The Mount St Helen’s thread made me start wondering this - I understand that Mount Ranier is an active volcano and that there are risks of lahars from an eruption.
How significant is the risk to Seattle?
PS - apologies to any Seattlites if this is a sensitive subject.
Here is a page that shows previous lahars. Some reached southern suburbs of Seattle.
Here is a page with a lahar hazard map. Note that the area doesn’t extend as far north as the historical ones.
I think there is a small chance under just the right circumstances of one making it all the way to the Duwamish mouth. Harbor Island and all that. For the people in West Seattle, it wouldn’t be all that bad by that time unless you were right on the shoreline (the people further south: not so good). But that is far, far less likely than Tacoma and Puyallup being taken out.
That first link shows Kent sitting in a body of water that doesn’t exist, I assume that must represent flooding from the Green River?
Probably not, but Tacoma could be history, as it lies in the historical Rainier lava flow.
Oops, I see ftg already noted that.
The biggest hazard for Seattle will be a tidal surge if the lahar hits Puget Sound with enough force. Where I use to live above the Kent/Auburn valley, I would have had a nice view of a lahar moving through the valley.
Topographically speaking, Tacoma climbs pretty steeply away from sea level. Since the likely flow would be down the Puyallup River, while the Tideflats and the Port of Tacoma would indeed be history (along with an indeterminate portion of downtown), most of the city would be safe from lava. To the disgust of every right-thinking Seattleite.
(Note that I specified from lava. Ash could be a definite problem, though the usual wind direction [from the S or W] would tend to blow the ash plume away.)
Hmmm Bellevue seems safe. Might even be good for property values. None of the tech companies (MFST, Amazon, Google, T Mobile or Travelocity?) should be majorly affected.
Microsoft is in Redmond, but saying Bellevue is close enough. Amazon, as far I know, has always had their offices in Seattle proper, and is building more. Not sure about Google, and I don’t think Travelocity has any offices in the region.
The Space Needle, a mile or two down-range from the city center, is 60 miles from the peak, in a straight line. The direct pressure wave + ash blast zone for MSH was something like 20 miles in radius (fanning from the breach). So, to have a serious impact on Seattle itself, Rainier would have to blow with at least nine times the force of MSH, and that is assuming the correct face yields the blast (it could blow toward Yakima or Centralia about as easily). And as far as lahar flows, it seems that the direction of the blast affects those as well (if it were to blow east, most of King and Pierce counties would experience a lesser effect).
Rainer has nearly 5000’ more height than MSH did, but it is not entirely clear what that means in terms of blast force. Probably something significant, but probably not 9x. So, Seattle is probably pretty safe.
I am up for correction here, but my understanding is the the explosiveness of the eruption has far more to do with the pressure and composition of what is below the surface than the size or shape of any lump sitting on top. (Notwithstanding the presence of bodies of water which can change everything.)
In other words, to adequately compare Ranier and St Helens you would want data on their entire eruption history to build up a pattern. My extremely limited knowledge of the two suggests that they are not too dissimilar although MSH’s last pop was a bit of an anomaly.
Some eruptions are bigger than others … something like Mt Mazama would be very very bad for Seattle, Portland, Spokane and Boise. Although it’s more likely a Cascadia Earthquake would happen and destroy Seattle.
Probably thinking Expedia, which does have it’s main office in downtown Bellevue.
Part of my point was that if Rainier blew off a similar amount of material, its shockwave blast (what flattened the forest around MSH) would originate at a higher elevation, thus would have potentially greater coverage. The crater on MSH is about a mile and a half across (N/S) and some 6000’ elevation – if Rainier blew off an area about 2 miles across, the floor of the crater would be around 10,000’, so the shockwave part of it would be less affected by intervening terrain. But, again, to reach Seattle with any effect, the explosion would have to be quite a bit more powerful than MSH, nearly an order of magnitude.
Fair enough.