Joshua Hamilton, of St Josephs School, Kaikoura, asks :-

Pippa Childs, of Cashmere School asked:-

How are earthquakes formed?

Terry Webb, a seismologist with the Institute of Geological and Nuclear Sciences, responded.

Earthquakes occur when the Earth's crust breaks under the strain created by the forces of plate tectonics. This is similar to what happens if you stretch a rubber band too far - it breaks. The rocks in the Earth do not stretch as easily as a rubber band, but under large pressures they do bend and stretch in much the same way. If they are bent too far they break. Most of these breaks occur on faults, which are the weakest places.

Fortunately, the large earthquakes that occur on major faults are relatively rare, so while you should be prepared for a large earthquake, there is no point in worrying about when it will happen.

So, what do I mean by "the forces of plate tectonics"? The inside of the earth is hotter than the outside. This difference in temperature causes convection currents to flow inside the earth, in much the same way as warm air rises off a heater. Inside the Earth, because it is rock that is moving, the currents are very slow, typically a few centimetres per year. These convection currents cause parts of the Earth's surface to sink, such as the floor of the Pacific Ocean beneath the North Island of New Zealand, and other parts to rise. Because of these movements, parts of the Earth's surface are pushed against each other, producing the forces that cause earthquakes.