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.