An expert writes in (with a few updates below):
As a geologist, I’m surprised that the discussion of extensive mountain-flattening in China has so far ignored the most scary potential consequence: widespread earthquake damage. The danger doesn’t come from removing the mountains; it comes from filling in the valleys. In brief, soils that are being placed for future building construction need to be deposited in thin “lifts” a few feet thick and compacted carefully before the next layer is placed. Soils that are just dumped into valleys, in layers tens of meters thick, can not be properly compacted. They are very likely to turn into Jello when shaken, especially if they’re wet when the shaking occurs.
If you want to see a marvelous example of this, look at the damage to the Mission District in San Francisco during the Loma Prieta quake. Buildings constructed on thick layers of poorly compacted fill were far more heavily damaged than those a few blocks away, which were built on native soils.
The scale of what the Chinese are doing dwarfs any comparison with San Francisco.
Some of these cities could double in area, with nearly all of the additional construction space consisting of poorly compacted soil. Even if the new buildings stay up for a few years, the soils at depth will remain uncompacted and may liquefy during the next earthquake. And central China, where this work is being conducted, is subject to frequent massive earthquakes, at least as frequent and as intense as anything California has to offer.
Update from a reader:
Mine is a small correction: it’s the Marina District that was built on landfill, not the Mission, and the damage from the ’89 earthquake was worst in the Marina. I was living on Telegraph Hill at the time. My home was shaken, a few books and records knocked out of their cabinets, but there was no real damage to my house or to those of anyone around me on the hill. We lived on rock. Most of the Mission, if not all of it, is solidly on land.
But the Marina and much of the Embarcadero are on landfill. (There’s a fine poem by Robert Hass about how some of this came about, called “The Harbor at Seattle“.) Currently our home is at the beach, built on sand, and everyone knows what that means, or thinks they do. But it’s survived two temblors with just one crack, a minor one, in the basement walk way. Liquefaction is a potential problem.
But the “give” of sand, in an earthquake, might be advantageous. It’s the tsunamis we have to watch for. There are signs everywhere out here instructing you where to run. (“To the hills!”)
Another:
Your first update was correct, in part: It was the Marina, not the Mission, that the first reader probably meant to reference. But the Mission did have soil failures in 1906, and we have liquefiable soil around the entire Bay edge and along old Mission creek.
But while we’re correcting earthquake references in the 25th anniversary year of Loma Prieta, may I just note, as a structural engineer in San Francisco, how often I have to shake my head when clients say “My building is on rock” or worse, point out how little damage they sustained in 1989 – from a short earthquake centered 50 miles away! When our earthquake comes, rock sites are going to shake plenty too, and a lot harder than they did in ’89. Soil can explain why a distant site still feels strong shaking, but the real culprit is a weak or ill-conceived structure above the ground. (San Francisco has a new mandatory retrofit program addressing the worst of these. Come here in the summer of 2018, and you won’t be able to walk to the nearest google bus stop without seeing a dozen retrofits in progress. Meanwhile, both the A’s and the Giants are currently in first place …)
As for the uncompacted fill (the first reader’s point about China), the earthquake issue there is not just amplified shaking, but settlement. Shake all that loose soil, and if it slumps or settles just a few inches, that’s enough to crack foundations, roads, runways, buried pipelines, etc. It doesn’t kill as many people as structural collapses, but it can really mess with your local and regional economy.
The Dish 