Earthquake!! Foreshock, Mainshock, or Aftershock? Which was it? @IRISEarthquakeScience

Uploaded February 2019 | Updated September 2026, 1 week ago

www.iris.edu/earthquake
There is often confusion surrounding the terminology of earthquakes. Large earthquakes are usually followed by hundreds and even thousands of smaller earthquakes, called aftershocks. In some earthquake sequences, a smaller earthquake called a “foreshock”, precedes the mainshock. Although seismologists have carefully analyzed foreshock/mainshock pairs of earthquakes, and earthquake triggering in general, there are no special characteristics of a foreshock that let us know it is a foreshock until the mainshock occurs. Rate of aftershocks was first studied in detail by Fusakichi Omori, Chairman of Seismology at Imperial University of Tokyo; one of the most important figures in the history of seismology.

Written & directed by Dr. Robert F. Butler, University of Portland
Animation & graphics: Jenda Johnson, Earth Sciences Animated
Narrated by Dr. Wendy Bohon, Education Specialist, IRIS
Reviewed by Dr. Peter Haeussler, U.S. Geological Survey,
Earthquake locations: IRIS Earthquake Browser, (IEB)
Maps: IEB, USGS, Google Earth
Photos from public domain sources
Video clips from FEMA
Music “J-sketches 1” by Andrew Menking
Made with support from the National Science Foundation (NSF)

Earthquake!! Foreshock, Mainshock, or Aftershock? Which was it?

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