GEOL 240Lg 1st Edition Lecture 17 Outline Current Lecture I Measuring Earthquake Magnitude How Big is Big II Magnitude III Intensity and damage of the earthquakes Current Lecture I INTENSITY AND MAGNITUDE II Intensity is a measure of the degree of shaking at a certain location during an earthquake III Therefore at distances further from the epicenter the intensity may decrease IV In the Modified Mercalli Intensity scale commonly called the MMI scale intensity is measured from I to 12 XII being the highest intensity V There has been major earthquakes of this magnitude in California Keep in mind of those earthquakes the professor mentioned VI It s a closed scale see the description of the levels of the MMI scale in your book An intensity of XII means a disaster VII Because intensity measures the degree of ground shaking during an earthquake it obviously depends on where you are located relative to the hypocenter Intensity information in the form of an isoseismal map is important for the assessment of damage from an earthquake An isoseismal map is a contoured map a given contour shows regions of equal intensity of shaking The lines separating these regions are called isoseismal lines which separate regions of different intensities Keep in mind these are very important so remember them It is important to note that intensity is NOT the same measure as magnitude Magnitude measures the size or the total energy release of an earthquake while intensity measures the degree of ground shaking at a given point due to an earthquake Each earthquake has only one magnitude but can have multiple intensities Magnitude is an open scale defined by the logarithm of the amplitude of the motion In fact the formula for determining local or Richter magnitude is ML logA 3log 8 tS tP 2 92 where A is the amplitude measured in millimeters Being an open scale it can range from to but is usually between 0 and 9 for most recorded earthquakes earthquakes When an earthquake is smaller than magnitude 3 are hardly consequential and earthquakes much larger than 9 are extremely rare see USGS chart on the final page of this handout Earthquakes with magnitude greater than 9 5 have not been recorded
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