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Chapter 3

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Environmental Science
Nick Eyles

Chapter 3- Earthquakes: • On February 5 , 1663, a violet earthquake struck the Charlevoix-Kamouraska area of Quebec - So strong and felt over entire eastern part of North America - No loss of life reported - So violent, and terrifying - This area of Quebec has experience many seismic events over the past two centuries and is named the Charlevoix-Kamouraska Seismic zone. - The October 20 , 1870, Baie-St-Paul earthquake was also in this zone. - These are among the earliest earthquakes to be documented in North America. What causes earthquakes? • Earthquake: 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). - Rock can deform only so far before it breaks  when a rock breaks, waves of energy are released & sent through the Earth  SEISMIC WAVES - Seismic waves  the waves of energy produced by an earthquake. - It is the seismic waves that cause the ground to tremble and shake during an earthquake. - Sudden release of energy may cause one huge mass of rock to slide past another mass of rock into a different relative position. - The break between 2 rock masses is a FAULT. • Classic explanation of why earthquakes take place is called the  elastic rebound theory • Elastic rebound theory  involves the 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. Initially the rock bends, lifts, or stretches but does not break. More & more energy is stored in the rock as the bending becomes more severe. • Eventually the energy stored in rock exceeds the breaking strength and the rock suddenly breaks causing an earthquake. • Two masses of rock move past one another along a fault. • Movement can be vertical, horizontal or both. • Strain on rock is released; energy is expended by moving the rock into new positions and by creating seismic waves. Classic model: - Implies that existing faults are strong; - A very large stress must act to break rocks along the fault. New idea: - faults are weak - need only a small stress to rupture and an earthquake • Brittle behavior of breaking rock is characteristic only of rocks near Earth’s surface. • Rocks at depth are subject to increased temperature & pressure, which tend to reduce brittleness. • Deep rocks deform plasticity (ductile behavior) instead of breaking (brittle behavior) • Hence, limit to the depth where faults can occur • Most earthquakes are associated with movement on faults • Earthquakes also occur during explosive volcanic eruptions and as magma forcibly fills underground magma chambers prior to many eruptions  these quakes may not be associated with fault movement at all. • The suggested cause of deep quakes is mineral transformations within downgoing rock as pressure collapses one mineral into a denser form. • Similar cause of deep quakes include the dehydration of water-containing serpentine and the conversion of serpentine into glass. Why do earthquakes cause so much damage? • The point within the earth where seismic waves first originate is called the FOCUS (or hypocentre) of the earthquakes. • This is the centre of the earthquake  the point of initial breakage and movement on a fault. • Rupture beings at the focus and then spreads rapidly along the fault plane. • The point on the earth’s surface directly above the focus is the EPICENTRE. • 2 types of seismic waves are generated during earthquakes. 1. BODY WAVES  are seismic waves that travel through the Earth’s interior, spreading outward from the focus in all directions. 2. SURFACE WAVES  are seismic waves that travel on Earth’s surface away from the epicenter, like water waves spreading out from a pebble thrown into a pond. • Rock movement associated with seismic surface waves dies out with depth into the Earth, just as water movement in ocean waves dies out with depth. Body waves- - 2 types of body waves 1. P Wave  a compressional (or longitudinal) wave in which rock vibrates back and forth parallel to the direction of wave propagation. - b/c it’s a very fast wave , travelling through near-surface rocks at speeds of 4-7 km per second (14 400 to more than 25,000 km/h), a P wave is the first (or primary) to arrive at a recording station following an Earthquake. 2. S Wave  secondary; is a slower, transverse wave that travels through near- surface rocks at 2-5 km per second - Shearing motion much like that in a stretched, shaken rope. - The rock vibrates PERPENDICULAR to the direction of waves are moving. • Both P waves and S waves pass easily though solid rock • A P wave can also pass through a fluid (gas or liquid) but a S wave CANNOT. Surface waves- - Slowest waves set off by earthquakes - Cause more property damage than body waves b/c they produce more ground movement and travel more slowly so they take longer to pass. - Two most important types of surface waves are LOVE WAVES and RAYLEIGH WAVES named after the geophysicist who discovered them. 1. Love waves – are most like S waves that have NO vertical displacement. - The ground moves side to side in a horizontal plane that is perpendicular to the direction the wave is travelling or propagating. - Like S waves, love waves do not travel through liquids and would not be felt on a body of water. - B/c of horizontal movement they tend to knock buildings off their foundations and destroy highway bridge supports. 2. Rayleigh waves – behave like rolling ocean waves - Unlike ocean waves, Rayleigh waves cause the ground to move in an elliptical path opposite to the direction the wave passes. - Rayleigh waves tend to be incredibly destructive to buildings because they produce more ground movement and take longer to pass. How do we know where earthquakes occur? - Instruments that could accurately record seismic waves - They measure the amount of ground motion and can be used to find the location, depth, and size of an earthquake. - SEISMOMETER  instrument to measure seismic waves - The principle of the seismometer is to keep a heavy suspended mass as motionless as possible – suspending it by springs or hanging it as a pendulum from the frame of the instrument. - When the ground moves, the frame of the instrument moves with it. - The inertia of the heavy mass suspended inside keeps the mass motionless to act as a point of reference in determining the amount of ground motion. - Seismometers usually placed in clusters of 3 to record the motion along the x, y, and z axes of 3-D space. - A seismometer by itself CANNOT record motion that it measures. - A SEISMOGRAPH is a recording device that produces a permanent record of Earth motion detected by a seismometer, in digital format that can be processed & displayed on computer terminals. - SEISMOGRAM is the paper record of the Earth’s vibration; can be used to measure strength of an earthquake. - Network of seismograph stations maintained all over the world to record and study earthquakes (& nuclear bomb explosions) Determining the location of an earthquake: - P & S waves start out from the focus of an earthquake at essentially the same time. - As they travel away from the quake, the two kinds of body waves essentially separate b/c they are travelling at different speeds. - On a seismogram, from a station close to the quake, the first arrival of P wave is separated from the first arrival the S wave by a short distance on paper recorded. - At a recording station far away from the quake, however the first arrivals of these waves will be recorded much farther apart on the seismogram. - The farther the seismic waves travel, the longer the time intervals between the arrivals of P and S waves and the more they are separated on the seismograms. - Since the first arrivals of P and S waves increases with distance from the focus of the earthquake, this interval can be used to determine distance from the seismograph station to a quake. - The increase in the P-S interval is regular with increasing distance for several thousand km and so can be graphed in a TRAVEL-TIME CURVE, which plots seismic-wave arrival time against distance. In the past: - A single analog station could determine only the distance to a quake, not the direction. - A circle was drawn on a globe, with the centre of the circle being the station and its radius the distance of the quake. - With information from 2 or more stations they could pinpoint the location of the quake. - If 3 or more stations determined the distance to a single quake, a circle was drawn for each station and the intersection of the circles located the epicenter. - Modern 3-component seismographs now record the intensity of the earthquake vibrations in their 3 orthogonal directions. - This allows scientists to determine both direction of the incoming wave and the distance from epicenter. - Analyses of seismograms also indicate at what depth it occurred. - Most quakes occur at the earth’s surface although a few occur much deeper. - Maximum DEPTH OF FOCUS is the distance between focus and epicenter  for earthquakes is about 670 km. - Quakes classified into 3 groups according to depth of focus: 1. Shallow focus  0-70 km deep 2. Intermediate focus  70-350 km deep 3. Deep focus  350 – 670 km deep - Shallow focus earthquakes most common; 85% of total quake energy released. - Intermediate (12%) & deep (3%) focus quakes are rarer b/c most deep rocks flow plasticity when stressed or deformed; they are unable to store and suddenly release energy as brittle surface rocks do. Measuring size of Earthquake- • Size measured in 2 ways 1. Find out how much and what kind of damage the quake has caused. - This determines INTENSITY which is a measure of an earthquake’s effect on people & buildings. - Intensities expressed as roman numerals (from I to XII which is 1-13) on the MODIFIED MERCALLI SCALE  higher numbers = greater damage. - Intensity as a measure has numerous drawbacks. o Different locations report different intensities o Damage to buildings & other structures depends on type of geologic material on which a structure was built as well as type of construction. o Damage estimates are also subjective (people may exaggerate) o Intensity maps cannot be drawn for uninhabited areas (the open ocean for instance) so not all quakes can be assigned intensities. o The one big ADVANTAGE of intensity ratings is that no instruments are required which allows seismologists to estimate the size of earthquakes that occurred before seismographs were available. 2. Calculate the amount of energy released by the quake - This method is usually done by measuring the height (amplitude) of one of the wiggles on a seismogram. - The larger the quake, the more the ground vibrates and the larger the wiggle. - After measuring a specific wave on a seismogram and correcting for the type of seismograph and for the distance from the quake, scientists can assign a number called the MAGNITUDE. - Magnitude is a measure of the energy released during the earthquake. - Magnitude reported on RICHTER SCALE, a mathematical scale of magnitudes. o Open-ended scale, meaning no earthquakes too large or too small to fit on the scale o Higher numbers indicated larger earthquakes. o Logarithmic scale o Very small ones can have negative magnitudes but these are rare. o Largest Richter magnitude measured so far is 8.6 o Smaller earthquakes are much more common than large ones. - Different magnitudes are sometimes reported for a single quake b/c different seismic waves (body or surface) can be measured to make the scale more useful over larger areas. - Complication  magnitudes calculated from seismograms tend to be inaccurate (usually too low) above magnitude 7. - A new method of calculating magnitude involves the use of the SEISMIC MOMENT of a quake which is determined from the strength of the rock, surface area of the rupture, and the amount of rock displacement along the fault. - The MOMENT MAGNITUDE is the most objective way of measuring the energy released by a large earthquake. - Richter scale  logarithmic  difference between 2 consecutive numbers on the scale means increase of 10 times in the amplitude of the earth’s vibrations particular below magnitude 5. Ex) Measured amplitude of vibration for certain rock is 1 cm during magnitude 4 quake then for a magnitude 5 quake the rock will move 10 cm ( because it increases by 10) - Estimated that a tenfold increase in the size of earth vibrations is caused by an increase of roughly 32 times in terms of ENERGY RELEASED. So a quake of magnitude 5 releases approx 32 times more energy than of magnitude 4. Magnitude 6 is about 1000 times (32 x 32) - Although a seismograph is usually required to measure magnitude, this measure has many advantages over intensity an indicator of earthquake strength. - A single magnitude number can be assigned to a single earthquake depending on the amount & kind of local damage. - Magnitudes can be reported for all quakes, even inhabited areas where there is no property to affect. Location and size of earthquakes in North America- - Only a few localities are relatively free of earthquakes - Most of the large earthquakes occur in western north America - Quakes in the province of British Columbia, and in western states including California, Nevada, Utah, Idaho, Montana, and Washington are related to known faults and usually (but not always) involve surface rupture of the ground. - Earth quakes in Alaska occur mainly below the Aleutian islands where the pacific plate is converging with and being subducted beneath the north amercian plate. - British Columbia is the most seismically active region of Canada & lies close to active plate margins along the Cascadia Subduction Zone.  the southwestern corner of B.C. experiences more than 200 earthquakes per year and 9 moderate to large ( M = 6-7) earthquakes have occurred in the region during historic times. - Queen Charlotte Islands  Canada’s largest historic earthquake, an M = 8.1 on August 22 , 1949. - East of rocky mountain  more rare and generally smaller and deeper than earthquakes in western Northern America. - Although large quakes are extremely rare in central and eastern North America, when they do occur they can be very destructive and widely felt b/c earth crust is older, cooler and more brittle in the east than in the west and seismic waves travel more efficiently. What kinds of damage can earthquakes cause? - Ground motion  trembling & shaking of the land that can cause buildings to vibrate (during small ones windows & walls may crack from such vibration) - In very large quakes, ground motion may be visible  can be strong enough to topple large structures such as bridges & office & apartments and buildings. - Building codes & location of buildings should be controlled. - Buildings built on soft sediment are damaged more than buildings on hard rock. - Fire  particularly serious problem just after an earthquake b/c of broken gas and water mains and fallen electrical wires. o Although fire was the cause of most of the damage to San Francisco in 1906, changes in building construction and improved fire fighting methods have reduced (but not eliminated) to fire
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