Plate tectonics
Plate tectonics is a scientific theory describing the large-scale motion of seven large plates and the movements of a larger number of smaller plates of the Earth's lithosphere, since tectonic processes began on Earth between 3 and 3.5 billion years ago. The model builds on the concept of continental drift, an idea developed during the first decades of the 20th century; the geoscientific community accepted plate-tectonic theory after seafloor spreading was validated in the late 1950s and early 1960s. The lithosphere, the rigid outermost shell of a planet, is broken into tectonic plates; the Earth's lithosphere is composed of many minor plates. Where the plates meet, their relative motion determines the type of boundary: convergent, divergent, or transform. Earthquakes, volcanic activity, mountain-building, oceanic trench formation occur along these plate boundaries; the relative movement of the plates ranges from zero to 100 mm annually. Tectonic plates are composed of oceanic lithosphere and thicker continental lithosphere, each topped by its own kind of crust.
Along convergent boundaries, subduction, or one plate moving under another, carries the lower one down into the mantle. In this way, the total surface of the lithosphere remains the same; this prediction of plate tectonics is referred to as the conveyor belt principle. Earlier theories, since disproven, proposed gradual expansion of the globe. Tectonic plates are able to move because the Earth's lithosphere has greater mechanical strength than the underlying asthenosphere. Lateral density variations in the mantle result in convection. Plate movement is thought to be driven by a combination of the motion of the seafloor away from spreading ridges due to variations in topography and density changes in the crust. At subduction zones the cold, dense crust is "pulled" or sinks down into the mantle over the downward convecting limb of a mantle cell. Another explanation lies in the different forces generated by tidal forces of the Moon; the relative importance of each of these factors and their relationship to each other is unclear, still the subject of much debate.
The outer layers of the Earth are divided into the asthenosphere. The division is based on differences in mechanical properties and in the method for the transfer of heat; the lithosphere is more rigid, while the asthenosphere is hotter and flows more easily. In terms of heat transfer, the lithosphere loses heat by conduction, whereas the asthenosphere transfers heat by convection and has a nearly adiabatic temperature gradient; this division should not be confused with the chemical subdivision of these same layers into the mantle and the crust: a given piece of mantle may be part of the lithosphere or the asthenosphere at different times depending on its temperature and pressure. The key principle of plate tectonics is that the lithosphere exists as separate and distinct tectonic plates, which ride on the fluid-like asthenosphere. Plate motions range up to a typical 10–40 mm/year, to about 160 mm/year; the driving mechanism behind this movement is described below. Tectonic lithosphere plates consist of lithospheric mantle overlain by one or two types of crustal material: oceanic crust and continental crust.
Average oceanic lithosphere is 100 km thick. Because it is formed at mid-ocean ridges and spreads outwards, its thickness is therefore a function of its distance from the mid-ocean ridge where it was formed. For a typical distance that oceanic lithosphere must travel before being subducted, the thickness varies from about 6 km thick at mid-ocean ridges to greater than 100 km at subduction zones. Continental lithosphere is about 200 km thick, though this varies between basins, mountain ranges, stable cratonic interiors of continents; the location where two plates meet is called a plate boundary. Plate boundaries are associated with geological events such as earthquakes and the creation of topographic features such as mountains, mid-ocean ridges, oceanic trenches; the majority of the world's active volcanoes occur along plate boundaries, with the Pacific Plate's Ring of Fire being the most active and known today. These boundaries are discussed in further detail below; some volcanoes occur in the interiors of plates, these have been variously attributed to internal plate deformation and to mantle plumes.
As explained above, tectonic plates may include continental crust or oceanic crust, most plates contain both. For example, the African Plate includes the continent and parts of the floor of the Atlantic and Indian Oceans; the distinction between oceanic crust and continental crust is based on their modes of formation. Oceanic crust is fo
Earth science
Earth science or geoscience includes all fields of natural science related to the planet Earth. This is a branch of science dealing with the physical constitution of its atmosphere. Earth science is the study of our planet’s physical characteristics, from earthquakes to raindrops, floods to fossils. Earth science can be with a much older history. Earth science encompasses four main branches of study, the lithosphere, the hydrosphere, the atmosphere, the biosphere, each of, further broken down into more specialized fields. There are both holistic approaches to earth sciences, it is the study of Earth and its neighbors in space. Some earth scientists use their knowledge of the planet to locate and develop energy and mineral resources. Others study the impact of human activity on Earth's environment, design methods to protect the planet; some use their knowledge about earth processes such as volcanoes and hurricanes to plan communities that will not expose people to these dangerous events. The earth sciences can include the study of geology, the lithosphere, the large-scale structure of the earth's interior, as well as the atmosphere and biosphere.
Earth scientists use tools from geography, physics, chemistry and mathematics to build a quantitative understanding of how the earth works and evolves. Earth science affects our everyday lives. For example, meteorologists study the watch for dangerous storms. Hydrologists warn of floods. Seismologists try to predict where they will strike. Geologists study rocks and help to locate useful minerals. Earth scientists work in the field—perhaps climbing mountains, exploring the seabed, crawling through caves, or wading in swamps, they measure and collect samples they record their findings on charts and maps. The following fields of science are categorized within the earth sciences: Physical geography covers aspects of geomorphology, soil study, meteorology and biogeography. Geology describes the rocky parts of its historic development. Major subdisciplines are mineralogy and petrology, geomorphology, stratigraphy, structural geology, engineering geology, sedimentology. Geophysics and geodesy investigate the shape of the Earth, its reaction to forces and its magnetic and gravity fields.
Geophysicists explore the earth's core and mantle as well as the tectonic and seismic activity of the lithosphere. Geophysics is used to supplement the work of geologists in developing a comprehensive understanding of crustal geology in mineral and petroleum exploration. Seismologists use geophysics to understand plate tectonic shifting, as well as predict seismic activity. Soil science covers the outermost layer of the earth's crust, subject to soil formation processes. Major subdivisions in this field of study include pedology. Ecology covers the interactions between the flora; this field of study differentiates the study of Earth from the study of other planets in the Solar System, Earth being the only planet teeming with life. Hydrology and limnology are studies which focus on the movement and quality of the water and involves all the components of the hydrologic cycle on the Earth and its atmosphere. "Sub-disciplines of hydrology include hydrometeorology, surface water hydrology, watershed science, forest hydrology, water chemistry."
Glaciology covers the icy parts of the Earth. Atmospheric sciences cover the gaseous parts of the Earth between the exosphere. Major subdisciplines include meteorology, atmospheric chemistry, atmospheric physics. Plate tectonics, mountain ranges and earthquakes are geological phenomena that can be explained in terms of physical and chemical processes in the earth's crust. Beneath the Earth's crust lies the mantle, heated by the radioactive decay of heavy elements; the mantle is not quite solid and consists of magma, in a state of semi-perpetual convection. This convection process causes the lithospheric plates to move, albeit slowly; the resulting process is known as plate tectonics. Plate tectonics might be thought of as the process; as the result of seafloor spreading, new crust and lithosphere is created by the flow of magma from the mantle to the near surface, through fissures, where it cools and solidifies. Through subduction, oceanic crust and lithosphere returns to the convecting mantle. Areas of the crust where new crust is created are called divergent boundaries, those where it is brought back into the earth are convergent boundaries and those where plates slide past each other, but no new lithospheric material is created or destroyed, are referred to as transform boundaries Earthquakes result from the movement of the lithospheric plates, they occur near convergent boundaries where parts of the crust are forced into the earth as part of subduction.
Volcanoes result from the melting of subducted crust material. Crust material, forced into the asthenosphere melts, some portion of the melted material becomes light enough to rise to the surface—giving birth to volcanoes; the troposphere, mesosphere and exosphere are the five layers which make up Earth's atmosphere. 75 % of the gases in the atmosphere are located within the lowest layer. In all, the atmosphere is made up of about 78.0% nitrogen, 20.9% ox
Seismometer
A seismometer is an instrument that responds to ground motions, such as caused by earthquakes, volcanic eruptions, explosions. Seismometers are combined with a timing device and a recording device to form a seismograph; the output of such a device — recorded on paper or film, now recorded and processed digitally — is a seismogram. Such data is used to locate and characterize earthquakes, to study the earth's internal structure. A simple seismometer, sensitive to up-down motions of the Earth, is like a weight hanging from a spring, both suspended from a frame that moves along with any motion detected; the relative motion between the weight and the frame provides a measurement of the vertical ground motion. A rotating drum is attached to the frame and a pen is attached to the weight, thus recording any ground motion in a seismogram. Any movement of the ground moves the frame; the mass tends not to move because of its inertia, by measuring the movement between the frame and the mass, the motion of the ground can be determined.
Early seismometers used optical levers or mechanical linkages to amplify the small motions involved, recording on soot-covered paper or photographic paper. Modern instruments use electronics. In some systems, the mass is held nearly motionless relative to the frame by an electronic negative feedback loop; the motion of the mass relative to the frame is measured, the feedback loop applies a magnetic or electrostatic force to keep the mass nearly motionless. The voltage needed to produce this force is the output of the seismometer, recorded digitally. In other systems the weight is allowed to move, its motion produces an electrical charge in a coil attached to the mass which voltage moves through the magnetic field of a magnet attached to the frame; this design is used in a geophone, used in exploration for oil and gas. Seismic observatories have instruments measuring three axes: north-south, east-west, vertical. If only one axis is measured, it is the vertical because it is less noisy and gives better records of some seismic waves.
The foundation of a seismic station is critical. A professional station is sometimes mounted on bedrock; the best mountings may be in deep boreholes, which avoid thermal effects, ground noise and tilting from weather and tides. Other instruments are mounted in insulated enclosures on small buried piers of unreinforced concrete. Reinforcing rods and aggregates would distort the pier as the temperature changes. A site is always surveyed for ground noise with a temporary installation before pouring the pier and laying conduit. European seismographs were placed in a particular area after a destructive earthquake. Today, they are concentrated in high-risk regions; the word derives from the Greek σεισμός, seismós, a shaking or quake, from the verb σείω, seíō, to shake. Seismograph is another Greek term from γράφω, gráphō, to draw, it is used to mean seismometer, though it is more applicable to the older instruments in which the measuring and recording of ground motion were combined, than to modern systems, in which these functions are separated.
Both types provide a continuous record of ground motion. The technical discipline concerning such devices is called seismometry, a branch of seismology; the concept of measuring the "shaking" of something means that the word "seismograph" might be used in a more general sense. For example, a monitoring station that tracks changes in electromagnetic noise affecting amateur radio waves presents an rf seismograph, and Helioseismology studies the "quakes" on the Sun. The first seismometer was made in China during the 2nd Century; the first Western description of the device comes from the French physicist and priest Jean de Hautefeuille in 1703. The modern seismometer was developed in the 19th century. In December 2018, a seismometer was deployed on the planet Mars by the InSight lander, the first time a seismometer was placed onto the surface of another planet. In AD 132, Zhang Heng of China's Han dynasty invented the first seismoscope, called Houfeng Didong Yi; the description we have, from the History of the Later Han Dynasty, says that it was a large bronze vessel, about 2 meters in diameter.
When there was an earthquake, one of the dragons' mouths would open and drop its ball into a bronze toad at the base, making a sound and showing the direction of the earthquake. On at least one occasion at the time of a large earthquake in Gansu in AD 143, the seismoscope indicated an earthquake though one was not felt; the available text says that inside the vessel was a central column that could move along eight tracks. The first earthquake recorded by this seismoscope was "somewhere in the east". Days a rider from the east reported this earthquake. By the 13th century, seismographic devices existed in the Maragheh observatory in Persia. French physicist and priest Jean de Hautefeuille built one in 1703. After 1880, most seismometers were descend
Subduction
Subduction is a geological process that takes place at convergent boundaries of tectonic plates where one plate moves under another and is forced to sink due to gravity into the mantle. Regions where this process occurs are known as subduction zones. Rates of subduction are in centimeters per year, with the average rate of convergence being two to eight centimeters per year along most plate boundaries. Plates include continental crust. Stable subduction zones involve the oceanic lithosphere of one plate sliding beneath the continental or oceanic lithosphere of another plate due to the higher density of the oceanic lithosphere; that is, the subducted lithosphere is always oceanic while the overriding lithosphere may or may not be oceanic. Subduction zones are sites that have a high rate of volcanism and earthquakes. Furthermore, subduction zones develop belts of deformation and metamorphism in the subducting crust, whose exhumation is part of orogeny and leads to mountain building in addition to collisional thickening.
Subduction zones are sites of gravitational sinking of Earth's lithosphere. Subduction zones exist at convergent plate boundaries where one plate of oceanic lithosphere converges with another plate; the descending slab, the subducting plate, is over-ridden by the leading edge of the other plate. The slab sinks at an angle of twenty-five to forty-five degrees to Earth's surface; this sinking is driven by the temperature difference between the subducting oceanic lithosphere and the surrounding mantle asthenosphere, as the colder oceanic lithosphere has, on average, a greater density. At a depth of greater than 60 kilometers, the basalt of the oceanic crust is converted to a metamorphic rock called eclogite. At that point, the density of the oceanic crust provides additional negative buoyancy, it is at subduction zones that Earth's lithosphere, oceanic crust and continental crust, sedimentary layers and some trapped water are recycled into the deep mantle. Earth is so far the only planet. Subduction is the driving force behind plate tectonics, without it, plate tectonics could not occur.
Oceanic subduction zones dive down into the mantle beneath 55,000 kilometers of convergent plate margins equal to the cumulative 60,000 kilometers of mid-ocean ridges. Subduction zones burrow but are imperfectly camouflaged, geophysics and geochemistry can be used to study them. Not the shallowest portions of subduction zones are known best. Subduction zones are asymmetric for the first several hundred kilometers of their descent, they start to go down at oceanic trenches. Their descents are marked by inclined zones of earthquakes that dip away from the trench beneath the volcanoes and extend down to the 660-kilometer discontinuity. Subduction zones are defined by the inclined array of earthquakes known as the Wadati–Benioff zone after the two scientists who first identified this distinctive aspect. Subduction zone earthquakes occur at greater depths than elsewhere on Earth; the subducting basalt and sediment are rich in hydrous minerals and clays. Additionally, large quantities of water are introduced into cracks and fractures created as the subducting slab bends downward.
During the transition from basalt to eclogite, these hydrous materials break down, producing copious quantities of water, which at such great pressure and temperature exists as a supercritical fluid. The supercritical water, hot and more buoyant than the surrounding rock, rises into the overlying mantle where it lowers the pressure in the mantle rock to the point of actual melting, generating magma; the magmas, in turn, rise. The mantle-derived magmas can continue to rise to Earth's surface, resulting in a volcanic eruption; the chemical composition of the erupting lava depends upon the degree to which the mantle-derived basalt interacts with Earth's crust and/or undergoes fractional crystallization. Above subduction zones, volcanoes exist in long chains called volcanic arcs. Volcanoes that exist along arcs tend to produce dangerous eruptions because they are rich in water and tend to be explosive. Krakatoa, Nevado del Ruiz, Mount Vesuvius are all examples of arc volcanoes. Arcs are known to be associated with precious metals such as gold and copper believed to be carried by water and concentrated in and around their host volcanoes in rock called "ore".
Although the process of subduction as it occurs today is well understood, its origin remains a matter of discussion and continuing study. Subduction initiation can occur spontaneously if denser oceanic lithosphere is able to founder and sink beneath adjacent oceanic or continental lithosphere. Both models can yield self-sustaining subduction zones, as oceanic crust is metamorphosed at great depth and becomes denser than the surrounding mantle rocks. Results from numerical models favor induced subduction initiation for most modern subduction zones, supported by geologic studies, but other analogue modeling shows the possibility of spontaneous subduction from inherent density differences between two plates at passiv
Mid-ocean ridge
A mid-ocean ridge is an underwater mountain system formed by plate tectonics. It consists of various mountains linked in chains having a valley known as a rift running along its spine; this type of oceanic mountain ridge is characteristic of what is known as an'oceanic spreading center', responsible for seafloor spreading. The production of new seafloor results from mantle upwelling in response to plate spreading; the buoyant melt rises as magma at a linear weakness in the oceanic crust, emerges as lava, creating new crust upon cooling. A mid-ocean ridge demarcates the boundary between two tectonic plates, is termed a divergent plate boundary. Mid-ocean ridges are geologically active, with continuing seismicity. New magma emerges onto the ocean floor and intrudes into the ocean crust at and near rifts along the ridge axes; the crystallized magma below it, gabbro. MORs are formed by two oceanic plates moving away from each other. Hydrothermal vents are a common feature at oceanic spreading centers.
Mid-ocean ridge basalt is a tholeiitic basalt and is characteristically low in incompatible elements. The rocks making up the crust below the seafloor are youngest along the axis of the ridge and age with increasing distance from that axis. New magma of basalt composition emerges at and near the axis because of decompression melting in the underlying Earth's mantle; the oceanic crust is made up of rocks much younger than the Earth itself. Most oceanic crust in the ocean basins is less than 200 million years old; the crust is in a constant state of "renewal" at the ocean ridges. Moving away from the mid-ocean ridge, ocean depth progressively increases; as the oceanic crust moves away from the ridge axis, the peridotite in the underlying mantle cools and becomes more rigid. The crust and the rigid peridotite below it make up the oceanic lithosphere; the bathymetry, or profile of a MOR is determined by the seafloor spreading rate at the ridge. Slow spreading ridges like the Mid-Atlantic Ridge have large, wide rift valleys, sometimes as wide as 10–20 km, rugged terrain at the ridge crest that can have relief of up to a 1,000 m.
By contrast, fast spreading ridges like the East Pacific Rise are narrow, sharp incisions surrounded by flat topography that slopes away from the ridge over many hundreds of miles. Ultra-slow spreading ridges, like the Southwest India and the Arctic Ridges form both magmatic and amagmatic ridge segments without transform faults; the spreading center or axis connects to a transform fault oriented at right angles to the axis. The flanks of mid-ocean ridges are in many places marked by the inactive scars of transform faults called fracture zones. At faster spreading rates the axes display Overlapping Spreading Centers that lack connecting transform faults; the depth of the seafloor is correlated with its age. The age-depth relation can be modeled by the cooling of a lithosphere plate or mantle mantle half-space in areas without significant subduction; the overall shape of ridges results from Pratt isostacy: close to the ridge axis there is hot, low-density mantle supporting the oceanic crust. As the oceanic plates cool, away from the ridge axes, the oceanic mantle lithosphere thickens and the density increases.
Thus older seafloor is deeper. The width of the ridge is hence a function of spreading rate – slow ridges like the MAR have spread much less far than faster ridges like the EPR for the same amount of cooling and consequent bathymetric deepening. Two processes, ridge-push and slab pull, are thought to be responsible for the spreading seen at mid-ocean ridges, there is some uncertainty as to, dominant. Ridge-push occurs when the growing bulk of the ridge pushes the rest of the tectonic plate away from the ridge towards a subduction zone. At the subduction zone, "slab-pull" comes into effect; this is the weight of the tectonic plate being subducted below the overlying plate dragging the rest of the plate along behind it. The slab pull mechanism is considered to be contributing more than the ridge push; the other process proposed to contribute to the formation of new oceanic crust at mid-ocean ridges is the "mantle conveyor". However, there have been some studies which have shown that the upper mantle is too plastic to generate enough friction to pull the tectonic plate along.
Moreover, unlike in the image above, mantle upwelling that causes magma to form beneath the ocean ridges appears to involve only its upper 400 km, as deduced from seismic tomography and from studies of the seismic discontinuity at about 400 km. The shallow depths from which the upwelling mantle rises below ridges are more consistent with the "slab-pull" process. On the other hand, some of the world's largest tectonic plates such as the North American Plate are in motion, yet are nowhere being subducted; the rate at which the mid-ocean ridge creates new material is known as the spreading rate, is measured in mm/yr. As a general rule, fast ridges have spreading rates of more than 90 mm/year. Intermediate ridges have a spreading rate of 50–90 mm/year
Earthscope
Earthscope is an earth science program using geological and geophysical techniques to explore the structure and evolution of the North American continent and to understand the processes controlling earthquakes and volcanoes. The project has three components: USARRAY, the Plate Boundary Observatory, the San Andreas Fault Observatory at Depth; the project is funded by the National Science Foundation, the data produced is publicly accessible in real-time. Organizations associated with the project include UNAVCO, the Incorporated Research Institutions for Seismology, Stanford University, the United States Geological Survey and National Aeronautics and Space Administration. Several international organizations contribute to the initiative. There are three EarthScope observatories, these include the San Andreas Fault Observatory at Depth, the Plate Boundary Observatory, the Seismic and Magnetotelluric Observatory; these observatories consist of boreholes into an active fault zone, global positioning system receivers, long-baseline laser strainmeters, borehole strainmeters and portable seismographs, magnetotelluric stations.
The various EarthScope components will provide integrated and accessible data on geochronology and thermochronology and geochemistry, structure and tectonics, surficial processes and geomorphology, geodynamic modeling, rock physics, hydrogeology. USArray, managed by IRIS, is a 15-year program to place a dense network of permanent and portable seismographs across the continental United States; these seismographs record the seismic waves released by earthquakes. Seismic waves are indicators of energy disbursement within the earth. By analyzing the records of earthquakes obtained from this dense grid of seismometers, scientists can learn about Earth structure and dynamics and the physical processes controlling earthquakes and volcanoes; the goal of USArray is to gain a better understanding of the structure and evolution of the continental crust and mantle underneath North America. The USArray is composed of four facilities: a Transportable Array, a Flexible Array, a Reference Network, a Magnetotelluric Facility.
The Transportable Array is composed of 400 seismometers that are being deployed in a rolling grid across the United States over a period of 10 years. The stations are placed 70 km apart, can map the upper 70 km of the Earth. After two years, stations are moved east to the next site on the grid – unless adopted by an organization and made a permanent installation. Once the sweep across the United States is completed, over 2000 locations will have been occupied; the Array Network Facility is responsible for data collection from the Transportable Array stations. The Flexible Array is composed of 291 broadband stations, 120 short period stations, 1700 active source stations; the Flexible Array allows sites to be targeted in a more focused manner than the broad Transportable Array. Natural or artificially created seismic waves can be used to map structures in the Earth; the Reference Network is composed of permanent seismic stations spaced about 300 km apart. The Reference Network provides a baseline for the Transportable Flexible Array.
EarthScope added and upgraded 39 stations to the existing Advanced National Seismic System, part of the Reference Network. The Magnetotelluric Facility is composed of seven permanent and 20 portable sensors that record electromagnetic fields, it is the electromagnetic equivalent of the seismic arrays. The portable sensors are moved in a rolling grid similar to the Transportable Array grid, but are only in place about a month before they are moved to the next location. A magnetotelluric station consists of a magnetometer, four electrodes, a data recording unit that are buried in shallow holes; the electrodes are oriented north-south and east-west and are saturated in a salt solution to improve conductivity with the ground. The Plate Boundary Observatory PBO consists of a series of geodetic instruments, Global Positioning System receivers and borehole strainmeters, that have been installed to help understand the boundary between the North American Plate and Pacific Plate; the PBO network includes several major observatory components: a network of 1100 permanent, continuously operating Global Positioning System stations many of which provide data at high-rate and in real-time, 78 borehole seismometers, 74 borehole strainmeters, 26 shallow borehole tiltmeters, six long baseline laser strainmeters.
These instruments are complemented by InSAR and LiDAR imagery and geochronology acquired as part of the GeoEarthScope initiative. PBO includes comprehensive data products, data management and education and outreach efforts; these permanent networks are supplemented by a pool of portable GPS receivers that can be deployed for temporary networks to researchers, to measure the crustal motion at a specific target or in response to a geologic event. The Plate Boundary Observatory portion of EarthScope is operated by Inc.. UNAVCO is a non-profit, university-governed consortium that facilitates research and education using geodesy; the San Andreas Fault Observatory at Depth consists of a main borehole that cuts across the active San Andreas Fault at a depth of 3 km and a pilot hole about 2 km southwest of San Andreas Fault. Data from the instruments installed in the holes, which consist of geophone sensors, data acquisition systems, GPS clocks, as well as samples collected during drilling, will help to better understand the processes that control the behavior of the San Andreas Fault.
Data collected from th
Seismogram
A seismogram is a graph output by a seismograph. It is a record of the ground motion at a measuring station as a function of time. Seismograms record motions in three cartesian axes, with the z axis perpendicular to the Earth's surface and the x- and y- axes parallel to the surface; the energy measured in a seismogram may result from an earthquake or from some other source, such as an explosion. Seismograms can record lots of things, record many little waves, called microseisms; these tiny microseisms can be caused by heavy traffic near the seismograph, waves hitting a beach, the wind, any number of other ordinary things that cause some shaking of the seismograph. Seismograms were recorded on paper attached to rotating drums; some used pens on ordinary paper. Today all seismograms are recorded digitally to make analysis by computer easier; some drum seismometers are still found when used for public display. Seismograms are essential for finding the magnitude of earthquakes. Prior to the availability of digital processing of seismic data in the late 1970s, the records were done in a few different forms on different types of media.
A Helicorder drum is a device used to record data into photographic paper or in the form of paper and ink. A piece of paper is wrapped around a rotating drum of the helicorder which receives the seismic signal from a seismometer. For each predefined interval of data, the helicorder will plot the seismic data in one line before moving to the next line at the next interval; the paper must be changed. In the model that use ink, regular maintenance of the pen must be done for accurate recording. A Develocorder is a machine; the machine was developed by Teledyne Geotech during the mid 1960s. It can automatically plot seismograms from 18 seismic signal sources and 3 time signals on a continuous reel of film; the signals from seismometers are processed by 15.5 Hz recording galvanometers which record the seismograms to a reel of 200 feet of film at the speeds between 3 and 20 centimetres per minute. The machine has self-contained circulating chemicals that are used to automatically develop the film. However, the machine takes at least 10 minutes from time of recording to the time that the film can be viewed.
After the digital processing has been used, the archives of the seismograms were recorded in magnetic tapes. The data from the magnetic tapes can be read back to reconstruct the original waveforms. Due to the deterioration of older magnetic tape medias, large number of waveforms from the archives in the early digital recording days are not recoverable. Today, many other forms are used to digitally record the seismograms into digital medias. Seismograms are read from left to right. Time marks show. Time is shown by half-hour units; each rotation of the seismograph drum is thirty minutes. Therefore, on seismograms, each line measures thirty minutes; this is a more efficient way to read a seismogram. Secondly, there are the minute-marks. A minute mark looks like a hyphen "-" between each minute. Minute marks count minutes on seismograms. From left to right, each mark stands for a minute; each seismic wave looks different. The P-wave is the first wave, bigger than the other waves; because P waves are the fastest seismic waves, they will be the first ones that the seismograph records.
The next set of seismic waves on the seismogram will be the S-waves. These are bigger than the P waves, have higher frequency. Look for a dramatic change in frequency for a different type of wave. Vertical seismic profile First break picking Linear seismic inversion How Do I Read a Seismogram? from Michigan Technological University REV, the Rapid Earthquake Viewer from the University of South Carolina
