Modified Mercalli Scale Anti-Earthquake Design

Anti-Earthquake Design

Modified Mercalli Scale, scale for measuring the intensity of earthquakes, adapted from the original Mercalli scale. The Mercalli scale was devised in 1902 by Italian seismologist Giuseppe Mercalli.

American seismologists Harry O. Wood and Frank Neumann created the Modified Mercalli scale in 1931 to measure the intensity of earthquakes that occur in California. The Modified Mercalli scale, or a scale similar to it, is now used worldwide. The scale has 12 levels of intensity. Each level is defined by a group of observable earthquake effects, such as shaking of the ground and damage to structures such as buildings, roads, and bridges. The levels are designated by the Roman numerals I to XII. Levels I through VI are used to describe what people see and feel during a small to moderate earthquake. Levels VII through XII are used to describe damage to structures during a moderate to catastrophic earthquake. On average, about one earthquake of level X to XII occurs worldwide every year; 10 to 20 earthquakes of level VII through IX occur each year; and over 500 earthquakes of level I to VI occur every year. Each year over 100,000 earthquakes occur that are not noticed by the human population and therefore are not rated on the Modified Mercalli scale.

Earthquake intensity is a measure of the effects of an earthquake in a particular place. Scientists who study earthquakes, known as seismologists, do not need special equipment or instruments to use an intensity scale. Seismologists can use recorded observations and intensity scales to compare the sizes of earthquakes that have occurred throughout history. However, they cannot use intensity scales to measure earthquakes that occur on the ocean floor because there are no people on the ocean floor to observe the effects of earthquakes there.

Seismologists use other scales to classify the magnitude of earthquakes. Magnitude is a measure of the strength of an earthquake, or the amount of strain that rocks in Earth’s crust release when an earthquake occurs. The Richter scale and the moment magnitude scale are used to measure the magnitude of earthquakes. Seismologists use sensitive instruments called seismographs to measure earthquake magnitudes and patterns and to locate the source of an earthquake. Seismographs also help to locate an earthquake’s epicenter, the point on the surface of the earth directly above the earthquake’s focus, the point within the earth where an earthquake originates. Seismologists even put seismographs in places where there are no people to observe the effects of earthquakes.

The surface effects of an earthquake, such as shaking of the ground, lessen with distance from the epicenter of the earthquake. Hence, an earthquake’s Modified Mercalli rating depends on where the earthquake is measured. Close to the epicenter of a moderate earthquake, ground motion is felt by nearly everyone. Landslides occur on steep slopes, ground cracks are widespread, and some buildings and other structures suffer significant damage. Farther from the epicenter, fewer people feel the earthquake. Level I on the Modified Mercalli scale is defined as “not felt except by a very few under favorable conditions.” Seismologists use level I to describe very small earthquakes. At the other end of the scale, level XII describes very large, catastrophic earthquakes that cause “total destruction.” Earthquakes of intensities II and III are roughly equivalent to earthquakes of magnitude 3 to 4 on the Richter scale. Intensity levels XI and XII on the Modified Mercalli scale are similar to magnitudes 8 to 9 on the Richter scale.

The main flaw of the Modified Mercalli scale is its subjectivity, or its reliance on the opinions of human observers. No instruments are used to measure ground motion. Instead, seismologists who use the Modified Mercalli scale gather information after an earthquake by means of letter questionnaires sent to earthquake victims or reports from the local population. After assigning Mercalli numbers to points in the affected areas, a seismologist draws contours, called isoseismal lines, on a map to separate places of equal intensity. The pattern of isoseismal lines indicates the regions of greatest shaking and provides evidence that can be used to locate the epicenter. If geological conditions such as soil composition and rock structure are similar near the epicenter, the isoseismal lines create a uniform, circular pattern. However, isoseismal lines are usually irregular in shape because the geology and soil conditions of the affected areas vary.