Earthquake Geology And What Causes an Earthquake
A Little History about the Haitian Earthquake
As the world was reminded only too clearly in January 2010, an earthquake can be a devastating event. On the 12th of January 2010, the Caribbean Island of Haiti was struck by a magnitude 7 earthquake. The disaster struck at 16:53 local time, which was a blessing since people were out and about rather than in bed in their homes. The quake was centered approximately 10 miles off of the capital Port-au-Prince at a depth of approximately 6.2 miles beneath the earth’s surface. It was the worst quake that the island had suffered for 200 years and is believed to have caused a quarter of a million fatalities. Almost 250 000 buildings were destroyed or rendered unsafe by the earthquake and 1.5 million people were left homeless by it. This article looks at the earthquake geology and explains where the devastating, destructive power of this natural disaster comes from.
Tectonic Plates
Haiti sits in the boundary region between the Caribbean Plate and the North American Plate. The theory of plate tectonics developed in the 1960s and 1970s. According to the theory, which is well established, the outer layer of the earth, the lithosphere, is composed of seven major tectonic plates. These plates are known as: the African, North American, South American, Eurasian, Australian, Antarctic, and Pacific plates (there are some minor plates, including the Caribbean Plate).
The plates are solid, rigid masses which float on the molten inner layer of the earth, allowing them to move about. A plate may drift by as much as 2 to 10 centimeters per year. Although a velocity of 0.00001 km per annum may not sound very fast, when you imagine the mass associated with these plates and remember that the kinetic energy they hold is the product of half of the mass times the square of the velocity (KE = 1/2mv2), it is easy to understand the tremendous energy that plates contain. It has been calculated that the energy associated with an 8.6 magnitude earthquake is equivalent to 10,000 times the explosive force of the nuclear weapon which was dropped on Hiroshima.
The plates come into contact with each other at the plate boundaries. They can continue to come together or drift apart for millennial – indeed if you look at a map of the world, you can see that most of the continental masses now separated by large distances were once part of a single land mass known as Gondwana. Geologists believe that the separation of this super continent took place 130 million years ago.
Plate Interactions
In terms of earthquake geology, there are three modes of interaction of the tectonic plates. The plates can come together (convergent boundaries); drift apart (divergent boundaries); and slide past each other (transform boundaries).
Earthquakes can be caused as a result of one plate slipping under another; a process called subduction. The process can involve parts of a plate fracturing into smaller pieces which get stuck. Eventually, the forces associated with the plates break. The fractions free, resulting in the violent release of energy associated with an earthquake. These types of quake can cause the Earth’s crust to move up by several meters.
While a transform boundary involves two plates that are slipping passed one another, plate fragments can become trapped and eventually released by the mechanism described above. One of the most famous transform boundaries is responsible for the San Andreas Fault and California’s earthquake activity.
Earthquakes may also be associated with volcanic activity.
References
- United States Geological Survey: https://earthquake.usgs.gov/earthquakes/recenteqsww/Quakes/us2010rja6.php#summary
- Gology.Com Map of Tectonic Plates: https://geology.com/plate-tectonics.shtml
- Gondwana: https://www.palaeos.com/Earth/Geography/Gondwana.htm