17. Earthquake simulations in the New Madrid Seismic Zone: The 1811–1812 Bicentennial

An important U.S. earthquake anniversary is quickly approaching: the bicentennial of the 1811–1812 New Madrid area earthquakes. Like the 1906 San Francisco earthquake centennial, the bicentennial of these earthquakes will be a critical time to reach user communities such as the insurance industry and engineers, as well as the general public, about central U.S. seismic hazard. At present only coarse models of potential ground motions expected from a repeat of the 1811–1812 events have been published, and they have been focused primarily on the Memphis or St. Louis urban areas excluding the territory in between. Also, the magnitudes of the events are not well constrained because they are based on non-instrumental data. We seek a better understanding of the temporal and spatial distribution of shaking and damage from St. Louis to Memphis that accompanied the great 1811–1812 earthquakes, as well as other historic events that occurred after 1812. Research under this project will advance the mission of the USGS by reducing uncertainty in potential damage estimates from a repeat of these earthquakes, as well as by helping to constrain the uncertainty regarding the magnitudes and source rupture models of these events. The results will be a part of a broad-based New Madrid earthquake scenario being planned by the Earthquake Engineering Research Institute, the Central U.S. Earthquake Consortium, and the USGS.

The primary objective of research under this opportunity will be construction of a new and comprehensive 3D geologic and seismic velocity model of the Mississippi embayment from existing data. The model, in turn, will permit 3D finite-difference (or finite element) simulations of the 1811–1812 ground motions at frequencies relevant for many engineered structures. The model will be validated using recordings from the central U.S. Advanced National Seismic System. Simulations for other earthquake ruptures in the New Madrid seismic zone such as repeats of the 1843 M6.3 event near Marked Tree, Arkansas, and the 1895 M6.6 event that occurred near Charleston, Missouri, will also be conducted. It is expected that the ground motion simulations will be viewable as a video on the web.

Because of the significant uncertainty about the 1811–1812 rupture models, a suite of possible 1811–1812 source models will need to be tested in order to evaluate the effects of rupture directivity and to model the observed shaking and damage. Inferring source characteristics of damaging CEUS earthquakes using instrumental recordings of large intraplate earthquakes in analog regions and of small, local earthquakes in the study area could be one approach.

A large percentage of the population in the New Madrid seismic zone resides on soft young sediments with low shear-wave velocity, thus, the simulations must incorporate “site effects,” the effect of shallow soft materials on higher frequency earthquake ground motions. The distribution of these materials, and their possible nonlinear behavior during strong shaking, is very important in controlling the pattern of shaking and damage. Recent studies have shown lower attenuation in the unconsolidated sediments of the Mississippi embayment than previously believed, therefore, consideration of alternative attenuation models will be crucial. To adequately address these factors, a new high-resolution 3D model of the embayment that permits simulations up to 2 Hz will be required; the latter, in turn, may lead to the use of a hybrid approach, where high frequencies are incorporated using a stochastic model.

We expect the model simulations to reveal previously unknown site-specific variability in ground motions due to scattering effects and alternative rupture models. The regional deterministic scenarios resulting from these simulations will aid local and regional planning and loss estimation efforts. In addition, earthquake time histories, a critical parameter used by earthquake engineers to estimate building response, will be available from the simulations. Using a ShakeMap type output, results of the new modeling will also describe the potential effects that a future 1811–1812 series of earthquakes may have on the region. During the 100th Anniversary of the 1906 earthquake, earthquake simulations were extremely popular products and were viewed by millions of people in the San Francisco Bay Area on television and on the web. By taking advantage of the raised public awareness associated with the bicentennial, the transfer of new scientific understanding from this research to external groups will be maximized.

Although generalized 3D geology and seismic velocity models of the Mississippi embayment currently exist, these models require significant modification to incorporate new data on shallow shear-wave velocities. 3D finite-element codes needed for the simulations were used and thoroughly tested by USGS seismologists for the 1906 simulations. In order to generate simulations at the frequencies needed by engineers and over the large spatial extent of the New Madrid seismic zone, extensive computer resources will be needed. These are available at both the USGS and the San Diego Supercomputing Center to which the postdoctoral fellow will be provided access. Innovations may be required to work around existing computational limitations.

Proposed Duty Station: Golden, CO

Areas of Ph.D.: Geophysics, seismology, computer science, geology, geotechnical engineering

Qualifications: Applicants must meet one of the following qualifications: Research Geophysicist, Research Geologist, Computer Scientist, Research Civil Engineer

(This type of research is performed by those who have backgrounds for the occupations stated above. However, other titles may be applicable depending on the applicant's background, education, and research proposal. The final classification of the position will be made by the Human Resources specialist.)

Research Advisor(s): Robert Williams, (303) 273-8636, rawilliams@usgs.gov; Steve Hartzell, (303) 273-8572, shartzell@usgs.gov; Eugene Schweig, (901) 678-4974, schweig@usgs.gov; Oliver Boyd, (901) 678-3463, olboyd@usgs.gov

Human Resources Office contact: Kathleen Scheich, (303) 236-9581, kscheich@usgs.gov

Summary of Opportunities