GEOL 240Lg 1st Edition Lecture 18 Outline Current Lecture I Why Earthquake Damage Occurs Where it Does Source Path and Site Effects II We will quickly go over the magnitude and the different intensities of different earthquake Current Lecture If time delay 2 seconds 10 km away If time delay 10 sec 100 Km away if time delay 18 sec 150 Km away Magnitude How big is big yes again earthquake magnitude The Ritcher Charles Ritcher 1930s professor Caltech assigns 1 number to an earthquake based on maximum amplitude of seismic waves recorded on a specific type of seismogram at a prescribed epicentral distance original Ritcher scale based on largest 1 sec period S wave recorded on a wood Anderson torisonal seismometer at a distance of 100km from epicenter measures twisting of ground good for measuring s0waves Ritcher based his scale on powers of 10 amplitude of largest S wave in a m 4 earthquake is 10 times bigger than in a m 3 earthquake he called this number magnitude term magnitude comes from astronomy measuring starlight also borrowed logirithonic scale the smallest S wave he could measure yes with a ruler was 1mm he called this Ao magnitude 0 smallest eq Ritcher could measure Ritcher scale is theoretically on open ended scale no max or min magnitude Mag in real earth however there are physical limits on max min eq size Magnitude of an earthquake is strongly dependent on area of fault that slips Practical limits on big and small end M 0 to M10 Ml based on high frequency 9 sec S waves but big eqs release much of their energy low frequency small earthquakes at high frequency high earthquakes release energy at low frequency terrible job at measuring big earthquakes above ML 6 5 7 it saturates above that it wildly underestimates magnitude surface wave magnitude MS 20 sec sometimes 40 sec surface waves Much better measuring bigger earthquakes and distant earthquakes low frequency energy are better
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