Earthquakes & Tsunamis

Earthquakes are a shaking of the ground. Some are slight tremors that barely rock a cradle. Others are so violent they can tear down mountains and cities. Small earthquakes can be set off by landslides, volcanoes or even just heavy traffic. Big earthquakes are set off by the grinding together of the vase tectonic plates.

Forces cause the plates to move, at a rate of 4-5 cm past each other in a year. In a slip that triggers a major quake plates can slip more than 1 m in a few seconds. In most quakes a few minor tremors (foreshocks) are followed by an intense burst lasting just 1 or 2 minutes. A second series of minor tremors (aftershocks) occur over the next few hours.

Faults are lines of weakness where the rock has been severed. Portions of the crust are under constant stress, like a bent bow. When the strain becomes intolerable the rock often collapse at a weak point, far beneath the surface. The rock may have been bending for hundreds of years before the earthquake happens.

As the crust makes sudden shifts along the fault lines, it releases pent-up energy in powerful waves, which causes the whole world to vibrate like a giant bell. These waves are detected on sensitive measuring instruments called seismographs.

The Richter Scale (is a seismograph) which measures the magnitude of an earthquake on a scale of 1 to 10. The most powerful earthquake was measured in China in 1960 at 9.5 on the RS. The longest was in Alaska in 1964, lasting 4 mins.

The starting point of an earthquake below ground is called the hypocenter or the focus. The epicentre of an earthquake is the point on the surface directly above the hypocenter. Earthquakes are strongest at the epicentre & gradually weaken farther away.

Shock waves begin at the epicentre & radiate outwards & upwards in circles. The greatest damage is at the epicentre, but the shock waves can radiate up to 400 km away. Certain regions are called earthquake zones, because they are prone to earthquakes and lie along the edges of tectonic plates.

Tsunamis Means “harbour wave” in Japanese. They are huge waves that begin when the sea floor is violently shaken by an earthquake, a landslide or volcanic eruption. In deep water, tsunamis travel almost unnoticed below the surface. Once they reach shallow coastal water the waves can reach 30+ meters.

Often mistakenly called tidal waves, but have nothing to do with tides. Before a Tsunami arrives, the sea may recede dramatically, like water draining from a bath. They can travel along the seabed as fast as a jet plane, at 700 km/h or more. A Tsunami that begins as an earthquake in Japan can arrive San Francisco 10 hours later.

 December 2004: On December 26, a 9.0 magnitude earthquake struck off the Indonesian island of Sumatra, triggering a tsunami that killed tens of thousands of people in Indian Ocean coastal communities. Much human and environmental devastation was left in its wake in Indonesia.