1. Earthquake magnitude Most related to maximum amplitudes in seismograms. Local Magnitude (M L ): Richter, 1930ies Noticed similar decay rate of log 10 A (displacement) versus distance Defined distance-independent magnitude estimate by subtracting a log 10 A for reference event recorded on a Wood- Anderson seismograph at the same distance M L =log 10 A(in 10 -6 m)-log 10 A 0 (in 10 -6 m) =log 10 A(in 10 -6 m)+2.56log 10 dist (in km) -1.67 for 10<dist<600km only

2. Geol 600 Notable Historical Earthquakes Source mechanisms and body wave radiation patterns http://www-rohan.sdsu.edu/~kbolsen/geol600_nhe_magnitude.ppt

4. Earthquake magnitude Body wave magnitude (m b ): (used for global seismology) m b =log 10 (A/T)+Q(h,  ) T is dominant period of the measured waves (usually P, 1s) Q is an empirical function of distance  and depth h (details versus amplitude versus range) Surface wave magnitude (M s ): (used for global seismology, typically using Rayleigh waves on vertical components) M s =log 10 (A/T)+1.66 log 10  + 3.3 = log 10 A 20 +1.66 log 10  + 2.0 (shallow events only)

5. Insert table showing saturation

6. Earthquake magnitude Saturation problem motivated the moment magnitude M w M w =2/3 log 10 M 0 -10.7 (M 0 moment in dyne-cm, 10 7 dyne cm=1Nm) = M w =2/3 log 10 M 0 -6.1 (M 0 moment in Nm) Scaling derived so M w agree with M s for small events More physical property, does not saturate for large events

7. Earthquake magnitude Intensity scale Measures damage to structures Often used is Mercalli I-XII Can be used to examine historic earthquakes without seismic records For example, eastern US.

9. Intensity Observed Effects I Not felt at all II Felt only by a few individuals, indoors and ﾊ at rest, usually on upper floors of tall buildings. III Felt indoors by many persons, but not necessarily recognized as an earthquake. Chandeliers and hanging plants swing. IV Felt both indoors and out. Feels like the vibration caused by a heavy truck or train passing. Windows rattle. V Strong enough to awaken sleeping persons. Small objects knocked off shelves. Beverages may splash out of cups or glasses on tables. VI Perceptible to everyone. May cause public fright. Pictures fall off walls. Weak masonry cracks. Some plaster may fall from ceilings. VII Difficult to stand upright. Ornamental masonry falls from buildings. Waves may be seen in ponds and swimming pools. VIII Mass panic may occur. Chimneys, smoke stacks and water towers may lean and fall. Unsecured frame houses slide off foundations. IX Panic is general. Heavy damage to masonry structures and to ﾊ underground pipes. Large cracks open in ground. X Many buildings collapse. Water splashes over riverbanks. XI-XII Virtually total destruction.

10. Radiated Energy Log 10 E s (ergs) ~ 5.8+2.4m b ~11.8+1.5M s E s (M s =7)/E s (M s =6)~32 E s (M s =7)/E s (M s =5)~1,000 E s ~0.5  D ave A=  M 0 /2   E s /M 0