4. Where do earthquakes occur ?

Earthquakes are strongly linked to the geological forces that build mountains and create oceans. Where plates meet each other, wide fault and fracture zones are concentrated. Earthquakes are thus usually concentrated along the edges of plates. Therefore, seismic risk mainly occurs in places where human settlements are located in:

  • Plate boundaries (convergent, transform and divergent, in order of expected seismic hazard)
  • Active volcanic areas, where the deformation of the volcano is responsible for low-energy/low-depth earthquakes usually threatening not very wide areas.
Distribution of earthquakes and volcanoes in the world. Credit: United States Geological Survey

Based on their geometry and on the field of forces responsible for plate movement, boundaries between plates can be schematically described by the following three simple characters:

  • Convergent boundaries (CB): plates collide so that subsequent compression buckles and deforms the Earth’s crust

Such a case is responsible for the creation of large volcanic arcs and mountain chains, depending mostly on the nature of the crust (continental and oceanic). CB are also described as “destructive” and are the seat of strong explosive volcanism and seismic activity.

  • Divergent boundaries (DB): plates move apart from each other.

New crust is created by the magma rising up through the fracture zones. Such boundaries are usually the place for quiet volcanism and seismic activity both in the oceanic spreading ridges and/or continental rift zones. As DB are the place of crust accretion, they are also named “constructive”.

  • Transform boundaries (TB): plates slide laterally along so-called transform faults.

Depending on the relative motion of the two plates, TB can then be named dextral or sinistral. As there is virtually no depletion or accretion of crust, such TB are also called “conservative”. They are mostly the seat of a large seismic activity. The San Andreas fault in California is the most known example of a transform boundary

Highly dangerous seismic zones are mostly seen in convergent and transform plate boundaries.

Types of Plate Boundaries. Divergent - Spreading Center - Constructive Margin. Characterized by ocean ridges and sea floor spreading. Convergent - Characterized by trenches and island arcs. Transform - Plates moving past one another along strike- slip faults. Source: http://geology.csupomona.edu

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