Transcript ppt
A-Introduction to Seismic Hazards What is an earthquake? Shaking or vibration of the ground rocks undergoing deformation break suddenly along a fault 1906 San Francisco earthquake Earthquakes: -affect more than 35 countries -Killed 750, 000 people in china, 1976 -Primary feature: ground shaking -secondary phenomena: fire, landslides,ground subsidence, snow and ice avalanche, floods. Where are earthquakes found? The Earth’s surface is composed of a number of mobile “tectonic plates” which are in constant motion Most earthquakes are found at plate margins Where are the world’s earthquakes in terms of plate tectonics? The great majority of earthquakes are located at plate margins This where magmatism, friction, faulting, etc., are most intense Earthquakes in plate interiors are comparatively rare The Pacific Rim of Fire This notorious zone is characterized by subduction zones Earthquakes and volcanoes here are particularly violent friction from subduction produces large destructive quakes B-Plate tectonics The constant movement of the plates is referred to as plate tectonics There are three main types of plate boundaries: divergent convergent transform Divergent margins Here two tectonic plates are in the process of being created Magma is injected into a crack, then cools and becomes new crust An example of a wide, mature divergent margin The middle of the Atlantic Ocean is a divergent margin which is being torn, or rifted, apart…the two plates are separating continuously at a rate of several cm/yr Convergent margins I Instead of two plates being created, they are being consumed… Here an oceanic plate slides beneath a continental plate, since the former is denser geologists refer to this process as subduction Large, destructive earthquakes occur here Convergent margins II If two continental plates collide, they do not subduct, because they are too buoyant Instead, intense compression occurs Large, destructive earthquakes also are generated in this situation Transform margins The third type of plate margin is called a transform boundary Here, plates are neither created nor destroyed… they simply slide by one another Faults associated with earthquakes Faults are planes of weakness along which the Earth has been broken Movements on a fault can be either slow (ductile deformation) or fast (brittle fracture) When a fault behaves in a brittle manner and breaks, earthquakes are generated Rock Behavior and Deformation stress force applied to material that tends to change its dimensions strain effect of stress shown by material strength limiting stress that a material can withstand without failing by rupture or continuous plastic flow Deformation •response of rock to stress depends on: type of stress amount of pressure temperature type of rock length of time rock subjected to stress Types of Stress A-compressional stress - forces directed toward one another - decreases volume of material -lithostatic pressure, example of all-sided confining pressure produced by burial B-tensional stress - stretching stress that tends to increase volume of a material Types of Stress C-shear stress force parallel, but in opposite directions results in displacement of adjacent layers along closely spaced planes Stress (force/area) and strain Types of strain Extension Compression Stretching Shortening Rock Response to Stress strain elastic deformation -strain is proportional to stress -rock returns to original volume/shape if stress removed -absorb, store, release energy Rock Response to Stress strain plastic deformation permanent deformation caused by flowing and folding at stresses above elastic limit high confining pressure and/or temperature warm rocks tend to deform plastically Absorb, internally consume energy, permanent deformation Rock Response to Stress strain brittle deformation -rock breaks if applied stress is too great -rocks at or near surface (cold, low pressure) tend to deform by brittle rupture Absorb, exceed and break Permanent deformation Three types of dominantly vertical faults A normal fault is the result of tensional forces (e.g., rifting) Reverse and thrust faults are the result of horizontal compression Faults whose movement is dominantly horizontal These faults are termed strike-slip faults They are a small-scale version of transform plate tectonic margins They are termed leftlateral (sinistral) or right-lateral (dextral) according to their movement Elastic rebound theory The elastic rebound theory is an explanation for how energy is released during earthquakes. As rocks on opposite sides of a fault are subjected to force, they accumulate energy and slowly deform until their internal strength is exceeded. At that time, a sudden movement occurs along the fault, releasing the accumulated energy, and the rocks snap back to their original undeformed shape. The deeper the rock in question, the more ductile it is warmer Rock that is deep enough to undergo plastic deformation will not generate What is an earthquake? An earthquake is the result of a sudden release of energy in the Earth's crust that creates seismic waves. 1906 San Francisco earthquake Violent versus gentle earthquakes -under compression rocks are very strong -under tension rocks are very weak D-Earthquake generation along a fault The earthquake focus is its point of origin along a fault plane Its epicenter is the vertical projection of the focus to the surface E-TYPES OF WAVES Earthquakes generate Body waves (travel through the earth) P and S waves Surface waves (travel along the surface) Rayleigh and Love waves Body Waves:Primary waves Primary waves, or P waves, travel through solid, liquid, and gas They are alternately compressional and expansive their speeds are ~5 km/s Rock vibrates parallel to the direction of wave propagation Body Waves: Shear waves Shear, or S waves, travel only through solids they push material at right angles to their travel path their speeds are 2-3 km/s Rock vibrates perpendicular to the direction of wave propagation A sample seismogram A seismogram is the graphical representation of Earth movement Surface waves: Love Surface waves, such as Love waves, are restricted to Earth’s surface They cause sideways shaking of the ground Their speed is slightly less than S waves Surface waves: Rayleigh Rayleigh waves are similar to Love waves But instead of causing shaking, they produce rolling motions of the ground Behave like ocean waves F-Sizes of earthquakes The size of an earthquake is the main factor in its destructiveness We will look at two ways to estimate size: Richter magnitudes Mercalli Index Richter magnitudes The Richter magnitude measures the maximum amplitude of ground shaking (vibrational energy) It is a logarithmic scale 1 Richter unit difference is x 10 for ground motion and x 33 for energy Globally, small earthquakes are more frequent than large: ~800,000/yr for events of magnitude 2.0-3.4 while an event of magnitude 8 occurs once every 5-10 years Richter magnitudes Earthquake Magnitude Scale Magnitude Earthquake Effects Estimated Number Each Year<B/> 2.5 or less Usually not felt, but can be recorded by seismograph. 900,000 2.5 to 5.4 Often felt, but only causes minor damage. 30,000 5.5 to 6.0 Slight damage to buildings and other structures. 500 6.1 to 6.9 May cause a lot of damage in very populated areas. 100 7.0 to 7.9 Major earthquake. Serious damage. 20 8.0 or greater Great earthquake. Can totally destroy communities near the epicenter. One every 5 to 10 years The modified Mercalli intensity scale Magnitudes do not necessarily describe the destructiveness of an earthquake… ...the earthquake may be close (more destructive) or distant (less destructive) from a population center… …and the event may be shallow (more destructive) or deep (less destructive) Mercalli The modified Mercalli intensity scale is used to assign a measure of destructiveness to an earthquake (degree of damage caused) It is qualitative and based upon observed effects on people and damage to buildings Mercalli I: very weak, not felt by people Mercalli XII: total destruction Mercalli Mercalli Intensity Magn (at Witness Observations itude epicenter ) I 1 to 2 Felt by very few people; barely noticeable. II 2 to 3 Felt by a few people, especially on upper floors. III 3 to 4 Noticeable indoors, especially on upperfloors, but may not be recognized as an earthquake. IV 4 Felt by many indoors, few outdoors. May feel like heavy truck passing by. V 4 to 5 Felt by almost everyone, some people awakened. Small objects moved. Trees and poles may shake. VI 5 to 6 Felt by everyone. Difficult to stand. Some heavy furniture moved, some plaster falls. Chimneys may be slightly damaged. VII 6 Slight to moderate damage in well built, ordinary structures. Considerable damage to poorly built structures. Some walls may fall. VIII 6 to 7 Little damage in specially built structures. Considerable damage to ordinary buildings, severe damage to poorly built structures. Some walls collapse. IX 7 Considerable damage to specially built structures, buildings shifted off foundations. Ground cracked noticeably. Wholesale destruction. Landslides. X 7 to 8 Most masonry and frame structures and their foundations destroyed. Ground badly cracked. Landslides. Wholesale destruction. XI 8 Total damage. Few, if any, structures standing. Bridges destroyed. Wide cracks in ground. Waves seen on ground. XII 8 or greate Total damage. Waves seen on ground. Objects thrown up into air. r G-Estimating the epicenter of an earthquake This requires data from at least three seismic stations Time difference between P wave and S wave is used to determine epicenter A sample seismogram A seismogram is the graphical representation of Earth movement Calculating the epicenter Finding the epicenter Example Here, travel time differences between P and S waves are used to calculate the distance of each seismic station from the epicenter Then the intersections of three circles determine the epicenter’s geographic location Case Study: San Andreas Fault The San Andreas fault Along much of the west coast, the plate boundary is a transform margin The San Andreas is a right-lateral strike-slip or transform fault I1-Case Study: San Andreas Fault Transform boundary between the Pacific plate and the North American plate Pacific plate moves northwest North American plate moves southeast (relative to fault) Some parts of the fault lock up and store energy Release it in abrupt motions, large earthquakes Other parts of the fault move smoothly Cause ground deformation, but only small earthquakes Over 1,200 km long At least 16 km deep in places One of the most studied faults in the world Has produced some infamous earthquakes Earthquakes on the San Andreas San Francisco area 1906 San Francisco quake M 7.7-7.9 ( magnitude) 3000 dead, $400 US damage 225,000 homeless (pop. at the time was 400,000) 1989 Loma Prieta quake M 6.9 ( magnitude) 57 dead $6 billion in damage Parkfield area 1857 Los Tejos quake M 8.0 (moment magnitude) 2 dead (it hit a then-sparsely populated part of California) After 1857, earthquakes M >6.0 occurred in 1881, 1901, 1922, 1934, 1966 and 2004 • Very active area of fault Loma Prieta 1989 The Marina district of San Francisco was very hard hit Unconsolidated, watersaturated materials were liquefied and mobilized by the shaking The lower picture shows a “volcano” of liquefied sand I2-Sudy case: Cascadia In the Pacific Northwest, the tectonic regime is subductionrelated, rather than transform as we have seen in California Cascadia Here, there is evidence for very large earthquakes over the last several thousand years…the most recent is 300 years ago I3-Quebec The St. Lawrence region has high levels of seismicity for a zone in the interior of a tectonic plate This seismicity may be related to old, aborted rifts about 200 Ma ago Map from Lamontagne (1999) What is an aborted or failed rift? -Failed rifts are ancient to modern features where continental rifting began, but then failed to continue. -Rifts are distinct from Mid-ocean ridges, where new oceanic crust and lithosphere is created by seafloor spreading. -In rifts, no crust or lithosphere is produced. If rifting continues, eventually a mid-ocean ridge may form, marking a divergent boundary between two tectonic plates. -There are three main groups of theories that have been proposed to explain the spatial occurrence of intraplate earthquakes: stress concentration, zone of weakness, and high heat flow. J-Effects of earthquakes: aftershocks Aftershocks normally occur after a major earthquake There may be many thousands of aftershock events over the space of months or even years Although their magnitudes generally decrease with time, aftershocks have potential to cause significant damage to already weakened materials (e.g., rocks, soils, buildings, power and gas lines) Effects: Liquefaction Occurs on sediments during earthquake shaking Groundwater can move upwards due to the shaking Water lubricates contact between sediment grains Weakens sediments Liquefaction has two consequences Amplifies shaking in structures Causes buildings to sink into sediment (Nelson, 2006) Effects: landslides The ground vibrations and severe shaking associated with an earthquake can induce landslides in mountainous areas This example in the Santa Susana Mtns. was caused by the 1994 Northridge event near Los Angeles Effects: tsunamis Tsunamis are ocean waves caused by displacements from earthquakes, landslides, etc. They can be devastating at great distances from the epicenter Tsunami damage in Hilo, Hawaii, as a result of the 22 May 1960 Chile earthquake Effects: building destruction Buildings are damaged or destroyed by ground vibrations and shaking The magnitude and duration of shaking are important factors in the extent of damage Liquefaction and aftershocks increase the damage Building damage near the epicenter of the 1989 Loma Prieta earthquake Effects on building materials Masonry is not capable of withstanding significant bending stresses Wood is more resistant because it is more yielding But wood is vulnerable to fires... Effects: fires The ground shaking will rupture power and gas lines… …and damage to water mains prevents or hinders fire fighting efforts the photo shows a broken gas line from the 1994 Northridge earthquake Francisco after the 1906 earthquake Effects: personal loss We are examining earthquakes from a scientific perspective… …but we must not forget the human element and the pathos conveyed by this photograph from the 1994 Northridge earthquake K-Mitigating earthquakes Seismic hazard maps and risk maps help to properly site and construct buildings Where to build your dream or trophy house - and where not to build Avoid unstable soils and unconsolidated materials... avoid mountainous terrain prone to landslides… and above all, avoid active faults ! Appropriate building codes which can withstand earthquake damage Bedrock foundations best Avoid asymmetrical buildings Bolt house firmly to foundations Appliances firmly bolted down Gas lines flexible Cupboards, shelving attached to walls Heavy objects at low levels; anchor heavy furniture Beds away from windows to avoid broken glass Earthquake-Resistant Building Diagonal cross-bracing Shear walls and a shear core – four shear walls in the center of a building that are connected to each other, level-to-level, providing resistance against floor collapse Base isolator NASA L-Earthquake Forecasting Short-term prediction Noticeable ground deformation can precede earthquakes Enough ground deformation can cause foreshocks Small earthquakes occurring before a large one 1975 Haicheng earthquake (China; moment magnitude 7.3) was successfully predicted because of foreshock observation • Area was evacuated days beforehand • City underwent significant destruction Changes in the water table can indicate fault movement preceding an earthquake Observing level in a well Earthquake Forecasting More short-term prediction Changes in electrical conductivity of rocks An increase in conductivity suggests groundwater movement groundwater conducts electricity better than rock Strange animal behavior Certain types of earth movement may produce sounds or vibrations that are detected by animals Increased radon gas emission Gas produced in the crust by the radioactive decay of uranium Increased emission can be due to presence of rock fracturing that might precede an earthquake Warning and prediction Precursory seismicity Precursory deformation Changes in physical properties of rocks near a fault Changes in water levels, soil gases Unusual behaviour of animals Earthquake prediction Important concepts: earthquake recurrence interval…seismic gap role of paleoseismology Yet our predictive ability is rudimentary, so we use probabilities e.g., 86% probability that a destructive quake of M>7 will hit southern California in the next 30 years (1994 estimate) Earthquakes - reading U.S. Geological Survey, 1999. Major quake likely to strike between 2000 and 2030. U.S. Geological Survey Fact Sheet 152-99, 4 pp. (http://pubs.usgs.gov/fs/1999/fs152-99/) Pelman, D., 2000. Tiny movements ease fault risk in East Bay; pressure builds up less in northern Hayward segment. San Francisco Chronicle, 18 August 2000. (http://www.sfgate.com/) Eastern Canadian seismicity: http://earthquakescanada.nrcan.gc.ca/historic_eq/20th/e_damaging_e.php Earthquakes - web Canadian seismicity: http://www.pgc.nrcan.gc.ca US seismicity: http://earthquake.usgs.gov/ San Francisco Bay area: http://www.abag.ca.gov/bayarea/eqmaps