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I"ln\ ''1 (' unr \ . ~ " l;" I The Magnitude of a seismic event is most commonly measured by the Richter Magnitude Scale, developed in 1935 by Charles Richter of the California Institute of Technology. Using the Richter Scale, Magnitude is expressed in whole numbers and decimals. An earthquake of 5.0 is a moderate event; 6.0 is a strong event; 7.0 is a major earthquake, and a great earthquake exceeds 8.0. The Intensity level is most commonly measured by the Modified Mercalli Intensity Scale (MMI). This scale is composed of 12 increasing levels of intensity ranging from imperceptible to catastrophic. These levels are an evaluation of the severity of ground motion at a given location measured relative to the effects of the earthquake on people and property. Surface faulting can occur as the result of an earthquake and is defined as the differential movement of the two sides of a fracture. Faults actually occur deep within the earth, but their effects on the surface can be severe. Surface faulting is a hazard to structures built across active faults. Surface faulting can damage railways and highways and buried infrastructure such as pipelines and tunnels. Ground failure is another potential effect of an earthquake due to liquefaction. Liquefaction is caused when clay-free soil deposits, primarily water-saturated sand and coarse silts, react to vibrations, temporarily lose strength, and behave as viscous fluids. Liquefaction takes place when seismic shear waves pass through a saturated granular soil layer, distort its granular structure, and cause some of the void spaces to collapse. I Deaths and injuries from surface faulting are unlikely, but casualties can occur through damage to structures. A variety of structures have been damaged by surface faulting including houses, apartments, commercial buildings, nursing homes, railroads, highways, tunnels, bridges, canals, storm drains, water wells, waterlines, gaslines, and sewer lines. Damage to these structures has ranged from minor to very severe. An example of severe damage occurred in 1952 when three railroad tunnels in California were badly damaged. As a result, traffic on a major line linking northern and southern California was stopped for 25 days, despite an around the-clock repair effort. The zone of greatest seismic activity in the United States is along the Pacific Coast in Alaska and California. However, the Central and Eastern States have also experienced seismic activity: the Boston vicinity (1755); the central Mississippi Valley at Madrid, MO; (1811- 1812) Charleston, SC (1880's) and Hebgen Lake, MT (1959). I Hays counly, Texas Miligalion Plan 90