Dahlen Home

Tony Dahlen
Professor of Geosciences
(Seismology)

Department of Geosciences
Princeton University

Professor Dahlen passed away on June 3rd, 2007. A fund will be set up in his memory. Details will be posted in these pages soon. Check made out to the "Trustees of Princeton University", with "Dahlen fund" written in the memo line can be mailed to: Ms. Debbie Fahey, 114 Guyot Hall, Princeton University, Princeton NJ 08544.

My current research activities are currently focused in the broad area of theoretical global seismology.  In collaboration with Professor Guust Nolet and graduate students Adam Baig and Raffaella Montelli, I am developing a new formalism for the inversion of frequency-dependent body-wave travel times, measured by cross-correlation.  The widespread availability of broadband digital seismic data makes the compilation of such cross-correlation travel-time datasets a very attractive option.  Our theory pertains to both absolute travel times obtained by cross-correlation of an observed and spherical-Earth synthetic seismogram, and to relative travel times obtained by cross-correlation of two arrivals, such as SS and S, on the same seismogram.  The Fréchet or sensitivity kernels, which are based upon the Born approximation, will allow such data to be incorporated into large-scale 3-D tomographic inversions.  The sensitivity kernels for an absolute travel-time measurement are small except in the vicinity of the associated geometrical ray, as expected; however, they have the curious property that they are identically zero on the ray itself.  This is an inherent figure of the finite-frequency scattering and diffraction effects that are ignored in ray-theoretical travel-time tomography.  We have dubbed our new constructs which account for these off-ray effects banana-doughnut kernels.

More recently, we have begun to extend our Born kernel formalism to non-geometrical body-wave phases, including core diffractions, as well as to a variety of other functionals that can be measured by analysis of seismic waveforms, including surface-wave phases and both body-wave and surface-wave arrival angles and amplitudes. This work is being carried out in connection with Professor (and ex-Princeton postdoc) Shu-Huei Hung of National Taiwan University, and with Professor Guust Nolet and graduate student Ying Zhou.

In past seismological work, I have been engaged in the development of improved computational methodologies for computing synthetic long-period synthetic seismograms on a 3-D Earth model.  The most accurate but time-consuming methods are based upon summation of the Earth's normal modes; eigenfrequencies and eigenfunctions are computed using a Rayleigh-Ritz variational method that requires the diagonalization of very large matrices.  Faster methods based upon a numerically stable variant of classical perturbation theory and JWKB-Maslov methods involving dynamic surface-wave ray tracing have also been developed.  In addition, I have been involved in a systematic investigation of the duality between the Earth's normal modes and propagating SH and P-SV body waves.  This work will hopefully provide the basis for a much more efficient means of computing complete synthetic seismograms and waveform sensitivity kernels in the future.

I have also conducted tectonic modelling studies of the mechanics and thermodynamics of steady-state mountain building, in collaboration with Professor John Suppe.  The essential premise that an actively deforming brittle-frictional mountain belt is mechanically analogous to a critically tapered wedge of soil or snow in front of a moving bulldozer.  The measured heat flow and geochronological and petrological data have been used to infer the coefficient of friction on the basal decollement fault beneath the active Taiwan fold-and-thrust belt.