Chapter 6 Earthquakes and Earth's Interior
Earthquakes
General features
Vibration of Earth produced by a sudden release of energy
Associated with movements along faults
Explained by the plate tectonics theory
Mechanism for earthquakes was first explained by H. Reid
Early 1900s
Rocks " spring back "
Phenomenon called elastic rebound
Vibrations (earthquakes) occur as rock elastically returns to its original shape
Fault creep
Often preceded by foreshocks
Often followed by aftershocks
Earthquake waves
Study of earthquake waves is called seismology
Earthquake recording instrument
Records movement of Earth
Instrument is a seismograph
Record is called a seismogram
Types of earthquake waves
Surface waves
Body waves
Primary waves
Secondary waves
Surface waves
Complex motion
Slowest velocity of all waves
Body waves
Primary (P) waves
Push-pull (compressional) motion
Travel through
Solids
Liquids
Gases
Greatest velocity of all earthquakes waves
Secondary waves (S) waves
" Shake " motion
Travel only through solids
Slower velocity than P waves
Locating an earthquake
Focus-the place within Earth where earthquakes waves originate
Epicenter
Point on the surface, directly above the focus
Located using the difference in the arrival times between P and S wave recordings, which are related to distance
Three station recordings are needed to locate an epicenter
Circle equal to the epicenter distance is drawn around each center
Point where three circles intersect is the epicenter
Earthquake zones are closely correlated with plate boundaries
e.g., Circum-Pacific belt
e.g, Oceanic-ridge system
Earthquake intensity and magnitude
Mercalli intensity scale
Assesses damage at a specific location
Depends on
Strength of the earthquake
Distance from the epicenter
Nature of the surface material
Building design
Magnitude
Concept introduced by Charles Richter in 1935
Measured using the Richter scale
Earthquake magnitude scale
Amplitude of largest wave recorded
Largest earthquakes near magnitude 8.6
Magnitudes less than 2.0 are usually not felt
Each unit of magnitude increase corresponds to
A tenfold increase in wave amplitude
About a 30-fold energy increase
Earthquake destruction
Factors that determine destruction
Magnitude of the earthquake
Proximity to population
Destruction from
Ground shaking
Liquefaction of the ground
Saturated material turns fluid
Underground objects may float to surface
Tsunamis, or seismic sea waves
Landslides and ground subsidence
Fires
Prediction
Short-range -- no reliable method yet devised for predicting
Long-range
Premise is that earthquakes are repetitive
Region is given a probability of a quake
Earths interior
Most of our knowledge of the interior comes from the study of P and S waves
Travel time of P and S waves through Earth vary depending on the properties of the materials
S waves travel only through solids
Earths interior structure
Crust
Thin outer layer
Varies in thickness
70 km in some mountainous regions
Less than 5 km in oceanic regions
Two parts
Continental crust - lighter, granitic rocks
Oceanic crust - basaltic composition
Lithosphere
Crust and uppermost mantle (about100 km thick)
Cool, rigid, solid
Mohorovicic discontinuity ( Moho) separates the crust from the mantle
Mantle
Below crust
2885 km thick
Composition similar to the igneous rock peridotite
Asthenosphere
Upper mantle
At a depth from 100 - 350 km
Hot, weak rock
Easily deformed
Up to 10 percent is molten
Key to explaining plate movement
Outer core
Below mantle
2270 km thick
Mobile liquid
Does not transmit S waves
Mainly iron and nickel composition
Related to Earths magnetic field
Inner core
1216 km radius
Solid
Iron and nickel composition
High density
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