EarthquakesEarthquakes DefinedEarthquake TerminologyThe Elastic Rebound Theory Refer to page 33-35Types of FaultsTypes of Tectonic StressSlide 7Dip-Slip Faults - Normal FaultsNormal Fault ExamplesNormal Fault Example – The Basin and Range of Nevada, Utah, and Adjacent AreasDip-Slip Faults - Reverse FaultsSubduction BoundariesReverse Fault Example –Subduction Zones Chile 1960Collision BoundariesReverse Fault Example – Collision Zones Bam, Iran 2003Strike-Slip Faults - Transform FaultsTransform Fault Example The San Andreas Fault SystemIntraplate EarthquakesEarthquake Seismic WavesP-wavesS-wavesSurface WavesSeismographsSeismogramLocating the Epicenter Via SeismogramsSlide 26Earthquake MeasurementEarthquake Measurement Richter Magnitude ScaleSlide 29Slide 30Modified Mercalli Scale Example: The 1994 Northridge, California EarthquakeModified Mercalli Scale vs. The Richter ScaleFault CreepEarthquake HazardsEarthquake Magnitude and Ground AccelerationEarthquake Magnitude and Shaking TimeMagnitude and Fault DisplacementDistance from the Epicenter Seismic waves attenuate with distance. Refer to page 52.Surface Faulting and Ground Rupture Land Uplift and SubsidenceSoil AmplificationLiquefactionLandslidesIntegrity of Structures and Type of ConstructionBase ShearSlide 45Slide 46Mitigation of Base Shear Refer to pages 79-81.Mitigation of Base ShearSlide 49Slide 50Slide 51Slide 52Building Vibration and OscillationIndoor HazardsAftershocksFiresSome of the Most Catastrophic Earthquakes in Terms of CasualtiesEarthquake Prediction / ForecastingPrecursor Events Refer to page 63-65.Precursor EventsSlide 61Migrating EarthquakesSiesmic GapsEarthquake RegularitySlide 65Paleoseismology1EarthquakesChapter 3Earthquakes and Their Damages: Shaking Ground, Collapsing BuildingsChapter 4Earthquake Prediction and Tectonic EnvironmentEarthquakes DefinedEarthquakes 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 33-35.23•The focus is the point on the fault where rupture occurs and the location from which seismic waves are released. •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.Earthquake Terminology4The Elastic Rebound TheoryRefer to page 33-35Types of FaultsThe majority of earthquakes (90%) are caused by rocks rupturing in response to tectonic stresses at active plate margins. Refer to pages 36-37.5Types of Tectonic Stress•Tensional Stress (extensional stress)•Compressional Stress•Shear StressRefer to page 36-37.6Types of FaultsFaults 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.78Dip-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 Valley9Left: 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 earthquakeDecember 16, 1954Normal Fault ExamplesNormal Fault Example – The Basin and Range of Nevada, Utah, and Adjacent AreasThe 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 41.10Lower 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.11Dip-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 boundariesLeft: Thrust Fault12Subduction 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 = 9.2Refer to pages 39-41.Reverse Fault Example –Subduction ZonesChile 1960On 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 39-41Case in Point page 55.13Left: 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.14Collision Boundaries•At collision boundaries two plates of continental lithosphere collide resulting in fold-thrust mountain belts.
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