Earthquakes

Earthquakes - A Violent Pulse: Earthquakes What is an...

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Unformatted text preview: A Violent Pulse: Earthquakes What is an Earthquake? Caused by a rapid but delayed release of energy Earthquakes destroy buildings and kill people Due to tectonic stress built-up Energy moves outward as an expanding sphere of waves Energy This waveform energy can be measured 3.5 million deaths in the last 2000 years Earthquakes are common Hundreds of thousands/year Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 8: A Violent Pulse: Earthquakes Seismicity Seismicity (earthquake activity) occurs due to … Motion along a newly formed crustal fracture Motion on an fault A sudden change in mineral structure Inflation of a magma chamber Volcanic eruption Giant landslides Meteorite impacts Nuclear detonations Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 8: A Violent Pulse: Earthquakes Earthquake Concepts Hypocenter (or focus) - The spot within the Earth where earthquake waves originate Usually occurs on a fault surface Earthquake waves expand outward from the hypocenter Earthquake Epicenter – Land surface above the hypocenter Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 8: A Violent Pulse: Earthquakes Faults and Earthquakes Most earthquakes occur along faults Most faults have a slope; Most others are vertical others The amount of movement The is termed displacement displacement Displacement is also Displacement called offset, or slip called Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 8: A Violent Pulse: Earthquakes Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 8: A Violent Pulse: Earthquakes Fault Initiation Tectonic forces add stress to unbroken rocks 1. 2. 2. 3. 3. 4. 4. The rock deforms slightly (elastic strain) The Continued stress will cause growth of cracks Continued Eventually, cracks grow to the point of failure Eventually, When the rock breaks, elastic strain transforms into When brittle deformation, releasing earthquake energy brittle Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 8: A Violent Pulse: Earthquakes Fault Motion Faults move in jumps Once movement starts, it quickly stops due to friction Eventually, strain builds up again causing failure This behavior is termed stick-slip behavior This Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 8: A Violent Pulse: Earthquakes Seismic Waves – Body Waves Body waves – Pass through Earth’s interior Compressional, or Primary (P), waves Compressional, Primary Push-pull (compress and expand) motion Push-pull Travel through solids, liquids, and gases Fastest Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 8: A Violent Pulse: Earthquakes Seismic Waves – Body Waves Body waves – Pass through Earth’s interior Pass Shear, or Secondary (S), waves Shear, Secondary waves “Shaking" motion Travel only through solids, not liquids Travel Slower Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 8: A Violent Pulse: Earthquakes Seismic Waves – Surface Waves Surface Waves – Travel along Earth’s surface Love waves – S waves intersecting the surface Love waves Move back and forth like a writhing snake Rayleigh waves – P waves intersecting the surface Rayleigh waves Move like ripples on a pond Move These waves are the slowest and most destructive Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 8: A Violent Pulse: Earthquakes Seismology Seismology is the study of earthquake waves Seismographs – Instruments that record seismicity Seismographs Worldwide, they detect earthquakes anywhere on Earth Seismology reveals size and location of earthquakes Seismology location Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 8: A Violent Pulse: Earthquakes Seismographs Seismographs Measure time of wave arrivals and magnitude of motion Straight line = background Waves arrive in sequence: P – S – Surface Waves Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 8: A Violent Pulse: Earthquakes Locating an Epicenter Difference between arrival of P, S, and Surface waves Difference varies with distance varies A travel-time graph plots the distance of each station to travel-time the epicenter the Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 8: A Violent Pulse: Earthquakes Locating an Epicenter Data from 3 stations can pinpoint the epicenter A circle is drawn around each station The radius is equal to the distance to epicenter Circles around 3 or more stations will intersect The point of intersection is the epicenter Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 8: A Violent Pulse: Earthquakes Earthquake Size Size is described by either intensity or magnitude Size intensity magnitude Mercalli intensity scale - Degree of shaking damage Roman numerals assigned to different levels of damage (I-XII) Damage occurs in zones Damage diminishes in Damage intensity with distance intensity Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 8: A Violent Pulse: Earthquakes Earthquake Size Magnitude – The amount of energy released Magnitude energy Several magnitude scales Several Maximum seismogram motion Maximum Normalized for distance Richter Moment Magnitude scales are Magnitude logarithmic logarithmic Increases of 1 unit = 10 fold increase in ground motion Increases of 1 unit = 30 fold Increases increase in energy released! increase Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 8: A Violent Pulse: Earthquakes Measuring Earthquake Size Earthquake energy release Earthquake can be calculated can M6.0 – Hiroshima bomb M8.9 – Annual energy M8.9 released by all other earthquakes earthquakes Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 8: A Violent Pulse: Earthquakes Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 8: A Violent Pulse: Earthquakes Earthquake Occurrence Earthquakes linked to plate tectonic boundaries Shallow – Divergent and transform boundaries Divergent Shallow, intermediate, and deep – Convergent boundaries Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 8: A Violent Pulse: Earthquakes Earthquake Focal Depths Shallow – 0-70 km Along the mid-ocean ridge Transform boundaries Shallow part of trenches Continental crust Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 8: A Violent Pulse: Earthquakes Earthquake Focal Depths Intermediate and deep earthquakes occur along the Intermediate subduction trace, the Benioff-Wadati zone subduction Intermediate – 70-300 km - Downgoing plate still brittle Deep - 300-670 km - Mineral transformations? Earthquakes rare below Earthquakes 670 km (mantle is ductile; 670 plate melting) Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 8: A Violent Pulse: Earthquakes Convergent Boundaries Cities near subduction zones have frequent earthquakes Most are minor Periodically, they are devastating Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 8: A Violent Pulse: Earthquakes Continental Earthquakes Earthquakes in continental crust: Earthquakes Transform faults (San Andreas; Anatolia, Turkey) Continental rifts (Basin and Range, East African Rift) Collision zones (Himalayas, Alps) Intraplate settings (ancient crustal weaknesses) Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 8: A Violent Pulse: Earthquakes San Andreas Fault … The Pacific plate meets the North American plate The San Andreas is a very active strike-slip fault The A very dangerous fault; hundreds of earthquakes per year San Francisco – Destroyed in 1906 Loma Prieta, 1989, World Series Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 8: A Violent Pulse: Earthquakes Intraplate Earthquakes 5% of earthquakes are not near plate boundaries Intraplate earthquakes are not well understood Remnant crustal weakness in failed rifts or shear zones? Stress transmitted inboard? Isostatic adjustments? Clusters: New Madrid, MO Charleston, S.C. Montreal, P.Q. Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 8: A Violent Pulse: Earthquakes Memorable Facts About Earthquakes: What are they? Where do they occur? How many earthquakes are there per year? What types of earthquake waves are there? Difference hypocenter (focus) and epicenter Relationship between depth of earthquakes and plate Relationship tectonic boundaries tectonic How can the size of earthquakes be measured? What increase in energy does one unit on the Richter What Scale represent? Scale Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 8: A Violent Pulse: Earthquakes ...
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