How Earthquakes Are Measured and Their Effects on St. Charles County

Earthquakes are commonly measured by their magnitude and intensity.

Magnitude is a measure of the total energy released during an earthquake. It is determined from a seismogram, which plots the ground motion produced by seismic waves.

On the Richter scale, each whole-number step represents an approximate thirty-two fold increase in released energy. For example:

6.0 is equal to 32 times the energy of a 5.0
1,000 times the energy of a 4.0 and
32,000 times the energy released by a 3.0

(small earthquakes do not release enough energy to prevent larger ones)

Two tenths rule:
Every two-tenths of a unit represents double the energy released at the focus.

5.0 to a 5.2 is twice as big
5.4 is four times as large as a 5.0
5.6 is eight times as large as a 5.0

Scientists also guage earthquakes by intensity, which is the degree of damage from an earthquake at a particular location. The intensity scale, the Modified Mercalli Scale, is divided into 12 degrees, each identified by a Roman numeral.

The effects of an earthquake will be dependant upon where you live. In St Charles County we have seen over the years that a smaller earthquake, such as a 4.3 can be felt by those who live close to the rivers, in the floodplain, and not felt by anyone else. This is due to the geological makeup of the ground, nearest the river more sand and more shallow water tables. When earthquake vibrations pass through soil which has a high liquid water content, the soil loses the properties of a solid and takes on those of a semi-liquid, like quicksand or pudding, this process is called liquefaction. The foundations of heavy buildings suddenly lose the support of the soil, and they may topple, or settle deeper into the Earth. Further west is more solid with clay and bedrock.

Within the past 15 years building codes have become more strict. Those buildings built within this period will have a better chance of riding out an earthquake, however, this does not mean that it will not sustain any damage. Certainly the older the structure the more likely it could see significant damage. But as we all know this area has not experienced a major earthquake since 1811-1812.

(The following is a reprint of "Modified Mercalli Intensity Scale of 1931 [adapted from Sieberg's Mercalli-Cancani Scale, Modified and Condensed]," Earthquake Information Bulletin, July-August 1977, 30-31)

1. Not felt -- or, except rarely under especially favorable circumstances. Sometimes birds, animals, reported uneasy or disturbed;
   Sometimes dizziness or nausea experienced;
   Sometimes trees, structures, liquids, bodies of water, may sway;
   Doors may swing, very slowly.
   
   
2. Felt indoors by few, especially on upper floors, or by sensitive or nervous persons.
   Sometimes hanging objects may swing, especially when delicately suspended;
   Sometimes trees, structures, liquids, bodies of water may sway, doors may swing very slowly;
   Sometimes birds, animals, reported uneasy or disturbed;
   Sometimes dizziness or nausea experienced.
   
   
3. Felt indoors by several, motion usually rapid vibration (vibration like that due to passing of light or lightly loaded trucks, or heavy trucks some distance away)
   Hanging objects may swing slightly.
   Rocked standing motor cars slightly.
   
   
4. Felt indoors by many, outdoors by few.
   Awakened few, especially light sleepers, frightened no one, unless apprehensive from previous experience.
   Vibration like that due to passing of heavy or heavily loaded trucks. Sensation like heavy body striking building, or falling of heavy objects inside.
   Rattling of dishes, windows, doors; glassware and crockery.
   Creaking of walls, frame structures.
   Hanging objects swung, in numerous instances.
   Slightly disturbed liquids in open vessels.
   Rocked standing motor cars noticeably.
   
   
5. Felt indoors by practically all, outdoors by many or most;
   Awakened many or most.
   Frightened few, slight excitement, a few ran outdoors.
   Buildings trembled throughout.
   Broke dishes, glassware, to some extent.
   Cracked windows, in some cases, but not generally.
   Overturned vases, small or unstable objects, with some falling.
   Hanging objects, doors, swung generally or considerably.
   Knocked pictures against walls or swung them out of place.
   Opened or closed doors, shutters, abruptly.
   Pendulum clocks stopped, started, or ran fast or slow.
   Moved small objects, furnishings, the latter to slight extent.
   Spilled liquids in small amounts from well-filled open containers.
   Trees, bushes, shaken slightly.
   
   A 6.0-6.9 Richter Magnitude could result in the following effects on St Charles County:
   
   
6. Felt by all, indoors and outdoors. Frightened many, excitement general, some alarm, many ran outdoors. Awakened all.
   Persons made to move unsteadily.
   Trees, bushes, shaken slightly to moderately.
   Liquid set in strong motion.
   Small bells rang, church, chapel, school, and so forth.
   Damage slight in poorly built buildings.
   Fall of plaster, in small amounts.
   Cracked plaster somewhat, especially fine cracks in chimneys and in some windows.
   Fall of knick-knacks, books, pictures.
   Overturned furniture in many instances.
   Moved furnishings of moderately heavy kind.
   
   A 7.0 - 7.9 Richter Magnitude could result in the following effects on St. Charles County:
   
   
7. Frightened all, general alarm, all ran outdoors.
   Some or many found it difficult to stand.
   Noticed by persons driving motor cars.
   Trees, bushes shaken moderately to strongly.
   Waves on ponds, lakes, running water.
   Water turbid from mud stirred up.
   Incaving to some extent of sand or gravel stream banks. (Along the rivers in the flood plain liquifaction could occur. This means that firm soil becomes semi-liquid, resembling quick sand and causing landslides, mudflows and sand blows. Fine wet sandy, alluvial [sandy soils with shallow water tables] are more prone to "liquifaction" .Areas along the Mississippi and Missouri rivers are most prone.)
   Rang large church bells, and so forth.
   Suspended objects made to quiver.
   Damage negligible of good design and construction, slight to moderate in well-built ordinary buildings, considerable in poorly built or badly designed buildings, adobe houses, old walls (especially where laid up without mortar), spires and so forth.
   Cracked chimneys and walls to some extent.
   Cracked plaster in considerable to large amounts, also some stucco.Broke numerous windows and furniture.
   Shook down loosened brickwork and tiles.
   Broke weak chimneys at the roof line, sometimes damaging roofs.
   Fall of cornices from towers, high buildings.
   Dislodged bricks and stones.
   Overturned heavy furniture, with damage from breaking.
   Damage considerable to concrete irrigation ditches.
   
   A 8.0 - 8.9 Richter Magnitude could result in the following effects on St. Charles County:
   
   
8. Fright general, alarm approaches panic.
   Disturbed persons driving motor cars.
   Trees shaken strongly, branches, trunks, broken off, especially palm trees.
   Ejected sand, mud, in small amounts.
   Changes temporary to permanent in flow of springs and wells; dry wells renewed flow; change in temperature of spring and well waters.
   Damage slight in structures (brick) built especially to withstand earthquakes. Considerable in ordinary substantial buildings, partial collapse; wooden houses in some cases; threw out panel walls in frame structures, broke off decayed pilings.
   Fall of walls.
   Cracked, broke solid stone walls seriously.
   Ground can become wet including on steep slopes.
   Twisting, fall of chimneys, columns, monuments, also factory stacks, towers.
   Moved conspicuously, overturned, very heavy furniture.
   
   
9. Panic general.
   Cracked ground conspicuously in (masonry) structures built especially to withstand earthquakes; threw out of plumb some wood-frame houses built especially to withstand earthquakes; great in substantial (masonry) buildings, some collapse in large part; or wholly shifted frame buildings off foundations, racked frames;
   Serious to reservoirs; underground pipes sometimes broken.
   
   
10. Cracked ground, especially where loose and wet, up to widths of several inches; fissures up to a yard in width ran parallel to canal and stream banks.
    Landslides considerable from river banks and steep coasts.
    Shifted sand and mud horizontally on beaches and flat land.
    Changed level of water in wells.
    Threw water on banks of canals, lakes, rivers, and so forth.
    Damage serious to dams, dikes, embankments.
    Severe to well-built wooden structures, bridges, some destroyed.
    Developed dangerous cracks in excellent brick walls.
    Destroyed most masonry, frame structures, also their foundations.
    Bent railroad rails slightly.
    Tore apart, crushed endwise, pipe lines buried in earth.
    Open cracks and broad wavy fold in cement pavements and asphalt road surfaces.