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After destructive November quake, Anchorage still faces onslaught of aftershocks

Traffic moves through downtown Anchorage, Alaska. Bloomberg photo by David Ryder

On Friday, November 30, a magnitude 7.0 earthquake struck Anchorage, Alaska. The initial jarring was destructive - and now, more than a month later, daily jolts continue to leave residents shaken.

Since the main shaking, about 350 aftershocks of magnitude 3 or greater have been registered on seismometers. Some - including the five 5.0 or worse quakes - have been large enough to cause additional damage. Though the aftershocks have slowly been winding down, the magnitude 5.0 tremor on Jan. 1 and magnitude 4.2 on Wednesday, Jan. 2, show that Alaskans are far from out of the woods yet.

That begs the question - how long will the earthquakes continue? We can turn to a little bit of seismology for the answer.

After a quake, swarms of aftershocks persist for weeks or even months. The bigger the main earthquake, the stronger and more frequent the aftershocks. The occurrence of aftershocks drops off exponentially as time progresses per a relationship known as Omori's Law . By fitting an equation to the number of observed aftershocks, we can extrapolate trends into the future.

Likewise, it's possible to forecast the intensity of said aftershocks. That comes through the Gutenberg-Richter equation . It breaks down the percentage of aftershocks that reach different levels of strength. It's like a pyramid - as you climb in magnitude, each tier gets more narrow. It's a logarithmic relationship.

Looking ahead, the U.S. Geological Survey estimates anywhere between 22 and 120 additional magnitude 3 or higher quakes may be felt in the next year.

Barbara Romanowicz is the former director of the Berkeley Seismological Laboratory and a professor at the University of California at Berkeley. She expects Anchorage's rumbling won't go away anytime soon. "Lots of small non-damaging earthquakes - magnitude 3.0 or greater - are very likely to continue" she said via email.

Most will be in the nearer term. In the next week, earthquake models suggest about one magnitude 3 or greater earthquake should occur every couple days. An average of one should reach magnitude 4 status every ten days.

Numbers should fall back a bit by the end of January, but even then there's no real end in sight.

There's an outside chance of more dangerous magnitude 5 quake transpiring within the month. The chances of this taking place are no more than 30 percent. Those odds climb ever so slightly to 41 percent during all of 2019 per USGS's numbers.

Five earthquakes topping magnitude 5.0 did already ensue, all within the first 24 hours following the "big quake" (in addition to the magnitude 5.0 aftershock on Jan. 1) The strongest - a 5.7 - jounced Anchorage six minutes after the main shock, startling residents just exiting their places of safe refuge. There shouldn't be any aftershocks stronger than this. Bath's Law states that the difference in magnitude of the biggest aftershock and the main quake in any earthquake should be about 1.1 or 1.2.

Quakes should start to wind down to more nuisance rumbles by the start of spring. While models struggle in this time range and relationships don't always hold true, it's a safe bet that only a few sporadic shakes will persist past then. May should feature merely a third the number of aftershocks as January.

The USGS also released a statement on the cause of the earthquake. "It occurred in the Alaska-Aleutian subduction zone on a fault within the subducting Pacific slab," it wrote. It did not stem from movement along the nearby interface of the Pacific and North American plates.

However, there is one fly in the ointment: the 7.0 may not be the main shock, but rather a foreshock - akin to an "appetizer" preceding a larger earthquake, but the odds are low. Romanowicz warns that "while another large earthquake is improbable in the near future, it could still happen." It's impossible to know for sure that the 7.0 was the main shock, and not a precursor to a larger slip along the fault.

This article was written by Matthew Cappucci, a reporter for The Washington Post.

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