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Southern California residents have long prepared against the threat of earthquakes, but a new study suggests that a different natural disaster could pose a bigger danger: tsunamis.

An estimated one-third of boats in California marinas would be damaged or sink if a massive tsunami hit the state, according to a new study. (credit: USGS)

If a magnitude-9.1 earthquake were to hit in coastal waters off the Alaskan Peninsula, waves could flood Southern California’s low-lying coastal areas, forcing up to 750,000 residents to evacuate, a federal study reported.

Damage to marinas, businesses and homes could cost the state between $3.5 billion and $6 billion.

The study, released by the U.S. Geological Survey Wednesday, was developed by dozens of scientists from USGS, California Geological Survey, among other organizations, and is the most extensive examination of the ramifications of a potential tsunami in California to date.

The experts modeled their “hypothetical yet plausible” scenario — a 9.1-magnitude Alaskan earthquake — after data from previous instances of tsunamis in the Pacific Ocean.

“The idea is to say: Look, these are not distant events, these could actually happen here,” Kathleen Tierney, director of the Natural Hazards Center at the University of Colorado Boulder, told the Los Angeles Times. “This is meant to get tsunamis on the public’s radar.”

According to the study, all 20 of California’s coastal counties — stretching from Del Norte to San Diego — would be impacted by a potential tsunami. Waves could hit Northern California in as little as four hours and would make their way down to San Diego in six hours.

A tsunami resulting from a large earthquake off the Alaskan Peninsula would have a devastating impact in California, according to a federal study. (credit: USGS)

Long Beach and San Diego were predicted to be among some of the most highly impacted regions. Low-lying coastal areas in southern Los Angeles and northern Orange counties, close to sea level and not protected by cliffs, would also be particularly vulnerable to damage.

“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, USGS Science Advisor for Risk Reduction, in a USGS news release. “The bad news is that the one-quarter at risk is some of the most economically valuable property in California.”

One-third of the boats in California’s marinas would sink or be damaged, and more than half of the docks could be destroyed, incurring an estimated $700 million in losses.

Of the estimated 750,000 evacuees, about 8,500 people would be left homeless due to extreme damage to their houses and would require emergency shelter, the study said.

This report comes two years after the 9.1-magnitude earthquake that devastated Japan. The ensuing tsunami, despite its far distance from California, made its way across the Pacific and hit the West Coast, costing the state between $50 million and $100 million in damage.

“Although this [damage] pales in comparison to the loss of lives and property in Japan, the U.S. Government must ask whether California, and the national economy, will someday face worse consequences from other distant-source tsunamis,” stated the USGS study. “Unfortunately, the answer is ‘yes.’”

USGS and the California Tsunami Hazard Mitigation Program plan to host a workshop series where they will explain their findings and discuss preparedness options with stakeholders in some of the state’s major coastal communities, including San Diego and Santa Barbara. A workshop was hosted in the Los Angeles neighborhood of San Pedro Wednesday.