The Dec. 5 tsunami warning along the Northern California coast left many local residents confused and questioning what to do. The good news is that the warning was canceled about an hour later.
More than 150 tsunamis have hit California’s shore since 1800, according to the California Geological Survey. Many were barely noticeable, but a few have caused fatalities or significant damage -- most recently a 2011 tsunami triggered by a devastating magnitude 9.0 earthquake off of Japan that caused $100 million of damage to California ports and harbors.
Summary:
- Know if you are in Tsunami Hazard Zone, make a plan to move out of the Tsunami Hazard Zone.
- Sign up for San Mateo County emergency alerts.
- Create a family emergency plan.
Areas along the San Mateo County’s coast and bay are at risk from an incoming tsunami. With that in mind, here are five facts you should know to help keep you and your family safe.
What is a tsunami?
Let’s start with a common understanding: A tsunami, a word that comes from the Japanese characters for harbor (tsu) and wave (nami), is a series of long waves that are often caused by undersea earthquakes.
Tsunamis radiate outward from the point of origin and can move across entire oceans. When they reach the coast, they can cause dangerous coastal flooding and powerful currents that can last for hours or days.
On Dec. 5, a magnitude 7.0 earthquake struck at 10:44 a.m. off the coast of Humboldt County, triggering a brief tsunami warning for the California coast from the Oregon border to south of San Mateo County.
What is the tsunami threat to San Mateo County?
The California Geological Survey in 2021 released new interactive maps that allow users to enter an address to determine whether a property is within an area that may be subject to tsunami flooding. Open the map.
Tsunami hazard areas represent the maximum inundation in a number of “extreme, infrequent, and realistic” scenarios, the California Geological Survey says.
What to do if a tsunami is imminent?
First, a few words about tsunami alerts.
Tsunami alerts are issued by federal tsunami warning centers to notify emergency managers and other local officials, the public and other partners about the potential for a tsunami.
There are four levels: warning, advisory, watch and information statement.
- If you receive a warning (like thousands did on Dec. 5, typically through a blaring message on your cell phone) Move to high ground or inland (away from the water).
- Advisory: Stay out of the water and away from beaches and waterways.
- Watch: Be Prepared — A distant earthquake has occurred. A tsunami is possible. Stay tuned for more information.
- Information Statement: Relax — An earthquake has occurred, but there is no threat, or it was far away and the threat has not been determined.
In all cases, the warning centers recommend following instructions from local officials.
Knowledge is power, and California emergency officials say learning “if you live, work, shop, vacation, or drop your children off for school in an area that could be impacted by a tsunami is the first step you can take to reducing your risk of injury or decreasing your property damage from the tsunami.”
If you don’t know whether you’re in a tsunami hazard zone in the event of a warning, local officials say to seek higher ground (at least 100 feet above sea level or the 4th floor of higher of a building) or to travel inland on foot at least one mile. Stay off the road as coastal highways will likely be gridlocked.
What triggers activation of tsunami warning sirens along the San Mateo County coast?
There are eight sirens on San Mateo County’s coast between Pacifica and Pescadero.
These sirens are used exclusively for tsunami warnings and are activated by County Officials, when a tsunami is expected.
Tsunami sirens are activated when a tsunami has been detected and may be approaching. If you hear a steady 3-minute siren, seek higher ground at least 100 feet above sea level, or seek refuge on the 4th floor or higher in a building or travel one mile inland and away from the coast.
Shortly after the Dec. 5 quake, cell phones across Northern California buzzed with the tsunami warning from the National Weather Service that said: “A series of powerful waves and strong currents may impact coasts near you. You are in danger. Get away from coastal waters. Move to high ground or inland now. Keep away from the coast until local officials say it is safe to return.”
In consultations with local, state and federal partners, officials from the San Mateo County Department of Emergency Management determined there was no imminent tsunami threat to the local coast or bayfront. And activating the Sirens would have caused unnecessary panic.
The quake was a strike-slip type of temblor that shifts more horizontally (think two hands rubbing together) and is less prone to cause tsunamis, unlike the more vertical types, officials said.
What can I do to get better prepared?
Familiarize yourself with the tsunami hazard zones.
Assemble an emergency “Go Bag” to take with you when evacuating.
Create a family emergency plan.
Sign up for alerts from:
San Mateo County SMC Alert
The National Oceanic and Atmospheric Administration Weather Radio
NOAA Wireless Emergency Alerts