When an earthquake occurs, understanding its size and potential for destruction requires two distinct measurements. Scientists quantify the total energy released at the source deep within the Earth, providing an objective measure of the event’s size (Magnitude). A different approach is necessary to document the varying degrees of shaking and damage experienced at different locations on the surface (Intensity). This dual system allows seismologists to characterize both the physical power of the earthquake and its localized impact.
The Instruments Used for Recording Motion
The study of earthquakes begins with highly sensitive equipment designed to detect and record ground motion. The core sensing device is the seismometer, which operates on the principle of inertia, typically using a mass suspended by a spring or pendulum. When the ground shakes, the instrument’s frame moves with the Earth, but the suspended mass remains still due to inertia. This relative motion is converted into an electrical signal proportional to the ground’s movement.
The seismograph is the complete system incorporating the seismometer, a data logger, and display components to record the signal. This system transforms the raw motion data into a visual or digital output known as a seismogram. The seismogram records the time and amplitude of the ground displacement, which is the foundational data set used by seismologists to calculate an earthquake’s size and pinpoint its origin.
Quantifying Energy Release (Magnitude)
The magnitude of an earthquake is a single number representing the energy released at the earthquake’s hypocenter, or source. Magnitude scales are logarithmic; each whole number increase represents a tenfold increase in the measured wave amplitude and approximately a 32-fold increase in the total energy released.
Historically, the Richter Scale, developed in 1935 by Charles F. Richter, was the first widely adopted system. This scale calculates magnitude based on the logarithm of the largest wave amplitude recorded. However, the Richter Scale has limitations for very large earthquakes (above magnitude 6.5) because the measurement method tends to “saturate,” failing to accurately differentiate their size.
Modern seismology primarily relies on the Moment Magnitude Scale (MMS or Mw), which overcomes these saturation limitations. The MMS calculates the seismic moment, a physical measure related to the total energy released. This calculation involves three factors: the area of the fault surface that slipped, the average distance the fault slipped, and the rigidity of the rocks involved. The MMS is the scientifically preferred measure, providing a more consistent and accurate measure of total energy release across the full spectrum of earthquake sizes.
Assessing Localized Impact (Intensity)
While magnitude measures the source energy, intensity measures the effects of the earthquake on the surface at a specific location. This measure is based on observable consequences, including how the shaking was felt by people and the degree of damage sustained by buildings and the environment. Because shaking diminishes with distance from the epicenter, a single earthquake produces many different intensity values across the affected area.
The scale used to quantify this localized effect is the Modified Mercalli Intensity (MMI) Scale, which employs Roman numerals from I (not felt) to XII (catastrophic destruction). Unlike magnitude scales, the MMI Scale is not based on instrumental measurements or mathematical formulas, relying instead on qualitative observations. Lower intensity values describe human perceptions, such as feeling the motion (II) or hanging objects swinging (III). Higher values focus on structural damage, ranging from slight damage (VII) to total destruction (XII). The MMI Scale is a valuable tool for emergency response and engineering analysis, offering a direct measure of the severity of shaking experienced locally.
Distinguishing Magnitude from Intensity
The fundamental difference between magnitude and intensity lies in what each is measuring: the source versus the effect. Magnitude is a single value for the entire event, representing the fixed energy released at the fault. This is analogous to the wattage rating of a lightbulb, a fixed property determined at the source.
Intensity describes how that energy manifests as ground shaking and damage at any given point on the surface. This measurement is variable; a single earthquake will have a uniform magnitude but multiple intensity ratings that change with distance, local geology, and building construction quality. Following the lightbulb analogy, intensity is like the brightness of the light measured in different rooms, which varies depending on distance and intervening factors.
For example, a large magnitude earthquake in a remote area might have a low intensity value (I or II) where it is unobserved. Conversely, a moderate magnitude earthquake beneath a densely populated city with poor building standards could produce very high intensity values (VIII or IX) due to severe localized shaking and damage.