Earthquakes

Earthquakes

• What is an earthquake?
• vibration of Earth caused by rapid relative movement of large bodies of rock
• Historic earthquakes
• 1975 - Liaoning Province, China: 7.5 magnitude earthquake kills only a few people
• 1976 - Tangshan, China: earthquake of magnitude 7.6 kills 650,000 people
• What made the difference?

How do rocks respond to stress? (Review)

• Strain - amount of deformation (Ex: percent shortening)
• Kinds of deformation
• Elastic deformation - returns to original shape after stress is removed
• Plastic deformation - does not return to original shape after stree is removed
• Brittle fracture - rupture, breakage

How do rocks respond to stress? 
Stress versus strain diagram

More about Earthquakes

• Elastic behavior key to understanding earthquakes
• Elastic Rebound Theory - fault remains locked while strain energy builds; sudden slip along fault releases energy yielding earthquake

• Focus - where slippage occurs
• Epicenter - point on Earth's surface above slippage

Measuring Seismic Waves

• Seismograph - measures seismic waves
• Horizontal motion seismograph
• Vertical motion seismograph (Fig. 16.8)

Types of Seismic Waves
Body Waves

• Body waves - travel within Earth's interior
• Primary or P waves
• Push-pull, compression-dilation (Fig. 16.10)
• First to arrive
• Analogous to sound waves: travel through solids, liquids, and gases
• Velocity depends on rigidity and density of travel medium
• Secondary or S waves (Shear waves)
• Particles move at right angles to wave direction (Fig. 16.11)
• Require rigid medium
• Velocity slower than P waves

Types of Seismic Waves
Surface Waves

• Surface waves
• Rayleigh waves - analogous to ocean waves
• Love Waves - side to side motion
• Waves reflected at crust-mantle boundary
• Cause the most damage

Locating Earthquakes

• Use difference in time between P and S waves
• The greater the difference, the farther the epicenter (Fig. 16.13)
• Need data from at least three stations to find epicenter (Fig. 16.14)

Earthquake Metrics

• Mercalli Scale - earthquake intensity
• based on damage
• I - XII (Table 16.1)
• Richter Scale - earthquake magnitude
• based on ground motion
• log scale--magnitude 6 thirty-times as much energy as magnitude 5 (see table)
• Moment magnitude - earthquake magnitude
• based on amount of slip and surface area of fault
• better representation of total energy released

Relation of Earthquake Magnitude
to Damage, Frequency, and Energy

Distribution of Earthquakes

• Follow plate boundaries (Fig. 16.15)
• Deep focus earthquakes (> 70 km) occur at subduction zones (Fig. 16.17)
• Benioff Zone - zone of earthquake foci along top of downgoing plate

Earthquake Prediction

• Historical record - earthquake risk map (Fig. 16.16)
• Earthquake risk map
• Earthquakes in Texas
• Seismic gap - gap in fault displacements (Ex: San Andreas)
• Tiltmeters, strain-gauges - strain (deformation) precursor to earthquake
• Buildup of stress
• Foreshocks
• Change in groundwater chemistry - radon (daughter product of ²³⁸U)
• Change in water well levels
• Behavior of animals - trained observers

Earthquake Info

• Triggering earthquakes
• Can trigger earthquakes by pumping water;
• Rocky Mtn Arsenal - pumped chemical wastes into 3600 m deep well
• Earthquake damage and deaths depend on: