Geophysics is a branch of science that applies physical principles to the study of the earth. Study of Earthquake, or Seismology, is a sub-branch of Geophysics.
Earthquakes represent an enormous threat to the Nation. Although damaging earthquakes occur infrequently, their consequences can be staggering. As recent earthquakes around the world have demonstrated, high population densities and development pressures, particularly in urban areas, are increasingly vulnerable. Unacceptably high loss of life and enormous economic consequences are associated with recent global earthquakes, like the one in Kashmir in Jan 2006, when 80,000 people lost their lives, or the submarine earthquake of Banda Aceh of the 26th of December, 2004, in which the Tsunami killed more than 300,000 people in Asia and Africa.
It is very important to improve the understanding of earthquakes and their effects. A large number of earthquakes are felt all over the globe every year. The small ones are unnoticed while the large ones are felt over thousands of kilometers. The big quakes are not necessarily the most destructive. Earthquakes have damaged and destroyed man’s work since time immemorial.
Here, I have tried to explain the nature, distribution, causes and effects of this terrifying natural event.
Natural calamities are the phenomenon which can’t be prevented, but we can take precautions. Natural Calamities such as Floods, Tornadoes, Volcanic Eruptions, Tsunamis, Earthquakes, causes disturbance to our day-to-day life. Today, with the help of Science and Technology, we may be able to counteract these natural events and avoid disasters, or even reduce its impact on people.
Tornadoes & Hurricanes
A Tornado is a violently rotating column of air extending from within a thundercloud down to ground level. The strongest tornadoes may sweep houses from their foundations, destroy brick buildings, toss cars and school buses through the air, and even lift railways from their tracks.
Floods mean an increase in water level in an area due to heavy rainfall or melting snow.
Floods are the natural disaster caused due to overflow of rivers due to heavy rainfall or melting of snow which causes a large destruction in cities and villages.
Shaking of the Earth’s surface caused by rapid movement of the Earth’s rocky outer layer is known as an Earthquake. Earthquakes occur when energy stored within the Earth, usually in the form of strain in rocks, is suddenly released.
What do you mean by Geophysics?
Geophysics is a branch of science that applies physical principles to the study of the earth. Geophysicists examine physical phenomena and their relationships within the earth; such phenomena include the earth’s magnetic field, heat flow, the propagation of seismic (earthquake) waves, and the force of gravity
Subdivision of the wide-ranging subject matter of geophysics into various branches involves categorizing specific endeavors. Strictly speaking, however, the discipline embraces all fields devoted to researching the earth’s interior, atmosphere, hydrosphere, and ionosphere.
Earthquakes are caused by active faults, which are, caused by the sudden movement of the two sides of a fault with respect to another. The occurrence of tectonic earthquakes can be explained by the theory of elastic rebound, first advanced by H. B. REID.
Elastic Rebound Theory
The motion along the fault is accompanied by the gradual buildup of elastic strain energy within the rock along the fault. The rock stores this strain energy like a giant spring being slowly tightened.
Eventually, the strain along the fault exceeds the limit of the rocks at that point to store any additional strain. The fault then ruptures – that is, it suddenly moves a comparatively large distance comparatively short amount of time. The rocky masses which form the two sides of the fault then snap back into a new position. This snapping back into position, upon the release of strain, is the “ELASTIC REBOUND” of Reid’s theory. The rupture of fault results in sudden release of the strain energy that has been built up over the years. The most important form which this suddenly released energy takes is that of seismic waves, which cause earthquakes and destruction.
Movement of Tectonic Plates
There are 4 different types of Tectonic movements
1. Strike-slip fault
Fault sliding against one another
2. Thrust fault
Both plates push upwards. It creates shorter & wider mountain ranges.
3. Down-dropped fault
Plates pull away from each other. It creates shorter & wider mountain ranges.
4. One plate pushes below the other plate
Volcanic earthquakes occur near active volcanoes but have the same fault slip mechanism as tectonic earthquakes. Volcanic earthquakes are caused by the upward movement of magma under the volcano, which strains the rock locally and leads to an earthquake. As the fluid magma rises to the surface of the volcano, it moves and fractures rock masses and causes continuous tremors that can last up to several hours or days. Volcanic eruptions give rise to earthquakes.
Earthquakes due to Man-made Activities
Human activities can also be the direct or indirect cause of significant earthquakes. Injecting fluid into deep wells for waste disposal, filling reservoirs with water, and firing underground nuclear test blasts can, in limited circumstances, lead to earthquakes. These activities increase the strain within the rock near the location of the activity so that rock slips and slides along pre-existing faults more easily.
Strong ground motion is also the primary cause of damages to the ground and soil upon which, or in which, people must build. These damages to the soil and ground can take a variety of forms: cracking and fissuring and weakening, sinking, settlement and surface fault displacement.
Sometimes, due to earthquake, there is tilting action in the ground. This causes plain land to tilt, causing excessive stresses on buildings, resulting in damage to buildings.
If a structure is built upon soil which is not homogeneous, then there is differential settlement, with some part of the structure sinking more than other. This induces excessive stresses and causes cracking.
During an earthquake, significant damage can result due to instability of the soil in the area affected by internal seismic waves. The soil response depends on the mechanical characteristics of the soil layers, the depth of the water table and the intensities and duration of the ground shaking. If the soil consists of deposits of loose granular materials it may be compacted by the ground vibrations induced by the earthquake, resulting in large settlement and differential settlements of the ground surface. This compaction of the soil may result in the development of excess hydrostatic pore water pressures of sufficient magnitude to cause liquefaction of the soil, resulting in settlement, tilting and rupture of structures.
Indirect Effects of Earthquakes
A tsunami is a very large sea wave that is generated by a disturbance along the ocean floor. This disturbance can be an earthquake, a landslide, or a volcanic eruption. A tsunami is undetectable far out in the ocean, but once it reaches shallow water, this fast-traveling wave grows very large. Tsunamis are very destructive, as this wall of water can destroy everything in its path.
Landslide means descent of a mass of earth and rock down a mountain slope. Landslides may occur when water from rain and melting snow sinks through the earth on top of a slope, seeps through cracks and pore spaces in underlying sandstone, and encounters a layer of slippery material, such as shale or clay, inclined toward the valley. Earthquakes and volcanic eruptions can also cause severe, fast-moving landslides.
Landslides that suddenly rush down a steep slope can cause great destruction across a wide area of habitable land and sometimes cause floods by damming up bodies of water.
Floods & Fires
The amount of damage caused by post-earthquake fire depends on the types of building materials used, whether water lines are intact, and whether natural gas mains have been broken. Ruptured gas mains may lead to numerous fires, and fire fighting cannot be effective if the water mains are not intact to transport water to the fires.
Earthquakes may also give rise to floods. Many times, large earthquakes can cause cracking in Dams. So, to contain the increased pressure, the authorities have to immediately release a lot of water to reduce the reservoir pressure. This gives rise to very heavy flooding in the region, causing great destruction.
Earthquake Engineering is a branch of Civil Engineering which covers the investigation and solution of problems to structures created by earthquakes. It includes planning, designing, constructing and managing earthquake-resistant structures and facilities.
Seismology: Study of Earthquakes
Seismology, basically, the science of earthquakes, involving observations of natural ground vibrations and artificially generated seismic signals, with many theoretical and practical ramifications (see Earthquake). A branch of geophysics, seismology has made vital contributions to understanding the structure of the earth’s interior.
Longitudinal, transverse, and surface seismic waves cause vibrations at points where they reach the earth’s surface. Seismic instruments have been designed to detect these movements through electromagnetic or optical methods. The main instruments, called seismographs, were perfected following the development by the German scientist Emil Wiechert of a horizontal seismograph about the turn of the century.
Some instruments, such as the electromagnetic pendulum seismometer, employ electromagnetic recording; that is, induced tension passes through an electric amplifier to a galvanometer. A photographic recorder scans a rapidly moving film, making sensitive time-movement registrations. Refraction and reflection waves are usually recorded on magnetic tapes, which are readily adapted to computer analysis. Strain seismographs, employing electronic measurement of the change in distance between two concrete pylons about 30 m (about 100 ft) apart, can detect compressional and extensional responses in the ground during seismic vibrations. The Benioff linear strain seismograph detects strains related to tectonic processes, those associated with propagating seismic waves, and tidal yielding of the solid earth. Still more recent inventions used in seismology include rotation seismographs; tiltmeters; wide-frequency-band, long-period seismographs; and ocean-bottom seismographs.