Welcome to The Deep science and technology column where we cover topics from the deep sea to deep space and beyond.

The day after Christmas 2004, our planet was rocked by the biggest earthquake in years.  The 8.9 earthquake struck 25 miles below the north coast of Indonesia and was the fifth largest quake since 1900.  It was so huge the Earth’s rotation rate was very slightly affected.  But the real destruction was caused by the massive tsunamis (tidal waves) generated by the earthquake. The tsunamis killed thousands of people including several in the African nation of Somalia, which is 3,000 miles away from the earthquake’s epicenter.  Many people died in Sri Lanka and in southern India and in many other parts of coastal southern Asia.

The tsunami caused by the December 26, 2004 earthquake strikes Ao Nang, Thailand.

Just last week we had another tsunami scare that generated some local controversy.  An 8.1 magnitude earthquake struck the Kuril Islands north of Japan and it generated tsunami waves that reached California, damaging docks and boats in the Crescent City Harbor near the Oregon state line. 

The harbor was hit with a series of surges that would cycle about every 15 minutes, changing the water level about five feet each time. The National Weather Service estimates the surges reached speeds of 30 miles an hour.  The Crescent City harbormaster said the repeated battering pulled apart at least two docks and set boats adrift, which banged into each other.  He estimated that two dozen or more boats were damaged, but none were sunk and no one was hurt.

Boat damage in Crescent City CA

Here on Guam, a tsunami alert was issued but most people were unaware of it, including some of the mayors, who are responsible for spreading such alerts to their villagers.  Two mayors said they weren't called by the Office of Civil Defense, and noted that the early warning sirens that had been utilized in such instances were destroyed by typhoons and never replaced.  Guam Homeland Security says that the island is “tsunami ready”, but many people dispute that claim.

So here are some questions to ask.  If the waves that were generated by last week’s tsunami were destructive in California, why wasn’t there any damage here?  Agana Harbor faces north where the waves came from.  Why weren’t there any reports of boat damage on Guam?  Did anyone notice five-foot waves pouring into the harbor?  Why weren’t beachside homes flooded?  Here on Guam, we’re surrounded by Mother Ocean.  How susceptible are we to the devastating effects of tsunamis, the deepest waves on Earth? 

Tsunamis are often called ‘tidal waves’ but they have nothing to do with tides.  They are caused by undersea earthquakes and landslides that displace huge volumes of water (and they have nothing to do with the weather).  These gigantic ripples spread across the ocean in excess of 500 miles per hour.  Because the water is disturbed at the ocean floor, these waves are as tall as the ocean is deep.  These waves aren’t dangerous in the open ocean; ships at sea may not even notice them. 
The problem occurs when the tsunami approaches land.  The bottom of the huge standing wave is pushed up by rising ground.  As the waves approach shallower water, they get taller (their amplitude increases) and run-up occurs.  Run-up is a measurement of the height of the water onshore as observed above a reference sea level. and the wave gets higher and higher above the water’s surface.  If the sea bottom rises gradually, the wave becomes a wall of water that may crash miles inland and destroys everything in its path.

However, most tsunamis don’t result in giant breaking waves (like normal curling surf at the shore).  Rather they behave as very strong and very fast tides and are actually a rapid local rise in sea level.  Tsunamis often travel much farther inland than normal waves.

The key to understanding tsunamis is that they are extremely deep waves and as they enter shallow water all that energy and all that water piles up in tremendous waves.  So what’s the point for Guam?  It’s that little phrase shallow water.

If you have a look at an underwater topographic of the island of Guam you’ll realize that there is virtually NO shallow water around Guam.  Our island rises more or less (mostly more) vertically from the abyssal depths (roughly 13,000 feet [almost three miles] in our area).  If you travel away from Guam on a boat, you don’t have to go very far before you’re over water that’s hundreds of feet deep.  Guam has no gradually rising slopes where run-up can occur and tsunamis simply curve around our island and travel on to their ultimate landfall.  Virtually the only place on Guam where a tsunami can occur is in Talofofo Bay (and a woman was swept out to sea from the shores of Talofofo River by a tsunami wave in the 1800’s). 

Having said this, DO NOT ignore tsunami warnings!  And if you’re ever at the beach and the water goes away (i.e. recedes dramatically away from the shore), GET AWAY FROM THE WATER.  It is ALWAYS better to be safe than sorry.  But in general, we may have to worry about earthquakes and typhoons here on Guam, but you can cross worrying about tsunamis off your list!

Cruise on over to the Deep Website at www.thedeepradioshow.com to learn more about tsunamis and many other topics.  Enjoy!