The 8.9 magnitude earthquake that struck Japan on Friday afternoon (11:46 p.m. CST Thursday) is the largest ever recorded there. The tsunami that occurred soon afterward washed away villages, tossed oceangoing vessels as if they were rubber ducks, damaged several nuclear power plants and caused unprecedented devastation. Aftershocks continue to rock the nation as emergency workers search for victims. Steve Marshak, a professor of geology and the director of the School of Earth, Society and Environment at Illinois, is a specialist in structural geology, tectonics and field geology. He was interviewed by Jeff Unger, of the U. of I. News Bureau.
What causes earthquakes like the kind that devastated parts of Japan today?
Earth's outer shell, called the lithosphere, is broken into about 20 pieces, which geologists refer to as plates. These plates move relative to one another, at very slow rates (between 1 and 5 inches per year). Most of the earthquakes on this planet happen along plate boundaries, the edges of plates - the interior of plates, away from the boundaries, are largely earthquake-free.
Geologists recognize three kinds of plate boundaries: 1) At mid-ocean ridges two plates move apart, and new ocean floor forms. The mid-Atlantic ridge is an example. 2) At transform boundaries, one plate slides sideways relative to its neighbor. The San Andreas fault in California is an example. 3) At trenches, one plate slides underneath another and sinks back into the Earth's interior. Japan sits right next to a trench, where the Pacific ocean floor (the Pacific Plate) is sliding underneath Asia (the Eurasian plate).
The movement along a plate boundary occurs in fits and starts. That is, no movement occurs for a while, because rock of the lithosphere can withstand a significant amount of force. But after a while, the forces become so large, rock breaks and a sudden movement takes place. During this sudden movement, rock fractures (breaks), sending shock waves (vibrations) through the Earth. The shock waves shake the ground surface, and that's what we feel as an earthquake. The process resembles what happens when you bend a stick until it suddenly snaps, and you feel the "snap" in your hands.
The fracture on which the slip occurs in the Earth is called a fault. During a small earthquake, a small amount of slip occurs over a small part of a fault. During a large earthquake, a large amount of slip occurs over a large part of the fault. There is a large fault at the boundary between the Pacific Plate and the Eurasian plate. A large portion of this fault slipped to cause the massive earthquake that took place today.
Besides the size, what's the difference between a tsunami and a normal ocean wave?
A tsunami and a storm wave (a "normal ocean wave") are very different.
Storm waves are generated by the shear between the wind and the surface of the sea. A storm wave can be very high (a few feet to 90 feet), and relatively long as measured parallel to its crest (a quarter mile long to a mile long), but is relatively narrow as measured perpendicular to the wave crest. If you look at waves crashing on a beach, there are only 10 to 100 feet between successive waves. This distance is called the wavelength. What this means is that the volume (amount) of water contained in a single wave is relatively small. So when a storm wave crashes on the shore, the water rushes up the beach only for 50 to 100 feet before it stops and gravity pulls the water back to the sea. In effect, a storm-wave runs out of water before it can travel very far.
A tsunami is caused by a sudden displacement of a broad area of the sea floor. The displacement of the sea floor surface may only be a few inches to perhaps 5 feet. But such a large area of sea floor moves that the event moves a huge amount of water. To picture the process, imagine placing your hand at the bottom of a bathtub full of water, and suddenly lifting it a couple of inches. The water on the surface of the tub rises up and then a set of waves starts moving outward from the temporary bulge. Such waves are like a tsunami.
Unlike a storm wave, an individual tsunami may be miles wide (as measured perpendicular to the wave crest). In other words, a tsunami can be more than 100 times wider than a storm wave. The waves travel across the ocean at almost 500 miles per hour. Out at sea, the wave may be inches to a couple of feet high. But when it approaches shore, the front part of the wave slows down, due to friction with the sea bottom, so the back part catches up - when this happens, water within the wave builds into a huge plateau of water that is a foot to 50 feet high. While this height is no different than that of a large storm wave, the volume of water involved is huge (since the wave is so wide), so when the wave reaches shore, it just keeps moving way beyond the edge of the beach and can reach a few miles inland. It washes away everything in its path. And when the water finally stops moving inland and starts rushing back to sea, it can carry debris back with it.
The word "tsunami" is Japanese for "harbor wave." The name comes from the fact that the waves get focused into harbors (which tend to be at the end of narrow bays) and thus may become particularly high in harbors, and cause a particularly large amount of damage. In English, they used to be called "tidal waves," because during a tsunami, the water rises and covers a region, like it does during a high tide. But the tides are largely due to the gravitational pull of the moon and sun; the origin of tsunami has nothing to do with such processes. Thus the old term, "tidal wave," is confusing and has been abandoned.
Could a quake that causes this much devastation happen in the Midwest?
There have been large earthquakes in historic time along the New Madrid fault system in southern Missouri. Three happened in 1811 and 1812. They may have been as large as Magnitude 8 (meaning, about 30 to 50 times smaller than today's earthquake in Japan). It is not possible to predict earthquakes accurately, but if earthquakes have happened in the past in a region, they are likely to occur again in the future. Geologists do not agree on how likely it is that a large earthquake could happen along the New Madrid during the next few centuries. Some geologists think that the next large earthquake won't happen in the Midwest for centuries or millennia, others think it could happen any time. Since the Midwest is not along a plate boundary, earthquakes occur much less frequently than they do in Japan.