20150509 - earthquake

Cause of Earthquakes

The Earth’s crust consists of several large lithospheric plates and numerous smaller fragments. These plates can converge, diverge or grind past each other (transform fault).

Earthquakes are caused by a sudden release of stress along faults in the crust. Due to continuous motion of tectonic plates, stress builds in the rock on both sides of a fault. When the stress exceeds the strength of the rock, it is released in a sudden, jerky movement. The released seismic energy propagates as waves through the earth and along its surface, and these waves cause the shaking that we feel.

The waves hit the surface first and most forceful at the epicenter, which explains why the damage is usually highest in this area. Displacement occurs at the fractures, which may be seen on the surface as well to a certain extent depending on the strength of the event.

Earthquakes which are caused by plate tectonics are called tectonic quakes. They account for most of the earthquakes worldwide and are in mostly concentrated at the margins of tectonic plates.

Volcanic quakes are associated with active volcanism. They are generally not as powerful as tectonic quakes and often occur close to the surface; thus they are felt only in close proximity to the hypocenter. They constitute only 3% of the total.

7% of all earthquakes are collapse quakes. These can be caused for instance by cave-in‘s, mostly in Karst areas („Karst quakes“) or close to mining facilities due to subsidence.

Induced quakes are caused by „direct outside influences“, such as subsurface explosions (e.g. nuclear tests), mining, oil and gas production, dam or tunnel construction. In geothermics such quakes are generally caused by an increase in fluid pressure. For the most part the seismic events caused by this are situated in two to five kilometers depth and are of low magnitude; they rarely cause any damage. 

 http://www.seismo.ethz.ch/eq_swiss/Ursache_Erdbeben/index_EN