Natural Disasters Exam #1

Natural Hazards

• A natural process or event that has the potential to cause harm to people, property, or the environment.

• Examples: earthquakes, hurricanes, floods, volcanic eruptions, droughts.

• A hazard doesn’t always cause damage—it’s just the risk or threat. 

Natural Disasters

• Happens when a natural hazard actually causes significant damage, destruction, or loss of life.

• A disaster depends on the hazard’s impact on society—how prepared people are, where they live, and how vulnerable they are.

• Example: An earthquake in the middle of the ocean is just a hazard, but if the same earthquake strikes a city and kills thousands, it becomes a disaster.

The economic cost of natural disasters is…

increasing

The number of lives lost in natural disasters is…

decreasing

Recurrence intervals

• The estimated average time between events of a certain size or intensity happening in a given location.

• It’s based on past records of natural hazards (like floods, hurricanes, or earthquakes).

• For example, if a flood has a 100-year recurrence interval, it means that, on average, such a flood has a 1% chance of happening in any given year.

•  In short: recurrence intervals describe the likelihood of a natural hazard event, not a schedule.

P-wave (Primary wave)

• Type: Compressional (longitudinal) wave

• Motion: Push–pull, particles move back and forth in the same direction as the wave.

• Speed: Fastest seismic wave, so it arrives first

• Can travel through solids, liquids, and gases.

S-wave (Secondary wave)

• Type: Shear (transverse) wave

• Motion: Side-to-side or up-and-down, particles move perpendicular to the wave’s direction.

• Speed: Slower than P-waves, so it arrives second.

• Medium: Can only travel through solids, not liquids or gases.

Earth’s radius

About 6,400 kilometers

The rocky mantle is made up of the…

Lithosphere and Asthenosphere

Lithosphere

• Cold outer layer

• strong and hard to deform

• source of earthquakes. 

• Lithosphere = tectonic plates

Asthenosphere

• Hotter material beneath the lithosphere. 

• Viscous/gooey and so flows/convects over millions of years.

• Allows the lithospheric plates to move or "float" on it.

• Important for mantle convection, which drives plate tectonics.

Inner core

Solid metal

Outer core

Liquid metal

Where and how is the Earth’s magnetic field generated?

• The Earth’s magnetic field is generated in the outer core:

• Deep inside Earth, in the liquid outer core, which is made mostly of molten iron and nickel.

• The movement of this electrically conducting liquid metal (caused by convection currents from heat escaping the inner core and Earth’s rotation) creates electric currents.

• These currents generate magnetic fields, and together they form a self-sustaining process called the geodynamo.

Geographic North Pole

• The fixed point at the very top of Earth’s axis of rotation (90°N latitude).

• Where the Earth “spins around.”

• Used for maps and navigation as true north.

Magnetic North Pole

• The point on Earth’s surface where the planet’s magnetic field points straight down.

• It moves over time because Earth’s magnetic field shifts (it can drift dozens of kilometers per year).

• A compass points to this pole, not the geographic pole.

The Moho

The base of Earth’s crust

Plate tectonics

• Plates move relative to one another and have deforming boundaries.

• These tectonic plates move at only a few cm per year. 

Three plate boundary types:

Divergent, Convergent, Transform

Divergent Plate Boundary

•  Plates move apart (in oceans: “seafloor spreading”)

• Example: Mid-Atlantic Ridge

Convergent Plate Boundary

• Plates move together at subduction zones. The highest and lowest points on Earth! 

• Example: The Himalayas

Transform Plate Boundary

• Plates slide past each other (aka ‘strike-slip boundaries’).

• Example: San Andreas Fault

Which types of plate boundaries are most susceptible to devastating earthquakes?

Convergent (subduction zones & continental collisions) and transform boundaries

What do we call mantle plume surface expressions

Hotspots

What do chains of hotspots tell us?

plate motion history (direction + rate)

Earthquakes occur on…

faults

Faults:

break in the crust where rocks can move and release energy.

Focus/Hypocenter

underground origin of earthquakes 

Epicenter

surface point above the earthquake

Fault terminology: hanging wall

• The block of rock above the fault plane.

• Think: if you could “hang” something from it, it would be the hanging wall.

Fault terminology: foot wall

• The block of rock below the fault plane

• Think: the part you could “stand on like a foot.”

Fault terminology: dip

• The angle at which the fault plane slopes into the Earth.

• Measured from the horizontal.

Fault terminology: fault scarp

• The steep cliff or step on the surface caused by vertical movement along a fault.

Fault terminology: right vs. left lateral

• Right-lateral: if you stand on one side, the other side moves to your right.

• Left-lateral: if you stand on one side, the other side moves to your left.

One diagnostic we can use to identify ancient earthquakes:

Paleoseismology evidence in sediments or rocks

• Example: fault scarps, displaced layers of sediment, or sand boils.

The Earthquake Cycle

Inter-seismic, co-seismic, post-seismic periods

The Earthquake Cycle: Inter-seismic period

• Time between earthquakes on a fault.

• Stress gradually builds up as tectonic plates continue to move.

• No major shaking occurs yet, but small foreshocks may happen.

The Earthquake Cycle: Co-seismic period

• The actual earthquake event.

• Sudden release of accumulated stress along the fault.

• Causes ground shaking, fault rupture, and possible surface displacement.

The Earthquake Cycle: Post-seismic period

• Time after the earthquake.

• Aftershocks occur as the crust adjusts.

• The fault and surrounding rocks slowly relax and redistribute stress

Strike-slip faulting (transform plate boundaries)

• Plates slide past each other horizontally.

• Example: San Andreas Fault, California.

Thrust faulting (subduction zones/convergent zones) 

• Plates push together, one plate may subduct beneath another.

• Example: 2011 Tōhoku earthquake, Japan (Pacific Plate subducting under Eurasian Plate).

• Creates some of the largest earthquakes and tsunamis.

Normal faulting (divergent plate boundaries)

• Plates pull apart, causing crust to stretch.

• Example: Mid-Atlantic Ridge earthquakes.

Earthquakes away from plate boundaries are called…

 Intraplate earthquakes

What can drive intraplate earthquakes?

• Fracking

• Ancient faults

Which type of plate boundary is associated with the biggest earthquakes?

convergent plate boundaries, especially subduction zones.

What specific type of fault occurs at convergent plate boundaries plate boundaries?

Thrust fault

Seismic waves are recorded at seismic stations as…

seismograms

Seismic waves:

Vibrations produced by earthquakes that travel through Earth

Seismograms:

Show the arrival times and amplitudes of different seismic waves 

The fastest seismic wave:

P-wave

The slowest seismic wave:

S-wave

Which seismic wave involves the greatest shaking:

S-wave

Wave terminology: Wavelength

• The distance between two successive crests or troughs of a wave

• Measured in meters.

Wave terminology: Amplitude

• The height of the wave from the rest position (middle line) to crest or trough.

• Related to the energy of the wave: bigger amplitude = stronger wave.

Wave terminology: Frequency

• The number of waves that pass a point per second.

• Measured in Hertz (Hz).

• Frequency and period are inversely related

Wave terminology: Period

• The time it takes for one complete wave to pass a point.

• Measured in seconds.

How to locate earthquakes?

Seismograph stations

Seismograph stations…

• Measure the arrival times of the P-wave and S-wave at each station.

• P-waves arrive first, S-waves arrive later.

• The time difference between them tells us the distance to the epicenter.

• Use at least three stations to triangulate the epicenter.

Intensity (i.e., ground shaking) due to earthquake depends on:

• Magnitude

• Location of earthquake

• Local geology

How does an Earthquake early warning system work?

• Seismic sensors detect the first waves (P-waves) of an earthquake.

• The system calculates the earthquake’s location, magnitude, and expected shaking in real time.

• Warnings are sent out to people, emergency services, and infrastructure before the more destructive S-waves and surface waves arrive.

• Warning time can be seconds to a minute or two, depending on distance from the epicenter.

Earthquake Forecasting:

Past recorded experiences produce statistical forecasts (constructed mainly from estimating recurrence intervals). 

Paleoseismology

The study of ancient or prehistoric earthquakes by examining geological layers, faults, and sediments.

Faulting complexities:

• Seismic gaps 

• Segmented faults 

• Migrating earthquakes

Retrofitting:

modifying existing buildings (can be expensive!)

Base isolation:

isolate building from shaking ground.

Situation that promotes a large EQ-generated tsunami: 

subduction zone earthquakes (megathrust) with shallow hypocenters. 

Following sudden seafloor movement, tsunami waves…

move out in both directions.

Alternative ways to trigger a tsunami:

• Volcanoes: A large eruption or a flank collapse.

• Landslides: Fast-moving rock displaces water and triggers waves. 

• Asteroids: E.g., the dinosaur-killing impact in Mexico.

As waves move towards shore (ocean floor depth shallows):

• Speed decreases

• Wavelength decreases

• Height increases!

What is the type of plate boundary and fault that produces most of the devastating tsunamis?

Convergent boundaries

What was the deadliest tsunami (in 2004)?

• Indian Ocean tsunami, caused by the M 9.1–9.3 Sumatra–Andaman earthquake.

• Date: December 26, 2004

• Location: Off the coast of Sumatra, Indonesia

• Deaths: Over 230,000 people across 14 countries

• Countries affected: Indonesia, Thailand, Sri Lanka, India, Maldives, and others

• Cause: A megathrust earthquake at a subduction zone caused massive vertical displacement of the seafloor, generating the tsunami

Potential future US tsunami hazards:

• Serious threat: Pacific northwest subduction earthquake. 

• Probably overblown: Flank collapse of Canary or Hawaiian volcanoes.

Natural mechanisms to increase size of land-bound waves?

• funneling by bays

• offshore canyons

• high tide 

Natural mechanisms to decrease size of land-bound waves?

• friction due to coastal corals

• vegetation.

Tsunami terminology: Run-up

The maximum vertical height a tsunami reaches as it moves inland above sea level.

Tsunami terminology: Trim

The change in water level along the coast caused by the passing tsunami wave, often used in modeling and mapping inundation.

Tsunami mitigation: Warning

• Far-field warning well developed

• Near-field is difficult due to timing.

Tsunami mitigation: Engineering

• Land zoning

• Sea walls 

Tsunami mitigation: Education. What to do in the event of an impending tsunami wave?

• Stay away from the beach

• Move to high ground

Volcano Terminology: Lava

Magma that reaches the Earth’s surface during an eruption.

Volcano Terminology: Magma

• Molten rock beneath Earth’s surface.

• Contains gases and crystals.

Volcano Terminology: Magma Chamber

Underground reservoir where magma collects before an eruption

Volcano Terminology: Vent

Opening at the Earth’s surface where magma/lava escapes.

Volcano Terminology: Conduit

Pipe or passage that connects the magma chamber to the vent.

3 main ways to melt rock:

• Increase temperature

• Reduce pressure (move rock towards the surface)

• Add water

3 main controls on volcanic behavior: 

• Viscosity

• Dissolved gases (volatiles). E.g., water.

• Magma type (felsic vs. mafic)

Felsic Rocks

(High SiO2 ): high viscosity, lots of gases -> Explosive 

Mafic Rocks

(Low SiO2 ): low viscosity, limited gases -> Effusive

Main types of volcanoes?

• Shield volcanoes

  • Shape: Broad, gently sloping

  • Lava: Low-viscosity basaltic lava (flows easily)

  • Example: Mauna Loa, Hawaii

• Stratovolcanoes

  • Shape: Steep-sided, tall, layered with lava and ash

  • Lava: More viscous, explosive eruptions common

  • Example: Mount Fuji, Japan; Mount St. Helens, USA

• Cinder cones (smaller type)

Volcanism occurs at: 

• Divergent boundaries (melting due to pressure decrease) 

• Subduction zones (melting due to addition of water)

• Hotspots (due to rising mantle plumes) 

• Flood basalts

  • the largest eruptions, which can cause mass extinctions, and also due to mantle plumes.

Main volcanic hazards: Lava flows 

• Description: Molten rock flowing downhill; usually slow-moving.

• Hazard: Can destroy buildings and roads, but rarely kills people due to slow speed.

• Example: 2018 Kīlauea eruption, Hawaii.

Main volcanic hazards: Ash and pumice falls 

• Description: Fine volcanic ash and pumice ejected into the air, settling over wide areas.

• Hazard: Can collapse roofs, damage engines, contaminate water, and cause respiratory issues.

• Example: 1980 Mount St. Helens, USA.

Main volcanic hazards: Pyroclastic flows 

• Description: Superhot gas, ash, and rock moving rapidly down slopes.

• Hazard: Extremely destructive and deadly, moving at hundreds of km/h.

• Example: 1902 eruption of Mount Pelée, Martinique.

Main volcanic hazards: Mud flows/lahar 

• Description: Volcanic debris mixed with water, forming fast-moving mudflows.

• Hazard: Can bury towns and infrastructure, triggered by heavy rain or melted snow.

• Example: 1985 Nevado del Ruiz, Colombia.

Main volcanic hazards: Poisonous gases 

• Description: Volcanoes release gases like CO₂, SO₂, H₂S, sometimes invisibly.

• Hazard: Can suffocate or poison people and animals.

• Example: 1986 Lake Nyos, Cameroon (CO₂ release).

Prediction and forecasting utilizes: The geological record (paleo-volcanology)

• Study of past eruptions through rock layers, ash deposits, and lava flows. 

• Helps determine:

  • Eruption frequency (recurrence interval)

  • Likely eruption style (explosive or effusive)

  • Hazard zones around the volcano

Prediction and forecasting utilizes: Monitoring (i.e., precursors)

• Observing current signs that may indicate an imminent eruption.

• Common precursors include:

  • Seismic activity (earthquakes beneath the volcano)

  • Ground deformation (swelling of the volcano from rising magma)

  • Gas emissions (increased CO₂, SO₂, or H₂S)

  • Temperature changes in fumaroles or hot springs

Precursors that we monitor for include: Seismic signals 

• What it is: Earthquakes beneath or around the volcano.

• Why it matters: Rising magma fractures rock, causing swarms of small earthquakes.

• Eruption clue: Increasing frequency or intensity of earthquakes can signal magma moving upward.

Precursors that we monitor for include: Temperature

• What it is: Changes in heat at fumaroles, hot springs, or crater areas.

• Why it matters: Rising magma warms surrounding rocks and groundwater.

• Eruption clue: Unusual heating can indicate magma is approaching the surface.

Precursors that we monitor for include: Gasses

• What it is: Emissions of volcanic gases like CO₂, SO₂, or H₂S.

• Why it matters: Magma releases gases as pressure decreases.

• Eruption clue: Sudden increases in gas output or changes in composition suggest magma is rising.