Aftershocks, even large ones, are completely normal
Since southcentral Alaska's 7.0 earthquake on Nov. 30, there have been more than 6,100 aftershocks. Most have been too small to feel but about 40 to date have been magnitude 4.0 or higher.
Aftershocks are a normal part of any earthquake. Based on the size and type of the initial earthquake scientists can issue an aftershock forecast. Seeing a number in this forecast does not mean that it will happen. It is simply the probability of another occurring in the future.
Here's an updated forecast from the Alaska Earthquake Center as of Dec. 21. It includes the forecast for this week and the next month. Unfortunately, there was no specific probability for an earthquake of magnitude 4 or higher, such as Thursday morning's 4.9 quake, in the forecast.
According to the United States Geological Survey (USGS):
Aftershocks are earthquakes that follow the largest shock of an earthquake sequence. They are smaller than the mainshock and within 1-2 rupture lengths distance from the mainshock. Aftershocks can continue over a period of weeks, months, or years. In general, the larger the mainshock, the larger and more numerous the aftershocks, and the longer they will continue.
This means that any earthquake we feel after the Nov. 30 event that is in the vicinity of the initial earthquake and less than magnitude 7.0 is actually an aftershock. To make matters more complicated, a strong aftershock can in turn have its own aftershocks -- which are still considered aftershocks of the initial earthquake.
A bit of solace after our latest rumble is that we've already experienced a big decline in the number of aftershocks since Nov. 30. Aftershocks typically decrease exponentially over time. Meaning that each day we will continue to feel fewer and fewer rumbles as the ground settles beneath our feet.
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