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Earthquake:
An earthquake is a ground shaking caused by a sudden release of energy in the Earth’s crust, typically from the movement of tectonic plates along fault lines.
Here’s a more detailed look at earthquakes:
Tectonic Plates:
Earthquakes are primarily caused by the movement of the Earth’s tectonic plates, which are massive slabs of rock that make up the Earth’s outer layer.
Fault lines are fractures in the Earth’s crust where these plates meet and move against each other.
Stress and Strain:
As plates move, they exert stress on the rocks along the fault lines, building up strain.
What they are:
Fault lines are fractures or zones of fractures in the Earth’s crust where rock blocks have moved relative to each other.
They are often found at plate boundaries, where tectonic plates interact, and can be caused by tensional, compressional, or shear forces.
When two blocks of rock along a fault suddenly slip past each other, it can release energy in the form of seismic waves, causing an earthquake.
The structure of the Earth
Earthquakes are the result of sudden movement along faults within the Earth. The movement releases stored-up ‘elastic strain’ energy in the form of seismic waves, which propagate through the Earth and cause the ground surface to shake. Such movement on the faults is generally a response to long-term deformation and the buildup of stress.
- Seismic waves from large earthquakes pass throughout the Earth. These waves contain vital information about the internal structure of the Earth. As seismic waves pass through the Earth, they are refracted, or bent, like rays of light bend when they pass through a glass prism.
- Because the speed of the seismic waves depends on density, we can use the travel-time of seismic waves to map change in density with depth and show that the Earth is composed of several layers.
Plate Tectonics
The Earth’s outermost layer is fragmented into about 15 major slabs called tectonic plates. These slabs form the lithosphere, which is comprised of the crust (continental and oceanic) and the upper part of the mantle. Tectonic plates move very slowly relative to each other, typically a few centimetres per year, but this still causes a huge amount of deformation at the plate boundaries, which in turn results in earthquakes.
Observations show that most earthquakes are associated with tectonic plate boundaries and the theory of plate tectonics can be used to provide a simplified explanation of the global distribution of earthquakes, while some of the characteristics of earthquakes can be explained by using a simple elastic rebound theory.
What drives the movement of tectonic plates?
- Below the tectonic plates lies the Earth’s 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 (buoyant upwelling mantle at mid-ocean ridges): 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
Elastic rebound theory
Elastic rebound theory was originally proposed after the great San Francisco earthquake in 1906 by the geologist Henry Fielding Reid, to explain the deformation caused by earthquakes.
Before an earthquake, the buildup of stress in the rocks on either side of a fault results in gradual deformation. Eventually, this deformation exceeds the frictional force holding the rocks together and sudden slip occurs along the fault. This releases the accumulated stress and the rocks on either side of the fault return to their original shape (elastic rebound) but are offset on either side of the fault.