Landforms Occurrence and Causes of Geomorphic Hazard – UGC NET – Notes

Earthquakes

Earthquakes can be categorized based on their origin and the geological activity that causes them. Mainly there are four types of earthquakes Tectonic Earthquakes, Volcanic Earthquakes, Collapse Earthquakes, Explosion Earthquakes, etc.

What is an Earthquake?

• An earthquake is the shaking of the surface of the Earth resulting from a sudden release of energy in the Earth’s lithosphere that creates seismic waves.
• Earthquakes can range in size from those that are so weak that they cannot be felt to those violent enough to toss people around and destroy whole cities.
• Seismicity, or seismic activity, of an area, refers to the frequency, type, and size of earthquakes experienced over a period of time. The word tremor is also used for non-earthquake seismic rumbling.
• Cause of earthquakes is often due to the movement of tectonic plates beneath the Earth’s surface.
• When these plates move past each other, they sometimes get stuck at their edges due to friction.
• When the stress on the edge overcomes the friction, there is an earthquake that releases energy in the form of seismic waves, which causes the ground to shake.
• The location below the Earth’s surface where the earthquake starts, is called the hypocenter, and the location directly above it on the surface is called the epicenter.
• Earthquakes can occur anywhere, but their most common sites are along tectonic plate boundaries.

Types of Earthquakes

On the Basis of Causative Factors:

Natural earthquakes are those which are caused by natural processes i.e., due to endogenic processes. These are further subdivided into the following sub-categories:

1) Tectonic Earthquakes

• They are caused by to dislocation of rock blocks during faulting activities.
• A tectonic earthquake occurs when two tectonic plates collide at a point called the boundary.
• A convergent plate boundary is formed when two plates push into each other.
• The oceanic Nazca Plate, for example, pushes into and is subducted into the South American Plate off the coast of South America along the Peru-Chile trench.
• The Andes mountains are formed as a result of this action, which rises the South American Plate.
• The Nazca Plate is broken into smaller pieces that are stuck in place for lengthy periods of time before shifting suddenly and causing earthquakes.
• The Mid-Atlantic Ridge, which stretches from the Arctic Ocean to beyond the southern tip of Africa, is an example of a divergent boundary, which happens when two plates shift away from each other, forming a new crust.
• It has produced thousands of kilometers of plate movement over millions of years.
• When plates slide horizontally past one another, they generate a transform boundary that neither destroys nor produces crust.
• Plate movement results in zigzagging plate edges and shallow earthquakes.
• These are also generated due to the sliding of rocks along a fault plane. Example: 2001 earthquake of Gujarat.

2) Volcanic Earthquakes

• Volcano earthquakes are caused by stress changes in the solid rock caused by the injection or withdrawal of magma (molten rock).
• They are caused due to volcanic eruptions of explosive and fissure types.
• The intensity and magnitude of the earthquake depend on the intensity and magnitude of volcanic eruptions.
• We can’t predict when the volcano will erupt, but it might happen at any time. Example: severe earthquakes caused by Karakatao volcano 1883.

3) Isostatic Earthquakes

• Isostatic or isostasy-related earthquakes are those associated with the buoyant behavior of the Earth’s crust as it undergoes vertical movements due to changes in surface loads.
• Isostasy refers to the equilibrium that exists between the Earth’s lithosphere (the crust and uppermost mantle) and the more fluid asthenosphere beneath it.
• The lithosphere essentially “floats” on the asthenosphere in a manner similar to how an iceberg floats on water.

• They are triggered due to sudden disturbances in isostatic balance at a regional scale.
• If a significant weight, such as a massive ice sheet, is added to or removed from the crust, the crust will respond by sinking or rebounding, respectively.
• A classic example of isostatic adjustment is seen in areas that were once covered by large ice sheets during the last Ice Age.
  • As these ice sheets melted, the previously compressed crust began to rise or “rebound” back to its original position.
  • This process, which can take thousands of years, can cause earthquakes, although they are typically of lower magnitude.
  • This rebound is often referred to as post-glacial or glacio-isostatic rebound.
• Mountains, after being built up due to tectonic forces, can undergo erosion over millions of years. As they erode, the crust can adjust isostatically, leading to earthquakes.

4) Plutonic Earthquakes

• They are deep-focus earthquakes whose centers lie at depths from 240 km to 600 km.
• They occur almost exclusively at convergent boundaries in association with subducted oceanic lithosphere.
• Herbert Hall Turner initially brought preliminary evidence for the presence of deep-focus earthquakes to the attention of the scientific community in 1922.
• Kiyoo Wadati demonstrated the existence of earthquakes deep into the lithosphere in 1928, debunking the myth that earthquakes only occur at shallow focal depths.
• Deep-focus earthquakes produce very few surface waves. Because of their focal depth, earthquakes are less likely to create a seismic wave motion with concentrated energy at the surface.
• Deep-focus earthquake seismic waves only pass through the heterogeneous upper mantle and highly changeable crust once on their way from the focus to the recording station.
• As a result, compared to seismic waves from shallow earthquakes, body waves experience less attenuation and reverberation, resulting in strong body wave peaks.

5) Artificial or Man Induced Earthquakes

• They are caused by anthropogenic activities such as underground explosions, mining, large reservoir-induced pressure, etc.
• Example: The enormous load exerted by dam reservoirs resulted in the strongest ever induced earthquake.
• The most notable fluid-induced earthquake in India happened in 1967 near Koyna, Maharashtra, and was ascribed to seismic activity caused by the Koyna dam’s impoundment.
• Earthquakes in Oklahoma’s tectonically quiet region have also been connected to oil and gas drilling activity.
• Such areas of man-made earthquake activity are thought to exceed the degree of seismic activity in hotspots like southern California.

On the Basis of the Focus of an Earthquake:

1) Shallow Focus Earthquake

• It has its seismic foci located at depths from the ground surface to 70 km.
• The majority of earthquakes have a shallow focal point. As a result, they’re also known as ‘crustal earthquakes.’
• Smaller earthquakes make up the majority of shallow-focus earthquakes (usual range of 1 to 5). However, a few are of greater magnitude and can create widespread devastation.
• They happen frequently and seemingly at random. However, because the majority of them are of minor magnitude or occur along submarine ridges, they are rarely felt.
• Shallow focus earthquakes are particularly common at divergent boundaries, such as mid-ocean ridges, and transform boundaries, like the San Andreas Fault in California.

2) Intermediate Focus Earthquake

• Intermediate focus earthquakes are seismic events that originate at depths ranging from 70 km to 300 km below the Earth’s surface.
• These earthquakes predominantly occur in subduction zones where one tectonic plate is being forced under another. As the subducting plate descends into the mantle, it can cause earthquakes at varying depths, including the intermediate range.
• While shallow focus earthquakes are the most common, intermediate focus events also occur frequently, especially in regions with active subduction zones.
• Examples include the Pacific “Ring of Fire”, which encircles the Pacific Ocean and features many subduction zones.
• The intensity of shaking at the Earth’s surface from an intermediate focus earthquake can be less than that of a similarly sized shallow focus earthquake.
• This is because the seismic waves have a longer distance to travel before reaching the surface, which can cause them to dissipate some of their energy.
• Examples: The 2011 earthquake off the Pacific coast of Tōhoku, Japan, which led to the devastating tsunami and Fukushima Daiichi nuclear disaster, was an intermediate focus earthquake. Another example is the 1976 Tangshan earthquake in China.

3) Deep Focus Earthquakes

• Deep focus earthquakes are seismic events that originate at depths ranging from 300 km to about 700 km beneath the Earth’s surface. It is also known as Wadati–Benioff Zone.
• They stand out because, at such profound depths, the Earth’s materials are expected to deform ductilely rather than fracture and slip, which is how earthquakes typically occur.
• Like intermediate focus earthquakes, deep focus earthquakes predominantly occur in subduction zones, where one tectonic plate is diving beneath another into the mantle.
• The occurrence of earthquakes at these depths is somewhat enigmatic because temperatures and pressures are so high that rocks are expected to deform plastically rather than breaking in a brittle fashion.
• Several mechanisms, such as phase transitions (changes in the crystal structure of minerals), have been proposed to explain these events.
• Deep focus earthquakes are particularly common in a few subduction zones around the Pacific “Ring of Fire.”
• Regions such as beneath the Fiji-Tonga area in the South Pacific and beneath the Sea of Okhotsk off the Russian Far East are known for these types of earthquakes.
• Like intermediate focus earthquakes, the intensity of shaking from a deep focus earthquake at the Earth’s surface can be less than that of a similarly sized shallow focus earthquake. They can, however, still be of high magnitude and potentially damaging.
• Examples: One of the most significant deep focus earthquakes in recorded history was the 2013 Sea of Okhotsk earthquake, with a depth of approximately 609 km and a magnitude of 8.3.
• Because deep focus earthquakes occur at depths inaccessible for direct study, their investigation offers valuable insights into the Earth’s deep interior, the properties of materials under extreme conditions, and the complex interactions in subduction zones.