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<br /> Anoka County 2019 <br />Multi-Jurisdictional <br />All Hazards Mitigation Plan <br /> <br /> 75 <br />46B4.2.1 Natural Hazards <br /> <br /> <br />79B4.2.1.1 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 /> <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 /> <br /> <br />80B4.2.1.2 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 <br />dam break floods. <br /> <br />Riverine floods result from precipitation over <br />large 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 /> <br />Areal floods Areal Flood Warning is issued for <br />flooding that occurs more gradually, normally from prolonged and persistent moderate to heavy <br />rainfall. <br /> <br />Dam break floods are usually the result of intense rainfall producing flooding larger than dam <br />design, faulty design, construction, or operational inadequacies. <br /> <br />