A collection of spaceship-like tripods popped up around Napa in the middle of October and left just as quickly as they came.
The devices weren’t otherworldly visitors. They were temporary earthquake monitoring stations installed by the U.S. Geological Survey.
“We are still measuring movement related to the earthquake” of August 2014, said USGS research geophysicist Jessica Murray, who is based at the Earthquake Science Center in Menlo Park. In order to do that, “you need measurements over time,” she explained.
Murray said approximately seven of the stations were set up at precise locations in Napa around Oct. 15 to measure how the ground is moving beneath them.
One such antenna was seen on Golden Gate Drive in south Napa. Two others were set up along Highway 29, including one at the southwest corner of Highway 29 and Trancas Street. Others were placed near the Veterans Home of California in Yountville, in the Carneros area, on Hagen Road and in American Canyon. Each was set up on a roadway shoulder, not private property.
The tripod holds an antenna that is connected to a plastic box containing a Global Positioning System (GPS) receiver. The GPS is similar to what is found in a car, “but a lot more precise and with better instrumentation,” Murray said.
The GPS device takes readings that are then downloaded by the scientists and interpreted for ground movement. The Napa data have not yet been processed, they said.
Right after the 2014 quake, such monitoring devices were left in place in Napa for weeks. This time, the seven units were only set up for about 48 hours.
“Since the post-earthquake motion decelerates over time, we are making less frequent surveys now” — about once every six months or so, said Murray, and soon they’ll probably only install them annually.
Ellen Phillips, also with the USGS Earthquake Science Center, helped install and monitor the temporary antennas in Napa. Phillips said the monitoring can be used to help assess hazards of future earthquakes.
“If you have a good sense of the shake likelihood, it can impact what kinds of structures are built” at a particular area, or appropriate building codes, Phillips said.
The data can also be used in the ShakeAlert earthquake early warning system that will be implemented on the West Coast, said Murray.
Sometimes the sites are vandalized, which is why the USGS locks the box and doesn’t like to advertise the monitoring locations.
“It’s unfortunate” when that happens, Murray said. “It’s valuable to us,” but the equipment can’t be reused for any other purpose, she said.
Murray asked that if Napans do see the stations in the future, “don’t tamper with them.”
“If someone bumps into it or nudges it a bit, it will throw off the measurements. Please stay back.” Jumping around the device shouldn’t affect it, she said. “It’s not like a seismometer,” but it’s best to avoid any contact with the devices.
Phillips said she enjoys collecting the data.
“I like seeing how things move and change,” she said. “I get to be a part of the scientific process which ultimately helps keep people safer.”
As to the antenna itself, she has heard that it does resemble a flying saucer.
“It’s definitely not intentional,” she said. The round rings are actually called a “choke ring” antenna and it helps filter out certain signals.