Explaining How Earthquakes work
and why we have them.
Interior of the earth
The Lithosphere (litho means rock) sphere contains the crust and upper portions of the upper most mantle. It is about 80 km thick on average - or 1.25% of the radius of the earth.
The interior of the earth is dimensionally similar to the dimensions of an egg. The yolk being the core and the crust being the shell. The shell, which is very thin and brittle is similar to how the crust behaves under similar forces. THE CRUST
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Source: http://www.aboutcivil.org/interior-of-the-earth.html
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How Plate Tectonics make Earthquakes
The inside of the earth is very hot and consists of 3 major areas:
- The inner and outer core (very hot - 13,000°C)
- The upper and lower Mantle (Pretty hot - 2-6000°C)
- The crust (~64°C)
Earthquakes occur because of plate tectonics. The theory of plate tectonics is based on the idea that the crust of the earth is sitting on top of a floating mantle of magma. Earthquakes, volcanoes, mountains, trenches and mid ocean ridges are created because of the movement of the broken up crust - also known as tectonic plates. These features are created because of the earths internal heat energy found in the core (through radioactive decay) which then creates convection currents throughout the mantle. The oceanic crust is an extension of the mantle - but in a more solid state. The continental crusts are similar to the scum found from processing iron and are less dense to the liquid iron below.
The liquid rock - aka the mantle - get's its heat from the core at the center of the earth through nuclear decay. That heat is then transferred through the mantle through convection. At the top of the convection cell, the liquid rock cools down to a solid state before coming back down towards the core. Along with it, it moves the crust above. Anytime the crust moves, it rubs against an adjacent piece of crust (aka - tectonic plate). That rubbing snaps and vibrates. This snap and vibration is what we commonly call an earthquake.
Sometimes pieces of crust collide towards each other. This causes the frontal parts of the colliding plates to buckle - this forms mountainous terrains - depending on what types of crust (oceanic or continental) are colliding together.
If it's continental to continental collisions - we get earthquakes and mountains. (i.e. Himalaya's)
If it's oceanic to continental collisions - we get trenches, volcanoes, mountains and earthquakes. (i.e. South America)
If it's oceanic to oceanic collisions - we get trenches, volcanoes and earthquakes - called island arcs (i.e. Japan, Aleutian Islands in Alaska etc.)
The liquid rock - aka the mantle - get's its heat from the core at the center of the earth through nuclear decay. That heat is then transferred through the mantle through convection. At the top of the convection cell, the liquid rock cools down to a solid state before coming back down towards the core. Along with it, it moves the crust above. Anytime the crust moves, it rubs against an adjacent piece of crust (aka - tectonic plate). That rubbing snaps and vibrates. This snap and vibration is what we commonly call an earthquake.
Sometimes pieces of crust collide towards each other. This causes the frontal parts of the colliding plates to buckle - this forms mountainous terrains - depending on what types of crust (oceanic or continental) are colliding together.
If it's continental to continental collisions - we get earthquakes and mountains. (i.e. Himalaya's)
If it's oceanic to continental collisions - we get trenches, volcanoes, mountains and earthquakes. (i.e. South America)
If it's oceanic to oceanic collisions - we get trenches, volcanoes and earthquakes - called island arcs (i.e. Japan, Aleutian Islands in Alaska etc.)
Before an earthquake happens, the magma deep below builds up tension along the surface between the two plates that are held together. Friction is the force that is keeping it together. When there is enough tension to overcome the friction, an earthquake occures and the fault ruptures - which allows the sides of each plate to move relative to each other.
In terms of energy, elastic energy is built up prior to it slipping (potential energy). Once it slips, mechanical energy is released (kinetic energy)
In terms of energy, elastic energy is built up prior to it slipping (potential energy). Once it slips, mechanical energy is released (kinetic energy)
Earthquakes define plate boundaries
Faults & Plate Boundaries
Where the plates interact - we have plate boundaries. At the plate boundaries - it is true that we have faults. But faults can also happen further away from the plate boundary - so let's determine the difference between the two.
Plate boundaries are determined by the interaction between two plates. These plates are either moving away from each other (divergent plate boundaries), or towards each other (convergent plate boundaries) - where they slam into each other - and lastly plates where they slide past one another (transform plate boundaries). |
The major faults lie on plate boundaries between plates. However, many faults can still form near and far from the plate boundary - such as the Wasatch Fault, or any other fault that is a spin off of the San Andreas fault in California. As shown here, Japan is experiencing many earthquakes that are not along the plate boundary that is shown dipping downwards. The shallow earthquakes that are happening at or to the sides of Japan indicate faulting due to regional compression - as the Pacific Plate moves towards Japan and as Japan moves towards the Pacific Plate.
There are three main kinds of faults:
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Click here to find out why the Pacific Plate is sub-ducting.
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The difference between a fault and a plate boundary
A model that shows what happens when a fault forms due to the compressional forces from above and below which squeeze the rock and breaks it.
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A reverse fault, much like the faults that formed most of Japan (not the plate boundary) forms when compressional forces squeeze the two blocks - that result in breaking the rock forming a fault.
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Normal faults, like the Wasatch Fault, happen when tension pulls at the blocks and breaks the rock forming the fault.
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A strike slip fault, like the San Andreas fault (which is a plate boundary) moves to the side relative to the other block instead of up and down.
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Reverse Faulting from Compression |
Normal Faulting from Tension |
Photos of existing faults
Faults Forces & Energy
Faulting happens because of several forces at play. Those forces also relate to energy.
Forces involved:
Energy Involved:
Tension and compression make up the stress along the fault. The fault is the plane of weakness |
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In order for an earthquake to occur, tensional and compressional forces must overcome frictional forces and the rocks rigidity - (or material strength).
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In this video - notice how he describes how stress (tension and compression) must overcome frictional resistance in order to get an Earthquake. Also notice the blue region which is relaxed - low stress, and the red regions that are high stress.
Also note that when an earthquake occurs, the fault is ripped - in this case, the fault ripped up to 75 miles, and ripped at a speed of 5000 mph. That's a very fast rip! I n class, I explain how this is very similar to a windshield crack and snapping your fingers. |
Notice in this video - that once the area that is relieved, it puts more stress on adjacent areas that have not snapped yet. This is where future earthquakes occurred - much like a windshield crack happens in spurts instead of all at once.
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Earthquakes & Seismic Waves:
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Different Seismic Waves
Basically, there are two different types of body waves - or waves that travel through material, and two surface waves - or waves that travel only on the surface.
Body Waves: P - Waves (Animation) S - Waves (Animation) Surface Waves: Love Waves (Animation) Rayleigh Wave (Animation) Source: http://www.geo.mtu.edu/UPSeis/waves.html |
How Seismic waves affect damage
This is a link that discusses how seismic waves damage buildings etc. (Earthquake Shaking)
How Earthquakes generate Tsunami's
Notice in this video how compressional forces of the Pacific Plate are pushing the overlying continental plate (i.e. like Japan). This pushing and subducting must overcome the frictional forces between the two. Once compression exceeds friction, slip happens and the water above is thrusted upwards - causing a huge wave to move outwards. This is how Japan's Tsunami was created.
Using Alaska's Earthquake to explain Japan's Earthquake
Why predicting is so difficult.
Predicting Earthquakes in relation to Japan's earthquakes
How earthquakes shape the earth over longer periods of time
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When an earthquake happens, the land moves is small short bursts. Each time the crust moves due to the mantle moving below, when the crust above breaks and moves, that rumbling we call an earthquake.
A part of Japan, after its last earthquake, moved 8-12 ft eastward. If we were to view it from a distance - it wouldn't look like a lot of movement in a global perspective. However, if over the course of millions of years of earthquakes, you might see gradual fluid like motion of the plates and continents moving, like the animations show above.
A part of Japan, after its last earthquake, moved 8-12 ft eastward. If we were to view it from a distance - it wouldn't look like a lot of movement in a global perspective. However, if over the course of millions of years of earthquakes, you might see gradual fluid like motion of the plates and continents moving, like the animations show above.
Additional Supplemental Videos
- PBS video on Plate Tectonics, "When plates collide" shows a great animation of what happens deep below.
- Anchorage Alaska Earthquake
- Seismograph animation through Norton