An earthquake is the shaking of the Earth's surface due to movement of tectonics plates. They are caused by the release of pressure between two tectonic plates. As plates push against each other, the rocks along the boundary become stressed. It's like bending a stick. As you bend it with more and more force, the stick becomes more and more deformed until eventually it breaks and each of the two pieces of the stick spring back to being more or less straight, but in a new position relative to each other. Earthquakes are known to sometimes cause a lot of damage, and aftershocks can cause even more damage.
Some tectonic plates move more than others, causing frequent small earthquakes. In areas where the tectonic plates don’t move as much, earthquakes wont happen as often but can be stronger. Earthquakes can happen along any type of plate boundary.
Earthquakes release energy waves called seismic waves. These spread out from the centre (or focus) of the earthquake. At the epicentre, the waves are felt most strongly, and typically, the further away you are from the epicentre, the less strong the shaking.
Earthquakes are measured using seismometers, which sense the movement caused by an earthquake. The seismometer records this movement on a seismograph. The size of an earthquake can then be measured on a scale called the ‘Richter scale’. Another way of describing an earthquake is to use the ‘Modified Mercalli scale’ which measures an earthquake based on the impact it has to people and their environment.
Hundred of earthquakes are measured every year along the East Coast but not all earthquakes are large enough to be felt by us. It depends on how deep or shallow the earthquake is and where it is located, as to whether or not you will feel it. Shallow earthquakes occur very close to the surface and can cause a lot of shaking, whereas deeper earthquakes are harder to feel as they are usually far below the surface.
You can have a look at all the recent earthquakes in New Zealand and find out their depth, magnitude and intensity at Geonet
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