facts and theories

facts and theories - Source path and site effects(biased...

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Source, path and site effects: (biased magnitude) Source effect: size, magnitude, distance, location, frequency content, directivity, duration(big eqs. tend to last longer) Path effect: for 1906 SF eq., most seismometers are located in northern Europe, and some in Japan and U.S. East coast magnitude overestimated due to path effect preferential energy propagation to Europe Site effect: one of the most basic controls of eqs. what’s 100 feet beneath you bedrock VS. soil basin (resonance), water-saturated sediments amplify and trap seismic waves, much longer and stronger shaking(amplitude of ground motion can be 10 times greater) 1812 New Madrid eq.: erroneous interpretation by people living along water- saturated site Loma Prieta eq.: bedrock MMI 6VS. MMI 9/10 in downtown Santa Cruz due to young, unconsolidated river gravels SF Marina District: on landfill and is susceptible to liquefaction during eqs. Shear wave velocity is a good proxy for stability: the slower the Vs the weaker the sediment 5 ma ago there’s no compressional component motion of Pacific Plate and NA plate are r-l s-s and slightly opening(normal) basin and range Liquefaction: shaking of water-saturated sediments particles and water originally held together by gravity during shaking incompressible H2O is pressurized and it in turn exerts outward force and coherence lost behave like a liquid, buildings tilted over and collapsed example: 1964 Nigata eq. and 1692 Port Royal, Jamaica(Caribbean Plate) eq. entire city submerged Mainshock: the largest event in a cluster that sometimes includes foreschocks and almost always includes aftershocks Foreshocks: smaller-magnitude event that precedes the mainshock, to be a foreshock, it must be in approximate location as the mainshock (within <1 fault length), within the same rough time period (seconds before mainshocks to months before mainshocks (?)) You do not know it’s a foreshock until/unless you have a subsequent mainshock (in the nearby area in the same amt. of time) example: April 1992, eq. in Joshua Tree (near the southern end of Lenders) Mw 6.1 then in July 1992: Lenders eq. Mw 7.3 Joshua Tress eq. is a classic foreshock of Lenders eq. S. Cali: 5% chance of having a foreshock Normal faults eqs usually have foreshocks, s-s faults sometimes, reverse rarely why? Reverse fault eqs commonly (not always) nucleate near the bottom of the seismogenic zone (i.e. near the bottom of the fault) so reverse flt. eqs tend to be deep eqs. Eqs with shallow hypocenters usually have foreshocks, deep eqs never have
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This note was uploaded on 01/28/2009 for the course GEOL 240 taught by Professor Sammis during the Fall '08 term at USC.

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facts and theories - Source path and site effects(biased...

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