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The Modified Mercalli Intensity Scale for Insurance Underwriting

June 11, 2012
modified mercalli intensity scale

A Seattle building after the 2001 Nisqually earthquake

The Modified Mercalli Intensity (MMI) scale is a seismic scale developed and used to assign a numerical designation to the intensity of an earthquake at a specific location. Unlike the Richter scale, which is a measure of the magnitude (energy released during an earthquake), the MMI scale measures the strength of shaking produced by the earthquake. The intensity is evaluated by taking into account the perceived or observable effects of an earthquake and by mathematical algorithms, ranging from an intensity level I – imperceptible shaking, to level XII – catastrophic destruction. The lower degrees on the scale relate to how the earthquake is felt by people, where as the higher degrees are determined by observed structural damage. Although many times there is a correlation between the MMI and the Richter scale, this isn’t always the case.  The depth of the earthquake, terrain, building construction, and population density, among other factors, may cause a discrepancy between the two. For instance, the May, 2011 earthquake in Southern California was a magnitude 0.7, but classified as an intensity III, according to the United States Geological Survey (USGS), because it was only 4 km deep, where as a magnitude 4.5 quake in Salta, Argentina was only classified as an intensity I, because it was 164 km deep and therefore was barely felt.

Because the MMI measures the actual effects of an earthquake, it’s much more useful to insurers in determining the level of damage done to structures. Unlike the Richter scale, the MMI differs depending on your distance from the epicenter, the further away you are the lower your intensity classification will be, so insurers can get a better picture of how a structure will be affected at a specific location.

Here’s how the numbers break down according to the USGS:

I. Not felt except by a very few under especially favorable conditions.

II. Felt only by a few persons at rest, especially on upper floors of buildings.

III. 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. Vibrations similar to the passing of a truck. Duration estimated.

IV. 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. Felt by nearly everyone; many awakened. Some dishes, windows broken. Unstable objects overturned. Pendulum clocks may stop.

VI. Felt by all, many frightened. Some heavy furniture moved; a few instances of fallen plaster. Damage slight.

VII. 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. Damage slight in specially designed structures; considerable damage in ordinary substantial buildings with partial collapse. Damage great in poorly built structures. Fall of chimneys, factory stacks, columns, monuments, walls. Heavy furniture overturned.

IX. 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. Some well-built wooden structures destroyed; most masonry and frame structures destroyed with foundations. Rails bent.

XI. Few, if any (masonry) structures remain standing. Bridges destroyed. Rails bent greatly.

XII. Damage total. Lines of sight and level are distorted. Objects thrown into the air.

For more information on how to use the Modified Mercalli Intensity Scale for your underwriting, contact Tracy Skinner at 206.273.7146.

Find out how to get the Modified Mercalli Intensity Scale through WSRB‘s PropertyEDGE™ software.

 

Article by: April LaRita Green

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

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