Why so many earthquakes in Eastern Idaho?

Central Idaho has seen more than its share of earthquakes over the past 20 months. So, what is going on? The US Geological Survey who keeps track of such things says these earthquakes are aftershocks of a M6.5 earthquake that occurred on March 31, 2020. That was the second largest earthquake recorded in Idaho. Fortunately, it caused very little damage, but its shaking was felt across the region.

Since that initial large earthquake, the USGS reports there have been over 4,300 aftershocks in the same area. After an earthquake they explain, the crust surrounding the ruptured fault will undergo large changes in stress.


The earthquake will reduce stress in some areas of the crust but will increase stress in other areas. As the crust adjusts to its new state of stress, it produces many smaller earthquakes—aftershocks—in the area around the mainshock. Aftershocks can last for tens or even hundreds of years, depending on the size of the mainshock. For example, in eastern Idaho, they are still recording aftershocks from the 1983 M6.9 Borah Peak earthquake, nearly 40 years later.

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