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 awayMagnitude.- 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 frequencyterrible 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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