Earthquakes Chapter 3 Earthquakes and Their Damages Shaking Ground Collapsing Buildings Chapter 4 Earthquake Prediction and Tectonic Environment 1 Earthquakes Defined Earthquakes are vibrations of the earth caused by the rupture and sudden movement of rocks that have been strained deformed beyond their elastic limit The forces that cause deformation and the build up of strain energy in the rock are referred to as stresses Earthquakes occur along faults Faults are fractures in the lithosphere where regions of rock move past each other displaced Refer to page 33 35 2 The focus is the point on the fault where rupture occurs and the location from which seismic waves are released Earthquake Terminology The epicenter is the point on the earth s surface directly above the focus When the fault ruptures waves of energy called seismic waves spread out in all directions Refer to page 44 3 The Elastic Rebound Theory Refer to page 33 35 4 Types of Faults The majority of earthquakes 90 are caused by rocks rupturing in response to tectonic stresses at active plate margins Refer to pages 36 37 5 Types of Tectonic Stress Tensional Stress extensional stress Compressional Stress Shear Stress Refer to page 36 37 6 Types of Faults Faults can be divided depending on the direction of relative displacement Relative displacement is largely a function of the type of tectonic stress the rock is under There are 2 main categories Dip slip faults where the displacement is vertical Strike slip faults where the displacement is horizontal 7 Dip Slip Faults Normal Faults Normal faults result from tensional stresses along divergent boundaries The hanging wall block moves down relative to the footwall block Low Richter magnitudes due to the tendency of rocks to break easily under tensional stress Shallow focus less than 20 km because the lithosphere is relatively thin along diverging plate boundaries Refer to pages 36 37 41 Examples all mid ocean ridges Continental Rift Valleys such as the basin and range province of the Western U S and the East African Rift Valley 8 Normal Fault Examples Left Fault scarp near Hebgen Lake Montana after the magnitude 7 1 earthquake of August 18 1959 shows a displacement of 5 5 to 6 0 m Right This section of the normal fault scarp was produced by the earthquake of October 28 1983 at Borah Peak Idaho Left Dixie Valley Fairview Peaks Nevada earthquake December 16 1954 9 Normal Fault Example The Basin and Range of Nevada Utah and Adjacent Areas The Faults of the Basin and Range occupy a spreading zone accompanying the Northwest drag of the Pacific Plate against the North American Plate which moves slightly south of west Refer to page 41 Lower Right The Wasatch Front is a high fault scarp east of the Salt Lake basin There has not been an earthquake of any consequence since the founding of Salt Lake city in 1847 However the Salt Lake City area should still consider themselves at high risk for major earthquakes 10 Dip Slip Faults Reverse Faults Reverse faults result from compressional stresses along convergent boundaries The hanging wall block has moved up relative to the footwall block A Thrust Fault is a special case of a reverse fault where the dip of the fault is less than 15o Refer to pages 39 41 There are two types of converging plate boundaries 1 Subduction boundaries 2 Collision boundaries Left Thrust Fault 11 Subduction Boundaries At subduction boundaries there is a continuum of stress along the subducting plate Shallow focus earthquakes can be generated near the trench but focal depths can reach down to 700 km as earthquakes are generated along the subducting plate Rocks are strong under compression and can store large amounts of strain energy before they rupture Therefore these earthquakes can be very powerful 1960 Southern Chili 9 5 1964 Alaska 9 2 Refer to pages 39 41 12 Reverse Fault Example Subduction Zones Chile 1960 On May 22 1960 the largest earthquake on record struck the coast of Chile with a Mw of 9 5 The earthquake ruptured along a 1 000 km length of the subduction zone In Chile the earthquake and the tsunami that followed took more than 2 000 lives From Chile the tsunami radiated outward killing 61 people in Hawaii and 122 in Japan Refer to pages 39 41 Case in Point page 55 Left Stuck to the subducting plate the overriding plate gets squeezed Right An earthquake along a subduction zone happens when the leading edge of the overriding plate breaks free and springs seaward raising the sea floor and the water above it This uplift starts a tsunami 13 Collision Boundaries At collision boundaries two plates of continental lithosphere collide resulting in fold thrust mountain belts Earthquakes occur due to the thrust faulting and range in depth from shallow to about 200 km Refer to pages 39 41 Example The Himalayas from the collision of India with Asia 14 Reverse Fault Example Collision Zones Bam Iran 2003 Below Before and after pictures of the 2 000 yr old citadel in the city of Bam the largest mudbrick structure in the world Map showing the AlpineHimalayan Belt Earthquakes in Iran and neighboring regions e g Turkey and Afghanistan are closely connected to their position within the active Alpine Himalayan belt near the convergence of the Arabian and Eurasian plate The Mw 6 5 Bam Earthquake destroyed nearly 80 of the adobe buildings killing more than 26 000 people 15 Strike Slip Faults Transform Faults Strike slip faults result from shear stresses acting on the lithosphere along transform boundaries Horizontal motion can be right lateral or left lateral Earthquakes along these boundaries tend to be shallow focus with depths usually less than about 100 km Richter magnitudes can be large Refer to pages 37 38 16 Transform Fault Example The San Andreas Fault System The San Andreas Fault is the main strand of a zone of parallel faults resulting from interaction between the relative northwest movement of the Pacific Plate compared with the North American Plate 17 Intraplate Earthquakes These are earthquakes that occur in the stable portions of continents Many of them occur as a result of re activation of ancient faults although the causes of some intraplate earthquakes are not well understood Refer to pages 42 44 Charleston South Carolina 1886 Mw 7 3 Mercalli Intensity of IX Right The Charleston Earthquake resulted from movements along a segment of the East Coast Fault system a series of faults trending Northeast near the boundary between continental crust and Atlantic Oceanic
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