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Fundamentals of seismic tomography

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.

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