Earthquakes | Destructive Phenomenon

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

Strike Slip Fault
Strike Slip Fault

2. Thrust fault

Both plates push upwards. It creates shorter & wider mountain ranges.

Thrust Fault
Thrust Fault

3. Down-dropped fault

Plates pull away from each other. It creates shorter & wider mountain ranges.

Down-dropped fault
Down-dropped fault

4. One plate pushes below the other plate

Up-Down Fault

Volcanic Earthquakes

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.

Effects of Earthquakes

Ground Sliding

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.

Ground Tilting

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.

Differential Settlement

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.

Liquefaction

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

Tsunami

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.

Tsunami
Tsunami

Landslides

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

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.

Studying Earthquakes

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.

Types of Earthquake Waves

What are Earthquake Waves?

A seismic wave is that which is propagated by some kind of elastic deformation, or, a change in shape that disappears when the forces are removed.

Body Waves

A seismic wave that can travel through the interior of the earth is a Body wave.
P-waves and S-waves are body waves.

Primary Waves

Primary Waves
Primary Waves

P waves are compression waves because the rocky material in their path moves back and forth in the same direction as the wave travels alternately compressing and expanding the rock. P waves are the fastest seismic waves; they travel in strong rock at about 6 to 7 km per second.

Secondary Waves

Secondary Waves
Secondary Waves

S waves, which shear, or twist, rather than compress the rock they travel through. S waves travel at about 3.5 km per second. S waves cause rocky material to move either side to side or up and down perpendicular to the direction the waves are traveling, thus shearing the rocks.

Both P and S waves help seismologists to locate the focus and epicenter of an earthquake.

Surface Waves

Earthquakes contain surface waves that travel out from the epicenter along the surface of the Earth. Two types of these surface waves occur: Rayleigh waves, named after British physicist Lord Rayleigh, and Love waves, named after British geophysicist A. E. H. Love. Surface waves also cause damage to structures, as they shake the ground underneath the foundations of buildings and other structures.

Rayleigh Waves

On the surface of the Earth, Rayleigh waves cause rock particles to move forward, up, backward, and down in a path that contains the direction of the wave travel. This circular movement is somewhat like a piece of seaweed caught in an ocean wave, rolling in a circular path onto a beach.

Love Waves

Love wave causes rock to move horizontally, or side to side at right angles to the direction of the traveling wave, with no vertical displacements.

Rayleigh and Love waves always travel slower than P waves and usually travel slower than S waves.

Earthquake Analysis and Measurement

What is Earthquake Magnitude?

Magnitude is a measure of the strength of an earthquake, or the amount of strain that rocks in Earth’s crust release when an earthquake occurs. The Richter scale and the moment magnitude scale are used to measure the magnitude of earthquakes.

What is Earthquake Intensity?

Earthquake intensity is a measure of the effects of an earthquake in a particular place. Earthquake intensity is the severity of shaking felt due to an Earthquake. It decreases with its distance from epicentre.

Recording an Earthquake

The vibrations produced by earthquakes are detected and recorded by instruments called seismographs. The time of occurrence, the duration of shaking, the locations of the epicenter and focus, and estimates of the energy released can be obtained from data from seismographs set up around the world

Richter Scale

Richter Scale, method of ranking the strength or size of an earthquake. The Richter scale, also known as the local magnitude scale, was devised in 1935 by the American seismologist Charles F. Richter to rank earthquakes occurring in California. Richter and his associates later modified it to apply to earthquakes anywhere in the world.

The Richter scale ranks earthquakes based on how much the ground shakes 100 km (60 mi) from the earthquake’s epicenter, the site on the earth’s surface directly above the earthquake’s origin. The amount of ground movement is measured by an instrument called a seismograph. Seismographs can detect movements as small as about 0.00001 mm (about 0.000004 in) to movements as large as about 1 m (about 40 in). In order to deal with numbers in such a broad range, the Richter scale is a logarithmic scale—each increase of 1 on the Richter scale represents a tenfold increase in movement. Thus, an earthquake registering 7 on the scale is 10 times as strong as an earthquake registering 6, and the earth moves 10 times as far.

Moment magnitude Scale (Mw)

Today, seismologists prefer to use a different kind of magnitude scale, called the moment magnitude scale, to measure earthquakes. Seismologists calculate moment magnitude by measuring the seismic moment of an earthquake, or the earthquake’s strength based on a calculation of the area and the amount of displacement in the slip. The moment magnitude is obtained by multiplying these two measurements. It is more reliable for earthquakes that measure above magnitude 7 on other scales that refer only to part of the seismic waves, whereas the moment magnitude scale measures the total size. The moment magnitude of the 1995 Kobe, Japan, earthquake was a 7.0 moment magnitude; in comparison, the Richter magnitude was 6.8 for that tremor.

Modified Mercalli Scale

Modified Mercalli Scale, scale for measuring the intensity of earthquakes, adapted from the original Mercalli scale. The Mercalli scale was devised in 1902 by Italian seismologist Giuseppe Mercalli. American seismologists Harry O. Wood and Frank Neumann created the Modified Mercalli scale in 1931 to measure the intensity of earthquakes that occur in California. The Modified Mercalli scale, or a scale similar to it, is now used worldwide. The scale has 12 levels of intensity. Each level is defined by a group of observable earthquake effects, such as shaking of the ground and damage to structures such as buildings, roads, and bridges.

The levels are designated by the Roman numerals I to XII. Levels I through VI are used to describe what people see and feel during a small to moderate earthquake. Levels VII through XII are used to describe damage to structures during a moderate to catastrophic earthquake. On average, about one earthquake of level X to XII occurs worldwide every year; 10 to 20 earthquakes of level VII through IX occur each year; and over 500 earthquakes of level I to VI occur every year. Each year over 100,000 earthquakes occur that are not noticed by the human population and therefore are not rated on the Modified Mercalli scale.

The Modified Mercalli Intensity Scale of 1931

I

Not felt except by very few people under especially favorable conditions.

II

Felt by a few people, especially those on upper floors of buildings. Suspended objects may swing.

III

Felt quite noticeably indoors. Many do not recognize it as an earthquake. Standing motorcars may rock slightly.

IV

Felt by many who are indoors; felt by a few outdoors. At night, some awakened. Dishes, windows and doors rattle.

V

Felt by nearly everyone; many awakened. Some dishes and windows broken; some cracked plaster; unstable objects overturned.

VI

Felt by everyone; many frightened and run outdoors. Some heavy furniture moved; some fallen plaster or damaged chimneys.

VII

Most people alarmed and run outside. Damage negligible in well constructed buildings; considerable damage in poorly constructed buildings.

VIII

Damage slight in special designed structures; considerable in ordinary buildings; great in poorly built structures. Heavy furniture overturned. Chimneys, monuments, etc. may topple.

IX

Damage considerable in specially designed structures. Buildings shift from foundations and collapse. Ground cracked. Underground pipes broken.

X

Some well-built wooden structures destroyed. Most masonry structures destroyed. Ground badly cracked. Landslides on steep slopes.

XI

Few, if any, masonry structures remain standing. Railroad rails bent; bridges destroyed. Broad fissure in ground.

XII

Virtually total destruction. Waves seen on ground; objects thrown into the air.