[iris-bulk] Mendenhall announcement for New Madrid earthquake simulations

[Oliver Boyd]

Earthquake simulations in the New Madrid Seismic Zone: The 1811-1812
Bicentennial


http://geology.usgs.gov/postdoc/2009/research.html

Opportunity #17

 

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 at usgs.gov;
Steve Hartzell, (303) 273-8572, shartzell at usgs.gov; Eugene Schweig, (901)
678-4974, schweig at usgs.gov; Oliver Boyd, (901) 678-3463, olboyd at usgs.gov

 

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

 

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