Theories of Geography Part 4 – Earthquakes


In Earthquake, there is a sudden release of energy in the Earth’s crust, which leads to a series of motions because of the waves created due to this energy (called seismic waves) released. These seismic waves originate in a limited region and spread in all directions.

Types of Earthquakes

Earthquakes can be generated by a number of sources, most of which are result of natural tectonic processes, usually caused by the interaction between two lithospheric plates. Other quakes can be generated by volcanoes as magma is injected into the Earth’s crust. For example, earthquakes in the island of Hawaii are generally volcanic earthquakes. Rest of the Earthquakes are artificially generated by nuclear test explosions. Thus, there are several types of Earthquakes such as:

  • Tectonic Earthquakes: Tectonic Earthquakes are most common and generated due to folding, faulting plate movement.
  • Volcanic Earthquakes: Earthquake associated with volcanic activity are called volcanic earthquake. These are confined to areas of volcanoes and pacific ring of fire is best example of these types of earthquakes.
  • Collapse Earthquakes: They are evident in the areas of intense mining activity, sometimes as the roofs of underground mines collapse causing minor tremors.
  • Explosion earthquakes: This is a minor shock due to the explosion of the nuclear devices.
  • Reservoir Induced Earthquakes: Large reservoirs may induce the seismic activity because of large mass of the water. They are called reservoir induced earthquakes

Foreshocks, Mainshocks and Aftershocks

The Earthquakes come in three forms of clusters called foreshocks, mainshocks, and aftershocks.

  • Foreshocks are quakes that occur before a larger one in the same location; around a quarter of all mainshocks happen within an hour of their foreshock.
  • Mainshocks and aftershocks are better known. Mainshocks are of the highest magnitude.
  • Aftershocks are smaller quakes that occur in the same general geographic area for days-and even years-after the larger, mainshock event.

Hypocentre and Epicentre

The point, where earthquakes are generated first, is called focus or hypocenter. A hypocenter is below the surface, where the first rock displaces and creates the fault.

Epicentre is the point on the Earth’s surface that is directly above the hypocenter or focus. This is the point where the shock waves reach the surface. Earthquakes originate at depths ranging from about 5 to 700 kilometers. Nearly 90 percent of all earthquakes occur at depths of less than 100 km. Shallower is the depth, more destructive an earthquake is.

Mechanism of Tectonic Earthquakes

Theory of plate tectonics explains that earth’s crust is formed by a number of large plates that move very slowly in various directions on the earth’s surface. These plates are 60-200 km thick and float on top of a more fluid zone, much in the way that icebergs float on top of the ocean.  Most earthquakes occur near a boundary between two plates. As one plate pushes past or moves over another, great stresses build up in the rock along the edges of the plates because friction prevents them from sliding past each other. Subsequently, the stresses become great enough so that the rocks can rupture. The edges of the plates slip a short distance in different directions, causing an earthquake. Greater the stresses, greater is the resulting earthquake. The movements are of three kinds


In divergent movements the plates move apart from each other. This is most common type of movement in mid-oceanic zones.


In convergent movements the plates move towards each other and the border overlap. This is most common type of movement in subduction zones where the dense oceanic plates collide and slide beneath the continental plates.


In this type of movement the plates move in opposite side, on parallel. Some earthquakes are caused by the movement of lava beneath the surface of the earth during volcanic activity.

Earthquake Belts

There are two major belts of earthquakes in the world. They are as follows:

Circum-Pacific Belt: This belt is along a path surrounding the Pacific Ocean This zone included the regions of great seismic activity such as Japan, the Philippines, and Chile. This path coincides with the “Pacific Ring of Fire”.

Alpine-Himalayan Belt: Another major concentration of strong seismic activity runs through the mountainous regions that flank the Mediterranean Sea and extends through Iran and on past the Himalayan Mountains. This zone of frequent and destructive earthquakes is referred to as the Alpine-Himalayan belt.

Earthquake Magnitude and Earthquake Intensity: Earthquake Magnitude and Earthquake Intensity are two terms often misunderstood. Earthquake magnitude is a measure of the size of the earthquake reflecting the elastic energy released by the earthquake. It is referred by a certain real number on the Richter scale (such as magnitude 6.5 earthquake). On the other hand, earthquake intensity indicates the extent of shaking experienced at a given location due to a particular earthquake. It is referred by a Roman numeral (such as VIII on MSK scale). Intensity of shaking at a location depends not only on the magnitude of the earthquake, but also on the distance of the site from the earthquake source and the geology / geography of the area.

We note here that the Isoseismals are the contours of equal earthquake intensity. The area that suffers strong shaking and significant damage during an earthquake is termed as meizoseismal region.


Richter Magnitude Scale

The concept of earthquake magnitude was first developed by Richter and hence the term “Richter scale”. The value of magnitude is obtained on the basis of recordings of earthquake ground motion on seismographs. Richter magnitude scale is a base-10 logarithmic scale obtained by calculating the logarithm of the shaking amplitude of the largest displacement from zero on Wood-Anderson torsion seismometer. It was developed in 1935 by Charles Richter in partnership with Beno Gutenberg, both of the California Institute of Technology.

Since in this scale, Earthquake magnitude is measured on a log scale, a small difference in earthquake recording on the instruments leads to a much smaller error in the magnitude. An increase of 1 in the Richter magnitude, there is a tenfold increase in the size of the waves also known as shaking amplitude. The Richter scale 5.0 is 10 times more shaking amplitude of 4.0. But there is a huge difference in energy. The energy release of an earthquake denotes the destructive power. It scales with 3/2 power of the shaking amplitude. A difference in magnitude of 1.0 is equivalent to a factor of 31.6. This is shown by the following equation:

A difference in magnitude of 2.0 is equivalent to a Factor of 1000. It is shown below:

With increase in magnitude by 1.0, the energy released by the earthquake goes up by a factor of about 31.6. Thus, a magnitude 8.0 earthquake releases about 31 times the energy released by a magnitude 7.0 earthquake, or about 1000 times the energy released by a magnitude 6.0 earthquake. There are no upper or lower bounds on earthquake magnitude. In fact, magnitude of a very small earthquake can be a negative number also. Usually, earthquakes of magnitude greater than 5.0 cause strong enough ground motion to be potentially damaging to structures. Earthquakes of magnitude greater than 8.0 are often termed as great earthquakes

Following table shows the exponential increase in earthquake energy on Richter scale:

Examples of the most devastating Earthquake recorded are Indian Ocean Earthquake 2004, which caused the 2004 Indian Ocean Tsunami and theValdivia earthquake (Chile), 1960. The Indian Ocean Earthquake was of 9.3 intensity in Richter scale while the Valdivia earthquake of Chile was 9.5. An earthquake of 10.0 on Richter scale has never been recorded by Humankind. The 2010, Haiti Earthquake was 7.0 on the Richter scale. The undersea megathrust earthquake off the coast of Japan that occurred on 11 March 2011 was 9.0 on Moment Magnitude Scale.

Moment Magnitude Scale

The Richter scale is denoted by ML. This scale was replaced in 1970s by the new Moment magnitude scale which is denoted as Mw. The scale is almost same and media uses the same term “Richter Scale” for the new MMS also. This is because medium earthquakes such as 5.0 are equal on both the scales. The Richter scale was based on the ground motion measured by a particular type of seismometer at a distance of 100 kilometers from the earthquake, and Richter scale has a highest measurable magnitude. The large earthquakes have a similar magnitude of around 7.0 on Richter scale.

The Richter scale measurement is also unreliable for measurements taken at a distance of more than about 600 kilometers from the earthquake’s epicenter. This problem is solved by the MMS (Moment magnitude scale). The Moment magnitude scale does not uses the ground motion, but used the physical properties of the Earthquake such as seismic moment. The scale was introduced by Thomas C. Hanks and Hiroo Kanamori in 1979. The US Geological survey uses the Moment magnitude scale for all large earthquakes. Drawback: Moment magnitude scale deviates at the low scale Earthquakes.

Shindo Scale

Shindo scale is also known as Japan Meteorological Agency (JMA) seismic intensity scale. It is used in Japan and Taiwan to measure the intensity of earthquakes. It is measured in units of Shindo which literally means degree of shaking. Unlike the moment magnitude scale, which measures the energy released by the earthquake, the JMA scale describes the degree of shaking at a point on the Earth’s surface. Thus it is similar to Mercalli intensity scale. The Shindo Scale ranges between Shindo-0 to Shindo-7.  Shindo-0 quake is not felt by most people, while Shindo-7 is most devastating earthquake. However, note that same earthquake has different Shindo numbers at different places. For example, 2011 Great Earthquake of Japan registered Shindo-7 at Kurihara, Miyagi Prefecture, while Shindo-6 at Fukushima, Ibaraki and Tochigi and Shindo-7 in Tokyo.

Medvedev-Sponheuer-Karnik scale (MSK-64)

Prior to the development of ground motion recording instruments, earthquakes were studied by recording the description of shaking intensity. This lead to the development of intensity scales which describe the effects of earthquake motion in qualitative terms. An intensity scale usually provides ten or twelve grades of intensity starting with most feeble vibrations and going upto most violent (i.e., total destruction). The most commonly used intensity scales are: Modified Mercalli (MM) Intensity Scale and the Medvedev-Sponhener-Karnik (MSK) Intensity Scale.

Both these scales are quite similar except that the MSK scale is more specific in its description of the earthquake effects. Medvedev-Sponheuer-Karnik scale denoted by MSK or MSK-64, is a macro seismic intensity scale which is used to evaluate the severity of ground shaking on the basis of observed effects in an area of the earthquake occurrence. It was proposed by Sergei Medvedev (USSR), Wilhelm Sponheuer (East Germany), and Vft Karnfk (Czechoslovakia) in 1964. MSK-64 is used in India, Israel, Russia, and throughout the Commonwealth of Independent States. In India the seismic zoning has been done on the basis of this scale. This scale has 12 intensity degrees expressed in Roman numerals, which are shown in the below graphics.

Seismic Waves

The waves generated by the earthquake are called Seismic waves.  The study of earthquake and seismic waves is called Seismology and the researchers are called Seismologists. Seismic waves are divided into two broad categories viz. Body Waves and Surface Waves.

Body waves

In Body waves the speed decreases with increasing density of rock and increases with increasing rock elasticity. Rock elasticity increases faster than density with depth. There are two kinds of body waves viz. P-waves and S-waves.


Primary Waves or P-waves

The Primary waves or Push waves are longitudinal / compressionwaves that vibrate parallel to the direction of wave movement. They have shortest wavelength, fastest speed {5-7 km/s} and can travel through solid, liquid and gas. They travel fast in denser, solid materials.

Secondary waves or S-waves

Secondary waves or Sheer waves or shock waves are transverse waveswhich create vibrations perpendicular to the direction of wave movement. The S waves only travel through solids because liquids and gases have no sheer strength. They have a medium wavelength and cause vibrations at right angles to the direction of propagation of waves. Their velocity is 3 to 4 km per second.

Surface Waves

Surface waves are of two types viz. Rayleigh Waves and Love waves

Rayleigh Waves or L-waves

L Waves or Surface Waves travel near the earth’s surface and within a depth of 30-32 kilometers from the surface.  These are also called Rayleigh waves after Lord Rayleigh who first described these waves. Behave like water waves with elliptical motion of material in wave. Generally slower than Love waves.

Love waves

Love waves make the ground vibrate at right angles to the direction of waves. They are a variety of S-waves where the particles of an elastic medium vibrate transversely to the direction of wave propagation, with no vertical components.  Involve shear motion in a horizontal plane. Most destructive kind of seismic wave.

How Seismic waves help in defining Earth’s interior?

The speed of the seismic waves varies with the composition of the medium. In earth crust their speed is around 2-8 kilometers per second, while in mantle the speed is up to 13 kilometer per second, because mantle is denser. In his observations, Mohorovičić found that when the focus of the Earthquake is not too deep, some waves are propagated along the surface and remains in the crust, while other set enters the mantle, speeds up and reaches the seismometer first. This means that for a seismograph stations located at about 150 Kilometers from a shallow focus earthquake epicentre received those waves first which came from beneath the ground via mantle. This was enough to conclude that there is something below earth crust which has a greater density and varied composition. It was later called Mohorovičić discontinuity or simply Moho.

The above finding led to determine that mantle is denser than crust and is viscous, semi-molten material. P-wave velocities are much slower in the outer core than in the deep mantle while S-waves do not travel at all in the liquid portion of the outer core.

Role of Seismic waves in determination of Epicentre

To determine the location of an earthquake, the following two things of info are required: Recorded seismograph of the earthquake from at least three seismographic stations at different distances from the epicentre of the quake. Time it takes for P-waves and S-waves to travel through the Earth and arrive at a seismographic station. As we know that the P waves reach the to the seismographs first at a station, the difference between the time of P waves and S waves is called S-P Interval. The S-P interval increases with increasing distance from the epicentre. At each station a circle on a map can be drawn which has a radius equal to the distance from the epicenter. Earthquake Shadow Zone Seismic waves recorded at increasing distances from an earthquake indicate that seismic velocities gradually increase with depth in the mantle. However, at arc distances of between about 105° and 140° no P waves are recorded. Furthermore, no S waves are record beyond about 105°. This is called Shadow zone.

Earthquakes in India

India has a very high frequency of great earthquakes (magnitude greater than 8.0) in comparison to the moderate earthquakes (magnitude 6.0 to 7.0). For example, during 1897 to 1950, India was hit by four great earthquakes. However, since 1950, only moderate size earthquakes have occurred in India which should be no reason to assume that the truly great earthquakes are a thing of the past. The reasons of high magnitude earthquakes in India are hidden in the tectonic setting of India. India is currently penetrating into Asia at a rate of approximately 45 mm/year and rotating slowly anticlockwise.  This rotation and translation results in left-lateral transform slip in Baluchistan at approximately 42 mm/year and right-lateral slip relative to Asia in the Indo-Burman ranges at 55 mm/year. At the same time, deformation within Asia reduces India’s convergence with Tibet to approximately 18 mm/year. Since Tibet is extending east-west, there is a convergence across the Himalaya that results in the development of potential slip available to drive large thrust earthquakes beneath the Himalaya at roughly 1.8 m/century.

Seismic Zoning of India

Indian subcontinent has a long history of devastating earthquakes, partially due to the fact that India is driving into Asia at a rate of approximately 47 mm/year. More than 50% area of Indian Subcontinent is vulnerable to earthquakes. According to the IS 1893:2002 (It is the latest code of Bureau of Indian Standards (BIS) which lays down the criteria of for earthquake resistant design of structures), India has been divided into four seismic zones viz. Zone-II, -III, -IV and -V unlike its previous version which consisted of five zones for the country. After some revisions in the previous zoning, Zone I was altogether removed. This zoning has been done on the basis of MSK-64 scale and a IS code Zone factor has been assigned by the BIS to each of them. The zone factor of 0.36 is indicative of effective (zero period) peak horizontal ground acceleration of 0.36 g (36% of gravity) that may be generated during MCE level earthquake in this zone. They are presented in the following table with IS code.



Impact of Earthquakes – Liquefaction

Earthquakes can cause soil liquefaction where loosely packed, water-logged sediments come loose from the intense shaking of the earthquake. The liquefaction is more prominent in areas such as river valleys, river plains and deltas.  The randomly bunched together soil particles have spaces have formed between them. These spaces, called pores, can be filled with water or air. The pressure of the material in the spaces holds the particles apart and stabilizing the soil in its present configuration. The effect of a seismic wave on granular soil and pore pressure is that it increases the water pressure and forces the particles apart as well as disrupts the contact point of the particles themselves. At this point in time the soil will flow like a liquid. The end product is the collapse of the particles so that there is less space between them. The water that was in that space is then forced upward.

Liquefaction should have the following conditions for it to take place:

  • Water table is less deep
  • The soil has pore spaces
  • The intensity of shaking in that area is viii or greater

The impacts of the Liquefaction are as follows:

The underlying layer of water rich sand compacts and sends a column of water and fine sand up and out onto the surface. This phenomenon is called Differential Compaction. At the same time, depth of lakes, ponds, borrow areas, and other depressions becomes lower, because the sand is pushed through the ground.  The buildings sink into the ground after the earthquake.

January 12, 2018

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