Earthquakes – Types and Classifications

World Tectonic Plates[3]
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The world’s earthquakes are not randomly distributed over the Earth’s surface. They tend to be concentrated in narrow zones and an explanation is to be found in plate tectonics.

Plate tectonics tells us that the Earth’s rigid outer shell (lithosphere) is broken into a mosaic of oceanic and continental plates which can slide over the plastic aesthenosphere, which is the uppermost layer of the mantle. The plates are in constant motion. Where they interact, along their margins, important geological processes take place, such as the formation of mountain belts, earthquakes, and volcanoes.[1]

A tectonic plate (also called lithospheric plate) is a massive, irregularly shaped slab of solid rock, generally composed of both continental and oceanic lithosphere. Plate size can vary greatly, from a few hundred to thousands of kilometers across; the Pacific and Antarctic Plates are among the largest. Plate thickness also varies greatly, ranging from less than 15 km for young oceanic lithosphere to about 200 km or more for ancient continental lithosphere (for example, the interior parts of North and South America).

Tectonic plates probably developed very early in the Earth’s 4.6-billion-year history, and they have been drifting about on the surface ever since-like slow-moving bumper cars repeatedly clustering together and then separating. Earthquake and volcanic activity is concentrated near these boundaries. The lithosphere is broken up into tectonic plates.[2]

There are three basic things that can happen where the edge of one plate meets another. The three types of plate boundaries are:[4]

Divergent Plate Boundary[5]

Divergent Plate Boundaries
If plates move apart, they form a divergent plate boundary. Almost all the Earth's new crust forms at divergent boundaries, but most are not well known because they lie deep beneath the oceans. These are zones where two plates move away from each other, allowing magma from the mantle to rise up and solidify as new crust.



Convergent Plate Boundary[6]

Convergent Plate Boundaries
Plates can push against each other, producing a convergent plate boundary. In this case, one plate is pulled beneath another (subduction), forming a deep trench. The long, narrow zone where the two plates meet is called a subduction zone.

Transform Plate Boundary[7]

Transform Plate Boundaries
When plates slip past each other, geologist refer to them as transform plate boundaries. All transform plate boundaries plates grind past each other side by side. This type of boundary separates the North American plate from the Pacific plate along the San Andreas fault, a famous transform plate boundary that's responsible for many of California's earthquakes.









Fault Types

A fault is a fracture or zone of fractures between two blocks of rock and allow them to move relative to each other. This movement may occur rapidly, in the form of an earthquake – or may occur slowly, in the form of creep. Faults may range in length from a few millimeters to thousands of kilometers. Most faults produce repeated displacements over geologic time.[8]

Fault creep is a slow, more or less continuous movement occurring on faults due to ongoing tectonic deformation. Faults that are creeping do not tend to have large earthquakes.[9]

Earth scientists use the angle of the fault with respect to the surface (known as the dip) and the direction of slip along the fault to classify faults. The following are the types of faults:[10]

Strike-Slip Fault[11]

Strike-Slip Fault
A strike-slip fault is a fault on which the two blocks slide past one another. The San Andreas Fault is an example of a right lateral fault.



Normal Fault[12]

Normal Fault
A normal fault is a dip-slip fault in which the block above the fault has moved downward relative to the block below. This type of faulting occurs in response to extension and is often observed in the Western United States Basin and Range Province and along oceanic ridge systems.

Thrust Fault[13]

Thrust Fault
A thrust fault is a dip-slip fault in which the upper block, above the fault plane, moves up and over the lower block. This type of faulting is common in areas of compression, such as regions where one plate is being subducted under another as in Japan.




  1. USGS – Earthquake and Plate Tectonics:
  2. USGS – What is a tectonic plate?:
  3. Image Source: [Accessed: January 14, 2014]
  4. USGS – The action is a the edges!:
  5. Image Source: [Accessed: January 14, 2014]
  6. Image Source: [Accessed: January 14, 2014]
  7. Image Source: [Accessed: January 14, 2014]
  8. USGS FAQs:
  9. USGS Earthquake Glossary – creep:
  10. USGS FAQs:
  11. Image Source: [Accessed: January 14, 2014]
  12. Image Source: [Accessed: January 14, 2014]
  13. Image Source: [Accessed: January 14, 2014]