Earthquakes
Introduction
- An earthquake, in simple words, is shaking of the earth. It is a natural event.
- It is caused due to release of energy, which generates waves that travel in all directions.
- The point where the earthquake starts is called the focus or hypocentre of an earthquake.
- The energy waves travelling in different directions reach the surface.
- The point on the surface, nearest to the focus, is called epicentre. It is the first one to experience the waves.
- It is generally, epicentre, which is shown as the location of earthquake in media and other channels.
Causes of Earthquakes
- Earthquakes are caused due to release of energy. The release of energy occurs along a fault. A fault is a sharp break in the crustal rocks. Rocks along a fault tend to move in opposite directions. As the overlying rock strata press them, the friction locks them together.
- However, their tendency to move apart at some point of time overcomes the friction. As a result, the blocks get deformed and eventually, they slide past one another abruptly. This causes a release of energy, and the energy waves travel in all directions.
This release of energy, along a fault line, may be due to several factors. They may be categorised as:
Natural causes
Tectonic earthquakes
- The earth has four major layers: the inner core, outer core, mantle and crust. The crust and the top of the mantle make up a thin skin on the surface of our planet.
- The Earth’s crust consists of seven large lithospheric plates and numerous smaller plates and the edges of the plates are called the plate boundaries. These plates move towards each other (a convergent boundary), apart (a divergent boundary) or past each other (a transform boundary).
- The plate boundaries are made up of many faults, and most of the earthquakes around the world occur on these faults. Earthquakes are caused by a sudden release of stress along these faults in the earth’s crust.
- As seen in below figure, most of the earthquakes take place along the plate boundaries. A rather more susceptible region around the pacific plate is called ‘ring of fire’ due to very high frequency of earthquakes in the region.
- The continuous motion of tectonic plates causes a steady build-up of pressure in the rock strata on both sides of a fault. It continues until the stress is sufficiently great that it is released in a sudden, jerky movement. The resulting waves of seismic energy propagate through the ground and over its surface, causing the shaking we perceive as earthquakes.
- There are mainly 3 types of faults along the plate boundaries as shown in figure
Volcanic earthquakes
- Volcanic earthquakes are caused by slip on a fault near a volcano. Volcanoes are often found in areas of crustal weakness and the mass of the volcano its self adds to the regional strain.
- They occur as a result of regional strain exerted in an area of weak faults. They can also be generated from changes of pressure under the volcano caused by the injection or removal of magma (molten rock) from the volcanic system.
- After the withdrawal of magma from a system, an empty space is left to be filled. The result is a collapse of surrounding rock to fill the void, also creating earthquakes.
- They are generally not as powerful as tectonic quakes and often occur relatively near the surface. Consequently, they are usually only felt in the vicinity of the hypocentre.
Anthropogenic causes or induced seismicity
- Induced seismicity refers to typically minor earthquakes and tremors that are caused by human activity that alters the stresses and strains on the Earth’s crust. Most induced seismicity is of a low magnitude.
- In the areas of intense mining activity, sometimes the roofs of underground mines collapse causing minor tremors. These are called collapse earthquakes.
- Ground shaking may also occur due to the explosion of chemical or nuclear devices. Such tremors are called explosion earthquakes.
- The earthquakes that occur in the areas of large reservoirs are referred to as reservoir induced earthquakes.
Distribution of Earthquakes
Earthquakes can strike any location at any time, but history shows they occur in the same general patterns year after year, principally in three large zones of the earth:
- Circum-Pacific seismic belt: The world’s greatest earthquake belt is found along the rim of the Pacific Ocean, where about 81 per cent of our planet’s largest earthquakes occur. It is also known as “Ring of Fire”.
- Alpide earthquake belt: It extends from Java to Sumatra through the Himalayas, the Mediterranean, and out into the Atlantic. This belt accounts for about 17 percent of the world’s largest earthquakes.
- Submerged mid-Atlantic Ridge: The ridge marks where two tectonic plates are spreading apart (a divergent plate boundary).
Frequency of Earthquake Occurances
- The earthquake is a natural hazard. As discussed above, not all the parts of the globe necessarily experience major shocks. The quakes of high magnitude, i.e. 8+ are quite rare; they occur once in 1-2 years whereas those of ‘tiny’ types occur almost every minute.
Earthquake Waves
- Release of energy during earthquake generates waves which are called Earthquake Waves. Earthquake waves are basically of two types — body waves and surface waves.
- Body waves: They are generated due to the release of energy at the focus and move in all directions travelling through the body of the earth. Hence, the name body waves. The body waves interact with the surface rocks and generate new set of waves called surface waves.
- Surface waves: These waves move along the surface. The velocity of waves changes as they travel through materials with different densities. The denser the material, the higher is the velocity. Their direction also changes as they reflect or refract when coming across materials with different densities.
There are two types of body waves. They are called P and S-waves.
- P-waves or ‘primary waves’ move faster and are the first to arrive at the surface. The P-waves are similar to sound waves. They travel through gaseous, liquid and solid materials.
- P-waves vibrate parallel to the direction of the wave. This exerts pressure on the material in the direction of the propagation. As a result, it creates density differences in the material leading to stretching and squeezing of the material.
- S-waves or secondary waves arrive at the surface with some time lag. They can travel only through solid materials. This characteristic of the S-waves is quite important. It has helped scientists to understand the structure of the interior of the earth.
- The direction of vibrations of S-waves is perpendicular to the wave direction in the vertical plane. Hence, they create troughs and crests in the material through which they pass. Surface waves are considered to be the most damaging waves.
Shadow zones
- There exist some specific areas where the waves are not reported by seismograph. Such a zone is called the ‘shadow zone’. The study of different events reveals that for each earthquake, there exists an altogether different shadow zone.
- Figure 6 shows the shadow zones of P and S-waves. It was observed that seismographs located at any distance within 105° from the epicentre, recorded the arrival of both P and S-waves. However, the seismographs located beyond 145° from epicentre record the arrival of P-waves, but not that of S-waves.
- Thus, a zone between 105° and 145° from epicentre was identified as the shadow zone for both the types of waves.
- The entire zone beyond 105° does not receive S-waves.
- The shadow zone of S-wave is much larger than that of the P-waves.
- The shadow zone of P-waves appears as a band around the earth between 105° and 145° away from the epicentre. The shadow zone of S-waves is not only larger in extent but it is also a little over 40 per cent of the earth surface.
Measuring Earthquakes
- The earthquake events are scaled either according to the magnitude or intensity of the shock.
- The magnitude scale is known as the Richter scale. The magnitude indicates energy released during the quake. It is expressed in absolute numbers 0-10.
- The intensity scale is named after Mercalli, an Italian seismologist. The intensity scale indicates the visible damage caused by the event. The range of intensity scale is from 1-12.
Effects of Earthquakes
Earthquakes have all encompassing disastrous effects on the area of their occurrence. Some of the important ones are listed in Table
| On ground | On manmade structures | On water |
| Fissures Settlements | Cracking, Slidings | Waves, Hydro-Dynamic Pressure |
| Landslides, Liquefaction | Overturning, Buckling, Collapse | Tsunami |
| Earth Pressure and Possible Chain-effects | Possible Chain-effects | Possible Chain-effects |
Earthquake Hazard Zone Mapping
National Geophysical Laboratory, Geological Survey of India has divided India into the following five earthquake zones:
- Very high damage risk zone
- High damage risk zone
- Moderate damage risk zone
- Low damage risk zone
- Very low damage risk zone
Earthquake Swarms
An Earthquake swarm is a sequence of mostly small earthquakes with no identifiable mainshock. Swarms are usually short-lived, but they can continue for days, weeks, or sometimes even months. They often recur at the same locations. Swarms are observed in volcanic environments, hydrothermal systems, and other active geothermal areas.
Occurrences of Swarms across the world:
- India: Since 11 November 2018, an earthquake swarm has been observed in the region of Dahanu, Maharashtra, an otherwise aseismic area. Ten to twenty quakes are felt daily, with magnitudes usually smaller than 3.5 (maximum magnitude 4.1 in February 2019).
- Philippines: An earthquake swarm occurred from early April 2017 to mid August 2017 in the Philippine province of Batangas.
- Europe:
- Czechia/Germany: The western Bohemia/Vogtland region is the border area between Czechia and Germany where earthquake swarms were first studied at the end of the 19th century. Swarm activity is recurrent there, sometimes with large maximum magnitudes,
- France: Ubaye earthquake swarms In Alpes-de-Haute-Provence, the Ubaye Valley is the most active seismic zone in the French Alps. Swarm activity in an area where usually only a few lowmagnitude events occur every year.
- Central America
- El Salvador: In April 2017, the Salvadoran municipality of Antiguo Cuscatlán, a suburb of San Salvador, experienced a sequence of close to 500 earthquakes within 2 days.
- Northern America
- United States: Between February and November 2008, Nevada experienced a swarm of 1,000 lowmagnitude quakes generally referred to as the 2008 Reno earthquakes The Yellowstone Caldera, a supervolcano in NW Wyoming, has experienced several strong earthquake swarms since the end of the 20th century.
- Atlantic Ocean: In El Hierro, the smallest and farthest south and west of the Canary Islands, hundreds of small earthquakes were recorded from July 2011 until October 2011.
Impact of earthquake swarms:
- Each earthquake within the swarm redistributes stress, which may in turn influence the subsequent swarm evolution, especially if the crust is in a critical state.
- Slider-block models have shown that earthquake swarms can result from a self-organized critical stress field without any external pore pressure source which can cause damage.
- Earthquake swarm activity also shares some common features with tectonic earthquake clusters, in particular embedded aftershock sequences which point to an important role for stress triggering collapse of structures.
- The potential for destruction from these events varies widely. Some cause considerable amount of damage but others are relatively harmless.
- Dhanau swarm caused casualties and damage to structures.
- Low intensity swarms which cause just shaking, maybe a cause of residents of inhabitants of the area. Especially in areas with reservoirs.
- They can be witnessed even in areas with no documented seismic activity in the recent past as was seen in Rhone Valley region.
Differences between an earthquake and earthquake swarm:
| Earthquake | Earthquake swarm | |
| Main Shock | Definite main shock | Not present |
| After shock | Generally occur after the mains shock | No after shocks |
| Occurrence Duration | One main shock but followed by Aftershocks, which become less frequent with time, although they can continue for days, weeks, months, or even years for a very large mainshock. | Usually short-lived, but they can continue for days, weeks, or sometimes even months. |
| Cause | Sudden release of energy in the Earth’s lithosphere that creates seismic waves. | Hydro-seismicity due to water percolation as well as seismic activity. |
| Frequency of occurrence | Regularly | Rare |
| Reoccurrence | Can happen at varied time intervals. | They can reoccur frequently. |
| Magnitude | Low to high | Low |
| Intensity | Low to high | Low |
| Destruction to life and property | Very high | Relatively less |
Reasons for the Earthquake proneness in India
- The major reason for the high frequency and intensity of the earthquakes is that the Indian plate is driving into Eurasia at a rate of approximately 47 mm/year.
- Himalayan belt: Collision between Indo-Austral plate with Eurasian plate and Burma Plate with Java Sumatra plate. This collision causes lots of strain in underlying rocks’ energy of which is released in the form of earthquakes.
- Andaman and Nicobar Islands: Seafloor displacement and underwater volcanoes which disturb the equilibrium of earth’s surface.
- Deccan Plateau: some earth scientists have come up with a theory of the emergence of a fault line and energy build-up along the fault line of the river Bhima (Krishna) near Latur and Osmanabad (Maharashtra).
- Increasing population and unscientific land use for construction make India a high-risk land for earthquakes.
