Earthquake Intensity vs. Earthquake Magnitude

Question description

Earthquake Intensity vs. Earthquake Magnitude

Several methods are used to describe the strength of an earthquake. While some methods characterize earthquake strength using a single numerical value (e.g. the moment magnitude or Richter scales), others describe the intensity of a quake using numerical values that can vary across the region. These methods commonly use earthquake damage reports to map out the effect of the quake over large areas.

Modified Mercalli Intensity Scale

Although numerous earthquake intensity scales have been developed over the last several hundred years, the one currently used in the United States is the Modified Mercalli Intensity Scale. This scale, composed of 12 increasing levels of intensity that range from imperceptible shaking to catastrophic destruction, is designated by Roman numerals (I to XII). It does not have a mathematical basis but is based on testimonies of those who experienced the earthquake and observable earthquake damage.

The Modified Mercalli Intensity value assigned to a specific site after an earthquake has a more meaningful measure of severity to the nonscientist than the magnitude because it refers to the effects actually experienced at that place. The maximum observed intensity generally occurs near the epicenter. Intensity values decrease with greater distance from the source. The intensity of shaking is also influenced greatly by the type of underlying material (i.e. soft sediments commonly shake more than hard rock). The lower numbers of the intensity scale generally deal with the manner in which the earthquake is felt by people. The higher numbers on the scale are based on observed structural damage.

October 31, 1895 Earthquake, Central United States

Magnitude 6.8

A moderately strong earthquake struck the Midwest at 5:07 AM (Central Standard Time) October 31, 1895. The quake was centered in the Mississippi valley region and had an estimated magnitude of 6.8 on the Richter scale. It was felt as far away as Washington, D.C. and Buffalo, NY.

Although this quake was widely felt throughout the Midwest, it caused serious damage only in the epicentral area near the confluence of the Ohio and Mississippi rivers. No substantial buildings collapsed, no one was killed, and there were few injuries. Descriptions of the earthquake intensity and damage begin on Page 3 of this exercise.

Exercise Procedure:

1. Beginning on Page 3 are 53 descriptions of the 1895 earthquake taken from newspaper accounts from the day after the event. Read each earthquake report and use the provided Modified Mercalli Scale (Page 2) to determine the intensity value for each location. IMPORTANT: When determining the intensity value for a location, use the descriptions of shaking or damage indicative of the highest Mercalli value. For example, the shaking that will knock down a brick wall (Mercalli VIII), will also cause hanging object to swing (Mercalli III).

3. One you have determined a Modified Mercalli Scale value, mark it down next to the VALUE: label underneath each account. Also list the value on the attached worksheet (Page 9).