Treasure trove of sand found among earthquake faults
The latest news from UCSD’s Scripps Institution of Oceanography is that offshore La Jolla does - far more than anyone imagined.
A breakthrough in photographic imaging of offshore, underwater sand has revealed huge deposits buried under 90 feet or more of water that are greater than 60-feet deep off La Jolla’s coastline.
Why is that important? Because it could mean there is a huge potential cache of usable sand from which to draw from in replenishing sand along San Diego’s coastline, a periodic activity made necessary by winter storms that can scour and deplete sand along area beaches.
Scripps geology professor Neil Driscoll said prominent earthquake faults bisecting La Jolla’s coastal areas are responsible for the treasure trove of sand which has built up over time in deep, underwater depressions. He said faulting is the result of the Pacific tectonic plate moving to the north with respect to the North American tectonic plate, which, in geologic terms, is an incredibly slow - but persistent - creep.
“This movement is on the order of about 40 millimeters per year, about the rate your fingernail grows,” said Driscoll, adding faults are straight, but have right and left bends and jogs in them. “What we have discovered, using photographic equipment that can penetrate and image layers beneath the sea floor, are basins, or receptacles, where sand can be stored on a timescale of thousands of years.”
These earthquake faults have created underwater basins or valleys that actually can preserve sediment. Said Driscoll: “One of these basins or depressions we imaged is right offshore north of Scripps Canyon, where there are 20-plus meters of sand. This is a potential borrow site for replenishment of sand along our beaches.”
The San Diego Association of Governments (SANDAG), a regional planning agency of the 18 cities and the county of San Diego, which is primarily concerned with transportation issues, is the governmental agency responsible for conducting sand replenishment, when necessary, along San Diego’s coastline. Rob Rundle, prinicipal regional planner for SANDAG, noted the discovery of huge sand deposits offshore in La Jolla is an important one. But he stopped short of describing it as a boon to potential future sand replenishment efforts.
“It’s a huge amount of sand,” said Rundle. “But we’d have to do further exploration to see what the quality and grain size of the sand are. It’s a potential source of sand for future beach nourishment, but we have to do some further investigation to see that it’s suitable.”
But Rundle noted early signs are encouraging that offshore La Jolla possesses a wealth of viable sand which could be used for future beach replenishment. He added, in 2001, that SANDAG oversaw the transfer of 2.1 million cubic yards of sand, at a cost of about $17.5 million, along 12 coastal beaches from Oceanside down to Imperial Beach, including the beach along Torrey Pines.
“We actually mined a very small amount, 76 million cubic feet, of some fairly fine-grained material from a small, discrete area (in La Jolla) for beach nourishment,” said Rundle. “It’s the ideal sand you’d want to use.”
Rundle said it costs an average of $5.50 per cubic yard to pump sand from the ocean floor and put it on coastal beaches. That cost does not reflect additional expenditures, such as environmetal review and the permit process required of such projects.
Geologist Driscoll said the outcome of this underwater study which revealed huge stores of sand points out how earthquake faults control what type of geology is on the seafloor, which in turn controls the biohabitats present there.
Driscoll also pointed out extraction of sand from newfound underwater deposits offshore in La Jolla could be less damaging to the environment. “Collecting the sand from deeper waters further offshore might have less negative impact when borrowing from these sites,” he said. “This might have the effect of bringing sand back onto the beaches, which is not available by natural processes such as waves.”
Driscoll noted tectonic faulting in San Diego is responsible for the uplift which, besides creating depressions which trap sand, also led to the creation of La Jolla’s Mount Soledad. Over recent geologic time, San Diego sea levels have alternately risen and fallen as glaciers on land have advanced and retreated, hugely impacting the region’s topography and environment. “Twenty thousand years ago the sea level was 125 meters lower than today,” he said. “Since 20,000 years ago, when the big glaciers were on land, the sea level has risen and flooded the coastal shelf that used to be exposed. Modern sands have accumulated above that surface.”