EOSC 114 Class notes Earthquake.docx

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University of British Columbia
Earth and Ocean Sciences
EOSC 114
Brett Gilley

EOSC, Class Notes, 9.Jan, 1. Earthquakes 09/01/2013 09:57:00 Seismology: the study of earthquakes - Plate Tectonics Earth surface consists of a number of lithosphere plates that include both the crust and upper mantle. These plates are moving around and they interact with each other. Parts of the mantle:  continental crust  oceanic crust  Lithosphere = crust + upper mantle  upper most mantle  Asthenosphere Plate fault happen at the interactions points of two lithospheres Plates are block of lithosphere floating on ductile asthenosphere Kinds of boundaries between lithospheres:  Divergent boundaries (plates are moving apart from another) o Warm rocks of the mantle push the plates apart, rifting o They start generating rocks between the plates o These rocks help push the plates away from each other o This produces a midoceaninc ridge o No massive earthquakes at divergent boundaries  Convergent boundaries (plates moving toward one another) o Stress can build up in large amounts and cause massive earthquakes o The places where the two plates meet is the subduction zone, where earth quakes happen  Transform boundaries (plates sliding along one another) -why plates move Ridge push. Movement for plates at ridges is propelled by sliding along the elevated lithosphere-asthenopshere boundary (Divergent margin) Slap pull: dense, subducting lithosphere ‘pulls’ the plate along with it (Converging margin) -different types of stress stress is the force (N) applied per unit of areas (m^2) stress is measure as N/m^2, also known as Pascals (Pa).  compressive (forces acting towards each other)  tensile (away from each other)  shear (next to each other in different directions) EOSC, Class Notes, 11.Jan, 2. Earthquakes 09/01/2013 09:57:00 Strain (deformation) A change in the shape or size of a body because of the application of a stress. Three kinds of deformation:  Elastic o Temporary change in shape or size o Recovers when stress is removed  Plastic o Permanent change in shape or size o Results in folding of rock (due to compression)  Brittle o Loss of cohesion due to stress o The material fractures, resulting in faults Stress vs. strains The type and extent of strain observed in a material is consequences of:  the type of stress  temperature  material properties Faults Large fracture in the earths crust. Fault are the point of origin for most earthquakes. Movement range is mm to km. Kinds of faults:  Normal Faults o Caused by tension (divergent margin) o Blocks move away o Hanging wall block moves down relative to footwall  Reverse Fault o Caused by compression (convergent margin) o Blocks move towards each other o Hanging wall block moves upwards relative to footwall  Thrust fault o Like reverse fault but at low angle  Strike Slip Fault o Caused by shear stress (transform margin) o Motion is lateral (like on a highway) o The fault itself is vertical (no hanging or foot wall) o Determination for movement to the right (right lateral) or the left (left lateral) of the original position (right or left lateral, strike slip fault) Boundaries and Earthquakes  Boundary -> Earthquakes Magintude o Divergent -> Small o Convergent -> small to very large o Transformation -> small to large o Intraplate Earthquakes Earthquakes can occur within plates  Less common  Can be large and damaging  Occur along ancient fault lines/plate boundaries Earthquakes take place when the stress at faults are to high ,at a place called focus then rupture takes place. The surface spot is the epicentre, directly above the movement. Due to the rupture high amounts of energy are released: * * * The earthquake cycle; elastic rebound theory Elastic drops after an earthquake and then slowly accumulate again until the next one. * * * Elastic rebound in a subduction zone (Vancouver island is rising every year due to elastic deformation) EOSC, Class Notes, 14. Jan, 3. Earthquakes 09/01/2013 09:57:00 In a subduction zone the overlying plate builds up stress on the underlying plate by movement towards it, which then ruptures. Seismic Waves  In an earthquake, energy is release as movement  *  *  Seismic waves radiate from the focus of an earthquake There are two types of seismic waves Each type of waves has a characteristic motion  Body waves: with the earth o P waves (Primary or pressure)  Compression movement  This waves always arrives first  6km/s o S waves (secondry or sheer)  Side to side sheering motion  Surface waves, translation of other waves into another kind of motion at the surface of the earth  Surface waves o Very slow o Moving at the surface of the earth o Cause a lot of damage  Raleigh waves  Rolling motion  Love waves  Side ways motion Seismograph (seismometers)  Records earthquakes waves Seismogram  Record of an earthquake Seismic waves can travel the globe At any given seismograph p-wa
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