From March 20 to March 27, California celebrates the 2016 Tsunami Preparedness Week! Though San Francisco’s tsunami risk is low, it’s very important to understand how and why tsunamis occur, and how we can prepare for them.
A tsunami is a series of ocean waves caused by movement of the sea floor. Usually they are caused by a large earthquake (Magnitude 7.5 and above). They can also be caused by a landslide under the water’s surface. Tsunamis can move up to 500 miles per hour in the open ocean, where they are barely noticeable! But as tsunami waves approach the shore, they slow to around 40 miles per hour and grow in height. Tsunami waves can reach as high as 30 feet; however, the size really depends on the topography of the coastline. For instance, a small tsunami on one beach can be much larger at a different beach a few miles away.
WHY ARE TSUNAMIS SO DANGEROUS?
Tsunami waves move the full column of water, from the sea floor to wave surface. That packs quite a punch, and is why tsunami waves can cause flooding in low-lying shoreline areas. Tsunamis also may cause strong, unpredictable ocean currents. For instance, the 2011 Tohoku Earthquake in Japan arrived in California during low tide, so it generated relatively small tsunami waves on the California coast, but resulted in strong currents in Santa Cruz, California that did serious damage to their harbor and boats.
Tsunami waves may fool people who think the danger is over after the first wave comes and goes. A tsunami is tricky: It produces a series of waves, and the first wave is almost never the largest.
Did you know that since 1850, 54 tsunamis have been recorded or observed in San Francisco? Luckily, San Francisco has seen zero injuries and no significant damage as a result of any of these tsunamis.
WHERE DO TSUNAMIS THAT REACH SF COME FROM?
San Francisco can experience three types of tsunamis:
- Distant-Source Tsunami: This is San Francisco’s primary tsunami threat. A distant source tsunami is caused by an earthquake on the Pacific Rim over 600 miles away (think Japan, Chile, or Alaska). A tsunami of this kind can reach our shores in 4 to 15 hours post-earthquake.
- Regional-Source Tsunami: These tsunamis originate from the Cascadia subduction zone (think Humboldt County). A tsunami of this kind can reach San Francisco shores within one hour.
- Near-Source Tsunami: These tsunamis can originate from an earthquake as close as Point Reyes Thrust Fault. A tsunami of this kind can reach our shores within 10 to 20 minutes.
WHAT ARE WE DOING ABOUT TSUNAMIS?
Here at the San Francisco Department of Emergency Management we have a team dedicated to updating our Tsunami Annex to our Emergency Response Plan. These are the plans City officials will use to guide our response efforts in the event a tsunami impacts San Francisco. Work is also underway to add to and improve tsunami signs around the city in areas of possible tsunami inundation. Many of these signs direct people where to go if a tsunami threat occurs. In addition, we are conducting public outreach and education in tsunami prone areas in the city. Our goal to make sure you know what to do and where to go in the face of such an emergency. Speaking of . . .
HOW WILL YOU KNOW IF A TSUNAMI IS COMING?
Here are some of the ways you may hear about a tsunami from us:
- The Outdoor Public Warning System, a network of over 120 loudspeakers placed throughout the city.
- Email and text messages from our emergency notification system AlertSF. To sign up for alerts text AlertSF to 888-777.
- Emergency alerts broadcast on local radio stations such as KCBS, 740 AM, and KQED, 88.5 FM; and local television stations.
- Social media updates via @SF_Emergency on Twitter & @SFDEM on Facebook.
Though it’s unlikely, a severe local earthquake may cause a near-source tsunami that would arrive within 10 to 20 minutes. Such an earthquake could damage the communications systems listed above, making it difficult for us to send out warning messages. That’s why you should also know “nature’s tsunami warning signs.”
Nature’s biggest warning sign is a strong earthquake, Magnitude 7.5 or above, that results in severe ground shaking lasting 20 seconds or more. “Severe” means it’s difficult to walk or stand while the shaking is going on. You may also see unusual ocean behavior, including a fast-rising flood or wall of approaching water, or unusual receding of water from the shoreline. You could also hear a loud roar, like a train or airplane, coming from the ocean.
WHAT DO YOU DO IF A TSUNAMI IS IMMINENT?
- If you are in or near the ocean or within the tsunami inundation area, move inland and to high ground immediately.
- If there is earthquake damage avoid fallen power lines and stay away from weakened structures.
- Once you reach a safe place, check back in with your radio, television, or mobile device for updates.
- Stay out of the tsunami inundation area until City officials tell you it is safe to return.
WHAT CAN YOU DO TO PREPARE FOR TSUNAMIS?
There is plenty you can do to prepare for tsunamis; the best part is that it isn’t that different from general preparedness! If you are prepared for an earthquake, flood, or fire, chances are you are well on your way to being prepared for a tsunami. Here are a few reminders and a few new tips that are tsunami specific:
- Find out if you reside, work, or if your children go to school in the San Francisco tsunami inundation zone:
- Make a family emergency plan that includes an out of state contact, a meeting place, and methods for communication between family members.
- Have a bag filled with the essentials that you will need (e.g., a change of clothes, medication, a small radio, extra cash, small first aid kit), which you can take with you if you have to leave home in a hurry.
- Have multiple ways of getting information from us, such as having a small radio and tuning into news stations like KCBS 740 AM, following us on social media (our accounts are listed above), and signing up for emergency notifications by registering for AlertSF.
Tsunamis can be scary, but the best way to confront them is with knowledge and a plan. We invite you to celebrate California’s Tsunami Preparedness Week by learning more about tsunamis with us as the week continues. Tune into our social media accounts to learn more about tsunamis. And if you’re up for it, our friend Sue can teach you a thing or two as well:
Edie Schaffer & Amy Ramirez