GEOL 108Lg 1st Edition Lecture 6Outline of Last LectureI. Earthquake HazardsII. Ground Shaking EstimatesIII. Omori’s Law, OverviewIV. Gutenberg Richter RelationshipV. Review of Magnitude, Moment, Intensity, Shear ModulusOutline Current Lecture I. Earthquake Hazards, Contd.a. Controlling Factors for Ground MotionII. Physical PrinciplesIII. Secondary HazardsIV. Earthquake Prediction/PreparationCurrent LectureEarthquakes Part IVPrimary Hazards: ground shaking – shaking damage; building responses to earthquakes (poorly constructed buildings often fall, despite their age)Building CodesWays to build houses such that they can withstand earthquakes (even strong quakes)Building codes, at times, are not following and thus buildings often collapse when they theoretically shouldn’t.Controlling Factors for Ground MotionsAttenuation-Decay of shaking with distance from Earthquake/ Fault (depending on regional settings)-Distributing energy of shaking earthquake over wider and wider area the further you go away from the fault. The further away, the less ground shaking you will experience. Site effects and amplification Dependence of shaking on local effectsLoose sediment shakes more; things on bedrock shake less (these can go by zip code or city blocks, different responses)Mercalli intensities of two earthquakes of about the same size on the West and East Coast were felt over wider area on east coast due to different rock sites – currently a stable margin on the east coast (oceanic plate) – the plate on the west coast is thinner/hotter, weaker, harder for waves to propagate and spread through a larger area so the earthquakes tend to be more large on the west coast.-“Did you feel it?” – maps are created to survey what people felt; shaking of an earthquake, the extent to which it was felt in certain areas. Amplification:Physical Principles Understanding how rocks deform helps you figure out what happens to a wave if it comes in from bedrock to a sedimentary basin. Shear stress = modulus x strain. Bedrock has a higher elastic modulus than sediment in basin. (The equation – wave comes in, rock is more solid, wavehas to push harder to get through the rock)Mrock > MsedimentThe stress is constant because the stress is a force per area. Rock and sediment stress must be the same, the force imbalance; the sediment would move and adjust until there was a medium. If you push at the base you’ll feel the same force at the surface. E=strainTrock=TsedimentTsed=Msed x Esed=Trock=Mrock x ErockMsed x Esed = Mrock x ErockThe sediment strain / rock strain = Mrock / M sed > 1Sediment strain is LARGER than the rock strain.The ground shaking has to increase when it goes through the sediment, because the sediment ismuch weaker. You drop the resistance to shaking in the sediment, you have to deform the rock alot to get the same stress. Both the rock and the sediments obey the elastic equation. In Los Angeles, we are sitting in a sedimentary basin—the vertical displacements/amplitudes go up drastically in sedimentary areas.Secondary Hazards: One disaster begets another, landslides.Liquefaction: soil has a lot of water, buildings fall overLiquefaction is caused by loose soils (sand or silt) and fluids. When shaken above a certain critical level, soils may then lose all strength and flow. Areas with high ground water and lose soils (saturated) …see slideTsunami: gravity waves in water that are generated by the up and down motion of an earthquake. Huge chunk of seafloor must be lifted (underground landslides, or thrust faults in a subduction zone). Wave propagates at large speed in deep water (speed of an airplane roughly) –the wave height in open water is very minimal, corresponds to the displacement of the ground. When you run into shallow water, you slow down the gravitational waves motion, the wave height increases as it slows down. It’s a basin wide effect that we cannot predict, but once one happened we have a lot of time to warn people sitting by the coast line.Fires: 1906 destroyed San Francisco, the fires created by ruptured gas linesEpidemics: after infrastructure is destroyed, plain drinking water Earthquake PredictionThere have been numerous suggestions for precursory phenomena. Animal behavior, seismicity increase or decrease before the main eventChanges in water levelsElectro-magnetic signalsChanges in seismic wave speedsHazard EstimatesA lot of earthquake hazard assessment has to do with common sense (historically earthquake, more likely to have earthquake – closer to fault, more shaking – attenuation)Plate Boundary Segments and Seismic GapsSeismic gap – you know plate boundary is moving - once in a while you have to make up a slip that is accumulated – if you haven’t had an earthquake in a while, you’re overdue, you need a quake to make up for slip deficit. It’s locked, it has to slide, has to catch up with rest of plate boundary. Resonance phenomenaThe frequency depends on the height ; the taller the building the larger the period. The seismic waves, period between waves – medium level buildings destroyed in Golchuk the frequency character of shaking is importantLateral force resisting systemsMitigation 1: make things strongerHow can we keep structures from collapsing? Make them stronger! Shear walls – less likely to collapse/strongerAbsorb Shaking: Passive Energy DissipationIntroduction of devices that dissipate the earthquake energy in a harmless manner such as friction or deformation of specially located materials. Taking out energy. Convert energy into one for another – structures in a building that are meant to fail - take energyfrom earthquake and turn it into controlled destruction, an element that will break into your building—take up the energy, crumple