An element of rock under stress

Fault mechanics is a field of study that investigates the behavior of geologic faults. Old fault exposed by roadcut near Harrisburg, Pennsylvania. ...

Behind every good earthquake is some weak rock. Whether the rock remains weak becomes an important point in determining the potential for bigger earthquakes. Global earthquake epicenters, 1963–1998 An earthquake is a sudden and sometimes catastrophic movement of a part of the Earths surface. ...

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). 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. A band of weak, fractured rock in a competent mass, can deform to resemble a classic geologic fault. Using seismometers and earthquake location, the requisite pattern of micro-earthquakes can be observed. Figure 1  Stress tensor In physics, stress is the internal distribution of forces within a body that balance and react to the loads applied to it. ... Old fault exposed by roadcut near Harrisburg, Pennsylvania. ... Seismometer (in Greek seismos = earthquake and metero = measure) are used by seismologists to measure and record seismic waves. ... The primary purpose of a seismometer is to locate the initiating points of earthquake hypocenters. ...

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. Under a following force, the seismic displacements eventually form a topographic feature, such as a mountain range. An interplate earthquake is an earthquake that occurs at the boundary between two tectonic plates. ...

Following forces creating a mountain range

Intraplate earthquakes do not have a following force, and are not associated with mountain building. 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]. Although the theory of plate tectonics well describes the mechanisms for interplate earthquakes (earthquakes at plate boundaries), there is the fact that very large intraplate earthquakes (earthquake within plates) can inflict heavy damage on towns and cities. ...

In fact, the seismic zone (such as the New Madrid Fault Zone) is ensured eternal life, by the action of water. 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. Seismic map New Madrid Seismic Zone - USGS The New Madrid Seismic Zone, also known as the Reelfoot Rift or the New Madrid Fault Line, is a major seismic zone, located in the mideastern United States. ...

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. All small earthquake zones eventually want to grow up to be like New Madrid or Charlevoix .[4]. Permeability has several meanings: In electromagnetism, permeability is the degree of magnetisation of a material in response to a magnetic field. ...

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