People have been observing and reporting on earthquakes throughout recorded history. Although the mechanisms of quakes have not been understood until recently, it has been possible to find out quite a bit about how they work by observing their effects.
Since the invention of the seismograph it has been possible to observe the different kinds of waves produced in the earth by quakes. Surprisingly, earthquake waves from a large quake can be detected around the world. This permits a study of how the waves move through the earth's interior. A comparison of readings from different places reveals that the different kinds of waves are affected in predictable ways by the density of the material through which they pass. The study of earthquake waves has shown definite layers inside the earth. See below for an explanation of how scientists have proven the existence of a solid core to the earth thousands of miles below our feet.
P-waves are the first to be felt in a quake. These are compression waves similar to sound waves that travel out directly from the focus straight through the earth. In fact, since they are similar to sound waves they can be heard. People often report hearing a sound like a train coming through the house or a sudden strong wind when there is no wind. This is the sound of the earthquake P-wave. S-waves follow with a side-to-side motion. Unlike the P-waves, S-waves follow the surface of the earth.
By studying the different kinds of waves as recorded on a seismograph, it is possible to gain a picture of the quake. Magnitude is determined by a study of the size of the waves. Location can be determined by comparing the direction of the quake from different stations. The directional lines from all the stations will converge on the epicenter. Interestingly, the nearest seismograph is usually not the most accurate for a large quake. Seismographs are very delicate instruments that can easily be disrupted by the very strong movements of a nearby earthquake. For instance, in the 2001 earthquake near Seattle the seismograph located in Seattle showed an initial magnitude of 4.7, compared to a more accurate 7.0 recorded in Colorado Springs (later adjusted to 6.8 after more study).
The mechanism for earthquakes, Plate Tectonics, was not discovered by studying earthquakes. There was a project to map the seafloor of the Atlantic Ocean, which discovered some very interesting things. They found an unexpected symmetry on both sides of the mid-Atlantic ridge. The rock on each side of the ridge got older as you moved away from the ridge. It was clear that the ocean floor was spreading from the middle. since new crust was being created at the mid-ocean ridge, it had to be absorbed back into the earth somewhere. This turned out to be in very deep trenches at the edges of continents. A map of earthquakes worldwide over a long period of time showed clear lines of activity that broke the earth's crust into a number of continent sized plates that were clearly in movement relative to each other. Earthquakes and volcanoes were mostly located at the borders of the plates, where they rubbed against each other. the most violent quakes were located near the trenches and were caused by one plate being forced under another. Other earthquakes were caused when two plates slid sideways relative to each other, such as the San Andreas fault in California, where the Pacific Plate is carrying a portion of the coast to the north relative to the rest of California. Yet others are the result of two plates colliding head on. India's collision with Asia has not only caused many severe quakes but has raised the Himalayas into the highest mountains on earth.
Seismic waves have probed to the center of the Earth.
For 60 years, geophysicists have suspected that the earth's inner core was solid - now they have proved it. By detecting special seismic tremors in the aftermath of a massive Indonesian earthquake, the seismologists from Northwestern University and the French Atomic Energy Commission have shown without doubt that at the heart of the Earth is a solid iron-nickel ball, 2400-km in diameter.
Emile Okal and his French colleague, Yves Cansi, used an eight-station French seismic network to study the earthquake occurring on the other side of the world. "The 1996 Flores Sea earthquake, which was a big earthquake at about 600-km depth, was perfect in geometry for recording in France," said Okal. It was a rare opportunity as massive, deep earthquakes are needed to probe the necessary 5000-km below the surface of the Earth but only happen infrequently.
Experts hailed the research, announced at the American Geophysical Union meeting in San Francisco, as a breakthrough. Professor Kathy Whaler, a geophysicist at Edinburgh University (Scotland), stated, "No-one has unambiguously taken these waves from the inner core before and there is a group in Utrecht (The Netherlands) who are getting the same result but using a different analysis of a different earthquake, 1994 in Bolivia - that gives us confidence."
As long ago as the 1930s, scientists predicted that despite a temperature of thousands of degrees, the crushing pressure at the Earth's center would cause the iron-nickel alloy to freeze but because the telltale signals are so weak, they have taken 60 years to detect. However, capturing the faint seismic tremors helped to dispelled many doubts.
The key to the breakthrough is the behavior of the two types of seismic waves. Pulse waves can travel through both liquids and solids as they move by compressing and then relaxing the material n the direction of travel. Shear waves, in contrast, can only pass through solids. They vibrate at right angles to the direction of travel and as liquids have no material strength the signal rapidly dissipates in the fluid.
Thus, detecting a shear wave coming all the way from the inner core would prove it was solid. But the scientists' task was made even more difficult by the fact that the liquid outer core, surrounding the inner core, blanks out all shear waves.
They, then, had to exploit the different speeds at which the waves travel. Imagine a seismic wave rumbling down through the Earth. When it reaches the outer core, all shear waves are lost and only pulse waves continue. When the pulse waves reach the inner core the waves are partly refracted and reflected.
This allows part of the energy to convert into shear waves that then travel through the inner core. Shear waves travel more slowly than pulse waves so they reach the opposite side of the inner core later. Here they are partly converted back to pulse waves.
It was these delayed pulse waves that the scientists detected. The reason it has taken 60 years to do so is because the conversions from pulse to shear to pulse sap the energy of the waves so the signals are exceedingly weak. Improvements in instrumentation over the last 15 years were crucial to the new finding, Okal said, as were computer capabilities, developed in France, to sort the signals from the noise.
"We look at the interior of the Earth because we want to know what is there," explains Okal simply. "But it may be interesting material scientists because it shows that under tremendous pressures, iron is behaving in a different way. This understanding might be applicable for other materials at not-so-heavy pressures."
Breakthrough Applauded
Wave Energy Converted
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