Earthquakes
A sudden release of stored energy in earth’s crust that creates seismic waves is termed as Earthquake.
Cause
- Most earthquakes are related to compressional or tensional stresses built up at the tectonic plates which make up the earth’s surface.
- This usually happens when the tectonic plates get locked up and are not able to slide past each other.
- It is estimated that only 10 percent or less of an earthquake’s total energy is radiated as seismic energy, the remaining energy is lost due frictional heat
Measuring the Giant
- The moment magnitude scale is a very sophisticated scale used now a days which for measuring the intensity or size of earthquake i.e. the amount of energy released.
- Earlier Richter scale was used for measuring the intensity of the earthquake.
- Seismologists assign a magnitude rating to earthquakes based on the strength and duration of their seismic waves.
- An earthquake measuring 3 to 5 is considered minor or light and is mostly imperceptible; 5 to 7 is moderate to strong; 7 to 8 is major; and 8 or more is catastrophic.
- The most recent large earthquake of magnitude 9.0 or larger was a 9.0 magnitude earthquake in Japan in 2011
- Intensity of shaking is measured on the Mercalli scale.
- There are two types of seismic waves viz; P waves and S waves.
P waves | S waves |
| Primary waves (P-waves) are compressional waves that are longitudinal in nature. | Secondary waves (S-waves) are waves that are transverse in nature. |
| Motion is along or parallel to the direction of wave propagation | Motion is perpendicular to the direction of wave propagation |
| P waves travel faster through the earth to arrive at seismometer first, hence the name “Primary”. | They arrive at seismometer after the faster-moving P-waves, hence the name “Secondary” |
| P-waves travel at around 6–7 km per second | S-waves travel at around 3.5–4 km per second. |
| P Waves can travel through any type of material, including fluids, and can travel at nearly twice the speed of S waves. | S-waves can travel only through solids, as fluids (liquids and gases) do not support shear stresses. |
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Terminology
1. Epicenter-That point on the Earth’s surface directly above the hypocenter (focus) of an earthquake.
2. Hypocenter-The point on the earth’s surface where the first sign of disturbance is reported i.e the site from where the waves originate.
