earthquake define
A road broken by an earthquake. The definition of an earthquake is the release of sudden and extreme energy that is caused by shifting in the Earth's crust. Facts About Earthquakes. A seismometer is used to record and measure the strength of an earthquake. The Mercalli scale is used to measure the earthquake. Anything seven or above is considered extremely dangerous. Earthquakes usually occur along fault lines, or cracks that occur within the Earthâs crust.