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What is an earthquake?
Faults
Anatomy of an earthquake
Strength and effects of earthquakes
Earthquakes
The distribution ofearthquakes.
Earthquake prediction
The Great Canadian Earthquake?
Earthquake: shockwaves generated by energy released as adjacent rock bodies suddenly move.
What is an Earthquake?
Sudden movement in response to building stress as forces act on the rock bodies.
Building of stress stores the energy that is released suddenly as the rocks "break", resulting in an earthquake.
All earthquakes are generated in the crust or the upper mantle.
Most earthquakes take place in response to motion along a fault.
Planes along which rock bodies are displaced in response to forces acting in opposite directions on either side of the plane
Faults
Fault plane: plane along which movement takes place.
Hanging wall: rock body above the fault plane.
Footwall: rock body below the fault.
Anatomy of an EarthquakeFocus (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).
Wave front: surface defining the front of the shockwaves emitting outward from the focus.
Ray path: the direction of propagation of the shockwaves.
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Types of ShockwavesP-waves (primary waves)
Compress and relax rock through which they pass.
Pass through liquids and solids.
Highest velocity seismic waves.
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.
L-waves (surface waves)
Travel along the surface like a water surface wave.Slowest waves but most destructive.
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.
Seismic Wave Velocities
VP=primary wave velocity.
K is the bulk modulus (incompressibility of the medium), n is the rigidity, and d is the density of the medium.
VS=secondary wave velocity.
In a fluid n=0, therefore VS=0: secondary waves do not pass through fluids.
Typical seismic wave velocities:
Crust P-waves <8 km/s
S-waves <4.5 km/s
Mantle P-waves 8-13.6 km/s
S-waves 4.5-7 km/s
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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.
Modified Mercalli Scale of Earthquake Intensity
Chinese philosopher Chang Hênginvented the first seismometer in 132 AD.
Seismometers, seismographs and seismograms
Seismometer: an instrument that detects the passage of a shock wave through the crust.
Seismograph: the instrument that records the passage of a shock wave.
Seismogram: the printed record of the shock waves.
2 metres in diameter with 8 balls oriented with the principle compass points.
Later seismographs were based on the following basic design:
Recording pen is on a free swinging weight that remains stationary as the recording chart moves in response to the shock waves.
Seismometers are mounted on bedrock as P and S waves are damped by soil, etc.
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Modern seismometers have electronic sensors and transmit their records digitally.
The seismogram is produced remotely from the seismometer.
http://www.mgs.md.gov/seismics/helicorder.php
Here’s the on-line seismogram from the Maryland Geological Survey:
Seismic Record Archive:
http://www.mgs.md.gov/seismics/eqarchv.shtml
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 Approximate Number Approx. energy(Richter Maximum Per Year release (Kg of TNTScale) Intensity equivalents).
1 2,900,000 20
2 II 360,000 6003 49,000 20,000
4 III 6,200 600,000
5 VI 800 20,000,000
6 VII 120 600,000,000
7 X 18 20 billion8 XII 1 60 billion9 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.
Ensuing fires devastating due to lack of water.
View of the San Francisco business district following the 1906 Earthquake (7.9M)
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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.
Can dam rivers to cause floods and/or mudslides.
Damage can be distant from area affected by surface vibration.
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.
Over 66,000 people were killed in a matter of minutes.
Liquefaction
Loss of sediment strength due to rapid upward flow of pore waters in response to vibration.
Sandy soil turns to quicksand.
Buildings sink and tilt.
The 1964 Niigata Earthquake (7.5M) liquefied the soil beneath these apartment buildings. They sank down into the slurry and tipped over.
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Tsunamis
Tsunamis are the second most powerful waves on the oceans.
Generated when ocean waters are displaced:
Underwater volcanic eruptions
Underwater earthquakes
Underwater landslides
Asteroid/comet impacts
Waves are generated above the disturbance and propagate outward from that point.
Earthquakes commonly generate tsunamis at the edge of the continental slope (associated with subduction).
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).
Initiation
Life of a Tsunami(US Geological Survey)http://temp.water.usgs.gov/tsunami/basics.html
Earthquake Induced Tsunamis
In some cases earthquakes cause displacement of the sea floor which, in turn, causes displacement of the water surface (i.e., generating a wave).
Many earthquakes are generated at trenches, close to shoreline and on the open ocean.
Split
The wave radiates outward from the epicentre.
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
Wave height: commonly up to 0.5 metres on the open ocean.
Celerity: up to 800 km/hr
Amplification
As the wave enters shallow water as it approaches land it becomes higher and shorter (amplified).
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Runup
As the wave propagates towards land the water level rises.
Runup is the measure of the height of the wave (with respect to sea level) when it passes over land.
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.
In most cases the wave does not form a “crashing” surf; the water rises and flows inland as a powerful current.
The first wave may or may not be the biggest and subsequent waves follow.
Largest recorded Tsunami at landfall:
85 metres in height (at an Island south of Japan)
(Niagara Escarpment is about 50 m high at Brock)
Following maximum runup the waters flow back offshore and may be followed by subsequent waves.
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.
Along low lying coasts of Java the waves washed 8 km onshore, dragging people along with them as they washed back to sea.
An estimated 36,417 people were killed by the tsunami alone.
The tsunami was recorded as a small rise in sea level as far away as the California coast (20 hours after the fourth blast).
The 1883 eruption of the volcano Krakatoa caused one of the worst tsunamis of historic time.
The largest witnessed tsunami to date was in Lituya Bay, Alaska.
A landslide created a wave that left splash marks 1720 feet above normal water level.
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/
World Propogation Animation
Tsunami arrival times
Earthquake: 0058 12/26 (UTC) 2004
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Tsunami run-up is the height above sea-level at the most inland location that the water penetrates.
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.
Tsunami run-up exceeded 10 m at land fall in some locations.
Video images of the tsunami
At landfall
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Inland Tsunami height (cm) on the Atlantic Ocean
Date Origin Effects Death Toll
June 7, 1692 Puerto Rico Port Royal Jamaica 2000Trench permanently submerged
Nov. 1, 1755 Atlantic Ocean Lisbon destroyed 60,000
Feb. 20, 1835 Peru-Chile Concepcion, Chile destroyed Not KnownTrench
Aug. 8, 1868 Peru-Chile Ships washed several miles inland 10-15,000Trench
Aug. 27, 1883 Krakatoa Devastation of East Indies 36,0000
June 15, 1896 Japan trench Swept the east coast of Japan 27,122with 30.5m waves
March 3, 1933 Japan Trench Wrecked 9,000 houses, 3,0008,000 ships
May 22, 1960 South-central Damage to Chile and Hawaii 1500, 61 in HawaiiChile
Aug. 23, 1976 Celebes Sea SW Philippines devastated 8,000
Dec. 26, 2004 Indonesia Indian Ocean Tsunami 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
Large tsunamis termed “megatsunamis” have been attributed to collapse of volcanoes in the past.
Tsunamis were generated by landslides as the side of the volcanoMauna Loa collapsed into the ocean.
Megatsunamis
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.
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The deposits are 4 miles inland and 1,600 feet above the position of the shoreline 120,000 year ago.
The island is subsiding do to the weight added to it by ongoing lava eruptions.
Such displacement of material to this elevation would require a 200 metre high wave.
Large landslides that could generate such tsunamis occur every 100,000 to 200,000 years and put most of the Pacific basin at risk.
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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Tsunami Detection Network
Established to provide forewarning of an incoming tsunami in theeastern Pacific basin.
http://www.pmel.noaa.gov/tsunami/Mov/DART_04.swf
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.
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.
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Real-time data
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.
Devastating Historic EarthquakesDate Location Magnitude Fatalities
1556 China 9.0 850,0001737 India ? 300,000
1908 Messina, Italy 7.5 86,9261920 China 8.6 100,000
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
1905 India ? 370,000
1923 Japan 8.3 200,000
Distribution of Earthquakes
Earthquakes occur where a variety of forces are at work:
Buoyant force: the vertical force exerted on the crust by the fluid upper mantle.
Lateral forces in the crust (driven by sea floor spreading).
Vertical forces due to the buoyancy of the crust floating on the upper mantle.
http://earthquake.usgs.gov/recenteqsww/index.html
For an up-to-the- minute answer to the question, where do earthquakes occur, go to:
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Magnitude Approximate Number Approx. energy(Richter Maximum Per Year release (Kg of TNTScale) Intensity equivalents).
1 2,900,000 20
2 II 360,000 6003 49,000 20,000
4 III 6,200 60,000
5 VI 800 20,000,000
6 VII 120 60,000,000
7 X 18 20 billion8 XII 1 60 billion9 Decades apart 20 trillion
http://www.gp.uwo.ca/
Recent seismic activity in Eastern Canada
Earthquakes are particularly common:
1. Along oceanic trenches.
2. In regions of continental collision.
3. Along oceanic ridges and transform faults.
4. Within plates, well away from plate margins.
And less common….
1. Oceanic trenches
Earthquakes occur beneath and within the obducting(over-riding) plate.
Subducting crust gets “stuck” as it descends, storing energy that is released to cause an earthquake.
Foci become deeper in the direction of subduction; to a maximum depth of 700 km.
Foci delineate the path of the subducting crust.
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Most occur along the top of or within the subducting platewith some associated with the island arc.
The top of the subducting crust is cooler than the bottom and generates more friction.
Earthquake activity along the Tonga Trench
Marianas Trench: western Pacific basin
3-D illustration of subducting crust along the Marianas/Japan/Kuriles Trench.
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A similar pattern of earthquakes occurs where oceanic crust subducts beneath continental crust.
Earthquakes also occur within the obductingcontinental crust due to compressive forces that cause movement along thrust faults.
Photo by George Ericksen, USGS
The distribution of earthquakes2. 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.
Northward thrusting of the Eurasian crust formed the highland ofthe Tibetan Plateau.
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.
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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.
Motion is periodic; rocks stick, accumulate energy and release it when they break.
Earthquakes are generated:
1. Along the ridge axis due to sea-floor spreading.2. Along transform faults due to lateral slipping between plates.
The San Andreas Fault is a transform that has been over-ridden by the North American Plate.
The fault experiences over 15,000 earthquakes per year.
Evolution of the San Andreas Fault
The distribution of earthquakes in eastern Africa suggest the possible formation of a new segment of oceanic ridge.
The East African Rift Valley experiences many shallow earthquakes that are distributed much like those along the oceanic ridge.
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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.
Continental rifting animation
The distribution of earthquakes
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.
Isostatic adjustments: the crust rises or sinks deeper into the mantle as weight is added to it or removed, respectively.
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.
Earthquakes are generated by slippage along old faults in the crust.
Animation
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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
Establishing construction standards based on risk.
Providing forewarning of an impending earthquake.
Evaluating risk for insurance purposes (rates higher in high-risk areas).
Risk-based site selection (e.g., for nuclear power plants)
Long term prediction:
Short term prediction:
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.
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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
Problem: the historical record is relatively short and in some areas earthquakes are clustered in time:
E.g., Major New Madrid earthquakes were clustered:1835-18471896-19111933-1942
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.
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.
Short term prediction
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 VSmeasured from induced shockwaves – explosive charges).
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1. VP/VS varies randomly by a few %.
The return to normal signals the impending earthquake.
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.
Why does VP/VS vary in this manner?
Building strain causes microfractures.
K = bulk modulus (incompressibility)n = rigidityd = 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.
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: longertime 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.
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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.
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
Microfractures involve the release of a small amount of energy, producing microearthquakes.
Microearthquakes can be measured with very sensitive seismometers.
As strain builds thousands of microearthqakes are generated.
Monitoring the microearthquakes can identify locations of strain accumulation risk of an earthquake.
Direct Sensors
Monitoring primary seismic waves directly.
Off the coast of Japan, near the trench, underwater seismometers send signals to shore when the first p-waves arrive.
P-waves arrive first so they give direct warning of incoming surface waves.
e.g., focus 200 km away, P-waves will arrive about 30 seconds before surface waves.
Sirens go off giving several providing tens of seconds to take action (get under furniture, etc.)..
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Place Earthquake When observed Description
China July 18, 1969 2 hours before Tiger depressed; pandas screamed; turtlesTientsin (7.4M) restless; yak did not eat; swans stayed Zoo away from the water.
China Feb. 4, 1975 1.5 months Snakes came out of hibernation.Haicheng (7.3M) 1-2 days Pigs did not eat and climbed walls.
20 minutes Turtle jumped out of water and cried
Japan Nov. 11, 1855 1 day before Wild cats cried; rats disappeared.Tokyo (7.3M)
Japan March 3, 1933 1 week before Rats disappeared;Sanriku (8.5M) 2-3 days before Rats and cats unusually quiet;
1 day before Seagulls left their usual habitat;Several hours Duck stayed away from usual sleeping
place.
Anecdotal evidence that animals may be able to forewarn of an Earthquake
Place Earthquake When observed Description
Italy Feb. 5, 1783 ? Geese cackled; dogs howled so unbearablyCalbria (?M) loudly that they had to be shot!
San April 18, 1906 Night before Dogs barked.Francisco (8.2M) Few seconds Horses and cows snorted and stampeded;
cats felt aftershocks before people.
US Aug. 17, 1959 Conflicting Water birds left lake area.(7.1M)
Italy May 6, 1976 2-3 hours before Cats left houses and villages;(6.7M) mice and rats left hiding places;
fowl refused to roost.
Earthquake Storms?
Along some faults when an earthquake takes place stress becomes concentrated elsewhere along the fault to produce another earthquake.
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.
“Earthquake Storms” are a series of earthquakes in sequence along a fault or fault system.
Earthquake storms are suspected along Turkey’s Anatolian Fault.
Given the pattern of earthquakes along the fault, the 1999 earthquake near Izmit (population 500,000) was predicted prior to the Earthquake.
The next major earthquake is predicted to take place near Istanbul a city with a population of 4 million people.
25,000 people were killed by the 1999 (7.4M) 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.
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.
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The Great Canadian Earthquake???(Evidence for a southern BC megaquake)
Evidence from:
Seismic gap
Tectonic setting
Geologic record
Japanese sea level records
Native Legend
Seismic Gap
There have been no major earthquakes since European settlers occupied the area (James Cook was the first European to visit in 1778).
Particularly no quakes associated with low angle thrust faults.
In the tectonic setting such earthquakes are expected.
Either the plates are slipping freely or they are stuck, building strain.
Tectonic setting
New crust is forming along the Juan de Fuca ridge, a few hundred kilometres west of Vancouver Island.
This crust is subductingat a trench, just 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.
Similar tectonic settings produce major earthquakes.
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.
Therefore, we can’t rely on the historical record of the west coast.
When plates lock during subduction they buckle: the region near the trench rises (uplifts) and the region inland sinks (subsides).
Geologic evidence
What happens when plates are locked in such a tectonic setting?
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When the plates release the strain that builds over time the reverse happens: coastal areas subside and the region inland rises back up.
Look for evidence of uplift and subsidence along the west coast of B.C.
This is a repeating cycle of strain buildup, release as an earthquake followed by repeated buildup of strain.
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.
Cores of sediment in the bogs indicate that there has been alternating periods of subsidence and uplift approximately every 300 years.
Inland of the coast are swamps that flooded over the past century (evidence for subsidence?).
Evidence of a repeated pattern of uplift along the coast and subsidence inland.
A Japanese Tsunami?
Japan keeps detailed, accurate measurements of sea level for the past several centuries.
A rise in sea level by about 15 m at 1700h on January 27, 1700 indicates that a tsunami had reached their coasts.
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.
The earthquake that would have produced the tsunami would have likely exceeded 9 M – a megaquake.
BC Native legend describes a winter’s night, not long before the Europeans arrived, that was rocked by catastrophe.
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?
Evidence suggests:
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!
