Lecture-4-F10

Lecture-4-F10 - EAS 111 General Announcements 9/1/10 •...

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Unformatted text preview: EAS 111 General Announcements 9/1/10 • Schedule Reminder Today – Earthquakes Part I - Earthquakes, Locating them, understanding the Monday 9/6 – NO CLASS – Labor Day Holiday -Have fun, be safe Tuesday 9/7 – Lab 3 starts for the week Weds 9/8 – Earthquakes Part 2 – CPS practice run - Earthquake hazards and The earth’s interior and structure imaged from seismic investigations Earth and Atmospheric Sciences 111 Fall 2010 Earthquakes, Part 1 -Shallow Earth structure -Why earthquakes happen, where and how -How we “see”, measure and locate earthquakes -Lecture 4 EAS 111: Earth Science News US Geological Survey (USGS) National Earthquake Information Center http://earthquake.usgs.gov/e qcenter/ Three Types of Relative Plate Motions Move apart: Move divergent boundary divergent Move toward each other: Move convergent boundary con Move horizontally Move past one another: transform boundary transform 03.03.b1-3 Continental Crust Oceanic Crust Mantle Lithosphere - 0–100 km depth (mechanically strong and rigid) Asthenosphere 100-200 km depth (mechanically weak-partially molten (~2%) Below 200 km – regular solid Mantle again 01.03.b1 Major North American Earthquakes New Madrid, New 1811-1812 1811-1812 12.07.a Alaska, 1964 Hebgen Lake, 1959 San Francisco, 1906 Northridge, 1994 Mexico City, 1985 Charleston, 1886 Major World Major Earthquakes Earthquakes Nicaragua, 1972 Japan, 1999 Armenia, 1988 12.08.a Chile, 1960 Turkey, 1999 Taiwan, 2004 Observe this data carefully: Observe where earthquakes occur Where are deep earthquakes found? A.Mid-Ocean Ridges B.Transform margins C.Subduction Zones D.Passive (non-active) margins Europe Europe Asia North North America America Africa Subduction Subduction zones zones South South America America Interior of Interior some continents continents Australia Other plate Other boundaries boundaries 12.03.a1 yellow - shallow; green - intermediate; red – deep 0-70 km 70-350 km 350-670 km 0-70 How Subduction Causes Earthquakes Trench and prism 12.03.c Volcanoes Thrusts South South America America Su bd uc tin g Continental crust pl at e Plate boundary Plate (megathrust) (megathrust) Lithospheric Lithospheric Ast mantle mantle hen osp her e Deep slab How could volcanoes generate seismic waves? Explosive eruption Movement Movement along faults along 12.01.c Slope failure Movement of Movement magma magma What else can cause seismic waves? Meteoroid impacts Landslides Explosions Earthquakes Along Mid-Ocean Ridges Observe the types of activity that occur along mid-ocean ridges Faulting Faulting near rift near Oceanic crust Lithospheric mantle Asthenosphere Mid-ocean ridge ridge Transform fault Faulting on Faulting transforms transforms Injection of magma 12.03.b What is a fault? • A fault (geologically speaking) is any fracture in the Earth, of any size, that shows offset. • A fracture that has simply opened up is a joint. • If that fracture shows some type of sideways slippage, then it becomes a fault. Describing Earthquakes Seismic waves Seismic radiate outward from hypocenter from Epicenter Epicenter Hypocenter Seismic Seismic stations: wave arrives at closer seismic stations first stations What causes most earthquakes? Normal faults Reverse and Reverse thrust faults thrust Strike-slip faults 12.01.a Strike-Slip Faults Rocks move Rocks horizontally 08.04.c Left lateral Right lateral Before, During, and After an Earthquake Slip and Slip earthquake earthquake Stress Stress increase and elastic strain increase Post-slip recovery Post-slip of elastic strain; cycle starts again cycle 12.02.a The Earthquake Cycle 1. Buildup of elastic strain from long-range, slowly applied stresses (such as plate motion) 2. Elastic limit is reached, rock breaks 3. Energy released (defines “magnitude”) as slip, heat, and ground shaking 4. Fault stabilizes and heals 5. Buildup of elastic strain begins again, cycle starts over! Observe how earthquake ruptures grow Starts as small Starts patch below surface patch Borah Peak scarp Migrates out, Migrates ruptures surface (fault scarp) (fault Fault scarp Fault lengthens lengthens Landers scarp 12.02.b Different Kinds of Seismic Waves Surface waves Vertical surface wave compresses materials Horizontal Horizontal surface wave shears material shears Primary body Primary wave (P-wave) compresses material; fastest material; Body waves Secondary body Secondary wave (S-wave) shears material shears 12.04.a How Seismic Waves How Are Recorded Are Modern Modern seismometer seismometer Basic Basic seismometer seismometer Waves on seismogram Seismogram 12.04.b-c A thought question for you and a partner P waves travel faster than S waves. How might this be a useful tool for locating earthquakes? A.If you detect a P wave, then a damaging earthquake is coming to your location B.The P wave will get larger with distance C.The time difference between a P wave arrival and an S wave arrival should increase with distance How do we determine the location of an earthquake? Earthquake recorded by Earthquake seismic network seismic Select earthquake records 12.05.a Estimate distance Triangulate the epicenter A thought question for you and a partner P waves travel faster than S waves. How might this be a useful tool for locating earthquakes? A.If you detect a P wave, then a damaging earthquake is coming to your location B.The P wave will get larger with distance A.The time difference between a P wave arrival and an S wave arrival should increase with distance Measuring the Size of an Earthquake For local magnitude For (Richter): Measure S-wave amplitude S-wave Connect amplitude and distance on plot and There are other ways we measure There the size of an earthquake 12.05.b Magnitude Vs. Intensity Does the same magnitude earthquake produce the same intensity of shaking everywhere? NO! A big earthquake a long ways away yields little shaking (large magnitude, small intensity) The same earthquake (same magnitude) can yield very different shaking intensities on different ground surface types (soft, hard, well packed, not well consolidated, etc. = variable intensities) EAS 111: Assignments for 9/8/10 ...
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This note was uploaded on 01/12/2011 for the course EAS 111 taught by Professor Dr.ericriggs during the Spring '10 term at Purdue.

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