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. (Here is a live view of s seismograph in action.) 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 at all. 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 eachother. Earthquakes and volcanos were mostly located at the borders of the plates, where they rubbed against eachother. the most violent quakes were located near the trenches and were caused by one plate being forced under another. Other earthquakes were casued when two plates slid sideways relative to eachother, 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 Himalyayas into the highest mountains on earth.
Seismology at the Science Fair
You might also want to look at this book: Janice Vancleave's Earthquakes: Mind-Boggling Experiments you can turn into Science Fair Projects for more science fair ideas.
