Seismic waves occur from actions within the Earth's core, including breaking and splitting rocks, explosions and volcanic eruptions. Seismic waves occur on land and in the ocean; they exist in two primary forms — surface waves and body waves. Body waves travel underground, below the Earth's surface, while surface waves travel across the surface.
Actions triggering seismic waves also determine their frequency and strength. Earthquakes, common sources of seismic waves, generate surface and body waves. Large earthquakes produce waves of greater force and length. Some geologists refer to seismic waves as ambient waves, as they derive from low-frequency energy sources. Seismic waves come in the form of primary waves and secondary waves. Secondary waves follow larger primary waves, delivering less force and power.