EOSC 114 Unit 2 Earthquakes.docx

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Earth and Ocean Sciences
EOSC 114
Leah May Ver

Unit 2: Earthquakes  Describe the global distribution of earthquakes and how often quakes of various magnitudes occur o Earthquakes tend to happen near plate boundaries  Plate = lithosphere = crust + top of upper mantle. Strong layer, about 100 km thick.  Plates float on the asthenosphere  Oceanic plates: fast moving (cm’s/year), young (less than 200 MY), created at mid-ocean ridges, destroyed at subduction zones  Continental plates: old (buoyant, not subducted), slower moving (mm to cm)  Relative motion of these plates results in…  Mid-ocean ridge spreading center: tension    Transform fault: shearing , slide past  Collision boundaries/subduction zones: compression    Understand the different types of faulting at different plate boundaries, and which plate boundaries produce the largest quakes 1. Divergent plate boundaries: Plates move apart due to tension (stretching), small-ish earthquakes 2. Convergent plate boundaries: Due to compressive forces  Continental collision: Continental plates collide, tends to form mountains  Subduction zone:  Oceanic + continental: oceanic plate is subducted under continental -> volcanic activity  Oceanic + oceanic: once plate subducted under another  Convergent boundaries cause small to very large quakes.  Subduction zones cause the greatest quakes – located in Japan, Mexico, Alaska, etc 3. Transform plate boundaries: Plates move sideways past eachother, leads to shearing motion  Many moderate to large quakes, but not as large as convergent margins  San Andreas fault, Queen Charlotte fault 4. Intraplate earthquakes: not at plate boundaries, can be devastating  Occur along ancient fault lines/plate boundaries which have been reactivated  Waves can travel far without getting smaller.  Many found in Ontario and Quebec.  Plate Boundaries near UBC 1. Queen Charlotte fault – north, near Vancouver island 2. Cascadia subduction zone: Juan de Fuca plate and North American plate  Divergent margin formed between Juan de Fuca and Pacific Plate 3. San Andreas fault much further, south.  Fastest plate on earth = Nazca plate 1. Creates mostly shallow quakes, biggest magnitudes are deep  In BC, most earthquakes (minor) happen around Vancouver and Queen Charlotte Islands. Some intraplates near Alberta.  Quakes follow factor of 10 rule -> minor ones common, occur 10 times as less for every increase in magnitude  Describe how the Earth builds, stores, and releases energy in earthquakes (elastic rebound) o Types of stress: compressional, tensional, shear = depends on direction of force and orientation of surface  Pressure: compressional stress that is the same in all directions (Cup example). NOT STRAIN.  Changes an object’s volume, not its shape  Stress can involve forces with different magnitudes -> changes shape. Force per area.  Change of shape under stress = strain  Understand concepts of (1) stress causing strain and (2) plastic versus brittle deformation o Responses to stress  Elastic deformation: relatively small stress, smalls trains but not permanent since material bounces back. Energy can pass as waves.  Plastic deformation: like putty, material strains but does not bounce back  Brittle deformation: material stores elastic energy but will eventually break. Catastrophic release. o Cold rock is brittle = upper crust. Can break if stress is large. o Hot rock is ductile = most of Earth’s interior  Elastic Rebound Theory o A pre-existing fault is locked by friction  BUT, elastic plates (“block”) on each side move slowly relative to each other (mm to cm per year)  This leads to the deformation of the blocks  Since they are elastic, shear stress gradually builds up  Earthquake starts at hypocenter/focus. At this points, two blocks slide past  In most cases, sliding stops and earthquake is tiny  Rupture occurs when elastic stresses exceed friction  However, in a large quake, a large part of the fault breaks and the blocks move relative to each other. Elastic strain and shear stress decrease. o At a depth, fault zones are ductile and not brittle. Relative motion of plates here is steady, so no quakes.  Describe how the rupture propagates from the focus and why shaking and damage are not necessarily greatest at the epicenter  Faults are weak surfaces, weaker than surrounding rock o They break repeatedly and may accumulate 100s of km of “slip” over millions of years o Rupture begins at hypocenter and travels two ways  Maximum slip is usually NOT at the hypocenter  The rupture propagates away at 2-3 km/sec  Shaking is greatest in the direction the rupture travels!  Foreshocks sometimes formed, aftershocks always, aftershocks decrease as 1/time o Biggest is usually 1 magnitude smaller than mainshock, however, distribution of SIZE does not vary o Aftershocks form because earthquake stress changes can affect probability on nearby faults  Describe the different types of seismic waves and how they move through the Earth o When elastic energy of rocks is released, energy goes to breaking rocks, generating heat o A tiny fraction of total energy causes seismic waves which can travel far from the hypocenter  Body waves: travel inside the Earth o P wave (Primary)  Compression and extension of a solid – like a sound wave.  Fastest type of seismic wave, about 6 km/s in continental crust. o S wave (Secondary)  Shearing distortion of the solid  Particle moves perpendicular to direction that the wave propagates  Slower than P wave, about 3.5 km/s. Cannot pass through fluids.  Surface waves: require an interface – ground-air, water-air, mantle-li
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