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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?
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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.