Earthquake

An earthquake is a sudden vibration or trembling in the Earth. More than 150,000 tremors strong enough to be felt by humans occur each year worldwide. Earthquake motion is caused by the quick release of stored potential energy into the kinetic energy of motion. Most earthquakes are produced along faults, tectonic plate boundary zones, or along the mid-oceanic ridges. At these areas, large masses of rock that are moving past each other can become locked due to friction. Friction is overcome when the accumulating stress has enough force to cause a sudden slippage of the rock masses. The magnitude of the shock wave released into the surrounding rocks is controlled by the quantity of stress built up because of friction, the distance the rock moved when the slippage occurred, and ability of the rock to transmit the energy contained in the seismic waves. The San Francisco earthquake of 1906 involved a 6 meter horizontal displacement of bedrock. Sometime after the main shock wave, aftershocks can occur because of the continued release of frictional stress. Most aftershocks are smaller than the main earthquake, but they can still cause considerable damage to already weakened natural and human constructed features.

Types of Earthquakes

There are two main types of earthquakes: natural and man-made. Naturally occurring (tectonic) earthquakes occur along tectonic plate lines (fault lines) while man-made earthquakes are always related to explosions detonated by man. Earthquakes are usually classified on the following bases:

(a) Cause of origin;

(b) Depth of focus; and

(c) Intensity and magnitude of earthquake.

(a) Cause of Origin:On the basis of the causes of earthquake, they are classified as:- (i) Tectonic and (ii) Non-tectonic earthquakes. The non-tectonic earthquakes are mainly of three types due to surface causes, volcanic causes and collapse of cavity roofs. The non-tectonic earthquakes due Volcanic to surface causes Earthquakes or Denudation earthquakes

(b) Depth of focus:As we know, the instrument designed to detect seismic waves is called seismometer and the seismograph is a seismometer to record the earth vibration. This record of earth vibration is known as seismogram. It has now become possible to estimate the depth of focus of an earthquake by analyzing seismograms. On the basis of the depth of focus, earthquakes are classified as:

(i) Surface-earthquakes

(ii) Shallow-focus earthquakes or normal earthquakes.

(iii) Intermediate-focus earthquakes, and

(iv) Deep-focus earthquakes.

(i)Surface-earthquakes: Surface-earthquakes are those in which the depth of the focus is less than 10,000 metres.

(ii)Shallow-focus earthquakes: The earthquakes with the hypocentre at a depth of 10 to 50 kms are known as shallow-focus earthquakes.

(iii)Intermediate-focus earthquakes :When the earthquake is originated at a depth of 50 to 300 Kms, it is called intermediate- earthquake.

(iv)Deep-focus earthquakes :The deep-focus earthquakes or the plutonic earthquakes are those with hypocentres located at depths more than 300 kms. Majority of the deep focus earthquakes originate between 500 and 700 kms.

Shallow-focus earthquakes constitute about 85 percent of all the earthquakes and the intermediate and deep-focus earthquakes account for 12 and 3 percent respectively of all the earthquakes. Thus it is seen that the intermediate and deep-focus earthquakes together account for only 15 percent of the earthquakes.

(c) Intensity and Magnitude of Earthquakes

As we know, the tremors caused by earthquake may be so feeble and imperceptible that they can only be registered by highly sensitive instruments and may be so vigorous to cause large scale devastation. The strength of an earthquake can be measured either by its intensity or by its magnitude.

Intensity of an earthquake is a measure of the degree of damage and destruction it can cause. These effects can be observed without the help of any instrument. It is also a fact that intensity of an earthquake diminishes outwards from the epicenter.

Therefore places in which the earthquake has manifested itself with equal intensity are contoured with - a line known as 'isoseismal'. Areas with one and the same intensity of earthquake restricted by isoseismal lines are known as isoseismal areas. There is a number of disadvantages in using intensity as a measure of the strength of a particular earthquake, some of the important disadvantages are as follows:

(i) The strength of an earthquake decreases with distance from its epicentre. Thus different degree of damage occurs at different distances for the same earthquake.

(ii) The degree of damage depends much on the geological characteristics of a particular area as well as the type of construction, population-density etc.

However, two scales of intensities are in vogue viz. (i) Rossi Forrel's Scale and (ii) Mercalli Scale.

(i)Rossi-Forrel's Scale

According to this scale, there exists ten distinct and well-defined intensities beginning from/ and ending with X of which / represents the most mild earthquake while X the most disastrous one. The scale is as follows:

Intensity Number

I. Imperceptible

II. Feeble

III. Very Slight

IV. Slight

V. Weak

VI. Moderate

Effects :

Recorded by sensitive instruments only. Recorded by all seismographs and felt by experienced persons only. Felt by several persons at rest. It is strong enough for the duration and direction to be recorded. Felt by persons in motion. Moveable objects disturbed. Affects window doors, ceilings etc. General alarm, ringing of bells, disturbance of furniture and beds etc.

General awakening of persons from sleep, stopping of clocks, visible oscillation of trees etc. Overthrows moveable objects, general panic, fall of plaster from the walls without damage to the building etc.

Fall of chimneys and cracks in the walls of buildings. Partial or total destruction of buildings. General destruction of buildings rock-falls and landslides in mountai- ncous regions.

(ii)Mercalli Scale

This scale is developed by Mercalli, an Italian seismologist, after he made studies of the intensity and regional effects of earthquakes. The scale had at first ten divisions but later on it was modified to a scale of 12 degrees. The higher the number of intensity the greater is the damage.

Earthquake Waves

Earthquakes are a form of wave energy that is transferred through bedrock. Motion is transmitted from the point of sudden energy release, the earthquake focus, as spherical seismic waves that travel in all directions outward. The point on the Earth's surface directly above the focus is termed the epicenter.

Fig 3.1: Movement of body waves away from the focus of the earthquake

Two different types of seismic waves have been described by geologists: body waves and surface waves shown in Fig. 3.1. Body waves are seismic waves that travel through the lithosphere. Two kinds of body waves exist: P-waves and S-waves. Both of these waves produce a sharp jolt or shaking. P-waves or primary waves are formed by the alternate expansion and contraction of bedrock and cause the volume of the material they travel through to change. They travel at a speed of about 5 to 7 kilometers per second through the lithosphere and about 8 kilometres per second in the asthenosphere. The speed of sound is about 0.30 kilometers per second. P-waves also have the ability to travel through solid, liquid, and gaseous materials. When some P-waves move from the ground to the lower atmosphere, the sound wave that is produced can sometimes be heard by humans and animals.

  1. S-waves or secondary waves are a second type of body wave. These waves are slower than P-waves and can only move through solid materials. S-waves are produced by shear stresses and move the materials they pass through in a perpendicular (up and down or side to side) direction.
  2. Surface waves travel at or near the Earth's surface. These waves produce a rolling or swaying motion causing the Earth's surface to behave like waves on the ocean. The velocity of these waves is slower than body waves. Despite their slow speed, these waves are particularly destructive to human construction because they cause considerable ground movement.

Earthquake Measurement

The strength of an earthquake can be measured by a device called a seismograph. When an earthquake occurs this device converts the wave energy into a standard unit of measurement like the Richter scale. In the Richter scale, units of measurement are referred to as magnitudes. The Richter scale is logarithmic as shown in Table 3.1. Thus, each unit increase in magnitude represents 10 times more energy released. Table 10m-1 describes the relationship between Richter scale magnitude and energy released. The following equation can be used to approximate the amount of energy released from an earthquake in joules when Richter magnitude (M) is known: Energy in joules = 1.74 x 10(5 + 1.44*M)

Table 3.1: Relationship between Richter Scale magnitude and energy released

S. no. / Magnitude in
Richter Scale / Energy Released
in Joules / Comment
2.0 / 1.3 x 108 / Smallest earthquake detectable by people.
5.0 / 2.8 x 1012 / Energy released by the Hiroshima atomic bomb.
6.0 - 6.9 / 7.6 x 1013 to 1.5 x 1015 / About 120 shallow earthquakes of this magnitude occur each year on the Earth.
6.7 / 7.7 x 1014 / Northridge, California earthquake January 17, 1994.
7.0 / 2.1 x 1015 / Major earthquake threshold.
7.4 / 7.9 x 1015 / Turkey earthquake August 17, 1999. More than 12,000 people killed.
7.6 / 1.5 x 1016 / Deadliest earthquake in the last 100 years. Tangshan, China, July 28, 1976. Approximately 255,000 people perished.
8.3 / 1.6 x 1017 / San Francisco earthquake of April 18, 1906.
9.3 / 4.3 x 1018 / December 26, 2004 Sumatra earthquake.
9.5 / 8.3 x 1018 / Most powerful earthquake recorded in the last 100 years. Southern Chile on May 22, 1960. Claimed 3,000 lives.

Figures 3.2 and 3.3 describe the spatial distribution of small and large earthquakes respectively. These maps indicate that large earthquakes have distributions that are quite different from small events. Many large earthquakes occur some distance away from a plate boundary. Some geologists believe that these powerful earthquakes may be occurring along ancient faults that are buried deep in the continental crust. Recent seismic studies in the central United States have discovered one such fault located thousands of meters below the lower Mississippi Valley. Some large earthquakes occur at particular locations along the plate boundaries. Scientists believe that these areas represent zones along adjacent plates that have greater frictional resistance and stress.

Fig. 3.2: Distribution of earthquakes with a magnitude less than fig 3.3: Distribution of earthquakes with a magnitude greater than

5on the Richter Scale.7 on the Richter Scale.

Earthquake Damage and Destruction

Earthquakes are a considerable hazard to humans. Earthquakes can cause destruction by structurally damaging buildings and dwellings, fires, tsunamis, and mass wasting. Earthquakes can also take human lives. The amount of damage and loss of life depends on a number of factors. Some of the more important factors are:

  • Time of day. Higher losses of life tend to occur on weekdays between the hours of 9:00 AM to 4:00 PM. During this time interval many people are in large buildings because of work or school. Large structures are often less safe than smaller homes in an earthquake.
  • Magnitude of the earthquake and duration of the event.
  • Distance form the earthquake's focus. The strength of the shock waves diminish with distance from the focus.
  • Geology of the area effected and soil type. Some rock types transmit seismic wave energy more readily. Buildings on solid bedrock tend to receive less damage. Unconsolidated rock and sediments have a tendency to increase the amplitude and duration of the seismic waves increasing the potential for damage. Some soil types when saturated become liquefied.
  • Type of building construction. Some building materials and designs are more susceptible to earthquake damage.
  • Population density. More people often means greater chance of injury and death.

The greatest loss of life because of an earthquake this century occurred in Tangshan, China in 1976 when an estimated 250,000 people died. In 1556, a large earthquake in the Shanxi Province of China was estimated to have caused the death of about 1,000,000 people.A common problem associated with earthquakes in urban areas is fire. Shaking and ground displacement often causes the severing of electrical and gas lines leading to the development of many localized fires. Response to this problem is usually not effective because shock waves also rupture pipes carrying water. In the San Francisco earthquake of 1906, almost 90% of the damage to buildings was caused by fire.

In mountainous regions, earthquake provoked landslides can cause many deaths and severe damage to built structures. The town of Yungay, Peru was buried by a debris flow that was triggered by an earthquake that occurred on May 31, 1970. This disaster engulfed the town in seconds with mud, rock, ice, and water and took the lives of about 20,000 people.

Another consequence of earthquakes is the generation of tsunamis. Tsunamis, or tidal waves, form when an earthquake causes a sudden movement of the seafloor. This movement creates a wave in the water body which radiates outward in concentric shells. On the open ocean, these waves are usually no higher than one to three meters in height and travel at speed of about 750 kilometres per hour. Tsunamis become dangerous when they approach land. Frictional interaction of the waves with the ocean floor, as they near shore, causes the waves to slow down and collide into each other. This amalgamation of waves then produces a super wave that can be as tall as 65 meters in height.

Prediction

Many methods have been developed for predicting the time and place in which earthquakes will occur. Despite considerable research efforts by seismologists, scientifically reproducible predictions cannot yet be made to a specific day or month. However, for well-understood faults the probability that a segment may rupture during the next few decades can be estimated. Earthquake warning systems have been developed that can provide regional notification of an earthquake in progress, but before the ground surface has begun to move, potentially allowing people within the system's range to seek shelter before the earthquake's impact is felt.

Preparedness

The objective of earthquake engineering is to foresee the impact of earthquakes on buildings and other structures and to design such structures to minimize the risk of damage. Existing structures can be modified by seismic retrofitting to improve their resistance to earthquakes. Earthquake insurance can provide building owners with financial protection against losses resulting from earthquakes. Emergency management strategies can be employed by a government or organization to mitigate risks and prepare for consequences.

Lecture Delivered by: Dr. Shahid-e-Murtaza