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Lecture 20 (November 11)

Lecture 20 (November 11) - how strong the ground is going...

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Read pg. 244-260, pg. 269-281 Path Effects o What happens to that energy from where the earthquake was to where you are o Energy propagates more efficiently through some parts of the earth rather than some other parts Ex. 1906 San Francisco Europe called it a M 8.3, but there was a specific path effect, when it really was a M 7.9 o Energy from California earthquakes propagates efficiently to Europe biased the magnitude Ex. The 3 rd New Madrid eq rang bells and moved furniture in Boston and the eastern U.S. Eastern part of the U.S. is known as a “shield” “Craton”: Extremely old, cold, stable, and rigid core of the continent o Accounted for the damage from the New Madrid eq Tomography o 3D study of Earth’s interior velocity structure Site Effects o One of the most basic controls on damage is bedrock vs. soils What’s directly 40 m or 100 m underneath you is a fundamental control on
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Unformatted text preview: how strong the ground is going to shake where you are • Weak sediments amplify the seismic shaking • 1989 Loma Prieta M W 6.9 o Probably occurred on the Sargent fault, a subsidiary strand of the SAF o Nucleated ~ 15 km NE of Santa Cruz • S.F. Bay o Avg. depth of the Bay is 4 ft, basically a big mudflat o The bay is a drowned, old river channel o Redwood poles in thick mud are the basis of the Bay Bridge because they couldn’t piledrive down to the bedrock • The Cypress Structure o A double-deck freeway in Emeryville (next to Oakland) o The slower the shear wave velocity, the weaker the sediment • 1868 “Great San Francisco Eq” M W 7.0—now called the Hayward Eq o Buildings on bayfill could not withstand earthquakes Relearned same lesson in 1906 Eq...
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