Earthquake Causes And Effects

Earthquake Causes and Effects

Earth is very important topic for UPSC. It is given in the UPSC syllabus. Here we will discuss Earthquake Causes and Effects:

An earthquake in simple words is the 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.

You can also check – Geography Notes for UPSC

Why does the earth shake?

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.

Terminology Used in the Study of Earthquakes

Focus and Epicenter

Focus or hypocenter: point within Earth where faulting begins.
Epicenter: the point directly above the focus on the surface is the epicenter it is nearest to focus.
- The intensity of the earthquake is highest at the epicenter and decreases with distance from the epicenter.
- A line connecting all points on the surface where the intensity is the same is called an isoseismic line.

Richter scale

Richter magnitude scale is the scale to measure the magnitude of energy released by an earthquake.
The number indicating magnitude ranges between 0 to 9
This scale was devised by Charles. F. Richter in the year 1935.

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

Seismograph – Instrument for graphical representation of earthquake wave – a curve of earthquake wave is recorded on it

Earthquake Waves

Earthquake waves are basically of two types — body waves and surface waves.

Body waves 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 a new set of waves called 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 the velocity.
Their direction also changes as they reflect or refract when coming across materials with different densities.

Body Waves

There are two types of body waves. They are called P and S-waves.

P- Waves

P-waves move faster and are the first to arrive at the surface.
These are also called ‘primary waves’.
The P-waves are similar to sound waves.
They travel through gaseous, liquid and solid materials.

S- Waves

S-waves arrive at the surface with some time lag.
These are called secondary waves.
An important fact about S-waves is that 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.

Surface Waves

The surface waves are the last to report on a seismograph.
These waves are more destructive.
They cause the displacement of rocks, and hence, the collapse of structures occurs.

Reflection causes waves to rebound whereas refraction makes waves move in different directions. The variations in the direction of waves are inferred with the help of their record on seismographs.

Propagation of Earthquake Waves

Different types of earthquake waves travel in different manners. As they move or propagate, they cause vibration in the body of the rocks through which they pass.

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.

The other three waves vibrate perpendicular to the direction of propagation. 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.

The emergence of Shadow Zone

Earthquake waves get recorded in seismo- graphs located at far-off locations. However, there exist some specific areas where the waves are not reported. 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.

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 the epicentre, record the arrival of P-waves, but not that of S-waves. Thus, a zone between 105° and 145° from the epicentre was identified as the shadow zone for both types of waves.

The entire zone beyond 105° does not receive S-waves. The shadow zone of the S-wave is much larger than that of the P-wave. 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’s surface.

To draw the shadow zone for any earthquake we need the location of the epicentre.

Earthquake Causes and Effects — S wave and P wave shadow zones

Types of Earthquakes

Tectonic Earthquake: The most common ones are tectonic earthquakes. These are generated due to the sliding of rocks along a fault plane.
Volcanic Earthquake: A special class of tectonic earthquake is sometimes recognised as a volcanic earthquake. However, these are confined to areas of active volcanoes.
Collapse Earthquake: In areas of intense mining activity, sometimes the roofs of underground mines collapse causing minor tremors. These are called collapse earthquakes.
Explosion Earthquakes: Ground shaking may also occur due to the explosion of chemical or nuclear devices. Such tremors are called explosion earthquakes.
Reservoir-Induced Earthquakes: The earthquakes that occur in the areas of large reservoirs are referred to as reservoir-induced earthquakes.

How to Measure 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 relates to the energy released during the quake.
The magnitude is expressed in numbers, 0-10. The intensity scale is named after Mercalli, an Italian seismologist.
The intensity scale takes into account the visible damage caused by the event.
The range of intensity scale is from 1-12.

Effects of Earthquake

Earthquake is a natural hazard. The following are the immediate hazardous effects of earthquakes:

Some effects bearings upon landforms are:
1. Ground Shaking
2. Differential ground settlement
3. Land and mudslides
4. Soil liquefaction
5. Ground lurching
6. Avalanches
Effects causing immediate concern to the life and properties of people in the region are:
- Ground displacement
- Floods from dam and levee failures
- Fires
- Structural collapse
- Falling objects
- Tsunami

The effect of the tsunami would occur only if the epicentre of the tremor is below oceanic waters and the magnitude is sufficiently high. Tsunamis are waves generated by the tremors and not an earthquake in themselves.

Though the actual quake activity lasts for a few seconds, its effects are devastating provided the magnitude of the quake is more than 5 on the Richter scale.

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 the 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 experts of 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 as 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.
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.

Seismic Zone II:
- Area with minor damage earthquakes corresponding to intensities V to VI of MM scale (MM-Modified Mercalli Intensity scale).
Seismic Zone III:
- Moderate damage corresponding to intensity VII of MM scale.
Seismic Zone IV:
- Major damage corresponding to intensity VII and higher of MM scale.
Seismic Zone V:
- Area determined by the seismics of certain major fault systems and is seismically the most active region.
- Earthquake zone V is the most vulnerable to earthquakes, where historically some of the country’s most powerful shocks have occurred.
- Earthquakes with magnitudes in excess of 7.0 have occured in these areas, and have had intensities higher than IX.

World Distribution of Earthquakes

The world’s distribution of earthquakes coincides very closely with that of volcanoes.
Regions of greatest seismicity are 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 (**Alpine Belt (Himalayas and Alps) and mid-continental 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 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.

Risk Recognition – Certain areas are more vulnerable to earthquakes than others, so risk recognition is the first step.
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.
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 prioritization of structures and to implement this, it is important to have an earthquake hazard map for various zones according to the vulnerability.