The magnitude 4.1 earthquake Tuesday night is considered an aftershock of the Nov. 30 quake that shook Southcentral. In fact, most notable rumblings in the next year will likely be aftershocks as well.

An aftershock is defined as an earthquake that follows the largest shock of an earthquake sequence, according to the United States Geologial Survey. 

Aftershocks can occur for years after an initial earthquake. The larger the initial earthquake, or mainshock, the stronger and more prevalent the aftershocks will be and the longer they will last.

To be considered an aftershock, the earthquake must be smaller than the mainshock and within one or two rupture lengths' distance from the mainshock.


When it comes to the Nov. 30 earthquake, earthquakes smaller than a magnitude 7.1 that fall within the region from about Point MacKenzie to near Big Lake will all be considered aftershocks.

Aftershocks from the Nov. 30 earthquake will likely last another 2 1/2 years. They will continue to weaken and occur less frequently. And a majority of them will never be felt.


In fact, there have been almost 70 aftershocks in just the past two weeks. Just three of those were a magnitude 3 or above. The last magnitude 4 or above happened Aug. 5. Before that, go all the way back to May 18 to find an aftershock greater than magnitude 4.    

Just because a majority of nearby recent earthquakes have been aftershocks doesn’t mean they all are. Earthquakes outside the aftershock area will still occur. The most recent example of this was the magnitude 3.4 earthquake in the Chugach Mountains that was felt in Anchorage on Aug. 16.

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