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## Intraplate Earthquakes

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**Intraplate Earthquakes**• New Madrid, Missouri: December 1811, January 1812, February 1812**Intraplate Earthquakes**• Eastern U.S. earthquakes less frequent, but more widely felt • Crust is older and transmits stress**Earthquake Size and Characteristics**• How big is it? • Answer in terms of perceived effects: intensity • Answer in terms of amount of energy released: magnitude**Earthquake Intensity**• Mercalli Intensity Scale developed in 1902 • Based on effects • Local small quake is similar to distant large quake**Felt effects using Mercalli Scale**http://www.scec.org/instanet/01news/images/NorthridgeSMap.gif**Felt effects using Mercalli Scale**Hypothetical M7.8 earthquake**Landers 1992 earthquake:**Felt intensity by zip code**Earthquake Magnitude**• Richter Magnitude Scale (ML) derived by Charles Richter in 1935 • Uses maximum amplitude of earthquake waves on seismograph • Logarithmic scale • Richter magnitude less accurate above M6.5**Problems with scales**• Moment magnitude is measure of total energy expended during earthquake • Determined from long-period waves • Moment = (shear strength of rocks) x (surface area of rupture) x (slip distance on fault) • Each number is 32X energy • Each 2 numbers =1000X • This is the most common scale for quakes >3.5**1995**Kobe earthquake, Japan**1989**Loma Prieta earthquake, SF Bay Area**Liquifaction**Water in sediment causes solid rock to behave like a liquid.**Secondary Ground Effects**• Surface rupture-scarp • Earthquakes often trigger landslides • Can also cause liquefaction • Soils become almost liquid when shaken, solidify when shaking stops • Significant damage to structures atop liquefied sediments • Fires**1906 earthquake surface rupture.**8’ fence offset above http://mnw.eas.slu.edu/Earthquake_Center/1906EQ/1906thumb.html And http://quake.wr.usgs.gov/info/1906/images/fenceoffset_big.html**Liquifaction**Water in sediment causes solid rock to behave like a liquid.**This residential and commercial building sank more than**three feet into the partially liquefied soil.**San Francisco 1906 M8.3**Secondary effects: Fire**Long term probability**• Probability of where and when an earthquake will strike used to construct risk map**Las Cruces:**15-18% chance of M5 in 50 yrs.