What is a tsunami?
A tsunami (pronounced soo-nah-mee) is a series of waves generated in a body of water by a rapid disturbance that vertically displaces the water. These changes can be caused by an underwater fault rupture (that generates an earthquake) or underwater landslides (typically triggered by earthquakes).
Tsunamis affecting the Bay Area can result from off-shore earthquakes within the Bay Area, or from very distant events. While it is most common for tsunamis to be generated by subduction faults such as those in Washington and Alaska, local tsunamis can be generated from strike-slip faults (such as the small one that was triggered by the 1906 earthquake ). The San Andreas fault runs along the coast off the Peninsula and the Hayward fault runs partially through San Pablo Bay, both of these are examples of local strike-slip faults. The 2011 Honshu, Japan earthquake caused tsunami damage in Santa Cruz, Crescent City, and Berkeley marinas. The 1964 Alaska earthquake caused extensive tsunami damage that flooded and heavily damaged coastal northern California near Crescent City.
Pacific Tsunami Warning Center
Tsunami Inundation Map for Coastal Evacuation
Source: Tsunami Inundation Map for Emergency Planning, CalEMA, CGS, and USC, 2009
Interactive tsunami inundation map Warning: This is not a hazard map. It is intended for coastal evacuation planning only.
These maps do not represent inundation from a single scenario event. They were created by combining inundation results from a suite of possible inundation scenarios, representing the worst-case at any given location. Read more.
Why are tsunamis so dangerous?
In the open ocean tsunamis travel approximately 500 mph. As they reach the shore they slow down to about 35 mph – which is much faster than you can run! Tsunami waves have an extremely long wavelength (that is, distance between the tops of the waves) that can be over 100 miles. The slowing down effect allows several incoming waves to bunch together and build up to great heights by the time they hit the shoreline. In the ocean, these waves can be only a few inches high and can’t be detected by a ship, but on shore they can have a run up height of several feet (that is the elevation to which the water will reach inland).
The energy and speed of a tsunami is very destructive when it reaches land, especially for wood frame buildings. Tsunamis also have a long periods (length of time between when the waves hit the coastline). Even after the first wave arrives, it is not safe to go back to the shoreline until the “all clear” has been issued because additional waves may still be arriving.