What Are the Vertical Spaces on a Seismograph?

Seismograph Basics

• The term seismograph generally refers both to the device used to measure the seismic waves of an earthquake and to the device used to record the data. More technically, a seismograph is the recording device, while a seismometer is the measuring device. In the past, both devices were typically coupled together. Today, the measurements from a seismometer are often transmitted to a seismograph, at a central location, for recording and monitoring. The physical or electronic recording is called a seismogram.

Vertical Space on a Seismogram

• The vertical space on a seismogram represents the displacement of the ground during an earthquake. This displacement can be measured vertically or horizontally. Typically, horizontal displacement is measured east-and-west or north-and-south. This displacement, recorded in the vertical space of the seismogram, defines the seismic wave's amplitude. The amplitude is directly proportional to the intensity of the vibration. Amplitude is measured in millimeters, though microns may be more appropriate for measuring tiny vibrations.

Layout of a Seismogram

• A typical seismogram will cover a 24-hour period of time. The width of the record will usually equal one hour of time, subdivided into ten- or fifteen-minute increments. The seismogram will display twenty-four recorded lines, one for each consecutive hour. However, seismograms can vary greatly. For example, a given seismograph may be programmed to record twelve rows, with each equal to two hours.

Vertical Space Identifies Wave Types

• The first wave to appear on a seismogram will be the P wave. The second wave to arrive, traveling at roughly half the speed of the P wave, will be the S wave. The higher intensity of this wave will be reflected in greater vertical movement on the seismogram. Because this type of seismic wave cannot travel through the Earth's fluid interior, it may not appear on the seismogram if the epicenter was located a long distance from the seismometer. The last waves to appear on the seismogram will be the surface waves. These waves have a lower frequency, meaning they will appear more spread out. If the epicenter of the earthquake was near the surface, they will be more intense than the P or S waves.

Calculating Earthquake Magnitude

• An earthquake's magnitude, measured according to the Richter scale, represents the amount of energy released by the quake. A quake's Richter magnitude is a function of the maximum amplitude, the vertical displacement on the seismogram, and the distance from the recording station to the epicenter. The distance can be measured by the time, or horizontal space, between the arrival of the P and S waves. Using these two measurements, you can calculate the magnitude of the earthquake.