Slumgullion movement due to atmospheric pressure?

Go to Dave's Landslide Blog for full details about this. I don't have access to the paper.

According to Dave Petley, there's a new paper in Nature Geoscience about the Slumgullion landslide. Slumgullion is in my greater neighborhood - it's in Colorado's San Juan Mountains, between Lake City (former home of Alferd Packer) and Creede (former home of Doc Holliday), and I think it's got the coolest name of any landslide (and possibly the coolest name of any geological feature). It's a strange landslide for its slow movement, and it's being monitored in excruciating detail by the US Geological Survey.

Warning: I haven't read the full paper - just the press release and Dave Petley's comments on it. So, with that caveat, here's why I think the paper is interesting, from the perspective of someone who teaches structural geology (faults, folds, etc.):

The paper concludes that changes in atmospheric pressure cause the small nightly movements of the slide. The explanation is that atmospheric pressure pushes down on both the soil and water. Increases in atmospheric pressure increase the friction that keeps the slide from moving; decreases in atmospheric pressure reduce friction, and allow the landslide to move. This implies an incredibly subtle balance of forces - atmospheric pressure isn't very large compared to the weight of water or soil or rock. And it also means that those things going on in the air are important to what happens below.

Dave pointed out that there was another paper in Nature this year that related changes in atmospheric pressure during typhoons to "slow earthquakes" in Taiwan. It's intriguing. (It does NOT support the concept of "earthquake weather" that Californians tend to bring up every October. Works for slow earthquakes on thrust faults during the extremely low air pressures of typhoons, because the orientation of the fault combined with the stress directions mean that pushing the rocks down decreases the chance of sliding. California's got the wrong fault orientations and stress directions for this to work.)

Intriguing, and perfect fodder for tomorrow's discussion of the stress conditions that reactivate faults. Thanks for the lecture help, Dave!

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Really!? What sort of pressure changes are they talking about? I mean, 1 atmosphere of pressure at sea level is equivalent to the force of ~6 ft of rock (assuming density of 2.5 gm/cm^3). A typical difference in pressure distinguishing a high pressure center from a low pressure one is just a couple of percent, or an inch or two of rock-equivalence. Even if a tropical depression passed over, we're talking about a change equivalent to less than 6 inches of rock, and I can't image the day/night fluxuations to be more than inch at best (in rock equivalence).

At what depth below the surface is sliding occuring at? I would think that below a few feet the relative change in pressure would be pretty slight.

Like I said, I don't have a copy of the paper, so I don't know what depth they're talking about. There was a USGS report on the work published a few years ago, though, which probably has the full details of the geometry of the slide - more than Nature Geoscience would have room for. Dave Petley (who studies landslides) found the results surprising, but thought the modeling looked reasonable. (He also thought that there are only rare cases where air pressure could make a difference - that the slide would need to be essentially at the point of failure all the time.)