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Earthquakes tend to occur where tectonic activities beneath the ground are very active. Since this site is an introduction to earthquake on an elementary scale, we do not focus on the mechanism of tectonic activities with detail here. In general, however, earthquakes occur in three main zones on the global scale:
An image of the world's plates and their boundaries. Notice that many plate boundaries do not coincide with coastlines.
The global distribution of earthquake prone areas is called seismicity. The most frequent and active tectonic activities take place along the belts of plate boundaries. Generally speaking, the earthquakes that occur along different plate boundaries all have some of their "unique" characteristics, this is due to the geological and geographic features of the places where they occur. The several types of plate boundaries are explained below.
Converging plate boundaries are where two plates run into each other. This results active compressional stresses and common reverse faults and thrust faults. Focus usually have an approx depth of 60km and the earthquakes usually have intermediate magnitudes.
Diverging plate boundaries are where two plates run away from each other. Tensional stress and normal faults tend to occur. Focus usually have an approx depth of 20km and the magnitude of the earthquakes are usually low.
Transform fault boundaries are where plates slide over horizontally with each other. Strike-slip faults tend to build up and the depth for the focus is approx 100km on average. Also, the magnitude of those earthquakes are usually large.
Collision boundaries are where two plates collide with each other, result a fold-thrust mountain belt. Earthquakes which are due to thrust faulting usually have a depth of 200km for the focus.
All of the characteristic plate boundaries that feature the occurrence of earthquakes are called "Interplate Earthquakes", since the earthquakes occur between plate boundaries. The other type of earthquake is called "Intraplate Earthquakes", where earthquakes occur in the interior of the continental plate.
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