An earthquake is a sudden movement of the Earth, caused by the abrupt release of strain that has accumulated over a long time. For hundreds of millions of years, the forces of plate tectonics have shaped the Earth as the huge plates that form the Earth's surface slowly move over, under, and past each other. While sometimes the movement is gradual, at other times, the plates are locked together, unable to release the accumulating energy. When the accumulated energy grows strong enough, the plates break free. If the earthquake occurs in a populated area, it may cause many deaths and injuries and extensive property damage.
Earthquake Fault Types
Faults are the location of most earthquakes. Faults are a fracture in rock in which the rock on one side of the fracture has moved with respect to the rock on the other side.
There are three main types of fault that may cause an earthquake: normal, reverse (thrust) and strike-slip.
- Normal - Normal events involve downward movement on a sloping fault as the fault's two sides move apart. They signify extension or stretching of the Earth's crust.
Many earthquakes are caused by movement on faults that have components of both dip-slip and strike-slip; this is known as oblique slip.
Measuring earthquakes (1)
The vibrations produced by earthquakes are detected, recorded, and measured by instruments call seismographs. The data from seismograms, help scientists determine the time, the epicenter, the focal depth, and the type of faulting of an earthquake and can estimate how much energy was released. The severity of an earthquake can be expressed in several ways. The magnitude of an earthquake, usually expressed by the Richter Scale, is a measure of the amplitude of the seismic waves. The moment magnitude of an earthquake is a measure of the amount of energy released - an amount that can be estimated from seismograph readings. The intensity, as expressed by the Modified Mercalli Scale, is a subjective measure that describes how strong a shock was felt at a particular location.
The Richter Scale, named after Dr. Charles F. Richter of the California Institute of Technology, is the best known scale for measuring the magnitude of earthquakes. The scale is logarithmic so that a recording of 6, for example, indicates a disturbance with ground motion 10 times as large as a recording of 5. A quake of magnitude 2 is the smallest quake normally felt by people. Earthquakes with a Richter value of 6 or more are commonly considered major. Great earthquakes have magnitude of 8 or more on the Richter scale.
For more information...
- U.S. Geological Survey Earthquake Hazards Program - This web site is provided by the United States Geological Survey’s (USGS) Earthquake Hazards Program as part of our effort to reduce earthquake hazard in the United States. On this site you can learn everything you want to know about earthquakes, how they are monitored, and what research scientists are doing to reduce earthquake hazards.
- Recent Earthquakes - Put together by the USGS. It provides information about recent and real-time earthquakes for the United States and the World. You can also sign up for earthquake notifications.
- FEMA for Kids! Disaster Connection Earthquakes - Information about earthquakes for kids put together by the Federal Emergency Management Agency.