After recent 7.0, Most Earthquake aftershocks within 60-mile radius of Anchorage

John Tetpon

The Alaska Earthquake Center has counted 5,000 aftershocks in the Anchorage area, with the last one occurring on Jan. 13

Parts of Alaska are earthquake and aftershock prone. But no one knows for sure why there’s a bigger than usual cluster of aftershocks in the Anchorage area in such a short time. Aftershocks have been happening almost daily near Anchorage ever since the big one on Nov. 30, which measured 7.0. And most of them are happening within a 60-mile radius.
The Alaska Earthquake Center has counted 5,000 aftershocks in the Anchorage area, with the last one occurring on the morning of Sunday, January 13, with a magnitude of 5.0 on the Richter scale. The center also reports that in one year, 50,000 earthquakes have been recorded in Alaska.

The 7:45 a.m. Sunday quake, or aftershock, was located about 10 miles northwest of Anchorage, near Point Mackenzie, which is located across the Cook Inlet from Anchorage. The Tsunami warning center says the quake was at a depth of 28 miles. The center said there was no tsunami danger. It's one of the stronger aftershocks felt by Alaska residents after the magnitude 7.0 shaker left hundreds of damaged homes and infrastructure.

Most of the aftershocks since then have measured at 1.8 and above, with many of them measuring about 3.0 to 4.0, not big enough to worry about. It’s that bigger jolt that really puts people on edge.

Earthquakes in the mountainous regions of the state are common, but some villages have recently begun to experience minor to moderate shakers where only a handful had ever happened before. No one can explain that yet.

On Aug. 12 last year, a magnitude 6.4 earthquake occurred 52 miles southwest of Kaktovik, Alaska. The Inupiat village is located on the Arctic Sea coast near the Canadian border. It was, by far, the largest earthquake ever recorded north of the Brooks Range in Alaska. This earthquake started a vigorous set of aftershock activity, including a magnitude 6.0 aftershock, the second largest ever recorded north of the Brooks Range.

The epicenter was in the remote Sadlerochit Mountains, about 25 miles south of the Beaufort Sea coast. There are no reports of damage or injuries, and Alyeska Pipeline Service Company reported there was no impact on pipeline operations. The earthquake was widely felt, with reports coming in from Kaktovik west to Nuiqsut and as far south as Fairbanks.
The Alaska Earthquake Center reports that there have been more than 4,400 earthquakes in this area since 1970, though none as large as the one on Aug. 12.

Then on Jan. 9, 2019, a smaller earthquake hit the same area with a magnitude 3.3 on the Richter scale. The epicenter was roughly 6 miles below the earth’s surface.

On Oct. 18, 2018, a magnitude 5.3 hit the Northwest Arctic city of Kotzebue in the Northwest Arctic Borough. It is the borough’s seat, by far its largest community and the economic and transportation hub of the sub-region of Alaska encompassing the borough.

Although the earthquake was only moderate, the perceived shaking was very strong, the USGS said.
There was no damage reported.

On Dec. 30, 2018, a small tremor struck the village of Shaktoolik on the shores of Norton Sound. Its recorded magnitude was 3.6 on the Richter scale and its depth was 4.4 miles below the earth’s surface. No damage was reported.

The Alaska Earthquake Center said: “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 main shock 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.”

Although there may be some connection to warmer weather thawing Arctic permafrost, it is still unclear if that has anything to do with the increased frequency of earthquakes in regions that have not experienced shaking ground.
The Alaska Earthquake Center also points out that the Nov. 30 quake and its aftershocks does not mean that a mega quake like the 1964 disaster will happen anytime soon. That quake, at 9.2 is considered the biggest shaker ever to strike North America.

“The Nov. 30 quake happened in a very different way. The1964 earthquake was a mega-thrust 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 mega-thrust,” the Center said.

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