Tsunami Physics and Trestles

28 02 2010

Waves On The Way To Trestles

With the tragic 8.8 earthquake in Chile generating a tsunami that traveled across the Pacific, the question arises again — “Just what is a tsunami (Japanese for ‘harbor wave’)?”  We add “How will it affect Trestles?”

The news cams and looky-loo’s run down to the beach looking for a Mav size set to wash away all the local fish taco stands, but are sorely disappointed when they find out the tsunami passed by without even a splash.  That’s because while tsunamis are waves from a physics standpoint, they don’t behave like the waves most people are used to seeing in news photos and movies like Big Wednesday.

Ocean waves range in velocity, wavelength and frequency from wind chop to twice-a-day tides.  Tsunamis fit in the ocean wave spectrum somewhere between surf beat and tides.   So, they behave more like a big surge.

Ordinary wind-driven waves, like the ones breaking yesterday at Snapper Rocks, range from about 5 to 15 seconds in period between waves.  Surf beat varies greatly, but it is the more correct term for what surfers call “sets.”  When I was a kid, the old pro’s on the beach would say that every 9th wave was a big one.  That was a wild guess.  Surf beat frequency and period actually depend on a lot of variables, like wave height, velocity, and frequency, not to mention the shape of the ocean bottom (bathymetry) across which they travel.   With that said, surf beat is a multiple of wave periods.  For example, every ninth 10-second wave would mean the surf beat is 90 seconds.  Tides average about 12 hours between peaks.   Tsunamis are somewhere in between.

Tsunami Wave Geometry

This diagram, borrowed from EnchantedLearning.com, shows the difference in wavelength and velocity between deep water travel and shallow water travel for tsunamis.  It is obvious that tsunamis are greatly affected by bathymetry and ocean depth.

For more on tsunami physics, here is a great link.  I especially like the video on one of the pages on this site that shows how the 1960 Chile earthquake tsunami traveled across the Pacific.  It shows graphically why tsunami effects are so difficult to predict.  That’s why it is better to be safe than sorry when responding to tsunami alerts.


In Southern California, nobody was really able to absolutely detect the tsunami as it passed, because it was mid-tide when it was predicted to occur.   But, if it had shown up a few hours earlier, at spring (seasonal high) tide, the story may have been a little more tragic for some low lying areas like Capistrano Beach or even Newport Beach.

As for Trestles, it was a Victory at Sea day, so a little more tsunami confusion would have just blended into the background.  The train tracks are still there.