Earthquake Notes PDF

Earthquake Volcanic Earthquakes Earthquake - are broad-banded vibratory ground  Earthquakes occurring in conjunction motions, resulting from a number of causes which with Volcanic Activity includes:...

Earthquake Volcanic Earthquakes Earthquake - are broad-banded vibratory ground  Earthquakes occurring in conjunction motions, resulting from a number of causes which with Volcanic Activity includes:  Induced by the movement (injection or withdrawal) of magma  Tectonic ground motions  Volcanism Collapse Earthquakes  Landslide  Small EQs occurring in regions of  Man-made explosions underground caverns and mines Of these, naturally occurring tectonic-related  Sometimes produced by massive land earthquakes are the largest and most important. sliding These are caused by the fracture and sliding of rock Man-made Earthquakes along faults within the Earth's crust and which might  Produced by the explosion of chemical be hundreds of kilometers long and depth of one to or nuclear devices over a hundred kilometers. Earthquake engineering -is the branch of civil Earthquake Initiate a number of phenomena or engineering that provides the principles and agents, called seismic hazards, which can cause procedures for the planning, analysis, and significant damage to the built environment. design of structures and facilities that are These hazards includes fault rupture, vibratory capable of resisting, to a preselected extend, ground motion, tsunami, liquefaction, fire, etc. the effects of earthquakes. What causes an Earthquake? EARTHQUAKE ENGINEERING Today, after the numerous scientific developments of Earthquake Engineering provides the the twentieth century and the many years of principles and procedures for geological and seismological studies, there seems to a) The selection of the proper location of be a clear understanding of what causes earthquakes, structures to minimize their exposure and where and how often they may occur. to earthquake hazards  Modern theories explaining the mechanisms b) The estimation of the earthquake that give birth to earthquake forces that may affect structures and  Phenomena that are deemed responsible for their surrounding environment in a these mechanisms given time interval c) The analysis of structures and the  Correlation between earthquake-generating surrounding environment under the mechanisms and features of the earth's effect of such forces to determine the surface maximum stresses and deformations Types of Earthquakes (According to the that may be imposed upon them Mode of Generation) d) The configuration, proportioning, and detailing of structures to make them Tectonic Earthquakes resist such stresses and deformations  The most common earthquake without collapse or failure of any of  Produced when rocks break suddenly their components in response to the various phological e) The improvement of soils and the forces stabilization of natural slopes to guarantee the stability of structures supported on weak sails or slopes.  He showed that the rupture was not only superficial but also happened in depth. This led During the 19 century systematic field studies to the generally accepted faulting origin of after earthquakes were started and the first earthquakes. attempts to relate them to tectonic processes were also made by Kato (Neodan, Japan, 1897) Harry Fielding Reid - Elastic Rebound among others Theory Professor Bunjiro Koto, Reid gave the five statements of his elastic  Professor of Geology, Paleontology & rebound theory in 1911: Mineralogy, Seismologist 1. The fracture of the rock, which causes  In 1891, a Japanese Seismologist Prof. a tectonic earthquake, is the result of B. Koto, after careful study of the elastic strains, greater than the Mino-Owan Earthquake noted, strength of the rock can withstand produced by the relative Fault displacements of neighbouring A fault is a fracture or zone of fractures between two portions of the earth's crust. blocks of rock. Faults allow the blocks to move 2. These relative displacements are not relative to each other. This movement may occur produced suddenly at the time of the rapidly, in the form of an fracture, but attain their maximum amounts gradually during a more or Faults may range in length from a few millimeters to less long period of time thousands of kilometers 3. The only mass movements that occur Most faults produce repeated displacements over at the time of the earthquake are the geologic time. sudden elastic rebounds off the sides of the fracture towards position of no During an earthquake, the rock on one side of the elastic strain; and these movements fault suddenly slips with respect to the other. extend to distances of only a few miles The fault surface can be horizontal or vertical or some from the fracture arbitrary angle in between. 4. The earthquake vibrations originate in the surface of fracture the surface from which they start has at first a very Professor Bunjiro Koto, Professor of Geology, small area, which may quickly become Paleontology & Mineralogy, Seismologist very large but at a rate of greater than the velocity of compressional elastic In 1891, a Japanese Seismologist, Prof. B. Koto, after waves in the rock careful study of the Mino-Chan Earthquake noted, it 5. The energy Liberated at the time of an can be confidently asserted that the sudden faulting earthquake was immediately before was the actual cause (and not the effect) of the the rupture, in the form of energy of earthquake The controversy at that time was to know elastic strain of the rock if faults were the origin or a consequence of earthquake. In a global sense, tectonic earthquakes result from motion between a number of Harry Fielding Reid- 1906 San Francisco EQ large plates comprising the earth's crust or lithosphere. The final proof came from geodetic observations after the 1906 San Francisco earthquake by Harry Fielding Reid (1859-1944)  Earth Inner Layers fr Ground Subsidence is a phenomenon in which the ground surface of a site settles or depresses as a result of the compaction induced by the vibratory effect of earthquakes. Sites with loose or compressible soils are the most likely to experience ground subsidence Landslides are often triggered by strong earthquakes, these landslides represent Tectonic plates are gigantic pieces of the Earth's crust the failure of slopes that are marginally and uppermost mantle. They are made up of oceanic stable before the earthquake and become crust and continental crust. Earthquakes occur unstable as a result of the violent shaking around mid-ocean ridges and the large faults which generated by the earthquake. mark the edges of the plates. Damaging Effects of Earthquake By Ground Motion Surface Faulting - is a geological feature (ground fissure) associated with the generation of earthquake During an earthquake, the two sides of a fault may slip relative to one another. If a structure lies across a surface fault, then the structure may be damaged Soil Liquefaction is a phenomenon by when the fault slips during an earthquake. which fine saturated granular soils Ground Cracking- is possible when the soil at the temporarily change from a solid to a surface loses its support and sinks, or when it is liquid state and as a result, lose their transported to a different location. ability to carry loads or remain stable. It occurs because when displaced, a soil layer breaks, It occurs when a deposit of loose soil is causing fissures, horsts, and grabens on the ground vigorously shaken or vibrated, and thus surface. it is commonly observed during earthquakes. It is caused by a water pressure build-up that is Earthquake Forces generated when a saturated soil is compacted by For structural engineers and from a conceptual the effect of the earthquake vibrations. point of view, Earthquakes represent just another force for which structures need to be designed. Damaging Effects of Earthquake Earthquake forces, however, possess several Other Effects characteristics be that make them unique in comparison with any forces, such as gravity, Tsunamis large sea waves generated by a sudden wind, or thermal forces. depression of the ocean floor. The dislocation of the ocean floor produced by the slippage of Earthquake forces, the result of a back and undersea earthquake faults is a common source forth and up and down, motion of the ground that supports a structure, can be exceptionally of pools tsunamis large in magnitude, can change rapidly and When a tsunami reaches a coastal area its height erratically during the duration of the may increase to catastrophic levels and strike the earthquake, and may radically different from area with a tremendous force earthquake to earthquake, from one other site to another, from one type of foundation soil to Seiche are long-period oscillating waves another, and from one structure to another. generated by distant earthquakes in enclosed Furthermore, earthquake forces depend on bodies of water such as bays, lakes, reservoirs, the properties of the structure. This means and even swimming pools. that if one modifies such properties, one also Seiches occur when the natural, frequency of a modifies the earthquake forces that will affect water body matches the frequency of the the structure. It also means that they can and usually do-change if the earthquake damages incoming earthquake waves, that is, when the the structure. Most importantly earthquake water body resonates with the earthquake forces are unpredictable. waves. Earthquake forces are also distinct from other forces in the sense that they affect the strength and behavior of structural materials. That is, the properties of structural materials under earthquake loads are different from the properties that are considered when designing, for example, for gravity loads. This is owed to the fact that earthquake forces are applied suddenly, are relatively short, and change in direction many times during the earthquake. Thus, the magnitude of the earthquake forces is only part of the information a structural engineer needs to know to properly design a structure against these forces. Design for Earthquake Forces h) Analysis of foundation soil to assess its susceptibility to earthquake effects Because of the unpredictability of earthquake i) Verification of analytical results using forces, the uncertainty of their occurrence, and laboratory tests of scaled models using the devastating effects they may produce, the shaking tables, or field tests of full- design of an earthquake-resistant structure is scale models using artificial means to an elaborate process that requires the generate ground vibrations participation of architects, seismologists, j) Configuration, proportioning, and geologists, soil engineers, foundation detailing of the members and engineers, and structural engineers. In general, connections of the structure by the it involves many of the following steps: estimated maximum internal forces a) identification of the sources where and deformations future earthquakes are likely to occur k) Improvement of foundation soil with the aid of historical information, properties to reduce soil’s seismological data, and geological susceptibility to earthquake effects studies b) Determination of the probable size of future earthquakes based on the History of study attributes of the identified seismic Since ancient times, human beings have sought sources to understand the formation and composition c) Definition of the distance and of the Earth. orientation of each seismic source concerning the structure’s location The earliest known case were unscientific is d) Establishment of semi-empirical nature taking the form of creation myths or equations that correlate ground religious fables involving the gods. motion characteristics with earthquake size, seismic source Most of the ancient theories about Earth orientation and distance, and site soil tended towards the "Flat- Earth" view of our conditions with the help of planet's physical forms. This was the view in instrumental and observational Mesopotamian culture, where the world was records from previous earthquakes portrayed as a flat disk afloat in an ocean. e) Dynamic analysis of the soil deposits at To the Mayans, the world was flat, and at it the structure’s site to quantify the corners, four jaguar (known as bacabs) held up ground motion amplification that may the sky. be induced as a result of their flexibility f) Selection or modification of structural The ancient Persians speculated that the Earth configuration, structural system, and was a seven-layered ziggurat (or cosmic structural materials to minimize mountain), while the Chinese viewed it as a undesirable structural responses and four- side cube. best resist the expected earthquake a) By the 6th century BCE, Greek forces philosophers began to speculate that g) Dynamic analysis of the structure and the Earth was in fact round, its components to estimate the b) 3rd century BCE, the idea of a spherical maximum values of the internal forces Earth began to become articulated as and deformations that may be a scientific matter. During the same generated by a ground motion with period, the development of a the established characteristics geological view of the Earth also began  to emerge, with philosophers j) 1770 - chemistry was starting to play a understanding that it consisted of pivotal role in the theoretical minerals, metals, and that it was foundation of geology, and theories subject to a very slow process of began to emerge about how the change. Earth's layers were formed c) However, it was not until the 16th and One popular idea had it that liquid 17th centuries that a scientific inundation, like the Biblical Deluge, understanding of planet Earth and its was responsible for creating all the structure truly began to advance. geological strata. Those who accepted d) in 1692, Edmond Halley (discoverer of this theory became known popularly Halley's Comet) proposed what is now as the Diluvianists or Neptunists. known as the "Hollow-Earth" theory. k) 1774- German geologist Abraham e) Halley's construct was a method of Gottlob Werner presented a detailed accounting for the values of the system for identifying specific minerals relative density of Earth and the Moon based on external charactheristics that had been given by Sir Isaac l) 1780s forward, which stated that Newton, in his Philosophiæ Naturalis instead of water, strata had been Principia Mathematica (1687) – which formed through heat (or fire). were later shown to be inaccurate. Those who followed this theory during However, his work was instrumental to the early 19th century referred to this the development of geography and view as Plutonism, which held that the theories about the interior of the Earth Earth formed gradually through the during the 17th and 18th centuries. solidification of molten masses at a f) 17th and 18th centuries about the slow rate. These theories together led authenticity of the Bible and the to the conclusion that the Earth was Deluge myth. This propelled scientists immeasurably older than suggested by and theologians to debate the true age the Bible. of the Earth, and compelled the search m) Early 19th century, the mining for evidence that the Great Flood had industry and Industrial Revolution in fact happened. stimulated the rapid development of Combined with fossil evidence, which the concept of the stratigraphic was found within the layers of the column – that rock formations were Earth, a systematic basis for identifying arranged according to their order of and dating the Earth's strata began to formation in time. Concurrently, emerge. geologists and natural scientists g) Modern mining techniques and began to understand that the age of growing attention to the importance fossils could be determined of minerals and their natural geologically (i.e. that the deeper the distribution also helped to spur the layer they were found in was from the development of modern geology. surface, the older they were). h) 1741, the National Museum of Natural n) During the imperial period of the 19th History in France created the first century, European scientists also had teaching position designated the opportunity to conduct research in specifically for geology. distant lands. One such individual was i) 1751- Encyclopédie by Denis Diderot, Charles Darwin, who had been the term "geology" was accepted recruited by Captain FitzRoy of the HMS Beagle to study the coastal land  of South America and give geological s) early 20th century- Then there was the advice. development of seismology, the study o) Darwin's discovery of giant fossils of earthquakes and the propagation of during the voyage helped to establish elastic waves through the Earth his reputation as a geologist, and his t) 1910 - Harry Fielding Ried put forward theorizing about the causes of their the "elastic rebound theory", based on extinction led to his theory of his studies of the 1906 San Fransisco evolution by natural selection, earthquake. This theory, which stated published in On the Origin of Species in that earthquakes occur when 1859. accumulated energy is released along p) During the 19th century, the a fault line, was the first scientific governments of several countries explanation for why earthquakes including Canada, Australia, Great happen, and remains the foundation Britain and the United States funded for modern tectonic studies. geological surveying that would u) 1926,- English scientist Harold Jeffreys produce geological maps of vast areas claimed that below the crust, the core of the countries. By this time, the of the Earth is liquid, based on his scientific consensus established the study of earthquake waves. age of the Earth in terms of millions of v) 1937- Danish seismologist Inge years, and the increase in funding and Lehmann went a step further and the development of improved determined that within the earth's methods and technology helped liquid outer core, there is a solid inner geology to move farther away from core. dogmatic notions of the Earth's age  In 1929 a large earthquake and structure. occurred near New Zealand. q) By the early 20th century, the Danish seismologist Inge development of radiometric dating Lehmann “the only Danish (which is used to determine the age of seismologist,” as she once minerals and rocks), provided the referred to herself—studied necessary the data to begin getting a the shock waves and was sense of the Earth's true age. By the puzzled by what she saw. turn of the century, geologists now  A few P-waves, which should believed the Earth to be 2 billion years have been deflected by the old, which opened doors for theories core, were in fact recorded at of continental movement during this seismic stations. vast amount of time.  Lehmann theorized that these Research into the ocean floor also led waves had traveled some directly to the theory of Plate distance into the core and Tectonics, which provided the then bounced off some kind of mechanism for Continental Drift. boundary. r) Geophysical evidence suggested  Her interpretation of this data lateral motion of continents and that was the foundation of a 1936 oceanic crust is younger than paper in which she theorized continental crust. This geophysical that Earth’s center consisted evidence also spurred the hypothesis of two parts: of paleomagnetism, the record of the  a solid inner core surrounded orientation of the Earth's magnetic by a liquid outer core, field recorded in magnetic minerals. separated by what has come  to be called the Lehmann  Asthenosphere Discontinuity.  Mesosphere  Lehmann’s hypothesis was  Core confirmed in 1970 when more sensitive seismographs Chemically (Chemical composition) detected waves deflecting off  Crust this solid core. w) Latter half of the 20th century,  Mantel scientists developed a comprehensive  Core theory of the Earth's structure and Chemical Layers dynamics had formed. As the century played out, perspectives shifted to a Crust more integrative approach, where  The outermost later geology and Earth sciences began to  Consists nearly entirely at rocky include the study of the Earth's silicate material, with se aluminium internal structure, atmosphere, and trace amounts of all the naturally biosphere and hydrosphere into one. occurring elements.  As the century played out,  It can be up to 50km thick, but in perspectives shifted to a more places is on thin as 5km. Considering integrative approach, where the Earth has a radius of some geology and Earth sciences 6400km, the crust is like a very thin began to include the study if eggshell. the Earth's internal structure,  There are two types of crust, atmosphere, bloophere and continental crust and oceanic crust. hydrosphere into one  The denser oceanic crust surrounds  This was assisted by the the whole Earth, with 'islands of less development of space flight, dense continental crust floating in it. which allowed for Earth's  The continents are made of atmosphere to be studied in continental crust, while the ocean detail, as well as photographs floors and below the continental crust take of Earth from space. are oceanic crust.  In 1972, the Landsat Program, a series of satellite missions jointly managed by NASA and the U.S. Geological Survey, began supplying satellite images that provided geologically detailed saps, and have be used to predict natural disasters and plate shifts. Mantle Layer can be divided in two ways  The mantle extends from the crust 2900km down and is composed of Mechnically-Rheologicall (Physical silicates with large amounts of iron and Properties) magnesium.  Lithosphere Core Asthenosphere  the core, extends a further 3400km to  is hotter and in a semi-liquid state. the centre of the Earth  Starting at around 80 to 100 km deep,  the core is primarily made of iron and the rock in the asthenosphere slowly nickel metals and is very hot - from flows in a plastic state moving in a 3200°C to 4000°C. It is the magnetic circular motion creating convection iron and nickel in the core that is currents of hot rock thought to be responsible for the  This moves heat from deep within the Earth's magnetic field. mantle towards the surface.  It is this movement which helps move the continents and creates volcanoes and lava flows Mesosphere  Comprising the inner part of the mantle, the mesosphere is a region of very hot solid rock.  Here, although hotter than the asthenosphere, the pressure is too high for liquid rock to form. Core  The core is divided into two parts, the liquid outer core, where temperature wins over pressure and the solid inner core where again the pressure is too Physical layers high for a liquid to form. As already mentioned, the temperature within Moving in from the surface to the centre of the the Earth increases the deeper you go, Earth you could expect to getter hotter and be reaching 4000°C at the centre. Pressure also subjected to ever increasing pressure, but you increases dramatically with depth. The would go from solid, through flowing semi- combination of these two factors creates five liquid plastic rock to solid, liquid and finally distinct layers or regions within the Earth solid again at the centre. alternating between solid, liquid and semi- liquid or "plastic" phases. Lithosphere  Thin, cool and solid, the lithosphere contains the crust and some of the mantle. Composed mainly of silicates, it "floats" on the underlying asthenosphere.  4) In the United States, interest in earthquakes and earthquake Historical Background engineering began after the 1906 1) Robert Mallet, an Irish civil engineer, is earthquake in San Francisco, California often cited as the first earthquake (1000 deaths), which caused great engineer, and his report on the 1857 damage and loss of lives. Naples earthquake is considered to be At that time, however, California was the first scientific investigation that still sparsely populated and, therefore, included observations of the the interest generated by this seismological, geological, and earthquake was not enough to engineering aspects of an earthquake. motivate public officials to develop 2) Modern research on earthquake- earthquake design regulations. resistant structures, however, began It was only after the 1933 earthquake in Japan in 1891, the year of the Nobi in Long Beach, California, that earthquake (7000 deaths; also known American engineers became fully as the Mino-Owari earthquake), with aware of the dangers of earthquakes, the formation of an earthquake and a great impetus was given to the investigation committee set up by the study of seismology and earthquake- Japanese government. It was this resistant designs. committee that first proposed the use As they became fully interested, the of a lateral force equal to a fraction of first inquiry was to find out the nature the total weight of a building to of the motion of the ground during an account for the forces exerted on earthquake. buildings by earthquakes. Special instruments were designed 3) Similar developments in Italy after the and deployed at various areas of high devastating Messina earthquake in seismicity to record such a motion 1908 (58,000 deaths) led to the permanently. appointment of a committee Congress charged the U.S. Coast and composed of practicing and academic Geodetic Survey with the engineers to study the earthquake and responsibility to study and report the formulation of practical strong earthquake motions. recommendations for the seismic At about the same time, new building design of buildings. codes were drawn up and enforced. In its report, this committee The California Legislature passed the recommended that the first story of a Field Act, which made it mandatory for building is designed for a horizontal all school buildings to be designed and force equal to 1/12 of the building built to resist earthquakes. weight above and that its second and 5) Shortly after, the State of California third stories be designed for a adopted the Riley Act, which made it horizontal force equal to 1/8 of the mandatory to design most buildings in building weight above. the state for a lateral load equal to 2% of the sum of their dead and live loads. These Japanese and Italian disasters thus gave The Pacific Coast Building Officials (to birth to practical considerations for the become later the International earthquake design of structures and Conference of Building Officials) earthquake engineering as a new branch of published the nation’s first seismic engineering. design provisions in 1927 in its Uniform Building Code.  Ever since earthquake engineering has V. Felt by nearly everyone, many unfolded at a steady pace and its principles awakened. Some dishes, windows, spread all over the world. It has rapidly etc., broken; a few instances of evolved into a science-based discipline, cracked plaster; unstable objects with a large body of knowledge and overturned. Disturbances of trees, institutionalized research and educational poles, and other tall objects are programs. sometimes noticed. Pendulum clocks may stop. Although learning takes place at a very VI. Felt by all, many frightened and run slow pace due to the infrequency of large outdoors. Some heavy furniture earthquakes, advances in methods of moved; a few instances of fallen dynamic analysis and experimental plaster or damaged chimneys. Damage research have provided engineers with slight. valuable data to gain, year after year, a VII. Everybody runs outdoors. Damage further understanding of earthquakes and negligible in buildings of good design the effects of earthquakes in civil and construction; slight to moderate in engineering structures and facilities, and well-built ordinary structures; to develop new devices and techniques to considerable in poorly built or badly protect these structures and facilities from designed structures; some chimneys such effects. As a result, cities around the broken. Noticed by persons driving world and the people living in them are motor cars. little by little becoming less vulnerable to VIII. Damage slight in specially designed the devastating effect of earthquakes. structures; considerable in ordinary substantial buildings, with partial collapse; great in poorly built structures. Panel walls are thrown out Modified Mercalli Intensity (MM) of frame structures. Fall of chimneys, Scale factory stacks, columns, monuments, walls. Heavy furniture overturned. I. Not felt except by a very few under Sand and mud ejected in small especially favorable circumstances amounts. Changes in well water. II. Felt only by a few persons at rest, Persons driving motor cars disturbed. especially on upper floors of buildings. IX. Damage considerable in specially Delicately suspended objects may designed structures; well-designed swing. frame structures thrown out of plumb; III. Felt quite noticeably indoors, great in substantial buildings, with especially on upper floors of buildings, partial collapse. Buildings shifted off but many people do not recognize it as foundations. The ground cracked an earthquake. Standing motor cars conspicuously. Underground pipes are may rock slightly. Vibration like the broken. passing of a truck. Duration estimated. X. Some well-built wooden structures IV. During the day felt indoors by many, destroyed; most masonry and frame outdoors by few. At night some structures destroyed with awakened. Dishes, windows, doors foundations; ground badly cracked. disturbed; walls make cracking sound. Rails bent. Landslides considerable Sensation like heavy truck striking from river banks and steep slopes. building. Standing motor cars rocked Shifted sand and mud. Water splashed noticeably over banks.  XI. Few, if any, (masonry) structures seismogram because seismographs are so remain standing. Bridges destroyed. sensitive that they can detect the ever-present Broad fissures in the ground. background noise of the earth. They are called Underground pipelines completely out microseisms and arise from local disturbances of service. Earth slumps and landslips such as traffic on the streets, the effect of in soft ground. Rails bent greatly. winds on trees, breaking of the surf on the XII. Damage total. Waves are seen on the beach, and other natural and human-made ground surface. Lines of sight and level disturbances. are distorted. Objects are thrown into the air. RICHTER OR LOCAL MAGNITUDE Besides providing information for the location of earthquakes, seismograms also provide the information that is needed to estimate the size SEISMOGRAMS or strength of an earthquake in terms of what is called earthquake magnitude. This  The records obtained from a instrumentally quantified measure of seismograph are called seismograms. earthquake strength is widely used nowadays  A seismogram is thus a record of the by seismologists, engineers, and even the variation with time of the general public. Although in some cases it fails displacement of the ground, magnified to give an accurate representation of the true by the magnification factor of the strength of an earthquake, it is still routinely seismograph, at the location where used to characterize the intensity of the seismograph is installed. earthquakes and remains a key parameter in  A typical seismogram is shown below. earthquake hazard analysis. The concept of The numbers in the middle of the earthquake magnitude was introduced by record indicate the hours referred to Charles Richter in 1935 to overcome the the GMT. limitations of the intensity scales, the only  The small deflections at regular method used back then to describe and intervals along the trace are time compare earthquakes. Following a marks at 1 min intervals. There are 60 fundamental idea first used by K. such marks in each line, so each line represents the motion recorded Wadati in Japan, Richter based his magnitude during 1 h. scale on a measurement of the wave motion recorded by a seismograph. He borrowed the term magnitude from astronomy as the relative brightness of stars (stellar magnitude) is referred to as magnitude. However, the analogy stops there because in astronomy a smaller magnitude means an increased brightness. Richter defined his scale in terms of the peak amplitude of the trace recorded by the then- standard Wood–Anderson seismograph, It should be noted that the traces in a which, as observed earlier, has a magnification seismogram are never without some little factor of 2800, a natural period of 0.8 s, and a ripples. These little ripples show up in a damping ratio of 80%. However, because such an amplitude can vary significantly from earthquake to earthquake, he used the logarithm of it, as opposed to the amplitude itself, to compress the range of the scale. Similarly, as the amplitude of seismic waves decreases with distance from the earthquake epicenter, he set the measurement of this amplitude at a standard distance of 100 km. Furthermore, he described such a peak trace amplitude about the peak trace amplitude that would be generated by a zero magnitude earthquake; that is, a barely perceptible earthquake. For this purpose, he defined a zero-magnitude earthquake as that which theoretically would produce a seismogram with a peak trace of 1 µm (10−6 m) at a distance of 100 km. As introduced by Richter, earthquake magnitude is thus defined as the logarithm to base ten of the peak wave amplitude measured in micrometers recorded by a Wood–Anderson seismograph at a distance of 100 km from the earthquake epicenter. That is,