A seismogram is the tangible output from a seismograph, an instrument designed to measure ground motion. It is a graphic record displaying the Earth’s shaking at a specific location as a function of time, typically appearing as a jagged, wavy line. The purpose of this record is to capture and quantify vibrations generated by seismic waves, primarily originating from earthquakes but also from volcanic activity or explosions. Analyzing the patterns on the seismogram allows scientists to determine the size and location of the seismic event.
How the Recording Instrument Works
The creation of a seismogram relies on the physical principle of inertia, the tendency of a mass to resist changes in motion. The instrument, called a seismometer, consists of a frame fixed to the ground and a suspended mass, often a pendulum, that is free to move. When seismic waves cause the ground to shake, the instrument’s frame moves with the Earth.
Due to inertia, the suspended mass momentarily remains stationary as the ground shifts around it. This creates relative motion between the moving frame and the static mass. Older, mechanical seismographs traced this motion onto a rotating drum, but modern instruments convert this relative movement into an electrical voltage.
This voltage is digitized and transmitted to a recording system, generating the digital seismogram. The instrument is designed with separate sensors to measure ground movement in all three dimensions: vertical, north-south, and east-west. Recording motion across these three axes captures a complete picture of the ground shaking.
Anatomy of a Seismogram
A seismogram trace is a chronological record of ground displacement, where the horizontal axis represents time and the vertical axis represents the amplitude of the ground motion. The first identifiable disturbance is the arrival of Primary waves (P-waves). P-waves travel fastest and are compressional, pushing and pulling the ground in the direction of wave travel. They usually register as the smallest, lowest-amplitude wiggles because their motion is less disruptive than subsequent waves.
The next distinct waves to arrive are Secondary waves (S-waves), which travel slower than P-waves. S-waves are characterized by a shearing motion, shaking the ground perpendicular to the direction of wave movement. The S-wave arrival is marked by a noticeable increase in the trace’s amplitude compared to the P-waves.
Following these body waves, which travel through the Earth’s interior, come the Surface waves, which propagate along the Earth’s surface. Surface waves, including Love waves and Rayleigh waves, travel the slowest but produce the largest oscillations. These waves are responsible for the most intense ground shaking and exhibit the highest amplitude on the trace. The distinct arrival times and amplitudes of these three wave types provide the data necessary to analyze the earthquake.
Determining Earthquake Location and Strength
The time difference between the P-wave and S-wave arrival, known as the S-P interval, determines the distance of the seismic station from the earthquake’s origin. Since P-waves travel faster than S-waves, the time gap between their arrivals increases with distance. Using a standardized travel-time curve, scientists relate the S-P interval to a specific distance and draw a circle around the recording station with that radius.
Because a single station only provides distance, the earthquake’s epicenter is determined using data from at least three different seismic stations. This technique is called triangulation. Three circles, each representing the distance from a different station, are drawn on a map, and the point where all three intersect marks the precise location of the epicenter.
To determine the earthquake’s strength, or magnitude, scientists measure the maximum wave amplitude recorded on the seismogram. The maximum amplitude, combined with the distance to the epicenter calculated from the S-P interval, allows for the calculation of the energy released. This data is used to assign a magnitude, such as on the Moment Magnitude Scale.