Chapter 8 Textbook Annotations (217-250)- 8.1 Introduction o Japan lies along a convergent plate boundary, where the Pacific Plate sinks backinto the mantle underneath the edge of the Eurasian Plate. The boundary between the two plates is a fault, a large fracture on whichsliding occurs, that slopes gently to the west. - Motion along a fault doesn’t happen continuously, rather for manyyears the rocks among the fault bends to accommodate the motiono Rock can only bend so far before it snaps. Shock waves are generated by the shearing andfracturing of rock along the fault. - When vibrations reach the land surface, theland surfaces lurches back and forth andbounce up and down.o Earthquake: an episode of ground shaking o The societal calamity due to an earthquake is a direct consequence of groundshaking, because many buildings collapse and crush their inhabitants and debristumbles down slopes in landslides. After the earthquake, the sea floor coast uplifts, a massive amount of waterwas displaced. - This water moved away from the site of the earthquake in immensewaves, 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 thegrowth of mountains, drift of continents, and the opening and closing ofocean basins. - Almost 1 million detectable earthquakes happen every year. o Most cause no damage or casualties, because they are toosmall, or they occur in unpopulated areas. - A few hundred earthquakes per year rattle the ground sufficientlyto damage buildings and injurie their occupants, and every 5 to 10years, a great earthquake occurs. - 8.2 What Causes Earthquakes?o Ancient cultures explanations for seismicity (earthquake activity), most of whichinvolved 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 slidingoccurs) 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 anexplosion 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 isthe 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 ormay be pulverized into tiny grains, due to the crushing and grinding thatcan accompany the slip, and the surface of a fault may be polished andgrooved as if scratched by a rasp. - In some localities the fault cuts through distinct marker; where thishappens, the end of the marker on one side of the fault is offsetrelative to the end on the other side. o The distance between the two ends of the marker, aremeasured along the fault surface in the direction of the slip,is called “displacement”. Many faults are completely underground and willbe visible only if exposed by erosion of overlyingrock. - Some faults intersect and offset the groundsurface, producing a step called a “faultscrap” o The ground surface exposure of afault is called the fault line or “faulttrace” o Rock masses above the sloping fault plane is called the hanging wall Because it is hung over headso 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 slopingfault by specifying the direction that the hanging wall moved inrelation 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 reversefault. If steep, and thrust fault id shallowly sloping.o Strike-slip faults are near vertical planes on which slipoccurs parallel to an imaginary horizontal line, called astrike 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 arecalled active faults. - Faults that last moved in the distant past and probably won’t moveagain 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 apreexisting 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 rockwould return to its original shape. Called elastic behavior. - If you bend the rock far enough, many small cracks or breaks startto form. o Eventually the cracks connect to one another to form afracture that cuts across the entire block of rock. The instant that this fracture forms, the block breaksin two and the rock on one side suddenly slides pastthe rock on the other side, and any elastic bendingthat had built up is released so the rock straightensour or rebounds. - Because sliding occurs, the fracture hasbecome a fault. o A fault can’t slip forever, for frictioneventually slows and stops themovement Friction is the force thatresists sliding on a surface, iscaused by the existence ofbumps on surfaces – thesebumps act like tiny anchorsand snag on the opposingsurface. Earthquakes due to slip on a preexisting fault- Once a fault comes into being, it is a scar in the Earth’s crust thatcan remain weaker than surrounding, intact crust. - When stress builds, it overcomes friction and the preexisting faultslips again. o This movement takes place before stress becomes greatenough to cause new fracturing of surrounding intact rock. After each slip event, friction prevents the faultfrom slipping again until stress builds again. - Such alternation between stress buildup andslip events
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