GSU GEOL 1122K - Chapter 8 Textbook Annotations (21 pages)

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Chapter 8 Textbook Annotations



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Chapter 8 Textbook Annotations

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Pages:
21
School:
Georgia State University
Course:
Geol 1122k - Introductory Geosciences Ii
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Chapter 8 Textbook Annotations 217 250 8 1 Introduction o Japan lies along a convergent plate boundary where the Pacific Plate sinks back into the mantle underneath the edge of the Eurasian Plate The boundary between the two plates is a fault a large fracture on which sliding occurs that slopes gently to the west Motion along a fault doesn t happen continuously rather for many years the rocks among the fault bends to accommodate the motion o Rock can only bend so far before it snaps Shock waves are generated by the shearing and fracturing of rock along the fault When vibrations reach the land surface the land surfaces lurches back and forth and bounce up and down o Earthquake an episode of ground shaking o The societal calamity due to an earthquake is a direct consequence of ground shaking because many buildings collapse and crush their inhabitants and debris tumbles down slopes in landslides After the earthquake the sea floor coast uplifts a massive amount of water was displaced This water moved away from the site of the earthquake in immense waves called tsunamis o Earthquakes have affected the Earth since the solid lithosphere formed Most are a consequence of plate movement they punctate each step in the growth of mountains drift of continents and the opening and closing of ocean basins Almost 1 million detectable earthquakes happen every year o Most cause no damage or casualties because they are too small or they occur in unpopulated areas A few hundred earthquakes per year rattle the ground sufficiently to damage buildings and injurie their occupants and every 5 to 10 years a great earthquake occurs 8 2 What Causes Earthquakes o Ancient cultures explanations for seismicity earthquake activity most of which involved the action or mood of an animal or a god o Scientific explanations for seismicity instead occurs for The sudden formation of a new fault fracture or rupture on which sliding occurs Sudden slip on an already existing fault A sudden change in the arrangement of atoms in rock minerals Movement of magma or explosion of a volcano A major landslide A meteorite impacts An underground nuclear bomb test o Most earthquakes are due to slips on faults plate movements o The place within the earth where tock ruptures and slips or a place where an explosion occurs is the hypocenter or focus of an earthquake Energy radiates from the focus The point on the surface of the earth lies directly above the focus is the epicenter Faults in The Crust o Faults may look simply like a fracture or break that cuts across rock or sediment The rock adjacent to the fault may be broken up into angular fragments or may be pulverized into tiny grains due to the crushing and grinding that can accompany the slip and the surface of a fault may be polished and grooved as if scratched by a rasp In some localities the fault cuts through distinct marker where this happens the end of the marker on one side of the fault is offset relative to the end on the other side o The distance between the two ends of the marker are measured along the fault surface in the direction of the slip is called displacement Many faults are completely underground and will be visible only if exposed by erosion of overlying rock Some faults intersect and offset the ground surface producing a step called a fault scrap o The ground surface exposure of a fault is called the fault line or fault trace o Rock masses above the sloping fault plane is called the hanging wall Because it is hung over heads o Rock masses below the fault plane is called the footwall Because it lies beneath their feet Describe the direction in which rock masses slipped on a sloping fault by specifying the direction that the hanging wall moved in relation to the footwall o When the hanging wall slips down the slope of the fault it s a normal fault o When the hanging wall slips up the slope it is a reverse fault If steep and thrust fault id shallowly sloping o Strike slip faults are near vertical planes on which slip occurs parallel to an imaginary horizontal line called a strike line on a fault plane No up or down motion takes place o Faults are found in many locations Not all of them are likely to be a source of earthquakes Faults that have moved recently or are likely to move soon are called active faults Faults that last moved in the distant past and probably won t move again soon are called inactive faults Generating Earthquake Energy Stick Slip o Earthquakes can happen either when rock breaks and a new fault forms or when a preexisting fault suddenly slips again Earthquakes due to fault formation Rocks are applied to stress o Stress is the pushing pulling or shearing At first the rock bends slightly but doesn t break o If you were to stop applying stress at this stage the rock would return to its original shape Called elastic behavior If you bend the rock far enough many small cracks or breaks start to form o Eventually the cracks connect to one another to form a fracture that cuts across the entire block of rock The instant that this fracture forms the block breaks in two and the rock on one side suddenly slides past the rock on the other side and any elastic bending that had built up is released so the rock straightens our or rebounds Because sliding occurs the fracture has become a fault o A fault can t slip forever for friction eventually slows and stops the movement Friction is the force that resists sliding on a surface is caused by the existence of bumps on surfaces these bumps act like tiny anchors and snag on the opposing surface Earthquakes due to slip on a preexisting fault Once a fault comes into being it is a scar in the Earth s crust that can remain weaker than surrounding intact crust When stress builds it overcomes friction and the preexisting fault slips again o This movement takes place before stress becomes great enough to cause new fracturing of surrounding intact rock After each slip event friction prevents the fault from slipping again until stress builds again Such alternation between stress buildup and slip events earthquakes are called stick slip behavior The breaking of rock that occurs when a fault slips generates earthquake energy o The concept that earthquakes happen because stresses build up causing rock adjacent to the fault to ben elastically until the slip on the fault occurs is called the elastic rebound theory The major earthquake or mainshock along a fault may be preceded by smaller ones called foreshocks which possibly


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