Seismic waves are vibrations that travel through the Earth, carrying energy released by sudden events like earthquakes, volcanic eruptions, or large explosions. These waves travel through the Earth’s interior and along its surface, providing geophysicists with data to understand the planet’s internal structure. The primary instrument designed to detect and measure these ground motions is the seismometer, which serves as the foundational sensor for all modern earthquake measurement.
The Primary Tool for Detection
The device used to measure ground motion is called a seismometer, which is the sensing component of the larger instrument known as a seismograph. The seismograph is the complete system that includes the seismometer and the attached recording system. To accurately measure the Earth’s movement, the instrument must be anchored rigidly to the ground so its frame moves perfectly with the shaking earth.
The core of the seismometer operates on the principle of inertia, which is the tendency of a mass to resist changes in its state of motion. The device consists of a heavy “proof mass” suspended by a spring or pendulum system within a frame. When the ground shakes, the outer frame and instrument housing move immediately with the Earth.
Due to inertia, the suspended proof mass tends to remain stationary while the frame moves around it. This creates a measurable relative motion between the mass and the moving frame. By measuring this differential movement, the seismometer accurately senses and quantifies the ground’s motion. Seismometers are sensitive, capable of detecting ground movements as minute as one ten-millionth of a centimeter at quiet sites.
How Seismic Waves Are Recorded
The relative motion between the mass and the moving frame is converted into a continuous record of ground displacement over time. In older, analog seismographs, this motion was mechanically transferred to a pen that traced a wavy line onto a rotating drum of paper. This visual record of the ground shaking is known as a seismogram.
Modern seismometers use electronic sensors and digital recording systems instead of mechanical pens and paper. The movement of the mass relative to the frame generates an electrical voltage, which is then digitized, stored, and processed by a computer. Today’s seismograms are digital plots where the horizontal axis represents time and the vertical axis represents ground displacement.
The seismogram displays the arrival times and amplitudes of different types of seismic waves. The first waves to arrive are the Primary or P-waves, which are compressional waves that travel fastest through the Earth’s interior. These are followed by the Secondary or S-waves, which are shear waves that travel at about 60% of the P-wave speed and typically show a larger amplitude. Finally, the slowest but often largest waves are the surface waves, which can cause the most significant shaking and damage.
Quantifying the Earthquake’s Strength
Once seismic waves are recorded, the raw data must be translated into numbers representing the earthquake’s overall size and impact. Historically, the Richter Scale, or Local Magnitude (\(M_L\)), was used, calculating magnitude from the logarithm of the largest wave amplitude recorded by a nearby seismograph. This scale was limited because it only worked well for moderate earthquakes in certain regions and tended to underestimate the true size of very large events.
Today, the most scientifically accurate and widely used measure of an earthquake’s size is the Moment Magnitude Scale (\(M_w\)). This scale is a physical measure that relates directly to the total energy released at the earthquake’s source. It is calculated using a formula that considers the physical properties of the rupture, such as the total area of the fault that slipped and the distance the fault moved.
A whole number increase on the Moment Magnitude Scale represents a tenfold increase in the measured wave amplitude and approximately 32 times more energy release. In contrast to magnitude scales, the Modified Mercalli Intensity (MMI) Scale measures the observed effects of shaking at a specific location. The MMI scale uses Roman numerals from I to XII and is based on eyewitness accounts and the level of damage to infrastructure. Since shaking intensity varies greatly across a region, a single earthquake will have one moment magnitude value but many different Mercalli intensity values.