EARS 5 – Natural Disasters & Catastrophes: Vocabulary Review

Objectives

• Scientific
  • Physical processes behind natural disasters
  • Data & tools used to study them
  • Prediction possibilities & limits

• Societal
  • Mitigation strategies
  • Communicating uncertainty
  • Geological vs. historical time perspectives

• General scientific thinking skills

Course Style & Policies

Natural Disasters: Key Definitions

• Natural process ≠ disaster until it impacts society

• Webster: “sudden & extraordinary misfortune”

• Working hierarchy
  • Hazard = probability a phenomenon occurs
  • Risk = probability of adverse consequences
  • Disaster = event causing significant damage
  • Catastrophe = very large deaths/injuries or economic loss

• Equation examples
  • Earthquake hazard: P(M>7 \;|\; \text{before 2020})
  • Personal risk: P(\$>10\,000\text{ damage}|\text{1 yr})

• Mitigation = strategies that lower risk (prediction, zoning, insurance, education …) — cost-benefit dependent

Major Disaster Types Discussed

• Geophysical: earthquakes, tsunami, volcanic eruptions

• Hydro-meteorological: floods, hurricanes, tornadoes, drought, wildfire, climate change, El Niño

• Mass-movement: landslides

• Extra-terrestrial: meteorite impacts

• Biological: pandemics (listed but not covered)

Why Study Natural Disasters?

• Humans often live in hazardous zones for cultural/economic reasons and quickly forget past events

• Population & asset growth amplify impacts

• Catastrophes disrupt temporary equilibria in Earth systems

Statistics & Trends (1980-2024)

• Rising counts of meteorological & hydrological events; geophysical event frequency relatively stable

• Economic losses now hundreds of \text{US}\$ 10^{11} annually – peaks >\$350\text{ B} (Our World in Data)

• Increasing insured vs. uninsured gap (Swiss Re)

• Magnitude–Frequency: M \propto \frac1f (large events rare)

Risk Perception

• Public often mis-ranks risks (e.g., nuclear power vs. motor vehicles)

• Probabilities of death per year
  • Motor vehicle 1!:!100 > smoking 10 cigarettes 1!:!200 > influenza 1!:!5000 > sports 1!:!25{,}000 > CA earthquake 1!:!2\,000,000

• Chicken Little vs. Alfred E. Neumann archetypes illustrate over- vs. under-reaction

Scientific vs. Policy Roles

• Scientists: reconstruct past, identify causes, develop forecasts & technologies

• Policy makers: zoning, codes, response plans, insurance; communication essential

Predicting Catastrophe

• Use past as key to future; recurrence interval statistics

• Plot magnitude vs. inter-event time — straight line on log–log plots enables extrapolation

• Many Earth features are fractal ⇒ small events inform large ones

• Overlapping cycles & feedbacks complicate prediction (e.g., tide + storm surge)

Earth Structure & Composition

Differentiation

• Early melting → iron-nickel core vs. silicate mantle

Compositional Layers

• Crust
  • Continental 20-70 km, \rho\approx2.7\times10^{3}\,\text{kg m}^{-3} (felsic)
  • Oceanic 5±3 km, \rho\approx3.0\times10^{3} (mafic)

• Mantle ≈ 2700 km thick, \rho=3.3$–$5.7\times10^{3}; rock = peridotite (ultramafic)

• Core
  • Outer liquid, \rho\approx9.7$–$14\times10^{3}
  • Inner solid, \rho\approx14$–$16\times10^{3}

Rheological Layers

• Lithosphere: cold, rigid, ~100 km

• Asthenosphere: warm, weak, convects (v\sim1\text{ cm yr}^{-1},\;\tau\sim10^{9}\text{ yr})

Geological Time

• Earth age: 4.55\times10^{9}\text{ yr} (Hadean → present)

• 24-hour & football-field metaphors illustrate how recent humanity is

Radiometric Dating Basics

• Radioactive decay: N(t)=N{0}(\tfrac12)^{t/t{1/2}}

• Parent → daughter ratio \dfrac{D}{P}=2^{t/t{1/2}}-1 ⇒ t=t{1/2}\,\dfrac{\log(D/P+1)}{\log2}

• Common systems & half-lives
  • ^{14}!C \rightarrow {}^{14}!N 5.7\times10^{3}\text{ yr}
  • ^{238}!U \rightarrow {}^{206}!Pb 4.5\times10^{9}\text{ yr} (Earth dating)

• Carbon-14 useless for rocks: short t_{1/2} and “clock” starts at organism death

Precision vs. Accuracy

• Precision = repeatability/significant figures (±½ LSB)

• Accuracy = closeness to true value

• Rule: final answers carry sig. figs. of least precise input

Plate Tectonics Essentials

• Lithosphere fragmented into ~8 major & many minor plates (map provided)

• Motion driven by mantle convection; GPS confirms \sim5\text{ cm yr}^{-1} typical

• Hypsographic curve: bimodal topography linked to crustal density differences

Plate Boundaries

Fault Mechanics & Earthquakes

• Elastic-rebound theory (Lawson 1908): stress builds, fault slips, energy released

• Fault terms: strike, dip, hanging-wall vs. foot-wall

• Types
  • Normal (HW ↓)
  • Reverse/thrust (HW ↑)
  • Strike-slip (left/right lateral)

• Magnitude related to rupture area and slip

Seismic Waves

• Body waves:
  • P (compressional, v\approx6\,\text{km s}^{-1})
  • S (shear, v\approx4\,\text{km s}^{-1}; no liquids)

• Surface waves: Love, Rayleigh (largest damage)

• Free oscillations: whole-Earth normal modes (T up to 10^{4}\,\text{s})

• Travel-time curves curved because velocity increases with depth & Earth curvature

Locating Epicenters

• Use \Delta t = tS - tP; distance d = \dfrac{\Delta t}{1/vS - 1/vP}

• Triangulate with ≥3 stations; networks yield real-time locations

Magnitude Scales

• Local (Richter) ML = \log{10}(A/A_0) (corrected to 100 km); each +1 ⇒ ×10 amplitude

• Surface-wave MS & body-wave mb (depth-biased)

• Moment magnitude MW = \tfrac23\log{10}M0 - 6 where M0 = \mu A S; each +1 ⇒ ×32 energy; global energy release since 1906 ≈ M_W 9.95

• Energy relation: \log{10}E = 11.8 + 1.5MW (ergs)

Intensity (Modified Mercalli)

• I (XII) based on effects; varies with depth, distance, local geology, population

• “Did You Feel It?” crowdsourcing enhances isoseismal mapping

Earthquake Effects

• Surface rupture, scarps, offset infrastructure

• Ground shaking, structural failure, resonance issues (bedrock vs. soft mud amplification)

• Liquefaction → sand blows, building tilting (Niigata 1964; Japan 2011 videos)

• Landslides (Loma Prieta 1989, Madison River 1959)

• Secondary: fires (SF 1906), dam failures, tsunamis

Mitigation & Early Warning

• Hazard maps (PGA %g) for zoning & building codes

• Early-warning networks: detect P-wave, alert before damaging S/surface waves (seconds)

• Personal preparedness (flashlight, secure furniture, reunification plans)

• Post-event: utilities shut-off, avoid phone, watch for aftershocks

Nuclear Test Monitoring

• Comprehensive Test-Ban Treaty Organization uses global seismic, hydro-acoustic, infrasound, radionuclide networks to distinguish explosions vs. quakes (different P/S signatures)

Volcanology Fundamentals

Terminology & Products

• Magma (sub-surface) vs. lava (surface)

• Pyroclast/tephra generic ejecta
  • Ash <2 mm • Lapilli 2–64 mm • Bombs/blocks >64 mm

Igneous Rock Series

• Mafic \sim50\% SiO₂ → basalt

• Intermediate \sim60\% → andesite/dacite

• Felsic \ge70\% → rhyolite

• Increasing silica ⇒ increasing viscosity

Volcano Morphologies

Magma Viscosity Controls

\eta = f(\text{SiO}2,\;T,\;H2O/\text{gas})

• Higher SiO₂ & lower T ⇒ higher \eta

• Dissolved volatiles decrease \eta but drive explosivity when exsolved

Explosivity Logic

1. Rising magma ↓P → volatiles exsolve → bubble volume ↑↑

2. If \eta high, bubbles trapped → fragmentation

3. Explosive volcanism = high \eta + high gas (e.g., rhyolite with 5-10 % H₂O)

Tectonic Settings & Magma Genesis

• Hotspot tracks (Hawaii–Emperor; Snake River Plain) trace plate motion over stationary plumes

• Flood basalts may form from initial plume-head arrival

Equations & Relationships Recap

• Hazard/Risk definitions (probability framework)

• Magnitude–frequency M \propto 1/f

• Radiometric dating \dfrac{D}{P}=2^{t/t_{1/2}}-1

• Seismic travel-time d = \dfrac{tS-tP}{1/vS-1/vP}

• Moment magnitude MW = \tfrac23\log{10}M_0 - 6

• Energy \log{10}E = 11.8 + 1.5MW

Ethical & Practical Considerations

• Communicating uncertainty vital for public trust & policy

• Insurance & economic inequities (insured vs. uninsured losses)

• Climate change and population growth feedback into disaster frequency and impact

• Importance of learning from past events to inform zoning, codes, and emergency planning