Natural disasters

Natural disasters

by HARA MARKAKI

Multiple Choice

1. What is an earthquake?

• The sudden movement of the ground

• The shake of the earth

• An earthquake is caused by a sudden slip on a fault. The tectonic plates are always slowly moving, but they get stuck at their edges due to friction. When the stress on the edge overcomes the friction, there is an earthquake that releases energy in waves that travel through the earth's crust and cause the shaking that we feel.

• A catastrophe

​How are earthquakes recorded?

Earthquakes are recorded by instruments called seismographs. The recording they make is called a seismogram. The seismograph has a base that sets firmly in the ground, and a heavy weight that hangs free. When an earthquake causes the ground to shake, the base of the seismograph shakes too, but the hanging weight does not. Instead the spring or string that it is hanging from absorbs all the movement. The difference in position between the shaking part of the seismograph and the motionless part is what is recorded.

​Deadliest Earthquake

​

1. Valdivia, Chile, 22 May 1960 (9.5)

This earthquake killed 1655 people, injured 3000 and displaced two million. It caused US$550 million damage in Chile, while the tsunami that it spawned caused deaths and damage as far away as Hawaii, Japan and the Philippines. The ‘rupture zone’ of the quake was more than 1000 km long. Two days after the initial quake, the nearby volcano Puyehue erupted, sending ash and steam up to 6 km into the atmosphere over a period of several weeks.