EESA06H3 Chapter Notes -Juan De Fuca Plate, 1989 Loma Prieta Earthquake, San Andreas Fault

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24 Apr 2012
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Chapter Three - Page 1 of 12
Chapter Three: Earthquakes
Feb 5, 1662 violent earthquake Charlevoix-Kamouraska area of Quebec
o Vast landslide along rivers in the St. Lawrence valley
o This area has received many seismic events over the past two centuries and is aptly
named the Charlevoix-Kamouraska Seismic Zone
o October 20, 1870 Baie-St-Paul earthquake (same zone) ripped open the ground along
several fissures near the epicentre in Baie-Se-Paul, while tremors were felt as far south
as Virginia
o Among the earliest documented earthquakes in NA
November 18, 1929 eastern coast of Canada
o Parts of the Atlantic ocean sea floor moved in the Grand Banks earthquake
o Large submarine landslide ruptured transatlantic cables, allowing scientists to record
the speed of movement and erosive capabilities of a turbidity current for the first time
o Large tsunami struck the Burin Peninsula of Newfoundland
28 deaths
November 25, 1988 largest earthquake in eastern Canada
o Saguenay region of Quebec
o Compromised the engineering integrity of structures built on glaciomarine clays
(quickclays) that underlie the region
April 18, 1906 part of California (coastal northern cali) slid abruptly past the rest of the state
o Visible scar 450km long
o Soil above rock was displaced
o The quake was located on a segment of the San Andreas fault near San Fran
o Buildings in SF toppled; broken gas mains; fires for three days; broken water mains
o 3000 deaths
o 90% of destruction caused by fires
October 17, 1989 San Fran
o Loma Prieta earthquake
o South on the San Andreas fault near Santa Cruz
o Didn’t tear ground surface but collapses buildings and structures built on the soft ‘bay
fill’ sediment in San Fran and Oakland
o Bay Bridge collapsed
o Raging fires; broken gas mains
o 63 deaths
March 27, 1964 southern Alaska Good Friday Earthquake
o Force was twice as strong as the 1906 San Fran earthquake but death/damage was low
because of Alaska’s small population
o 15 deaths from the shaking
o Section of earth was raised and another sank
o Slight horizontal movement
o Landslides in Anchorage
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Chapter Three - Page 2 of 12
o Tsunami responsible for most deaths (100)
o Few people drowned in Oregon and Northern Cali from the Pacific Ocean waves
o Also caused damage in Port Albemi, BC
Jan 17, 1994 Northridge Earthquake San Fernando Valley (north of LA)
o Near Cali State University
o Lots of damage
o 16 deaths
November 3, 2002 largest earthquake recorded in the interior of Alaska
o Rupture mainly along the Denali fault
Propagated eastward at more than 11,000km/h, offset streams and glaciers,
thousands of landslides
o No deaths; minimal damage (remote area)
o Trans-Alaska Pipeline suffered minor damage (didn’t break due to engineering design)
December 26, 2004
o 9.3 magnitude
o Second largest recorded since 1900
o Deformed the Indian ocean floor off the western coast of Northern Sumatra
o Quake was so severe that it slightly changed the shape of the planet, reduced the length
of the day by almost 3 microseconds, and moved the north pole by several centimetres
o Tsunami caused more casualties than any other ever recorded
Detected on tide gauges in the Indian, Pacific, and Atlantic oceans
Quake epicentre was located to the east of the Sunda Trench, where the india
plate is subducted below the Burma plate
Caused by thrust faulting along the boundary between the two plates
This type of destructive megathrust earthquake is possible along the western
coast of Canada and the US Pacific Northwest where the Juan de Fuca plate is
being subducted beneath the NA plate
Feb 8, 2001 Nisqually earthquake
o 6.8 magnitude
o South-west of Seattle
o Deep quake with such movement of the downgoing Juan de Fuca plate
o Damage caused to unreinforced brick and concrete structures
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 deep in the earth may put a stress on the rock, which may bend or change in shape
(strain)
When a rock breaks, waves of energy are released and sent through the Earth (seismic waves)
o These waves cause the ground to shake
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Chapter Three - Page 3 of 12
The sudden release of energy may cause one mass of rock to slide past another into a different
position
o The break between these two masses is a fault
Elastic rebound theory
o Sudden release of progressively stored elastic strain energy in rocks, causing movement
along a fault
o Deep seated internal forces (tectonic forces) act on a mass of rock over many decdes
o Initially, the rocks bends, lifts, or stretches
o When the rock breaks, there’s an earthquake
o Two masses of rock move past each other along a fault movement can be vertical,
horizontal, or both
Elastic rebound theory revised
o Suggests that faults are weak, and need only a small stress to cause rupture and an
earthquake
o The weak fault model poses problems for earthquake predictions
Brittle behaviour of breaking rock is only for rocks near the surface
o Rocks at depths are subject to heat and pressure which reduces brittleness
o Deep rocks deform plastically (ductile) instead of breaking (brittle); hence, there’s a limit
to the depth where faults can occur
Most earthquakes are linked with movements on faults
Most in eastern NA aren’t associated with surface displacement
Earthquakes can also occur during volcanic eruptions as magma forcibly fills underground
magma chambers prior to many eruptions; these have no fault movement at all
Deep earthquakes (100-670km below the surface) are found on cold, subducting plates slides
down into the mantle
o Although the downgoing plates are colder than the surrounding rock, the high
temperature/pressure suggests that the rock in the plates should behave plastically
rather than break in a brittle manner
o Suggested cause of these quakes is mineral transformations within the downgoing rock
as pressure collapses one mineral into a denser form
Indonesia/Sumatra Earthquake and Tsunami
250 000 deaths in south Asia and east Africa
Caused by release of energy along a portion of the boundary between the India plate and the
subducting Burma plate
Boundary forms a large thrust fault that allows movement between the two colliding plates
Ocean floor above the thrust fault was uplifted several metres and created a large tsunami wave
in all directions
Wave height reduces over time as it spreads across the ocean and loses energy
Two hours after the quake, it was 60cm and travelling at 750km/hr
9 hours after the quake, it was between 5-10cm and had spread across most of the Indian ocean
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