Seismic Shear Wave Propagation Animations

Created by Saadia A. Baqer and Michael E. Wysession
Department of Earth and Planetary Sciences, Washington University, St. Louis, Missouri

Funded by the National Science Foundation (DUE-9455417), and the David and Lucile Packard Foundation

Description

A series of animations is presented here, which show the way that seismic shear waves propagate through the mantle of the earth from an earthquake. Because of the complexity of the mantle's seismic structure, the initially spherical S wave front that leaves the earthquake rupture becomes broken up into many different segments that become the later shear wave arrivals such as sS, SS, ScS, sScS, etc. The manner in which this happens is fascinating, and is shown in a series of computer-generated animations. Red colors represent motions out of the screen, and blue colors represent motions into the screen. The intensities of the colors are proportional to the amplitude raised to a power of 0.8, to improve visibility.

As a comparison to the wave animations, we also show the suite of shear waves in their more traditional manner of representation, which is done using geometric ray-tracing. Each method has it's separate advantages. The waveform animation gives an intuitive sense of the true wave propagation, including relative wave amplitudes. The ray-tracing, however, is helpful in following individual packets of energy, and helps in understanding the generation of travel time curves, which show the arrival times of phase as functions of distance traveled.

Method

The animations are done through the analysis of large numbers of synthetic seismograms created using normal mode summation. For all torsional modes (28,585) of free Earth oscillation with periods of 12 seconds or above, the eigenfunctional responses are computed for depths ranging from the surface to the core-mantle boundary. For a given strike, dip, slip, earthquake depth and azimuth, displacement seismograms (72,846) are computed along a two-dimensional grid of points within the mantle. The grid is then smoothed in a series of time slices that are combined using Macromedia Director ©. Individual seismograms that would be recorded at the surface at distances of 30, 60, and 90 are included for reference.

Wave Propagation Animations

Homogeneous Mantle Model ( 13.1MB Feb1 11) - This movie presents the seismic waves propagating through a homogeneous mantle with a constant shear velocity of 7 km/sec. The earthquake depth is 600 km and the focal mechanism has a dip of 45 degrees, a slip of 0 degrees, and strike perpendicular to the direction of propagation.
PREM model ( 15.1MB Feb1 11) - In this case we used the same focal mechanism and depth (600 km) as the homogeneous model, changing only the velocity of the model. Here we used the Preliminary Reference Earth Model (PREM).
PREM: 300 km depth source ( 9.4MB Feb4 11) - Here the depth of the earthquake is changes to 300 km only, still using the PREM velocity model.
PREM: 20 km depth source ( 7.4MB Feb1 11) - In this case, we changed the earthquake depth to 20 km, still using the PREM velocity model.
Azimuthal Variation ( 14.6MB Feb1 11) - In this case, we used the 600 km focal depth and the PREM velocity model. However, we change the azimuthal direction of wave propagation. We used 100 degrees instead of 280 degrees. It shows how the waveform changes with azimuth.

Ray Path Animations

Mantle SH-wave ray tracing: S, SS, SSS ( 4.4MB Feb1 11) - This is an example for the S, SS and SSS ray-tracing. A time travel plot shows the arrival times of the waves as a function of the distances traveled.
ScS and Sdiff ( 2.1MB Feb1 11) - This is an example for the ScS and S-diffracted waves with their travel time plot.
sS+sSS ( 3.3MB Feb4 11) - An example of sS and sSS waves with their travel time plot.
sScS+sSd ( 2.9MB Feb4 11) - An example of sScS and Ss-diffracted waves with their travel time plot.
Many S phases ( 7.7MB Feb1 11) - Here we present the ray-tracing for a selection of the waves shown in earlier cases, using the PREM velocity model. Only a selection of take-off angles are used.

Related References

• Baqer, S. A. and M. E. Wysession, Visualizing Seismic Wave Propagation, IRIS workshop, 1999. (Abstract)
• Wysession M. E. and Shore P. J. (1994), Visualization of Whole Mantle Propagation of Seismic Shear Energy Using Normal Mode Summation, Pure and appl. geophys. vol. 142, pp.295-310.