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Southern earthquakes


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Pickens SC
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With our "big one" in upstate SC some liquefaction would happen, but not sand boils. We're got granite interspersed with igneous rocks covered by hard clay; the clay is our problem as it will 'jellify' bringing some landslides and settlement of filled-in areas and also riverine plains which have a higher silt content. Once again old buildings will be the big problem as almost all of those are built on piers, not foundation walls, with too-small footings under them. Even modern pier footings, with about double the surface area of support would be at risk. But worst of all is that there's pockets of Bentonite here which will essentially collapse under any loading, becoming like quicksand. I know of pockets of Bentonite at least the size of a bus plus numerous smaller pockets. Almost no builders or code inspectors have a clue about what it is and how it acts, yet in my work of doing home repairs I've encountered it and watched the center of a living room floor rise and fall over 2 1/2" according to how much groundwater at the surface there was. I've seen it at depths of 4' below grade and I don't know if it goes larger or deeper- if it does we will have Florida-like sinkholes happening too. Our taller buildings all sit on bedrock, rarely more than 60' deep, so no problems there.

Halfway downstate towards and parallel to the coast is the "Fall line" where elevation suddenly drops and the soil becomes much sandier with bedrock being much deeper. Liquefaction will be a huge problem there as Charleston discovered with their "big one". Back in those days almost everything between the coast and mid-state was farmland with tiny towns of mostly wood construction. Since then there's been a lot of growth including larger heavier buildings on insufficient foundations and footings, and a lot of that is also unreinforced masonry; the worst combination possible. Also the water table is higher there with the soil draining faster through their sandy substrate. Just above the 'fall line' the bedrock formations are weak, full of lesser faults, and tending to 'break off' and sink through the normal degradation process of ancient mountain ranges. Elgin, where we've been getting our quake swarm, it right there among the mess. We're in nowhere near the danger of SoCal and the Pacific ledge, but we're also nowhere near as prepared for anything significant to happen. Only the newest largest buildings have considerations made for seismic acvtivity and much of the older urban construction is unreinforced masonry everywhere, so things could be really bad, although our generally mild climate and abundance of all things nature will allow for sustaining life far more easily until the rebuilding is done. That one point alone has allowed SC to get through all disasters man-made and natural in decent shape; hopefully we won't need to test our resilience again.