My apologies for being so absent this month, my darlings. It’s been an eventful several weeks: we had to find a new place to live quickly due to issues with our old place. After a frantic scramble, we found a beautiful new home – and the day before we moved into it, my roommate ended up in the hospital for two weeks. Then I got sick. So yeah, it’s been a bit fraught.
But we’re through the worst of it now, and I can get back to talking about the good science of rock-breaking!
We’ll start with a bit of a geology grab-bag. I’ve been collecting links over the month. I think you’ll like the shiny things I’ve been tossing in the sample bag for ye.
Why Geology Matters: Dam-Building Edition
You may have watched with awe and trepidation last year as California’s Oroville Dam nearly failed. This year, we have the report on what happened, and it turns out that geology matters one heck of a lot when it comes to dams. You ignore it at your peril.
An investigation into last winter’s near catastrophe at Oroville Dam uncovered a litany of problems with how the dam was built and maintained, but one of them stands out: Even as workers built the dam, they were raising alarms about the eroded, crumbling rock on which they were directed to lay concrete for the 3,000-foot-long main flood control spillway.
Construction reports from the fall of 1966 showed an abundance of loose clay, “shot rock” and “very little solid rock.” The surface was so crumbly, according to a state engineer overseeing the work, that a laborer at one point refused to do any more prep work until he got clearance from his boss. The contractor told the California Department of Water Resources it needed to dig deeper to find stronger rock.
But DWR limited the additional excavation work proposed by the contractor, a decision that investigators now say might have been motivated by money.
Because no one scraped away the rotting rock and installed the dam and spillway on solid stone, the spillway weakened and cracked, causing some pretty dramatic erosion and near-catastrophic failure.
Why Geology Matters: Fires and Mudslides Edition
Also in California: hillsides whose vegetation burned in the massive, horrific fires late last year are now slipping and sliding after being hit by winter rains, killing nearly two dozen people at last count. This, too, was utterly predictable if you know your geology.
As mountains rise, erosion tears them down. And Southern California’s mountains are rising fast, squeezed up by the action of the region’s active faults. This produces steep slopes that erode quickly, though much of that erosion happens in infrequent events, such as big rainstorms right after big wildfires.
We know that risks vary across the terrain and that some places in landslide-prone zones are more dangerous than others. In some regions the riskiest areas are well downslope or downstream of slide-prone slopes, in the places where debris runs out and comes to rest. Unfortunately, few people are aware of these risks when developers build in and around landslide-prone mountains.
If we want to make our citizens safer and our towns and cities less prone to disaster, we’re going to have to start paying attention to less enthralling geologic phenomenon like earthquakes and volcanoes. We’re going to have to accept that the places we want to live and work can be incredibly dangerous if we don’t properly map and mitigate their hazards.
Speaking of landslides, check out this earth in motion!
In my own home state, another landslide is causing government agencies quite a bit of worry. There’s some pretty dramatic drone footage of it now.
This landslide appears to be in the secondary creep phase at present. In some slopes the transition to a tertiary creep phase does not occur, and the slope deforms slowly through time. In others the slope accelerates to failure. This behaviour is controlled primarily by the characteristics of the materials forming the landslide, which (as the images above show) are accumulating damage at present in the case of the Rattlesnake Hills rockslide. In many cases this damage leads to a weakening, such the resistance to movement declines, allowing the slope to collapse rapidly. In other cases, the materials are able to maintain their strength as they deform, and no transition to tertiary creep occurs. Other factors controlling this behaviour will include the geometry of the slope (which in this case is very complex) and the response of the mass should significant rainfall occur. In many cases the best guide might be the behaviour of other rockslides in the same materials, with evidence being drawn from landslide deposits in the region.
Until that landslide decides where it wants to go, how fast, and when, I don’t think I’m going to be driving the interstate near it…
Earthquakes from Space!
After all this gloom and doom, let’s end on a more exciting note. Did you know some earthquakes come from space? They do! They happen when meteorites fall down and go boom, and one happened very recently in Michigan. !
On January 18th at 8:08 PM local time hundreds of people in Michigan, and about six surrounding U.S. states and southernmost Canada, witnessed a spectacular sight in the sky. A meteor travelling at an estimated speed of 58,000 km/hr (from NASA) broke up across southeastern Michigan, creating a light show that dazzled all who were lucky enough to see it. Any large object travelling that fast creates a shockwave as it moves through the atmosphere, and the shockwave from this meteor just happened to be recorded by an array of sensitive instruments on the ground in the area.
Sensitive infrasound instruments were co-deployed with seismometers across the central and eastern U.S. as part of a National Science Foundation funded U.S. Array Experiment, and 158 of the instruments were left permanently when the rest of them moved on to another region. The normal seismometers recorded the shockwave energy that was transferred into the ground, and the measurement was the equivalent of a magnitude 1.8 earthquake. Infrasound instruments, however, measure acoustic (sound) waves in the air by sensing a change in the air pressure.
This is just sheer awesome right here. There’s much more information at the link. So amazing when astronomy and geology collide!