The Earth’s crust is a collection of massive, moving tectonic plates that constantly grind against each other. An earthquake is the sudden release of accumulated strain energy within the Earth, occurring when rocks along a fault plane fracture and slip past one another. Measuring this complex phenomenon requires standardized methods to accurately compare events globally for scientific analysis and public safety.
Instruments of Detection
The foundational tool for detecting and recording ground motion is the seismometer, housed within a larger assembly known as a seismograph. This device operates on the principle of inertia, the tendency of an object to resist changes in motion. A seismometer utilizes a heavy “proof mass,” suspended within a frame that is firmly anchored to the ground.
When seismic waves reach the instrument, the ground and the attached frame move vigorously. The heavy, suspended mass momentarily resists this motion due to inertia, remaining relatively stationary. The difference in motion between the moving frame and the mass is converted into an electrical signal and recorded. The resulting visual record, called a seismogram, represents the raw data of the ground’s displacement, velocity, or acceleration over time.
Quantifying Energy Release
The most important measure of an earthquake is its magnitude, which quantifies the total seismic energy released at the source of the rupture. Modern seismology uses the Moment Magnitude Scale (\(M_w\)) as the standard for this measurement. The Moment Magnitude Scale is a calculation based on the seismic moment, a physical property reflecting the size of the rupture.
Seismic moment is determined by three factors: the rigidity of the rocks involved, the total area of the fault that slipped, and the average distance the fault slipped. This approach provides a direct measure of the mechanical work performed by the earthquake, offering a more accurate assessment of large events. An increase of one whole number on the logarithmic magnitude scale represents a factor of about 32 times more energy released.
The Moment Magnitude Scale superseded the older Richter scale, which was developed in the 1930s and based on the amplitude of seismic waves recorded by a specific type of seismograph. The Richter scale “saturates” at higher magnitudes, meaning it underestimates the true size of large earthquakes, typically those above magnitude 7.0. The Moment Magnitude Scale overcomes this limitation because it is based on fundamental physical parameters of the fault rupture, making it the standard for reporting significant seismic events worldwide.
Assessing Local Impact
While magnitude provides a single value for the energy released at the source, the effects felt on the surface vary dramatically by location. This localized impact is measured using the Modified Mercalli Intensity (MMI) scale. The MMI scale is descriptive, using Roman numerals from I (not felt) to XII (catastrophic destruction) to categorize the severity of ground shaking.
Intensity is not calculated from instrumental readings but is assigned based on observed effects, such as the reactions of people, movement of objects, and the extent of damage to buildings. A single earthquake has only one magnitude value, but it has many different intensity values across the affected area. Factors like the distance from the epicenter, the depth of the earthquake, and the local geology—such as soft sediment versus hard bedrock—influence the intensity of shaking experienced.
The Upper Limit of Earthquake Size
The maximum size an earthquake can reach is governed by the physical properties and dimensions of the Earth’s tectonic plates. Since magnitude is directly related to the area of the fault rupture, the maximum size is constrained by the total length and width of existing fault systems. A larger earthquake requires a longer fault segment to rupture, and the rupture must propagate deeper into the crust.
The largest earthquake ever recorded was the 1960 Valdivia earthquake in Chile, which registered a Moment Magnitude of 9.5. This event involved a fault rupture hundreds of kilometers long and tens of kilometers deep along the subduction zone boundary. For an earthquake to reach magnitude 10.0, the rupture would theoretically need to span a length of approximately 1,000 kilometers and involve an unprecedented amount of slip.
Because the Earth’s crust is not a single continuous fault, the size of a potential rupture is limited by the boundaries between major tectonic plates. While the Moment Magnitude Scale is open-ended, the physical constraints of the planet’s geology—the finite length of plate boundaries and the mechanical strength of the rocks—suggest that an earthquake much larger than magnitude 9.5 is highly improbable, placing the practical upper limit close to magnitude 10.0.