On Shaky Ground: The New Madrid Earthquakes of 1811-1812

Appendix 1: Mercalli Intensity Scale

APPENDIX 1 Mercalli Intensity Scale The Mercalli scale for measuring the intensity of earthquakes was developed in 1902 by an Italian priest and geologist, Giuseppe Mercalli. It was modified in 1931 to include descriptions ofdamage for modern structures. The Mercalli scale is a method ofmeasuring, using personal judgment, the intensity ofground movement and putting it into categories on a scale of I to XII. Thus, at its epicenter a quake may measure XI on the Mercalli scale, but as the movement spreads away from the center, the intensity of movement lessens until it is barely felt, and the Mercalli numbers assigned to the intensity levels decrease. Each level ofintensity on the Mercalli scale is described below. As the numbers increase on the Mercalli scale, the damage for that category includes that listed for that number plus that of all the numbers before it. Movement is not felt except in unusual circumstances. Sometimes animals seem uneasy. II Some movement is felt indoors by a few people, especially on upper floors. Light hanging objects may swing. III Atremor can be felt indoors, as ifa heavy truck had passed on the street, but most people do not recognize the movement as an earthquake. It may be noticeable on upper floors ofbuildings. 95 96 Appendix 1 IV The tremor is felt indoors by many, outdoors by some. It rattles dishes, and people can see movement ofliquids in open containers, such as toilets. v Motion is felt indoors by most, outdoors by many. It awakens people from sleep, overturns small objects, and may stop pendulum docks. Trees shake; there is some broken glass and cracked plaster. VI Movement is felt by everyone, indoors and out. Liquids are set in strong motion, and trees shake. Cracked plaster damages some buildings, and fine cracks may appear in chimneys. Xshaped cracks in brickwalls covered in stucco are typical. Poorly built or older buildings may be heavily damaged. VII This temblor frightens everyone. Brick walls may collapse, and some people find it difficult to stand. Trees are shaken strongly, and waves appear on open water. Damage to poorly built buildings is considerable. Chimneys may be cracked, windows broken, and heavy furniture overturned. VIII Tremorcauses general fright approachingpanicand can befelt by persons driving automobiles. Trees and branches are broken. Sand and mud are spurted in small amounts from the earth. Damage from partial collapse of sturdy buildings occurs. Solid stone walls will be cracked or broken. IX Movement causes general panic. Some frame houses built to withstand earthquakes are thrown out of plumb. Masonry Mercalli Intensity Scale 97 buildings will be damaged as well. Some underground pipes will be broken, and there is serious threat to reservoirs. x Motion cracks the ground. Landslides occur from river banks and steep coasts. The level ofwater in wells is changed; water is thrown up on the banks ofrivers and lakes. There are dangerous cracks in brick walls; most masonry and frame structures are destroyed. Serious damage to dams and embankments occurs, as do open cracks in cement and asphalt roads. XI Quake causes great damage to frame buildings and to most masonry structures as well. Well-builtbridges are destroyed, and pipelines buried in the ground are not usable. Broad fissures appear in the ground, and water mixed with sand and mud is spurted from the ground in large amounts. XII At this intensity, damage is total. Almost every human-made structure is damaged or destroyed. Landslides, rock falls, and falling ofriverbanks occur. Land may be seen to wave. All water bodies, underground and surface, are disturbed and changed. Note: The sources for this explanation of the scale are Otto W. Nuttli, The Effects ofEarthquakes in the Central United States, 2nd ed. (Cape Girardeau, Mo.: Centerfor EarthquakeStudies, 1990), pages 2-3; and William Atkinson, The Next New Madrid Earthquake : A Survival Guide for the Midwest (Carbondale: Southern Illinois University Press, 1989), pages 157-61. ...