What is a Tsunami?
A tsunami is a series of long period ocean waves usually caused by the sudden displacement of the ocean floor by an undersea earthquake, volcanic eruption, or landslide. As tsunamis enter shallow water near land, they increase in height and can cause unusually strong currents and severe flooding, which can be devastating and deadly.
There are two types of tsunamis: distant source and local source tsunamis.
- Distant source tsunamis come from faraway seismic events over 621 miles away, such as near Alaska or Japan. These tsunamis may take more than 3 hours to reach the shore, allowing more time to move to higher ground.
- Local source tsunamis are caused by seismic events near the California coast and can arrive within 15 to 20 minutes after the earthquake. Though relatively rare, these events leave little time to respond. You may need to act fast and move to safety before an official alert is given.
Signs of a Possible Tsunami
When you are on the Coast, pay attention for natural warning signs of a possible tsunami. If you are knocked off your feet by a very strong earthquake lasting 20 or more seconds, hear an unusually loud roar from the ocean, or see a rapid and unusual rise or fall in sea level, a tsunami could be approaching.
Ground shaking may be the only warning you have. In the event of a strong earthquake, avoid low-lying areas located within the Tsunami Hazard Zone and travel on foot to higher ground.
- Low-lying areas are regions near sea level or lower than the surrounding areas such as beaches, sand dunes, harbors, floodplains, creeks, rivers, and oceanside sections of highways. This can be simplified to areas within the Tsunami Hazard Zone.
- High ground is areas inland, uphill and away from areas prone to flooding. This can be simplified to areas beyond the Tsunami Hazard Zone.
Large waves may continue to come ashore for several hours, do not return to low-lying areas until the official "all clear" message.
Tsunami Alerts
1. Tsunami Sirens
On every first Wednesday at 10 AM, San Mateo County conducts a test of the Coastside Tsunami Warning Sirens at full volume. You do not need to respond.
There are 8 Tsunami Warning Sirens on San Mateo County’s coastline: 3 in Pacifica, 4 along the Midcoast, and 1 in Pescadero. In San Mateo County, these Sirens are activated only when a tidal surge from a seismic event is imminent. They serve to let anyone in the Tsunami Hazard Zone to know to head to high ground immediately. This can be done by travelling by foot, not by car, away from low-lying areas.
Listen to the Tsunami Siren Wail Sound
If you hear a steady 3-minute wailing siren, avoid low-lying areas and seek higher ground. Then listen to your local radio or news channel for official information.
2. Emergency Alert Notification
If a Tsunami Warning is issued by the National Weather Service (NWS) after an earthquake, it will be announced over multiple alert and warning channels:
- Wireless Emergency Alert (WEA) alert message to geotargeted mobile devices
- Emergency Alert System (EAS) via television, radio, and satellite
- Non-Weather Emergency Message (NWEM) through NOAA weather radios
3. Warning Message
If SMCEM receives reliable and applicable updates from the NWS, we will also provide county-specific information to affected San Mateo County residents and visitors using:
- The county’s mass notification system (SMC Alert) to registered users using text, email, and voice messages (register for SMC Alert here)
- Wireless Emergency Alerts (WEA) to geotargeted mobile devices in affected areas
No Alert?
Despite County alert systems, you may not be within range of a Tsunami Warning Siren or a geotargeted alert. Do not wait for official warning to avoid low-lying areas, especially if you have young children or family with physical disabilities that may need additional time to move. Trust Your Gut! Travel on foot to high ground and inland immediately. Then, listen to your radio, television, or NOAA weather radio for official information.
Before a Tsunami
Minutes count! The best way to prepare is knowing if you are within the Tsunami Hazard Zone, registering for SMC Alerts, and making an emergency kit and plan.
- Familiarize yourself with the Tsunami Hazard Zones. If you are on the Coastside, take note of Tsunami Hazard Zones and Evacuation Site official signage.
- Tsunami Hazard Zones: These zones show the areas that may need to avoid if a tsunami occurs.
- Tsunami Evacuation Sites: Locations away from the hazard zones where people can assemble.
- Pre-assemble an emergency "Go Kit" to take with you when evacuating. Include in your kit:
- Essential medications, first aid kit and reference guide for its use
- Bottled water, packaged food, snacks, pet food
- Blankets or sleeping bags, cash, portable radio, flashlight, and batteries.
- Have a designated meeting place and evacuation route and practice your plan.
- Check the Tsunami Hazard Area Dashboard to see if your child's school is in a tsunami hazard area and know you child's school evacuation plan.
During a Tsunami
- Protect yourself until the earthquake is over – drop, cover, and hold on!
- If you receive an emergency alert, hear the Tsunami Warning Sirens, or see natural tsunami warning signs, avoid low lying areas and travel by foot to higher ground immediately. If you are unable to leave the Tsunami Hazard Zone, relocate to 4th floor or higher of a building.
- Travel on foot if at all possible. In most locations you only need to walk a few blocks inland to leave the Tsunami Hazard Zone. Traffic will become gridlocked along coastal highways. Please keep roadways clear for emergency vehicles.
- Monitor local radio and news stations for local information. Wait for an “all clear” from local emergency officials before returning to low-lying areas like the beaches.
After a Tsunami
- Tsunami waves and hazardous conditions may last up to several hours. Only return to low-lying areas when you receive an official "all clear" message.
Tsunami Preparedness for Boaters
If you are on a boat when you receive a tsunami warning, your response time will depend on the size of the tsunami, the currents it produces, where you are, how much time you have before the first wave arrives and the weather at sea. If you are a boat owner or captain:
- Make sure you have a way to receive tsunami warnings when you are on the water. The U.S. Coast Guard will issue urgent marine information broadcasts on your marine VHF radio’s channel 16. Additional information will be available from NOAA Weather Radio.
- If you are on a boat in a harbor when you receive a tsunami warning, in general, you should plan to leave your boat and move quickly to high ground, or inland, away from the water. If you are at sea, you should plan to move to a safe depth of at least 180 feet and stay away from harbors under warning until officials tell you the threat has passed. Your harbormaster, port captain, the US Coast Guard, San Mateo County's Department of Emergency Management (SMCEM), and state emergency management offices are the best sources for tsunami safety information and regulations for boaters in San Mateo County.
- Make a plan and assemble a disaster supplies kit to keep on board your boat. Be aware that shore facilities may be damaged, so if you are out to sea during a tsunami, you may not be able to return to the harbor you left. Be prepared to remain at sea for a day or more.
Tsunami Preparedness Flyers
More Information
San Mateo County Tsunami Hazard Map
San Mateo County Harbor District: Tsunami Preparedness
NOAA Tsunami Warning Center FAQ; Tsunami Program; Tsunami Information; Tsunami Warning System; Tsunamis: Know the Signs; The Tsunami Zone (tsunami inundation maps)
International Tsunami Information Center
FEMA – Tsunami / Special Conditions / On A Boat
KQED – What Would Really Happen if a Tsunami Hit the Bay Area?