Chapter 11 – Earthquakes & Earthquake Hazards

Tsunami Example: Tohoku, Japan (March 11, 2011)

• Subduction zone

• Mw 9.0 9.1 undersea megathrust eq

• generated a major tsunami

Megathrust earthquakes

very large earthquakes produced by sudden slip along a subduction zone at a deep-sea trench where one tectonic plate is pushed beneath another

Slip

occurs along plate boundaries

Deep-sea trench

Makes subduction zone

Large aftershock

Follow major rupture 

What causes an earthquake? 

Movement of the lithosphere plates → large forces 

Earthquake

Sudden failure/slip along a geologic fault 

Friction

Prevents slip until stress > friction

Lithosphere

Crust + uppermost mantle

Hypocenter (Focus)

Below the surface, EQ starts here

Epicenter

Above focus on earths surface

Slip releases

Stored elastic strain energy as seismic waves

Elastic Rebound Theory

1. Stress builds → crust deforms but remains locked

2. Cracks form & grow

3. Rupture occurs (fault slips) 

4. Energy released & rock rebounds (EQ happens) 

Elastic Rebound Theory: Step 1

Stress builds → crust deforms but remains locked

Elastic Rebound Theory: Step 2

Cracks form & grow

Elastic Rebound Theory: Step 3

Rupture occurs (fault slips) 

Elastic Rebound Theory: Step 4

Energy released & rock rebounds (EQ happens) 

No elastic energy

Rupture just happened

Some elastic energy

Partial recovery over time

High elastic energy

Rupture hasn’t happened yet

Elastic Rebound Theory Example: 1906 San Francisco (San Andres Fault)

• Fence displaced 8.5ft

• M 7.9 

• 3,000 dead; >80% city destroyed 

Focus & Epicenter

Waves originate at focus & travel outward

Body waves (travel through earth)

• P-waves 

• S-waves 

P-waves (primary)

• Fastest 

• Compressional (push-pull) 

• Motion parallel to direction 

S-waves (secondary)

• Slower (1/2 P-wave speed)

• Shear motion

• Motion perpendicular to direction 

Surface waves (travel on surface)

• Rayleigh waves

• Love waves 

Surface waves are

The most destructive

Rayleigh waves

vertical rolling

Love waves

horizontal waves 

Wave arrival

P → S → surface waves

Difference in arrivals time

Increase with distance

Measuring earthquakes

• Seismometer 

• Epicenter triangulation 

Seismometer

• Detects ground motion (very small scale) 

• Modern instruments are electronic; measure all directions

Epicenter triangulation

• Difference between P & S arrive time = distance

• Use 3 stations → draw circles → intersection = epicenter 

Seismicity sources

• Sudden slip on faults (most destructive) 

• Magma inflation 

• Volcanic eruption 

• Giant landslide 

• Meteorite impact

• Human activity (induced seismicity) 

Induced earthquakes

Caused by fluid injection

Fluid injection

• Wastewater injection 

• Hydro fracturing (fracking) 

• Enhanced oil recovery 

Induced earthquakes mechanism 

• Increases fluid pressure in faults 

• Loosens fault zones → easier slip

• even inactive faults can slip

Wastewater

• Salty/polluted water injected deep underground 

• Mostly from oil & gas production 

Hydraulic fracturing (fracking) 

• High-pressure water cracks rocks 

• Causes micro earthquakes (small)

• Some fluid reused, some becomes wastewater

Enhanced oil recovery

• Inject steam/water/CO₂ to push oil toward wells

• Operates long-term, produces large wastewater volumes

Stable continental interiors (SCI)

Shaking felt over large areas

Stable continental interiors (SCI) Example

New Madrid (1811-1812)

1811–1812 New Madrid EQs

• ~Mw 7.2–7.3 (Dec. 16, 1811)

• ~Mw 7.0 (Jan. 23, 1812)

• ~Mw 7.4–7.5 (Feb. 7, 1812)

• Plus major aftershock ~7.0

Damage factors

• Intensity + duration 

  • 1964 Alaska EQ (M 9.2) shock  for 3-4 min 

• Construction quality 

• Local rock & sediment behavior 

Intensity (Mercalli Scale)

• Measures shaking effects/damage

• 12 point scale (roman numeral)

• Based on building damage 

Magnitude

Measures energy release

Richer scale

Largest ground motion at a set distance

Moment magnitude

Size of fault rupture

+1 magnitude

10x increase in fault areas

Effects of earthquakes 

• Ground shaking 

• Surface faults 

• Ground failure (3 types) 

• Tsunamis 

Ground failures 

• Lateral spreads

• Flow failures 

• Liquefaction 

Ground failures - Lateral spreads

Soil blocks slide sideways

Ground failures - Flow failures

Landslide

Ground failures - Liquficication

EQ shaking → grains lose contact → soil acts like liquid

• <10,000 year old sediments + groundwater <30ft

Liquification conditions

• Loose sediments/soil

• Saturated pore space

• Shaking (EQ or construction) 

Liquification example

• Christchurch, New Zealand (2011, M 6.2)

• Aftershock of 2010 Canterbury EQ (M 7.1)

Tsunamis

Seismic sea waves

Tsunamis caused by

seafloor topography change due to fault slip

Tsunamis travel

At jetliner speeds

Tsunamis is deep water

Low height, long wavelength

Tsunamis near shore 

• Wave slows due to friction

• Wave height increases (10–15 m)

The largest earthquakes occur

at convergent plate boundaries (subduction zones) at deep-sea trenches, producing megathrust earthquakes