The intraslab, a fault-less quake that can provide a big shake

Before time dulls memory, Alaskans who rode out the end of November earthquake that shook the Anchorage metropolitan area to its core might want to take a look back at what got smashed in homes and offices to better start preparing for the next shake, say the experts.



Because Anchorage could get another earthquake of this nature at any time, and possibly even closer to the city center than the last which had its epicenter only about seven miles north.

“It could happen right under Anchorage,” Peter Haeussler, a research geologist with the U.S. Geological Survey observed Monday.

Anchorage is built upon a very geologically active corner of the Gulf of Alaska, and though most concerns about earthquakes in the U.S. focus on strike-slip events along known geologic rifts such as the famous San Andreas Fault in California, there is another issue of concern in Alaska, one which geologists have been talking about for a long time.

This is the invisible, underground, “intraslab” earthquake caused by the pressure placed on tectonic plates of rock floating on the core of molten rock at the earth’s core. One of these plates – the Pacific plate – steadily drifts northwest at the rate of a couple of inches a year and then forces itself under the North American plate, as the University of Alaska Fairbanks Alaska Earthquake Center notes.

All is fine when this subduction proceeds smoothly, but it doesn’t always proceed smoothly. The process tends to be herky-jerky, which causes tremors in the ground beneath our feet as the majority of Alaskans living in the state’s urban core discovered in a big way on Friday when the ground shook to the magnitude of 7.0.

That quake was unpredicted and unpredictable, but also wholly expected by geologists.

“In our calculations, the rate of intraslab earthquake activity dominates the seismicity rates near Anchorage and this is consistent with the historical record. There have been at least 15 earthquakes of M6.5 (magnitude 6.) and greater within 200 km (125 miles) of Anchorage since 1898 that we believe have an intraslab source…,” concluded a 2010 evaluation of seismic hazards in the city. 

Unfortunately, there appears no pattern to the timing of those earthquakes and to intraslab earthquakes in general. Anchorage could go decades without another event like the one that ended the month of November this year, according to Hauessler, or it could be hit by another in January.

“They are not regular,” as John Vidale, a University of Washington Earth and space sciences professor and director of the UW seismic network, explained on the 10th anniversary of the Nisqually earthquake.

Nisqually rocked Puget Sound to the tune of 6.8 in 2001. An intraslab earthquake much like the one which hit Anchorage, the Nisqually quake was about 40 miles south of Seattle, but still caused serious damage in that city.

Bricks rained down from buildings to flatten cars and trucks. Four hundred people were injured. One person died of a heart attack. And damages were eventually estimated at $2 billion, KING5-TV late reported.

Limited shaking

The good news, if there is good news in any earthquake, is that the Anchorage area got off a lot lighter than the Seattle area. No one is reported to have died, and the few reported injuries are minor.

Damages are expected to be in the millions, not the billions.

Haeussler and others credited a better Municpality of Anchorage building code and better engineering of buildings and infrastructure throughout the region since geologists started talking seriously about the dangers posed to Anchorage by intraslab quakes in the early 1990s.

It didn’t hurt either that the maximum intraslab shakes usually reach only into the low 7s in magnitude, Haeussler said. These quakes can be intense, but they don’t run as long as major slip-strike earthquakes.

“There is the strength of shaking and then there’s the duration,” the geologist said. Alaskans this time got a taste of the ’64 power, but the quake ended a lot sooner.

Everyone in Anchorage on Nov. 30 now has a general idea of what a big one feels like, a startling 30 to 40 seconds of the land moving as if it were the sea. Scary as this might have been in the moment for some, the shaking pales compared to the legendary 9.2 earthquake that flattened Anchorage on Good Friday in 1964.

That time the shaking went on for at least six times as long. If you’re an Alaskan reading this, look around and imagine what else might have come loose in your home or office if last week’s rumbling had continued past the minute mark and then the two-minute mark and then the three-minute mark and then the four-minute mark.

Then there might indeed have been the “panic” the New York Times reported sweeping the state.

Even in 40 seconds, Anchorage residents reported cabinets opening, books falling, dishware breaking, furniture toppling, hot water heaters tipping over, pictures dropping off the walls, ceiling tiles raining down and more.

After the shake ended, Haeussler noted on his Facebook page, he found picture hangers bent as a result of the shaking sent art bouncing up and down.

Every Alaskan reading this in the Anchorage metro area should be able to come up with a way in which their home could have been better prepared.

Think maybe of that full-curl, shoulder mount Dall sheep head that broke free from the wall and almost hit one of your kids. Or the can of bear spray that rattled off a counter and crashed onto a concrete floor but luckily didn’t puncture.

Imagine the mess a spewing can of bear spray could make of your home in the midst of an earthquake.

Admittedly, some damage in a significant earthquake might be impossible to prevent. Expensive Reidel wine glasses, for instance, are fragile, and if you had some bouncing around in a cupboard and none broke, consider yourself lucky.

They should probably be stored in a foam case when not being used if you’re really worried about earthquakes, but who wants to go to such trouble?

Dishware that bounces out of cupboards and crashes to the floor and breaks is likely something most people are willing to put up with, too, rather than outfitting all their cupboards with RV cabinet latches, though that might not be a bad idea if you have family heirloom china.

Still, there are many small things that can be done minimize damage in a 7.0 or greater earthquake. The Anchorage building code now requires water heaters to be anchored to a wall. If yours isn’t, it should be.

If yours moved around so much in the quake it broke pipes, as some reportedly did, you might want to anchor it better. The same goes for tall furniture or filing cabinets that toppled.

The QuakeSmart Community Resilience Program has a handy guide to the many steps that can be taken to prevent damage in a quake. 

The guide goes so far as to suggest fragile or expensive art or collectibles should be secured with “safety straps, mats, small enclosures, base isolation
platforms, and other devices.”  That might be overkill for anything but the most valuable personable possessions, but there are other simple suggestions that don’t cost much or get in the way on a daily basis that are worth thinking about:

  • Computers, printers, monitors and other electronic equipment put on skidproof pads or fastened down with something as simple as adhesive locks are much more likely to survive a quake.
  • Earthquake straps are readily available to fasten tall hutches or bookcases to a stud in a wall.
  • A nail driven into framing makes a much better hanger for heavy photographs or artwork than a lightweight picture hook.
  • Safety wires from lights in suspended ceilings will prevent the fixtures from coming down if ceiling tiles are shaken loose. You might even consider a remodel to get rid of the suspended ceiling prone to earthquake damage.

Meanwhile, every Alaska home should contain some sort of emergency survival kit not just for earthquakes but in case of whatever emergencies might arise. Wind and snowstorms can sometimes shut down parts of the city as effectively as an earthquake.

The Red Cross provides good advice on what is needed in such a kit, but leaves off what could be one key item in these times – a powerbank or maybe two for recharging phones. The cellphone charger the Red Cross suggests isn’t much help if the power is out as it was during Anchorage’s last earthquake.

Despite that, many managed not only to stay in contact with friends and family during the power outage but surf the internet for the latest damage in the news as it was being posted in real-time on Facebook and elsewhere by people moving about the city.

Those really concerned about the next big earthquake might even want to invest in a satellite phone, which can talk globally if local cellular connections fail, or maybe even a satellite internet connection (they’re getting better by the day) and a generator for power so you can stay on the internet even if the whole rest of the city is shutdown.

Just don’t forget extra fuel for the generator in case the shutdown runs into days instead of just hours as in the last quake.

But give some thought now to being better prepared next time because the next quake is coming. It’s not a matter of if, the geologists, agree.

It’s only a matter of when.








15 replies »

    • thanks for the credit. the paper is an interesting read, and it’s nice to be credited for journalistic research, which – given the internet – one would expect to find the norm in journalism today.

      but alas….

  1. Thank God!!! I thought all this was the result of Fracking. Plates, faults, volcanos, etc.. I knew there was a rational explantion to all this.

  2. Its really quite simple: Global warming causes increased pressure on the ocean plates, causing them to move more aggressively, increasing earthquakes, uplift, displacement, and volcanism. Also the plates have over millennia regularly bottled up their movement until a plate breakaway occurs. This totally changes the ocean circulation and the climate thereof. Now we have the Maunder Minimum to consider. Will our civilization last as long?

    • Anyone else noticed that the Earth is overdue for a magnetic field reversal? So does that mean we we have a period of zero magnetic field? Incinerating us?

    • Chris, I thought Steve was bad. Now Global Warming (whatever that is) causes earthquakes? Please tell me you are joking..Right? Here I thought Global Warming was man-made and no oceanic made. You do realize how silly all this nonsense sounds right?

  3. Craig,
    So in summary, one of the most common seismic hazards to Anchorage, intraslab earthquakes, are currently unpredictable with unknown recurrence intervals? And a future intraslab earthquake epicenter could be shallower and closer to Anchorage causing even more damage?

  4. How would we design a Knik Arm bridge for such a quake??
    “It didn’t hurt either that the maximum intraslab shakes usually reach only into the low 7s in magnitude, Haeussler said. These quakes can be intense, but they don’t run as long as major slip-strike earthquakes.” While this conversation is relative to “interslab” quakes, Anchorage and Knik Arm may be subjected those major “slip-strike” earthquakes, too.
    What bridge engineer is going to want to be involved in design of such a bridge? Maybe an old one who won’t be around long enough to see it get a shake-up.

    • In rethinking this, I’m reminded of San Francisco’s bridges that are designed to handle large quakes. Most likely just comes down to money and how much $ it takes to build a bridge that can withstand the sort of quake that’s expected.
      Not sure what this added concern will do to the costs associated with Knik Arm bridge. Last i heard was it was expected to cost $1.5 bil.

      • The Frisco bridge is cable stayed, allowing flexibility when seismic stresses occur.
        The Bay Area bridge failed as it is a classic fixed truss design, being inherently stiffer. Both bridges are supported on bedrock. The real challenge of the KAB is
        very deep silt not allowing for proper support. This is further exacerbated by plans to put a train rail on the bridge. This will dictate a fixed truss design, which is not a good design for the seismic onditions. A tunnel would be even scarier.

        Anyway, the bridge will probably never happen. I would bet that the one billion plus bucks will go to yet another attempt to get the Port up to snuff. Design badly, spend crazily, experience failure, make excuses, repeat several times…that’s the governmental mantra.

  5. This earthquake made me realize that we are likely missing an important earthquake-related building code. Just like with hot water heaters, it seems wood stoves should be anchored to something. Like bolted to the floor, or strapped to the wall. I’ve had the muni at my house relatively recently to inspect a new hot water heater and a new wood stove. Water heater needed to be anchored. Not the case with a wood stove. It was good to go, free-standing on skinny legs on slippery tiles.

    In an earthquake, it would not be good if your wood stove was burning away and then started hopping around, shook loose from the stove pipe and tipped over. The house would quickly fill with smoke and if the stove door or vent opened, the fire would get lots of oxygen and go to max flame. Then the chances of your house catching on fire would go up dramatically.

    I have a modern Jotul wood stove. But when I look at the legs, there is no way you could bolt them to the floor for earthquake security. Seems like this is a basic safety design oversight on part of the wood stove industry. You’d think all stove legs or stove pedestals should be able to be anchored to the floor. Not the case. And building codes don’t require this.

    How to change this? Well, as soon as a house fire is linked to a wood stove tipping over during an earthquake, and insurance companies become aware of this risk, then anchoring of stoves will be required if you want house insurance. Soon after, building codes will follow.

    • I recall reading of one family log home, during 64 quake, that was stood on one end with everything inside it (including occupants) sliding to one end. Then it was stood on the other end with everything sliding towards the other end, including the wood stove that was hot at the time. It would really take some anchoring to stop such violent shakes but your idea is a good one and probably should be done for boilers, etc., IMO.

      • Not all M7’s are the “same.” An intraslab M7 with an epicenter under anchorage may cause more damage than a distal M8 megathrust quake*. I don’t think anyone has modeled the shake potential-damage potential of a M7 intraslab earthquake with an epicenter under Anchorage at a shallow dept. Not sure if we want to see the results.

        *Intraslab earthquakes normally won’t lead to a tsnumai (except landslide generated) unlike a megathrust earthqauke. So there is more than just shake potential that needs to be considered in seismic hazard potential.

    • What about pouring a concrete slab a couple of inches thick and installing the stove legs in it before the concrete sets? Or making holes for the legs in the hearth pad and using an epoxy anchoring adhesive like Simpson makes? Just thinking out loud here…

      • never thought about that Tim, but oddly enough my old, cheap-ass airtight that heats the workshop is bolted to the slab. can’t even remember why i did it. maybe because i’m always banging around in there with with stuff and didn’t want it to move if i ran into it with a snowgo or something?

        and, no, i don’t work on machines with gasoline engines in the building with the woodstove burning or even embers in there. it might be air tight, but i’m not that trusting. wood heat is nice, but there are inherent dangers.

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