GEOL 240Lg 1st Edition Lecture 20 Outline Current Lecture I Source Path side effects continuation II Northridge Earthquake III Chi Chi Earthquake IV Source Puente Hills thrust fault directivity wave will hit Los Angeles because motion etc parallel V 94 Mw 6 7 Northridge a fault is only in LA system that dips south b Ruptured up the dip of the thrust fault North NW away from Los Angeles why city was not really damaged c 99MW 7 6 Chi Chi c i not really strong directivity Current Lecture Path Effects I 11 Mw 5 8 Virginia a affected an area 4x the size of an earthquake in CA that s 2x the magnitude a i Energy propagates much more efficiently there because old not broken by a bunch of faults cold rigid II 12 Mw 7 5 New Madrid a dirt shakes more strongly than bedrock b most reports coming from people living on dirt next to rivers c affected move all of E C than Mw 7 6 San Francisco 06 d Seismic energy propagates easier in EC d i time most seismeters in EV estimate SF was 8 4 because due to path EC Mississippi path can generate earthquakes affecting all the states in Midwest etc hazard is low but risk is very high California is the opposite Side Effects how local geography controls ground shaking bedrock vs soil sites 89 Mw 7 0 Loma Prieta USGS Mw 6 9 adjacent to SAF mud shakes incredibly strongly 10x higher than bedrock III S houses built on mud in Marina District of SF 2 were destroyed IV until 1915 marina lagoonvo V earthquake happened 100Km away VI Cypress structure double decker freeway a pancake partly built on mud so b vs sheer wave velocity used by engineers to see how ground will shake c bay bridge built on wooden piles into mud c i new one not c ii 1868 Great SF earthquake Mw 7 0 c iii occurred on Hayward Fault c iv as early as 1949 mud used to make land c v sedimentary basins bowls of Jell O VII trap amplify seismic shaking ie Energy LA covered by gravel from San Gabriel Mountains
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