GEOL 240Lg 1st Edition Lecture 20 Outline Current Lecture I. Source, Path side effects continuation II. Northridge EarthquakeIII. Chi-Chi EarthquakeIV. Source- Puente Hills thrust fault directivity wave → will hit Los Angeles because motion, etc parallelV. 94 Mw 6.7 Northridgea. 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 citywas not really damaged)c. 99MW 7.6 Chi-Chic.i. not really strong directivityCurrent LecturePath EffectsI. 11 Mw 5.8 Virginiaa. 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 + rigidII. 12 Mw ~7.5 New Madrida. dirt shakes more strongly than bedrockb. most reports coming from people living on dirt next to riversc. 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 + lagoonvoV. → 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 shakec. bay bridge built on wooden piles into mudc.i. new one notc.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
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