Earthquakes occur because of a sudden release of stored energy. This energy has built up over long periods of time as a result of tectonic forces within the earth. This sudden motion causes shock waves (seismic waves) to radiate from their point of origin called the focus and travel through the earth.
It is these seismic waves that can produce ground motion which people call an earthquake. Each year there are thousands of earthquakes that can be felt by people and over one million that are strong enough to be recorded by instruments. Strong seismic waves can cause great local damage and they can travel large distances.
What are Earthquake (seismic) waves
A seismic wave is simply a means of transferring energy from one spot to another within the earth. Although seismologists recognize different types of waves, we are interested in only two types:
P (primary) waves (similar to sound waves)
S (secondary) waves (shear wave)
Within the earth, P waves can travel through solids and liquids, whereas S waves can only travel through solids.
The speed of an earthquake wave is not constant but varies with many factors.
Speed changes mostly with depth and rock type. P waves travel between 6 and 13 km/sec. S waves are slower and travel between 3.5 and 7.5 km/sec.
Types of tectonic Earthquakes
There are three main plate tectonic environments: extensional (divergent), transform, and compressional (convergent). Plate boundaries in different localities are subject to different inter-plate stresses, producing these three types of earthquakes. Each type has its own special hazards.
Earthquakes at divergent plate boundaries
Credit: The Why Files
During an earthquake at a divergent fault line, tectonic plates separate. In many cases, magma rises from the mantle to fill in the gap or one plate slide down to fill in the space. These types of earthquakes usually occur in ocean.
Earthquakes at Convergent plate boundaries
An Earthquake occurs at a convergent fault line when tectonic plates collide. Typically, at such a collision, subduction zones form as a result of a denser, older plate sliding under the other. A trench, sometimes 70 miles wide, is found at a subduction site, the result of the older plate descending into the earth.
Credit: The Why Files
Earthquakes at transform plate boundaries
In an earthquake at a transform fault line, energy stored in tectonic plates is released. This release of energy causes the two plates to rub against each other.
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