Maryland Department of Natural Resources

IRIS Educational Materials

 Education & Outreach Series No. 3 
How Often Do Earthquakes Occur?

Earthquakes are always happening somewhere.

Large earthquakes occur about once a year. Smaller earthquakes, such as magnitude 2 earthquakes, occur several hundred times a day. To create a mountain system might take several million medium size earthquakes over tens of millions of years.

Earthquake Energy Equivalents

We describe the size of an earthquake using the Richter Magnitude Scale, shown on the left hand side of the figure above. The larger the number, the bigger the earthquake. The scale on the right hand side of the figure represents the amount of high explosive required to produce the energy released by the earthquake.

    The 1994 earthquake in Northridge, California, for example, was about magnitude 6.7. Earthquakes this size occur about 20 times each year worldwide. Although the Northridge earthquake is considered moderate in size, it caused over $20 billion in damage. The earthquake released energy equivalent to almost 2 billion kilograms of explosive, about 100 times the amount of energy that was released by the atomic bomb that destroyed the city of Hiroshima during World War II.

The IRIS Consortium Logo.

IRIS is a university research consortium dedicated to monitoring the Earth and exploring its interior through the collection and distribution of geophysical data.   IRIS programs contribute to scholarly research, education, earthquake hazard mitigation, and the verification of the Comprehensive Test Ban Treaty.

Support for IRIS comes from the National Science Foundation, other federal agencies, universities, and private foundations.

The seismic monitor was developed by the IRIS Consortium, US Geological Survey, University of Colorado, and Reel Illusions Multimedia, Inc.

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This educational material is reproduced with permission from the IRIS Consortium.

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updated 02/10/14