Earthquakes

1. Introduction to an Earthquake

An Earthquake may be defined as the shaking and trembling of the Earth’s surface caused by sudden releases of stress with the crust of Earth. Thus an earthquake releases the energy that has been gradually stored through the stress of increasing deformation of rocks. The released energy takes the form of seismic waves that radiate in all directions from the epicenter of the earthquake. An earthquake can originate near the surface, deep inside the crust, or even in the layer called the upper mantle. the exact place of the origin of an earthquake is called the focus and the point above the focus is termed an Epicenter. The intensity of the earthquakes has many variations. these range from tremors so small that these are hardly detectable to freat shocks which can play havoc in a region.

Ten causes of an Earthquake (See Detail)

Types of Earthquakes:

On the basis of the depth of their origin, there are three types of an earthquake:

(1). Shallow-focus earthquakes (30 miles deep)

(2). Medium-focus earthquakes (30-150 miles deep)

(3). Deep-focus earthquake (150-450 miles deep)

Earthquake Zones:

Major Earthquake zones of the world include countries located on the Asian coast of the Pacific, the eastern coast of the Pacific covering the west coast of North America and Latin America. Detail of the major zones is as under:

  1. The heaviest concentration of earthquakes is along the circum-pacific belt. It includes countries of Japan, Philippines, Indonesia, New Zealand to the west, Aleutian Islands and southern Alaska to the North, and the USA, Mexico, Colombia, Ecuador, Peru, and Chile to the east of the Pacific.
  2. Trans-Eurasian Belt is another zone of Earthquake. It extends eastward from the Mediterranean Sea through South-west Asia and the Himalayas into Afghanistan, Northern Pakistan, North India and Nepal.
  3. The third major zone comprises mid-oceanic ridges.
  4. Moreover, many epicenters also lie scattered in interior Asia, in eastern Africa (Ethiopia to South Africa) , and in the North America east of the West Coast.

 

2. Earthquake Terminology

Focus: The point of origin of an earthquake within the earth’s crust is called the ”focus”. From the focus, the earthquake waves radiate in all directions.

Epicenter: The point lying vertically above the earth’s surface, directly above the focus, is called the ”epicenter”. In the epicenter, the shaking is most intense. The intensity gradually decreases outwards.

Isoseismal Lines: The imaginary lines connecting points of equal intensity on the ground surface are called ”isoseismal lines”. If the focus is a point, the isoseismal lines will be the circle, but as the focus is commonly a line, the isoseismals are generally elliptical. From the distribution of the lines, the epicenter of an earthquake can be determined

Earthquake intensity: It is a degree of destruction caused by an earthquake. It is expressed by a number as given in the earthquake intensity scale.

3. Measurement of an Earthquake

Generally, earthquakes occur at depths of lesser than 100 kilometers under the ground. The earthquakes are frequently noted by the instruments like seismography on daily basis. The location of the earthquakes are also determined by the seismographs. The point at which an earthquake is noted is known as the focus-point or hypocenter. Just like ripples in a pool of water, the vibrations spread out all around from the hypocenter. The location of the earth’s surface quite above the hypocenter is known as the epicenter. It is the epicenter, which normally bears the brunt of the destructive power of the earthquake vibrations. The epicenter is the place of maximum damage, being the nearest point to the hypocenter. The extent of the damage is directly dependent on the intensity of vibration or energy associated with the earthquake. Moreover, the destruction also depends on the density of the population near the epicenter. In the case of densely populated cities, an earthquake of high magnitude may be catastrophic, while if there is few people living around the epicenter, an earthquake will kill fewer people and bring lesser destruction. Another point must be noted that earthquakes do not directly kill people and animals, but the cause of deaths is our own constructed structures, which can not survive earthquakes. An earthquake is a natural phenomenon, which will happen at all costs. We can not make earthquakes not happen. We should construct buildings in a way that may resist earthquakes. We should follow the building codes in earthquake-prone zones.

Note: The nature of the soil is another factor, which determines the extent of the damage. If the soil is loose and damp, the damage is greater, but if it is hard and firm, it will bring lesser damage. Keeping in view the damage caused by earthquakes, an earthquake can be measured in two ways; in terms of its magnitude, and in terms of its intensity.

Measuring the magnitude of the earthquake (Richter Scale):

The energy of vibration determines the magnitude of an earthquake. If the energy released is higher, the earthquake will be of higher magnitude, while in the case of lower vibratory energy, the magnitude will be lower. The magnitude of an earthquake is measured by a seismograph on a scale known as the Richter Scale. The range of this scale starts from zero and the maximum magnitude on this scale can be recorded is 10. The energy of the vibrations increases by steps of about 30 on this scale. Each ascending scale on it will be multiplied by 30 in order to determine the amount of energy released. For example; vibrations of an earthquake of a magnitude 5, are 30 times more energetic than those of a quake measuring 4. Similarly, vibrations of an earthquake of magnitude 7 would be 30 times greater than the vibrations of an earthquake of magnitude 6. The following chart will clearly indicate the rate of change in energy at each ascending scale.

    • 1 magnitude     =0.5 kg of TNT
    • 2 magnitude     =0.5 ✕ 30            = 15kg of TNT
    • 3 magnitude     =15 ✕ 30             = 450 kg of TNT
    • 4 magnitude     =450 ✕ 30           = 13500 kg of TNT
    • 5 magnitude     =13500 ✕ 30        = 40,5000 kg of TNT
    • 6 magnitude     =40,5000 ✕ 30     = 12,150,000 kg of TNT
    • 7 magnitude     =12,150,000 ✕ 30 = 364,500,000 kg of TNT
    • (at each upper-scale the energy would be multiplied with 30)

Earthquakes measuring 9 or more on this scale are rare. Those measuring from 8 to 8.9 are quite devastating, while those between 7 and 7.9 are considered major. Even moderate (5 to 5.9) and strong (6.0 to 6.9) earthquakes are quite destructive in densely populated areas.

Measuring the intensity of earthquakes (Modified Mercalli scale):

The impact of an earthquake is known as its intensity. This scale measures the intensity of an earthquake over a range of I to XII. Earthquakes of intensity level I, termed instrumental, are merely recorded by instruments and are hardly felt by anyone. An earthquake of intensity level XII is termed catastrophic. The intensity of an earthquake varies from point to point. For example, the intensity of an earthquake of 8 magnitudes is higher at a point nearby the epicenter, than at a point far from the epicenter. The closer we are to the epicenter, the higher the intensity will be.

4. Effect of Earthquakes

Effects of the Earthquakes:

Major earthquakes cause great damage to life and property. Earthquakes caused by movement along faults are very disastrous, especially near the cities and areas of congested population. They demolish buildings and damage the infrastructure severely. Vulnerable infrastructure may be railway lines, roads, electricity lines, telephone systems, and bridges. The after-effects of the earthquakes are also very destructive because the tsunami tidal waves play havoc in the coastal areas. In recent history, coastal areas of Java, Sumatra, and Sri-Lanka have witnessed colossal damages caused by these waves.

On November 1, 1755, tidal waves created by an earthquake in the ocean floor of the Atlantic West of Lisbon were 35 feet high. These waves not only razed most of the buildings in the coastal areas near Lisbon but also killed 60 thousand people. Similarly, on 1st September 1923, an earthquake destroyed half a million houses in the Japanese cities of Tokyo and Yokohama and killed 250,000 people. The effects of earthquakes are summarized in the following points;

  • Buildings are damaged and people get frightened.
  • Roads are fissured, railway lines are twisted, and bridges are destroyed.
  • In cities, ground waves disrupt underground services and start fires.
  • Rivers change their courses. Fissures are opened up in the ground which may cause spring.
  • Permanent tilting of the landmasses may occur in certain areas. Landslides may occur in high lands.

5. How to protect from the effect of an earthquake

Earthquakes bring many changes to the natural and cultural landscape, but the greatest damage caused when buildings fail to survive an earthquake. Earthquake does not directly kill people, but people are killed by their own constructed buildings, which are vulnerable to an earthquake. Avoiding constructing buildings while fearing earthquakes is irrational. So in order to protect ourselves from the hazards of earthquakes, we need to take two precautionary measures; one is to design buildings in such a way, that they could survive earthquakes, and the second is a general safety measure that people can take during an earthquake. Before discussing the safety measures that are necessary during an earthquake, we are going to discuss the design and construction of the buildings to protect them from damage.

Protection of buildings:

An earthquake vibrates the ground, and as a result, the buildings also start vibrating. The bottom of the buildings moves to-and-fro as per the vibrations of the ground. But due to an increase in length with the height of the building the upper part of the building sways back and forth and gets pushed and pulled with high displacement. The whole building does not move back and forth. The bottom of the building just vibrates slightly because of being fixed with the ground, and the top sways a lot in the back and the fourth direction. Which may result in the building crack or even collapse.

This movement sets up a lot of stress in the building, and when the stress becomes saturated, the building cracks and collapse. It is somewhat like holding a stick at one end firmly and pushing the other end too much, the stick will ultimately break. In case both ends of the stick are openly moved, the stick will never break.

There are two ways of protecting buildings; buildings should be stronger enough, which could resist an earthquake, and buildings should move accordingly to the vibration.

Strengthening the buildings:

There are many ways to achieve strength in the buildings. One way is to insert diagonal pieces in the walls and divide them into rectangular areas. This protects the walls from getting quashed when there is a sideways force exerted on them.

Let’s take the example of an empty matchbox, which can easily be quashed when there are no match sticks inside it. If we try to quash the match box filled with match sticks, it will require greater force to get quashed. This is how the diagonal pieces inside the walls and roof can protect them from squashing.

Allowing the buildings to move according to the earthquake t vibrations:

Because the buildings are fixed by their bottom in the ground, and the top of the buildings sways during seismic vibration, stress arises. Especially in earthquakes of high magnitude, the upper part of the buildings starts swaying more as compared to the bottom. The excessive swaying creates more stress in the skeleton of the building. So in order to prevent the buildings from stress, it is reasonable to allow the whole mass of the building to sway equally. In modern architecture, the buildings can be placed on the bearings, which allow the building to freely move back and forth. This is done by placing the strong bearings on the thick (fixed) prestressed concrete foundation. The buildings placed on the bearings can move both forth-and-back, and sideways as per vibration that is caused by the earthquakes. By this method, the bottom of the building also moves equally to the top of the building, thus reducing the swinging of the building. This method is modern and mechanically advanced, and the cost of this method is also higher if compared to the construction of the fixed buildings.

What safety measure can be taken in order to protect ourselves during an earthquake?

In case we are trapped in a building during an earthquake, the foremost preference should be to save, somehow, our lives. Life is the most precious thing in the world. We should follow the following measures in order to protect ourselves from the lethal consequences of the earthquakes.

1. Stay away from non-fixed item in the room, like windows, glass objects, mirrors and things that can fall like book cases and cabinets.

2. Crawl under a table or bed or crouch near an inner wall or doorway and protect your head and face with your arms.

3. In order to get support hold on something which is unlikely to fall.

4. Never use the lift.

5. Turn off the power supply.

6. In case, you are in outdoors, just stay away from the buildings, poles, bridges, and trees.

7. Generally after shocks follow an earthquake. You should be ready for the aftershocks.

8. People in earthquake-prone areas should be particular about fixing tall, heavy furniture to the wall and anchoring heavy appliances well. They should avoid placing breakable and heavy objects on high shelves.

6. Causes of Earthquakes

An earthquake is a natural phenomenon, which has many causes behind it. Some of these are natural while others are man-made. Among these causes, the most importants are as follow;

(1). Tectonic Movement causes Earthquakes: Earth’s crust is divided into large segments (fragments) separated along ridges. These segments are called plates (Lithospheric Plates). The term tectonic plates are used to describe the active mobile nature of these plates. There is a continuous process of convection within the lower mantel due to the extremely high temperature and its molten state. The convection causes the lower magma to move upward and the upper magma to move again downward. The type of movement is known as convection. Convection may be divergent as well as convergent. The convection within the mantle put an impact on the solid crust and made the crust crack into many pieces. Each cracked piece of the crust is known as the tectonic/ lithospheric plate. The convection of the mantle moves these plates. The plates under which the convection is divergent, move the plate apart from each other, while the plates, which are above the converging magma, move convergently (one plate overrides the other plate). There is a third type of tectonic movement, which is known as a transverse movement. The transverse movement of the two plates moves them horizontally across each other. The part of the crust where two plates transversely move across each other is known as the transverse boundary. Two transversely moving plates collide with each other and cause stress between the boundaries of the plates. When this stress is (abruptly) released, a huge amount of seismic energy is produced. The seismic energy results in an earthquake by sending strong seismic waves across the earth.

(2). Volcanic Eruption: Volcanic eruption is one of the major causes of earthquakes. These eruptions result in the expulsion of huge amounts of lava, stones, steam, and ashes from the crust of Earth. Thus the surface of Earth near the volcano is shaken. This phenomenon is called an earthquake. In 1883, an earthquake took place in Krakatoa (Indonesia) as a result of the volcanic eruption. It destroyed many villages around Java and Sumatra.

(3). Internal Movement of Earth: Internal movements of Earth comprise of the effects of all the internal forces which build up the features of the crust of our planet. The internal movements of Earth are also called the Tectonic Movements. These include diastrophic forces (faulting, folding, depression, and uplift) and vulcanicity. These forces are the major source of the internal movement of the Earth. Mostly tensional forces and compressional forces are responsible for an earthquake. The best example of an earthquake caused by the internal forces of Earth is the one, which shook Bihar (India) in 1934.

(4). Isostasy: Isostasy means the condition of vertical equilibrium between the floating landmasses and the asthenosphere beneath them. This system of equilibrium is maintained despite the many internal forces, which are constantly at work to change the landmasses. The isostasy adjustments cause earthquakes. In 1949, an isostatic Earthquake shook Hindukush mountainous region causing great destruction. Thus isostasy is maintained between the Sialic (Silicon Aluminium) crust and the Simatic (Silicon Magnesium) substratum resulting in earthquakes.

(5). Landslides: In mountainous regions, landslides also cause minor earthquakes. In this case, a large mass of rocks and soil travel downhill very rapidly. The downslope movement of the landmasses causes tremors.

(6). Ejection of Steam: Sometimes the huge volume of hot steam is ejected or moves underneath the surface of Earth. The pressure developed through this movement causes tremors in the region.

(7). Cavern Collapses: The cavern collapses are another source of minor earthquakes.

(8). Displacement of Ice Blocks: In snow-covered mountains, the fall of huge ice blocks into the valley cause minor tremors and shocks.

Read a list of ten causes of the earthquakes

7. Magnitude of Earthquake

The intensity of an earthquake is assessed on the Rickter Scale, which was developed by geophysicist Charles Richter in 1935. This scale assigns a number to an earthquake based on the severity of the ground motion It ranges from 0 to 8+. It is logarithmic so that an earthquake with a magnitude 4 causes 10 times as much ground motion as an earthquake of magnitude 3. Similarly, an earthquake of magnitude 4 causes 100 times much ground motion as an earthquake of 2 magnitudes shakes the ground.

Magnitude of an Earthquake: It is the most common measurement of an earthquake’s size. It is a measurement of the size of the earthquake source and is the same number no matter where you are or what the shaking feels like. These days, the Richter Scale has become an outdated device for measuring the magnitude of an earthquake. The Richter Scale is no longer being used by the United States Geological Survey for large, and teleseismic earthquakes. The Richter scale measures the largest wiggle (amplitude) on the recording, but other magnitude scales measure different parts of the earthquake. The United States Geological Survey currently reports earthquake magnitudes using the Moment Magnitude scale, though many other magnitudes are calculated for research and comparison purposes.

Intensity of an Earthquake: The value of the intensity of an earthquake changes from location to location. Intensity of the earthquake is a measurement of shaking and destruction caused by an earthquake. Earthquake size can be judged through its intensity which reflects its impact on the structures, people, and their activities. Intensity of an earthquake is measured on Mercalli Scale which was developed in 1905 by Italian geologist Giuseppe Merealli. Later on, it was modified in 1931. This scale assigns a number to an earthquake ranging from I to XII Roman numbers. For example, an earthquake of intensity I is negligible and can be detected by instruments only. Similarly, an earthquake of intensity II is also feeble, an earthquake of the intensity of III is slight, IX is ruinous. X is disastrous. XI is very disastrous, and an earthquake of intensity XII is catastrophic.

8. Types of Earthquake Waves

The Earthquake waves are classified into two main types; (i) Body Waves, and (ii) Surface Waves.

Body waves include primary waves and secondary waves, while the surface waves include ”long waves”, and the ”rayleigh waves”. Detail of these waves can be read in this article: Types of Earthquake waves

 

 

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