activity.”
“What is subduction?” Kristen asked.
“The earth is covered with a semirigid crust called the lithosphere. This crust floats on a molten mantle of liquid rock and is broken up into a number of geologic plates. These plates are constantly moving, albeit slowly. Where these tectonic plates meet, one of several things can happen. First, they can push together to form an undersea ridge—a mountain range. Or, second, they might move away from each other,allowing the mantle below to rise and create new crust. Third, they can slip by each other in a lateral movement, or, fourth, one plate may subduct, that is, have its leading edge pushed down and under an adjoining plate. Sometimes this happens slowly, others times quickly. Anytime there is a sudden movement there is an earthquake.”
“But this didn’t happen at a plate boundary?” David asked.
“No. It was fairly close to the center of the plate. There have been a few earthquakes in the center, but not many. This is surprising. If I were a betting man, I would have wagered against such a thing happening at that location. Nevertheless, it did. That’s one thing about science: You can be surprised at any time.”
“So this earthquake caused the tidal wave?” Kristen inquired.
“Tidal wave is the wrong word. The wave that struck the coastal regions of the Bay of Bengal had nothing to do with the tides. It was the result of a sudden seismic occurrence. The accurate term is
tsunami.
It’s a Japanese word that means ‘harbor wave.’ But to answer your question, yes, the earthquake caused the tsunami.”
Osborn paused for a second. “Let me explain. Imagine an area of ocean floor. Above it is several miles of heavy seawater. Think of it as a column of water over the seabed. Have you all got the picture in your mind?” Ten heads in the room nodded. “OK, here’s what probably happened in the Indian Ocean. Centuries of stress fractures a piece of the crust, hundreds of miles long, and thrusts it upward twenty or thirty feet. Now what happens to that column of water above it?”
“It has to move too,” Kristen offered.
“Exactly, but here’s the kicker. Water can’t be compressed. So when this massive sliver of crust is pushed up, it pushes up all that water above it. In this case, over twenty-one thousand feet of water was suddenly pushed upward. Immediately after that, the ocean surface seeks its own level. Gravity insists on that. That sudden rise and fall of the ocean over the earthquake causes a wave, and that wave moves out away from its point of origin in a circle, like what you’d see if you threw a pebble into a pond.”
“How big is this wave?” Bob Connick, the CFO, asked.
“At sea, about three feet high.”
The room filled with incredulous murmuring. “Three feet?” Connick objected. “That’s not big enough to capsize a rowboat.”
Osborn shook his head. “We need to forget images from movies like
The Poseidon Adventure.
Don’t let that fool you. It’s true that if we were on a cruise in the Indian Ocean and a tsunami came by we wouldn’t notice it. It wouldn’t even spill the drink in your glass. Ships at sea seldom notice a tsunami. That’s because the ocean is so deep that the wave travels below the surface with only a slight bulge above. The danger occurs when the massive wave hits shallow water.” He held up his forearm to simulate a wave and slowly moved it in front of his body. “The wave continues on, sometimes altered by undersea geography, until it reaches shallow water. In the deep ocean the wave can move at jetliner speeds—upward of five hundred miles an hour.”
“Five hundred miles an hour?” Kristen said with disbelief.
“Yes, and unlike waves you see at the beach, a tsunami can cross an ocean in a few hours, hit a continental shelf, and bounce back across the ocean. In 1960, for example, atsunami created by an earthquake off Chile reverberated through the Pacific for over a
“What is subduction?” Kristen asked.
“The earth is covered with a semirigid crust called the lithosphere. This crust floats on a molten mantle of liquid rock and is broken up into a number of geologic plates. These plates are constantly moving, albeit slowly. Where these tectonic plates meet, one of several things can happen. First, they can push together to form an undersea ridge—a mountain range. Or, second, they might move away from each other,allowing the mantle below to rise and create new crust. Third, they can slip by each other in a lateral movement, or, fourth, one plate may subduct, that is, have its leading edge pushed down and under an adjoining plate. Sometimes this happens slowly, others times quickly. Anytime there is a sudden movement there is an earthquake.”
“But this didn’t happen at a plate boundary?” David asked.
“No. It was fairly close to the center of the plate. There have been a few earthquakes in the center, but not many. This is surprising. If I were a betting man, I would have wagered against such a thing happening at that location. Nevertheless, it did. That’s one thing about science: You can be surprised at any time.”
“So this earthquake caused the tidal wave?” Kristen inquired.
“Tidal wave is the wrong word. The wave that struck the coastal regions of the Bay of Bengal had nothing to do with the tides. It was the result of a sudden seismic occurrence. The accurate term is
tsunami.
It’s a Japanese word that means ‘harbor wave.’ But to answer your question, yes, the earthquake caused the tsunami.”
Osborn paused for a second. “Let me explain. Imagine an area of ocean floor. Above it is several miles of heavy seawater. Think of it as a column of water over the seabed. Have you all got the picture in your mind?” Ten heads in the room nodded. “OK, here’s what probably happened in the Indian Ocean. Centuries of stress fractures a piece of the crust, hundreds of miles long, and thrusts it upward twenty or thirty feet. Now what happens to that column of water above it?”
“It has to move too,” Kristen offered.
“Exactly, but here’s the kicker. Water can’t be compressed. So when this massive sliver of crust is pushed up, it pushes up all that water above it. In this case, over twenty-one thousand feet of water was suddenly pushed upward. Immediately after that, the ocean surface seeks its own level. Gravity insists on that. That sudden rise and fall of the ocean over the earthquake causes a wave, and that wave moves out away from its point of origin in a circle, like what you’d see if you threw a pebble into a pond.”
“How big is this wave?” Bob Connick, the CFO, asked.
“At sea, about three feet high.”
The room filled with incredulous murmuring. “Three feet?” Connick objected. “That’s not big enough to capsize a rowboat.”
Osborn shook his head. “We need to forget images from movies like
The Poseidon Adventure.
Don’t let that fool you. It’s true that if we were on a cruise in the Indian Ocean and a tsunami came by we wouldn’t notice it. It wouldn’t even spill the drink in your glass. Ships at sea seldom notice a tsunami. That’s because the ocean is so deep that the wave travels below the surface with only a slight bulge above. The danger occurs when the massive wave hits shallow water.” He held up his forearm to simulate a wave and slowly moved it in front of his body. “The wave continues on, sometimes altered by undersea geography, until it reaches shallow water. In the deep ocean the wave can move at jetliner speeds—upward of five hundred miles an hour.”
“Five hundred miles an hour?” Kristen said with disbelief.
“Yes, and unlike waves you see at the beach, a tsunami can cross an ocean in a few hours, hit a continental shelf, and bounce back across the ocean. In 1960, for example, atsunami created by an earthquake off Chile reverberated through the Pacific for over a
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