Earth 20 - Lecture 5 - Lecture 4 Earthquakes cont Subduc&on zone earthquakes ring of fire Oceanocean Oceancontinent Subduc&on zone earthquakes Ques&on 0

Earth 20 - Lecture 5 - Lecture 4 Earthquakes cont Subduc&on...

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Unformatted text preview: Lecture 4) Earthquakes cont. Subduc&on zone earthquakes “ring of fire” Oceanocean Oceancontinent Subduc&on zone earthquakes Ques&on 0: What type of faults occur at subduc&on zones? •  •  •  •  •  A) normal faults B) [email protected]­‐lateral strike slip faults C) right-­‐lateral strike slip faults D) thrust faults E) no faults 2011 Tohoku > earthquake M 9.0 Magnitude 9.0 NEAR THE EAST COAST OF HONSHU, JAPAN Friday, March 11, 2011 at 05:46:23 UTC Earthquake occurred 130 east of Sendai,, Japan Images courtesy of the US Geological Survey Rate of convergence at this plate boundary is high: about 83 mm/yr Magnitude 9.0 NEAR THE EAST COAST OF HONSHU, JAPAN Friday, March 11, 2011 at 05:46:23 UTC Map on the right: historic earthquake activity near epicenter (star) from 1990 to present. 2011 Earthquake hypocenter was 32 km underground. Ruptured patch of earth’s crust 150 miles long and 50 miles across Seismicity Cross Section across the subduction zone showing the relationship between color and earthquake depth. Images courtesy of the US Geological Survey Magnitude 9.0 NEAR THE EAST COAST OF HONSHU, JAPAN Friday, March 11, 2011 at 05:46:23 UTC Earth’s axis of rotation shifted 25 cm Honshu coastline moved 2.5 m to the East Devastating earthquake and tsunami killed 15,867 people, injured 6,109 people, and left 2,909 people missing! 383,429 homes were destroyed Level 7 meltdowns at 3 nuclear reactors Estimated cost of $250 billion – most expensive natural disaster in world history Magnitude 9.0 NEAR THE EAST COAST OF HONSHU, JAPAN at 05:46:23 UTC Friday, March 11, 2011 Subduction Zones in U.S.A. Alaska and Northwest U.S. Great 1964 Alaskan Earthquake Mw 9.2 – Second or third largest earthquake ever recorded 4 min. of shaking 38 ft. of vertical displacement in some locations 220 ft. tsunami 143 deaths Intraplate Earthquakes •  Occur away from plate boundaries •  Example: Salt Lake City along Wasatch fault New Madrid seismic zone •  Series of earthquakes •  4 large events –  M 7.3 to 7.7 •  felt over a very large area New Madrid seismic zone •  recent seismicity •  90% likelihood of M 7.0 EQ in 50 years •  [email protected] sediments in Mississippi river valley Ques&on 1: What causes intraplate earthquakes? •  •  •  •  •  A) shortening of eastern North America B) ancient [email protected] being reac&vated C) ac&ve transform fault along the Mississippi D) South American plate pushing northward E) major flooding events in the Mississippi New Madrid seismic zone Lecture 5: Earthquakes Seismology, Magnitude, and Mitigation Outline I.  Seismic Waves II.  Determining magnitude III. Lessons learned IV.  Predictions and hazard mitigation The Earthquake Sequence or Cycle: 1) Tectonic loading of faults >> stress >> strain 2) Earthquakes >> release of strain 3) Seismic waves >> energy propagation ? 4) Shaking (ground motion) >> energy transferred onto materials 5) Structural failure >> energy produces brittle material response 1995, KOBE, JAPAN" Seismic Waves Wave Attributes Properties of Seismic Waves: •  •  •  •  Amplitude: height of wave Wavelength: distance between successive wave peaks Period: time between wave peaks (= 1/frequency) Frequency: number of wave peaks in one second Seismic Waves: Overview •  Body Waves: –  P-­‐Wave: Primary Wave (compressional; push-­‐and-­‐pull) –  S-­‐wave: Secondary Wave (shear wave) •  Surface Waves: –  Both Love and Rayleigh waves. Have high amplitudes •  Velocity of seismic wave depends on material through which it is moving –  Faster through hard rocks/materials –  Slower through soft rocks/materials P (primary) waves •  Fastest of all waves; Zirst to reach a recording station •  Move as push-­‐pull – alternating pulses of compression and extension •  Travels through all materials (solid, liquid, gas) S (secondary) waves •  Second to reach a recording station (after primary) •  Exhibit transverse motion – shearing particles at right angles to the wave’s path •  Travel only through solids –  S wave is reelected back or converted if reaches liquid; larger than P-­‐waves Surface Waves •  Only travel near the Earth’s surface (solids only) •  created by body waves disturbing the surface •  Usually larger and more destructive than body waves. •  Travel farther than body waves Two types: 1.  Rayleigh Waves 2.  Love Waves Rayleigh Waves • Backward-­‐rotating, elliptical motion (similar to ocean waves). • Travel through solids only Love Waves •  Similar motion to S waves, but move side-­‐to-­‐side in horizontal plane •  Travel faster than Rayleigh waves •  Travel only through solids Summary of Seismic Waves P-­‐wave (body wave) S-­‐wave (body wave) Rayleigh wave (surface wave) Love wave (surface wave) Seismic Waves in the Earth •  Wave signals from large earthquakes can pass through or around the entire Earth and be recorded all over the world •  Waves do not follow straight paths through the Earth but change velocity and direction as they encounter different layers Earthquakes generate Seismic Waves that travel around the globe and tell us about the Earth’s interior. Source of several animations related to seismicity from IRIS: Ques&on 2: How are earthquakes detected? •  •  •  •  •  A) Seismographs B) Seismometers C) Seismograms D) Sesimologs E) Seism Seismographs Measures horizontal shaking Measures vertical shaking Seismogram (a recording of the motion of the ground) Maximum Amplitude = 540 mm P Tp=14s S Ts=23s Distance to Earthquake Epicenter P-­‐wave eirst S-­‐wave second Surface waves last •  Time lag between P-­‐ and S-­‐wave arrival is called Δt. notice how large Ques&on 3: If the &me between the P and S waves is 5 minutes, then the distance to the earthquake is: A) Close to 1,000 km B) Close to 4,000 km C) Close to 8,000 km D) Close to 10,000 km E) I have no idea Intersection of 3 Circles •  Need distance of earthquake from three stations to pinpoint location of earthquake: –  Visualize circles drawn around each station for corresponding distance from station –  Intersection of circles at earthquake’s location Figure 4.23 Cartoon: Locating Earthquakes Radius of circle directly proportional to S-­‐P time S-­‐P time Cartoon: Locating Earthquakes Radius of circle directly proportional to S-­‐P time S-­‐P time S-­‐P time Cartoon: Locating Earthquakes Radius of circle directly proportional to S-­‐P time S-­‐P time S-­‐P time S-­‐P time Measures of an Earthquake’s Size & Energy •  Richter Magnitude: Relative Size of an Earthquake -­‐-­‐based on distance and amplitude •  Seismic Moment/Moment Magnitude: Absolute Size of an Earthquake -­‐-­‐based on energy released •  ModiZied Mercalli Intensity: Impact of an Earthquake on the built environment -­‐-­‐based on damage and degree of shaking -­‐-­‐Roman Numerals I through XII Magnitude (Classic Method) Richter Scale Local Magnitude ML: •  Devised in 1935 to describe magnitude of shallow, moderately-­‐sized earthquakes located in southern California •  Works best in near-­‐by events with shallow foci •  Bigger earthquake à more shaking à greater amplitude on a seismogram •  Logarithmic = rapid increase in energy! Richter Scale Magnitude –  For every 10 fold increase in recorded amplitude, Richter magnitude increases one number –  With every one increase in Richter magnitude, the energy release increases by about 30-­‐40 times Moment Magnitude Mw Based on Seismic Moment Mo Mo = μSA μ is the shear modulus, an indicator for rock strength S is the average slip on the faulted area A is the area of the fault plane over which slip has occurred Mo can be estimated from shaking recorded by seismograms Absolute Size of an Earthquake -­‐-­‐based on energy released Earthquake Intensity Scale •  Mercalli Intensity Scale was developed to quantify the degree of shaking 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 Intensity Map for the 2010 Haiti EQ USGS Shaking Intensity The earthquake occurred about 10 miles west of the capital of Haiti, Port-­‐au-­‐Prince, and caused extreme shaking. Perceived ModiZied Mercalli Intensity Shaking Extreme Violent Severe Very Strong Strong Moderate Light Weak Not Felt Magnitude – Frequency Relation Earthquake Magnitude Scale Magnitude Estimated Number Each Year 3.0 100,000 4.0 5.0 1,000 6.0 100 7.0 10 8.0 1 9.0 1 in every 30-­‐40 years 10,000 Note that these are approximate numbers. Relationship Between Seismic-­‐Wave Frequencies and Damage: •  High frequency waves (like body waves) cause much damage at epicenter but die out quickly with distance from epicenter •  Low frequency waves (like surface waves) travel great distance from epicenter so do most damage farther away Ground Motion During Earthquakes •  Buildings are designed to handle vertical forces (weight of building and contents) > vertical shaking in earthquakes is usually safe •  Horizontal shaking during earthquakes > can do massive damage to buildings •  Acceleration –  Measured as acceleration due to gravity (g) –  Weak buildings can be damaged by as little as 0.1 g –  At isolated locations, peak ground acceleration can be as much as 2.7 g (2011 Honshu Earthquake, Japan) Why are some regions more prone to shaking and destruction than others? Shaking Effects: 1. Amplieication 2. Liquefaction 3. Building Resonance Amplifica&on •  Velocity of seismic wave depends on material through which it is moving –  Faster through hard rocks/materials –  Slower through soft rocks/materials Waves passing from hard to soft rocks SLOW DOWN •  increase their amplitude (carry same amount of energy) •  Leads to greater shaking •  Shaking stronger at sites with softer ground foundations (basins, valleys, reclaimed wetlands, etc.) AmpliZication: East Bay, 1989 Loma Prieta Earthquake LA basin filled with [email protected] sediments Amplieication Map for SoCal Amplieication Liquefaction •  Caused by shaking during an earthquake –  Occurs in poorly consolidated sediment –  Seismic waves cause increased eluid pressure in space between rock grains –  Rock or soil particles become buoyant forming a liquid-­‐like slurry –  Lead to much destruction 1964 Alaska, Turnagain Heights Turnagain Heights, 1964 Alaska Earthquake Ground Motion and RESONANCE 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 3 seconds per cycle •  Building materials affect building periods –  Flexible materials (wood, steel) à longer period of shaking –  Stiff materials (brick, concrete) à shorter period of shaking •  If the period of the wave matches the period of the building, shaking is ampliZied and resonance results –  Common cause of catastrophic failure of buildings –  Simulation video Resonance: Mexico City, 1985 •  “Earthquakes Don’t Kill, Buildings Do” •  Resonance between seismic waves, soft lake-­‐sediment foundations, and improperly designed buildings, all vibrating most intensely at 1 to 2-­‐second periods, destroyed many buildings Figure 4.16 Earthquake Predic&on First, it helps to understand a li1le more about fault movement Elastic rebound theory Buildup of stress on the fault overcomes friction on the fault and the rocks on both sides of the fault move in an earthquake •  After earthquake, all stress on the fault has been removed and buildup begins again •  Lessons from Landers Earthquake, CA, 1992 Sequence of several strong earthquakes: •  1992-­‐-­‐Joshua Tree: Mw 6.1 •  1992-­‐Two months later —Landers: Mw 7.3 •  Followed few hours later —Big Bear: Mw 6.3 •  1999—Hector Mine: Mw 7.1 Lessons from Landers, CA, 1992 What was learned about how faults move? •  Only small portion of fault slipped at any one time •  Patches of the faults without much movement became origin for later earthquakes •  Amount of slip varied from cm to several m •  Amount of fault movement at the ground surface differs from that at depth Predic&ng Earthquakes: Seismic-­‐ Gap Method •  If some segments of a fault have moved recently, it is reasonable to expect that unmoved portions will move next, to eill the gaps •  Yields expectations, not guarantees! Segments may move in two or more earthquakes before adjacent unmoved segments move once But how do we know about all these old earthquakes? Paleoseismology •  Fault movements can result disturbances and offsets in sediment deposition in down-­‐dropped basins •  Determine size of earthquakes by amount of offset •  Dates of prehistoric earthquakes from radioactive carbon (C14) •  use principle of crosscutting relationships Earthquake Probability for the Bay Area California EQ Prediction •  Probability of a 6 or greater EQ happening before 2032 •  Based on historic records, carbon dating of offset sediments, and GPS Remember, these are long-­‐term probabilities! •  Paleoseismology can show how big and how often earthquakes have occurred on a given fault. • Short-­‐term prediction of earthquakes: ELUSIVE! • Detailed behavior of complex fault systems are still unpredictable Hazard Mi&ga&on: Building in Earthquake Country “Earthquakes Don’t Kill, Buildings Do” (and tsunamis, but that’s for a later lecture) Building in Earthquake Country •  Eliminate resonance: –  Change height/shape of building –  Move weight to lower eloors –  Change building materials –  Change attachment of building to foundation •  Avoid weak Zirst-­‐story construction Building in Earthquake Country •  Shear Walls –  Designed to receive horizontal forces from eloors, roofs and trusses and transmit to ground –  Lack of shear walls typically cause structures like parking garages to fail in earthquakes Good! not so good Building in Earthquake Country •  Bracing (above photo) •  Base Isolation –  Devices on ground or within structure to absorb part of earthquake energy –  Use wheels, ball bearings, shock absorbers, etc. to isolate building from worst shaking 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 Earthquake Early Warning (EEW) How big of a difference can a few seconds make? Earthquake Early Warning (EEW) Ques&on 4: EEW is . a.  Already being implemented in CA, we just haven’t had big enough earthquakes since the system was established b.  Already implemented in many countries worldwide, but hasn’t been a priority in CA c.  not yet implemented anywhere, but actively being developed by scientists in CA and Japan. ...
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