Since the Nov. 30 Anchorage earthquake, we’ve fielded far more questions about the aftershock sequence than we have about the magnitude 7.1 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.
How do you know this was an aftershock? And what is the difference between an aftershock and an earthquake?
Aftershocks are earthquakes. The magnitude 7.1 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.
This is unavoidably imprecise. Some small number of those earthquakes would have happened anyway, and some presumed aftershocks just outside the rupture patch could be unrelated. But we know that the mainshock caused the very elevated rate of earthquakes in that area, so we can say that the great majority of those earthquakes are aftershocks.
That magnitude 4.9 was the largest aftershock since November 30. Is this normal?
If you look at a plot of the cumulative number of aftershocks over time, it forms an arc that begins steep and gradually flattens out as the aftershocks decrease. But that’s the long view, and that regular curve includes lulls and clusters of fewer and more earthquakes. They’re not perfectly distributed.
The larger aftershocks are not evenly distributed, either. As an aftershock sequence goes on, it’s typical for strong aftershocks to mostly taper off but to still happen sporadically, sometimes in twos or threes and sometimes separated by weeks or months. This is expected, and it would be far stranger if we did not experience a scattering of larger aftershocks as the sequence goes on. We are still recording magnitude 4+ earthquakes from the Offshore Kodiak magnitude 7.9 in January and the North Slope magnitude 6.3 in August.
Does the M4.9 increase the chances of an earthquake larger than the mainshock?
How do you know?
Aftershock forecasts are based on statistical modeling. There are different approaches to this, but they all draw from observations of past sequences and what they reveal about how aftershocks tend to play out. This means that when the USGS aftershock forecast puts the probability of another magnitude 7 at less than 1 percent over the next year, that reflects how rare it is for an apparently normal aftershock sequence to precede a larger quake.
When we answer questions about larger earthquakes happening in the future, we choose our words carefully because we need to. Large earthquakes can happen at any time in Alaska, so we can’t say that it’s impossible. We can say that nothing has happened to make us expect a larger earthquake, and everything we know leads us not to expect one.
One important caveat. While there is only a very small chance of another major earthquake happening as part of this sequence, Alaska’s overall seismic hazard remains unchanged. Alaska has had 27 earthquakes of magnitude 7 or greater in the last 50 years, which shows that we are always at risk for a big one. The forecast for the Nov. 30 earthquake only applies to the small area west of Anchorage around the rupture patch.
I’ve heard that this activity resembles the buildup to the M9.2 earthquake in 1964. Is that true?
That is not true.
First, the Nov. 30 quake happened in a very different way. The1964 earthquake was a megathrust quake caused by a 500-mile-long rupture along the interface of the Pacific and North American plates. The Nov. 30 quake happened well away from the plate interface and much deeper, inside the Pacific plate. It’s a very common type of earthquake in Southcentral Alaska, and these intermediate-depth quakes are not associated with great earthquakes on the megathrust.
Second, there was no earthquake sequence resembling this one prior to the 1964 quake. There was magnitude 6.9 near Chirikof Island several weeks before, but that was hundreds of miles away and unrelated. Earthquake monitoring was minimal in Alaska at that time, but a sequence like this one would have been well documented through media accounts, written observations, and recordings of the mainshock and largest aftershocks. There is no record of such a sequence. This is just a baseless rumor.
When will it stop?
We don’t know. We will consider the aftershock sequence effectively over once the background seismicity level returns to normal—around 10 earthquakes per day—and stays at that rate for a month. We don’t expect that to happen for a year or more, but it’s impossible to know exactly how long it will take. Earthquakes large enough to feel should taper off before that, but we expect that to take months rather than weeks.
We wish we could make more definite statements about what you should expect. We can say that this aftershock sequence has behaved like an entirely ordinary aftershock sequence, and it’s done nothing to surprise or alarm us.
Note: On March 8, we revised the magnitude of the November 30 Anchorage earthquake upward, from 7.0 to 7.1. This piece has been edited to reflect that revision.