Late this year, motorists and residents on some of the main roads in the coastal areas of the county are in for a strange sight.
Four huge trucks — each weighing 65,000 pounds — will stop nose to tail on the road and lower large pistons onto the pavement. The pistons will vibrate, first at a low frequency and then transitioning to a higher one.
People standing about 50 feet from the trucks will feel the ground shake beneath their feet, similar to a large dump truck passing by. Portable sensors arrayed on either side of the road will record echoes of the vibrations as they pass through the shallow parts of the Earth’s crust.
After about five minutes, the trucks will raise the pistons, drive from 100 to 300 feet down the road and repeat the process. This will happen on 120 miles of road during September and November of this year.This work will be the land-based component of seismic studies being conducted by PG&E to better map the earthquake faults around Diablo Canyon nuclear power plant.
The utility put on two public demonstrations of the vibration trucks Wednesday. The earlier demonstration attracted about 50 members of the public.
People asked the PG&E experts a variety of questions. These ranged from where the sensors will be deployed to the effect the vibrations might have on pets.
PG&E’s senior seismologist Stuart Nishenko said the vibrations would be kept to less than a fifth of an inch of movement per minute, which is the nuisance threshold.
“If we do our job right,” Nishenko said, “you won’t even know we’re there.”
Between 1,000 and 2,000 sensors will be arrayed in configurations around the areas to be vibrated. These are short yellow cylinders mounted on spikes driven into the ground.
Much of the work will be done on land owned by PG&E surrounding the nuclear plant. Public roads in the study are Los Osos Valley Road, South Bay Boulevard and Highway 1 through the Five Cities area into Oceano.
The work with the vibration trucks will be coupled with seismic work PG&E is doing in offshore areas near the nuclear plant. The utility is currently doing low-energy offshore seismic surveys. High-energy surveys using very loud sounds emitted into the ocean are planned for late next year.
This seismic work was prompted in part by a devastating March 11 earthquake and tsunamis in Japan that crippled three nuclear reactors there. State, local and federal agencies and elected officials pressed PG&E to do the studies.
“We don’t want to repeat the same mistakes that were made in Japan,” Nishenko said.