The variation in energetic state of a biatomic molecule can be described in a simple model in which the bond joining the two atoms vibrates, so that the energy of the bond varies as the length of the bond varies. The change in energy with bond length is given by Hooke's Law. A true theory of Elasticity and Springiness.

(Historical aside- Robert Hooke was a contemporary or Newton's, and afraid that he would be scooped on his Law. He hid a preliminary formulation (CEIIOSSOTTUU, an anagram) up the chimney in his house,- a coded version of the Latin "ut tensio sic vis" - "as the extension, so is the force".
In Hooke's Law, the relation between energy and bond length gives a parabolic curve, and provides the framework for discussion of the dependence of energy on vibrational state, and hence on temperature. As the temperature increased, the increased vibrational energy allows the molecule to "swing" along the parabola, so that it visits the higher energy levels more frequently.

1755 - Lisbon Earthquake

The 1755 Lisbon earthquake, also known as the Great Lisbon Earthquake, took place on November 1, 1755, at 9:40 in the morning. It was one of the most destructive and deadly earthquakes in history, killing between 60,000 and 100,000 people (though the exact number is uncertain). The earthquake was followed by a tsunami and fire, resulting in the near-total destruction of Lisbon. The earthquake accentuated political tensions in Portugal and profoundly disrupted the country's eighteenth-century colonial ambitions.

The event was widely discussed by European Enlightenment philosophers, and inspired major developments in theodicy and in the philosophy of the sublime. As the first earthquake studied scientifically for its effects over a large area, it signaled the birth of modern seismology. Geologists today estimate the Lisbon earthquake approached magnitude 9 on the Richter scale, with an epicenter in the Atlantic Ocean about 200 km (120 mi) west-southwest of Cape St. Vincent.

The birth of seismology

The prime minister's response was not limited to the practicalities of reconstruction. The marquis ordered a query sent to all parishes of the country regarding the earthquake and its effects. Questions included:

• how long did the earthquake last?
• how many aftershocks were felt?
• what kind of damage was caused?
• did animals behave strangely? (this question anticipated studies by modern Chinese seismologists in the 1960s)
• what happened in wells and water holes?

The answers to these and other questions are still archived in the Tower of Tombo, the national historical archive. Studying and cross-referencing the priests' accounts, modern scientists were able to reconstruct the event from a scientific perspective. Without the query designed by the Marquis of Pombal, this would have been impossible. Because the marquis was the first to attempt an objective scientific description of the broad causes and consequences of an earthquake, he is regarded as a forerunner of modern seismological scientists.
It is said that many animals sensed danger and fled to higher ground before the water arrived. The Lisbon quake is the first documented reporting of such a phenomenon in Europe.

The geological causes of this earthquake and the seismic activity in the region continue to be discussed and debated by contemporary scientists. Some geologists have suggested that the earthquake may indicate the early development of an Atlantic subduction zone, and the beginning of the closure of the Atlantic ocean. Indeed, the only other recorded earthquakes of this size have been megathrust earthquakes involving subduction, making it all but certain that the Lisbon event was a megathrust earthquake as well.

1830 - Discovery of P & S waves

The early part of the 19th century was an extraordinary time for mathematics.
French mathematicians Navier and Cauchy developed equations for elasticity.

Then in 1830, Poisson published a paper showing that there were two fundamental elastic waves: P and S waves.

Poisson's ratio, which is measure to P velocity, is widely used in seismology today.

Consider an isotropic elastic medium with two different modes of elastic waves:
P - waves: (Primary) alternate compression and expansion, go through all states of matter, faster than other waves, longitudinal waves- (motion of the medium is in the same direction as the wave)

S - waves: (Secondary) shear, not passed through liquids, slower than P waves, transverse waves- (motion of the medium is at right angles to the wave direction).

1862 - Robert Mallet FRS ,The First Seismologist

Von Rebeur-Paschwitz obtained the first recording of a teleseism in 1889. In the next decade, investigators in Italy, Germany, and England studied the waves from distant earthquakes and constructed the first teleseismic travel-time charts. Wiechert introduced a seismometer with viscous damping in 1898.

Theory seems to have been neglected in the early development of the seismograph. Theoretical studies of forced damped harmonic-oscillator seismographs were presented by Perry and Ayrton, and Lippmann, but these had little effect on the construction of seismographs. In the 1890's, the importance of tilt was much debated. By 1900, many seismologists had become convinced that the effect of tilting on seismograph response could usually be neglected.

1906 - The Great San Francisco Earthquake
5:12 AM - April 18, 1906

Photograph from: http://earthquake.usgs.gov

The California earthquake of April 18, 1906 ranks as one of the most significant earthquakes of all time. Today, its importance comes more from the wealth of scientific knowledge derived from it than from its sheer size. Rupturing the northernmost 296 miles (477 kilometers) of the San Andreas fault from northwest of San Juan Bautista to the triple junction at Cape Mendocino, the earthquake confounded contemporary geologists with its large, horizontal displacements and great rupture length. Indeed, the significance of the fault and recognition of its large cumulative offset would not be fully appreciated until the advent of plate tectonics more than half a century later. Analysis of the 1906 displacements and strain in the surrounding crust led Reid (1910) to formulate his elastic-rebound theory of the earthquake source, which remains today the principal model of the earthquake cycle.

At almost precisely 5:12 a.m., local time, a foreshock occurred with sufficient force to be felt widely throughout the San Francisco Bay area. The great earthquake broke loose some 20 to 25 seconds later, with an epicenter near San Francisco. Violent shocks punctuated the strong shaking which lasted some 45 to 60 seconds. The earthquake was felt from southern Oregon to south of Los Angeles and inland as far as central Nevada. The highest Modified Mercalli Intensities (MMI's) of VII to IX paralleled the length of the rupture, extending as far as 80 kilometers inland from the fault trace. One important characteristic of the shaking intensity noted in Lawson's (1908) report was the clear correlation of intensity with underlying geologic conditions. Areas situated in sediment-filled valleys sustained stronger shaking than nearby bedrock sites, and the strongest shaking occurred in areas where ground reclaimed from San Francisco Bay failed in the earthquake. Modern seismic-zonation practice accounts for the differences in seismic hazard posed by varying geologic conditions.

As a basic reference about the earthquake and the damage it caused, geologic observations of the fault rupture and shaking effects, and other consequences of the earthquake, the Lawson (1908) report remains the authoritative work, as well as arguably the most important study of a single earthquake. In the public's mind, this earthquake is perhaps remembered most for the fire it spawned in San Francisco, giving it the somewhat misleading appellation of the "San Francisco earthquake". Shaking damage, however, was equally severe in many other places along the fault rupture. The frequently quoted value of 700 deaths caused by the earthquake and fire is now believed to underestimate the total loss of life by a factor of 3 or 4. Most of the fatalities occurred in San Francisco, and 189 were reported elsewhere.

1923 - The Tokyo Earthquake

On September 1, 1923, just before noon, an earthquake of magnitude 8.3 occurred near the densely populated, modern industrial cities of Tokyo and Yokohama, Japan. The epicenter was placed in Sagami Bay, just southwest of Tokyo Bay. Destruction ranged from far up into the Hakone mountains, home to popular tourist resorts, to the busy shipping lanes of Yokohama Bay, north to the city of Tokyo.

Though not the largest earthquake to ever hit Japan, the proximity to Tokyo and Yokohama and the surrounding areas, with combined populations numbering 2 million, made it one of the most devastating quakes ever to hit Japan. Tokyo's principle business and industrial districts lay in ruins.

At a time when thousands of homes and restaurants had lit fires, mostly gas ranges, for noon-day meal preparation, the quake hit, demolishing buildings and toppling contents of the traditional wood and paper Japanese houses. Flamable materials in the industrial plants and explosions at a munitions factory helped fuel the flames at such a pace that the normally well-prepared firefighters could not keep up. Broken water mains made water unavailable to fight the fires.

Deaths were estimated at nearly 100,000, with an additional 40,000 missing. Hundreds of thousands were left homeless in the resulting fires. Fires in the Honjo and Fukagawa districts of Tokyo surrounded over 30,000 people who took refuge in a large open area. The meager possessions they had fled with became additional fuel for the firestorm and they were literally incinerated on this spot.

The quake is remembered by Japanese authors as the Great Kanto Earthquake, Kanto being the name of the region which includes Tokyo. The year of the quake, 1923, is referred to as Year 12 of the Taisho Era, the 12th year of Emperor Taisho's reign which lasted from 1912 - 1926.

1928 - Wadati-Benioff zone: The Occurance of Deep earthquakes

The structure of the earth's interior is now fully understood as follows:

His inspiration for the technique was the apparent magnitude scale used in astronomy to describe the brightness of stars and other celestial objects. Richter arbitrarily chose a magnitude 0 event to be an earthquake that would show a maximum combined horizontal displacement of 1 micrometre on a seismogram recorded using a Wood-Anderson torsion seismometer 100 km from the earthquake epicenter. This choice was intended to prevent negative magnitudes from being assigned. However, the Richter scale has no upper or lower limit, and sensitive modern seismographs now routinely record quakes with negative magnitudes.

 

Because of the limitations of the Wood-Anderson torsion seismometer used to develop the scale, the original ML cannot be calculated for events larger than about 6.8. Many investigators have proposed extensions to the local magnitude scale, the most popular being the surface wave magnitude S and the body wave (seismology)|body wave magnitude Mb. These traditional magnitude scales have largely been superseded by the implementation of methods for estimating the seismic moment and its associated moment magnitude scale.

Richter magnitudes

The Richter magnitude of an earthquake is determined from the logarithm of the amplitude of waves recorded by seismographs (adjustments are included to compensate for the variation in the distance between the various seismographs and the epicenter of the earthquake). Because of the logarithmic basis of the scale, each whole number increase in magnitude represents a tenfold increase in measured amplitude; in terms of energy, each whole number increase corresponds to an increase of about 32 times the amount of energy released.

The following describes the typical effects of earthquakes of various magnitudes near the epicenter. This table should be taken with extreme caution, since intensity and thus ground effects depend not only on the magnitude, but also on the distance to the epicenter, the depth of the earthquake's focus beneath the epicenter, and geological conditions (certain terrains can amplify seismic signals).

1960 - The Chilean Earthquake

On 22 May 1960 at 19:11 GMT Chile in South America experienced by far the largest recorded earthquake in history measuring 9.5 on Ritcher Scale with several after shocks in the form of Tsunamis and volcanic eruption of Cordón Caulle.

The Great Chilean Earthquake was preceded by a smaller earthquake in Arauco Province at 06:02 on 21 May 1960. Telecommunications to southern Chile were cut off and President Jorge Alessandri had to cancel the traditional ceremony of the Battle of Iquique memorial holiday to oversee the emergency assistance efforts. The government was just beginning to organize help to the affected region when the second earthquake occurred at 14:55 UTC on 22 May in Valdivia.

The second earthquake affected all of Chile between Talca and Chiloé Island, more than 400,000 square kilometers. Coastal villages, such as Toltén, disappeared. At Corral, the main port of Valdivia, the water level rose 4 meters before it began to recede. At 16:20 UTC, an eight-meter wave struck the Chilean coast, mainly between Concepción and Chiloe. Ten minutes later, another wave measuring 10 meters was reported.

Hundreds of people were already reported dead by the time the tsunami struck. Ships, like the Canelo, that were at the mouth of Valdivia River sank after being moved 1.5 km backward and forward in the river. The mast of the Canelo is still visible from the road to Niebla.

A number of Spanish-colonial forts around Valdivia were completely destroyed. Soil subsidence also destroyed buildings, deepened local rivers, and created wetlands in places like the Río Cruces and Chorocomayo, a new aquatic park north of the city. Extensive areas of the city were flooded. The electricity and water systems of Valdivia were totally destroyed. Witnesses reported underground water flowing up through the soil. Despite the heavy rains of 21 May, the city was without a water supply. The river turned brown with sediment from landslides and was full of floating debris- including entire houses. The lack of potable water became a serious problem in Chile's most rainy region.

A transform fault is a geological fault that is a special case of strike-slip faulting which terminates abruptly, at both ends, at a major transverse geological feature. Also known as a conservative plate boundary.

Transform faults comprise one of the three types of plate boundaries in plate tectonics. This term was proposed by J. Tuzo Wilson in 1965 and he particularly recognized the concept in the case of the transverse strike-slip faults along which mid-oceanic ridges are off-set.

1976 - The Tangshan Earthquake, China

The deadliest earthquake of the 20th century

On July 28, 1976 at 3:42 am, an earthquake of magnitude 7.6 struck near the east coast of China. The epicenter was near Tangshan, an industrial city with a population of about 1 million people. (Yong et al., 1988). This quake's destruction was worsened by the fact that it struck in the middle of the night. Almost everyone in the city was asleep, and many people were probably crushed to death without even waking up. Many more who lay injured in the rubble died before they could be rescued. The quake knocked out power through the city, making rescue efforts by shocked residents of the city impossible in the dark. A smaller number of people were trapped in nearby coal mines. Many were rescued, but not until hours or days later (Yong, et al., 1988).

Officially, the Chinese government estimated between 240,000 and 250,000 people were killed (Yong et al., 1988). In the decades since the quake, the death toll is estimated closer to half a million. Either number would make this the most deadly quake in the twentieth century, and the strongest since the Alaska quake in 1964 which was of magnitude 8.4 (Bolt, 1993b).

Geologic Setting

Below the surface lies China's complex geology. Forces pushing in two different directions are squashing the Asian continent. The combination of forces has made China a very active location for earthquakes throughout history. Earthquakes have also played a significant part in Chinese science and culture. The Chinese were the first to develop functioning seismometers, and a record of major quakes in the region has been reconstructed dating back to 1831 BC (Yong et al., 1988).

Tangshan lies on a block of continental crust bounded by major faults. The faults can be hundreds of kilometers in length and have experienced large displacements in the terrain over time. The faults are locked together, grinding against each other as tectonic forces build up. Eventually, the forces will be great enough to break the fault and the the crustal blocks will slip past each other. When the blocks slip, a great amount of energy is released. The energy moves outward much like the ripples created by throwing a rock into a pond. It is this energy that shakes and deforms the earth surface and that we record on seismometers.

China is being squeezed as the Indian plate moves northeastward. As India pushes on the Asian continent, the crust wrinkles and bends, often causing cracks in the cool, rigid crust. The force of this movement was enough to raise the Himalayan mountains, the highest in the world (Sullivan, 1976).

The Pacific plate is also squeezing China, but from the west. This oceanic plate is being subducted beneath the Asian continent at a steep angle because the oceanic crust is old and dense. The plate has had time to cool since its formation. The steep angle of subduction causes a strong horizonal force which acts on the continent. The area of northern china hit by the Tangshan earthquake is recognized as being particularly prone to the westward movement of the Pacific plate. This particular quake occured on a fault in the Tancheng-Lujiang wrench fault system (or Tan-Lu) which is a fault zone, or collection of approximately parallel faults. The zone is very wide and has more than 750 km of displacement (Jiawei, 1993).

"SHAKEMAP" - The Advanced Global Seismic System

ShakeMap is created using data from about several seismic instruments spread across the territory of the earthquake.

One purpose of ShakeMaps is to provide rapid information to aid emergency managers in responding to a quake. Another purpose of ShakeMaps is to reveal local variations in shaking that engineers can use to better design buildings to withstand earthquakes.As building failures kill the most people in earthquakes, the data will be crucial in saving lives in future quakes.

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