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Earthquakes Chapter 6

Earthquakes – Vibrations of the earth’s crust
Rocks under stress suddenly shift along a fault
Elastic Rebound Theory
Theory geologist use to explain earthquakes
 Rocks on each side of a fault are moving slowly
 Locked fault causes stress to increase
 Stressed pass certain point rocks fracture
 Rocks separate at weakest point
 Spring back to original shape – Rebound
 As rocks fracture and slip into new positions, rocks
along fault release energy
 Energy released in form of vibrations along the fault
called seismic waves
 This is what causes aftershock – a series of small
tremors from other rocks along the fault that fracture
and spring back
 Focus – Area along fault where slippage first occurs
 Epicenter – Point on earth’s surface directly above
the focus
 Seismic waves ripple outward in all directions from
the focus
 90% of earthquakes have a shallow focus w/n 70 km
of earth’s surface
 Intermediate focus occur at a depth of 70 km to 300
 Deep – focus earthquakes occur in the subduction
zones at depths between
300 km and 650 km
3 major earthquake zones
1. Pacific Ring of Fire
West coasts of North and South America, the east coast
of Asia, and western Pacific islands of the Philippines,
Indonesia, New Guinea, and New Zealand
2. along the mid-ocean ridges
Spreading motion along ridges creates stress in rocks
along major ocean ridges
3. Eurasion – Melansian Mountain Belt
Belt of mountains formed by collision of the Eurasian
plate with the African and Indian plates.
Fault Zone – formed along plate boundaries because of
stress that results:
 Plates separate
 Collide
 Subduct
 Slide past one another
 Example San Andreas Fault zone
Earthquake Activity in center of United States
 New Madrid, MS 1812 earthquake felt as far away as
Boston, MA
 Ancient fault zone deep w/n earth’s crust Mississippi
River region
Seismograph – Seismic waves can be detected and
recorded by using
 3 separate sensing devices
 One records vertical motion of the ground
 Other two record horizontal motion of the ground
 Records motion by tracing wave – shaped lines
 Translating motion into electronic signals
 Electronic signals recorded on magnetic tape
 Seismic waves directly loaded into computer and
Types of Seismic Waves
 Primary Waves (P waves) move fastest, first
recorded, travel through solids and liquids
 P waves move fastest through rigid material
 P waves cause rock particles to move together and
apart along direction of wave
 Secondary Waves (S waves) second waves to be
recorded, travel only through solid material, not
detected on other side of earth from earthquake
because they don’t travel through the earth’s liquid
 S waves cause rock particles to move at right angles
to the direction the wave is traveling
 Long Waves (L waves) – slowest moving waves,
last recorded, movement similar to ocean waves
 L waves cause surface to rise and fall, destructive
through loose earth
Locating an Earthquake
(Earthquake Search) How did you find the epicenter?
Earthquake Measurement
Richter Scale – expresses magnitude of earthquake
 Magnitude measure of the energy released
 Each increase of one whole number represents
release of 31.7 times more energy
M=1 to 3: Recorded on local seismographs, but
generally not felt
M=3 to 4: Often felt, no damage
M=5: Felt widely, slight damage near epicenter
M=6: Damage to poorly constructed buildings and
other structures within 10's km
M=7: "Major" earthquake, causes serious damage up
to ~100 km (recent Taiwan, Turkey, Kobe, Japan, and
California earthquakes).
M=8: "Great" earthquake, great destruction, loss of
life over several 100 km (1906 San Francisco, 1949
Queen Charlotte Islands) .
M=9: Rare great earthquake, major damage over a
large region over 1000 km (Chile 1960, Alaska 1964,
and west coast of British Columbia, Washington,
Oregon, 1700).
The Richter scale is logarithmic, that is an increase of 1 magnitude unit
represents a factor of ten times in amplitude. The seismic waves of a
magnitude 6 earthquake are 10 times greater in amplitude than those of a
magnitude 5 earthquake. However, in terms of energy release, a magnitude 6
earthquake is about 31 times greater than a magnitude 5.
The diagram below demonstrates how to use Richter's original method to measure
a seismogram for a magnitude estimate in Southern California: