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Chapter 3

Chapter #3 Notes


Department
Environmental Science
Course Code
EESA06H3
Professor
Nick Eyles
Chapter
3

Page:
of 3
EESA06 Chapter #3 Textbook Notes
Notable Historical Earthquakes
Charlevoix-Kamouraska Seismic Zone in Quebec, 1663 and 1870 (First earthquakes ever
documented in North America)
1929 Eastern Canada, Triggered a tsunami killing 28, scientist we able to record speed of the
movement.
1906, a 450km long scar was visible on northern California coast. 3000 people died and $400
million of damage was caused by the earthquake. Most of the damage was caused by fires.
1989, earthquake hit San Francisco. Death toll 63 people and $.6 billion in damages.
1964, earthquake rocked southern Alaska. This earthquake was twice as strong as 1906 San
Francisco earthquake. 15 people died and $300 million in damages. Tsunami was created and
felt across Pacific Ocean.
1994, San Fernando Valley, north of Los Angeles was hit with earthquake. Highways and
buildings were damaged. 16 people died when apartment collapsed and $25 billion in damages.
2002, Alaska on the Denali fault was the largest earthquake ever recorded. No deaths and
minimal damage.
2004, 9.3 magnitude earthquake deformed the Indian Ocean and caused more casualties than
any other earthquake before.
What Causes Earthquakes
Terms
Earthquakes
Trembling or shaking of ground caused by sudden release of energy stored in
rocks in Earth’s interior.
Seismic Waves
Waves of energy produced by earthquakes.
Elastic Rebound
Theory
The sudden release of progressive stored elastic strain energy in rocks, causing
movement along a fault.
Classic Model
Called the elastic rebound theory in which tectonic forces act on a mass of rock over many
decades. As more energy is stored onto the rock, the energy stored in the rock eventually
exceeds the breaking strength of the rock. The rock breaks suddenly, causing an earthquake.
Implied that existing forces are strong, a very large stress must act to break rock along a fault.
Improved Model
Faults are weak, and need only a small stress to cause rupture and earthquake.
Suggestive idea but not conclusive. This weak-fault model poses serious problems for
earthquake prediction.
Earthquakes in eastern North America are not associated with surface displacement. Earthquakes can
occur during volcanic eruptions and as magma forcibly fills underground magma chambers prior to
eruptions. Earthquakes don’t always have to be associated with fault movements.
Earthquake Components
Structure
Focus
The point within earth where seismic waves first occur.
Epicentre
Point on Earth’s surface directly above focus.
Seismic Waves
Body Waves
Primary waves (P Waves) is compression or longitudinal wave in which rocks
vibrate back and forth parallel in direction to wave propagation
Secondary waves (S Waves) is slower transverse wave that travels through
near surface rocks. Cannot pass through a fluid.
Surface Waves
Travel on earth’s surface away from epicentre. Causes more damage than body
waves. Produce Love or Rayleigh waves.
Study of Earthquakes
The use of a seismometer is used to measure seismic waves. Usually placed in group of 3 to record in 3-
dimensional space. Seismic waves can pick up seismic waves in order of P waves, S waves and then
Surface Waves. By analyzing the seismograms on a seismometer, the location and size of the
earthquake can be determined.
Richter scale is a numerical scale of magnitude in which earthquakes are identified. This tends to be
inaccurate for magnitudes above 7. New method of calculating magnitude is called moment magnitude
which currently is the most objective way of measuring energy released by large earthquake.
Types of Damage Earthquakes Cause
Ground Motion
Fire
Landslide
Liquefaction
Permanent Displacement
Foreshocks and aftershocks
Tsunami
Possible Earthquake Hotspots
Circum-Pacific belt: Encircles rim of Pacific Ocean
Mediterranean-Himalayan belt: From Mediterranean Sea and meets at Circum-Pacific belt
Shallow focus earthquakes near crest of mid-oceanic ridges
Isolated volcanoes such as Basaltic volcanoes in Hawaii
Benioff Zones: Places with inclined seismic activity
Island Arc: Line of islands formed by Andesitic volcanoes
Earthquakes at Plate Boundaries
Interplate earthquakes (Edge of plates)
Divergent: Shallow earthquakes, low magnitude
Transform: Shallow earthquakes
Convergent: Two types, collision and subduction
Intraplate earthquakes (Middle of plates)
Occur in areas of thinned or weakened crust such as continental margins, aulacogens or ancient
suture zones. Triggered by build-up of stress between lithosphere and asthenosphere as plates
move.
Earthquake Prediction
Monitoring slight changes in rock next to a fault line before rock breaks or moves.
Before earthquake, small cracks open within rocks causing tremors or microseisms to increase.
Change in rock magnetism.
Radon emissions increase from wells prelude to earthquake.
Surface of Earth tilts and change elevation.
Animal Behaviour where animals run away or freak out.
Study of seismic gaps to predict future earthquakes by looking at historical earthquakes.