Earthquakes

• An earthquake is a result of a sudden release of energy in the earth’s crust that creates seismic waves.
   
• Earthquakes are short-lived episodes of ground shaking produced when blocks of Earth suddenly shift.
   
• They typically last for a few seconds (small earthquakes) to several minutes (largest earthquakes) and produce several types of seismic waves that propagate through the Earth.
   
• The shaking caused by earthquakes can result in landslides, volcanic activity and even Tsunami. When a large earthquake occurs in the oceans, the ocean floor can suffer sufficient displacement to cause a tsunami (e.g. 2004 Indian Ocean tsunami).
   
• Earthquakes are usually caused by rupture of geological faults, but can also be caused by volcanic activity, landslides, mine blasts and nuclear experiments.
   
• The point of initial rupture of an earthquake is called its hypocentre, while the point on the surface directly above it is called the epicentre.

Mechanism of action

• Earthquakes can occur anywhere within the earth where there is stored elastic energy sufficient enough to drive fault propagation along a fault plane. Most earthquakes are caused indirectly by plate tectonics.
   
• Almost all earthquakes occur along plate boundaries because plate boundaries are the loci of horizontal forces that push and stretch rocks, causing them to break and produce earthquakes. Earthquakes are produced at all three types of plate boundaries. Locations far from plate boundaries experience few earthquakes.
   
• Tectonic plates move past each other smoothly only if there are no irregularities and asperities. Most plate boundaries do have asperities and this leads to stick-slip behaviour.
   
• Once the boundary has locked into a relative stable position, continued relative motion between the plates leads to increased stress and stored strain energy.
   
• This continues until the stress rises sufficiently to break through the relative stable position, suddenly sliding over the locked position of the fault and thereby releasing the stored energy.
   
• The energy is released as a combination of elastic seismic waves, frictional heating of the surface and cracking of rock, thereby causing an earthquake.
   
• This process of gradual build up of stress and sudden release of energy in the form of earthquakes is called elastic-rebound theory.
   
• It is estimated that less than 10 % of the total energy of an earthquake is radiated as seismic energy. Most of the earthquake’s energy is used to power fracture growth or is converted as heat generated by friction.

SEISMIC WAVES

All natural earthquakes take place in the lithosphere. 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 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 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 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 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. 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 seismograph.
   
• The surface waves are the last to report on seismograph. These waves are more destructive. They cause displacement of rocks, and hence, the collapse of structures occurs.

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. 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.

EMERGENCE OF SHADOW ZONE

Earthquake waves get recorded in seismographs 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 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.

TYPES OF EARTHQUAKES

(i) The most common ones are the tectonic earthquakes. These are generated due to sliding of rocks along a fault plane.

(ii) A special class of tectonic earthquake is sometimes recognised as volcanic earthquake. However, these are confined to areas of active volcanoes.

(iii) In the areas of intense mining activity, sometimes the roofs of underground mines collapse causing minor tremors. These are called collapse earthquakes.

(iv) Ground shaking may also occur due to the explosion of chemical or nuclear devices. Such tremors are called explosion earthquakes. 

(v) The earthquakes that occur in the areas of large reservoirs are referred to as reservoir induced earthquakes.

EFFECTS OFEARTHQUAKE

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

♦ Ground Shaking

♦ Differential ground settlement

♦ Land and mud slides

♦ Soil liquefaction

♦ Ground lurching

♦ Avalanches

♦ Ground displacement

♦ Floods from dam and levee failures

♦ Fires

♦ Structural collapse

♦ Falling objects

♦ Tsunami

The first six listed above have some bearings upon landforms, while others may be considered the effects causing immediate concern to the life and properties of people in the region. The effect of 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 itself. 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

MEASURING EARTHQUAKES

The absolute magnitude of a quake is reported on the Moment Magnitude scale (MMS), while perceived magnitude is reported on the Modified Mercalli (MM) scale. The Richter scale is another scale that measures the absolute magnitude – it is no longer used in academic circles but is still used in popular parlance.

As a rule of thumb, the distance to the earthquake epicentre is the number of seconds between the P and S waves multiplied by 8.

Induced seismicity

• While most earthquakes occur due to natural movement of the earth’s tectonic plates, human activity can produce earthquakes as well
   
• Four main human activities that contribute to earthquakes include. 
  
  ♦ Large dams  
  
  ♦ Drilling and injecting liquids into wells  
  
  ♦ Coal mining    
  
  ♦ Oil drilling
   
• For instance, the 2008 Sichuan earthquake in China is believed to have been caused by the Zipingpu dam which caused the pressure of a nearby fault to fluctuate, increasing the movement of the fault and the magnitude of the earthquake

Earthquakes and volcanic activity

• Earthquakes often occur in volcanic areas
   
• They are caused both by tectonic faults and the movement of magma in volcanoes
   
• Such earthquakes can serve as early warning of impending volcanic eruptions. Eg: Mount St Helens eruption of 1980 (USA)

Measurement: Mercalli Scale vs Richter Scale

• Earthquakes are recorded with a seismograph and are reported on a magnitude of the Richter scale. The Richter scale describes the earthquake's magnitude by measuring the seismic waves that cause the earthquake.
   
• In general, earthquakes of magnitude less than 3 are imperceptible, and more than 7 can cause serious damage. The most powerful earthquake ever recorded is the Valdivia earthquake in Chile in 1960. It measured 9.5 on the Richter scale.
   
• The intensity of an earthquake can also be measured on the Modified Mercalli (MM) scale. The MM scale quantifies the effect an earthquake has on humans, natural objects and man-made structures based on observation.
   
• The two scales have different applications and measurement techniques. The Mercalli scale is linear and the Richter scale is logarithmic. i.e. a magnitude 5 earthquake is ten times as intense as a magnitude 4 earthquake.
   

Parameters	Mercalli Scale	Richter Scale
Measures:	The effects caused by earthquake	The energy released by the earthquake
Measuring Tool:	Observation	Seismograph
Calculation:	Quantified from observation of effect on earth’s surface, human, objects and man-made structures	Base-10 logarithmic scale obtained by calculating logarithm of the amplitude of waves.
Scale:	I (not felt) to XII (total destruction)	From 2.0 to 10.0+ (never recorded). A 3.0 earthquake is 10 times stronger than a 2.0 earthquake.
Consistency:	Varies at different distances from the epicenter	Varies at different distances from the epicenter, but one value is given for the earthquake as a whole.

• The Medvedev–Sponheuer–Karnik scale, also known as the MSK or MSK-64, is a macroseismic intensity scale used to evaluate the severity of ground shaking on the basis of observed effects in an area of the earthquake occurrence.
  

Distribution of Earthquakes

• In general, earthquakes can occur almost anywhere (even away from plate boundaries).
   
• The relationship between frequency and intensity of earthquakes is roughly exponential i.e. for instance, there are roughly 10 times as many earthquakes of magnitude 4 as of magnitude 5
   
• Most of the world’s earthquakes occur in Pacific Ring of Fire seismic belt. Massive earthquakes occur along other plate boundaries too, such as the Himalayas.
   
• According to a moderate estimate about 30,000 earthquakes occur every year. But most of these are so slight that we cannot feel them. There is no visible damage from them. But every year there are some earthquakes of great intensity and magnitude.
   
• Every year hundreds of earthquakes pass unnoticed because they occur in areas where there is no possibility of any loss of human life and damage to property.
   
• Earthquakes have a definite distribution pattern. There are three major belts in the world which are frequented by earthquakes of varying intensities. These belts are as under:
  
  1. The Circum-Pacific Belt
  
  2. The Mid-Atlantic Belt
  
  3. The Mid-Continental Belt

1. THE CIRCUM-PACIFIC BELT:  This belt is located around the coast of the Pacific Ocean. In this belt the earthquakes originate mostly beneath the ocean floor near the coast. The Circum- Pacific Belt represents the convergent plate boundaries where the most widespread and intense earthquakes occur.

This belt has about 66 percent of the total earthquake that are recorded in the world. Most of the earthquakes occurring in this belt are shallow ones with their focus about 25 km deep.

It may be pointed out that these belts being the zones of convergent plate boundaries (the subduction zones) are isostatically very unstable. Japan alone experiences about 1500 earthquakes per year.

2. THE MID-ATLANTIC BELT:

This belt is characterised by the sea floor spreading which is the main cause of the occurrence of earthquakes in it. This earthquake belt runs along the mid- oceanic ridges and the other ridges in the Atlantic Ocean.

In this belt most of the earthquakes are of moderate to mild intensity. Their foci are generally less than 70 km deep. Since the divergent plates in this belt move in opposite directions and there is splitting as well, transform faults and fractures are created.

3. THE MID-CONTINENTAL BELT:

• This belt extends along the young folded Alpine mountain system of Europe, North Africa, through Asia Minor, Caucasia, Iran, Afghanistan and Pakistan to the Himalayan mountain system. This belt continues further to include Tibet, the Pamirs and the mountains of Tien Shan etc. The young folded mountain systems of Myanmar, China and eastern Siberia fall in this belt.
   
• This belt happens to be the subduction zone of continental plates. It is in this belt that the African as well as Indian plates sub-duct below the Eurasian plate.
   
• It experiences about 20 per cent of the earthquakes in the world having shallow and intermediate origin. However, sometimes earthquakes of great violence occur in this belt.

It may be pointed out that more than 50 percent of all earthquakes are associated with the young folded mountains which are said to be still growing.

The Andes, Himalayas and Coast Ranges of the United States are the specific examples. It is worthwhile to remember that this girdle of young fold mountains has no correspondence with the line of active volcanoes like the Circum-Pacific earthquake zone.

There are some regions on the earth's surface which are relatively immune from violent and vigorous earthquakes. This is so because diastrophism and volcanism are either absent or only moderately active. But the infrequent occurrence of minor shocks in such regions is not ruled out. Such shocks may occur due to local causes like the subterranean movement of imprisoned gases or liquids.

The most glaring example of the occurrence of minor earthquakes in quite unexpected places is the Koyna earthquake which shook Koynanagar on September 13 and 14, 1967.
 

Major earthquakes.

S. No.	Date	Location	Magnitude
1.	1960	Valdivia, Chile	9.5
2.	Dec 2004	Sumatra, Indonesia	9.3
3.	1964	Alaska, USA	9.2
4.	1952	Kamchatka, Russia	9.0
5.	1700	Cascadia Subduction Zone (Pacific Ocean rim)	9.0

Earthquake Zoning in India:

Effects of an earthquake is measured by descriptive scale namely Modified Mercalli intensity scale or the Medvedev-Sponheuer-Karnik scale.

• Based on the likelihood of occurrence of damaging earthquakes, a seismic zone map has been prepared to spot the critical regions in India .
  
• The seismic zone map has been subdivided India into 5 zones – I, II, III, IV and V. The seismic shaking intensity is expected maximum in zones marked as V and higher.
  
• The seismic zone maps are updated regularly with strict reference to geology, the seismotectonics and the seismic activity in the country.

Zone 5

• Zone 5 covers the areas with the highest risks zone that suffers earthquakes of intensity. It is referred to as the Very High Damage Risk Zone. The state of Kashmir, the western and central Himalayas, the North-East Indian region and the Rann of Kutch fall in this zone. Generally, the areas having trap or basaltic rock are prone to earthquakes.

Zone 4

• This zone is called the High Damage Risk Zone. The Indo-Gangetic basin and the capital of the country (Delhi), Jammu and Kashmir fall in Zone 4. In Maharashtra the Patan area (Koyananager) is also in zone 4.

Zone 3

• The Andaman and Nicobar Islands, parts of Kashmir, Western Himalayas fall under this zone. This zone is classified as Moderate Damage Risk Zone.

Zone 2

• This region is classified as the Low Damage Risk Zone. 

Zone 1

• Rest of the country. Very low damage risk zone.