Friday, July 6, 2007
Normal Faulting on Whidbey Island
Normal Faulting is "Normal"
Mathematically, an action is normal, when it is perpendicular to that seen previously. But for faulting, Normal not only is similar (a block falls perpendicularly to the earth's surface), but this is the NORMAL action. Most faulting in the interior is in this category- only at large tectonic events is there significant thrusting or REVERSE faulting.
The cliff walls at the beach in Bush Point Bay (between Lagoon Point and Bush Point S32 T30N R2E)show normal faulting, but the crack shown above could have such small displacement downward, that it would be classified as a FRACTURE. Notice how the vegetation has masked the appearance of the fracture as it continues to the upper left. This is also normal, since there would be percolation of water down along the fracture surface (the opposite of compression, where movement of fluids is reduced). This category of faulting occurs for the case of the crust (or shallow layers of the ground) being under tension (extension could also be the classification).
This is of major mechanical importance, since the earth's crust moves only in three ways or is in three categories:
1. Under EXTENSION, or being stretched or pulled apart (Normal);
2. Under COMPRESSION, as with crushing or shoving (Reverse, or Thrust); and
3. Under SHEAR, as two opposing forces moving alongside each other- TORSION, or twisting, could create a similar effect. Faulting in this category is hard to see, since vegetation or rubble on the surface obscures the line of shear (Lateral faulting).