(CNN) -- The violent waves that injured several spectators at a California surf competition over the weekend technically weren't "rogue waves" as they've been called in some media reports, but they were unexpected and dangerous, a weather and water expert said.
Spectators at the Mavericks Surfing Contest were standing on the beach at Princeton-by-the Sea, California, on Saturday morning when large waves knocked them down. More than 12 people were injured, according to local reports; video of one of the waves was recorded by CNN affiliate KRON of San Francisco.
They were "sneaker waves," waves near the shore that are unexpectedly and significantly larger than others occurring at the time, said Larry Smith, a National Weather Service meteorologist based in Monterey, California.
Though some may loosely define them as rogue waves, those that hit the spectators Saturday don't technically fit the National Oceanic and Atmospheric Administration's definition, Smith said. Rogue waves, according to NOAA, occur offshore -- not near it -- and are loosely defined as being twice the size of surrounding waves, sometimes coming from a direction different than the prevailing wind.
Saturday's sneaker waves were the result of several factors, including a storm in the Gulf of Alaska, Smith said. The storm, many hundreds of miles north of Princeton-by-the-Sea, was kicking up high waves -- 18 to 22 feet -- well off California's shore.
"And the frequency in which they occurred was pretty long -- 20 to 30 seconds. Any time you have a wave that powerful, you can expect dangerous conditions in the surf zone," with waves in that surf zone potentially getting higher than 30 feet, Smith said.
The power produced by some of those waves pushed water over the beach's sea wall in a few instances, knocking over some of the surfing contest's spectators.
"Days like that don't happen very often," said Mavericks CEO Keir Beadling, whose company has hosted the surfing competition at the beach five times. "Those storms have to be born thousands of miles away. When they get there, the tide needs to be right, the wind needs to be right, and it really is a perfect storm."
The unique geography of Princeton-by-the-Sea, about 30 miles from San Francisco, also came into play, Beadling said.
"The reality is that there are very few places in the world where you have waves that break the way they do there [in Princeton-by-the-Sea]," Beadling said.
The area "acts like a catcher's mitt from waves that originate off the coast of Alaska and Japan," he said. "The way that the seafloor is shaped there acts like a funnel. It goes from 80 feet to 20 feet deep in a short distance."
Beadling said while the spectator injuries were regrettable, people were told not to approach the rocks along the shoreline.
"We had the police department there, the fire department, and yet people disregarded warnings and signs and got themselves in tricky spots," Beadling said.
A high surf warning from the weather service was in effect when the injuries happened, Smith said. Such a warning indicates that dangerous battering waves will pound the shoreline, resulting in dangerous swimming conditions and deadly rip currents, he said.
In contrast to sneaker and rogue waves, tsunamis are produced specifically by underwater earthquakes or landslides, according to NOAA's Web site. They are "a series of waves that can travel at speeds averaging 450 (and up to 600) mph in the open ocean," according to NOAA.