A new tsunami hazard map for Orange County expands evacuation zones in Newport Beach and reduces the hazard areas in Huntington Beach, while threats to south Orange County are largely limited to the coastline west of Pacific Coast Highway.
Evacuation zones in the county include Seal Beach’s downtown, Sunset Beach, Huntington Harbour, the Bolsa Chica wetlands, the Balboa Peninsula, Newport Beach’s islands and Back Bay, Laguna Beach’s Main Beach and part of its downtown, and Dana Point Harbor.
The new map, released July 8, uses new data to update the 2009 zones. Additionally, it now bases evacuation boundaries on a worst-case, 1,000-year tsunami that anticipates an ocean surge equivalent to a 9.3 magnitude earthquake in Alaska’s Aleutian Islands.
The 2009 map was based on a variety of scenarios, but the new map follows the lead of Japan, which established the 1,000-year threshold after experiencing the once-in-a-millennium, 9.0 Tohoku earthquake and accompanying tsunami in 2011. It was the world’s fourth largest recorded earthquake and took 19,747 human lives, with 90% of those being drowning deaths that were mostly attributed to the tsunami. It was also the cause of the Fukushima nuclear plant disaster.
“So, we’re taking a very conservative approach and using a 1,000-year scenario as the baseline for our new maps, hoping to avoid the tragic loss of life experienced in Japan,” said Steve Bohlen, head of the California Geological Survey.
“While damaging tsunamis are infrequent in California, they have (happened) and do happen. If you live on or visit the coast, you need to be aware of this potential hazard.”
State’s tsunami history
Ten hours after the 2011 tsunami was generated off the coast of Japan, the surge hit the California coast. One person was killed in the state and there was $100 million in damage, primarily in the harbors and neighboring areas of Crescent City and in Santa Cruz.
In Orange County the damage was negligible, although beaches, harbors and piers were closed for several hours until the threat passed.
“More than 150 tsunamis have hit California’s shore since 1800,” according to the California Geological Survey. “Most were barely noticeable, but a few have caused fatalities or significant damage. The most destructive to hit California occurred March 28, 1964.”
That tsunami was the result of a 9.2 earthquake in Alaska, the world’s second largest recorded temblor.
“Several surges reaching 21-feet high swept into Crescent City four hours after (the earthquake), killing 12 and leveling much of the town’s business district,” according to the California Geological Survey.
Despite taking a more conservative approach with the new map, the changes are relatively minor for Orange County.
The modest expansion of the hazard zone in Newport Beach, intended to make evacuations more efficient, is the most noteworthy addition. Meanwhile, hazard zones around the Bolsa Chica wetlands and next to the Santa Ana River have shrunk.
That reduction is a result of improved modeling that shows the levees along the Santa Ana River and Bolsa Chica’s Wintersburg Channel, along with basins and marshes at the Bolsa Chica and Huntington Beach wetlands, would buffer the impact of a tsunami.
The new maps are based on mean high tide levels, meaning that a tsunami during low tide could have less of an impact. The state has completed 2021 tsunami map updates for 13 coastal counties, with seven more to go. The plan is to update the maps again in 10 years, taking into account any sea-level rise that occurs in the interim, according to Nicholas Graehl for the California Geological Survey.
What to do
While there are signs along the coast identifying tsunami evacuation zones, residents are urged by officials to look at the new maps to identify whether they live or work in a hazard area and if so, to locate an evacuation route. As for wildfire and earthquake preparations, an emergency “to-go” bag with 72 hours-worth of supplies is also encouraged.
Tsunamis can be preceded by either official alerts or natural warnings — or both.
A tsunami generated by a nearby earthquake is unlikely to have as much advance alert time as one traveling from Japan or Alaska. So, for local quakes, people should pay attention to the sensation of a strong earthquake, observation of a sudden rise or fall of the ocean, or a loud roar from the ocean as possible cues to evacuate.
A more distant event of major magnitude — a tsunami from an Aleutian Islands earthquake would take about six hours to reach California shores — is likely to be preceded by official alerts calling for evacuations.
While many people would likely consider jumping in their car to get out of harm’s way, the California Geological Survey says that moving inland or to higher ground by foot may be the best option — especially in areas where there are likely to be traffic jams.
“The United States Geological Survey has performed pedestrian evacuation modeling on Balboa Island and it shows that residents can walk off the island within one hour, but it may take up to five hours for everyone to take their car because of traffic,” Graehl said. “Five hours is about the time it takes for a tsunami to arrive from the Aleutian Islands. Although it is up to the individual, walking out of the tsunami hazard area is the best way to ensure safety.”
To see the new tsunami evacuation maps and other tsunami information, visit www.conservation.ca.gov/cgs/tsunami.
Source: Orange County Register