The burning question on the minds of many residents in Southcentral Alaska is, “When will the aftershocks stop?” This is a tricky question, but enough time has passed since the November 30 magnitude 7 earthquake for us to make an informed estimate.

We usually say that aftershocks have stopped when the seismicity in the ruptured area returns to its background level, which is the average rate of earthquakes prior to the mainshock. Because people mostly want to know when they will stop feeling aftershocks, we’ll estimate when we expect a return to background level for earthquakes greater than magnitude 3.

When we look at plots showing the number of magnitude 3 and larger earthquakes each day, we can see that those daily counts start high and decrease rapidly. This is a defining characteristic of all mainshock-aftershock sequences. Mathematically speaking, the decrease is logarithmic and asymptotic. In other words, the declining rate of aftershocks can be graphed as a curve that falls steeply at first and then levels off gradually without ever quite reaching zero. Because of this relationship, we can plot the data on a log/log scale (1).