The energy radiates outward from the fault in all directions in the form of seismic waves like ripples on a pond. The cartoon sketch of the seismograph shows how the insrument shakes with the earth below it, but the recording device remains stationary instead of the other way around. Earthquakes are recorded by instruments called seismographs. The recording they make is called a seismogram. The seismograph has a base that sets firmly in the ground, and a heavy weight that hangs free.
When an earthquake causes the ground to shake, the base of the seismograph shakes too, but the hanging weight does not. Instead the spring or string that it is hanging from absorbs all the movement. The difference in position between the shaking part of the seismograph and the motionless part is what is recorded. 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 figure 5. 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.
Seismograms come in handy for locating earthquakes too, and being able to see the P wave and the S wave is important. P waves are also faster than S waves, and this fact is what allows us to tell where an earthquake was.
Light travels faster than sound, so during a thunderstorm you will first see the lightning and then you will hear the thunder. If you are close to the lightning, the thunder will boom right after the lightning, but if you are far away from the lightning, you can count several seconds before you hear the thunder. The further you are from the storm, the longer it will take between the lightning and the thunder. P waves are like the lightning, and S waves are like the thunder.
The P waves travel faster and shake the ground where you are first. Then the S waves follow and shake the ground also. If you are close to the earthquake, the P and S wave will come one right after the other, but if you are far away, there will be more time between the two. P Waves alternately compress and stretch the crustal material parallel to the direction they are propagating. S Waves cause the crustal material to move back and forth perpendicular to the direction they are travelling.
A seismic wave is a type of elastic wave. A zone of fractures or breaks in rocks, where movements occur. Earthquakes often occur along faults because they are weak zones in the rock. More-or-less symmetrical splays into sub-faults near the intersection of the main fault with the ground surface. An earthquake that is smaller than, and precedes, a "mainshock". Foreshocks tend to occur in the same area as the mainshock.
Foreshocks have not been observed before damaging earthquakes in British Columbia. A crustal block of rock generally long and narrow, that has dropped down along boundary faults relative to the adjacent rocks. Discontinuity in seismic velocity that marks the boundary between the core and the mantle; named after seismologist Beno Gutenberg. The subsurface location focus at which the energy of an earthquake is released. Earthquakes generally occur at depths less than about 30 km, but may occur to a depth of km or more in some areas.
Central solid region of the Earth's core, probably mostly iron; radius about kilometers, discovered by Inge Lehmann in The Modified Mercalli Scale is a numerical scale used to catagorize earthquakes based on descriptions of how the earthquake was felt.
These effects may range from I not felt except by a very few under especially favourable conditions and XII total damage. Earthquake with its focus on a plate boundary.
Offshore earthquakes of western Canada are of this type. Earthquake with its focus within a tectonic plate. Eastern Canadian earthquakes are of this type. A line connecting points on the Earth's surface at which earthquake intensity is the same.
It is usually a closed curve around the epicentre. An abrupt movement of geological materials downhill in response to gravity. Landslides can be triggered by an earthquake or other natural causes.
Undersea landslides can cause tsunamis, such as the one triggered by the Grand Banks earthquake. The location of a point north or south of the equator. Latitude is shown on a map or globe as east-west lines parallel to the equator. A strike-slip fault on which the displacement of the far block is to the left when viewed from either side. The process in which a granular solid soil takes on the characteristics of a liquid as a result of an increase in pore pressure and a reduction in stress.
In other words, solid ground loses cohesion and starts flowing like a liquid. The outer, rigid shell of the Earth above the asthenosphere. It contains the crust, continents, and plates. A surface wave which travels through the continental crust.
This wave type is the one which causes damage during large Eastern Canadian earthquakes. The location of a point east or west of the prime meridian. Longitude is shown on a map or globe as north-south lines left and right of the prime meridian, which passes through Greenwich, England. A major type of surface wave having a horizontal motion that is shear or transverse to the direction of propagation travel.
It is named after A. Love, the English mathematician who discovered it. Any layer in the Earth in which seismic wave velocities are lower than in the layers above and below. Magnitude is a measure of the amount of energy released during an earthquake.
It may be expressed using the Richter scale. The largest earthquake in a "cluster" of earthquakes. Mainshocks are sometimes preceded by "foreshocks", and generally followed by aftershocks. The main bulk of the Earth, between the crust and the core, ranging from depths of about 40 to kilometres. It is composed of dense silicate rocks and divided into a number of concentric shells. Under Eastern Canada, it can be found at around 40 km depth. A more or less continuous motion in the Earth that is unrelated to an earthquake and that has a period of 1.
It is caused by a variety of natural and artificial agents. The division of a town or county into smaller areas according to their variation in seismic hazard. The Mercalli scale rates the intensity of shaking from an earthquake. The ratings vary from I felt only under especially favourable circumstances to XII total destruction.
The boundary surface or sharp seismic-velocity discontinuity pronounced Mo-ho-ro-vi-chich that separates the Earth's crust from the underlying mantle. Named for Andrija Mohorovicic, the Croatian seismologist who first suggested its existence. A measure of earthquake size related to the leverage of the forces couples across the area of the fault slip. The rigidity of the rock times the area of faulting times the amount of slip. Dimensions are dyne-cm or Newton-metres.
A dip-slip fault in which the rock above the fault plane has moved downward with respect to the rock below. Outer liquid shell of the Earth's core, probably iron with some oxygen; inner radius, kilometres, outer radius, kilometres.
Also called primary, longitudinal, irrotational, push, pressure, dilatational, compressional, or push-pull wave. P waves are the fastest body waves and arrive at stations before the S waves, or secondary waves. Their velocity in the crust varies between 5. The waves carry energy through the Earth as longitudinal waves, moving particles in the same line as the direction of the wave. P waves can travel through all layers of the Earth.
P waves are generally felt by humans as a bang or thump. Continental margin formed during initial rifting apart of continents to form an ocean; frequently has thick sedimentary deposits. The time interval between successive crests in a sinusoidal wave train; the period is the inverse of the frequency of a cyclic event.
The onset of a displacement or oscillation on a seismogram, indicating the arrival of a different type of seismic wave. The crust and upper mantle of the earth are made up of about a dozen large plates and several smaller ones that are constantly moving. The movements are very slow - only a few centimetres per year. Where the plates rub against one another, strain builds up, especially at the edges.
When the strength of the rock is exceeded, the earth's crust may break and suddenly shift by several metres, causing an earthquake.
A change in the geological or geophysical conditions that is a forerunner to earthquake generation on a fault. Precursors cannot reliably be recognized as such beforehand. The forecasting in time, place, and magnitude of an earthquake; the forecasting of strong ground motions. Currently, there is no reliable method of predicting earthquakes. The number of cases that meet a given description divided by the total number of equally likely cases possible. A type of surface wave having a retrograde elliptical motion at the Earth's surface.
These are the slowest, but often the largest and most destructive, of the wave types caused by an earthquake. They are usually felt as a rolling or rocking motion and, in the case of major earthquakes, can be seen as they approach. Named after Lord Rayleigh, the English physicist who predicted its existence. The rock above the fault plane the "hanging"wall moves up and over the rock below "foot" wall. Region where the crust has split apart, usually marked by a rift valley e.
A strike-slip fault on which the displacement of the far block is to the right when viewed from either side. The Queen Charlotte fault is one of this type of fault. An index of the resistance of an elastic body to shear. The ratio of the shearing stress to the amount of angular rotation it produces in a rock sample.
The probabilistic risk is the odds of an earthquake occurring and causing damage within a given time interval and region. The Rossi-Forel scale is a measure of intensity of shaking from an earthquake. This scale was replaced by the Mercalli intensity scale. The elevation of the water level above the immediate tide level when a tsunami runs up onto the coastal land. The area of the Earth through which faulting occurred during an earthquake.
For very small earthquakes, this zone could be a few millimetres long, but in the case of a great earthquake, the rupture zone may extend several hundred kilometres in length and tens of kilometres in width.
Also called shear, secondary, rotational, tangential, equivoluminal, distortional, transverse, or shake wave. These waves carry energy through the Earth in very complex patterns of transverse crosswise waves. These waves move more slowly than P waves, but in an earthquake they are usually bigger. S waves cannot travel through the outer core because these waves cannot exist in fluids, such as air, water or molten rock.
A narrow geological depression found in strike-slip fault zones. Those that contain water are called sag ponds. A free or standing wave oscillation of the surface of water in an enclosed basin, that is initiated by local atmospheric changes, tidal currents, or earthquakes. Similar to water sloshing in a bathtub. An elongated earthquake zone, for example, circum-Pacific, Mediterranean, Rocky Mountain.
Seismic waves are vibrations generated by sudden movements of rock. After earthquakes occur, the seismic waves propagate from the hypocentre to the surface of the Earth. Some waves have a high enough frequency to be audible; others have a very low frequency corresponding to periods of several seconds or minutes. Earthquakes generate two main types of waves: compressive P waves, and transverse S waves. The two types of waves travel through the interior of the Earth from the hypocentre, but only the compressive waves travel through the part of the Earth called the outer core, which is composed of molten matter.
Compressive waves travel faster; they are first to arrive at the surface. This is why they're called primary or P waves. Transverse waves do not travel as fast; they are therefore called secondary or S waves. Sometimes the first indication of an earthquake is a a sudden low sound, indicating the arrival of the P waves.
Then, the S waves reach the surface and cause a more violent shaking. A very sensitive instrument used to record and measure earthquakes.
During an earthquake, vibrations initiated by fracturing of the earth's crust radiate outward from the point of fracture and are detected by seismographs. The visual record produced is called a "seismogram". The comparison between the amplitude of the seismic signal and the amplitude of noise caused by seismic unrest and or the seismic instruments.
Growth in the amplitude of earthquakes when seismic waves pass from rock into less rigid material such as soil. The layout of seismometer or geophone groups from which data from a single shot the explosive charge are recorded simultaneously. He has been a fisheries scientist with the Massachusetts Division of Marine Fisheries since and currently heads up the Massachusetts Shark Research Program. For more than 30 years, Greg has been actively involved in the study of life history, ecology, and physiology of sharks.
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He has written dozens of scientific research papers and has appeared in a number of film and television documentaries, including programs for National Geographic, Discovery Channel, BBC, and numerous television networks. His most recent book, The Shark Handbook, is a must buy for all shark enthusiasts.
Robert D. He served in the U. Navy for more than 30 years and continues to work with the Office of Naval Research. A pioneer in the development of deep-sea submersibles and remotely operated vehicle systems, he has taken part in more than deep-sea expeditions.
In , he discovered the RMS Titanic , and has succeeded in tracking down numerous other significant shipwrecks, including the German battleship Bismarck , the lost fleet of Guadalcanal, the U. He is known for his research on the ecology and evolution of fauna in deep-ocean hydrothermal, seamount, canyon and deep trench systems. He has conducted more than 60 scientific expeditions in the Arctic, Atlantic, Pacific, and Indian Oceans. Sunita L. Her research explores how the larvae of seafloor invertebrates such as anemones and sea stars disperse to isolated, island-like habitats, how larvae settle and colonize new sites, and how their communities change over time.
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