Fractures in rocks cause seismic anistropy Flashcards
What are characteristics of fractured rocks?
Fractured rocks are much less rigid perpendicular to the fractures than along them, because the gaps in between the fractures are filled with air or fluids.
How are seismic velocities affected by fractured rocks?
Along-fracture elastic moduli and seismic velocities are therefore larger than across-fracture
The amplitude of seismic waves wll also attenuate more across-fracture as lots of energy is used to compress the air or fluid gaps between the solid rock
What does more attenuation mean?
The more attenuation the lower seismic wave frequency will become, as discussed previously. This is because higher frequencies are attenuated first and lower frequencies are prevailed
What does attenuation and frequency depend on?
Both attenuation and frequency therefore depend on the number and size of fracture
What are low velocities due to?
Low velocities in central vent field due to greatly elevated temperature and fracturing
What are high velocities due to?
High velocity bands (and at depth) due to infilling of fractures by mineral precipitation from hydrothermal vents/activity
What are low velocities also due to?
Low velocities on outer flanks of topography highs due to lava extrusion and downward flow
What is this evidence for?
This is good evidence for hydrothermal activity closes cracks shut in between sheeted dykes through precipitation which gives the oceanic crust a lot of strength and sheeted dykes are cemented together
What creates abyssal plains?
Once material cools down, it contracts and becomes heavier and isostasy starts to pull it down
New lithosphere cools, contracts, and subsides away from the ridge
This is what creates the abyssal plain on either side of the mid-ocean ridge
What is a function of temperature?
Age vs. depth of sea floor is fundamentally a function of temperature
