New Madrid Seismic Zone: Overview of Earthquake Hazard and Magnitude Assessment Based on Fragility of Historic Structures
The assessment of earthquake hazard has been a long-standing concern in areas known to be prone to earthquakes. While housing construction in the United States is generally considered to be earthquake-resistant in comparison to many forms of construction found worldwide, the assessment of seismic hazard has significant implications with regard to the balance of housing affordability and safety. Seismic hazard assessments affect building code design requirements (i.e., mapped design ground motions), construction guidelines, building costs, insurance rates, expected consequences of future earthquake activity, and regional economies as a whole. The seismic design provisions of the International Building Code (IBC-2000) and the International Residential Code (IRC-2000) are both currently being considered for adoption by local political jurisdictions across the U.S., and have generated much concern and controversy as to the accuracy and validity of the new seismic provisions in the Central and Eastern United States, and particularly in the New Madrid Seismic Zone (NMSZ).
This study provides an overview of the seismic hazard characterization procedures used in the NMSZ and implemented in the IBC-2000 and IRC-2000. Furthermore, a series of structural fragility evaluations of historic accounts of building damage are conducted to provide additional and independent constraints on the magnitude estimates of the 1811-1812 earthquakes. This approach to magnitude assessment is particularly appealing given that the magnitude estimate is ultimately used for regulation of building construction through the use of seismic hazard maps that are integral with seismic design provisions in modern building codes. The study aims to communicate the level of uncertainties involved in the seismic hazard assessment procedures and their impact on the assignment of seismic design ground motions and categories and to provide recommendations regarding implications associated with adoption or modification of newer seismic hazard provisions found in the IBC-2000 and IRC-2000.
Results of this study further confirm the high level of seismic hazard in the NMSZ and the need for continued attention to and consideration of adequate mitigation measures. This high level of seismic hazard is evidenced by large earthquakes that have repeatedly occurred in the past reaching destructive magnitudes. The study makes several concludes that deterministic capping of ground motions in the immediate vicinity of the modeled faults in the NMSZ should be considered for establishing design level ground motions, and hazard de-aggregation represents a useful and practical tool for communicating concepts and sources of seismic hazard to the stakeholders in a more transparent manner. Additionally, recent studies of paleoseismology and magnitude estimation have improved the understanding of the past behavior of the NMSZ, whereas questions remain open as to the future behavior of the NMSZ. The study makes recommendations for further research and implementation, including methods to perform future post-earthquake damage assessments and building evaluations.