TsunamisTsunamis It Comes in Waves
Tsunamis pose a real threat to the California coast, even if the triggering earthquakes occur elsewhere. And it doesn’t take a 50-foot tsunami out of a science-fiction film to inflict severe damage. Researchers are helping ensure coastal cities are ready.
“Tsunami magnet”: that’s what Lori Dengler, Ph.D., professor emeritus from Humboldt State University Department of Geology, has called Crescent City. This port city just north of the Humboldt County campus has been hit by tsunamis after earthquakes in Japan, Alaska and South America.
But it doesn’t take a 50-foot tsunami out of a science-fiction film to inflict severe damage. In fact, the Crescent City Harbor was partially destroyed by a five- to six-foot tsunami in 2006 following an 8.3-magnitude earthquake in Japan, and completely destroyed by an eight-foot tsunami in 2011 following a 9.1-magnitude earthquake in Japan. “It was simply the really strong currents coming in and coming out that ripped apart the docks, that ripped the boats away from their mooring,” Dr. Dengler says.
What Is a Tsunami?
A tsunami occurs when there is a sudden, large-scale movement of the sea floor that displaces the water above it. While earthquakes are the most common cause, volcanoes and landslides can also trigger such an event (#1 in the illustration). Tsunamis move the entire column of water from surface to sea floor—unlike storm waves, in which wind only moves the water’s surface. Multiple waves or surges then ripple out in all directions, moving at speeds of 500 miles per hour or more in deep water (#2).
As the tsunami approaches the coast, it slows down significantly to about 25 to 30 miles per hour, causing the water behind it to build up and the tsunami to grow higher (#3). Because the tsunami carries more water at higher speeds than storm waves, the water penetrates the landscape further inland with greater force, resulting in more flooding, property damage and other destructive effects. Tidal height, coastal topography and sea floor depth may further amplify its effects (#4).
“A really large tsunami looks like a sudden increase in the height of the tide, because the water keeps coming in,” Dengler explains. “It comes in for 15 minutes, for 20 minutes, for 30 minutes, rather than seven seconds or 14 seconds [like normal wind-driven waves].”
A Wave of Support
Outside of Dengler’s research and teaching roles, much of her work has centered on outreach—particularly after the 1992 7.2-magnitude Cape Mendocino earthquake, its large aftershocks, a three-foot tsunami and the resulting $66 million in damages.
