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The Modified Mercalli Intensity Scale for Earthquake Insurance Underwriting

Posted by Joe Nolan on March 27, 2018

This past St. Patrick’s Day, you may have felt — or heard about — the small earthquake centered in Mercer Island. Though small, it reminds us that we’re in a region prone to quakes.

In this blog post, we’ll be discussing an underwriting tool that will help you to better understand potential damage to your risks: the Modified Mercalli Intensity (MMI) scale.


Aerial view of Seattle bridge and Mercer Island


The MMI scale was developed to assign a number to designate the intensity of an earthquake at a specific location. Unlike the Richter scale, which is a measure of the magnitude of energy released during an earthquake, the MMI scale measures the strength of shaking produced by the earthquake at a specific location.

The scale takes into account the perceived or observable effects of an earthquake, using mathematical algorithms to assign a Roman numeral value. The MMI ranges from level I (imperceptible shaking) to level XII (catastrophic destruction). The lower degrees reflect how much people feel the quake, the higher numbers are determined by observed structural damage.

The MMI and Richter scales often correlate with one another, but not always. When assigning an MMI number, man-made factors, like building construction and population density, come into play. The depth of the quake and the terrain where it’s felt (hard rock, landfill etc.) are also taken into account. These factors may result in a discrepancy between the Richter magnitude and MMI level.

For example, the April, 2012 Baja California/Mexico earthquake was a magnitude 7.0 on the Richter scale, and an intensity VIII on the MMI, according to the United States Geological Survey (USGS). This was because it was 13 km deep. The March 2018 magnitude 5.7 earthquake in Ndoi Island, Fiji, was 538.7 km deep. Because it was barely felt, the Fiji quake was classified as an intensity II.


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Because MMI measures the actual effects of an earthquake, it is much more useful to insurers in determining the level of damage to structures. Unlike the Richter scale, the MMI varies depending on distance from the earthquake’s epicenter. The further away the structure, the less intense the shaking.

Here’s how the numbers on the MMI scale break down, according to the USGS:

  1. Not felt, except by a very few under especially favorable conditions.
  2. Felt only by a few persons at rest, especially on upper floors of buildings.
  3. 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, people may experience vibrations similar to the passing of a truck.
  4. Felt indoors by many, outdoors by few during the day. At night, some people are awakened. Dishes, windows, and doors are disturbed; walls make a cracking sound. People describe a sensation like a heavy truck striking the building. Standing motor cars are rocked noticeably.
  5. Felt by nearly everyone; many are awakened. Some dishes and windows are broken. Unstable objects are overturned, and pendulum clocks may stop.
  6. Felt by all, many are frightened. Some heavy furniture is moved; a few instances of fallen plaster. Slight damage.
  7. Damage is negligible in buildings of good design and construction. Slight to moderate damage in well-built ordinary structures, and considerable damage in poorly built or badly designed structures. Some chimneys are broken.
  8. Damage is slight in specially designed structures; considerable damage in ordinary substantial buildings with partial collapse. Damage is great in poorly built structures. Fall of chimneys, factory stacks, columns, monuments, and walls. Heavy furniture is overturned.
  9. Damage is considerable in specially designed structures. well-designed frame structures are thrown out of plumb. Damage is great in substantial buildings, with partial collapse. Buildings are shifted off foundations.
  10. Some well-built wooden structures are destroyed; most masonry and frame structures are destroyed with foundations. Rails are bent.
  11. Few, if any (masonry) structures remain standing. Bridges are destroyed, rails are bent greatly.
  12. Damage is total. Lines of sight and level are distorted. Objects are thrown into the air.

The Modified Mercalli Intensity Scale is a valuable tool for underwriting buildings in earthquake-prone areas.  It is available through PropertyEDGE as part of your WSRB subscription. Simply input an address into this valuable tool and the information panel provides access to the MMI Score report.


MMI Image.jpg


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Be sure to check out WSRB’s PropertyEDGE for MMI scoring, fault information, and other tools for assessing risk in the event of an earthquake. For more information on how to improve your underwriting with PropertyEDGE, contact us.

Topics: Insurance underwriting, insurance, earthquake