sf-earthquake_21_600x450What Causes Earthquakes?

Earthquakes are caused by portions of the earth’s crust (tectonic plates) suddenly slipping past one another on a fault. The plates that make up the earth’s surface are in continuous slow motion, constantly rubbing up against one another, sometimes slipping beneath each other. This sudden lateral or vertical movement is called faulting.

When faulting occurs, energy from the event is released in waves; an earthquake is the shaking that radiates from the release of energy into the earth’s crust. The epicenter of an earthquake is the place on the earth’s surface directly above the point on the fault where the earthquake rupture began.

Magnitude vs. Intensity

Earthquakes are described (or measured) in two ways: Magnitude and Intensity.

Magnitude measures the maximum ground motion recorded by a seismometer; therefore, the amount of seismic energy released during an earthquake is related to its magnitude. The measure used to determine the total energy released by an earthquake is the Moment Magnitude Scale (which supersedes the outdated Richter Scale).

Earthquakes_Richter Scale_U.S. Geological Survey

Intensity describes what people experience during an earthquake, the effects of shaking on structures and the extent of damage. The scale used to measure intensity is called the Modified Mercalli Intensity Scale. Intensity values tend to be higher closer to the epicenter of the earthquake and decrease with distance from the source.

Earthquake Intensity

Hazards Related to Earthquakes:

  • Landslides and Debris Falls
  • Fire from broken gas or power lines
  • Flooding from ruptured dams or levees
  • Tsunami

For more earthquake information:

U.S. Geological Survey Earthquakes

The Great Hawaii Shake Out