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Could quake have struck here?

By The Mercury

Maybe folks who thought spending upwards of $150 million in the last decade to make Tuttle Creek Dam earthquake-proof was an expensive boondoggle will reconsider now that a Virginia earthquake has rattled much of the East Coast.

Earthquakes are rare in that region. Still, as was proved Tuesday afternoon when a quake measuring 5.8 — something Californians might shrug off — caused modest damage and led to evacuations even in New York City.

Tuesday’s quake is about the magnitude of a quake that experts think could emanate someday from the Humboldt Fault Zone. That zone is a set of north-south fault lines that from Omaha to Oklahoma City. Tuttle Creek Dam is within 15 miles of the zone.

Geologists and the U.S. Army Corps of Engineers who studied the issue acknowledged that the likelihood of an earthquake strong enough to damage Tuttle Creek Dam — 5.7 to 6.6 — was small. Still, the chance of such a quake during the expected life of the dam was sufficient enough to merit attention.

Before the recent improvements to the dam, an earthquake of that magnitude could have turned sand under the dam to quicksand; that could have caused the dam to expand and its top to sink by as much as three feet. Although the dam would still have been above lake level, water would have seeped through the dam until it ultimately failed.

The worst case scenario was bad indeed, with almost 6,000 homes and more than 12,000 residents in the path of the flood. (Both figures are higher now.) Water at the rate of up to 300,000 cubic feet per second (five times the rate at which it was released when the gates were opened during the 1993 flood) would have inundated parts of Manhattan.

The levee wouldn’t have held the water back, and it would have been up to 10 feet deep in the vicinity of Manhattan Town Center. Such a flood today of course, would flood the downtown redevelopment. Catastrophic is not too extreme a term.

Virginia and Washington, D.C. didn’t escape damage from Tuesday’s earthquake, but more through luck than planning, those areas were able to dodge a natural disaster.

As for Manhattan, we’ll take luck if we have to, but, happily, recent work on the dam lengthened the odds of a catastrophe here.

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