When the earth quakes…

Through the movements in the inner mantels of the earth, the earth plates also move. First the plates only hook onto each other and over years and centuries nothing happens but the building up of high tension. At some point however, one of the plates jerks suddenly. This causes the rock masses to move to one another along a fracture zone and this in turn releases enormous amounts of energy. (Ill A.) The resulting sudden shaking is the earthquake.

The rock masses concerned can generally move by one another in three different modes, whereby combinations are possible (Ill. B):

• a) In a strike-slip fault, (or transform fault) two rock masses move by each other laterally.
• b) A thrust fault occurs through lateral pressure, whereby a rock mass shifts over another mass along the fracture zone. In this case, there is a compression of the earth’s crust.
• c) A gravity fault occurs through tension stress, whereby a rock mass on a fracture zone slides off relative to another rock mass. Here there is an expansion of the earth’s crust.

The place where the rock breaks in the underground and energy is released is called the hypocenter. From there the released shock spreads out as earthquake waves in the interior and on the earth’s surface. The point above the hypocenter on the earth’s surface is called the epicenter. (Ill. C)

An earthquake with a hypocenter not very deep is more severe at the surface than one that occurs deep in the earth’s crust. The earthquake waves come to the surface faster before the power of destruction has diminished,