TEST THREE OUTLINES

TEST THREE OUTLINES - 6 Earthquakes ← “We learn geology...

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Unformatted text preview: 6 - Earthquakes 10/25/07 ← “We learn geology in the morning after the earthquake, on ghastly diagrams of cloven mountains, upheaven plains, and the dry bed of the sea” – Ralph Waldo Emerson ← ← USGS estimates about 1 million quakes annually ← Most earthquakes are concentrated along plate boundaries ← ← Stress and Strain • Stress: force exerted per unit area • Strain: deformation caused by stress o compressive inward o tensile opposite directions o shear two different directions = tumbling • Rock Strength: ability to withstand stress before rupture • EARTHQUAKE: GROUND-SHAKING CASUED BY THE SUDDEN RELEASE OF STRAIN ENERGY CAUSED BY ROCK RUPTURE (FAULTING) All earthquakes occur on faults! ← ← Earthquake Cycles • Drop in strain after event, takes time to rebuild • Four Stages: (cycle can take hundreds to thousands of years) o Inactivity o Increase seismicity, accumulated strain o Foreshocks o Major earthquake, aftershocks ← ← Earthquake Focus and Epicenter • Seismic waves propagate outwards from the focus • Epicenter, point on surface, directly above the focus of the earthquake • Fault plain = As rupture grows bigger and grows up, it releases energy of strain and releases it in all directions in form of seismic waves ← ← Seismic Waves • P-Waves o Travels through all material o Faster in dense material • S-Wave o Travels only through solids dissipates in gases or liquids o ½ the speed of P-Wave • Surface Waves o Seismic waves that intersect with the surface o Travel much slower o Can cause more damage to buildings ← ← Frequency • As waves move out from the focus, high frequencies attenuated (lose their power and disappear fast) • High F = low building vibrates • Low F = high building vibrates ← ← Measuring Earthquakes • Seismograph or seismometer • Amplitude of seismic waves: ground vibration • First arrival of seismic waves: o Time of earthquake o Distance to epicenter • Wave Speeds As distance to the focus increases, so does the time gap between the arrival of the first P-Wave and the first S-Wave • An earthquakes occurs 500 km. from a seismometer. The fist P-Wave arrives after 70 seconds, the first S-Wave arrives in 150 seconds. Locating Epicenter • Locating an epicenter depends on the different velocities of P and S Waves • Compare P and S wave arrival times from a minimum of 3 seismic stations • A circle with a radius = distance to the epicenter is drawn around each station • Triangulation!...
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TEST THREE OUTLINES - 6 Earthquakes ← “We learn geology...

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