EESA06H3 Chapter Notes - Chapter 3: Love Wave, Juan De Fuca Plate, San Andreas Fault

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21 Apr 2012
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Chapter 3 - Earthquakes
Historical Earthquakes
Charlesvoix-Kamouraska (Quebec) – February 1663
Some of the earliest to be documented in North America
Names the Charlesvoix-Kamouraska Seismic Zone because of repeated earthquakes
No deaths in 1663 but was very violent = caused landslides
Eastern Canada – November 1929
Seafloor beneath Atlantic Ocean moved
Earthquake triggered submarine landslide – allowed scientists to record speed of movement of
current for first time
Tsunami resulted from quake in Newfounland – 28 people died
Largest earthquake in eastern Canada hit Saguenay region of Quebec in 1988.
San Francisco California – April 18, 1906
Great earthquake – California slid abruptly past the rest of the state
Visible scar 450 km where Earth was torn open
Originated on San Andreas fault near San Francisco
Ground shook for one minute = damage, broken gas mains = fire
3000 people died and $400 mill spent
90% of destruction caused by fires
AGAIN San Francisco 1989
Loma Prieta earthquake on the San Andreas fault near Santa Cruz
Ground was NOT torn but buildings collapsed
Section of Bay Bridge collapsed
Fires by broken gas mains again; hard to extinguish since water mains were broken too
Severe damage in small towns NEAR THE CENTRE of the quake
Death toll = 63, Damage $= 6 billion.
Alaska 1964
Earthquake for 3 minutes
Force was twice as strong as 1906 San Francisco but loss of life and property was low because
of SMALL population
Only 15 died due to shaking and damage cost $300 million
Greatest loss of life due to tsunami generated by earthquake – almost 100 drowned
Northridge Earthquake in San Fernando Valley (Los Angeles, California)
40 seconds
Building & schools damaged
16 killed
Alaska 2002
Greatest earthquake ever recorded in the interior of Alaska
Ruptured the ground surface
Triggered thousands of landslides
No deaths and minimal damage since it occured in a remote area
Pipeline did not break due to good engineering design
December 26, 2004
Earthquake – 9.3 magnitude
2nd largest recorded since 1900 - Deformed Indian Ocean floor
So severe that it slightly changed the shape of the planet, reducing the length of day by 3
microseconds
The quake epicentre was located to the east of the Sunda Trench, where the India plate is
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subducted below the Burma plate, and was caused by thrust faulting along the boundary
between the two plates
This type of megathrust earthquake is also possible along the western coast of Canada and
U.S. Pacific Northwest where the Juan de Fuca plate is being subducted beneath the North
American plate.
What Causes Earthquakes?
An earthquake is a trembling or shaking of the ground caused by the sudden release of energy stored
in the rocks beneath Earth's surface
Great forces acting deep in the Earth may put a stress on the rock, which may bend or change in
shape (strain)
Strain = beding of the rock/change in shape
Bending stick analogy
Rock can deform only so far before it breaks
Seismic Waves: the waves of energy that are released when rock breaks or in other words the waves
of energy produced by an earthquake.
Eslastic Rebound Theory
Classic explanation of why earthquakes take place
Sudden release of progressively stored elastic strain energy in rocks, causing movement along a
fault
Deep-seated internal forces (tectonic forces) act on a mass of rock over many decades
First rock bends, or stretched but not stretches
More energy is built up – eventually rock breaks causing an earthquake.
Energy is expended by moving the rock into new positions and by creating seismic waves
Two masses of rock move past one another along a fault
Diagram (p 77)
2 Models for Fault Behaviour
Some say faults are strong to begin with, other says it's weak, so a small amount of stress is
enough to trigger movement along the fault and subsequent earthquake
Ductile Behaviour vs. Brittle Behaviour
Breakage of rocks is usually near the Earth's surface since they're brittle and the rocks that are
deeper are more flexible (ductile); they don't break
Although most earthquakes are associated witrh movement on faults, some are not. Some are
associated with buried thrust faults, and even volcanic eruptions
Most earthquakes in eastern North America are not associated with surface displacement
Reasons for deep earthquakes
All deep earthquakes are found on cold, subducting plates sliding down into the mantle
Since then theres high temperature and pressure at depth, rocks in plates SHOULD behave
plastically rather than breaking
So what's the cause?
Maybe mineral transformation within the downgoing rock, as pressure collapses one mineral
into a denser form
Why Do Earthquakes Cause So Much Damage?
Focus (hypocentre)
Point within the Earth where seismic waves first originate
This is the point of initial breakage and movement on a fault
Rupture begins at the focus and then spreads along the fault plane.
Epicentre
Point on the Earth's surface directly above the focus is the epicentre
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2 Types of Waves generated during earthquakes
Body waves
Seismic waves that travel through the Earth's interior, spreading outward from the focus in
all directions
P Wave (Spring)
A compressional (longitudinal) wave in which rock vibrates back and forth PARALLEL
to the direction of wave propoagation.
Very fast wave, travelling through near-surface rocks at speeds of 4 to 7 km per second
(14 400 – 25 000 km per hour)
First/primary wave to arrive at a recording station following an earthquake
CAN pass through fluid (gas or liquid)
S Wave (Vertical motion of Rope)
Secondary wave
Slower, transverse wave that travels through near-surface rocks at 2 to 5 km per second
This wave is propagated by a shearing motion much like that in a stretched, shaken rope
(Think of playing snake with a jump-rope)
The rock vibrates PERPENDICULAR to the direction of wave propogation; that is,
crosswise to the direction the waves are moving
CANNOT pass through fluid (gas or liquid)
Both P waves and S waves pass easily through solid rock.
Surface waves
Seismic waves that travel on Earth's surface away from the epicentre, like water waves
spreading out from a pebble thrown into a pond. Similar to water movement (pebble thrown
in a pond)
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