An element of rock under stress

Fault mechanics is a field of study that investigates the behavior of geologic faults. In Geology a fault, or fault line, is a planar rock fracture which shows evidence of relative movement

Behind every good earthquake is some weak rock. An earthquake is the result of a sudden release of energy in the Earth 's crust that creates Seismic waves Earthquakes are recorded with a Seismometer Whether the rock remains weak becomes an important point in determining the potential for bigger earthquakes.

On a small scale, fractured rock behaves essentially the same throughout the world, in that the angle of friction is more or less uniform (see Fault friction). The angle of repose is an Engineering property of Granular materials The angle of repose is the maximum angle of a stable slope determined by friction cohesion Fault friction describes the relation of Friction to Fault mechanics. A small element of rock in a larger mass responds to stress changes in a well defined manner: if it is squeezed by differential stresses greater than its strength, it is capable of large deformations. Stress is a measure of the average amount of Force exerted per unit Area. A band of weak, fractured rock in a competent mass, can deform to resemble a classic geologic fault. In Geology a fault, or fault line, is a planar rock fracture which shows evidence of relative movement Using seismometers and earthquake location, the requisite pattern of micro-earthquakes can be observed. Seismometers (from Greek Seism - "the shakes" - and Metro - "I measure" are instruments that measure and record motions of the ground including The primary purpose of a Seismometer is to locate the initiating points of Earthquake Hypocenters The secondary purpose of determining the 'size' or Moment

Penny-shape cracks in rock

For earthquakes, it all starts with an embedded penny-shaped crack as first envisioned by Brune [1] . As illustrated, an earthquake zone may start as a single crack, and grow to form many individual cracks, and collections of cracks along a fault. The key to fault growth is the concept of a ‘following force’, as conveniently provided for interplate earthquakes, by the motion of tectonic plates. An interplate earthquake is an Earthquake that occurs at the boundary between two Tectonic plates If one plate is trying to move past the other they will be locked Under a following force, the seismic displacements eventually form a topographic feature, such as a mountain range.

Following forces creating a mountain range

Intraplate earthquakes do not have a following force, and are not associated with mountain building. An intraplate earthquake is an Earthquake that occurs in the interior of a Tectonic plate, whereas an Interplate earthquake is one that occurs at a plate Thus, there is the puzzling question of how long any interior active zone has to live. For, in a solid stressed plate, every seismic displacement acts to relieve (reduce) stress; the fault zone should come to equilibrium; and all seismic activity cease. You can see this type of arching ‘lockup’ in many natural processes [2].

In fact, the seismic zone (such as the New Madrid Fault Zone) is ensured eternal life, by the action of water. This article is about the seismic zone in southeastern Missouri As shown, if we add the equivalent of a giant funnel to the crack, it becomes the beneficiary of stress corrosion (the progressive weakening of the crack edge by water) [3] . If there is a continuing supply of new water, the system does not come to equilibrium, but continues to grow, ever relieving stress from a larger and larger volume.

Fresh water continually being injected

Thus, the prerequisite for a continuing seismically active interior zone is the presence of water, the ability of the water to get down to the fault source (high permeability), and the usual high horizontal interior stresses of the rock mass. Permeability in the Earth sciences (commonly symbolized as κ, or k) is a measure of the ability of a material (typically a rock or unconsolidated All small earthquake zones eventually want to grow up to be like New Madrid or Charlevoix . [4].

References

1. James N. Brune, Tectonic stress and the spectra of seismic shear waves from earthquakes, J. Geophys. Res. 75:4997-5009, 1970. , review written in 1987. http://www.garfield.library.upenn.edu/classics1987/A1987J040600001.pdf retrieved Aug. 01, 2005
2. Arches National Park, http://www.exploratorium.edu/ronh/adventure/arches.html retrieved Aug. 01,2005
3. Stress Corrosion Cracking of Rock in a Chemical Environment, http://www.nire.go.jp/annual/1998/28.htm retrieved Dec. 9, 2005
4. Maurice Lamontagne, last modified 2003-12-22, The Charlevoix-Kamouraska* Seismic Zone, Canada - Natural Resources, http://www.seismo.nrcan.gc.ca/historic_eq/charpage_e.php Retrieved Aug. 01,2005