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Chapter 4 - Natural Hazards: An Overview

Chapter 4 - Natural Hazards: An Overview

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Chapter 4 - Natural Hazards: An Overview

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  1. Chapter 4 - Natural Hazards: An Overview

  2. Effects of hazards on humans • scope: $50 billion/year • average of 150,000 dead/year • social loss - employment, anguish, productivity • humans located in the way of natural processes

  3. Problems • hazardous zones: geologically active • good vs bad - depends on POV • few if any places are free from all hazards

  4. magnitude and frequency • magnitude: size of event • frequency: recurrence interval • % chance per year • hi magnitude, low frequency usually most dangerous

  5. catastrophe potential • Latin and Greek - overturn or overthrow • extraordinary or violent change • any great or sudden calamity, disaster, or misfortune • any event that disturbs or overthrows the order of things • complex response & threshold crossings • dramatic effect of “small” hazard • geologic importance is debated by geologists • table p 106

  6. Evaluation of hazards • purpose - to minimize loss • methods: identify susceptible areas based on: • past events - history of area • studies of process • physical location

  7. Evaluation of Hazards • media - human impact • scientists • conservative • reluctant to make statements without disclaimers • based on • it is likely • lack 100% agreement • communication problems

  8. forecast general location magnitude range chance of occurrence not specific ratio = 1:100 or 100 yr flood percent - 50% over next 15 yrs prediction warning – this will happen specific time place magnitude based on precursors ie heavy rain = flood non or pseudo science - beware often wrong certain to be correct occasionally dangers boy who cried wolf affects people and businesses Evaluation of Hazards

  9. Risk assessment • probability x consequences • qualitative - determine factors • quantitative • assign # values to risk • # values may be hard to determine • Acceptable risk • based on • personal control • public perception • problems • opportunities

  10. Impact of and recovery from disasters • impact • direct • indirect • recovery - figure p 115 • emergency work • restoration • reconstruction I: recovery to pre-disaster • reconstruction II: may plan to decrease effects of repeat disaster

  11. Adjusting to hazards • reactive - after the fact • proactive - before the fact • avoidance • identification and probability • predictions and forecasts • risk assessments • land use planning • hazard studies and zoning • insurance • evacuation plans • disaster preparedness • bear the loss - ride it out • artificial control • deflect/redirect the hazard • stabilize problem areas

  12. Climate change, land use change, and hazards • effects floods, erosion, landslides, drought, fires • alters locations and probabilities • normal, long-term change

  13. Population increase and natural hazards • increases demands on land and resources • pushes people into marginal areas

  14. Chapter 5: Earthquakes & Related Phenomena

  15. EQ features • epicenter • hypocenter (focus) • seismic waves • fault • rupture • below ground • surface

  16. Magnitude • amount of shaking • normalized to set distance • Richter magnitude • largest amplitude S-wave • logarithmic scale • energy is 30X for each level • Moment magnitude • seismic moment based on • average amount of slip on fault • area actually ruptured • strength of rx that failed • more quantitative and accurate

  17. Intensity • based on personal observations of severity of shaking • quantifies damage – mag. doesn’t • Shows variation for different areas affected by EQ • modified Mercalli scale

  18. Faults • cause • plate boundary - may be far from actual boundary • intraplate - weak zones • former plate boundaries • Addition or removal of material • types • Dip slip • normal • reverse & thrust • Strike slip - right lateral, left lateral • oblique slip • buried/blind faults - no surface trace • zone - related faults may be of several types

  19. EQ causes • EQ cycle - Elastic rebound theory • stress builds up • exceeds strength • rocks snap back • vibrations = EQ • recurrence depends on rock strength • Human induced EQs • addition of water • reservoirs (increases pressure and lubricates fault • fluid injection • explosions & nuclear tests

  20. Seismic activity • Identification • plot foci • date movements of soils and other features • study stress field and measure stain • tectonic creep - constant movement (small or no EQs) • classification (table p 137 • active fault zone - Holocene (10K yr) • potentially active - Quaternary (2M yr) • inactive - no activity for 2M yr

  21. Seismic Waves • Body waves - hi freq 05 -20hz • P-wave • fastest • S-wave • thru solid only • Surface waves - lo freq <1hz • Love - shear (side to side) • Rayleigh - oscillation - fig p 139

  22. Seismology • Measuring seismic waves • seismograph • seismic station • seismogram • Location by triangulation • S&P wave arrivals • Distance radios for 3 stations

  23. Shaking • frequency • building vs EQ wave • harmonics - natural freq of vibration • low building - hi freq • tall buildings - low freq • materials - natural freqs vary • distance • hi freq wave decay most quickly • tall bldgs are damaged at greater distances

  24. Shaking • amplification • material - most intense in unconsolidated material!!! • directivity - most intense in direction of fault rupture • ground acceleration • acceleration of ground as EQ waves pass • horizontal & vertical • distance • depth of focus • horizontal distance

  25. Primary Effects of EQs • ground motion • Fault rupture - very localized • Shaking • collapse buildings • knock things down • bend things

  26. Secondary effects of EQs • liquefaction • water saturated material • material acts as a liquid • landslides • fires - broken power and gas lines - result loss of life • water bodies • tsunamis - long wavelength, fast • seiches • changes in land elevation • disease

  27. Estimation of seismic hazard • Max. magnitude/intensity • effect at surface • estimated fault location

  28. EQ forecast • recurrence interval • expected magnitudes • all based on • fault assessment • historical record • earth materials • stress field measurements

  29. EQ prediction • Precursors - don’t always occur • micro earthquake swarms • preseismic deformation of ground surface • rates of uplift or subsidence • radon gas release may increase • seismic gaps (locked fault • magnetic fluctuations • electrical resistivity • varies with earth materials, groundwater, and others • changes before EQ • animal behavior • not reliable • could relate to other precursors

  30. EQ hazard reduction • mapping • active fault zones • earth materials sensitive to shaking • research to predict and control EQs • develop and improve adjustment • building design • land-use planning & hazard assessment • siting assessment for new facilities • hazard assessment for existing facilities • Insurance and relief • warning systems • small seismic sensors • 15sec - 1min warning

  31. EQ Hazard perception • denial • acceptance • why? • education • experience • response • move away • prepare

  32. Chapter 6: Volcanic Activity

  33. Volcanoes • Magma rises to surface • eruption • lava • pyroclastics • gas • landform: Paricutin • vent • cone • caldera • rift

  34. volcano types and eruption manner - table p 176 • factors • Gas content (hi gas = explosive) • Si content (hi Si content = explosive) • hi viscosity = explosive • types • Shield - quiet • Cinder - explosive • Composite - quiet/explosive • Volcanic domes - explosive • Flood basalts - quiet

  35. Origins: plate tectonics • mid-ocean ridge • hot spots • subduction zones

  36. Volcano Effects • Lava flows • Aa, slow blocky • Pahoehoe, fast ropey

  37. Volcano Effects • Pyroclastic activity • tephra blown from vent into air • ash fall • wide spread • buries, contaminates H2O, collapses structures, respiratory problems, kills vegetation • ash flow • supported by gas • huee ardente • lateral blast (one type Mt St Helens • cloud collapse

  38. Volcano Effects • gases • types • water vapor • CO2 • CO, SO2, H2SO4 • emission • during eruption • during dormancy • 1986 Lake Wios, Cameroon • heavier than air • dissolved in H2O • released quickly due to agitation

  39. Volcano Effects • debris flows and mudflows (lahars) • ash and water esp. from snow and/or ice • landslide hazard • may be large and fast • may dam rivers or more far downstream • during eruption and after eruption • Fires

  40. Volcano Effects • Caldera - forming eruptions • vary in size eg Crater Lake 7K yrs ago, Yellowstone, 600K yrs ago • massive release of material • collapse of overlying material • dormant result may linger for a long time • Long Valley, CA • hot springs & geysers

  41. Identification of volcanic hazard • activity • active • dormant • inactive • hazardous areas • identify effects of previous eruptions • examine current conditions

  42. prediction of eruptions • Geophysical monitoring • seismic monitoring • magnetic • thermal • hydrologic • topographic changes • tilting • gas emissions • geochemistry • quantity • geologic history

  43. Adjustment to and perception of hazard • mapping - land use planning • evacuation • warning system: table p 201 • diversion of lava flows • bombing - of lava in a channel - blocks channel • water - chilling creates lava wall • walls

  44. Chapter 7: Rivers & Flooding

  45. Basics of rivers • flowing surface water within a channel • source of water – precipitation via: • overland flow • groundwater

  46. Basics of rivers • basin (watershed) • area drained by stream • characteristics • size • drainage density • relief

  47. Basics of rivers • channel • shape - width and depth • gradient • velocity • discharge - volume/time • pattern • braided - bars • sinuous/meandering - fig p 217 • pools and riffles

  48. Basics of rivers • sediment load • suspended load • bed load • dissolved load • erosion and deposition

  49. Basics of rivers • dynamic equilibrium • describes relationship between all of the above • disturbing one disturbs all • stream will alter until a new balance is reached • land use change - fig p 215 • dam - fig p 216

  50. Flooding • overbank flow • causes • precipitation rate (or snowmelt rate) exceeds infiltration capacity, affected by • soil/rock type • preceding rainfall • freezing • dam failure