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.
In 2006, 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.
We have just had the “Pacific Week of Despair” with multiple deaths from typhoons, mudslides, floods earthquakes and tsunamis. The death toll from the tsunami in the Samoas has risen to almost 200.
We have earthquakes here, we have typhoons here and well, do we have tsunamis here? There was a tsunami alert called on Guam for both the quake in Samoa and the quake in Indonesia. Lots of people are really worried about tsunamis here on Guam. Should you be?
Well, here are some questions to ask yourself. Have we had a tsunami here in your lifetime? If the tsunami that was generated by the earthquake in the Kurils in 2006 was 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? We had one of the most powerful earthquakes in the world off our western shore in 1993. Do you remember that it generated a large tsunami that destroyed lots of houses? 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 the rising ocean floor. 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.
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.
This is an underwater map of the area surrounding the Samoas with the epicenter of the earthquake marked. Have a look at the lighter areas on this map. They represent shallow water less than a thousand feet deep. The earthquake occurred in very deep water and as the tsunami it generated reached the shallow water surrounding the islands, it piled up and reached far inland, wiping out whole villages and killing many people.
Map of Samoa region (made in GeoMapApp) showing epicenter of Sept 29, 2009 8.0 earthquake
Now have a look at an underwater topographic map of the island of Guam. As you can see, there’s 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 and even thousands 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).
Map generated by Pam Eastlick’s Hands-On-Science students. Flat seafloor regions indicate areas of no data for those depths.
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!