Download Earthquakes

Earthquakes
Faults
Building of stress stores the energy that is released
suddenly as the rocks "break", resulting in an earthquake.
Anatomy of an earthquake
The distribution of
earthquakes.
Earthquake: shockwaves generated by energy released as
adjacent rock bodies suddenly move.
Sudden movement in response to building stress as forces
act on the rock bodies.
What is an earthquake?
Strength and effects
of earthquakes
What is an Earthquake?
Earthquake prediction
The Great Canadian
Earthquake?
Faults
All earthquakes are generated in the crust or the upper
mantle.
Most earthquakes take place in response to motion along a
fault.
Fault Animation
Planes along which rock bodies are displaced in response
to forces acting in opposite directions on either side of the
plane
Fault plane: plane along
which movement takes
place.
Hanging wall: rock body
above the fault plane.
Footwall: rock body below
the fault.
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Anatomy of an Earthquake
Focus (or hypocentre): the center of energy release.
Epicentre: the point on the ground surface
immediately above the focus (closest point on the
surface to the focus).
Types of Shockwaves
P-waves (primary waves)
Compress and relax rock through which they pass.
Pass through liquids and solids.
Highest velocity seismic waves.
Wave front: surface defining the front of the
shockwaves emitting outward from the focus.
Ray path: the direction of propagation of the
shockwaves.
S-waves (secondary or shear waves)
Cause side to side motion of rocks through which they
pass.
Velocity slower than P-waves.
Travel through solids, not fluids.
2
L-waves (surface waves)
Seismic Wave Velocities
Travel along the surface like a water surface wave.
Slowest waves but most destructive.
VP=primary wave velocity.
Height up to > 0.5 m. Length up to > 8m.
Last for 3 to 4 minutes.
Due to the combination of P-waves and S-waves at the
surface.
K is the bulk modulus (incompressibility of the
medium), n is the rigidity, and d is the density of the
medium.
Typical seismic wave velocities:
VS=secondary wave velocity.
Crust
Mantle
P-waves
<8 km/s
S-waves
<4.5 km/s
P-waves
8-13.6 km/s
S-waves
4.5-7 km/s
In a fluid n=0, therefore VS=0: secondary waves do not
pass through fluids.
3
Modified Mercalli Scale of Earthquake Intensity
Intensity and Magnitude of Earthquakes
Intensity: a measure of the effect.
Depends on energy released, distance from epicentre,
type of bedrock.
Magnitude: measure of the energy released.
Mercalli Intensity Scale
Based on an estimate of the damage caused by an
earthquake.
Maximum at the epicentre, decreasing with distance
from it.
I. Instrumental
Detected only by seismographs
II. Feeble
Noticed only by sensitive people.
III. Slight
Resembling vibrations caused by heavy traffic.
IV. Moderate
Felt by people walking; rocking of free standing objects.
V. Rather strong
Sleepers awakened and bells ring.
VI. Strong
Trees sway, some damage from overturning and falling objects.
VII. Very strong
General alarm, cracking of walls.
VIII. Destructive
Chimneys fall and there is some damage to buildings.
IX. Ruinous
Ground begins to crack, houses begin to collapse and pipes
break.
X.Disastrous
Ground badly cracked and many buildings are destroyed. There
are some landslides.
XI.Very Disastrous
Few buildings remain standing; bridges and railways destroyed;
water, gas, electricity and telephones out of action.
XII.Catastrophic
Total destruction; objects are thrown into the air,much heaving,
shaking and distortion of the ground.
Seismometers, seismographs and seismograms
Later seismographs were based on the following basic design:
Seismometer: an instrument that detects the passage of a shock wave
through the crust.
Recording pen is on a free swinging weight that remains
stationary as the recording chart moves in response to the
shock waves.
Seismograph: the instrument that records the passage of a shock
wave.
Seismometers are mounted on bedrock as P and S waves are
damped by soil, etc.
Seismogram: the printed record of the shock waves.
Chinese philosopher Chang Hêng
invented the first seismometer in
132 AD.
2 metres in diameter with 8 balls
oriented with the principle
compass points.
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Seismograph Animation
Modern seismometers have electronic sensors and transmit their
records digitally.
Here’s the on-line seismogram from the Maryland Geological Survey:
The seismogram is produced remotely from the seismometer.
http://www.mgs.md.gov/seismics/helicorder.php
Seismic Record Archive:
http://www.mgs.md.gov/seismics/eqarchv.shtml
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Richter Scale
Based on the amplitude of seismic waves measured on
a seismograph, corrected for distance from the
epicentre.
Proportional to the amount of energy released at the
focus.
Value does not vary with distance from epicentre.
Logarithmic: a 1 unit increase in the scale represents
an increase in energy release by a factor of 31.
Magnitude
(Richter
Scale)
Approximate
Maximum
Intensity
1
Number
Per Year
Approx. energy
release (Kg of TNT
equivalents).
2,900,000
II
4
III
6,200
5
VI
800
20,000,000
6
VII
120
600,000,000
7
X
8
XII
9
360,000
49,000
20
2
3
18
600
20,000
600,000
20 billion
1
60 billion
Decades apart
20 trillion
Upper limit, about 9.5 on the Richter Scale.
Rocks are not strong enough for greater magnitude
earthquakes.
Damage due to earthquakes
Surface vibration
Caused by rolling surface waves.
Amplified on soft sediment.
Major damage to buildings, water mains, sewers, etc.
View of the San Francisco business district following the
1906 Earthquake (7.9M)
Ensuing fires devastating due to lack of water.
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Oakland Freeway, 1989
Landslides
Surface waves send loose debris moving down slopes.
Dangerous in areas of high relief.
Rock and debris slides down slope.
The town of Yungay, Peru,
was buried beneath over 2
million m3 of debris after
7.5 M earthquake.
The landslide traveled down
the slopes at over 160 km
per hour.
Can dam rivers to cause floods and/or mudslides.
Damage can be distant from area affected by surface
vibration.
Over 66,000 people were
killed in a matter of
minutes.
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Liquefaction
Loss of sediment strength due to rapid upward flow of
pore waters in response to vibration.
The 1964 Niigata Earthquake (7.5M) liquefied the soil
beneath these apartment buildings. They sank down into
the slurry and tipped over.
Sandy soil turns to quicksand.
Buildings sink and tilt.
Earthquakes commonly generate tsunamis at the edge of the continental slope
(associated with subduction).
Tsunamis
Tsunamis are the second most powerful waves on the oceans.
Part of the wave travels over the adjacent shelf reaching shore quickly.
The other part travels across the ocean basin (12-14 hours across the Pacific
Basin).
Generated when ocean waters are displaced:
Underwater earthquakes
Underwater volcanic eruptions
Underwater landslides
Asteroid/comet impacts
Waves are generated above the disturbance and propagate outward from that
point.
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Life of a Tsunami
(US Geological Survey)
http://temp.water.usgs.gov/tsunami/basics.html
Earthquake Induced Tsunamis
Initiation
Split
In some cases earthquakes cause displacement of the sea floor which, in
turn, causes displacement of the water surface (i.e., generating a wave).
The wave radiates outward from the epicentre.
Many earthquakes are generated at trenches, close to shoreline and on
the open ocean.
The shoreward part of the wave travels a short distance to the shoreline.
The ocean-ward wave travels at high velocity across the basin.
Wave celerity is proportional to the square root of water depth so that it
travels fastest over the open ocean (up to 6000m depth).
On the open ocean:
Wave length: 160 km
Amplification
As the wave enters shallow water as it approaches land it becomes higher
and shorter (amplified).
Wave height: commonly up to 0.5 metres on the open ocean.
Celerity: up to 800 km/hr (the speed at which the waves travel)
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Run-up
As the wave propagates towards land the water level rises.
Following maximum runup the waters flow back offshore and may be
followed by subsequent waves.
In some cases a trough of the wave reaches land first and the water
recedes from the shoreline and then returns as the wave reaches land.
The first wave may or may not be the biggest and subsequent waves follow.
Run-up is the measure of the height of the wave (with respect to sea
level) when it passes over land.
In most cases the wave does not form a “crashing” surf; the water rises
and flows inland as a powerful current.
The 1883 eruption of the volcano Krakatoa caused one of the worst
tsunamis of historic time.
Largest recorded Tsunami at landfall:
85 metres in height (at an Island south of Japan)
(Niagara Escarpment is about 50 m high at Brock)
The largest witnessed tsunami to
date was in Lituya Bay, Alaska.
Coral blocks up to 600 tons were washed ashore.
This steamship was carried almost 2
km onto the land and dropped 10 m
above sea level.
A landslide created a wave that left
splash marks 1720 feet above
normal water level.
Along low lying coasts of Java the
waves washed 8 km onshore,
dragging people along with them
as they washed back to sea.
The tsunami was recorded as a small rise in sea level as far away as
the California coast (20 hours after the fourth blast).
An estimated 36,417 people were killed by the tsunami alone.
10
The 2004 Indonesian tsunami was triggered by a 9.0 magnitude
earthquake (energy = 20 Trillion Kg of TNT).
http://neic.usgs.gov/neis/eq_depot/2004/eq_041226/
Tsunami arrival times
Earthquake: 0058 12/26 (UTC) 2004
World Propogation Animation
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Tsunami run-up is the height above sea-level at the most inland
location that the water penetrates.
Tsunami run-up exceeded 10 m at land fall in some locations.
Inundation distance is the distance inland from the normal shoreline
that the water penetrates.
Inundation distance depends on the slope of the land extending down
to the shoreline and the run-up elevation.
Video images of the tsunami
At landfall
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Inland
Tsunami height (cm) on the Atlantic Ocean
Date
Origin
Effects
June 7, 1692
Puerto Rico
Trench
Port Royal Jamaica
permanently submerged
Nov. 1, 1755
Atlantic Ocean Lisbon destroyed
60,000
Feb. 20, 1835
Peru-Chile
Trench
Concepcion, Chile destroyed
Not Known
Aug. 8, 1868
Peru-Chile
Ships washed several miles inland 10-15,000
Trench
Aug. 27, 1883
Krakatoa
Devastation of East Indies
36,000
June 15, 1896
Japan trench
Swept the east coast of Japan
with 30.5m waves
27,122
March 3, 1933 Japan Trench
Wrecked 9,000 houses,
8,000 ships
May 22, 1960 South-central
Chile
Damage to Chile and Hawaii
Aug. 23, 1976
Celebes Sea
SW Philippines devastated
Dec. 26, 2004
Indonesia
Indian Ocean Tsunami
Death Toll
Megatsunamis
2000
Large tsunamis termed “megatsunamis” have been attributed to
collapse of volcanoes in the past.
Tsunamis were generated by landslides as the side of the volcano
Mauna Loa collapsed into the ocean.
3,000
1500, 61 in Hawaii
8,000
283,100 people were killed,
14,100 are still listed as missing
1,076,350 were displaced
Source (in part): http://www.pmel.noaa.gov/tsunami-hazard/majortsunamischart.pdf
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Alika 2 landslide involved 120 cubic miles of debris. (Mt. St.
Helen’s landslide involved less than 1 cubic mile).
The sediments just on top of the debris are 120,000 years old.
On land deposits of coral debris
that is about 120,000 years old
have been found on the side of
Kohala volcano.
Deposited by the massive
tsunami that was generated by
the landslide.
The deposits are 4 miles inland
and 1,600 feet above the
position of the shoreline
120,000 year ago.
Such displacement of material
to this elevation would require
a 200 metre high wave.
The island is subsiding do to the
weight added to it by ongoing
lava eruptions.
Large landslides that could generate
such tsunamis occur every 100,000 to
200,000 years and put most of the
Pacific basin at risk.
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Tsunami Risk from the Canary Islands?
At least a dozen major landslides on volcanoes have taken place
over the past several million years along the Canary Islands.
From: Ward, S.N. and Day, S.,2002, Cumbre Vieja Volcano—Potential collapse and tsunami at La Palma, Canary Islands.
GEOPHYSICAL RESEARCH LETTERS, VOL. 28, NO. 17, PAGES 3397–3400, 2001
La Palma Island has been identified as a volcanic island that may
have the geological conditions for a major landslide.
If such a landslide takes place up to 500 km3 of debris will enter
the ocean.
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Date
Origin
Effects
June 7, 1692
Puerto Rico
Trench
Port Royal Jamaica
permanently submerged
Nov. 1, 1755
Atlantic Ocean Lisbon destroyed
60,000
Feb. 20, 1835
Peru-Chile
Trench
Concepcion, Chile destroyed
Not Known
Aug. 8, 1868
Peru-Chile
Ships washed several miles inland 10-15,000
Trench
Aug. 27, 1883
Krakatoa
Devastation of East Indies
36,000
June 15, 1896
Japan trench
Swept the east coast of Japan
with 30.5m waves
27,122
March 3, 1933 Japan Trench
Wrecked 9,000 houses,
8,000 ships
May 22, 1960 South-central
Chile
Damage to Chile and Hawaii
Aug. 23, 1976
Celebes Sea
SW Philippines devastated
Dec. 26, 2004
Indonesia
Indian Ocean Tsunami
Death Toll
Tsunami Detection Network
2000
Established to provide forewarning of an incoming tsunami in the
eastern Pacific basin.
3,000
1500, 61 in Hawaii
8,000
283,100 people were killed,
14,100 are still listed as missing
1,076,350 were displaced
Source (in part): http://www.pmel.noaa.gov/tsunami-hazard/majortsunamischart.pdf
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Goal: to detect tsunamis as small as
1 cm in 6000m of water.
Small changes in water pressure are
measured at the Sea floor to detect
changes in water depth.
http://www.pmel.noaa.gov/tsunami/Mov/DART_04.swf
During “Tide Mode” the
buoys transmit data once per
hour.
If the water level exceeds 1 to
3 cm of the expected level the
buoy goes into “Tsunami
Mode” and transmits data
every minute for three hours.
Following the 2004 tsunami plans have been made to expand the system
across the Pacific and to deploy new buoys in the Atlantic.
Real-time data
17
Bedrock geology and physiography are important in
setting risk:
Areas of high relief may experience landslides.
Low areas with sandy soil may undergo liquefaction.
Coastal areas may experience tsunamis.
Distribution of Earthquakes
Earthquakes occur where a variety of forces are at
work:
Lateral forces in the crust (driven by sea floor
spreading).
Devastating Historic Earthquakes
Date
Location
Magnitude
Fatalities
1556
China
9.0
850,000
1737
India
?
300,000
1905
India
?
370,000
1908
1920
Messina, Italy
China
7.5
8.6
86,926
100,000
1923
Japan
8.3
200,000
1970
Northern Peru
7.7
66,794
1988
Armenia
6.8
55,000
1999
Turkey
7.8
15,000
2001
India
7.7
20,103
For an up-to-the- minute answer to the question, where do
earthquakes occur, go to:
http://earthquake.usgs.gov/recenteqsww/index.html
Vertical forces due to the buoyancy of the crust
floating on the upper mantle.
Buoyant force: the vertical force exerted on the crust
by the fluid upper mantle.
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Magnitude
(Richter
Scale)
Approximate
Maximum
Intensity
1
Number
Per Year
Approx. energy
release (Kg of TNT
equivalents).
2,900,000
2
3
II
4
5
20
360,000
49,000
600
20,000
III
6,200
60,000
VI
800
20,000,000
6
VII
120
60,000,000
7
X
18
20 billion
8
XII
9
1
60 billion
Decades apart
20 trillion
http://www.gp.uwo.ca/
Recent seismic activity in Eastern Canada
19
Earthquakes are particularly common:
1. Oceanic trenches (Animation)
1. Along oceanic trenches.
Subducting crust gets “stuck” as it descends, storing
energy that is released to cause an earthquake.
2. In regions of continental collision.
Earthquakes occur beneath and within the obducting
(over-riding) plate.
3. Along oceanic ridges and transform faults.
And less common….
4. Within plates, well away from plate margins.
Foci become deeper in the direction of subduction; to a
maximum depth of 700 km.
Foci delineate the path of the subducting crust.
Most occur along the top of or
within the subducting plate
with some associated with the
island arc.
The top of the subducting crust
is cooler than the bottom and
generates more friction.
20
Marianas Trench: western
Pacific basin
Earthquake activity along the Tonga Trench
21
3-D illustration of subducting crust along the
Marianas/Japan/Kuriles Trench.
A similar pattern of
earthquakes occurs
where oceanic crust
subducts beneath
continental crust.
Earthquakes also occur
within the obducting
continental crust due to
compressive forces that
cause movement along
thrust faults.
The distribution of earthquakes
Photo by George Ericksen, USGS
22
2. In regions of continental collision.
Mostly shallow to intermediate depth earthquakes.
Due to compressive forces as two continental land masses collide.
The modern Himalayan Mountains are an extensive mountain belt
that formed when the Indian plate collided with the Eurasian plate.
Mountain building continues as India is driven northeastward with
respect to Eurasia due to spreading along the oceanic ridge to the
west of India.
As the two continental masses collided thick slices of the leading
edge of India moved southward along several thrust faults.
Thrusting thickened the crust to form the Himalayan mountains.
Northward thrusting of the Eurasian crust formed the highland of
the Tibetan Plateau.
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Most earthquakes are generated with the movement related to
thrusting.
Deep earthquakes have also occurred (several hundred km) due to
the ongoing subduction of oceanic crust from before the collision.
3. Oceanic Ridges
Largely shallow
earthquakes (foci
above 70 km depth)
along the length of the
ridge.
Earthquakes are generated:
1. Along the ridge axis due to sea-floor
spreading.
2. Along transform faults due to lateral slipping
between plates.
Motion is periodic;
rocks stick,
accumulate energy and
release it when they
break.
24
The San Andreas Fault is a
transform that has been overridden by the North American
Plate.
The distribution of
earthquakes in eastern
Africa suggest the possible
formation of a new segment
of oceanic ridge.
The fault experiences over
15,000 earthquakes per year.
The East African Rift Valley
experiences many shallow
earthquakes that are
distributed much like those
along the oceanic ridge.
Evolution of the San Andreas Fault
The Red Sea and the Gulf of Aden
formed with the development of a
new segment of oceanic ridge that
split the north-eastern area of
Africa apart.
The East Africa Rift Valley is the
third “arm” of the same ridge
system.
It may evolve to split eastern
Africa off of the continent to form
a new sea over the next 10 million
years, or so.
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The distribution of earthquakes
Continental rifting animation
4. Within plates, well away from plate margins.
Most “intracratonic”
earthquakes are due to
“isostatic adjustments” of the
Earth’s crust.
Intracratonic: within the
craton, the oldest, tectonically
stable portion of any continent.
Earthquakes in the Appalachians:
Once a high, thick mountain belt that has undergone
millions of years of erosion
Thousands of metres of rock have been eroded (reducing
the weight of the crust).
Buoyant forces are pushing the
mountains upwards.
Animation
Isostatic adjustments: the crust
rises or sinks deeper into the
mantle as weight is added to it
or removed, respectively.
Earthquakes are generated by
slippage along old faults in the
crust.
26
Earthquakes along the glacial limit:
As continental glaciers retreated about 10,000 years ago a
great weight was removed from the crust.
Glacier thickness ranged to over 2 of kilometres.
Added weight pushed the crust
into the mantle by up to 300
metres (about 50 metres in
southern Ontario).
Since the glaciers retreated the
crust has rebounded (as it is
pushed upwards by buoyant
forces in the mantle).
Vertical movement results in earthquakes, especially along
regions with old faults (e.g., the St. Lawrence valley).
Along the glacier limit there is strong differential uplift
(north of the limit crust is rising whereas south of the limit
it is not).
Results in small intensity
earthquakes.
New Madrid, Missouri, has
experienced the highest
magnitude quakes in central
North America (estimated at
> 8.5M)
Earthquake prediction is important for several reasons
Long term prediction:
Establishing construction standards based on risk.
Evaluating risk for insurance purposes (rates higher in
high-risk areas).
Risk-based site selection (e.g., for nuclear power plants)
Short term prediction:
Providing forewarning of an impending earthquake.
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Long term prediction
Probabilistic approach: What is the likelihood of an
earthquake of a given intensity taking place at a given
location?
Based on the frequency, magnitude and spatial
distribution of historical earthquakes.
Recurrence rates
Prediction of earthquake risk based on the probability that
an earthquake of a given magnitude will happen at some
location over a given period of time.
Assumes that earthquakes will take place in future at the
same rate and in the same locations that they have in the
past.
http://www.gp.uwo.ca/docs/eqmapp3.html
28
Problem: the historical record is relatively short and in
some areas earthquakes are clustered in time:
E.g., Major New Madrid earthquakes were clustered:
1835-1847
1896-1911
1933-1942
Recurrence rates assume uniform distribution of
earthquakes over time.
For New Madrid, the probability of a major earthquake
varies with time.
Seismic Gaps
Regions where earthquakes do not take place with the
frequency and/or magnitude expected for the tectonic
setting.
Gaps are regions
where powerful
earthquakes may
be expected despite
the historical
record.
Short term prediction
In gaps plates may be sliding smoothly past each other
(no earthquakes).
OR
Plates may be stuck and storing strain energy to be
released as a large magnitude earthquake.
Seismic risk may be extreme in some seismic gaps.
A variety of methods that identify precursor events,
events that occur prior to earthquakes that can be
used to provide warning.
Seismic wave velocities
Changes in the ratio of VP/VS over time (VP and VS
measured from induced shockwaves – explosive
charges).
29
1. VP/VS varies
randomly by a few %.
2. VP/VS decreases
sharply by 6-15%.
3. Just prior to the
event VP/VS returns to
normal.
The magnitude of
the resulting
earthquake
increases with the
duration of the time
over which VP/VS is
reduced.
The return to normal
signals the impending
earthquake.
Why does VP/VS vary in this manner?
Building strain causes microfractures.
K = bulk modulus (incompressibility)
n = rigidity
d = density
Microfractures reduce K and n.
VP varies with K and n whereas VS varies only with n.
Therefore there is a net decrease in VP/VS.
30
VP/VS increases prior to earthquake as fractures fill
with water (increasing both n and K).
Water also may lubricate the fault and help generate
the earthquake.
Greater intensity with
increasing duration: longer
time to build energy.
Ground level deformation
Microfractures cause the rock to dilate (increase in
volume).
On the ground surface above the site of strain
accumulation the dilation causes the ground to rise.
Measuring ground level elevation in seismically active
areas can show where strain is accumulating and
where earthquakes may occur.
Not feasible when strain is accumulating deep in the
crust.
Vertical movements along
the west coast of Japan near
the June 1964 Niigata
earthquake.
Groundwater Chemistry
Microfractures can release gases from rocks.
These gases dissolve in groundwater and turn up in
well water.
Monitoring concentrations of gases can provide
evidence for microfractures and the strain
accumulation that causes them.
Radon gas is one such gas that has been useful in some
areas.
It forms in the rock as a product of the radioactive
decay of Uranium-238.
31
Just prior to an earthquake the concentration of radon
has been observed to increase sharply in deep water
wells.
Other elements have shown similar behaviour:
Microearthquake swarms
Direct Sensors
Monitoring primary seismic waves directly.
Microfractures involve the release of a small amount
of energy, producing microearthquakes.
P-waves arrive first so they give direct warning of
incoming surface waves.
Microearthquakes can be measured with very sensitive
seismometers.
e.g., focus 200 km away, P-waves will arrive about 30
seconds before surface waves.
As strain builds thousands of microearthqakes are
generated.
Off the coast of Japan, near the trench, underwater
seismometers send signals to shore when the first pwaves arrive.
Monitoring the microearthquakes can identify
locations of strain accumulation risk of an earthquake.
Sirens go off giving several providing tens of seconds
to take action (get under furniture, etc.).
.
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Anecdotal evidence that animals may be able to
forewarn of an Earthquake
Place
Earthquake
When observed
Place
Earthquake
When observed
Description
San
April 18, 1906 Night before
Francisco
(8.2M)
Few seconds
Tiger depressed; pandas screamed; turtles
restless; yak did not eat; swans stayed
away from the water.
Dogs barked.
Horses and cows snorted and stampeded;
cats felt aftershocks before people.
US
Aug. 17, 1959 Conflicting
(7.1M)
Water birds left lake area.
Italy
May 6, 1976
(6.7M)
2-3 hours before Cats left houses and villages;
mice and rats left hiding places;
fowl refused to roost.
Italy
Calbria
Feb. 5, 1783
(?M)
?
Description
China July 18, 1969
Tientsin
(7.4M)
Zoo
2 hours before
China Feb. 4, 1975
Haicheng (7.3M)
1.5 months
1-2 days
20 minutes
Snakes came out of hibernation.
Pigs did not eat and climbed walls.
Turtle jumped out of water and cried
Japan
Tokyo
Nov. 11, 1855
(7.3M)
1 day before
Wild cats cried; rats disappeared.
Japan
Sanriku
March 3, 1933 1 week before
(8.5M)
2-3 days before
1 day before
Several hours
Geese cackled; dogs howled so unbearably
loudly that they had to be shot!
Rats disappeared;
Rats and cats unusually quiet;
Seagulls left their usual habitat;
Duck stayed away from usual sleeping
place.
Earthquake Storms?
Earthquake storms are suspected along Turkey’s Anatolian Fault.
Along some faults when an earthquake takes place stress
becomes concentrated elsewhere along the fault to produce
another earthquake.
“Earthquake Storms” are a series of earthquakes in
sequence along a fault or fault system.
First recognized on the San Andreas Fault in 1992.
Two earthquakes occurred within 3 hours of each other
when the stress of the first earthquake was transferred to
the location of the second.
Such earthquakes could follow hour, weeks, months or
years following the initial earthquake.
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Given the pattern of earthquakes along the fault, the 1999
earthquake near Izmit (population 500,000) was predicted prior to
the Earthquake.
Evidence of a similar chain of earthquakes circa 1200 BC have led
to suggestions that their cumulative devastation may have lead to
the end of the Bronze Age.
25,000 people were killed by the 1999 (7.4M) earthquake.
The next major earthquake is predicted to take place near Istanbul a
city with a population of 4 million people.
Concentration of major cities along active plate boundaries which
experience earthquake storms may have led to a weakening of their
stability and infrastructure.
Extensive and repeated earthquake damage left the cities
vulnerable to military defeat.
The Great Canadian Earthquake???
(Evidence for a southern BC megaquake)
Evidence from:
Seismic Gap
There have been no major earthquakes since European
settlers occupied the area (James Cook was the first
European to visit in 1778).
Seismic gap
Tectonic setting
Particularly no quakes associated with low angle thrust
faults.
Geologic record
In the tectonic setting such earthquakes are expected.
Japanese sea level records
Native Legend
Either the plates are slipping freely or they are stuck,
building strain.
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Tectonic setting
New crust is forming along the Juan de Fuca ridge, a few
hundred kilometres west of Vancouver Island.
The subducting crust is very young, light and buoyant and
descends at a low angle, exerting an upward force on the
base of the obducting North American Plate…increasing
frictional resistance to subduction.
This crust is subducting
at a trench, just west of
Vancouver Island.
Similar tectonic settings produce major earthquakes.
Geologic evidence
What happens when plates are locked in such a tectonic
setting?
e.g., Chile, 1960, experienced a
9.5 M earthquake in the same
tectonic setting.
The time interval between such
high magnitude earthquakes is on
the order of 200 years.
When plates lock during subduction they buckle: the
region near the trench rises (uplifts) and the region inland
sinks (subsides).
Therefore, we can’t rely on the historical record of the
west coast.
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This is a repeating cycle of strain buildup, release as an
earthquake followed by repeated buildup of strain.
When the plates release the strain that has built over time
and earthquake takes place and coastal areas subside and
the region inland rises back up.
Look for evidence of uplift and subsidence along the west
coast of B.C.
Raised beaches: beaches at
elevations above the local
water line because the land
surface has uplifted.
Such raised beaches are
known to have developed
over the past few hundred
years along the west coast of
B.C.
Inland of the coast are swamps that flooded over the past
century (evidence for subsidence?).
Cores of sediment in the bogs indicate that there has been
alternating periods of subsidence and uplift approximately
every 300 years.
Evidence of a repeated
pattern of uplift along the
coast and subsidence inland.
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A Japanese Tsunami?
Japan keeps detailed, accurate measurements of sea level
for the past several centuries.
BC Native legend describes a winter’s night, not long
before the Europeans arrived, that was rocked by
catastrophe.
A rise in sea level by about 15 m at 1700h on January 27,
1700 indicates that a tsunami had reached their coasts.
Another legend speaks of the sea emptying from a bay
followed by its return to cover an entire island….an
account of a massive tsunami?
Hindcasts of the source of the tsunami points to the west
coast of British Columbia or Washington, about 11:00 PM
on January 26, 1700.
Evidence suggests:
The earthquake that would have produced the tsunami
would have likely exceeded 9 M – a megaquake.
Megaquakes likely do occur on the west coast of BC with a
recurrence interval of about 300 years.
The last big earthquake was approximately 300 years ago!
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