Today a research vessel will finish mapping more than four square miles of ocean floor off Avila Beach so geologists can better understand an earthquake fault near Diablo Canyon nuclear power plant.
The marine research vessel Bluefin of Seattle has spent nearly most of December towing a sophisticated array of instruments in a grid pattern through the waters of San Luis Obispo Bay.
The result will be a detailed map of the southern end of the Shoreline Fault, which runs northwest from the bay, brushing the coastline just off Diablo Canyon.
The mapping work is part of a three-year effort that, using the latest advances in seismology, will make the land and ocean around Diablo Canyon some of the most intensely studied geography in the United States, said Stuart Nishenko, senior seismologist with PG&E, which operates the plant.
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“Keeping up the science is key,” he said. “This is not static.”
When the study is done at the end of 2012, geologists will have a more complete picture of the earthquake faults that bracket the nuclear plant. A clear understanding of the threat these faults pose to the plant is a prerequisite for approving PG&E’s request to renew Diablo Canyon’s two operating licenses for 20 more years to 2044 and 2045.
Concern about earthquake safety at nuclear plants reached new heights last March after a powerful earthquake and subsequent tsunamis that crippled four nuclear reactors in Japan, releasing large amounts of radiation into the environment.
Mapping with sound
On the Bluefin’s after-deck, workers in hardhats and life vests hurry in the dawn light to deploy a seismic instrument array that extends nearly 500 feet behind the ship. The process takes nearly an hour.
Phil Hogan, a geologist with Fugro Consultants of Ventura, the firm hired by PG&E to perform the surveys, explains how it works. The instruments consist of a sled, called a triple plate boomer, which emits a bang as loud as a gunshot or firecracker into the ocean.
Trailing behind the boomer is a line of 14 streamers, each containing eight hydrophones, he said. Once the instrument array is deployed, the Bluefin begins steaming in the grid formation across the study area, a process that Hogan likens to mowing the lawn.
The boomer emits sound into the ocean every 10 feet the ship travels or about every second and a half, Hogan said. The sound penetrates several hundred feet into the seafloor before bouncing back and being picked up by the hydrophones.
In a compartment on the ship called the instrument room, half a dozen technicians stare at computer screens that display the raw data the hydrophones pick up. On a day when the weather cooperated, the ship and crew spent 10 uninterrupted hours surveying.
All the while, biologists on the bridge of the ship scan the horizon with binoculars, looking for marine mammals. A 525-foot safety zone is maintained around the ship while it operates.
Work can be stopped if a whale or seal shows signs of distress as a result of the sounds emitted by the boomer, said Marques Humpal, a biologist hired by consultants Padre Associates for the survey work.
Three separate studies
This month’s work on the Bluefin was the second time in as many years that the Shoreline Fault has been mapped. In October 2010, a 6½-mile swath of ocean from Diablo Canyon to Point Buchon, containing the northern end of the Shoreline Fault, was seismically surveyed.
All of this work is a dress rehearsal for much more elaborate three-dimensional offshore fault mapping from Point San Luis to Cambria that will be done late next year. That mapping involves emitting pulses of sound into the ocean that are nearly twice as loud as the triple-plate boomer and require a special permit from the state.
An environmental impact report on these high-energy studies is due out in January, Nishenko said. It will examine the possible harmful effect of the loud sounds on marine life.
The benefit of the high energy studies is that they penetrate up to six miles into the Earth’s crust and produce a three-dimensional view of the main faults off Diablo Canyon — the Shoreline and much larger Hosgri Fault. The images produced by the low-energy mapping are comparable to an X-ray, while the high-energy mapping will produce images like a CT scan.
“With the high energy, you can spin the images around and slice it and dice it any way you want,” Nishenko said. “Each technology tells you a little something different at different depths.”
Although most of the work is being done offshore, in November PG&E also completed an onshore component, mostly in the Irish Hills that surround Diablo Canyon.
Using large trucks that vibrate the ground or drop heavy weights, shock waves were sent into the ground along 120 miles of road. Echoes from the shock waves were picked up by hundreds of sensors arrayed through the area.
Are faults connected?
In all, PG&E customers will pay $64 million for the three years of seismic study. Just bringing the vessel here that will do the high-energy work next year will cost $1 million.
One of the main goals of all this work is to determine whether any of the faults around Diablo Canyon connect with one another. One of the absolutes in seismology is that size matters — the larger the area over which a fault ruptures, the larger the earthquake it will produce, Nishenko said.
Diablo Canyon is designed to withstand a 7.5-magnitude quake. Alone, none of the faults around the plant are thought capable of delivering that kind of a jolt. However, if two faults interconnect, they could rupture over a greater area and that margin of safety could be lost.
The Shoreline Fault is of particular interest to seismologists because it is so close to the plant. It runs through the surf zone at several spots south of Diablo Canyon.
Seismologists are hoping the mapping done by the Bluefin will tell them just how active the fault has been. To do this, they will be looking at a prehistoric section of San Luis Obispo Creek that bisected the Shoreline Fault.
During the last ice age, some 20,000 years ago, sea levels off the coast were 400 feet lower and San Luis Obispo Creek extended three miles longer. Channels incised by the creek into the bedrock are still visible on the ocean floor.
Every time a quake occurs along a fault, doglegs are created in creeks that cross it. Using techniques perfected by geologists working at Wallace Creek along the San Andreas Fault on the Carrizo Plain, seismologists will be able to measure how much San Luis Obispo Creek was offset by the Shoreline fault.
Now that the Bluefin’s work is done, Nishenko and other seismologists will spend months analyzing the data collected. They will also be getting ready for next year’s high-energy survey work.
“It’s all a matter of finding the right tools to answer the right questions,” Nishenko said.