Earthquake intensity describes the effects of ground shaking on people, buildings, and the environment at a specific location. It measures the severity of an earthquake’s impact, which is fundamentally different from the energy released by the event itself. Understanding intensity helps scientists and the public assess the actual danger and potential damage an earthquake poses across a geographical area.
Defining Intensity vs. Magnitude
Earthquake intensity and magnitude are often confused, but they describe two separate properties of an earthquake event. Magnitude measures the total energy released at the source of the earthquake, deep underground, and is calculated using instrumental recordings like seismographs. This value, typically expressed on the Moment Magnitude Scale, is constant and singular for any given earthquake, representing its overall size.
In contrast, intensity measures the effects of the shaking at any specific point on the Earth’s surface and is highly variable across a region. A single earthquake produces a wide range of intensity values depending on the distance from the fault and local ground conditions. Intensity is determined by observational data, such as human perception, the movement of objects, and the degree of structural damage.
The difference can be thought of using a lightbulb analogy: magnitude is the wattage, which is fixed and represents the total energy available. Intensity is how brightly that light illuminates various rooms, changing based on distance and local factors. Seismologists use magnitude to understand the rupture physics and intensity to understand the practical impact on communities.
The Modified Mercalli Scale
The most commonly used method for quantifying earthquake intensity is the Modified Mercalli Intensity (MMI) Scale. This descriptive, non-mathematical ranking system uses Roman numerals from I to XII to categorize the observed effects of ground shaking. The ratings are based purely on qualitative observations of what people felt and the resulting damage to structures and the natural environment.
The lower end of the scale describes how people perceive the shaking. Intensity I is not felt except by a very few people under favorable conditions. Intensity V, a mid-range value, is felt by almost everyone, awakens sleeping people, and overturns small objects. These lower numbers primarily rely on human reports gathered through public surveys.
The higher Roman numerals focus on structural damage, providing a measure of destructive power. Intensity VII indicates difficulty standing and causes slight damage to specially designed structures, but considerable damage to poorly built ones. At the highest end, Intensity X describes a scenario where most masonry and frame structures are destroyed with their foundations. Structural engineers frequently contribute information to accurately assign values of Intensity VIII and higher.
Factors Driving Local Shaking
The intensity value assigned to a location is determined by a combination of variables that influence the seismic waves as they travel. A primary influence is the distance from the earthquake’s hypocenter—the point where the rupture begins—as seismic waves naturally lose energy the farther they travel. The depth of the hypocenter also matters; a shallower earthquake produces more intense surface shaking compared to a deeper one of the same magnitude.
Local geological conditions are a significant factor in determining the final intensity reading at a specific site, often called the site effect. When seismic waves transition from hard bedrock to softer materials like thick layers of sediment or loose fill, the shaking can be amplified. The soft soil vibrates more intensely and for a longer duration than the underlying rock, substantially increasing the severity of the shaking.
Areas built on soft, water-saturated soils may experience liquefaction, where the ground temporarily loses its strength and behaves like a liquid, leading to extreme intensity ratings. Consequently, two nearby neighborhoods can experience vastly different intensity values for the same earthquake simply because one is built on solid rock and the other is built on landfill. These local factors create the irregular pattern of shaking intensity observed in earthquake maps.