Earthquakes are described using two distinct measurements: magnitude and intensity. While both relate to the size and effect of a seismic event, they measure separate aspects of the phenomenon. Magnitude quantifies the physical energy released at the earthquake’s source. Intensity describes the severity of the shaking and the resulting damage experienced at a specific location on the Earth’s surface. This distinction explains why a single earthquake yields one magnitude value but can produce numerous intensity values across a region.
Magnitude: Measuring Energy at the Source
Magnitude is a fixed, objective number representing the total energy released at the point where the fault ruptures deep within the Earth’s crust. It is a measurement of the earthquake itself and remains constant regardless of where the measurement is taken on the surface. Seismologists calculate this value using instruments that record the seismic waves generated by the event.
The standard measurement used today is the Moment Magnitude Scale (\(\text{M}_\text{w}\)). This scale replaced the older Richter Scale for reporting large earthquakes, typically those above magnitude 4. The Moment Magnitude Scale is based on the seismic moment, a physical quantity that incorporates the rigidity of the rocks, the area of the fault that slipped, and the average distance of that slip.
This scale is logarithmic, meaning that each whole-number increase represents a significant jump in energy release. For example, an increase of one whole number corresponds to approximately 32 times more energy released. A magnitude 7.0 event, therefore, releases about 1,000 times more energy than a magnitude 5.0 event.
Intensity: Measuring the Impact on the Ground
Intensity is a descriptive measure of the severity of ground shaking and the resulting effects on people, buildings, and the environment at a specific location. Unlike magnitude, intensity is not a single value for an earthquake; it varies widely depending on the distance from the source and the local ground conditions. It is a measure of the effect, not the cause.
The severity of the shaking is assessed using the Modified Mercalli Intensity (MMI) Scale. This scale assigns a Roman numeral from I (not felt) to XII (catastrophic destruction) to describe the observed effects. The MMI scale is based on qualitative observations, such as whether people were awakened, the movement of furniture, or the degree of structural damage.
The MMI Scale has no mathematical basis but provides a relatable measure of severity for the public because it directly ties to the experience of the shaking. Seismologists use reports from the affected area, including observed damage and human accounts, to assign the appropriate intensity value for a specific community or site. The highest intensity is typically found near the earthquake’s epicenter, gradually decreasing with distance.
Why Local Conditions Affect Intensity
A major earthquake with a single, fixed magnitude can produce different intensity values across a region due to varying local conditions. The distance from the earthquake’s hypocenter, the point of origin below the surface, is a primary factor, as seismic wave energy naturally dissipates the farther it travels. The geology of the surface layers also plays a significant role in modifying the shaking.
Areas built on soft sediments, such as sand or unconsolidated fill, often experience greater shaking than areas built on solid bedrock. The soft materials can amplify the seismic waves, increasing the intensity of the ground motion, a phenomenon known as site amplification. Furthermore, water-saturated, loose granular soils can be susceptible to liquefaction, where the ground temporarily loses its strength and behaves like a liquid.
The quality of construction heavily influences the final intensity value observed in a community. Modern, earthquake-resistant buildings might experience minimal damage compared to older, unreinforced structures built nearby, which could suffer partial or total collapse. These local variations mean that an earthquake of magnitude 6.0 might cause a strong MMI VI in a town built on bedrock but a destructive MMI IX in a neighboring town built on soft river sediment.
A Side-by-Side Comparison
Magnitude and intensity serve distinct and complementary purposes in the study and reporting of earthquakes. Magnitude is an instrumental measure that quantifies the inherent size of the event by calculating the energy released at the source. It is represented by a single, objective number, such as \(\text{M}_\text{w}\) 7.2, determined using seismographs.
Intensity, conversely, is a descriptive measure that quantifies the effects and damage caused by the shaking at a particular surface location. It is represented by a range of values, typically Roman numerals from I to XII on the Modified Mercalli Intensity Scale. While magnitude is fixed for a given event, intensity changes based on distance, local geology, and building vulnerability.