A seismogram is the graphical output generated by a seismograph, which is an instrument designed to detect and record ground motion. This record is the fundamental tool seismologists use to analyze the disturbances caused by earthquakes, explosions, or other sources of shaking. Reading this patterned trace of Earth’s movement is the process through which scientists can determine precisely when an event occurred, where it originated, and how strong it was. Understanding the components of this chart is the first step toward interpreting the powerful forces at work beneath the planet’s surface.
The Anatomy of a Seismogram
A seismogram is essentially a two-dimensional graph that records the movement of the ground over time. The horizontal axis of the chart represents time, moving from left to right in increments like minutes or seconds. This time axis allows analysts to pinpoint the exact moment a seismic wave arrived at the recording station.
The vertical axis, conversely, measures the amplitude of the ground motion, which represents the degree of displacement from a resting position. This resting position is represented by a flat, central line called the baseline, which indicates a period of no ground movement. Any deviation, either upward or downward, from this baseline trace signifies movement, with the height of the wave directly relating to the intensity of the shaking. Modern seismographs often record motion in three dimensions—vertical, north-south, and east-west—producing three separate seismograms for a single event to capture the full complexity of the ground displacement.
Identifying the Key Seismic Waves
The trace on the seismogram is defined by the arrival of three main types of seismic waves, each with distinct visual characteristics and arrival times.
The first waves to appear are the Primary, or P-waves, which are compressional waves that travel fastest through rock and can pass through solids and liquids. On the seismogram, the P-wave arrival is typically marked by a small, sudden deflection from the baseline, representing the initial, relatively gentle jolt of the earthquake.
Following the P-waves are the Secondary, or S-waves, which are shear waves that move particles perpendicular to the direction of wave travel. S-waves travel slower than P-waves and can only move through solid material, making them a useful tool for studying the Earth’s interior structure. Their arrival is usually characterized by a larger, more pronounced wave pattern on the seismogram compared to the initial P-wave.
The final and often largest recorded movements belong to the Surface waves, which are confined to the Earth’s surface and arrive last because they travel the slowest. These waves, which include Love and Rayleigh waves, cause the most substantial ground shaking and are responsible for the majority of earthquake damage. On the seismogram, the Surface waves exhibit the largest amplitude and the longest duration, often appearing as a sustained, high-energy oscillation that gradually tapers off.
Calculating Distance to the Epicenter
A core application of reading a seismogram is determining the distance from the recording station to the earthquake’s epicenter. This calculation relies entirely on the measurable time difference between the arrival of the faster P-wave and the slower S-wave, known as the S-P interval. Because P and S waves are generated simultaneously at the earthquake’s source and travel at different speeds, the time interval between their arrivals increases the farther the station is from the epicenter.
To convert this measured time delay into a distance, seismologists use a tool called a travel-time curve. This curve is a standardized graph plotting the travel times of P and S waves against the distance from the epicenter. By physically measuring the S-P interval on the seismogram and finding where that same time span fits vertically between the P-wave and S-wave curves on the chart, the corresponding horizontal distance can be read directly. This single distance measurement defines a circle around the recording station; the precise location of the epicenter is then found by triangulating the intersecting points of three or more such distance circles from different seismic stations.
Determining Earthquake Magnitude
The final step in interpreting earthquake data is determining the magnitude, which quantifies the energy released at the earthquake’s source. This calculation combines the distance to the epicenter, which was derived from the S-P interval, with the measurement of wave intensity. The intensity is determined by measuring the maximum wave amplitude, which is the greatest vertical excursion above or below the baseline on the seismogram, often found within the S-wave or the Surface wave train.
These two pieces of data—epicentral distance and maximum amplitude—are then used together, typically with a specialized chart called a nomogram, to find the magnitude. The procedure involves drawing a straight line connecting the distance value on one scale to the maximum amplitude value on another. Where this line intersects the central magnitude scale indicates the earthquake’s size, expressed using scales like the Moment Magnitude Scale. Since these scales are logarithmic, each whole number increase in magnitude corresponds to a tenfold increase in the measured wave amplitude, reflecting the immense power contained within seismic events.