Shear wave splitting
These measurements reflect the degree of anisotropy and lead to a better understanding of the area's crack density and orientation or crystal alignment.
The time delays observed between the slow and fast shear waves give information about the density of cracks in the medium.
When plotted using polarization diagrams, the arrival of split shear waves can be identified by the abrupt changes in direction of the particle motion (Fig.3).
This area was chosen for this study because oceanic basins are made of large, relatively uniform homogeneous rocks.
Hess found that the slow compressional waves propagated perpendicular to the plane of slip and the higher velocity component was parallel to it.
This study focused on shear wave splitting recorded near the Chubu Volcanic Area in Japan.
They noted the arrivals of two distinct shear waves with different polarizations (N-S, fast and E-W, slow) approximately 0.7 seconds apart.
It was concluded that the splitting was not caused by the earthquake source but by the travel path of the waves on the way to the seismometers.
This theory is based on the fact that microcracks between the grains or crystals in rocks will open wider than normal at high stress levels.
This phenomenon of cracks opening and closing in response to changing stress conditions is called dilatancy.
The correct identification of body and surface wave polarizations is the key to determining the degree of anisotropy.
[7] The difference in the travel velocities of the two shear waves can be explained by comparing their polarizations with the dominant direction of anisotropy in the area.
The interactions between the tiny particles that make up solids and liquids can be used as an analogue for the way a wave travels through a medium.
In a liquid, the particles are much less tightly bound and it generally takes a longer time for the energy to be transmitted.
If there is a shear wave that is polarized perpendicular to the liquid-filled cracks or elongated olivine crystals present in the medium, then it would act upon these particles like those that make up a liquid or gas.
[7] Mathematical Explanation (Ray theory)[8] The equation of motion in rectangular Cartesian coordinates can be written as where t is the time,
There is one incident shear wave (blue) traveling vertically along the center grey axis through an isotropic medium (green).
To date, this is the best method to gain in situ information about the fracture network present in a hydrocarbon reservoir.
[10] The best production in a field is associated with an area where there are multiple small fractures that are open, allowing for constant flow of the hydrocarbons.
Based on this information, an alert was sent to the Iceland Meteorological Office (IMO) on October 27 and 29, warning of an approaching earthquake.
This technique was not successful again until 2008 due to the lack of appropriate source-geophone-earthquake geometry needed to evaluate changes in shear wave splitting signatures and time delays.
More observations are needed to confirm whether the increase and decrease time delay pattern is universal for all volcanic eruptions or if each area is different.
[15] Shear wave splitting is very sensitive to fine changes in the pore pressure in the Earth's crust.
[7] Shear wave splitting may be caused by more than just one layer that is anisotropic and located anywhere between the source and the receiver station.
[17] Due to the nature of split shear waves, when they are recorded in typical three-component seismograms, they write very complicated signatures.
[7] Shear waves that propagate along the ray path at a group velocity have polarizations that are only orthogonal in a few specific directions.
Polarizations of body waves are orthogonal in all phase velocity directions, however this type of propagation is generally very difficult to observe or record.
Because recorded shear wave time delays and polarizations vary in their incidence angle and azimuth of radio propagation, stacking these arrivals will degrade the signal and decrease the signal to noise ratio, resulting in a plot that is noisy and hard to interpret at best.
As our knowledge improves in this area, there will invariably be better ways of recording and interpreting these measurements and more opportunities to use the data.
With better equipment and more densely spaced recording stations, we have been able to study the signature variations of shear wave splitting over earthquakes in different regions.
