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Chapter Earthquake Geology and Seismology Lecture PowerPoint Copyright © The McGraw-Hill Companies, Inc Permission required for reproduction or display • The Lisbon Earthquake of 1755 Morning of November 1, 1755: Lisbon experienced two major earthquakes in close succession, the first of which caused widespread fires and the second of which caused sea waves which swept many away • A few hours later, Lisbon was again shaken by an earthquake in Fez, Morocco (550 km away) • 70,000 people killed and 90% of structures destroyed or damaged • Changed people’s attitudes about the world What is an Earthquake? • An event in which the Earth quakes, and vibrations are felt or recorded • Caused by volcanic activity, meteorite impacts, undersea landslides, explosions of nuclear bombs or most commonly, by movement of the Earth across a fault • Fault: fracture in the Earth across which the two sides move relative to each other • Stresses build up until enough to cause rocks to fracture and shift, sending off waves of seismic energy, felt as earthquake Figure 3.2 Faults and Geologic Mapping century recognition that fault movements cause • 19th earthquakes led to identification of earthquakehazard belts • Understanding faults begins with understanding rock relationships, formalized by Steno: – Law of original horizontality : sediments are originally deposited in horizontal layers – Law of superposition: in undeformed sequence of sedimentary rock layers, each layer is younger than the layer beneath it and older than the layer above it Faults and Geologic Mapping – Law of original continuity : sediment layers are continuous, ending only against a topographic high, by pinching out from lack of sediment, or by gradational change from one sediment to another • If sedimentary layer ends abruptly, may have been eroded by water action or truncated by fault passing through layer • Identifying truncated sedimentary layers and recognizing their offset continuation allows determination of fault length • Length of fault determines size of earthquake possible on fault (longer fault ruptures create bigger earthquakes) • Understanding fault offset can also have financial rewards, if ore-bearing unit exists two different places on either side of fault (example of gold-bearing gravels 840 km apart in New Zealand) Faults and Geologic Mapping Figure 3.5 Figure 3.4 Types of Faults • Jointing – brittle lithospheric rocks fracture and crack • Large stress differential on either side of a fracture results in movement: fracture becomes a fault • Movement ranges from millimeters to hundreds of kilometers, resulting in tilting and folding of layers • Use strike and dip to describe location in 3D space of deformed rock layer Types of Faults • Use strike and dip to describe location in 3D space of deformed rock layer • Dip: angle of inclination from horizontal of tilted layer • Strike: compass bearing of horizontal line in tilted layer Figure 3.7 Types of Faults Dip-slip faults : • Dip-slip faults are dominated by vertical movement • Ore veins often form in fault zones, so many mines are actually dug out along faults • Miners refer to the block beneath them as the footwall (block beneath the fault) and the block above them as the hangingwall (block above the fault) Figure 3.8 Types of Faults Dip-Slip Faults: • Caused by pushing or pulling force • Where dominant force is extensional (pulling), normal fault occurs when hangingwall moves down relative to footwall, and zone of omission results Figure 3.9 Ground Motion During Earthquakes Periods of Buildings and Responses of Foundations: • Buildings have natural frequencies and periods • Periods of swaying are about 0.1 second per story – 1-story house shakes at about 0.1 second per cycle – 30-story building sways at about seconds per cycle • Building materials affect building periods – Flexible materials (wood, steel)  longer period of shaking – Stiff materials (brick, concrete)  shorter period of shaking • Velocity of seismic wave depends on material through which it is moving – Faster through hard rocks, slower through soft rocks Ground Motion During Earthquakes Periods of Buildings and Responses of Foundations: • When waves pass from harder to softer rocks, slow down • Must therefore increase their amplitude in order to carry same amount of energy  greater shaking • Shaking tends to be stronger at sites with softer ground foundations (basins, valleys, reclaimed wetlands, etc.) • If the period of the wave matches the period of the building, shaking is amplified and resonance results – Common cause of catastrophic failure of buildings • Earthquake Intensity – What We Feel During an Earthquake Mercalli intensity scale was developed to quantify what people feel during an earthquake • Used for earthquakes before instrumentation or current earthquakes in areas without instrumentation • Assesses effects on people and buildings • Did You Feel It?: Maps of Mercalli intensities can be generated quickly after an earthquake using people’s input to the webpage http://pasadena.wr.usgs.gov/shake Insert Table 3.6 here In Greater Depth: What To Do Before and During an Earthquake • Before an earthquake: – Inside and outside your home, visualize what might fall during strong shaking, and anchor those objects by nailing, bracing, tying, etc – Inside and outside your home, locate safe spots with protection – under heavy table, strong desk, bed, etc • During an earthquake: – – – – Duck, cover and hold Stay calm If inside, stay inside If outside, stay outside Earthquake Intensity – What We Feel During an Earthquake Mercalli scale variables: • Earthquake magnitude – Bigger earthquake, more likely death and damage • Distance from hypocenter – Usually (but not always), closer earthquake  more damage • Type of rock or sediment making up ground surface – Hard rock foundations vibrate from nearby earthquake body waves – Soft sediments amplified by distant earthquake surface waves – Steep slopes can generate landslides when shaken Earthquake Intensity – What We Feel During an Earthquake Mercalli scale variables: • Building style – Body waves near epicenter are amplified by rigid short buildings – Low-frequency surface waves are amplified by tall buildings, especially if on soft foundations • Duration of shaking – Longer shaking lasts, more buildings can Insert Table 3.7 here be damaged A Case History of Mercalli Variables San Fernando Valley, California, Earthquake of 1971 • Earthquake magnitude – Magnitude 6.6, with 35 magnitude 4.0 or higher aftershocks in minutes after main shock • Distance from epicenter – Bull’s-eye damage pattern • Building style – ‘Soft’ first-story buildings were major problem – Hollow-core bricks at V.A Hospital caused collapse – Collapse of freeway bridges Figure 3.27 A Case History of Mercalli Variables San Fernando Valley, California, Earthquake of 1971 • Duration of Shaking – Strong ground shaking lasted 12 seconds – Lower Van Norman Reservoir failed by landslides until stood only ft above water – had shaking continued seconds longer, dam would have failed, homes of 80,000 would have flooded • Learning from the Past, Planning for the Future – 1994 Northridge earthquake caused Figure 3.30 Building in Earthquake Country • Eliminate resonance : – – – – – – Change height of building Move weight to lower floors Change shape of building Change building materials Change attachment of building to foundation Hard foundation (high-frequency vibrations)  build tall, flexible building – Soft foundation (low-frequency vibrations)  build short, stiff building Building in Earthquake Country • Shear Walls – Designed to receive horizontal forces from floors, roofs and trusses and transmit to ground – Lack of shear walls typically cause structures like parking garages to fail in earthquakes Figure 3.32 Figure 3.31 Building in Earthquake Country • Braced Frames – Bracing with ductile materials offers resistance • Retrofit Buildings – Increase resistance to seismic shaking Figure 3.33 Figure 3.34 Building in Earthquake Country • Base Isolation – Devices on ground or within structure to absorb part of earthquake energy – Use wheels, ball bearings, shock absorbers, ‘rubber doughnuts’, etc to isolate building from worst shaking Figure 3.35 Building in Earthquake Country • Retrofit Bridges – Bridges combine steel with concrete, materials with different properties in earthquakes – Rebuild with alternating layers of steel and concrete Figure 3.36 Building in Earthquake Country • Houses – Modern 1-, 2-story wood-frame houses perform well in earthquakes – Additional support can be given by building shear walls, bracing, tying walls and foundations and roof together – Much damage as interior items are thrown about • Bolt down water heaters, ceiling fans, cabinets, bookshelves, electronics Figure 3.37 End of Chapter
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