Earthquakes Notes.docx

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Earth and Ocean Sciences
EOSC 114
Leah May Ver

Earthquakes Learning Goals A. Describe the global distribution of earthquakes and how often quakes of various magnitudes occur  Earthquakes occur where tectonic plates are colliding or moving apart  They do not occur with equal likelihood everywhere, but do seem to occur in specific areas more often  Larger earthquakes occur to the west of Vancouver Island  Earthquakes can have negative value because of magnitude is in logarithmic scale, basically before converting the value was between 0 and 1  Seismogram is a graph which measures ground motion related to time  Majority of the earthquakes occur at the margins of tectonic plates  Largest occur where oceans meet continents B. Understand the different types of faulting at the plate boundaries, and which plate boundaries produce the largest quakes  There are 9 major plates in lithosphere, there are two types of plates: oceanic plates and continental plates o Oceanic plates are fast-moving, centimeters per year, and young, 200 million years old, formed at mid-ocean ridges, destroyed at subduction zones o Continental plates are slow-moving, millimeters or centimeters per year, much older than oceanic plates, do not get subducted as they are more buoyant than oceanic plates  Sheer – sliding past  Tension – moving apart  Compression – colliding  Plate boundaries and faults: o Divergent: Plates move apart at mid-ocean ridges and spreading centers, small earthquakes, normal dip-slip faults o Transform: Plates move past each other, shearing forces between the plates, many earthquakes occur, medium to large sized o Convergent I: Plates move toward each other and collide, compression forces, one of the plates is less dense than the other causing one of the plates to dive under or subduct under the other plate, largest earthquakes occur at subduction zones o Convergent II: Plates move toward each other and collide, don’t subduct, plates crumple up like a rug pushed together, small to large earthquakes occur  Two forces drive motion of the plates: o 1. Gravity pulls on the denser portions of the plates that are diving under others o 2. Heat within the Earth causes convection cells to cycle within the mantle  Earthquakes primarily occur at plate boundaries  The largest earthquakes occur where the largest amount of energy is accumulated, which are at collision zones  Pacific Northwest is the region from Northern California to Central British Columbia and is referred to as Cascadia o This region has a relatively high earthquake risk  Seismic echo sounding is an observation techniques that involves generating strong pulses that travel through the Earth like sound waves and they produce a pattern of echoes which can be processed to create images of the subsurface Lesson 2  Three types of ground motions can be observed: o Permanent shifts in ground position o Slow plastic movement o Short oscillations after which ground returns to it’s origin  Elastic deformation means the shape is restored when force is removed and energy passes as waves  Plastic deformation means materials change shape permanently and there is no storage of energy  Brittle deformation means material stores energy and accumulates stress and breaks when too much stress is applied o There is catastrophic release of energy in the form of sound, heat, and motion o These are seismic waves What are faults?  Fault is a region where rocks have broken and the two sides move relative to each other, three types of faults exist: o Dip-slip faults  Vertical motion along a slanting plane  Reverse faults (side leaning over moves up), normal faults (side leaning over moves down due to gravity)  Pulled away from each other in normal divergent boundaries, pushed towards each other in reverse convergent boundaries o Strike-slip faults  Motion that is horizontal  Two types depending on which side they move  Rocks being sheared occur at transform boundaries o Oblique faults  Motion that is a combination of vertical and horizontal motion How does stress change before, during, and after an earthquake?  After an earthquake stress increases the likelihood at the ends of the zones where faults moved  An accelograph records the change in speed of the ground o Similar to seismograph, but responds to much larger variations in ground  The hypocenter or focus is the point where the energy is released and the felt zone is where the ground motion was felt Describe the different types of seismic waves and how they move through the Earth  The released energy is converted into wave energy  Two types of seismic waves: o Body Waves – travel through the interior of the materials, two types based on particle motion associated with the wave  Pressure, Primary, P-Waves:  When particles move back and forth in line with the direction of the wave  Travel 20 times faster in rock than air  Secondary, S-Waves:  When particles move side to side, perpendicular to
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