6 comments on “The M7.0 Anchorage Earthquake of 2018

  1. Wow, an eyewitness report, thank you! I hope you all have recovered from it by now. It must be a strange feeling living on a piece of land where you know another piece of “land” is creeping under, deep down below your feet. It’s always active, always burrowing forward, always ready to break – but you don’t know when… I haven’t heard much about landslides in your area, have there been any larger ones due to the quake?

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    • Every place you live on this planet has its own special little natural surprise – storms, drought, flooding, rogue waves, tsunamis, excess snowfall, tornadoes, hurricanes, volcanoes, earthquakes, and the ever popular bolide impact / airburst. You pays your money and you takes your chances. Is being surprised by a quake superior to having 3-4 days to think about an incoming storm? Dunno. But we all pay some sort of price for our choices of where we choose to live. As long as you understand that it is a choice, and accept that you have made that choice accepting / embracing that risk, everything seems to work out ok.

      There were landslides, but the largest immediate problem in that regard were the rockslides closing the southbound Seward Highway out of town. They are still going on. We were fortunate that there wasn’t a large snowpack on the lower mountains, as all of that would have come down. Our population is neither particularly large nor densely packed and spread out over a relatively large area, so if there are a lot of landslides, which there were in more remote areas, they don’t necessarily do anything but rearrange the local treeline.

      All in all, it has been an interesting journey and we have been very fortunate not to get anyone badly hurt. As exciting as the quake was, I think I have come to despise the aftershocks. Typically an aftershock is smaller than the main quake, and the time between them stretches out over time. That works right up to the point where God decides that the main quake wasn’t really the main quake at all, but a smaller foreshock instead. You generally don’t find that out until the main quake hits. My understanding is these are somewhat uncommon events, but all you need is the doubt in the back of your mind to make things more interesting afterwards than they need to be. Cheers –

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  2. Very interesting to read the eye witness account. I was impressed by how well Anchorage withstood the shaking. The damage is remarkably little. It may be worse across the bay, perhaps (I have heard rather little about that area in the news reports). But overall, this shows how effective preparation can be. This particular fault should be save for some time now (decade), but there may be a risk from smaller events on faults near the surface. And looking at the the history, I think the recovery phase from the mega quake of the 1960’s is now over. Major shaking was much more frequent in the century before that event, and a significant quake once a decade may become the new normal in your area.

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    • Howdy Albert –

      One of the strange things about the quake was that the parts of the area on relatively poor soils didn’t do all that badly, though there was a cul-de-sac of homes that sunk about 10-15 cm or so. The structures on the better ground on the Hillside and in Eagle River to the north did a bit worse. This is opposite of our normal performance.

      The MatSu is the area across the Inlet to the NW of Anchorage. It got beat up pretty well, partly because the soils are worse and partly because it was a bit closer to the epicenter.

      The 2010 Wang et al paper (linked below) describes 3 primary types of quakes up here – megathrust (1964), intraplate (this one and the 2 at Iliamna), and strike-slip (Denali 2002, and Gulf of Alaska Jan 2018). It then explores possibilities for future quakes. Might be worth a read.

      I don’t know that I understand the failure mechanisms of an intraplate quake all that well, other than rock under pressure rearranges itself into a more compact configuration from time to time. There is no fault line in play with this one that I know of. Depictions of the Pacific Plate under ANC following this suggest it is travelling close to parallel before diving into the mantle to the west of town. Don’t know how to square it with megathrust action of the plate making the dive into the mantle as it scrapes the Yakutat terrane off on the southern part of the North American Plate.

      What I do know is that we have experienced 4 pretty large quakes in the last 4 years, one of them a long ways away, and 5 in the last 16 years. No reason to believe things are going to calm down any time soon, though it would be nice. Cheers –

      http://sciencedocbox.com/Geology/70620894-Evaluating-the-seismic-hazard-in-anchorage-alaska-abstract.html

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  3. The quake was on the interface between the pacific plate and the Alaskan continental crust. It was thrust (but not megathrust). The interface is not entirely horizontal but indeed much less steep than it is further in-land. The recemt big quakes have not been underneath Anchorage (until this event). There is a risk for more local large earthquakes in the future, at least if the situation from before 1964 comes back. Keep the building code up – Anchorage seems remarkable robust.

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