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Weeks 3 and 4.docx

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Western University
Earth Sciences
Earth Sciences 2240F/G

Weeks 3 and 4 Chapter 4: Earthquakes, the Basics  1201: Syrian/ Egyptian earthquake killed 1.1 million people  1556: china, 830000 killed in Shansi province  2004 tsunami: dead and missing of 250000  2010 Haitian quake: maybe 316000 dead, but there is no census Seismic Energy Source  An earthquake is the ground shaking that accompanies sudden movement on a fault, but also may be the shaking produced by motion of magma underground, a fast-moving landslide or an underground nuclear explosion  The boundaries of plates are active regions, and most of that action takes the form of: o Normal faults o Reverse and thrust faults o Strike-slip or transform faults  Just because there is a break or fracture through a rock doesn’t meant that the blocks on either side move readily, but if they do that’s a ‘creep’ o Normally there is enough friction from the rocks on either side of the break to allow stress energy to build o When the resistance across the break is overcome, the blocks ‘jump’ along the fault plane to new positions, and it is the release of that energy that produces an earthquake  Stress energy vs. strain: at low applications of stress, the rock will deform, but as soon as you release the stress, the deformation disappears (elastic). If you impose greater stress, the rock will deform to its elastic limit, and when the stress is released, not all the deformation disappears (plastic deformation). Even greater stress will result in rock fracture o Cold/brittle rocks, which are close to earth’s surface have virtually zero elasticity, and break with the application of very little stress (but deeper rocks can handle more stress) Seismic Energy Expression  Creation of a reverse fault (reverse evidence as a scarp/cliff). It is pretty rare to find rock that’s uniformly homogenous, so some spot along the fracture plane is weaker than other spots, and this is where the failure/rupture will start (called the focus or hypocenter) o The point vertically above the focus is the epicenter o The failure on the fault plane will be greatest at the focus, and will gradually fade out in all directions as the release is consumed by moving the rock blocks  The seismic energy waves travel outward from the focus in all directions, but there are different kinds of motion to those waves o ‘Jolt’ and sound of an energy wave called ‘primary’ or P-wave. The P-waves travel as a series of fast compression waves. As the front of each wave passes, there is momentary compression of atoms or molecules in everything it passes through  More brittle material will probably be cracked  P-waves’ velocity depends on the density of the material they pass. In dense rock =8+km/s, less dense =5-6km/s. P-waves go through anything! o Secondary waves, S-waves, shear waves, are slower than P-waves. They oscillate up and down, in the range of 4.5-3.5km/s  Don’t go through liquids o Together, P- and S-waves are body waves o Surface waves are the really destructive events: they are slow because instead of going through earth, they go on top, at 2-3km/s. Large amplitudes  Love waves move forward across the surface, side to side oscillation  Rayleigh waves move forward, up and down backward rotating motion Measurement: Location and Magnitude  Inertia: resistance to motion. The base of a measurement system is going to vibrate because it’s resting on Earth’s surface, so we attach heavy masses to the actual recorder part of the instrument, knowing that inertia will give us a short breathing spell before the vibration of the earthquake can get those big masses to move  Seismographs: one aspect measures the horizontal motion, another aspect the vertical motion Location  To locate the epicenter, you need to record an earthquake on a minimum of three seismographs, and use triangulation o Measure the time interval between the arrival of the first P- and the first S-waves at each seismograph  We get distances from three location, of the distance from epicenter. BUT this assumes that we know exactly how fast the seismic energy waves move through all the hugely different rocks between the site of the seismograph and the epicenter of the earthquake (impossible) o We use a table of average seismic wave travel distance per unit time, compiled from experimental results Magnitude  Before Richter, the strength of earthquakes was measured subjectively, by the amount of observed damage  Modified Mercalli Intensity Scale o Tells little about an earthquake in a region essentially devoid of people and buildings  Richter Magnitude Scale, 1935: worked within 100km of an earthquake, termed the ‘local magnitude’ scale, designation is M L  Nomograph: three strips of paper such that a straight line joined 100km on the distance scale in log units, 3 on the arithmetic magnitude scale, and 1mm on the log amplitude scale  the standard earthquake o Now, it was easy to get the magnitude of any earthquake: you just needed the distance away from your seismograph, the amplitude from your instrument’s graph, and you could read the magnitude from the straight line joining those two  This is a logarithmic magnitude scale: magnitude 6 is 10x more energetic than 5. This 10x factor doesn’t hold all the way through the scale, it really only holds until about 7  the frequency of shaking seems to peak somewhere between magnitudes 7 and 8, and from there, higher magnitude numbers mean the shaking has affected bigger areas, with greater duration of shaking o So the Richter Scale saturates at 8, becoming unreliable after Moment Magnitude Scale  1979, M W based on the total energy released by an earthquake, never saturates or becomes non-scalar, based upon seismograph instrumental recordings. Total energy is calculated from: o Total measured area of the fault that has ruptured o Amount of offset along the fault (how far did the rocks move) o The strength of the rocks involved  This is where the Richter Scale falls inaccurate, the moment scale can calculate up to 9.5, just about the maximum earthquake magnitude that can be generated by any plate tectonic process Destruction- Acceleration, Period and Resonance  Acceleration: rate of change of velocity with respect to time, measured in g, where 1 g = 32 feet/sec/sec, or 980cm/sec o Two components, horizontal and vertical  Acceleration in the horizontal sense: 0.1-0.2g of horizontal acceleration means trouble standing (standing on a train, sharp sway), at 0.1g structural damage begins. At horizontal acceleration of 1.8g, destruction is pretty much total o Vertical acceleration isn’t really an issue  A flagpole moves back and forth with a period of 2 seconds. If a seismic wave also has a period of 2seconds, the pole will resonate, moving more and more. If the flagpole is sitting in a chunk of concrete, and the dimensions/constructions of the base mean that it starts resonating at a 2- second period, the whole pole will vibrate to pieces o Mexico 1985, buildings resonated and smashed together Liquefaction  Wet sand supports your weight: but if you jump up and down, in seconds you’ll sink. The sand sediment weight is partially supported by the water in the pore spaces between the sand grains, but when the sand is shaken, the grains are pushed apart, and so the sand-water mixture now behaves as a liquid (quicksand) o During earthquakes, the sand can turn from a solid to a liquid causing destruction Plate Boundary Quakes- Divergent Boundaries  Divergent: stress action pulls the opposite sides apart, quakes are numerous, close to surface, low magnitude o Therefore, rocks breaking apart at the Mid-Atlantic Ridge won’t initiate worrisome tremors, but continental crust is much thicker, so it takes more stress to pull apart, and so a higher magnitude o Normal faults, extension to the crust Convergent Boundaries  Subduction zones: strongest, in fact the 1960 Chile quake was alone responsible for 30-45% of th all the energy released by all quakes in the 20 century  The vast majority of ruptures in a falling slab of crust occur at shallow depth, where the slab is being bent  reverse faults, where a shortening of crust results o Shallow ruptures, but the earthquakes aren’t small  There’s a scattering of earthquakes at deeper levels. The upper ones result from ‘shuffling’ and breakage of the cold rock as it is subjected to stress by activity within the asthenosphere. Gradually the slab becomes a bit
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