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Earthquakes Forces and Faults Think About It… What happens to a rubber band that is stretched past its elastic limit? A. Stress and Strain… What are earthquakes? • natural vibrations of the ground caused by movement along gigantic fractures in Earth’s crust Stress and Strain… 1. When do most earthquakes occur? • when rocks fracture (break) deep within Earth 2. When do fractures form? • when stress exceeds the strength of the rocks involved Stress and Strain… What is stress? … the forces per unit area acting on a material … There are three types of stress. 3. Stress Compression decreases the volume of a material – occurs at convergent boundaries http://rst.gsfc.nasa.gov/Sect2/05-01_reverse_fault-jyougashima_DSC8766.jpg Stress! Tension pulls a material apart – at divergent boundaries http://rpmedia.ask.com/ts?u=/wikipedia/commons/thumb/b/b1/Boudin_vein.jpg/180px-Boudin_vein.jpg Stress! Shear (horizontal) causes a material to twist – at transform boundaries. Strain! 5. What is strain? • the deformation of materials in response to stress • It is represented by the solid line on your diagrams. Elastic Limit a. b. A slab of rock’s elastic limit… • … is the point at which it will undergo permanent deformation At failure… • … a material breaks! • We call this failure an earthquake! Faults… 1. Rocks will fail and form a fault…when stress is applied too quickly or when stress is great 2. A fault is the resulting fracture or system of fractures along which movement occurs. Strike-Slip Faults vs Dip-Slip Faults Normal Fault – The Sierra Nevadas http://fela2fela.files.wordpress.com/2012/02/sierra-nevadas.jpg http://www.pbs.org/weta/thewest/places/states/california/ca_sierra.jpg http://geology1a-1.wikispaces.com/file/view/NormalFault.gif/90969209/NormalFault.gif Normal Fault… Is caused by… tension (at a divergent boundary) Movement of crust… block above fault moves down http://www.iris.edu/gifs/animations/faults.htm Reverse Fault – The Himalayas! http://www.geographicguide.net/earth/pictures/himalayas-asia.jpg http://img.wylio.com/flickr/500/494666903 Reverse Fault… Is caused by… compression (at a convergent boundary) Movement of crust… block above fault moves up and over http://www.iris.edu/gifs/anim ations/faults.htm http://homepage.ufp.pt/biblioteca/GlossarySaltTectonics/PlatesJPG/Fig.R018-ReverseFault.jpg http://7bcore3.wikispaces.com/file/view/StrikeSlipLLFault%5B1%5D.gif/193098468/StrikeSlipLLFault%5B1%5D.gif Strike-slip Fault… Is caused by… horizontal shear (at a transform boundary) Movement of crust… blocks slide past each other horizontal (no vertical movement) http://www.iris.edu/gifs/animations/faults.htm The San Andreas Fault… http://www.sanandreasfault.org/ A Video About Earthquakes https://www.youtube.com/watch?v=Yukp 0bPkQxs Earthquakes Seismic Waves and Earth’s Interior The Focus of an Earthquake… 1. 2. The focus of an earthquake is where the rocks break and it is located at least several miles beneath Earth’s surface. The epicenter is on Earth’s surface just above it. http://www.em.gov.bc.ca/Mining/Geoscience/SurficialGeologyandHazards/Earthquakes/PublishingImages/eq-2b.gif Earthquake Waves… 1. What are seismic waves? • vibrations of the ground during an earthquake 2. How do the following types of seismic waves move rocks? a. Primary (P) waves – squeeze and pull rocks in the same direction along which the waves are traveling b. Secondary (S) waves – cause rocks to move at right angles in relation to direction of waves c. Surface waves – move in 2 directions – up and down and side to side (slowest and most damaging!) d. See next slide… http://gomyclass.com/geology10/files/lecture9/html/web_data/file22.htm Earthquake Waves… http://aspire.cosmic-ray.org/Labs/SeismicWaves/ Seismogram… http://www.geo.mtu.edu/UPSeis/reading.html “So which wiggles are the earthquake? The P wave will be the first wiggle that is bigger than the rest of the little ones (the microseisms). Because P waves are the fastest seismic waves, they will usually be the first ones that your seismograph records. The next set of seismic waves on your seismogram will be the S waves. These are usually bigger than the P waves. The surface waves are the other, often larger, waves marked on the seismogram. Surface waves travel a little slower than S waves (which, in turn, are slower than P waves) so they tend to arrive at the seismograph just after the S waves.” Travel-Time Graph (not time travel!) 1. A travel-time graph shows… The average travel time of P and S waves for different distances 2. One can calculate the distance to the epicenter using the arrival times of the p and s waves. • http://mtweb.mtsu.edu/cribb/100eq.html Travel-Time Curves Suppose the waves recorded on a seismogram at Station A are farther apart than they are at Station B… The epicenter is farther from Station A. Focus Question… How are earthquakes measured and located? Earthquake Magnitude and Intensity How is the amount of energy released by an earthquake measured? • by its magnitude Haiti, 1/2010 (7) The Richter Scale… a. The Richter scale is based on… The size of the largest seismic waves generated by the quake b. Each successive number in the represents… An increase in seismic-wave size of a factor of 10 c. Each increase in magnitude corresponds to… A 32-fold increase in seismic energy d. A magnitude 8 EQ releases 32 times more energy than a magnitude 7 EQ. The Richter Scale Haiti, 1/2010 (7) Japan, 3/2011 (9) http://sahanasingh.wordpress.com/2011/05/17/failing-to-prepare/ The Modified Mercalli Scale… a. The intensity of an earthquake can be determined by… The amount of damage done to the structures involved b. The modified Mercalli scale is used to determine the intensity of an EQ by… Rating the types of damage and other effects of an earthquake as noted by observers This scale uses Roman numerals from I to XII. It was developed by Giuseppe Mercalli in 1902. http://www.geo.mtu.edu/UPSeis/intensity.html Modified Mercalli Scale… http://scearthquakes.cofc.edu/images/MercalRichtscale.gif Modified Mercalli Scale… d. A seismic intensity map is made by… • … compiling the MMsI values (Modified Mercalli scale intensity values), plotting them on a map, & joining points of similar intensity. • Mercalli values at distances very far from the epicenter… Decrease to I Time to put the Mercalli Scale into practice! Think About It… What is the strongest earthquake that has happened in your lifetime? Depth of Focus… e. Another factor that determines the intensity of an earthquake is… • The depth of an EQ’s focus Shallow-focus events will produce catastrophic quakes with high intensity values. Seismic Intensity Map… Northridge, California Earthquake http://seismo.berkeley.edu/blogs/seismoblog.php/2009/01/17/today-in-earthquake-history-northridge-1994 Damage in Northridge, Ca… http://www.weldreality.com/northridge-earthquake-4.jpg More Damage in Northridge, Ca… Locating an Earthquake… 1. The distance to an earthquake’s epicenter is determined by… 1. the P-S wave separation (the time difference b/tw the arrival of the P-wave and the S-wave) 2. This distance is called… 1. Epicentral distance 3. Three seismograph stations are needed to determine an EQ’s epicenter. See next slide… Locating an Earthquake… 5. A travel-time graph determines the exact time of an EQ. http://www.geo.mtu.edu/UPSeis/images/triangle.gif Seismic Belts… 1. 2. Most earthquakes are associated with… 1. Tectonic plate boundaries 80% of earthquakes occur in… 1. 3. Another 15% of earthquakes occur at the 1. 4. The Circum-Pacific Belt Mediterranean-Asian Belt Most of the remaining earthquakes occur in narrow bands that run along the crests of ocean ridges. http://earthquake.usgs.gov/ http://oceanworld.tamu.edu/resources/oceanography-book/earthquakes.htm Lab 9 USA Map… http://www.worldatlas.com/geoquiz/mississi.gif Think About It… Why do developing countries such as Haiti suffer such devastation from earthquakes? Focus Question… How do earthquakes affect the four spheres of Earth? Structural Failure… 1. The types of buildings that sustain the most severe damage from earthquakes are… • Typical building in Iraq unreinforced made of stone, concrete, or other brittle materials Church, Armenia, 1988 http://www.siampinoy.net/forum/55-ano-ba-ang-latest-post-it-here/88292-chile-earthquake-may-have-shortened-days-on-earth.html Structural Failure… 2. The types of buildings that sustain the least amount of damage from earthquakes are… • wooden • high-rise, steel-framed Kobe, Japan 1995 Structural Failure… 3. • Pancaking is… when supporting walls of the ground floor collapse and cause the upper floors to fall and collapse Pancaked Building in Turkey Structural Failure… 4. Building with 515 stories may collapse because… • their natural sway has the same period of vibration as the EQ http://www.wbdg.org/resources/seismic_design.php Land and Soil Failure… 5. Earthquakes in sloping areas can trigger… • landslides! El Salvador, 2001, mag 7.6 Land and Soil Failure… Soil liquefaction is… • when subsurface materials liquefy and behave like quicksand due to the seismic vibrations of an EQ 7. Seismic waves are amplified in soft materials because… • they have little resistance to deformation 6. http://en.wikipedia.org/wiki/Soil_liquefaction Land and Soil Failure… 7. Because Mexico City is built on soft sediments, there was much damage to structures there during the 1985 EQ. http://seismo.berkeley.edu/blogs/seismoblog.php/2008/09/19/title Fault Scarps… 8. A fault scarp is… • 9. an area of great vertical offset where the fault intersects the ground surface. The fault scarp shown created a… • waterfall Tsunamis… 10. A tsunami is caused by… • the vertical motions of the seafloor displacing the entire column of water over the fault 11. A tsunami in the open ocean is usually… • 1 meter high • You probably wouldn’t notice it if you were there! Tsunami strikes Ao Nang, Thailand http://en.wikipedia.org/wiki/2004_Indian_Ocean_earthquake_and_tsunami Tsunamis… 12. A tsunami is dangerous when it enters shallow water because… • it moves very quickly • it can be very high – up to 30m! Sri Lanka, 2004 Tsunamis… 13. The most recent devastating tsunami was in… • Japan, March 2011 Seismic Risk…p. 510 Most earthquakes occur at… • tectonic plate boundaries Seismic Risk… 14a. Five states with the greatest seismic risk… • • • • • Alaska California Hawaii Utah Nevada The San Andreas Fault is a transform boundary. http://pubs.usgs.gov/fs/2005/3052/images/map.jpg Seismic Risk… 14c. The seismic risk of the NC Piedmont and mountains… • moderate 14d. The seismic risk of the coastal plains of NC… • minor Earthquake Prediction… 15. Earthquake recurrence rates indicate … • that the fault ruptures repeatedly at regular intervals to generate similar quakes http://upload.wikimedia.org/wikipedia/commons/8/8d/San_Andreas.jpg Earthquake Prediction… 21. Seismic gaps are • sections of active faults that haven’t experienced significant EQs for a while • An earthquake is likely to happen here! The San Andreas Fault Earthquake Prediction… 17. Why is earthquake prediction important? … to save lives and prevent property damage!! EQ Shaking Hazards in the USA… F. Clues to Earth’s Interior (p. 503) 1. Where does the knowledge of Earth’s interior come from? • The study of seismic waves 2. How do P-waves and S-waves move through the mantle? • They follow fairly direct paths 3. What do P-waves do when they strike the core? • They bend 4. Why don’t S-waves enter Earth’s core? • They cannot travel through liquids Clues to Earth’s Interior 5. What does the disappearance of Swaves tell us about Earth’s outer core? • That it must be liquid 6. What shows us that Earth’s inner core is solid? • Studies of how P waves are deflected deep within Earth http://www.columbia.edu/~vjd1/ray_paths.gif G. Earth’s Internal Structure… Crust of lithosphere – granite (continental crust), basalt (oceanic crust) Upper mantle – peridotite (intrusive, bumpy, igneous) Asthenosphere – peridotite Lower mantle – oxides of iron, silicon, and magnesium Core – iron and nickel (magnetic field is generated here! G. Earth’s Internal Structure … http://astronomy.nju.edu.cn/~lixd/GA/AT4/AT407/HTML/AT40703.htm Let’s pause for a moment. What does all of this have to do with North Carolina? Have you heard of “fracking”? What does this have to do with earthquakes? http://www.youtube.com/watch?v=qm7e5 53S7fg http://www.youtube.com/watch?v=btRIH GKKKes http://www.youtube.com/watch?v=NW_xJ qPjE_I