Earthquakes and Volcanoes

The theory of tectonics can explain both earthquakes and volcanoes. The earth’s crust consists of a series of plates. There are seven main plates and lots of smaller ones. Some plates contain continental crust, while others are made mostly of oceanic crust.

Earthquakes

Convectional activity causes the plates to move. The sides of the plates are called plate margins. There are three sorts of plate margins. The plates move together at a destructive boundary, but the plates move apart at a constructive boundary. At a conservative boundary, the plates move side by side.
At a constructive boundary, molten rock or magma rises to the surface, forming a new crust. This forces the existing crust apart, causing seafloor spreading. This causes a geological phenomenon. At destructive margins, one plate is forced under another into the subduction zone.

Terminology Used in the Study of Earthquakes

• Earthquake intensity
• Earthquake magnitude
• Richter Scale
• Mercalli Scale
• Fault
• Focus
• Epicenter
• Seismic wave
• Seismograph

Seismic waves, as a result of plate movement, cause earthquakes. The main target of an earthquake may be a fault deep within the earth’s crust. The shock waves move out from the focus and reach the earth’s surface at the epicenter. Most earthquakes occur along plate margins.

The effect of an earthquake can be measured on the Richter or Mercalli scales. On a scale of 1 to 10, the Richter scale measures strength. An earthquake measuring seven on the Richter scale is 100 times stronger than one measuring five. The Mercalli scale measures
the physical effects of an earthquake on a scale of 1–12.
LEDCs suffer the greatest loss of life from earthquakes. This is because buildings aren’t as strong and emergency services aren’t as efficient. The economic cost of earthquakes is often greater in MEDCs because the economic life of an MEDC suffers greater disruption.
There are many attempts to reduce the consequences of earthquakes. More accurate forecasting of earthquakes allows earlier evacuation. The use of cross-bracing and the installation of rubber shock absorbers in foundations make buildings more shock resistant.

Focus and Epicenter

• The point within Earth where faulting begins is the focus or hypocenter.
• The point directly above the focus on the surface is the epicenter. The intensity of the earthquake is highest at the epicenter and decreases with distance from the epicenter.

Richter scale

• Richter magnitude scale is the scale to measure the magnitude of energy released by an earthquake.
• This scale was devised by Charles. F. Richter in the year 1935.
• The number indicating magnitude ranges between 0 to 9
• An earthquake that registers 5.0 on the Richter scale has a shaking amplitude 10 times that of an earthquake that registered 4.0, and thus corresponds to a release of energy 31.6 times that released by the lesser earthquake.

Mercalli scale

• The Mercalli intensity scale is a seismic scale used for measuring the intensity of an earthquake.
• It measures the effects of an earthquake
• The number indicating intensity ranges between  1 to 12

Seismic Waves

• Seismic waves are waves of energy caused by the sudden breaking of rock within the earth.
• They are the energy that travels through the earth and is recorded on seismographs.
• The two main types of waves are body waves and surface waves.

Body waves

• Primary waves ( P-waves)
• Secondary waves ( S-waves)

Surface Waves

• Love Waves (L-waves)
• Rayleigh waves

Primary waves (longitudinal wave)-

• The first kind of body wave is the P wave or primary wave.
• This is the fastest kind of seismic wave.
• The P wave can move through gaseous, solid rock and fluids, like water or the liquid layers of the earth.
• It pushes and pulls the rock, it moves through just like sound waves push and pull the air.

Secondary waves (transverse wave)

• The second type of body wave is the S wave or secondary wave.
• An S wave is slower than a P wave and can only move through solid rock.
• This wave moves rock up and down, or side-to-side.
• S-waves arrive at the surface with some time Lag.

Love Waves

• The first kind of surface wave is called a Love wave, named after A.E.H. Love, a British
  mathematician.
• It’s the fastest surface wave and moves the ground from side to side.

Rayleigh Waves

• The other kind of surface wave is the Rayleigh wave, named after Lord Rayleigh.
• A Rayleigh wave rolls along the ground just like a wave rolls across a lake or an ocean.
• Because it rolls, it moves the ground up and down and side-to-side in the same direction that the wave is moving.
• Most of the shaking felt from an earthquake is due to the Rayleigh wave, which can be much larger than the other waves.

Earthquake Predicting

Classification of earthquake

1. On basis of causative factors
   • Natural
     • Volcanic
     • Tectonic
     • Isostatic
     • Plutonic
   • Artificial
2. On basis of depth of focus
   • Moderate(0-50km)
   • Intermediate(50-250km)
   • Deep focus( 250-700km)
3. On basis of human casualties
   • Moderate (deaths<50,00)
   • Highly hazardous(51,000-1,00,00)
   • Most hazardous(>1,00,00)

World Distribution of Earthquakes

• The world’s distribution of earthquakes coincides very closely with that of volcanoes.
• Region of greatest seismicity is the Circum-Pacific areas, with the epicenters and the most frequent occurrences along the ‘Pacific Ring of Fire’.
• It is said that as many as 70% of earthquakes occur in the Circum-Pacific belt.
• Another 20% of earthquakes take place in the Mediterranean-Himalayan belt including Asia Minor, the Himalayas, and parts of north-west China.
• The remaining occur in the interiors of plates and on spreading ridge centers.

Earthquake Causes

Earthquakes are caused mainly due to dis-equilibrium in any part of the crust of the earth.

A number of causes have been assigned to cause dis-equilibrium or isostatic imbalance in the earth’s crust.

(a). Natural Reasons

• Volcanic eruption
• Faulting and folding
• Upwarping and down warping
• Gaseous expansion and contraction inside the earth.
• Plate Movement
• Landslides

(b). Man-made/Anthropogenic Reasons

• Deep underground mining
• Blasting of rock by dynamites for construction purposes.
• Deep underground tunnel
• Nuclear explosion
• Reservoir-Induced Seismicity (RIS) (E.g. Koyna Reservoir witnessed Earthquake in 1967 due to RIS)
• The hydrostatic pressure of man-made water bodies like reservoirs and lakes.

Plate tectonics provides the most logical explanation of volcanoes and earthquakes.

There are 3 types of plate boundaries along which earthquake occurs

1. Convergent
2. Divergent
3. Transform

Earthquake-prone areas in India

• Earthquake of mild intensity takes place daily. Strong tremors causing large-scale destructions are, however, less frequent. Earthquakes are more frequent in the areas of plate boundaries, especially along convergent boundaries.
• In India, the region of convergence of the Indian Plate and the Eurasian Plate is more vulnerable to earthquakes. E.g. the Himalayan Region.
• The peninsular part of India is considered to be a stable block. Occasionally, however, some earthquakes are felt along the margins of minor plates. The Koyna earthquake of 1967 and the Latur earthquake of 1993 are examples of earthquakes in peninsular regions.
• The experts in Indian Seismology have divided India into Four seismic zones namely Zone-II, Zone-III, Zone-IV, and Zone-V. It may be observed that the entire Himalayan region, the states of North-East India, Western and Northern Punjab, Haryana, Uttar Pradesh, Delhi, and parts of Gujarat belong to the highest and the high-risk categories zone, named zone V and IV.
• The remaining parts of the northern plains and western coastal areas fall in the moderate-risk zone and a large part of the peninsular region lies in the low-risk zone.

Consequences of Earthquake

Damage to human life and property

• The deformation of the ground surface because of the vertical and horizontal movement of the earth’s crust causes huge damage and destruction to human establishments and structures.
• Example: – An urban disaster case study of the Nepal earthquake of 2015. This earthquake was of 7.8 magnitudes and was 8.2 Km deep. The Nepal earthquake caused heavy casualties because of unplanned urban construction; poorly designed buildings and unscientifically designed structures.
• Urban areas of Kathmandu suffered heavy damages with a death toll of 8 thousand people and an economic loss of 10 billion USD.

Landslides and Avalanches

• Tremors especially in mountain areas can cause slope instability and slope failure leading to debris down the slope causing landslides.
• The huge masses of ice may fall down snow-covered peaks due to earthquakes causing Avalanches.
• Example: – The Nepal earthquake of 2015 resulted in several avalanches on and around Mount Everest peak. The Sikkim earthquake of 2011 caused landslides and serious damage to life and property, especially the Singik and Upper Teesta hydel projects.

Floods

• The earthquake can lead to devastating disturbances to dams, and reservoirs and can cause flash floods. Landslides and Avalanches may block the river course, leading to floods.
• Example: – The Assam earthquake of 1950 produced a barrier in the Dihang River due to the Accumulation of huge debris causing flash floods in the upstream section.

Tsunami

• Tsunamis are the waves produced due to the disruption of the ocean basin and displacement of the huge volume of water. Seismic waves of an earthquake can displace the sea floor and generate high sea waves such as Tsunamis.
• Example: – The Tsunami of 26th December 2004 in the Indian Ocean was caused by an earthquake off the coast of Sumatra. It happened because of the subduction of the Indian plate under the Burmese plate. It killed about 2.4 lakh people in the countries in and around the Indian Ocean.
• Fukushima Nuclear Accident – The massive Tohoku earthquake in Japan in 2011 resulted in Tsunami waves of 10m which was caused due to an undersea earthquake of magnitude 9. This destroyed the emergency generators cooling the reactors and leading to the nuclear meltdown and the radioactive fallout from Fukushima Daiichi became a worldwide concern.

Earthquake Management

Earthquake management is the organization and management of the resources and responsibilities for dealing with all humanitarian aspects of emergencies. The aim is to reduce the harmful effects of the hazards. Earthquake management includes steps from pre-earthquake risk reduction to post-earthquake recovery.

1. Risk Recognition – Certain areas are more vulnerable to earthquakes than others, so risk recognition is the first step.
2. Earthquake monitoring system/Early warning system– Making a precise forecast about the occurrence of an earthquake in a region is still a difficult proposition. Seismologists are increasingly concentrating on the aspect of earthquake forecasting.
   • It will help in reducing the impact of upcoming disasters.
   • Example: – Japan has an earthquake early warning system that uses electronic signals that reach faster than earthquake waves.
3. Structural Solution– Past earthquakes show that over 95% of the lives lost were due to the collapse of buildings that were not earthquake resistant. But, the construction of such quake-resistant buildings is more expensive than ordinary buildings. Therefore, a cost-effective solution remains a challenge for a country like India. Seismic strengthening can be done through the prioritization of structures and to implement this, it is important to have an earthquake hazard map for various zones according to the vulnerability.

Volcanoes

Volcanoes occur where there’s a weakness within the earth’s crust. This enables magma to move to the surface, where it forms lava. A “lively” volcano is one that has erupted in living memory. A dormant volcano is one that last erupted in historic times. It can never be assumed that a volcano is extinct.

Mount Pinatubo in the Philippines erupted violently in 1991 after having been dormant for 600 years.
Magma can also bubble up to the earth’s surface through fissures or cracks, forming lava plateaux.

The buildup of material from a series of eruptions forms a volcanic cone. The shape of the cone depends on the type of material and the chemical composition of the lava. Viscous lava forms a steep-sided cone. Thin, non-viscous lava produces a low-angle, shield volcano. Many cones are composite, as they contain layers of ash and lava.

Other volcanic hazards include nueés ardent (glowing clouds), which are superheated clouds of gas and mud, lahars, mudflows, ash, pumice, and toxic gas.
Despite the danger, people still live close to volcanoes. Volcanic soils are very fertile. Tourists wish to see volcanic hot springs, geysers, and boiling mud. Geothermal energy produces electricity. Precious stones and minerals are often found in extinct volcanoes.

Formation of Volcano

• The majority of volcanoes in the world form along the boundaries of Earth’s tectonic plates—massive expanses of our planet’s lithosphere that continually shift, bumping into one another.
• When tectonic plates collide, one often plunges deep below the other in what’s known as a subduction zone.
• As the descending landmass sinks deep into the Earth, temperatures and pressures climb, releasing water from the rocks.
• The water slightly reduces the melting point of the overlying rock, forming magma that can work its way to the surface—the spark of life to reawaken a slumbering volcano.
• Not all volcanoes are related to subduction,
• Another way volcanoes can form is what’s known as hotspot volcanism.
• In this situation, a zone of magmatic activity—or a hotspot—in the middle of a tectonic plate can push up through the crust to form a volcano.
• Although the hotspot itself is thought to be largely stationary, the tectonic plates continue their slow march, building a line of volcanoes or islands on the surface. This mechanism is thought to be behind the Hawaii volcanic chain.

Location of Volcano

Around the ring of fire, a 25,000-mile-long horseshoe-shaped region that spans from the southern tip of South America around the West Coast of North America, through the Bering Sea to Japan, and on to New Zealand, is located approximately 75% of the world’s active volcanoes.
The Pacific and Nazca tectonic plates’ margins meet a variety of other tectonic plates in this area. Importantly, though, there is no geological link between the ring’s volcanoes. To put it another way, an eruption of volcanic material in Indonesia is unrelated to one in Alaska and cannot cause the infamous Yellowstone supervolcano to erupt.

The Ring of Fire is a string of volcanoes and sites of seismic activity, or earthquakes, around the edges of the Pacific Ocean. Roughly 90% of all earthquakes occur along the Ring of Fire, and the ring is dotted with 75% of all active volcanoes on Earth.

The Ring of Fire isn’t quite a circular ring. It is shaped more like a 40,000-kilometer (25,000-mile) horseshoe. A string of 452 volcanoes stretches from the southern tip of South America, up along the coast of North America, across the Bering Strait, down through Japan, and into New Zealand. Several active and dormant volcanoes in Antarctica, however, “close” the ring.

Plate Boundaries

Plate tectonics is the cause of the Ring of Fire. Huge slabs of the Earth’s crust called tectonic plates fit together like puzzle pieces. The mantle, a layer of solid and molten rock, is on top of the moving plates, which are not fixed. These plates can occasionally slide adjacent to one another, clash, or move apart. These geologically active zones in the Ring of Fire are where the most tectonic activity takes place.

Volcanic Landform

Depending on whether magma cools below or above the crust, volcanic landforms are classified as either extrusive or intrusive. Igneous rocks are those created by either plutonic cooling of magma within the crust or volcanic cooling of lava above the surface.

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