Harvard Seismology: Surface Waves

We have automatically measured phase dispersion and arrival angles on seismograms from a database of over 100,000 digital records. The measurements are in the range 32s to 150s. This data set is inverted for global laterally heterogeneous phase velocity structure. We have developed theory for interpreting the measurements in terms of anisotropic phase velocity.

Active participants in this project are Göran Ekström, Jeroen Tromp, and Erik Larson.

Recent Developments

• We have calculated group velocity values from the phase dispersion curves at a number of periods, and then determined maps of global group velocity.
This maps are isotropic models, up to degree 40 spherical harmonics. We also compare this models to recent group velocity models of Eurasia in Ritzwoller and Levshin (1998)
• We have analyzed our dataset of arrival angle measurenments to determine misreported azimuths of seismometers worldwide.
• Our large data set of Rayleigh wave and Love wave phase measurements has been inverted for isotropic phase velocity maps, expanded in spherical harmonics to degree 40.
• An automated method for making measurements of arrival angle has been developed.
• We have developed a complete ray theory for anisotropic propagation of surface waves, including terms to predict phase, arrival angle, and amplitude.
• A comparison of ray perturbation theory to exact ray theory shows that perturbation theory is sufficiently accurate (for phase and arrival angle) for use in large scale inversions for anisotropic phase velocity structure.
• Maps of phase velocity which allow only isotropic structure can be biased, predicting small scale isotropic structure in place of true anisotropic structure.

Publications

• Surface-wave propagation on a rotating anisotropic earth (Tromp)
• Measurements and global models of surface wave propagation (Ekström, Tromp, Larson)
• Effects of slight anisotropy on surface waves (Larson, Tromp, Ekström)
• Global models of surface wave group velocity (Larson & Ekström)