One aspect of seismology is concerned with measuring the speeds at which seismic waves travel through the earth. Past earthquake studies have shown that P, or primary/compressional, waves travel fastest through the earth; S, or secondary/transverse, waves cannot pass through liquids, allowing scientists to discern the earth's many boundary layers known as the crust, mantle, and core. For example, the disappearance of S waves below 1,800 mi (2,900 km) shows that the outer core of the earth is liquid. Seismologists also prepare seismic risk maps for earthquake-prone countries; these indicate the degree of seismic danger. In addition, seismologists use earthquake data to determine plate boundaries (see plate tectonics); active earthquake areas generally coincide with plate margins, both destructive and growing, and transform faults.
An important commercial application of seismology is its use in prospecting for oil deposits. The first oil field to be discovered by this method was found in Texas in 1924. A portable seismograph is set up in the area to be investigated, and an explosive energy source is activated nearby; formerly, explosives such as dynamite were used to create the seismic waves, but they have been largely replaced by high-energy vibrators on land and air-gun arrays at sea. The waves generated are received by detectors known as geophones; on land, these are commonly placed in a fan-shaped pattern on the ground. From an interpretation of the waves created by the energy source and recorded by the seismograph, the detection of geological structures in which oil may be trapped is possible.
Seismic methods are sometimes used to locate subsurface water and to detect the underlying structure of the oceanic and continental crust. With the development of underground testing of nuclear devices, seismographic stations for their detection were set up throughout the world. Under the Comprehensive Test Ban Treaty (signed 1996 but not yet in force) an international monitoring system has been set up which includes many seismic stations; the detailed data collected is also used by contributing nations for purposes other than monitoring nuclear tests.