What is MMI?

Scientists and engineers use acceleration or velocity to describe how violently the ground shakes. The Modified Mercalli Intensity scale is used to present shaking severity in a more accessible manner.

The Modified Mercalli Intensity Scale

Acceleration or velocity may accurately describe how violently the ground shakes, but they don't help much when trying to understand what you'll experience. The Modified Mercalli Intensity scale is used to explain shaking severity in a more accessible manner. It is based on the observed effects of an earthquake.

I Not felt Not felt except by a very few under especially favorable conditions.
II Weak Felt only by a few persons at rest, especially on upper floors of buildings. Delicately suspended objects may swing.
III Weak Felt quite noticeably by persons indoors, especially on upper floors of buildings. Many people do not recognize it as an earthquake. Standing motor cars may rock slightly. Vibration similar to the passing of a truck. Duration estimated.
IV Light Felt indoors by many, outdoors by few during the day. At night, some awakened. Dishes, windows, doors disturbed; walls make cracking sound. Sensation like heavy truck striking building. Standing motor cars rocked noticeably.
V Moderate Felt by nearly everyone; many awakened. some dishes, windows broken. Unstable objects overturned. Pendulum clocks may stop.
VI Strong Felt by all, many frightened. Some heavy furniture moved; a few instances of fallen plaster. Damage slight.
VII Very strong Damage negligible in buildings of good design and construction; slight to moderate in well-built ordinary structures; considerable damage in poorly built or badly designed structures; some chimneys broken.
VIII Severe Damage slight in specially designed structures; considerable damage in ordinary substantial buildings with partial collapse. Damage great in poorly built structures. Fall of chmineys, factory stacks, columns, monuments, walls. Heavy furniture overturned.
IX Violent Damage considerable in specially designed structures; well-designed frame structures thrown out of plumb. Damage great in substantial buildings, with partial collapse. Buildings shifted off foundations.
X+ Extreme Some well-built wooden structures destroyed; most masonry and frame structures destroyed with foundations. Rails bent.

Source: The Severity of an Earthquake, a U. S. Geological Survey General Interest Publication. U.S. GOVERNMENT PRINTING OFFICE: 1989-288-913

Magnitude vs. Intensity

The magnitudeDescribes the total amount of energy released by an earthquake. is used to describe the severity of earthquakes. But knowing the magnitude tells you little about how the earthquake might affect you. Every earthquake has a single magnitude, but many different intensities. Intensity describes how violently the ground shakes. The intensity you experience is based on:

  • The earthquake's magnitude
  • How far you are from the fault rupture
  • The type of soil at your location

The table below shows the relationship between acceleration, velocity, and MMI.

Modified Mercalli Intensity Scale