The distribution of earthquakes is closely related to plate boundaries as they are triggered by the motion of the Earth’s crust when the plates move either towards each other, away from each other or slide past each other.
Divergent Plate Boundaries
At divergent plate boundaries, the magma upwells through the opening between plates. This boundary mostly marks the mid-oceanic ridge and may be an active or extinct spreading ridge. The basaltic lava spews out from the ocean floor and produces a line of volcanic vents. In these spreading centres, shallow-focus earthquakes are common. The rupture that produces the earthquakes is up to 70 kilometres in depth.
Earthquakes are common in the Continental rift valley, with divergent boundaries developing within a continent. One such example is the Rift Valley of Africa. As the Arabian plate moves away from the African plate formation of the Red Sea and the Gulf of Eden is taking place. This 3000 kilometres long East African Rift System extends from the Afar triple junction to Mozambique. Scientists believe this continental rift valley will develop into oceanic spreading centres like the mid-oceanic ridge.
Transform Plate Boundaries
There is a great deal of seismic activity when the lithospheric plates slide past each other laterally across vertical fractures commonly referred to as transform faults. This zone is famous for the occurrence of shallow-focus earthquakes.
The transform fault mostly occurs along short offsets related to slight bends in the mid-oceanic ridge system. But in some cases, such as San Andreas Fault in California, the transform fault extends through the continental lithosphere.
On April 18, 1906, an earthquake of magnitude 7.9 struck California. Other examples of transform faults in the continental lithosphere are the Alpine Fault in New Zealand, the Queen Charlotte Fault in North America and the Anatolian Fault in Turkey, which witness several shallow focus earthquakes. An earthquake of magnitude 7.8 struck Canada along the Queen Charlotte fault in 2012.
Convergent Plate Boundaries
The lithospheric plates collide as they move towards each other, forming convergent boundaries. These are also called destructive margins, as here, there is compression or removal of the earth’s crust.
Convergent boundaries can be of three types:
- Oceanic-Continental Convergence
- Oceanic-Oceanic Convergence and
- Continental-Continental Convergence
In oceanic-continental and oceanic-oceanic convergence, subduction of the denser stratum takes place with a great deal of seismic activity.
In oceanic-continental convergence, the oceanic crust being dense, subducts under the continental plate leading to shallow focus earthquakes at the trench that is formed and deep focus earthquakes at more than 700 kilometres occur as the subducting plate descends into the asthenosphere. These earthquakes are produced by slipping on the subduction thrust fault as the oceanic plate is pushed into the mantle. This zone is also referred to as Benioff Zone.
The above Map shows the concentration of shallow focus, intermediate focus and deep focus earthquakes along the circum-pacific belt, also referred to as the Ring of Fire. The Pacific plate subducts into the North American plate, forming the Aleutian trench – where earthquakes are a common feature. The Peru-Chile trench is created as the Nazca plate subducts beneath the South American plate.
In the Indian Ocean, the Tsunami of 2004 was triggered by an earthquake in Sumatra, Indonesia of a magnitude of 9.1, along the megathrust where the Indo-Australian Plate subducts under the Burma Plate, which is considered part of the Eurasian plate. Large subduction zone earthquakes have continued to occur along this belt – Nias Island earthquake of magnitude 8.6 in 2005, 8.5 and 7.9 magnitude earthquakes in 2007 and a 7.8 magnitude earthquake on a portion of megathrust west of the Mentawai Islands in 2010.
In the case of oceanic-oceanic convergence, one oceanic plate subducts under the other, leading to the formation of oceanic trenches. The earthquakes, both shallow and deep focus, occur during this convergence. The Mariana Trench is the result of the convergence of the fast-moving Pacific plate against the slow-moving Philippine plate, which is also a zone of earthquakes.
The continental-continental convergence has no subduction due to the buoyancy of the continental crust, but shallow-focus earthquakes are quite common. The formation of huge mountain ranges, such as the Alps and the Himalayas, is attributed to this type of convergence. Earthquakes in the Himalayas are common as the Indian plate, and the Eurasian plates continue to converge at a relative rate of 40-50mm/year.
In Nepal on 25th April 2015, a shallow focus earthquake of magnitude of 7.8 occurred that devastated the small Himalayan nation. The epicentre was located 36 kilometres East of Khudi in Nepal and was followed by aftershocks of magnitude 6.1 and 6.6 on April 25th and 6.7 on April 26th 2017. In 2016, on 3rd January, a 6.7 magnitude earthquake occurred 30 kilometres East of Imphal in India.
Read More in Geomorphology
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- Epeirogenic Earth Movements
- Orogenic Earth Movements
- Cymatogenic Earth Movements
- Concept of Stress and Strain in Rocks
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- Fault in Geography
- Mountain Building Process
- Morphogenetic Regions
- Isostasy: Concept of Airy, Pratt, Hayford & Bowie and Jolly
- Continental Drift Theory of Alfred Lothar Wegener (1912)
- Plate Tectonics: Assumptions, Evidences, Plate Boundaries and Features Formed
- Volcanoes: Process, Products, Types, Landforms and Distribution
- Earthquakes: Processes, Causes and Measurement
- Plate Tectonics and Earthquakes
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- Artificial Sources to Study Earth’s Interior
- Natural Sources to Study Earth’s Interior
- Internal Structure of Earth
- Chemical Composition and Layering of Earth
- Weathering: Definition and Types
- Mass Wasting: Concept, Factors and Types
- Models of Slope Development: Davis, Penck, King, Wood and Strahler
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- Development of Slope
- Elements of Slope
- Interruptions to Normal Cycle of Erosion
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- River Capture or Stream Capture
- Stream Channel Pattern
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