A seismogram is the graphical output generated by a seismograph, an instrument designed to record the ground’s motion over time. This record is the primary tool seismologists use to analyze the energy released by an earthquake. The horizontal axis represents time, while the vertical axis shows the amount of ground displacement. By studying the distinct patterns and waves traced on this record, scientists can characterize a seismic event, including its origin, time of occurrence, and size.
Interpreting the Seismic Wave Arrivals
The first information revealed by a seismogram is the sequence of seismic waves that travel through the Earth from the earthquake’s source. These waves arrive at the recording station in a distinct order based on their travel speed. The initial disturbance marks the arrival of the Primary waves (P-waves), which are the fastest and travel through the Earth by compression and expansion.
Following the P-waves are the Secondary waves (S-waves), which travel slower and move the ground in a shearing, side-to-side motion. S-waves are generally recorded with a greater amplitude than P-waves, making them easily distinguishable. The final waves to appear are the surface waves, which travel along the Earth’s crust. Surface waves are the slowest but typically have the largest amplitude on the recording, and because they cause the most severe ground movement, they are usually responsible for the majority of the damage felt during an earthquake.
Pinpointing the Earthquake’s Location
The different travel speeds of the P-waves and S-waves provide the means to calculate the distance from the recording station to the earthquake’s origin. This process relies on measuring the time interval between the arrival of the first P-wave and the first S-wave, known as the S-P interval. Since the P-wave consistently outruns the S-wave, the time difference between their arrivals increases the farther the seismic station is from the earthquake’s source.
Seismologists use pre-calculated travel-time curves, which plot the known travel times of P and S waves against distance, to convert the measured S-P interval into an exact distance. This distance calculation only tells scientists how far away the earthquake occurred, not the direction it came from. Therefore, the calculated distance establishes a radius, and the earthquake’s location could be anywhere on the circumference of a circle drawn around the recording station.
To find the precise surface location, or epicenter, scientists use a method called triangulation. This technique requires distance calculations from at least three different seismic recording stations. A circle is drawn around each station with a radius equal to the calculated distance. The point where all three circles intersect marks the exact location of the epicenter. Once the epicenter is found, seismologists use the travel times and wave speeds to determine the depth of the earthquake’s source, called the hypocenter.
Quantifying the Earthquake’s Magnitude
After the location is determined, the seismogram is used to quantify the earthquake’s magnitude, or size. This is achieved by measuring the maximum wave amplitude—the height of the largest deflection recorded, usually from the slower surface waves. The amplitude is a measure of the ground motion caused by the seismic energy at the recording station.
This maximum amplitude measurement is then correlated with the calculated distance from the station to the epicenter. This two-part process is essential because seismic waves naturally diminish (attenuate) as they travel away from the source. By factoring in the distance, scientists can correct for this energy loss, providing a more accurate assessment of the energy released at the earthquake’s source.
The final magnitude is determined using a logarithmic scale, such as the Moment Magnitude Scale. This means that each whole number increase on the scale represents a tenfold increase in the measured wave amplitude. A one-unit increase in magnitude corresponds to the release of about 32 times more seismic energy. The seismogram’s amplitude and the calculated distance are the fundamental data points that feed directly into these formulas, providing a standardized measure of the earthquake’s strength.