Did you know geologists can look at rocks deep beneath the surface using sound, just like a doctor uses an ultrasound machine to see a developing baby?
Ultrasounds and seismic waves are both types of sound waves, and they need a medium like air, liquid or rock to travel through. Because of that, when they come to a boundary between two substances, some of the sound gets reflected.
Two Types Of Seismic Waves
With medical ultrasound, the waves reflect off the surface of the baby's body. There are two types of seismic waves. P-waves travel through liquids and solids, and S-waves only travel through solids. When they hit boundaries between different substances they change directions, showing the earth's structure.
The earth is its own ultrasound machine. All scientists have to do is record when and where the waves occur.
Scientists measuring layers of rock in an area of the Pacific floor called the Nankai Trough discovered why earthquakes trigger tsunamis from that area.
A major fault runs from six miles deep right up to the sea floor. When an earthquake happens there, the fault lets the sea floor move up and down, creating large waves. Other scientists in the San Francisco Bay area have mapped the area to determine where faults are located and where earthquakes will occur.