A seismogram is the tangible record of ground motion that occurs during an earthquake or other seismic event. This graphical output is created by a seismograph, the instrument designed to detect and record these vibrations. The seismogram provides a visual history of the earth’s movement over time, which scientists use to analyze the characteristics and location of the seismic source. Interpreting the distinct patterns and measurements captured on this record allows for a practical understanding of the raw data that forms the basis of seismology.
The Physical Components of a Seismogram
A seismogram is essentially a chart with two fundamental dimensions that quantify the earth’s movement. The horizontal axis represents time, typically marked in seconds or minutes, allowing for the precise measurement of wave arrival sequences. The vertical axis represents the amplitude, which directly correlates to the amount of ground displacement or velocity at the recording station. This amplitude is the measure of the height of the wave oscillation from the resting position, or baseline.
The baseline is the relatively flat line on the seismogram that indicates the ground is at rest or experiencing only minor background noise. The “trace” is the wiggling line that moves away from this baseline to record motion. Historically, these records were drawn with pens on rotating paper drums, but modern seismographs use digital sensors to create electronic records. The data is often recorded across three components—up-down, north-south, and east-west—to capture the full three-dimensional movement of the ground.
Recognizing Distinct Seismic Wave Signatures
Earthquakes generate different types of seismic waves, and each leaves a distinct signature on the seismogram based on its speed and motion style. These waves are categorized into faster-traveling body waves and slower, more destructive surface waves.
The first signal to arrive at a seismic station is the Primary wave (P-wave), which travels fastest and is characterized by a push-pull, compressional motion in the direction of travel. On the seismogram, the P-wave appears as a sudden, small-amplitude wiggle, often most visible on the vertical component trace.
Following the P-wave is the Secondary wave (S-wave), which moves slower and shakes the ground with a shearing motion, perpendicular to the direction of wave travel. S-waves appear on the seismogram as oscillations with a significantly larger amplitude than the P-waves. Since S-waves can only travel through solid material, their presence or absence in a record provides information about the material they have passed through.
The last group of signals to arrive are the surface waves, which are confined to the Earth’s surface and consist of Love and Rayleigh waves. These waves travel the slowest but are responsible for the largest ground motion and the most damage during an earthquake. On the seismogram, surface waves are marked by the largest, most sustained amplitudes, creating a long-duration train of oscillations. The distinct change in amplitude allows a reader to visually identify the arrival of each wave type in sequence.
Calculating Earthquake Distance and Magnitude
Interpreting the time difference between the arrival of the two body waves allows for the calculation of the distance between the recording station and the earthquake’s epicenter. P-waves travel faster than S-waves, meaning the time difference between their arrivals, known as the S-P interval, increases with distance from the source.
To find the epicentral distance, the time interval between the first P-wave arrival and the first S-wave arrival is measured directly from the seismogram. This measured S-P interval is then compared to a standard time-travel graph, which converts the time difference into a distance measurement.
The maximum amplitude of the seismic waves is the second crucial measurement needed to determine the earthquake’s magnitude. This is typically measured as one-half the distance between the highest peak and the lowest trough of the largest wave, which is often the S-wave or a surface wave. The magnitude is then determined by combining this maximum amplitude measurement with the calculated epicentral distance. This calculation often involves a conceptual tool called a nomogram, which graphically relates the distance, the maximum wave amplitude, and the earthquake magnitude.
The magnitude scales, such as the Richter or Moment Magnitude scale, are logarithmic, meaning a small increase in the calculated number represents a large increase in the measured amplitude of ground shaking. Each whole number increase in magnitude corresponds to a tenfold increase in the measured maximum wave amplitude on the seismogram. This combination of time-interval analysis for distance and amplitude measurement allows the seismogram to serve as a complete source of data for characterizing an earthquake event.