by Ian Dickson
Since the Nov. 30 Anchorage earthquake, we’ve fielded far more questions about the aftershock sequence than we have about the magnitude 7.0 mainshock that started them. All of those questions popped up again Friday after the magnitude 4.9 aftershock, which was the largest since the night of Nov. 30, and grew more insistent after the magnitude 5 aftershock on New Year’s Eve. Below, we will try to answer them as plainly and non-technically as possible.
First, the short version: There is nothing unusual about this aftershock sequence. We are not surprised that strong aftershocks are still happening, and they do not suggest that a larger earthquake is on its way. We cannot say when it will end, but we can say that the aftershocks have already grown far less frequent.
A little perspective: While aftershocks can cause a great deal of anxiety for many, they are nothing compared to the mainshock in terms of destructive power. Taken together, the 6,000 aftershocks still account for only 10 percent of the energy released during the sequence, while the mainshock accounts for 90 percent. Also, the aftershocks have already slowed considerably. Consider that out of 40 aftershocks of magnitude 4 or greater, 17 happened in the first 72 hours. Before yesterday’s magnitude 4.9, eleven days had passed since the previous magnitude 4. In other words, things are moving in the right direction.
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How do you know this was an aftershock? And what is the difference between an aftershock and an earthquake?
Aftershocks are earthquakes. The magnitude 7.0 earthquake on Nov. 30—what we call the mainshock—ruptured an area inside the subducting Pacific plate roughly 20 miles deep and running from south of Point MacKenzie up to Big Lake. That rupture changed the distribution of stress in the rock throughout that area.
The aftershocks are smaller ruptures that happen in response to those new stresses. What distinguishes aftershocks from ordinary earthquakes is this causal relationship. The mainshock creates a new set of conditions, which causes the aftershocks.
Because aftershocks are just ordinary earthquakes, they have no special characteristics that help us to label them as aftershocks. Instead, we define them based on when and where they happen. For as long as the area around the rupture has an elevated rate of earthquakes, we will label the earthquakes in that area as aftershocks.