An earthquake (or tremors, tremors) is shaking the surface of the earth, caused by a sudden movement in the earth's crust. They can be extremely violent. Earthquakes are usually quite brief, but they can be repeated. They are the result of a sudden release of energy in the earth's crust. This creates seismic waves, which are energy waves that travel through the Earth. The study of earthquakes is called seismology. Seismology studies the frequency, type and size of earthquakes over a period of time.
There are great earthquakes and small earthquakes. Large earthquakes can destroy buildings and cause death and injury. Earthquakes are measured using seismometer observations. The magnitude of an earthquake, and the intensity of the shaking, is generally reported on the Richter scale. On the scale, 3 or less is barely perceptible, and magnitude 7 (or more) causes damage over a wide area.
An earthquake under the ocean can cause a tsunami. This can cause as much death and destruction as the earthquake itself. Slips can also occur. Earthquakes are part of Earth's rock cycle.
Earthquakes sometimes hit cities and killed hundreds of thousands of people. Most earthquakes occur along the Pacific Ring of Fire, but the largest occur mainly in other places. Tectonically active places are places where earthquakes or volcanic eruptions are frequent.
The ancient Chinese used a device that looked like a jar with dragons on top surrounded by frogs with their mouths open. When an earthquake occurred, a ball in the mouth of each dragon fell from the dragon's mouth into that of the frog. The position of the frog that received a ball indicated the direction of the earthquake. This was one of the first tools to help find out where an earthquake originated.
Earthquake, any sudden jolt of soil caused by the passage of seismic waves through the rocks of the Earth. Seismic waves occur when some form of energy stored in the earth's crust is suddenly released, usually when the rock masses are tensed against each other suddenly they fracture and "glide." Earthquakes occur most frequently along geological faults, narrow areas where rocky masses move in relation to each other. The main fault lines of the world are in the strips of the enormous tectonic plates that form the terrestrial crust.