INTRODUCTION TO EARTHQUAKE
- An earthquake is the rapid vibration of the earth created by a sudden movement of large sections of rock.
- An earthquake is a sudden motion or trembling in the earth caused by the abrupt release of slowly accumulated energy.
- An earthquake is sudden ground movement caused by the sudden release of energy stored in rocks, called the elastic rebound theory.
Types of Earthquakes
- Tectonic Earthquakes
The earth’s crust is composed of loose, cracked fragments of lands referred to as tectonic plates. These plates are capable of moving slowly and gradually. The movement of these plates occurs in different forms; towards each other, away from each other, sliding past each other or colliding with each other. A huge tremor occurs when 2 moving tectonic plates slide over one another. This type of earthquake is known as tectonic earthquake. Tectonic earthquakes are the most prevalent kinds of earthquakes in the world. Its magnitude may be small or large. Tectonic earthquakes have caused most of the planet’s mass destruction. Tremors triggered by tectonic earthquakes are always severe, and if their magnitude is high, they are capable of bringing down an entire city in seconds.
- Volcanic Earthquakes
Compared to tectonic earthquakes, volcanic earthquakes are less prevalent. They typically take place before or after an eruption. Volcanic earthquakes come in two forms: long-period volcanic earthquakes and volcano-tectonic earthquakes. Volcano-tectonic earthquakes usually happen after a volcanic eruption. During an earthquake, magma erupts from inside the earth’s crust leaving a space behind. The space left after magma eruption must be filled. To fill it, rocks move towards the space resulting in severe earthquakes.
On numerous occasions, magma blocks the vents during a volcanic activity. This means that the high pressure fails to be released. The buildup of pressure becomes unbearable and releases itself with a massive explosion. The massive explosion results in a ruthless earthquake. On the other hand, the long period volcanic earthquake takes place after a volcanic eruption. Some days prior to the massive explosion, the magma inside the earth’s crust experiences rapid changes in heat. The change in heat triggers seismic waves, resulting in an earthquake.
- Explosion Earthquakes
These are caused by nuclear explosions. They are, essentially, man triggered kind of earthquakes and represent the biggest impact of modern day nuclear war. During the 1930s nuclear tests conducted by the United Sates, numerous small towns and villages were devastated as a result of this grave act.
- Collapse Earthquakes
These kinds of earthquakes are generally smaller and most commonly occur near underground mines. They are sometimes referred to as mine bursts. Collapse earthquakes are instigated by the pressure generated within the rocks. This kind of earthquake leads to the collapse of the roof of the mine instigating more tremors. Collapse earthquakes are prevalent in small towns where underground mines are located.
Why does earth shake during earthquake?
While the edges of faults are stuck together and the rest of the block is moving, the energy that would normally cause the blocks to slide past one another is being accumulated.
When the force of the moving block finally overcomes the friction of the jagged edges of the fault and it breaks free, all that stored up energy is released. The seismic waves shake the earth as they move through it, and when the waves reach the earth’s surface, they shake the ground and anything on it, like our houses and us.
The energy radiates outward from the fault in all directions, in the form of seismic waves- like ripples on a pond.
- What Is Seismology?
Seismology is the study of earthquakes and seismic waves that move through and around the earth. A seismologist is a scientist who studies earthquakes and seismic waves.
- What Are Seismic Waves?
Seismic waves are the waves of energy caused by the sudden breaking of rock within the earth or an explosion. They are the energy that travels through the earth and is recorded on seismographs.
- Types of Seismic Waves
There are several different kinds of seismic waves, and they all move in different ways. The two main types of waves are body waves and surface waves. Body waves can travel through the earth’s inner layers, but surface waves can only move along the surface of the planet like ripples on water. Earthquakes radiate seismic energy as both body and surface waves.
1. BODY WAVES
Traveling through the interior of the earth, body waves arrive before the surface waves emitted by an earthquake. These waves are of a higher frequency than surface waves.
- P WAVES
The first kind of body wave is the P wave or primary wave. This is the fastest kind of seismic wave, and, consequently, the first to ‘arrive’ at a seismic station. The P wave can move through solid rock and fluids, like water or the liquid layers of the earth. It pushes and pulls the rock it moves through just like sound waves push and pull the air. Have you ever heard a big clap of thunder and heard the windows rattle at the same time? The windows rattle because the sound waves were pushing and pulling on the window glass much like P waves push and pull on rock. Sometimes animals can hear the P waves of an earthquake. Dogs, for instance, commonly begin barking hysterically just before an earthquake ‘hits’ (or more specifically, before the surface waves arrive). Usually people can only feel the bump and rattle of these waves.
P waves are also known as compressional waves, because of the pushing and pulling they do. Subjected to a P wave, particles move in the same direction that the the wave is moving in, which is the direction that the energy is traveling in, and is sometimes called the ‘direction of wave propagation’.
- S WAVES
The second type of body wave is the S wave or secondary wave, which is the second wave you feel in an earthquake. An S wave is slower than a P wave and can only move through solid rock, not through any liquid medium. It is this property of S waves that led seismologists to conclude that the Earth’s outer core is a liquid. S waves move rock particles up and down, or side-to-side–perpindicular to the direction that the wave is traveling in (the direction of wave propagation).
2. SURFACE WAVES
Travelling only through the crust, surface waves are of a lower frequency than body waves, and are easily distinguished on a seismogram as a result. Though they arrive after body waves, it is surface waves that are almost enitrely responsible for the damage and destruction associated with earthquakes. This damage and the strength of the surface waves are reduced in deeper earthquakes.
- LOVE WAVES
The first kind of surface wave is called a Love wave, named after A.E.H. Love, a British mathematician who worked out the mathematical model for this kind of wave in 1911. It’s the fastest surface wave and moves the ground from side-to-side. Confined to the surface of the crust, Love waves produce entirely horizontal motion.
- RAYLEIGH WAVES
The other kind of surface wave is the Rayleigh wave, named for John William Strutt, Lord Rayleigh, who mathematically predicted the existence of this kind of wave in 1885. A Rayleigh wave rolls along the ground just like a wave rolls across a lake or an ocean. Because it rolls, it moves the ground up and down, and side-to-side in the same direction that the wave is moving. Most of the shaking felt from an earthquake is due to the Rayleigh wave, which can be much larger than the other waves. Click here to see a Rayleigh wave in action.
Recordings of seismic waves from earthquakes led to the discovery of the earth’s core and eventual maps of the layers of the Earth’s inside. Just as the prism below refracts light at its faces, seismic waves bend, reflect and change speed at the boundaries between different materials below the Earth’s surface.
Can Earthquakes Be Predicted
Till date, scientists have not been able to predict earthquakes. Many modern techniques have been used, unfortunately none of them has worked. Is any such toll is built to predict earthquakes, many lives could be saved in future.
The only thing that you can do is to educate yourself about earthquake management and be vigilant in times disasters. You can also do precautionary measures by buying properties which are not located in known earthquake-prone areas or fault lines. The occurrence of earthquake can happen anytime and we will never be ready for it and the imminent danger it brings. But with earthquake preparedness measures and awareness, it can make you alert and quick in making sound decisions in times of danger.
How do scientists measure the size of earthquakes?
The size of an earthquake depends on the size of the fault and the amount of slip on the fault, but that’s not something scientists can simply measure with a measuring tape since faults are many kilometers deep beneath the earth’s surface. So how do they measure an earthquake? They use the seismogram recordings made on the seismographs at the surface of the earth to determine how large the earthquake was. A short wiggly line that doesn’t wiggle very much means a small earthquake, and a long wiggly line that wiggles a lot means a large earthquake. The length of the wiggle depends on the size of the fault, and the size of the wiggle depends on the amount of slip.
The size of the earthquake is called its magnitude. There is one magnitude for each earthquake. Scientists also talk about the intensity of shaking from an earthquake, and this varies depending on where you are during the earthquake.
Earthquakes are measured by the amount of force or energy they produced. This done through Richter scale. This tool was developed by Charles F. Richter of the California Institute of Technology. Many times you must have heard or read about this tool in news or internet. Richter scale uses the information produced through seismograph to calculate the magnitude of the earthquake.
Magnitude of an earthquake gives you an idea of the effect of an earthquake. Earthquakes occurring above 7 on Richter scale are known to have much devastating effect and can cause severe damage to life and property. Earthquakes occurring below 3 on Richter scale can’t be felt. Earthquakes occurring between 3 and 6 are said to be of mild type. Countries like Japan are prone to earthquakes as they come in high seismic zone. When an earthquakes occurs in sea, it paves way for Tsunami. One of the most devastating Tsunami occurred in Indian Ocean on Dec. 26th 2004.
- Moment Magnitude Scale
The moment magnitude scale (abbreviated as MMS; denoted as Mw or M) is used by seismologists to measure the size of earthquakes in terms of the energy released.
The scale was developed in the 1970s to succeed the 1930s-era Richter magnitude scale (ML). Even though the formulae are different, the new scale retains a similar continuum of magnitude values to that defined by the older one. As with the Richter magnitude scale, an increase of one step on this logarithmic scale corresponds to a 101.5 (about 32) times increase in the amount of energy released, and an increase of two steps corresponds to a 103 (1,000) times increase in energy. Thus, an earthquake of Mw of 7.0 releases about 32 times as much energy as one of 6.0 and 1,000 times that of 5.0.
The magnitude is based on the seismic moment of the earthquake, which is equal to the rigidity of the Earth multiplied by the average amount of slip on the fault and the size of the area that slipped.
Since January 2002, the MMS has been the scale used by the United States Geological Survey to calculate and report magnitudes for all modern large earthquakes.
- The Richter scale, a former measure of earthquake magnitude
Developed in the 1930s by seismologists Charles Francis Richter (April 26, 1900 – September 30, 1985) and Beno Gutenberg (June 4, 1889 – January 25, 1960) of the California Institute of Technology, the Richter magnitude scale (also Richter scale) assigns a magnitude number to quantify the size of an earthquake. It is a base-10 logarithmic scale: as measured with a seismometer, an earthquake that registers 5.0 on the Richter scale has a shaking amplitude 10 times greater than an earthquake that registered 4.0 at the same distance.
The intensity of an earthquake will typically measure between 2 and 10 on the Richter scale. Any earthquakes registering below a 5.5 are fairly minor; they may shake the ground a bit, but are seldom strong enough to cause much damage. Earthquakes with a Richter rating of between 5.5 and 7.9 are much more severe, and any quake above an 8 is likely to cause massive damage.
Richter Magnitudes and Earthquake Effects
- Less than 3.5: Generally not felt, but recorded.
- 3.5 – 5.4: Often felt, but rarely causes damage.
- 5.5 – 6.0: At most slight damage to well-designed buildings, but can cause major damage to poorly constructed buildings.
- 6.1 – 6.9: Can be destructive in areas up to about 100 kilometers across where people live.
- 7.0 – 7.9: Major earthquake. Can cause serious damage over larger areas.
- 8 or greater: Great earthquake. Can cause serious damage in areas several hundred kilometers across.
Geographical Distribution of Earthquakes
It is true that the earthquakes can happen in any part of the world. But in the areas of faulting and folding or of crustal weakness, the frequency of earthquakes is more than anywhere else. The earthquakes are concentrated in two main belts.
- Circum-Pacific Earthquake Belt: This belt includes all the coastal areas around the vast pacific ocean. This belt extends as an isostatically sensitive zone through the coasts of Alaska, Aleutian Islands, Japan, Philippines, New Zealand, North and South America. This zone accounts for 68% of all earthquakes on the surface of the earth. The most talked about earthquake areas in this zone include Japan, Chile, California and Mexico.
- Mediterranean-Asia Earthquake Belt: This belt begins from Alps mountain range and passes through Turkey, Caucasus Range, Iran, Iraq, Himalayan mountains and Tibet to China. One of its branches passes through Mongolia and Lake Baikal and another branch extends to Myanmar. About 31% of world’s earthquakes are located in this region.
- Other Areas: These include Northern Africa and Rift Valley areas of the Red Sea and the Dead Sea. In addition to these, the ocean ridges are also active earthquake zones.
Effects of Earthquakes
- Damage to buildings
High magnitude earthquakes can lead to complete collapse of buildings. Debris from collapsing buildings is the main danger in the course of an earthquake because the falling effects of huge, heavy objects can be deadly to humans. High magnitude earthquakes result in shattering of mirrors and windows, which also present danger to humans.
- Damage to infrastructure
Earthquakes can cause electricity lines to fall. This is dangerous because the exposed live wires can electrocute humans or start fires. Major earthquakes can cause rupturing of roads, gas lines, and water pipelines. Broken gas lines can cause gas to escape. Escaping gas can result in explosion and fires, which may be difficult to contain.
- Landslides and rockslides
When an earthquake occurs, large rocks and sections of earth located uphill can be dislodged, consequently, rolling rapidly down into the valleys. Landslides and rockslides can cause destruction and death to the people living downstream.
- Can result in floods
High magnitude earthquakes can instigate cracking of dam walls, collapsing in the long run. This would send raging waters into nearby areas leading to massive flooding.
- Earthquakes can trigger tsunamis
A tsunami is a series of long high sea tremors sparked by an earthquake or volcanic eruptions under the sea. A tsunami can wipe out an entire surrounding coastal area population. A typical example is the March 11, 2011, earthquake and tsunami that struck the coast of Japan leaving more than 18, 000 people dead in its wake.
- Leads to liquefaction
Liquefaction is a phenomenon where the soil becomes saturated and loses it strength. When sediments consisting of high water content are subjected to constant trembling, water pressure held in the sediment pores slowly increase. Ultimately, the sediments lose almost all cohesive strength and start acting like liquids. Buildings and other structures built on top of this liquefied soil overturn or sink into the ground. Earthquakes are responsible for most of the liquefaction occurring across the world. A typical example of liquefaction phenomenon is the earthquake of 1692 in Jamaica that resulted in the devastation of the town of Port Royal.
- Sometimes the earthquakes cause formation of hot springs which are very useful to people.
- The earthquakes sometimes cause submergence in coastal land, and result in formation of inlets, bays and gulfs which help to develop of fishing and shipping etc.
- Sometimes, the earthquakes cause emergence of costs and bring fertile shore out of water to give chance to develop crop production.
Earthquakes have claimed millions of lives in the last 100 years, and improvements in technology have only slightly reduced the death toll.
- 12 November 2017
A magnitude-7.3 earthquake, the fourth largest in 2017 up to that point, strikes the Iran-Iraq border. About 440 people are killed and another 10,000 injured as the quake is felt in Israel and across the Gulf.
- 19 September 2017
At least 369 people die – most in and around Mexico City – during a magnitude 7.1 earthquake. It follows a more powerful but less deadly earthquake 12 days before; the 7 September quake was a magnitude 8.1, the most powerful to hit the country in a century, but its epicentre was offshore.
- 24 August 2016
At least 298 people are killed when a magnitude 6 earthquake strikes central Italy. Worst hit is Amatrice, where many of the town’s historic buildings collapse. Italy rushes to help homeless after earthquake
- 16 April 2016
A powerful 7.8 magnitude earthquake strikes Ecuador’s coast, killing more than 650 people. More than 16,000 people are hurt and some 7,000 buildings destroyed.
- 26 October 2015
Almost 400 people are killed when a magnitude 7.5 earthquake strikes north-eastern Afghanistan. Most of those killed are in Pakistan, but the quake is also felt in northern India and Tajikistan.
- 25 April 2015
A 7.8-magnitude earthquake kills more than 8,000 people and leaves hundreds of thousands homeless, in the worst natural disaster to strike Nepal since 1934. In some parts of the country, the quake flattens 98% of all homes in hillside villages.
- 3 August 2014
Approximately 600 people are killed in a 6.1-magnitude earthquake that strikes Yunnan province in China. Thousands of houses are destroyed and landslides are triggered. More than 2,400 people are injured.
- 15 October 2013
More than 200 people are reported to have died after a magnitude 7.2 earthquake strikes centrally-located Bohol and Cebu in the Philippines.
- 25 September 2013
More than 300 people are killed as a 7.7-magnitude quake flattens entire villages in Pakistan’s remote south-western province of Balochistan, mainly in the district of Awaran.
- 20 April 2013
A powerful 6.6-magnitude earthquake kills at least 160 people and injured at least 5,700 in China’s rural south-western Sichuan province.
- 11 August 2012
At least 250 people are killed and more than 2,000 injured in north-west Iran by two powerful quakes which strikes within minutes of each other near the towns of Tabriz and Ahar.
- 23 October 2011
More than 200 people are killed and 1,000 are injured in a powerful 7.2-magnitude earthquake which hits south-eastern Turkey; many of the victims are in the town of Ercis, where dozens of buildings collapse.
- 11 March 2011
A devastating magnitude-8.9 quake strikes Japan, leaving more than 20,000 people dead or missing. The tremor generates a massive tsunami along the Japanese coast and triggers the world’s biggest nuclear disaster since Chernobyl in 1986.
- 22 February 2011
A magnitude-6.3 earthquake shatters the New Zealand city of Christchurch, killing more than 160 people and damaging some 100,000 homes.
- 14 April 2010
At least 400 people die after a magnitude 6.9 earthquake strikes western China’s Qinghai province.
- 27 February 2010
A magnitude-8.8 earthquake hits central Chile north-east of the second city, Concepcion, killing more than 700 people.
- 12 January 2010
About 230,000 people die in and around the Haitian capital Port-au-Prince as a 7.0-magnitude earthquake strikes the city.
- 30 September 2009
More than 1,000 people die after an earthquake strikes the Indonesian island of Sumatra.
- 6 April 2009
An earthquake hits the historic Italian city of L’Aquila, killing 309 people. Life after L’Aquila’s heart was ripped out
- 29 October 2008
Up to 300 people are killed in the Pakistani province of Balochistan after an earthquake of 6.4 magnitude strikes 45 miles (70km) north of Quetta.
- 12 May 2008
Up to 87,000 people are killed or missing and as many as 370,000 injured by an earthquake in just one county in China’s south-western Sichuan province. The tremor, measuring 7.8, struck 57 miles (92km) from the provincial capital Chengdu during the early afternoon.
- 15 August 2007
At least 519 people are killed in Peru’s coastal province of Ica, as a 7.9-magnitude undersea earthquake strikes about 90 miles (145km) south-east of the capital, Lima.
- 17 July 2006
A 7.7-magnitude undersea earthquake triggers a tsunami that strikes a 125-mile (200km) stretch of the southern coast of Java, killing more than 650 people on the Indonesian island.
- 27 May 2006
More than 5,700 people die when a magnitude 6.2 quake hits the Indonesian island of Java, devastating the city of Yogyakarta and surrounding areas.
- 8 October 2005
An earthquake measuring 7.6 strikes northern Pakistan and the disputed Kashmir region, killing more than 73,000 people and leaving millions homeless.
- 28 March 2005
About 1,300 people are killed in an 8.7-magnitude quake off the coast of the Indonesian island of Nias, west of Sumatra.
- 22 February 2005
Hundreds die in a 6.4 magnitude quake centred in a remote area near Zarand in Iran’s Kerman province.
- 26 December 2004
Hundreds of thousands are killed across Asia when an earthquake measuring 9.2 triggers sea surges that spread across the region.
- 24 February 2004
At least 500 people die in an earthquake which strikes towns on Morocco’s Mediterranean coast.
- 26 December 2003
More than 26,000 people are killed when an earthquake destroys the historic city of Bam in southern Iran.
- 21 May 2003
Algeria suffers its worst earthquake in more than two decades. More than 2,000 people die and more than 8,000 are injured in a quake felt across the sea in Spain.
- 1 May 2003
More than 160 people are killed, including 83 children in a collapsed dormitory, in south-eastern Turkey.
- 24 February 2003
More than 260 people die and almost 10,000 homes are destroyed in Xinjiang region, in western China.
- 31 October 2002
Italy is traumatised by the loss of an entire class of children, killed in the southern village of San Giuliano di Puglia when their school building collapses on them.
- 26 January 2001
An earthquake measuring magnitude 7.9 devastates much of Gujarat state in north-western India, killing nearly 20,000 people and making more than a million homeless. Bhuj and Ahmedabad are among the towns worst hit.
- 12 November 1999
About 400 people die when an earthquake measuring 7.2 on the Richter scale strikes Ducze, in north-west Turkey.
- 21 September 1999
Taiwan is hit by a quake measuring 7.6 that kills nearly 2,500 people and causes damage to every town on the island.
- 17 August 1999
A magnitude-7.4 earthquake rocks the Turkish cities of Izmit and Istanbul, leaving more than 17,000 dead and many more injured.
- 30 May 1998
Northern Afghanistan is hit by a major earthquake, killing 4,000 people.
- May 1997
More than 1,600 are killed in Birjand, eastern Iran, in an earthquake of magnitude 7.1.
- 27 May 1995
The far eastern island of Sakhalin is hit by a massive earthquake measuring 7.5, which claims the lives of 1,989 Russians.
- 17 January 1995
The Hyogo quake hits the city of Kobe in Japan, killing 6,430 people.
- 30 September 1993
About 10,000 villagers are killed in western and southern India.
- 21 June 1990
About 40,000 people die in a tremor in the northern Iranian province of Gilan.
- 7 December 1988
An earthquake measuring 6.9 on the Richter scale devastates north-west Armenia, killing 25,000 people.
- 19 September 1985
Mexico City is shaken by a huge earthquake which razes buildings and kills 10,000 people.
- 4 March 1977
Some 1,500 people are killed in an earthquake that hit close to the Romanian capital, Bucharest.
- 28 July 1976
The Chinese city of Tangshan is reduced to rubble in a quake that claims at least 250,000 lives.
- 23 December 1972
Up to 10,000 people are killed in the Nicaraguan capital Managua by an earthquake that measures 6.5 on the Richter scale. The devastation caused by the earthquake is blamed on badly built high-rise buildings that easily collapsed.
- 31 May 1970
An earthquake high in the Peruvian Andes triggers a landslide, burying the town of Yungay and killing 66,000 people.
- 26 July 1963
An earthquake measuring 6.9 on the Richter scale strikes the Macedonian capital of Skopje, killing 1,000 people and leaving 100,000 homeless.
- 22 May 1960
The world’s strongest recorded earthquake devastates Chile, with a reading of 9.5 on the Richter scale. A tsunami 30ft (10m) high eliminates entire villages. Death toll reports vary widely, but many settle on the 2,000 mark.
- 1 September 1923
The Great Kanto earthquake, with its epicentre just outside Tokyo, claims the lives of 142,800 people in the Japanese capital.
- 28 December 1908
Earthquake about 7.1 magnitude and subsequent tsunami in Italy’s Messina Strait, badly affecting the cities of Messina and Reggio Calabria. Deaths estimated at 70,000-80,000.
- 18 April 1906
San Francisco is hit by a series of violent shocks which last up to a minute. Between 700 and 3,000 people die either from collapsing buildings or in the subsequent fire.
- 13 August 1868 – Arica, Peru (now part of Chile)
Hawaii also felt the force of the tsunami created by this pacific basin earthquake, but here the destruction was just as heavy in South America with the city of Arequipa destroyed and 25,000 killed. The quake was felt as far away as La Paz in Bolivia. Four hours after the first shocks, waves as high as 16 metres inundated the coast and carried one US gunboat two miles inland to rest precariously on the edge of a 60m cliff.
- 26 January 1700 – North Pacific coast of America
Magnitude 9 (estimated)
The only North American account of one of the continent’s largest earthquakes comes from the oral history of native Americans near Vancouver island which describes how the large community of Pachena bay was wiped out by a huge wave. Across the pacific, the quake was accurately recorded by Japanese observers of the large tsunami that struck Japan on 27 January 1700. The power of that inundation has been used by historians and seismologists to pinpoint the magnitude of the Vancouver quake.
- 13 January 1906 – coast of Ecuador
Emanating from the ocean off Ecuador and Colombia, the quake generated a tsunami that killed between 500 and 1,500 people along a coastline from Central America to San Francisco. To the west in Hawaii, rivers suddenly drained about 12 hours after the first shocks, then were submerged as a series of successively larger waves flooded the coast.
- 1 November 1755 – Lisbon
The near-total destruction of Lisbon and the deaths of a quarter of the city’s population were caused by an earthquake, followed by a tsunami and fire, that was felt in north Africa, France and northern Italy. In the age of enlightenment, the cultural impact of the quake spread even further afield as the horrors of Lisbon provided inspiration for sensationalist artworks and philosophical tracts. Voltaire penned a poem on the catastrophe and scientists found a wealth of written first-hand accounts to advance their understanding of the physical world.
Definitions and Glossary
- Focus: Focus is the location within the earth where underground rock moves and sends out earthquake waves.
- Epicenter: The epicenter is the location on the surface of the earth directly above the focus of an earthquake.
- Earthquake waves (seismic waves): Earthquake waves are the shock waves created at the focus of an earthquake and sent out in all directions through the earth.
- Seismograph: A seismograph is a device that records earthquake waves.
- Seismogram: A seismogram is the picture drawn by a seismograph.
- Plate tectonics: According to the theory of Plate Tectonics, the outer layer of the earth is broken up into large, brittle plates of rock that float on warmer soft rock below.
Preparing structures (either new or old) for earthquakes is expensive and the level of investment is a social and political decision. The choice of building design is a compromise between appearance, function, structure, strength, and of course, cost. Standards are instituted through the establishment of Building Codes, which regulate the design and construction of buildings. Most of our building codes are designed to protect first the building occupants, and second the building integrity. Building codes are usually drafted to meet the demands of the expected shaking in a given region that are summarized by seismologists and earthquake engineers in hazards maps. Hazard maps are constructed by examining:
- The earthquake history of the region to estimate the probability of an earthquake.
- The expected shaking intensity produced by the earthquake (often expressed as a peak acceleration).
- The frequency of the shaking, the distance from the fault.
- The regional geology and site conditions.
to estimate the maximum level of shaking expected during the lifetime of a building. Constructing accurate hazard maps is a challenge and remains the focus of much Geoscience research.
Preparing Structures for Shaking
The first step in preparing structures for shaking is to understand how buildings respond to ground motions- this is the field of study for earthquake and structural engineers.
When the ground shakes, buildings respond to the accelerations transmitted from the ground through the structure’s foundation. The inertia of the building (it wants to stay at rest) can cause shearing of the structure which can concentrate stresses on the weak walls or joints in the structure resulting in failure or perhaps total collapse. The type of shaking and the frequency of shaking depends on the structure. Tall buildings tend to amplify the motions of longer period motions when compared with small buildings. Each structure has a resonance frequency that is characteristic of the building. Predicting the precise behavior of buildings is complicated, a rule of thumb is that the period of resonance is about equal to 0.1 times the number of stories in the structure. Thus Macelwane Hall resonates at about 0.3 seconds period, and Griesedeck at about 1.4 seconds.
Taller buildings also tend to shake longer than short buildings, which can make them relatively more susceptible to damage. Fortunately many tall buildings are constructed to withstand strong winds and some precautions have been taken to reduce their tendency to shake. And they can be made resistant to earthquake vibrations.
In many regions of limited resources and/or old structures, the structures are not very well suited to earthquake induced strains and collapse of adobe-style construction has caused thousands of deaths in the last decade.
Direct Shaking Hazards and Human-Made Structures
Most earthquake-related deaths are caused by the collapse of structures and the construction practices play a tremendous role in the death toll of an earthquake. In southern Italy in 1909 more than 100,000 people perished in an earthquake that struck the region. Almost half of the people living in the region of Messina were killed due to the easily collapsible structures that dominated the villages of the region. A larger earthquake that struck San Francisco three years earlier had killed fewer people (about 700) because building construction practices were different type (predominantly wood). Survival rates in the San Francisco earthquake was about 98%, that in the Messina earthquake was between 33% and 45%) (Zebrowski, 1997). Building practices can make all the difference in earthquakes, even a moderate rupture beneath a city with structures unprepared for shaking can produce tens of thousands of casualties.
Although probably the most important, direct shaking effects are not the only hazard associated with earthquakes, other effects such as landslides, liquefaction, and tsunamis have also played important part in destruction produced by earthquakes.