H-AVILA BEACH

Jul 23, 2013 8:12 PM by Cameron Polom, KSBY News

Seismic sensors dropped into ocean

How many earthquakes occur along the Hosgri fault every year? Researchers may soon have that answer.

PG&E hopes to get a better understanding of earthquakes near Diablo Canyon through a new research project currently underway.

On Tuesday, researchers dropped sensitive seismic sensors to the sea floor in hopes of learning how many and where earthquakes happen along the offshore fault lines.

Unlike the high frequency and low frequency testing that was proposed and subsequently denied by the Coastal Commission in October of last year, these devices do not emit sound waves into the earth's crust. They merely sit on the ocean floor and listen for vibrations from both large and micro earthquakes.

"So we're headed about three nautical miles offshore to a vessel called the surveyor," said Blair Jones of PG&E.

On Tuesday, PG&E and a team of researchers placed the third of four seismic sensors on the ocean floor near Diablo Canyon.

"The crews on the Surveyor are preparing to lift up one of these ocean bottom seismometers, lift it down into the water and gently place it on the ocean bottom," said Jones.

That's easier said than done. Researchers have mapped out the best locations to obtain the data they are seeking. When those locations are 110 meters below the surface of the ocean, mother nature presents some challenges.

One of the things they were worried about were the ocean swells, but Tuesday seemed to be very calm and work went smoothly.

Crews hook lines onto the 1.2 ton concrete domes that protect the sensors.
Using GPS and some expert driving, the concrete domes are slowly lowered in place.

"It's going to be connected by a cable to other seismometers on the ocean floor as part of this project," said Jones.

The sensors will relay any detection of seismic movement along the cables up to researchers on shore.

Once in place each of the four sensors will stay there for up to ten years.

If everything goes as planned, all four sensors should be in place within the next few weeks and will immediately begin feeding seismic data to scientists on shore.

PG&E says the sensors are completely non-invasive. They don't emit any sound and only listen for vibrations from the off-shore faults and relay it to researchers.

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