An earthquake is a sudden, powerful release of stored energy within the Earth’s crust, typically caused by shifting tectonic plates. This energy radiates outward from the source point, called the hypocenter, as ground vibrations. These shock waves traveling through the Earth are collectively known as seismic waves. The study of these waves is fundamental to seismology, allowing scientists to locate earthquakes and probe the planet’s deep, layered interior.
Seismic Waves: The Main Classifications
All seismic waves fall into two major groups based on their travel path: body waves and surface waves. Body waves travel through the Earth’s interior layers, penetrating the crust, mantle, and core. They travel faster and are the first to arrive at a seismic station following an earthquake.
Surface waves are confined to the outermost layer of the planet, traveling along the near-surface of the crust. This difference in travel path dictates their characteristics, including speed and the damage they inflict. Surface waves generally arrive later than body waves but possess a larger amplitude, causing the most significant ground shaking.
Body Waves: Rapid Transmission Through the Interior
The first group of seismic waves consists of Primary waves (P-waves) and Secondary waves (S-waves). P-waves are the fastest seismic waves, earning the name “Primary” because they are the first recorded by seismographs. They travel through material by compression and expansion, moving particles parallel to the wave’s direction, similar to sound waves.
P-waves can travel through solids, liquids, and gases, allowing them to pass through every layer of the Earth, including the liquid outer core. S-waves, or “Secondary” waves, arrive next because they are slower, traveling at about 60% of the speed of P-waves in any given material. S-waves move material perpendicular to the wave’s direction of travel, causing a side-to-side or up-and-down shearing motion.
Unlike P-waves, S-waves cannot propagate through liquids or gases because these states lack the rigidity to support shear stress. The absence of S-waves passing through the Earth’s center provides definitive evidence that the outer core is liquid. The different speeds and capabilities of P and S waves are fundamental to both structural analysis of the Earth and the precise location of earthquakes.
Surface Waves: Slow Moving and Destructive
Surface waves are the slowest seismic waves, arriving after both P and S body waves. They are responsible for the most intense and prolonged ground shaking. Their energy is concentrated near the Earth’s surface, which is why they cause the majority of structural damage to buildings and infrastructure. This category is divided into two types: Love waves and Rayleigh waves.
Love waves move the ground in a horizontal, side-to-side shearing motion, perpendicular to the direction of wave travel. This strong lateral motion puts stress on building foundations and is highly destructive, often causing structures to be knocked off their supports. Love waves are typically faster than Rayleigh waves.
Rayleigh waves create a distinctive rolling motion, similar to ocean waves, moving the ground in an elliptical path that is both up and down and back and forth. This complex motion shakes structures in multiple directions simultaneously, increasing the risk of cracking, swaying, and toppling. Their large amplitude and longer duration make them the main culprits behind visible destruction during an earthquake.
Tracking Earthquakes Using Wave Data
Seismologists use the arrival times of body waves to accurately locate the earthquake source, known as the epicenter. Because P-waves are faster than S-waves, the time difference between their arrival at a seismic station, called the S-P interval, increases with distance from the source. Measuring this interval on a seismogram allows scientists to calculate the precise distance to the epicenter from that single station.
One station’s data only provides a distance, meaning the earthquake could have occurred anywhere on a circle around that station. To pinpoint the exact location, seismologists use triangulation, which requires data from at least three different seismic stations. By drawing a circle on a map for each station, with the radius equal to the calculated distance, the point where all three circles intersect marks the earthquake’s precise epicenter.