What are faults and why do they generate earthquakes

Energy released after movement along the fault plane is the cause of most earthquakes

by Lorenzo Ciotti
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What are faults and why do they generate earthquakes
© David McNew / Staff Getty Images

The fault is a fracture of rocks in the earth's crust, with relative movement between the two rock masses divided by it. The rocks near a fault are often intensely crushed and in this case we speak of fault rocks such as cataclasites or mylonites.

Energy released after movement along the fault plane is the cause of most earthquakes. The fault plane can form, with respect to the horizontal plane, strike-slip faults and strike-slip faults.

Faults and earthquakes

Often in nature it is possible to detect faults with complex movements resulting from both vertical and horizontal displacement.

The vertical and horizontal component of the dislocation, which can be obtained for example from stratigraphic markers, is called rejection. Strike-slip faults occur if the plane is vertical or oblique with relative horizontal displacement of the rock masses.

These faults are divided into right-handed or left-handed; the direction can be determined by placing yourself on the fault line and observing the direction of the dislocation of the rocky block in front of you. An example of this type of fault is the very famous San Andreas Fault.

Direct faults occur when the roof drops relative to the wall. In this case the sector of the earth's crust is affected by an extensional or divergent tectonic regime, such as for example during the opening of a rift.

San Andreas Fault© David Paul Morris / Stringer Getty Images

Reverse faults occur if the roof rises above the wall.

In this case the crust sector is shortened due to a compressive tectonic stress field. Their meaning is opposite to what appears simply by observing the movement of the various ridge sectors. The identification of active faults defines the seismic hazard of a region.

Seismogenic zones are characterized by tectonic lineaments whose movement determines releases of quantities of energy which propagate in the form of seismic waves. For a seismogenic area, the cataloging of historical earthquakes is fundamental in order to determine, on a probabilistic basis, the severity of the earthquakes that have occurred and the return time of a seismic event.

The definition of the seismic hazard of an area and the seismic risk associated with it is the basis of any microzonation study.