A Natural part of our planet’s workings, earthquake can be terrifying and destructive events. Some trigger powerful Ocean waves called tsunamis.
Earth’s outer shell is made up of huge slabs called tectonic plates. These plates are constantly moving, and push past each other with hard jerky movements. In some places, the opposing masses of rock become locked together by friction. In these periods, there is a gradual build-up of strain in the Locked-up area. Eventually, the pressure becomes so high that there is a sudden shift between the blocks of rock, or a massive break, usually on or near lines on Easth’s surface called faults. As this happens, energy is released in the form of powerful shockwaves, or vibrations, causing an earthquake. When an earthquake happens under the sea floor, it can create a tsunami.
How a tsunami happens?
Many types of events can cause tsunamis, including big volcanic eruptions near or in the sea, landslides into the ocean and even asteroid impacts. However, the most common cause is a huge earthquake under the sea floor,usually where the edge of one tectonic plate rises above another.
Inside a fault
Faults at the boundaries of tectonic plates are prone to earthquakes. Here, two plates move past each other in opposite directions. Occasionally, the movement becomes stuck and stress builds up between the plates. Eventually, the build-up of stress causes a sudden shift or rupture, releasing vast amount of energy.
A large rupture below the seafloor causes an earthwuake. At the same time, a huge black of seabed is suddenly thrust upwards. This in turn pushes up the seawater above, triggering a tsunami wave at the ocean surface.
At sea surface, the sudden upthrust of a mass of water from below sets off a series of high-energy waves. These start travelling over the sea surface at a speed of over 800 kph. (500mph).
When a Tsunami wave approaches a shore, it slows down and its height increases. The upward- sloping seabed creates a resistance to the water movment-it pushes the water so that the wave is “amplified” (gets bigger).
When a Tsunami wave hits the Shore, it doesn’t usually break and collapse. Instead, it continues to surge forward for a considerable distance, flooding the whole coast. The powerful Rush of water can smash buildings and carry cars and people away.
Earthquakes produce two types of shock waves,called P and S waves,which travel through parts of Earth’s interior. Scientist can work out where and when a quake happened by detecting these waves as they arrive back at the surface.
Living on the edge
Some countries are more affected by earthquakes than others, because they sit on the boundaries between tectonic plates. This map shows the 10 countries with most earthquakes fatalities.
Some facts :
- The typical number of small earthquakes that occur in southern California every day.
- The largest earthquake ever recorded occured off the coast of Chile in 1960. It caused a devastating tsunami.
- (0.3 in)- the distance that the whole planet vibrates back and forth in space during the very largest earthquakes.
- The length of the rupture under the seafloor that caused the 2004 Indian Ocean tsunami was 1,600 km.
Knowledge Encyclopedia, DK Publications.