Lab 5 Course Material
- These animations were created by Saadia A. Baker and Micheal E. Wysession, Department of Earth and Planetary Sciences, Washington University, St. Louis, Missouri.
- 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 its separate advantages. The waveform animation gives an intuitive sense of the true wave propagation, including the 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 travelled.
- 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 point; 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 o3: 0°, 60°, and 90° are included for reference.
- Wysession M. E. and Shore P. J. (1994), Visualization of Whol Mantle Propagation of Seismic Shear Energy Using Normal Mode Summation, Pure and appl. geophys. vol. 142, pp.295-310.
Homogeneous Earth Model
- This movie presents the seismic waves propagation through a homogeneous mantle with a constant shear velocity of7 km/sec. The earthquake depth is 600 km, and focal mechanism has a dip of 45°, a slip of O°. And strike perpendicular to the direction of propagation. We used a real earthquake sound, from the 1992 Landers earthquake in southern California, magnitude of 7.3. Here is a link to the file embedded below (mp4).
PREM Velocity Model
- In this case we used same focal mechanism and depth (600 km), changing only the velocity model. Here we used the Preliminary Reference Earth Model (PREM). Here is a link to the file embedded below (mp4).
PREM 300 km Depth Model
- Here the depth of the earthquake is changed to 300 km only, and still using PREM velocity model. Here is a link to the file embedded below (mp4).
PREM 20 km Depth Model
- In this case, we change the earthquake depth to 20 km, and still using PREM as a velocity model. Here is a link to the file embedded below (mp4).
Varying Azimuth Model
- This case, we used the same focal depth (600 km).and velocity model (PREM). However, we change the azimuthal direction of wave propagation. We used 100°, instead of a 280°. It shows how the shear waves waveform change with azimuth. Here is a link to the file embedded below (mp4).
Ray-Tracing- All Phases Model
- Here we present the ray-tracing for a selection of the waves shown in earlier cases, using the PREM velocity model. Only a sampling of take-off angle are used. Here is a link to the file embedded below (mp4).
- This is an example for the S, SS, and SSS ray-tracing. A travel time plot shows the arrival times of the waves as a function of the distances traveled. Here is a link to the file embedded below (mp4).
- This is an example for the ScS and S-diffracted waves with their travel time plot. Here is a link to the file embedded below (mp4).
- An example of sS and sSS waves with their travel time plot. Here is a link to the file embedded below (mp4).
- An example of sScS and sS-diffracted waves with their travel time plot. Here is a link to the file embedded below (mp4).
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