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Anoka County Multi- Jurisdictional <br />All Hazards Mitigation Plan <br />4.2.1 Natural Hazards <br />4.2.1.1 Earthquake <br />An earthquake is a naturally induced shaking of <br />the ground, caused fractures and sliding of rock <br />within the Earth's crust. Earthquake magnitude <br />is determined by the dimensions of the rupturing <br />fracture (fault) and the amount of displacement <br />that takes place. The larger the fault surface and <br />displacement, the greater the energy produced. <br />This energy produces shaking and a variety of <br />seismic waves that radiate throughout the Earth. <br />Earthquake magnitude is measured using the <br />Richter Scale Table (referenced at 4.3.1.1) and <br />earthquake intensity (how strong an earthquake <br />was felt at a given site) is measured using the <br />Modified Mercalli Intensity Scale. <br />Most property damage and earthquake- related deaths are caused by the failure and collapse of <br />structures due to ground shaking. The level of damage depends upon the amplitude and <br />duration of the shaking, which are directly related to the earthquake size, distance from the fault, <br />site, and regional geology. Other damaging earthquake effects include landslides and <br />liquefaction. Earthquakes can affect hundreds of thousands of square miles, cause damage to <br />property measured in the tens of billions of dollars, loss of life and injury to hundreds of <br />thousands of persons, and disrupt the social and economic functioning of the affected area. <br />4.2.1.2 Flooding /Flash Flooding <br />Flooding is an overflowing of water onto normally <br />dry land and is one of the most significant and <br />costly of natural disasters. Three principle types <br />of floods are riverine floods, flash floods, and dam <br />break floods. <br />Riverine floods result from precipitation over large <br />areas and occur in river systems whose <br />tributaries may drain large geographic areas. <br />Flash floods usually result from a torrential rain <br />on a relatively small drainage area and produce <br />localized floods of great volume and short <br />duration. <br />Dam break floods are usually the result of intense <br />rainfall producing flooding larger than dam <br />design, faulty design, construction, or operational inadequacies. <br />4.2.1.3 Landslides /Mudslides <br />Landslides (rockslides, mudslides, etc.) are among the most common natural hazards. Unlike <br />most natural hazards, however, most damage is not caused by extreme events, but by <br />uncounted (and often unreported) minor events. <br />76 <br />