Jose Pujol, PhD
Department of Earth Sciences
The University of Memphis
October 3, 2007
4:00 p.m. in Manning Hall room 201
At a global scale the standard earth models assume spherical symmetry. In other words,
the earth properties (such as pressure, temperature, wave velocities, etc.) are a
function of radial distance. At a local scale (surface distances of up to about 200
km) a Cartesian coordinate system is generally used and the typical earth models are
based on horizontal homogeneous layers. Although these models are highly valuable,
they generally do not represent the actual variations of earth properties, which usually
follow 3-D patterns.
Seismic tomography refers to a number of techniques designed to determine some of
these variations using arrival times and/or waveforms from natural and artificial
sources. The most common product of a tomographic study is a velocity model, although
other parameters, such as attenuation, are also studied. The importance of seismic
tomography stems from two facts. One, it generally has higher resolution than that
provided by other geophysical methods. Two, it provides information that (a) can help
solve fundamental problems concerning the internal structure of the earth at a global
scale, and (b) has been used in tectonic and seismic hazards studies at a local scale.
In the most general terms, seismic tomography problems are inverse problems, and
before the word “tomography” entered the seismological literature the term inversion
was used. The most common form of tomography is based on the use of wave arrival times
and will be addressed in this talk, but regardless of the data used, seismic tomography
involves the solution of a linear system of equations that generally is ill-posed.
To solve this type of system two approaches are used, a regularization approach and
a Bayesian approach. They will be discussed here, as well as applications to an area
near Los Angeles, California, and Taiwan.