 American Geophysical Union Fall annual meeting, December 5-9, 2005, in San Francisco, CA. The title of his presentation is “Role of the Kentucky Geological Survey in Implementing IBC-2000 and IRC-2000 in Kentucky : Better Communication of Seismic Hazard and Risk Assessment.”
The design ground motions were based on the national seismic hazard maps, produced by the U. S. Geological Survey, that have a 2 percent chance of being exceeded during a 50 year period. The ground motion is commonly measured in terms of peak ground acceleration (PGA) and expressed as a percentage of the acceleration of gravity (g). Damage to buildings and loss of life mainly come from ground shaking during an earthquake. According to the maps, many counties in western Kentucky , the Jackson Purchase region in particular, could experience extremely high ground motions (0.6 to 1.6g PGA).
Overly conservative design ground motions are not consistent with what is known of the geology and seismology of the region. “Although we continue to examine the geologic record, 10 years of investigation in the northern Jackson Purchase region has not produced any evidence of a large, local earthquake during the past 10,000 years,” said Edward Woolery, an assistant professor in the Department of Geological Sciences at the University of Kentucky . Drs. Wang and Woolery are leading an earthquake and seismic hazards research group at UK with support from the U.S. Department of Energy (DOE), the U.S. Geological Survey, and the Commonwealth of Kentucky .
By contrast, the highest design ground motion for buildings in earthquake-prone California is capped at 0.4g. The high design ground motions for western Kentucky have created many problems for compliance with seismic safety regulations affecting home construction in the region as well as environmental clean-up at the superfund site at the Paducah Gaseous Diffusion Plant. For example, a prospective homeowner would need to enlist professional engineering design services in order to construct an average two-story home in Paducah if the high ground motion is required. Also, environmental clean-up at the superfund site becomes much more difficult for the same reason.
The research at UK has shown that the most conservative estimates of seismological parameters (i.e., the location and maximum magnitude of earthquakes in the New Madrid Seismic Zone and the ground motion attenuation in particular) for the New Madrid region were used in the national hazard maps. Attenuation refers to the reduction in the energy of seismic waves as they pass through the ground. Furthermore, the research has also shown that the method used to construct the national hazard maps, known as probabilistic seismic hazard analysis (PSHA), has several intrinsic or inherent problems. Therefore, the use of the most conservative input parameters and the method (PSHA) result in extremely high estimates of ground motions for western Kentucky , the Jackson Purchase region in particular.
The results from the research at UK are helping the Commonwealth to understand the difficulties of estimating seismic hazards and to choose more appropriate hazard estimates for seismic hazard mitigation in Kentucky . For example, by working with the Kentucky Department of Housing, Building and Construction and the Structural Engineers Association of Kentucky, KGS helped the State to understand that the use of ground motions with 5 percent chance of being exceeded in 50 years is more appropriate. Consequently, the State adopted the 2000 International Residential Code as the 2002 Kentucky Residential Code with a modification of design ground motion for western Kentucky . The researchers at UK have also helped the State to develop more appropriate design ground motions for highways and bridges in Kentucky .
Estimation of seismic hazards has significant implications for economic development in Kentucky . Either overestimating or underestimating the hazards may have detrimental consequences to society. “We do not come to any conclusion on seismic hazard lightly because safeguarding the public is our most important responsibility,” said James Cobb, the director of the Kentucky Geological Survey and state geologist. “These also demonstrate why the research at UK is important and how it benefits the Commonwealth,” added Cobb.
PSHA has been widely used to assess seismic hazards for public policies and engineering designs in the U.S. and other countries in the world. For example, the seismic regulations and designs for critical facilities, such as nuclear power plant and waste repository, in the U.S. have been mostly based on PSHA methodology. But the research at UK demonstrated that the ground motion derived from PSHA does not have a clear physical or statistical meaning and it may not be appropriate to use. The use of PSHA may result in either overly conservative or unsafe policies or engineering designs against risk from seismic hazards.
“The results from the research at UK not only affect Kentucky , but could also have an impact on the nation and other countries in the world as well,” said Wendy Baldwin, Executive Vice President for Research at UK . Seismologists at UK are working together with their counterparts in other states, the USGS, and other countries to solve problems related to seismic safety.
For more information about the earthquake and seismic hazards research at UK , please go to: http://www.uky.edu/KGS/geologichazards/.