Earthquakes: Causes & Distribution, Shallow & Deep Quakes

PMF IAS Environment

Earthquakes

  • An earthquake is shaking or trembling of the earth’s surface, caused by the seismic waves or earthquake waves that are generated due to a sudden movement (sudden release of energy) in the earth’s crust (shallow-focus earthquakes) or upper mantle (some shallow-focus and all intermediate and deep-focus earthquakes).
  • A seismograph, or seismometer, is an instrument used to detect and record earthquakes.

Focus and epicentre

  • The point where the energy is released is called the focus or the hypocentre of an earthquake.
  • The point on the surface directly above the focus is called epicentre (first surface point to experience the earthquake waves).
  • A line connecting all points on the surface where the intensity is the same is called an isoseismic line.

The focus of an Earthquake (Eround1, via Wikimedia Commons); Epicentre (AnsateSam, via Wikimedia Commons)

Foreshocks and aftershocks

  • Usually, a major or even moderate earthquake of shallow focus is followed by many lesser-size earthquakes known as aftershocks.
  • A mild earthquake preceding the violent shaking movement of an earthquake is known as a foreshock.

Swarms

  • Large numbers of small earthquakes may occur in a region for months without a major earthquake.
  • Such series of earthquakes are called earthquake swarms.
  • Earthquakes associated with volcanic activity often occur in swarms.
  • Earthquake swarms can serve as markers for the location of the flowing magma throughout the volcanoes.

Causes of Earthquakes

  • Fault Zones
  • Plate tectonics
  • Volcanic activity
  • Human Induced Earthquakes

Fault Zones

  • The immediate cause of most shallow earthquakes is the sudden release of stress along a fault rupture (crack) in the earth’s crust.
  • Sudden slipping of rock formations along fault rupture in the earth’s crust happens due to the constant change in volume and density of rocks due to intense temperature and pressure in the earth’s interior.

Deformation and Rupturing (Steven Earle)

  • The longer the length and the wider the width of the faulted area, the larger the resulting magnitude.
  • The longest earthquake ruptures along thrust faults (convergent boundary) are approximately 1,000 km.
  • The longest earthquake ruptures on strike-slip faults (transform fault) are about half to one third as long as the lengths along the thrust fault.
  • The fault ruptures along normal faults (divergent boundary) are shorter.

Plate tectonics

  • Slipping of land along the faultline along convergent, divergent and transform boundaries cause earthquakes.
  • Reverse faults (convergent boundary) are associated with the most powerful earthquakes, megathrust earthquakes, including almost all of those of magnitude 8 or more.
  • Megathrust earthquakes occur at subduction zones, where one tectonic plate is forced underneath another. E.g. 2004 Indian Ocean earthquake.
  • Strike-slip faults, particularly continental transforms, can produce major earthquakes up to about magnitude 8.
  • San Andreas Fault is a transform fault where Pacific plate and North American plate move horizontally relative to each other causing earthquakes along the fault lines.
  • Earthquakes associated with normal faults (divergent boundary) are generally less than magnitude 7.

Volcanic activity

  • Volcanic activity also can cause an earthquake, but the earthquakes of volcanic origin are generally less severe and more limited in extent than those caused by fracturing of the earth’s crust.
  • Earthquakes in volcanic regions are caused by the consequent release of elastic strain energy both by tectonic faults and the movement of magma in volcanoes.
  • Such earthquakes can serve as an early warning of volcanic eruptions, as during the 1980 eruption of Mount St. Helens
  • There is a clear correspondence between the geographic distribution of volcanoes and major earthquakes, particularly in the Circum-Pacific Belt and along oceanic ridges.
  • Volcanic vents, however, are generally several hundred kilometres from the epicentres of most major shallow earthquakes, and many earthquake sources occur nowhere near active volcanoes.

Human Induced Earthquakes

  • Human Induced Earthquakes refers to typically minor earthquakes and tremors that are caused by human activity like mining, large scale petroleum extraction, artificial lakes (reservoirs), nuclear tests etc.

Reservoir-induced seismicity

  • The pressure offered by a column of water in a large and deep artificial lake alter stresses along an existing fault or fracture. Also, the percolation of water weakens the soil structure and lubricates the faults.
  • Loading and unloading of water can significantly change the stress. This significant change in stress can lead to a sudden movement along the fault or fracture, resulting in an earthquake.
  • The 6.3 magnitude 1967 Koynanagar earthquake occurred near the Koyna Dam reservoir in Maharashtra and claimed more than 150 lives. There have been several earthquakes of smaller magnitude since then.
  • Some geologists believe that the earthquake was due to reservoir-triggered seismic activity.
  • The 2008 Sichuan earthquake, which caused approximately 68,000 deaths, is another possible example. It is believed that the construction and filling of the Zipingpu Dam may have triggered the earthquake.

Earthquakes based on the depth of focus

  • Earthquakes can occur anywhere between the Earth’s surface and about 700 kilometres below the surface.
  • For scientific purposes, this earthquake depth range of 0 – 700 km is divided into three zones: shallow, intermediate, and deep.
  • Shallow focus earthquakes are found within the earth’s outer crustal layer, while deep focus earthquakes occur within the deeper subduction zones of the earth.
  • Shallow earthquakes are 0 – 70 km deep.
  • Intermediate earthquakes are 70 – 300 km deep.
  • Deep earthquakes are 300 – 700 km deep.
  • Of the total energy released in earthquakes, about 12-15 per cent comes from intermediate earthquakes, about 3-5 per cent from deeper earthquakes and about 70-85 per cent from the shallow earthquakes.
  • A quake’s destructive force depends not only on the energy released but also on location, distance from the epicentre and depth.
  • On 24 August 2016, a 6.2 earthquake rocked Central Italy killing about 300 people. An even bigger 6.8 hit Myanmar the same day killing just a few people.
  • Italy’s quake was very shallow, originating within 10 kilometres underground. By contrast, the quake in Myanmar was deeper ― 84 kilometres.

Shallow-focus earthquake

  • The great majority of earthquakes have shallow-focus. Hence, they are also called as ‘crustal earthquakes.’
  • Majority of the shallow focus earthquakes are of smaller magnitudes (usual range of 1 to 5). But a few can be of a higher magnitude and can cause a great deal of destruction.
  • They occur quite frequently and at random. However, as most of them are either of smaller magnitudes or occur along submarine ridges, they are often not felt.
  • Though comparatively of low magnitude, shallow focus earthquakes can cause relatively greater damage at the surface (as the whole energy is directed towards a small area) compared to their deep-focus counterparts.

Deep-focus earthquake

  • In general, the term “deep-focus earthquakes” is applied to earthquakes deeper than 70 km.
  • The deeper-focus earthquakes commonly occur in patterns called Benioff zones that dip into the Earth, indicating the presence of a subducting slab (zone of subduction).
  • Hence, they are also are known as intraplate earthquakes (triggered by the collision between plates).
  • They happen as huge quakes with larger magnitudes (usual range of 6 to 8), as a great deal of energy is released with the forceful collision of the plates.
  • But the earthquakes alone may not cause much destruction as the foci of the quakes lie at great depths and the energy of the quakes dissipates over a wide area.
  • The strongest deep-focus earthquake in seismic record was the magnitude 8.3 Okhotsk Sea earthquake that occurred at a depth of 609 km in 2013.
  • The deepest earthquake ever recorded was a 4.2 earthquake in Vanuatu at a depth of 735.8 km in 2004.

Wadati–Benioff zone: Earthquakes along the Convergent boundary

  • Wadati Benioff zone is a zone of subduction along which earthquakes are common. The most powerful earthquakes occur along this zone (most powerful earthquakes occur along the convergent boundary).
  • Differential motion along the zone produces numerous earthquakes, the foci of which may be as deep as about 700 kilometres.
  • Wadati–Benioff zones can be produced by slip along the subduction thrust fault (Himalayan Region – C-C convergent boundary) or slip on faults within the downgoing plate (O-O and C-O convergent boundary).

Distribution of Earthquakes

  • Earth’s major earthquakes occur mainly in belts coinciding with the margins of tectonic plates.
  • The most important earthquake belt is the Circum-Pacific Belt, which affects many populated coastal regions around the Pacific Ocean—for example, those of New Zealand, New Guinea, Japan, the Aleutian Islands, Alaska, and the western coasts of North and South America.
  • The seismic activity is by no means uniform throughout the belt, and there are many branches at various points.
  • Because at many places the Circum-Pacific Belt is associated with volcanic activity, it has been popularly dubbed the “Pacific Ring of Fire.”
  • The Pacific Ring of Fire accounts for about 68 per cent of all earthquakes.
  • A second belt, known as the Alpine Belt (Himalayas and Alps). The energy released in earthquakes from this belt is about 15 per cent of the world total.
  • The mid-world mountain belt (Alpine Belt) extends parallel to the equator from Mexico across the Atlantic Ocean, the Mediterranean Sea from Alpine-Caucasus ranges to the Caspian, Himalayan mountains and the adjoining lands.
  • There also are striking connected belts of seismic activity, mainly along oceanic ridges—including those in the Arctic Ocean, the Atlantic Ocean, and the western Indian Ocean—and along the rift valleys of East Africa.

Distribution of Earthquakes

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