lecture11 earthquakes

Earthquakes

Global Seismicity

Continental Drift Animationbased on paleomaps from University of Texas Arlington

Plate tectonics: predicting the future

Plate tectonics in the future: 50 my

• Australia will straddle the equator• E. Africa will form new continent• Antarctica will swing off the south pole• Mediterranean will close off• Atlantic Ocean will grow• Pacific Ocean will shrink

Global Seismicity

Rocks are stressed too…

Rock stress: from tectonic forces, gravity, and weight of rocks above

Types of stress4. Tension (stretching)5. Compression (shortening)6. Shear (twisting or tearing)

Strain

= how rocks respond to stress

2 types4. folding (bending)5. faulting (breaking)

depends on rock composition, pressure, and rock characteristics (brittle vs. ductile)

Types of folds1. Anticline

• simple upfold• layers slope down from

axis• younger on outside

2. Syncline• simple downfold• layers slope up from

axis• younger on inside

younger

older

younger

older

Types of folds

synclineanticline overturned

anticline

Types of Faults

Tension

Compression

Shear

Stretching

Shortening

Twisting

Faulting

= rocks on either side of a fracture are displaced relative to each other

Earthquake: caused by sudden release of energy along a fault

Anatomy of a faultFault scarp = steep cliffs that make up the

edge of a displaced block

3 Types of Faults

1. Normal• tension

2. Reverse• compression

3. Strike-slip (transcurrent)• shearing• lateral displacement

3 Types of Faults

3 Types of Faults

Divergent Plate Boundaries

3 Types of Faults

Convergent Plate Boundaries

3 Types of Faults

Transform Plate Boundaries

Earthquakes

= vibration in the Earth• produced by shockwaves from sudden

movements along faults

Seismic= pertaining to earthquakes

Focus vs epicenterFocus = center of fault motionEpicenter = surface directly above focus

Seismometer

Seismogram

Richter ScaleScale is exponentialFor every increase of 1 in the Richter scale, the wave amplitude increases x10

Exponential decay

Exponential growth

y = y0e-kx y = ekx

a = 10M

Where M is Richter magnitude, and a is wave amplitude

M = log10(a)Which is the same as

Richter ScaleScale is exponentialFor every increase of 1 in the Richter scale, the wave amplitude increases x10

But wave amplitude is not the same as energy released. (see pg 408 in Strahler)

E = 10(4.8+1.5M) = 104.8 ·101.5M

Where M is Richter magnitude, and E is energy released

So for every increase of 1 in the Richter magnitude, energy increases x 32

Richter Scale

How much energy is released?For example, the earthquake that caused the Indian Ocean tsunamis on Boxing Day 2004 (12/26/04) measured 9 on the Richter scale

How much energy was released by this quake?

E = 10(4.8 + 1.5M)

The Boxing Day Quake

How much energy was released by this quake?

E = 10(4.8 + 1.5M)

= 10(4.8 + 1.5(9))

= 10(4.8 + 13.5)

= 10(18.3) = 1.99 x 1018J

For comparison, the average U.S. electric power consumption rate is 3 x 1012J/sec