Rapid Earth Movements: Earthquakes

Rapid Earth Movements: Earthquakes: MOVEMENTS OF EARTH A large scale and sudden release of energy in the interior of the earth gives rise to rapid earth movements. As...

Earthquakes

MOVEMENTS OF EARTH

A large scale and sudden release of energy in the interior of the earth gives rise to rapid earth movements.

As speed of these movements is high so they are called rapid movements.

 

 

 

 

Shaking of the Earth’s surface caused by rapid movement of the Earth’s rocky outer layer, is called Earthquake . Earthquakes occur when energy stored within the Earth, usually in the form of strain in rocks, suddenly releases. This energy is transmitted to the surface of the Earth by earthquake waves. The study of earthquakes and the waves they create is called seismology (from the Greek seismos, “to shake”). Scientists who study earthquakes are called seismologists.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

EARTHQUAKE

Due to the movements taking place below the earth surface, the crust is subjected to stress and when this stress exceeds a critical limit, the energy is released suddenly. As a result, the earth surface shakes. This sudden shaking of the land is called Earthquake.

 

Earthquake occurs due to:-

• Movement of the plates.

• Their passage.

• Their collusion with one another.

• Subsidence.

• Eruption of volcanoes.

• Faulting…etc

The release of energy can be showed by the following diagram. The first waves that are generated are the Primary waves or P-waves. The secondary or S-waves are generated later. On the surface of Earth the waves generated are known as Surface waves.





PRIMARY WAVES
These waves first reach the surface of earth after the energy is released. They move in radial direction. The rocks move forward and backward. These P-waves are able to travel through both solid rock such as granite mountains and liquid material such as volcanic magma or the water of the oceans.

 

 

 SECONDARY WAVES
These waves are reported after the primary waves. They radiate on all directions from focus. These waves are very destructive. They can make the rocks around them move up and down. S-waves can not propagate in the liquid parts of earth like oceans and lakes.

 

 

P & S WAVES DIRECTION

 

 

 

 

 

SURFACE WAVES

After the Primary and secondary waves reach the surface of earth, a new set of waves are generated at the surface called the surface waves.

They are spread along the circumference of the earth, They are so powerful that they can move the rocks up, down and sideways.

They are the most destructive waves.

Surface waves can be divided into two types

• Love waves
• Rayleigh waves.

LOVE WAVES

The first waves are known as Love Wave.  It's motion is essentially that of S waves that have no vertical displacement; it moves the ground from side to side in a horizontal plane but at right angles to the direction of propagation. The horizontal shaking of Love waves is particularly damaging to the foundations of structures.

 



 

 

 

RAYLEIGH WAVES

The second type of surface wave is known as Rayleigh wave. Like rolling ocean waves, Rayleigh waves move both  vertically and  horizontally in a vertical plane pointed in the direction  in which the waves are travelling.

 

 

 

 

Love waves generally travel faster than Rayleigh waves.

Love waves (do not propagate through water) can effect surface water only in the sides of lakes and ocean bays pushing water sideways like the sides of a vibrating tank, whereas Rayleigh waves, because of their vertical component of their motion can affect the bodies of water such as lakes.

 

The destruction an earthquake causes depends on its magnitude and duration, or the amount of shaking that occurs. A structure’s design and the materials used in its construction also affect the amount of damage the structure incurs. Earthquakes vary from small, imperceptible shaking to large shocks felt over thousands of kilometers. Earthquakes can deform the ground, make buildings and other structures collapse, and create tsunamis (large sea waves). Lives may be lost in the resulting destruction.

How Earthquake Is Measured?

The magnitude of the earthquake is measured in Richter’s scale with the help of an instrument called a seismograph.

SEISMOGRAPH

Seismographs are the principal tool of scientists who study earthquakes.

A Seismograph is a simple pendulum. When the ground shakes, the base and frame of the instrument move with it, but inertia keeps the pendulum bob in place. It will then appear to move, relative to the shaking ground. As it moves it records the pendulum displacements as they change with time, tracing out a record called a seismogram.

FREQUENCY

Earthquakes, or seismic tremors, occur at a rate of several hundred per day around the world. A worldwide network of seismographs (machines that record movements of the Earth) detects about 1 million small earthquakes per year. Very large earthquakes, such as the 1964 Alaskan earthquake, which caused millions of dollars in damage, occur worldwide once every few years. Moderate earthquakes, such as the 1989 tremor in Loma Prieta, California, and the 1995 tremor in Kōbe, Japan, occur about 20 times a year. Moderate earthquakes also cause millions of dollars in damage and can harm many people.

PRECAUTIONS

In the last 500 years, several million people have been killed by earthquakes around the world, including over 240,000 in the 1976 T’ang-Shan, China, earthquake. Worldwide, earthquakes have also caused severe property and structural damage. Adequate precautions, such as education, emergency planning, and constructing stronger, more flexible, safely designed structures, can limit the loss of life and decrease the damage caused by earthquakes.

Before the Earthquake:

1. Be prepared to act. Know how to act so your response is automatic. Identify safe places in your work area to ‘Drop, Cover and Hold On.’ Know at least two ways to exit the building safely after an earthquake.

2. Stock up on emergency supplies. Keep the basics: flashlight, first-aid kit, whistle, gloves, goggles, blankets and sturdy shoes. Coordinate supplies with your work group or department. Plan as if food and water may not be available for about 24 hours and other supplies for up to 3 days.

3. Arrange your work area for safety. Make sure that bookcases, large file cabinets and artwork are anchored. Store heavy objects on low shelves. Store breakable objects in cabinets with latches. Use normal work order process to get furniture anchored.

During an Earthquake:

4. Remain calm as the quake occurs – others will respond to your actions. A cool head can prevent panic. If you are indoors when the shaking occurs, stay there. Move away from windows and unsecured tall furniture. Drop, cover and hold on under a desk, a table or along an interior wall. Protect your head, neck and face. Stay under cover until the shaking stops and debris settles.

5. If you are outdoors, move to an open area away from falling hazards such as trees, power lines, and buildings. Drop to the ground and cover your head and neck.

After an Earthquake:

6. Remain calm and reassuring. Check yourself and other for injuries. Do not move injured people unless they are in danger. Use your training to provide first aid, use fire extinguishers, and clean up spills. In laboratories, safely shut down processes when possible.

7. Expect aftershocks. After large earthquakes, tremors and aftershocks can continue for days.

8. Be ready to act without electricity or lights. Know how to move around your work area and how to exit in the dark. Know how to access and use your emergency supplies. Be aware of objects that have shifted during the quake.

9. If you must leave a building, use extreme caution. Continually assess your surroundings and be on the lookout for falling debris and other hazards. Take your keys, personal items and emergency supplies with you if safe to do so. Do not re-enter damaged buildings until an all-clear is given.

10. Use telephones only to report a life-threatening emergency. Cell and hard-line phone systems will be jammed. Text messages take less band width and may go through when voice calls can’t be made.