Anchorage is still shaking: aftershock after aftershock is rocking the city and the wider region, several days after the November 30th magnitude 7.0 earthquake.
The latest count sounds incredible at first. According to USA Today, there have been at least 1,406 above a magnitude 1.0 – not nearly large enough to be felt. In addition, there have been 172 aftershocks above a magnitude 3.0, per the United States Geological Survey (USGS), and five of a magnitude 5.0 or above, the latter of which are powerful enough to cause surface damage.
The quantity of aftershocks has understandably generated plenty of headlines, because 1,000 sure sounds like a big number. So is there anything unusual about this number of aftershocks in just a handful of days? Happily enough, no – this is precisely what seismologists would expect after a magnitude 7.0 earthquake. Here’s why.
Aftershocks are earthquakes being generated as a result of the original fault that slipped, but which is still slipping and releasing stress, albeit in smaller packages. Aftershocks can also be generated by faults nearby to the original that have been loaded with additional stress after the primary earthquake took place.
Aftershocks are also only assigned after the fact, because earthquakes don’t occur in isolation. The mainshock, the most energetic earthquake in a sequence, is only definitively known as the mainshock sometime after it occurs; before then, it can be hard to tell whether or not it was the highest of magnitudes.
On the day of the event, the USGS estimated that there was just a 3 percent chance that a more powerful quake would occur within seven days. That means there was a very high chance that the 7.0 event was the mainshock.
At the time of writing, the USGS says there is less than a 1 percent chance of a quake with a magnitude larger than the original in the next week. They’ve even officially named the 7.0 event as the 2018 Anchorage earthquake, so it looks like all bets are now off: every earthquake post-7.0 in that area can be referred to as an aftershock.
The agency carefully notes that the “USGS and other scientists cannot predict the exact time, location and magnitude of any specific earthquake.” Saying that, plenty of knowledge about previous earthquakes and seismic systems means they can say something about what’s to happen in the near future.
In general, the more energetic the original earthquake, the more powerful the following aftershocks will be, and the longer they will go on for. In some cases, they can keep cropping up for years after the original earthquake, although always trending toward a decreased frequency and magnitude.
A magnitude 7.0 quake is pretty darn energetic, and although no-one can say for sure when the aftershocks will stop, they are expected to continue for some undetermined amount of time. This is certainly being validated in and around Anchorage; after the magnitude 5.7 aftershock that took place just seven minutes after the mainshock, the ground has been shaking in a variety of ways ever since, with plenty knocking things off shelves.
On November 30th, the USGS said that they’d expect plenty of magnitude 3.0 or higher aftershocks – anywhere between 84 and 610 – within the next week. If near the epicentre, these will be felt by the local population. Now, with new data coming in, that forecast is coming in at 47 to 110 of a magnitude 3.0 or higher over the next week.
The fault slippages will also generate far more smaller earthquakes, meaning that there will be hundreds more that won’t be felt by the local population. Altogether, this means that it’s entirely unusual that the area has experienced more than 1,400 aftershocks above a magnitude 1.0. There’s nothing to worry about in this respect.