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Chapter 2

Chapter 2.docx

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Alireza Ghaffari

Risks and Disasters Reading #2 (Module 1) Chapter 2 - Earthquakes = greatest natural hazards - Consequences depend on: o Its magnitude and depth o Distance from populated areas o Nature of local earth materials o Engineering and construction practices - Large differences in # of casualties and damage b/c they occurred at different depths within Earth o Deeper = less damage - Also building and zoning regulations AND poor land use (landslide in El Salvador after earthquake) 2.1 Introduction to Earthquakes - Earthquakes: rupture of rocks along a fracture in Earth’s crust cause rocks on opposite sides to move suddenly o Energy is released as waves - Epicenter: the point on the face of the earth directly above the fault rupture - Focus/Hypocenter: location of the initial rupture along the fault directly below the epicenter o Seismic waves radiate outward in all directions from the focus - Seismologist: scientists who study earthquakes - Magnitude: express amount of energy and earthquake releases - Intensity: measure of the effects of a quake on people and structure - Seismograph: instrument that records earthquake ground motions Earthquake Magnitude - Expressed as a decimal number - Richter scale: quantifies the magnitude of local earthquakes as the logarithm to the base 10 of the maximum signal wave amplitude recorded on seismogram at distance of 100 km from the epicenter o First quantitative magnitude scale - Newer scales: o Body-Wave scale (M )b– based on strength or a type of earthquake wave that travels through Earth (P wave) and is used to measure the magnitude of deep earthquakes o Surface-wave scale (M s – based on earthquake waves that travel along the Earth’s surface - Most commonly used = Moment magnitude (M ) W o Determined from an estimate of the area that ruptured along a fault plane + amount of movement/slippage along fault + rigidity of the rocks near the focus o 1 whole number increase = 10x the amount of shaking, 32x the amount of energy o Equal in magnitude to Richter scale EXCEPT for very large earthquakes - Earthquakes given adjectives: o Major = M 7 – 7.9  most damaging o Strong = M 6 – 6.9  just as damaging abut smaller area o Great (M8) & Giant (M9)  uncommon o Minor (M3)  VERY common Earthquake Intensity - Intensity of an earthquake at any location depends on magnitude, distance from the epicenter and the nature of the ground at the site - Modified Mercalli Intensity Scale: measures degree to which earthquake affects people, property and the ground o 12 categories assigned w/ Roman numerals o Each category has description of how people perceive shaking/damage to buildings - Modified Mercalli Intensity maps o Shows where damage is most severe - @ CA, UT – there are dense networks of seismograph stations transmit direct measurements of instrumental intensity to immediately make shake map - Shake maps = important to emergency response teams 2.2 Earthquake Process Process of Faulting - Faulting: the process of fault rupture = sliding two rough boards past each other o Lithospheric plates moving past each other are slowed by friction o Friction = force on rocks = strain or deformation of rocks o If stress @ rock exceeds breaking point/strength = rocks move along fault - Rupture starts @ focus and propogates up, down and laterally - Rupture = waves of vibrational energy (seismic waves_ - Faults = seismic source Fault Types - Two basic types = distinguished by the direction of displacement rocks or sediment bordering them - Displacements = horizontal @ strike-slip faults & vertical @ dip slip faults - 3 types of dip-slip faults o Reverse fault: Hanging wall (block above) moved up relative to the footwall (at feet) along a plane inclined at an angle steeper than 45 degrees o Thrust faults: similar to reverse but angle of fault plane is 45 degrees or less o Normal fault: dip-slip fault in which hanging wall has moved down relative to footwall - Some faults are buried and don’t reach ground surface Fault activity - Active fault = if its moved in the last 11,600 years (Holocene Epoch) - Potentially active = moved during Pleistocene Epoch (last 2.6 million years) - Inactive fault = faults that haven’t moved in 2.6 million years - Have to look at prehistoric history of earthquakes (paleoseismicity of fault) Tectonic Creep - Tectonic creep/fault creep: gradual movement along a fault without accompanying felt earthquakes o E.g., winery along Calaveras fault near Hollister, CA being pulled apart at rate of 1 cm/year - Periodic slow earthquake that is not felt Seismic Waves - Some waves travel through body of the earth or surface - Body waves: include P waves and S waves - P waves: Compressional/primary waves = faster of the two types o Travel through any type of material, although their velocity in solids is much higher than in liquids o Avg velocity in P waves @ Earth’s crust is 6 km/s vs 1.5 km/s in water - S waves: Sheer/secondary waves, travel through solid materials o Avg velocity through Earth’s crust of 3 km/s (half the speed of P waves) o Produce back and forth motion at right angles  whipping motion of long jump rope o S waves can’t travel through liquid (water can’t spring back from sideways shear) - Surface waves: form when P and S waves reach Earth’s surface and move along it o Travel more slowly than P or S waves and cause much of the damage near the epiicentre o Produce complex horizontal/vertical movement - rolling motion  @ epicenter can see rippling ground o Love wave: causes horizontal shaking (v. damaging to foundations) o Rayleigh wave: travels with an elliptical motion like rolling ocean waves  Surface materials move vertically as wave moves forward 2.3 Earthquake Shaking - Four factors determine the shaking during an earthquake and the damage the earthquake causes: o Magnitude o Distance to epicenter/focal depth o Direction of rupture o Local soil and rock conditions Distance to the Epicenter and Focal Depth - Shaking decreases further from epicenter - Can locate epicenter using p and s waves o P waves travel faster than S waves so they show up faster @ seismogram o Difference b/w arrival times used to determine distance of epicenter from seismograph - Can locate epicenter using p and s times from seismographs at DIFFERENT locations via triangulation - Depth of earthquake = amount of shaking - Seismic waves lose some of their energy before they reach the surface - Greater the focal depth, the less intense the shaking at surface - Loss of energy = attenuation Direction of Rupture - Directivity: when earthquake energy is focused in the direction of the rupture, contributing to amplification of waves and increased shaking Local Soil and Rock Conditions - Dense = transmit earthquake energy efficiently - Seismic energy/ground shaking = diminish rapidly away from epicenter o Move slower through unconsolidated sediment then @ bedrock o Waves move to MORE UNCONSOLIDATED (looser) materials, slow b/c higher water content o The slowing = some of P and S wave energy Is transferred the surface waves (amplification) = increased ground motion - Local geographical structures can amplify shaking o E.g., synclines/fault-bounded sedimentary basins can focus seismic waves like a magnify lens 2.4 The Earthquake Cycle - Earthquake cycle: elastic strain drops abruptly after an earthquake and then slowly accumulates until the next - Strain: deformation resulting from stress - Elastic strain: temporary deformation  when stress is released, elastically deformed goes back to normal o If stress continues to increase, deformed material will eventually rupture making deformation permanent - Elastic rebound - Typical cycle has 3 or 4 stages 1. Long period of inactivity 2. Accumulated elastic strain = small earthquakes 3. Characterized by foreshocks (this stage can be absent) 4. Mainshock (major earthquake + aftershocks) 2.5 Geographic Regions at Risk from Earthquakes - Risk = probability that a damaging earthquake of a specified magnitude will occur Plate Boundary Earthquakes - Plate-boundary earthquakes: occur on faults separating lithospheric plates – 3 types: o Strike slip earthquakes @ transform faults o Thrust earthquakes @ convergent plate boundaries o Dip slip earthquakes @ divergent plate boundaries - Strike Slip earthquakes’ - Thrust earthquakes o Subduction earthquakes: occur on the large thrust faults that separate subducting and overriding lithospheric plates  Strain energy accumulates in rocks adjacent to the plate-boundary fault until it is suddenly released during a subduction earthq
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