A tsunami generated by a massive earthquake off Alaska could cause major damage to California's economy and force 750,000 people to evacuate, a report published Wednesday warned.

One third of all boats in California's marinas could be damaged or sunk, costing some $700 million in losses, while major ports would struggle to get huge cargo vessels out to sea in time to avoid being buffeted by tsunamis.

Experts from the US Geological Survey (USGS) based their damage assessment on the scenario of a 9.1 magnitude quake off Alaska's Pacific Coast, which it said was "hypothetical but plausible."

"In this scenario approximately 750,000 people would need to be evacuated, with 90,000 of those being tourists and visitors," said the report, co-published by the USGS and the California Geological Survey.

The number of tourists -- who would be more at risk because they may be less prepared for what to do -- would increase to millions in the event of a tsunami in summer months, when visitors flock to California's beaches.

"The good news is that three-quarters of California's coastline is cliffs, and thus immune to the harsher and more devastating impacts tsunamis could pose," said Lucy Jones, who led the study.

She also welcomed findings that neither of California's two nuclear power plants, both near the coast, would be risk under the scenario studied.

"The bad news is that the one-quarter at risk is some of the most economically valuable property in California," she added.

The report highlighted the potential impact on the ports of Los Angeles and Long Beach, two of the largest trade gateways on the US West Coast. "Larger vessels that remain in ports might also be vulnerable," it said.

"For example, given the short time between the tsunami warning ... and the first wave arrival" -- 3.5 hours in LA and Long Beach -- it may be difficult or impossible" to get ships out to sea, where they would be at less risk.

"Damage to vessels in the ports is possible. Other ports in San Francisco Bay and San Diego Bay are also likely to be damaged in such a scenario," it added.

California has long braced for the Big One, a monster 8.5 plus magnitude earthquake expected to occur on one of the seismological weak spots under the US state itself, notably the San Andreas fault east of Los Angeles.

But a tsunami generated from a quake further away has been taken more seriously, notably since the March 11, 2011 9.0-magnitude temblor off Japan that killed some 19,000 people and triggering a nuclear calamity.