Saturday, July 18, 2015

Cascadia quake and climate correlations

New Yorker's article on the exposure of the Pacific Northwest to a massive quake and tsunami is well worth the read. I used to live there and knew they're equivalently vulnerable to quakes as we are in California, and much less prepared. I didn't know about the tsunami danger.

The correlations I see between this issue and climate are in the seriousness of risk, timescale, and effort needed to respond to the problem. Overall Cascadia may have an even greater problem with the quake and tsunami than they/we have with climate change - the long tail of risk includes immediate catastrophe as a possibility. If they/we are lucky for the next 50 years, taking action on both mitigating emissions and tsunami-proofing our coastal areas without a catastrophe hitting, my guess is the cost of tsunami-proofing in the region would be higher than mitigating and adapting to climate change.

One other overlap is that one way to adapt to tsunamis is to relocate away from sea level, at least with your most vulnerable communities and critical infrastructure, and that will help with sea level rise as well. Finally, they are doing something to deal with this problem, just not doing enough.

Having said all that, one thing that did bother me with the article is that some things seemed exaggerated. Getting knocked over by ankle-deep running water? Call me skeptical. Same thing with Sacramento's alleged vulnerability to tsunamis - I'd like to get learn more about this, but I do know that there are two constriction points between the city and the ocean (Golden Gate and Carquinez Strait) and lots of room for water to spread. Hopefully the other risks aren't exaggerated.


Tom said...

Some things seem exaggerated? How unlike climate change...

Barton Paul Levenson said...

"Getting knocked over by ankle-deep running water?"

BPL: Try it. Try going one foot deep into a beach just as the tide is going out. The undertow is only a few inches deep along the bottom. But believe me, it can knock you over, especially if you're not ready for it.

Mal Adapted said...

Brian: "Same thing with Sacramento's alleged vulnerability to tsunamis - I'd like to get learn more about this, but I do know that there are two constriction points between the city and the ocean (Golden Gate and Carquinez Strait) and lots of room for water to spread."

Keep in mind that a tsunami results when an area of ocean floor is displaced, along with the entire column of water above it:

"It will not look like a Hokusai-style wave, rising up from the surface of the sea and breaking from above. It will look like the whole ocean, elevated, overtaking land."

That means that thousands of cubic kilometers of water will pour in through the Golden Gate. The incoming water will move up the Sacramento and San Joaquin rivers, filling the lowest elevations first of course.

Well, Sacramento's downtown, at the confluence of the Sacramento and American rivers, is less than 10 meters above MSL, and is protected from river flooding by levees less than three meters high. Do you still think the article exaggerates the city's vulnerability?

Anonymous said...

Warnings about fast water knocking you of your feet are common.
Dynamic pressure equals p_d = 0.5 * rho * v²
At 6.7 mph and assuming a lowball area of 10 * 10 cm² for your feet, it's equivalent to 5 kg pulling at each foot. I think that can knock you over.

Brian said...

Not the world's most important issue, but I don't think 10 kg of lateral force is going to knock a grown man over, especially if you're at all prepared for it.

I did think that standing on a steeply sloped beach with the sand eroding out from under your feet might be a different situation, but then it's a different situation.

I've crossed many fast-moving glacial streams in Alaska where I did much of my backpacking. I never had a problem with streams where the water was below my knees (discounting slippery rocks, again a different situation). Granted, I'm not sure they moved at 6.7 mph, that's really fast, but still....

Re Sacramento, no I'm not convinced. I did a cursory search for tsunamis and Sacramento and didn't find anything, but maybe should've put more than a few minutes into it. I do know that the Japanese tsunami caused millions in damages in Santa Cruz and Half Moon Bay while making barely a ripple in Oakland.

The wave energy coming through the constriction of the Golden Gate gets dispersed throughout the entire SF Bay. A second constriction limits energy moving upstream of Carquinez, and then it's got well over 30 miles of channel to fill up, along with what's left of the Delta, before getting to Sacramento. If it's still got enough energy to overtop levees, then it's also got an area larger than SF Bay to disperse into before it hits Sacramento. If it doesn't have enough energy to overtop downstream levees, I don't see why it should break out over 30 miles upstream at Sacramento.

Steve Bloom said...

"To see the full scale of the devastation when that tsunami recedes, you would need to be in the international space station. The inundation zone will be scoured of structures from California to Canada. The earthquake will have wrought its worst havoc west of the Cascades but caused damage as far away as Sacramento, California—as distant from the worst-hit areas as Fort Wayne, Indiana, is from New York."

I don't take that as implying a tsunami at Sacramento, just shaking rather. Indeed, I think it's impossible since the Cascadia fault ends far north of the Golden Gate and vertical displacement is what's needed for the tsunami. The San Andreas fault just offshore is big enough to generate one, but doesn't have the correct motion to do so (not a very big one, anyway, which in any case would attenuate greatly once in the Bay). But lest we rest easy, WP says:

"Recent studies of past earthquakes indicate that there is a correlation in time between seismic events on the northern San Andreas Fault and the southern part of the Cascadia subduction zone (which stretches from Vancouver Island to northern California). Scientists believe quakes on the Cascadia subduction zone may have triggered most of the major quakes on the northern San Andreas within the past 3,000 years. The evidence also shows the rupture direction going from north to south in each of these time-correlated events. However the 1906 San Francisco earthquake seems to have been the exception to this correlation because the plate movement was moved mostly from south to north and it was not preceded by a major quake in the Cascadia zone."

I read elsewhere that worst-case San Andreas quakes are doubles involving north and south (apparently the movie got that right anyway), so the real Big One might involve the entire west coast system going at once.

Interestingly the northern San Andreas anyway seems to have been a subducting fault until relatively recently, when what had been the spreading center between the Pacific and Farallon plates got subducted and the fault motion changed.

Steve Bloom said...

The supercontinent cycle is interesting. We're at about maximum dispersal now, and early signs of the reversal have been observed (although apparently on the opposite side of the Atantic from where it had been predicted, meaning that the incipient continental motion will follow a different pattern).

After the remaining bits of the Farallon plate have disappeared under North America (the Juan de Fuca plate that has the Cascadia subduction zone on its eastern margin is one of several), I'm not sure what's supposed to happen here (other than that the northward rotation of the Pacific plate will continue to grab off bits of California and plaster them onto southern Alaska -- so long, Point Reyes!). Possibly it will remain a passive margin and the Pacific expansion to once again become the World Ocean (the Atlantic and Indian oceans close entirely as the supercontinent reforms) will occur on the other side (Asia and Australasia).

Apparently an implication of all of this is that the current icehouse climate state (enabled by relatively low tectonic CO2 emissions and relatively high weathering of exposed rock) will come to an end relatively soon (on a scale of millions of years) and not recur until the supercontinent has re-formed ~300 my in the future (they being prone to glaciation).


"Getting knocked over by ankle-deep running water?"

Is a lot easier than water skiing barefoot.

Hank Roberts said...