The string of four sensors will be in place for at least 10 years. They will allow seismologists with PG&E and the U.S. Geological Survey to better understand earthquake activity along the Hosgri Fault and the newly discovered Shoreline Fault, said Marcia McLaren, a senior seismologist with the utility company.
The project is part of PG&Es ongoing efforts to study the earthquake hazard facing Diablo Canyon, which is located in an area crisscrossed by several active faults. The dominant fault is the Hosgri Fault, about three miles offshore of the plant.
Diablo Canyon is designed to withstand a magnitude 7.5 quake, the largest jolt the Hosgri Fault is likely to deliver. However, seismologists are studying whether a quake along both the Hosgri and Shoreline faults could deliver a more powerful jolt.
The sensors will be deployed along an 11-mile semicircular line starting about a mile offshore of Diablo Canyon and extending to a point about two miles offshore of Spooners Cove in Montaña de Oro State Park. They will join a series of land-based seismometers maintained by PG&E and the USGS in the region since 1986.
It should take PG&E about a week to deploy the sensors. Four temporary seismometers will also be deployed for two weeks.
The eight permanent and temporary sensors will provide a brief but very detailed picture of seismic activity in the area that can be compared to the long-term data collected by the permanent sensors. The sensors passively record the vibrations caused by even very small earthquakes and do not emit any sound or energy themselves, McLaren said.
The sensors will be placed on areas of the ocean with sandy bottoms in order to minimize their impact to the environment, said Horst Rademacher, a senior seismologist with Guralp Systems, a contractor hired by PG&E for the project. Each will have a 1-ton concrete cap placed on top of it to stabilize and protect it and reduce any ambient noise it might pick up from the ocean.
Data collected by the sensors will be brought ashore via a small cable running along the ocean floor to Diablo Canyons cooling water intake cove. The data will then be transmitted to PG&Es headquarters in San Francisco and to the USGS.
The 10-year duration of the project should give enough information to help clarify how active and connected the Hosgri and Shoreline faults are. PG&E could decide to apply to state regulators to keep the permanent sensors in place longer than 10 years if they perform well and do not experience any breakdowns, McLaren said.