A seismogram is a graphical recording of ground motion detected by a seismograph. This visual document displays the amplitude and frequency of ground shaking over time at a specific location. By analyzing the patterns recorded, seismologists can precisely pinpoint the earthquake’s origin point, known as the epicenter. The process relies on the fact that different types of seismic waves travel through the Earth at different speeds, creating a measurable time difference on the seismogram.
Understanding the Key Data: P-Waves and S-Waves
The information needed to locate an earthquake is contained within two primary types of body waves: Primary waves (P-waves) and Secondary waves (S-waves). P-waves are compressional, pushing and pulling material in the same direction the wave is moving, similar to sound waves. Traveling the fastest, P-waves are always the first to register on a seismogram.
S-waves are shear waves that move the ground perpendicular to the direction of propagation, causing side-to-side or up-and-down motion. These waves travel slower than P-waves, typically at about 60% of the P-wave speed, and arrive second at a seismic station. The physical properties of S-waves prevent them from traveling through liquids.
Because P-waves travel faster than S-waves, the time gap between their arrivals at a recording station relates directly to the distance traveled. For a station close to the source, the arrival time difference is small. The farther a station is from the epicenter, the greater the separation, or lag, between the P-wave and S-wave arrival times becomes. This increasing time interval is the fundamental measurement used to calculate the distance to the earthquake source.
Determining the Time-Distance Relationship
The first analytical step involves accurately measuring the time interval between the arrival of the P-wave and the arrival of the S-wave on the seismogram. This measurement is known as the S-P interval, or lag time. Seismologists determine this interval precisely, which represents the accumulated travel time difference between the two waves.
The measured S-P interval is then converted into a distance value using standardized travel-time curves. These are pre-calculated graphs showing how the arrival times of P and S waves change with distance. These curves are built upon decades of data and models of Earth’s internal structure, providing a reliable correlation between the measured time lag and the distance from the station to the earthquake.
A longer S-P interval indicates the station is farther away from the source, while a shorter interval points to a closer location. Once the S-P interval is plotted on the travel-time curve, the corresponding distance is read directly from the graph. This single distance value defines a radius, meaning the earthquake’s epicenter lies somewhere on the circumference of a circle centered at the recording station.
Locating the Epicenter
A single distance measurement, derived from one seismogram, only provides a circle of potential locations for the earthquake source. To narrow this down to a single point, seismologists employ a geometric technique known as triangulation. This method requires distance calculations from at least three different seismic recording stations.
For each of the three stations, a circle is drawn on a map using the calculated distance as the radius, centered at the station. A second circle, drawn from a different station, will intersect the first circle at two distinct points.
The third circle is then drawn, based on the distance calculated from the third station’s seismogram. It should intersect the other two circles at a single, common point. This unique intersection point where all three circles overlap is the precise geographic location of the earthquake’s epicenter. The use of three separate data points eliminates ambiguous possibilities, providing a definitive coordinate for the earthquake’s origin.