Earthquakes, Why and How?
Earthquake are a shaking activities of the ground, some are slight tremors that only just rock a cradle, while others are so violent they can tear down mountains and cities. Small earthquake my be set off by landslides, volcanoes or even just heavy traffic. Bigger earthquakes are set off by the grinding together of the vast tectonic plates that make up the Earth’s surface.
During an earthquake, shock waves radiate in circles outwards and upwards from the focus of the earthquake. The damage caused is greatest at the epicentre, where the waves are strongest, but vibrations may be left 1000 km away.
Tectonic plates are moving all the time, radiating minor tremors as they grind past each other. Every now and then they get jammed, then the pressure builds up until they suddenly lurch on again, sending out vibrations called shock waves, in all directions and creating major earthquakes. Tectonic plate typically slide 4-5 cm past each other in year, in a slip that triggers a major quake they can slip more than 1 m in a few seconds.
In an earthquake, shock waves radiate out in circles from its origin or hypocentre called focus, shock waves vibrate throughout ground, but it is at the surface that they do most damage. Damage is most severe at the epicentre – the point on the surface directly above the focus – where the shock waves are strongest, but they can often be felt up to thousands of kilometres away.
In 1906, San Francisco in California, USA, was shaken by an earthquake that lasted three minutes. The earthquake started fires that burned the city almost flat. The earthquake was so strong that its effects were detected thousands of miles away. More than two-thirds of San Francisco’s population were left homeless. In most quakes a few minor tremors (foreshocks) are followed by an intense burst lasting just one or two minutes, a second series of minor tremors (aftershocks) occur over the next few hours, while aftershocks are less powerful than the main quake they can often add to the damage.
Seismometer (scientists who study earthquakes) measure the strength of the shock waves with a device called a Seismometer. They then grade the severity of the quake on the Moment Magnitude Scale. Experts also rate an earthquake on the Mercalli scale, which assesses the damage on a scale of one (barely noticeable) to 12 at total destruction. The Moment Magnitude Scale tells us how much energy an earthquake has – but the damage it does depends on how far the pace is from the centre of the earthquake.
Each year around 800.000 earthquakes are detected by sensitive instruments worldwide, about 40.000 to 50.000 can be felt but don’t do any damage, and less than 1000 actually cause damage.
What is Tsunami? Undersea earthquakes can produce huge waves called tsunamis. These waves move at up to 800 km per hours but may not be noticed in the open sea. In shallow waters however, the wave builds into a colossal wall of water up to 30 m high, which rushes inland and drowning everything in its path.
Out in the profundities of the sea, tidal wave waves don’t significantly increment in stature. In any case, as the waves travel inland, they develop to ever more elevated statures as the profundity of the sea diminishes. The speed of torrent waves relies upon sea profundity instead of the separation from the wellspring of the wave. Wave waves may go as quick as fly planes over profound waters, just backing off when achieving shallow waters. While torrents are frequently alluded to as tsunamis, this name is debilitated by oceanographers since tides have little to do with these mammoth waves.