The land near the Washington Monument has sunk by 2 millimeters, according to a team of federal land measurement experts surveying the Mall this week.
The discovery is a bit unexpected; the area typically sinks annually at rates in the tenths of a millimeter. But whether the new sinkage is due to last summer’s 5.8-magnitude earthquake is still up in the air.
“The findings are very preliminary at this stage,” said the National Geodetic Survey’s chief surveyor David Doyle said Wednesday.
Bottom line: They haven’t discovered anything Monument and Mall-lovers should be worried about, he said.
“When we have the final data, we’ll be able to sit down and say what part of this is attributable to the normal subsidence rate and what part of this is attributable to the quake,” he said.
Doyle said he expects that to be only about a millimeter or less. Large earthquakes like those in San Francisco can cause land to sink by as much as a few feet, he said.
The ground is built on a “landfill” — sand and gravel taken from the Potomac River. Doyle said that an earthquake can cause these sediments to liquefy and speed up the rate at which the land sinks, but it doesn’t appear that this is happening.
The federal agency has been surveying the monument grounds since 1884. This year they’ll be analyzing the entire National Mall, from the Capitol building to the Tidal Basin and even benchmark areas near the White House.
The survey is expected to be completed some time next week, at which point the agency will hand over its findings to the National Park Service. The NPS analysis will take a couple of months, according to spokeswoman Carol Johnson.
“This is something that happens annually because it’s on a landfill,” she said. “This is pretty standard in the Washington Monument area.”
