Foreshocks, aftershocks - what's the difference?

"Foreshock" and "aftershock" are relative terms.

Foreshocks are earthquakes that precede larger earthquakes in the same location. An earthquake cannot be identified as a foreshock until after a larger earthquake in the same area occurs. 

Aftershocks are smaller earthquakes that occur in the same general area during the days to years following a larger event or "mainshock." They occur within 1-2 fault lengths away and during the period of time before the background seismicity level has resumed. As a general rule, aftershocks represent minor readjustments along the portion of a fault that slipped at the time of the mainshock. The frequency of these aftershocks decreases with time. Historically, deep earthquakes (>30 km) are much less likely to be followed by aftershocks than shallow earthquakes. (modified from Univ. of Washington)

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Do earthquakes occur in Antarctica?

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Date published: May 12, 2015

Magnitude 7.8 Earthquake in Nepal Aftershocks

A magnitude 7.8 earthquake struck Nepal on April 25, 2015 at 06:11:26 UTC. Visit the USGS event page to learn more about this earthquake.

Date published: February 10, 2015

Virginia Earthquake Aftershocks Identify Previously Unknown Fault Zone

RESTON, Va.-- Aftershocks from the 2011 Virginia earthquake have helped scientists identify the previously unknown fault zone on which the earthquake occurred.

Date published: September 26, 2012

Rare Great Earthquake in April Triggers Large Aftershocks All Over the Globe

MENLO PARK, Calif. — Large earthquakes can alter seismicity patterns across the globe in very different ways, according to two new studies by U.S. Geological Survey seismologists. Both studies shed light on more than a decade of debate on the origin and prevalence of remotely triggered earthquakes.

Date published: January 21, 2010

USGS Issues Assessment of Aftershock Hazards in Haiti

RESTON, Va.—The aftershock sequence of the magnitude-7 earthquake that struck near Port-au-Prince, Haiti, on Jan. 12, 2010, will continue for months, if not years. The frequency of events will diminish with time, but damaging earthquakes will remain a threat.

Date published: March 1, 2001

Minor Aftershock hits Seattle

A minor aftershock struck the Seattle area early Thursday morning, March 1, 2001. The aftershock, which struck at 1:10 a.m. local time, had a preliminary magnitude of 3.4 and was felt throughout the Seattle-Tacoma-Olympia area.

Date published: October 8, 1999

ENTER AT YOUR OWN RISK...But Check the Aftershocks Probabilities

When is a badly damaged, but stable building safe to enter after an earthquake? That is a question that safety-response and building-department officials have to answer in order to let occupants retrieve important possessions and business records, and to let contractors begin emergency repairs

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colored map of part of southern Alaska, and part of Gulf of Alaska, showing a cluster of yellow and orange dots offshore.
January 25, 2018

60 aftershocks greater than or equal to M4.0 occurred in the first 48 hours after the mainshock. The blue dot is the location of the main Jan 23 earthquake. Yellow and orange dots are aftershock epicenters.

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