What Are Earthquake Waves
Earthquake waves, more commonly known as seismic waves, are vibrations generated by an earthquake and propagated within Earth or along its surface. There are four principal types of elastic waves: two, primary and secondary waves, travel within Earth, whereas the other two, Rayleigh and Love waves, called surface waves, travel along its surface. In addition, seismic waves can be produced artificially by explosions.
How Can We Define And Explain Earthquakes
An earthquake is the sudden and violent shaking of tectonic plates and is caused by the sudden release of energy due to a buildup of stress between tectonic plates. This buildup of stress is a consequence of rocks from the Earth’s Tectonic Plates getting caught on each other and generating friction . The strain eventually overrides the elasticity of the rocks, which results in the release of stress, leading to a shaking motion on the surface.
Two important definitions that you need to know are the focus and epicentre of an earthquake.
- The focus of an earthquake is the point between the tectonic plates where the rocks break off and the earthquake starts. The waves of energy spread from this point, with the greatest damage being close to the focus.
- The epicentre is the point on the Earths surface, above the earthquakes focus.
What Are Intraplate Earthquakes And How Are They Caused
Approximately 5% of earthquakes occur within the plates instead of at the plate margins. These earthquakes that take place away from the plate boundaries are called intraplate earthquakes. Plates travel over a spherical surface, and this creates areas of weakness. An earthquake occurs at these zones of weakness.
Examples of intraplate earthquakes are those caused by the New Madrid Seismic Zone on the Mississippi River. Here, earthquakes reach magnitudes of up to 7.5, even though the zone is thousands of kilometres from any plate margins.
Dont get confused between intraplate and interplate earthquakes! Intraplate earthquakes take place within the inside of a tectonic plate, whereas interplate earthquakes happen at the boundary between two plates.
How Do Scientists Measure The Size Of Earthquakes
The size of an earthquake depends on the size of the fault and the amount of slip on the fault, but thats not something scientists can simply measure with a measuring tape since faults are many kilometers deep beneath the earths surface. So how do they measure an earthquake? They use the seismogram recordings made on the seismographs at the surface of the earth to determine how large the earthquake was. A short wiggly line that doesnt wiggle very much means a small earthquake, and a long wiggly line that wiggles a lot means a large earthquake. The length of the wiggle depends on the size of the fault, and the size of the wiggle depends on the amount of slip.
The size of the earthquake is called its magnitude. There is one magnitude for each earthquake. Scientists also talk about the intensity of shaking from an earthquake, and this varies depending on where you are during the earthquake.
Earthquakes And Volcanic Activity
Earthquakes often occur in volcanic regions and are caused there, both by tectonic faults and the movement of magma in volcanoes. Such earthquakes can serve as an early warning of volcanic eruptions, as during the 1980 eruption of Mount St. Helens. Earthquake swarms can serve as markers for the location of the flowing magma throughout the volcanoes. These swarms can be recorded by seismometers and tiltmeters and used as sensors to predict imminent or upcoming eruptions.
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What Drives The Movement Of Tectonic Plates
Below the tectonic plates lies the Earths asthenosphere. The asthenosphere behaves like a fluid over very long time scales. There are a number of competing theories that attempt to explain what drives the movement of tectonic plates. Three of the forces that have been proposed as the main drivers of tectonic plate movement are:
- mantle convection currents: warm mantle currents drive and carry plates of lithosphere along a like a conveyor belt
- ridge push : newly formed plates at oceanic ridges are warm, so they have a higher elevation at the oceanic ridge than the colder, more dense plate material further away gravity causes the higher plate at the ridge to push away the lithosphere that lies further from the ridge
- slab pull: older, colder plates sink at subduction zones because, as they cool, they become more dense than the underlying mantle and the cooler, sinking plate pulls the rest of the warmer plate along behind it
Research has shown that the major driving force for most plate movement is slab pull, because the plates with more of their edges being subducted are the faster-moving ones. However, ridge push is also presented in recent research to be a force that drives the movement of plates.
How Do Earthquakes Happen
Earthquakes happen when tectonic plates slip. Tectonic plates are constantly moving over or horizontally past one another. As a result, the rocks from the plates get caught on each other and generate friction. The strain eventually overrides the elasticity of the rocks, which results in the release of stress, leading to a shaking motion on the surface.
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Intensity Of Earth Quaking And Magnitude Of Earthquakes
Quaking or shaking of the earth is a common phenomenon undoubtedly known to humans from the earliest times. Before the development of strong-motion accelerometers that can measure peak ground speed and acceleration directly, the intensity of the earth-shaking was estimated based on the observed effects, as categorized on various seismic intensity scales. Only in the last century has the source of such shaking been identified as ruptures in the Earth’s crust, with the intensity of shaking at any locality dependent not only on the local ground conditions but also on the strength or magnitude of the rupture, and on its distance.
The first scale for measuring earthquake magnitudes was developed by Charles F. Richter in 1935. Subsequent scales have retained a key feature, where each unit represents a ten-fold difference in the amplitude of the ground shaking and a 32-fold difference in energy. Subsequent scales are also adjusted to have approximately the same numeric value within the limits of the scale.
Although the mass media commonly reports earthquake magnitudes as “Richter magnitude” or “Richter scale”, standard practice by most seismological authorities is to express an earthquake’s strength on the moment magnitude scale, which is based on the actual energy released by an earthquake.
Where Do Earthquakes Occur
Earthquakes can occur anywhere, but they occur mainly along fault lines , where compressional or tensional forces move rocks on opposite sides of a fracture. Faults extend from a few centimetres to many hundreds of kilometres. In addition, most of the worlds earthquakes occur within the Ring of Fire, a long horseshoe-shaped belt of earthquake epicentres, volcanoes, and tectonic plate boundaries fringing the Pacific basin.
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Most Seismically Active Areas On Earth
There are three major zones where earthquakes are more likely to happen: the Circum-Pacific belt, the Alpide Belt, and the mid-Atlantic Ridge belt.
The Ring of Fire, also known as the Circum-Pacific belt, is an earthquake-prone zone that surrounds the Pacific Ocean, accounting for over 90% of all earthquakes worldwide.
The belt runs along tectonic plate boundaries, where plates of predominantly oceanic crust descend beneath another plate. Slip between plates and rupture within plates produce earthquakes in these subduction zones.
The worlds largest earthquake, the 9.5 magnitude Chilean earthquake in 1960 occurred in this zone. The largest earthquake in the United States, the 1964 9.2 Alaskan tremor that occurred in the Prince William Sound region is also part of the Ring of Fire.
The Alpide belt is the next most seismically active region .
The submerged mid-Atlantic Ridge belt is the area that designates the separation of two tectonic plates . The majority of the mid-Atlantic Ridge is submerged and far from human settlement, although Iceland, which stands directly above the ridge, has seen earthquakes as strong as M6.9.
Earthquakes In The United States
Alask is the U.S. state that experiences the most earthquakes each year. A magnitude 7 earthquake occurs virtually every year in Alaska, and a magnitude 8 or higher earthquake occurs every 14 years on average.
California was the second most active state for number of earthquakes until 2014. An increase in seismicity in Oklahoma in 2014 has the state now ranked second for number of earthquakes. In Oklahoma, there were 585 earthquakes of magnitude 3 or larger in 2014, and roughly 200 in California.
Only four states experienced no earthquakes between 1975 and 1995: Florida, Iowa, North Dakota, and Wisconsin.
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How Do You Keep Safe During An Earthquake
|Scheme 1 – Earthquake Survival Kit. Click on the picture to the right to enlarge it. Identify five things that you recognise and write down why they would be useful to help you to survive after a quake.Click here to be taken to the task sheet.|
You live in Japan. Following on from the last major earthquake in 2011, the plate boundary on which Japan lies has been showing signs of activity. The Japanese Prime Minister has told all local people to put together an earthquake emergency kit that would be useful in the event of another earthquake.Extension Task plenty of visualsScheme 2Designing An Earthquake Resistant Building. Background Research Videos
Shaking And Ground Rupture
Shaking and ground rupture are the main effects created by earthquakes, principally resulting in more or less severe damage to buildings and other rigid structures. The severity of the local effects depends on the complex combination of the earthquake magnitude, the distance from the epicenter, and the local geological and geomorphological conditions, which may amplify or reduce wave propagation. The ground-shaking is measured by ground acceleration.
Specific local geological, geomorphological, and geostructural features can induce high levels of shaking on the ground surface even from low-intensity earthquakes. This effect is called site or local amplification. It is principally due to the transfer of the seismic motion from hard deep soils to soft superficial soils and the effects of seismic energy focalization owing to the typical geometrical setting of such deposits.
Ground rupture is a visible breaking and displacement of the Earth’s surface along the trace of the fault, which may be of the order of several meters in the case of major earthquakes. Ground rupture is a major risk for large engineering structures such as dams, bridges, and nuclear power stations and requires careful mapping of existing faults to identify any that are likely to break the ground surface within the life of the structure.
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Episodic Tremor And Slip
Episodic tremor and slip is periodic slow sliding along part of a subduction boundary. It does not produce recognizable earthquakes but does produce seismic tremors . It was first discovered on the Vancouver Island part of the Cascadia subduction zone by Geological Survey of Canada geologists Herb Dragert and Garry Rogers.
The boundary between the subducting Juan de Fuca Plate and the North America Plate can be divided into three segments . The cold upper part of the Juan de Fuca Plate boundary is locked. The plates are stuck and dont move, except with very large earthquakes that happen approximately every 500 years . The warm lower part of the boundary is sliding continuously because the warm rock is weaker. The central part of the boundary isnt cold enough to be stuck but isnt warm enough to slide continuously. Instead, it slips episodically, approximately every 14 months for about 2 weeks, moving a few centimetres each time.
Since 2003, ETS processes have also been observed on Mexico, New Zealand, and Japan subduction zones.
Earthquake Waves: What Are Surface Waves
There are different types of surface waves, but the two we focus on here are L waves and Rayleigh waves.
- L waves travel through the Earths crust and have a lower frequency. Despite travelling slower than body waves, L waves cause most of the damage made by earthquakes. L waves move the ground from side to side.
- Rayleigh waves roll similarly to waves on water . Although they are slower than the L waves, most of the ground shaking that is felt during an earthquake is due to Rayleigh waves.
How Do The Physical Processes Of Tectonic Plates Impact The Magnitude Of Earthquakes
The different physical processes, which are based on the type of plate margin, impact the magnitude of tectonic hazards, including earthquakes.
Earthquakes at divergent plate margins
Earthquakes at divergent plate margins often have low magnitudes and a shallow focus .
Earthquakes at convergent plate margins
The frequency of earthquakes at convergent plate margins dependson the types of tectonic plates that meet.
Earthquakes at conservative plate margins
Earthquakes at conservative plate margins often have a shallow focus and reach magnitudes up to 8. They can be very destructive and often have aftershocks due to additional stress along the fault.
Earthquake Waves: What Are Body Waves
Body waves are of higher frequency and travel through the interior of the ground. They travel faster than surface waves.
- P waves travel the fastest and move through solid rock and fluids . They are similar to the motion of sound waves. P waves compress and dilate rocks and can also travel through the air.
- S waves are the slower waves that can only travel through solid rock.
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Read A Brief Summary Of This Topic
earthquake, any sudden shaking of the ground caused by the passage of seismic waves through Earths rocks. Seismic waves are produced when some form of energy stored in Earths crust is suddenly released, usually when masses of rock straining against one another suddenly fracture and slip. Earthquakes occur most often along geologic faults, narrow zones where rock masses move in relation to one another. The major fault lines of the world are located at the fringes of the huge tectonic plates that make up Earths crust.
East African Rift System
The East African Rift System is a 50-60 km wide zone of active volcanics and faulting in eastern Africa that stretches north-south for more than 3000 km from Ethiopia in the north to Zambezi in the south.
The East African Rift System is a rare example of an active continental rift zone, in which a continental plate attempts to split into two plates that are moving away from one another.
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The Earthquake Through My Eyes
|Objective: To try to use our imaginations to write about and draw the damage that an earthquake could cause. Task 1 – How would you feel? – Study the presentation below carefully and then watch the video below. Complete the first adjectives activity on a digital copy of worksheet 1.Task 2 – Completing the drawing task using a paper copy of worksheet 2 below. Task 3 – Complete your written task for a local newspaper. Go back to worksheet 1 and use the space provided to complete the activity.||
What Causes An Earthquake
An earthquake is the shaking and vibration of the Earth’s crust due to movement of the Earth’s plates . Earthquakes can happen along any type of plate boundary.
Earthquakes occur when tension is released from inside the crust. Plates do not always move smoothly alongside each other and sometimes get stuck. When this happens pressure builds up. When this pressure is eventually released, an earthquake tends to occur.
The point inside the crust where the pressure is released is called the focus. The point on the Earth’s surface above the focus is called the epicentre.
Earthquake energy is released in seismic waves. These waves spread out from the focus. The waves are felt most strongly at the epicentre, becoming less strong as they travel further away. The most severe damage caused by an earthquake will happen close to the epicentre.
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What Are The Consequences And Effects Of Earthquakes
The consequences of earthquake waves include ground shaking and crustal fracturing. Crustal fracturing occurs within the earth but can impact the surface through buckling and fractures. The secondary hazards caused by earthquakes include tsunamis, landslides, liquefaction, subsidence, and fires. The 2004 Indian Ocean earthquake mentioned at the beginning of the article caused a devasting tsunami.
Why Does Earth Shake During Earthquake
While the edges of faults are stuck together and the rest of the block is moving, the energy that would normally cause the blocks to slide past one another is being accumulated.
When the force of the moving block finally overcomes the friction of the jagged edges of the fault and it breaks free, all that stored up energy is released. The seismic waves shake the earth as they move through it, and when the waves reach the earths surface, they shake the ground and anything on it, like our houses and us.
The energy radiates outward from the fault in all directions, in the form of seismic waves- like ripples on a pond.
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