GLY1000 Exam 2 Study Guide Rheology and Deformation Stress Strain Relationship and elastic moduli Bulk and Shear o Stress a force producing or tending to produce deformation in a body Measured as F A the force divided by the area over which the force is applied Defined in such units as dunes per square centimeter or pounds per square feet etc o Strain refers to the deformation resulting from the stress i e to the change in the dimensions or shape of the body relative to the original dimensions or shape o Defining the strain Tensional Stress Stress which stretches rocks in two different directions Strain can be defined as D d D Compressional Stress Stresses directed toward center of rock mass Strain can be defined as D d D Shear Stress Stress parallel to the plane Strain can be defined as the tangent of angle q Hydrostatic Stress Force is applied on a body in all directions which can result in a volume strain defined as V v V GLY1000 Exam 2 Study Guide o Elastic Modulus important physical property of material refers to the proportionality constant of the ratio of stress to strain Bulk Modulus a measure of the incompressibility of a material Refers to hydrostatically strained spheres The more incompressible a material is the smaller will be the value V v and hence the greater the value of the Bulk Modulus Shear Modulus a measure of the rigidity of a material The more rigid a material is the smaller will be the value tan q and thus the larger the value of shear modulus Factor effecting type of strain i e brittle versus ductile strain o Brittle Breaks easily under stress o Ductile Instead of a material breaking it absorbs the stress by flowing o Whether solids are brittle or ductile depends on their temperature and strain rate They can be brittle at low temperatures and ductile at higher temperatures They can be brittle under high strain rates and ductile under low strain rates Normal reverse thrust and strike slip faults orientation of fault planes and stresses o Normal fault A fault in which the hanging wall has moved downward relative to the footwall Occurs were two blocks or rock are pulled apart as by tension Arrows give direction of primary tensional stress o Reverse thrust Fault A fault in which the hanging wall appears to have been pushed up along the footwall Arrow gives direction of primary compressional stress GLY1000 Exam 2 Study Guide o Strike Slip Fault A fault in which rock strata are displaced mainly in a horizontal direction parallel to the line of the fault Arrows give orientation of shear couple Earthquakes Understanding of what causes earthquakes in terms of stresses elastic strain slip along faults o Regions of the Earth s crust and lithosphere that are subjected to sufficient stresses can undergo elastic strain recall that it is the nature of elastic strain that it is proportional to the stress applied This strain can continue to build up over long periods of time If the stress is suddenly released as they might be when rocks slip along a fault the stored elastic strain energy is released some in the form of seismic waves that travel outward away from the point of origin or focus of the earthquake What are the major hazards of an Earthquake o The effect of ground shaking Buildings can be damaged by the shaking itself or by the ground beneath them settling to a different level than it was before the earthquake Subsidence Grounds shaking can even cause buildings to sink into the ground if soil liquefaction occurs Liquefaction mixing of sand or soil and ground water water underground during the shaking of a moderate or strong earthquake When water and soil mixed it becomes soft and acts like quicksand sometimes causing a building to lean tips over or sink Common is areas that have groundwater near the surface and sandy soil Ground shaking can also cause landslides mudslides and avalanches on steeper hills or mountains o Ground Displacement Ground movement along a fault If a structure is built across a fault the ground displacement during an earthquake could seriously damage or rip apart that structure o Flooding An earthquake can rupture dams or levees along a river the water from the river or the reservoir would then flood the area damaging buildings and maybe sweeping away or drowning people Tsunami Huge wave caused by an Earthquake under the ocean GLY1000 Exam 2 Study Guide Seiches small tsunamis they occur on lakes that are shaken by an earthquake and are usually only a few feet high but can still flood knock down houses and tip trees o Fire These fires can be started by broken gas lines and power lines or tipped over wood or coal stoves Can become a serious problem especially if they are near water lines that feed the fire hydrants are broken too Great San Francisco Earthquake in 1906 city burned for 3 days city destroyed 250 000 people left homeless Some great faults like San Andreas periodically generate earthquakes along some segments of their lengths but not along other segments why What can account for an almost periodical instead of random occurrence of earthquakes along parts of the San Andreas Fault o Continental transform fault that extends 810 miles through California Forms tectonic boundary between the Pacific Plate and the North American Plate Distribution and depth where do most earthquakes occur and how deep are the foci o The point below the surface where the rock breaks is called the earthquake focus o The seismic waves from an earthquake are usually strongest at the epicenter the point on the surface right above the focus o The great majority of earthquake foci are shallow o Shallow Up to 70 km below the surface crust o Intermediate 70 to 300 km o Deep Greater than 300 km Why do earthquakes originate in the lithosphere but not in the asthenosphere o The cold lithosphere bends and breaks but does not flow the hot asthenosphere flows but does not break How are earthquake depths associated with types of plate boundaries and why o Shallow earthquakes create the most damage and of total energy released of earthquakes in the world They unlike intermediate and deep focus earthquakes shallow earthquakes occur at all type of plate boundaries like ocean trenches subduction zones etc Transform convergent and divergent boundaries o Deep earthquakes can happen only where there is solid rock hundreds of kilometers below the surface That only occurs at convergent boundaries where plates are sometimes pushed deep beneath the surface and into the mantle o
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