Academic journal article Science Scope

Investigating the Mercalli Intensity Scale through "Lived Experience"

Academic journal article Science Scope

Investigating the Mercalli Intensity Scale through "Lived Experience"

Article excerpt

Shortly after almost any perceptible earthquake, the news reports the magnitude of the event based on the Richter scale. While this measure is specific and has deep meaning to most seismologists, the public has very little understanding of how this exponential energy measurement scale relates to the destruction they see via the media or firsthand. Most people understand that a Richter magnitude 7.0 quake will do more damage than a magnitude 6 quake, but they are often puzzled by the variation in damage they see between earthquakes of the same magnitude. (For example, the difference between the Haiti and New Zealand earthquakes that both had magnitudes of 7.0 on the Richter scale.) In addition, there is only one Richter magnitude for each earthquake, which makes no reference to distance from the epicenter. Richter magnitudes are absolute measures of the energy released by an earthquake; geologists also measure an earthquake's intensity, or its effect on the Earth's surface (USGS 2012). There are many intensities for an earthquake, and this scale does not have a mathematical basis. Instead, it is based on recording physical damage and qualitative observations made during or shortly after an earthquake.

As early as 1883, Michele Stefano De Rossi and Francois Alphonse Forel developed a 10-step scale to quantify the intensity of earthquakes based on observed ground motion, destruction, and personal accounts (Bolt 1999). In 1902, the Italian volcanologist Giuseppe Mercalli developed the 12-step Mercalli scale as an improvement to the 10-step Rossi-Forel scale (Musson 2009). In 1931, Mercalli's scale was modified by Harry Wood and Frank Neumann (USGS 2012) into the scale we currently use in the United States. The modified Mercalli (MM) intensity scale is composed of 12 increasing levels of intensity that range from imperceptible shaking to catastrophic destruction and is designated by Roman numerals I through XII. Although qualitative in nature, it can provide a more concrete model for middle and high school students striving to understand the dynamics of earthquake behavior. In Europe, seismologists have continued to refine the Mercalli scale (see generalized time line, Figure 1) and have used the European macroseismic scale since 1992 (Musson 2009).

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How is the scale used?

Immediately after an earthquake, trained observers go into an earthquake-affected area and note the degree of physical destruction and record the observations of eyewitnesses. For example, this is an eyewitness account of the 2010 Christchurch, New Zealand, earthquake:

"The building started to shake, my computer screen started to flicker and then the books started to fly off the shelves so I just dived under my desk. I have experienced a few aftershocks before this, but this was totally different. When I came outside the city looked like a bomb had hit it. There was dust and smoke in the air and bits of glass and rubble falling from the tops of buildings. People were walking around covered in blood and in tears--it was just shocking."

--Christopher Ratcliffe, 27 (Birmingham Post 2010)

These observations and various degrees of destruction are evaluated according to their positions on the modified Mercalli intensity scale and plotted very carefully on a map of the affected area. The points of equal Mercalli intensity are then connected to from isoseismal lines (lines of equal seismic intensity). What commonly occurs is a relatively simple pattern of concentric "contours," with the greatest intensity at the epicenter of the earthquake and progressively lesser intensities at greater distance from the epicenter. Frequently, the pattern of isoseismal lines is elongated along the trace of the fault that caused the earthquake. This allows geologists to deduce the approximate position of the fault and locate it on the map, and is something students will do as part of this activity. …

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