Earthquakes – Introduction

San Andreas Fault[9]
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An earthquake is a term used to describe ground shaking caused by the sudden release of accumulated strain by an abrupt shift of rock along a fracture in the earth or by volcanic or magmatic activity, or other sudden stress changes in the earth.[1]

Earthquakes occur on faults. When an earthquake occurs on one of these faults, the rock on one side of the fault slips with respect to the other. The fault surface can be vertical, horizontal, or at some angle to the surface of the earth.[2]

The epicenter is the point on the earth’s surface vertically above the hypocenter (or focus), point in the crust where a seismic rupture begins.[3]

The seismic activity of an area refers to the frequency, type and size of earthquakes experienced over a period of time.[4]

Seismology is the study of earthquakes and the structure of the earth, by both naturally and artificially generated seismic waves. A seismic wave is an elastic wave generated by an impulse such as an earthquake or an explosion.[5]

Earthquakes affect the normal activities of people through ground shaking, surface rupture, and tectonic deformation. Secondary effects include landslides, liquefaction, tsunamis, seiches, and fires.[6]

While most earthquakes are caused by movement of the Earth’s tectonic plates, human activity can also produce earthquakes. Four main activities contribute to this phenomenon: storing large amounts of water behind a dam (and possibly building an extremely heavy building), drilling and injecting liquid into wells, and by coal mining and oil drilling.[7][8]



  1. USGS Science Topics – Earthquakes:
  2. USGS FAQs:
  3. USGS Earthquake Glossary – epicenter:
  4. USGS Earthquake Hazards Program – The Science of Earthquakes:
  5. USGS Earthquake Glossary – seismology:
  6. National
  7. USGS Earthquake Facts and Fantasy:
  8. USGS Science Features – Man-Made Earthquakes Update:
  9. Image Source: [Accessed: January 14, 2014]