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?

Stress

• What causes rock to move?
• Stress (Directed pressure)
• tension - pull apart
• compression -push together
• shear - scissor action

• How do rocks respond to stress?
• deformation

How do rocks respond to stress?

• 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 and Strain

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 and Epicenter

• Focus - where slippage occurs (Fig. 10-5, 10-26)
• Epicenter - point on Earth's surface above slippage (Fig. 10-5, 10-26)

Measuring Seismic Waves

• Seismograph - measures seismic waves
• horizontal motion seismograph
• vertical motion seismograph (Fig. 10-10)

Types of Seismic Waves
Body Waves

• Body waves - travel within Earth's interior
• Primary or P waves
• push-pull, compression-dilation (Fig. 10-6)
• 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. 10-7)
• require rigid medium
• velocity slower than P waves

Types of Seismic Waves
Surface Waves

• Surface waves (Fig. 10-8)
• 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. 10-13)
• Need data from at least three stations to find epicenter (Fig. 10-14)

Earthquake Metrics

• Mercalli Scale - earthquake intensity
• based on damage
• I - XII (Table 18.1)
• 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. 10-21)
• Deep focus earthquakes (> 70 km) occur at subduction zones (p. 210)
• Benioff Zone - zone of earthquake foci along top of downgoing plate

Earthquake Prediction

• historical record - earthquake risk map
• 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: