A rotten egg smell which had Californians turning up their noses was caused by dead fish or algae in a nearby lake, and nothing more sinister, experts said Tuesday.

More than 200 people called authorities Monday after the sulphurous odor was detected over a 150 mile area, leading to online speculation including that it was some kind of geo-thermic event preceding a long-feared mega earthquake experts believe will shake California.

But experts at the South Coast Air Quality Management District said the source was almost certainly the Salton Sea, a huge lake a couple of hours east of San Diego in southern California.

"We now have solid evidence that clearly points to the Salton Sea as the source of a very large and unusual odor event," said the air monitoring body's executive officer Barry Wallerstein.

Hydrogen sulfide, "a product of organic decay such as that occurring in the Salton Sea, has an unmistakable rotten-egg odor," it noted.

Its experts believe "that strong winds pushed surface waters aside and allowed water from the bottom of the shallow sea -- rich with decaying and odorous bacteria -- to rise to the surface," it said.

"While hydrogen sulfide concentrations at the Salton Sea yesterday were higher than normal, they were not high enough to cause irreversible harm to human health," it added.

California's perennial quake jitters have been heightened this month, after a "quake swarm" which shook part of southern California with hundreds of moderate temblors in quick succession, and two jolts centered on Beverly Hills.