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Earthquakes Plate Boundaries Sketch the 3 type of plate boundaries Pacific-North American Plate Boundary Earthquake Study • Seismology – the study of earthquakes (#27) • Seismologist – person who studies earthquakes Where? • Most occur near tectonic plate boundaries Causes • Deformation – bending, tilting, and breaking of Earth’s crust (#28) • Caused by plates pushing, pulling & sliding • 2 kinds of deformation – Plastic – bends like piece of soft clay • No earthquake – Elastic – stretches like a rubber band • Earthquake Elastic Rebound • Elastic rebound – sudden return of elastically deformed rock to its undeformed shape – Like a stretched rubber band that breaks and returns to its unstretched shape, now in 2 pieces – Energy released as seismic waves Normal Reverse Divergent Convergent Strike-slip Transform Body Waves • Seismic waves – waves of energy that travel through Earth, away from an earthquake in all directions (#29) • Two types of body waves that travel through interior of Earth – P waves • • • • • Primary waves (#30) Pressure waves (#30) Back and forth motion (#30) Go through solid, liquid, gas Fastest wave – S waves • • • • • Secondary waves (#31) Shear waves (#31) Side to side motion (#31) Go through solids only Slower wave, 2nd to arrive More Waves • Surface waves – waves that move in top few km of the crust – Up, down, circular motion – Back and forth motion • Travel slower than body waves • More destructive Draw pictures of P and S Waves Measurement • Seismograph – instrument that records ground vibrations to find location and strength of earthquake • Seismogram – tracing of earthquake motion created by a seismograph • Epicenter – point on Earth’s surface directly above an earthquake (#32) • Focus – point on a fault where earthquake motion first occurs (#33) Earthquake Location Finding Epicenter – Step 1 • Scientists use the S-P Time Method – Use a time-distance graph – S and P curves are already calculated – Determine distance away from epicenter for at least 3 seismograms Finding Epicenter – Step 2 Earthquake Strength • Richter Magnitude Scale – – Created in 1930s by Charles Richter – Magnitude measures strength of earthquake by ground motion adjusted for distance from epicenter – Each unit represents 10x increase in strength • 5.0 is 10x stronger than a 4.0 • 6.0 is 100x stronger than a 4.0 Richter Magnitude Scale Earthquake Intensity • Intensity – degree to which earthquake is felt and amount of damage caused • Modified Mercalli Intensity Scale – Numerical scale from I to XII – I not felt by people – XII total destruction • Intensity values highest near epicenter Earthquake Hazards • How likely area is to have a damaging quake in the future Earthquake Forecasting Gap Hypothesis • Areas on active faults that haven’t had many earthquakes are likely to be sites for future strong earthquakes. – Known as seismic gaps Earthquake Preparations • Resistant buildings – Mass damper – Active tendon system – Base isolators – Cross braces – Flexible pipes • Retrofitting – Reinforce columns – Fasten to foundation What to do in an Earthquake • Before shaking – Put heavy objects down low – Have an emergency meeting place – Store emergency supplies • During shaking – Crouch or lie under a strong table or desk – Outside – lay down away from buildings, trees, power lines – Stop car and stay inside • After shaking – – – – Stay calm and think Get away from danger areas Prepare for aftershocks Follow emergency plan Earthquake Examples • Normal • Reverse • Strike slip Normal Fault Example Dixie Valley-Fairview Peaks, Nevada earthquake December 16, 1954 Thrust Fault Example Thrust Fault Example Strike-slip Fault Example Strike-slip Fault Example 1906 San Francisco Earthquake Earthquake Effects • • • • • • Ground shaking Liquefaction – solid earth turns to liquid Surface faulting Landslides Fires Tsunami Earthquake Effects - Ground Shaking Loma Prieta, CA 1989 KGO-TV News ABC-7 Earthquake Effects - Ground Shaking Kobe, Japan 1995 Earthquake Effects - Liquefaction Source: National Geophysical Data Center Niigata, Japan 1964 Earthquake Effects - Surface Faulting Landers, CA 1992 Earthquake Effects - Landslides Source: National Geophysical Data Center Turnagain Heights, Alaska,1964 (upper left inset); Santa Cruz Mtns, California , 1989 Earthquake Effects - Fires Loma Prieta, CA 1989 KGO-TV News ABC-7 Earthquake Effects - Tsunamis 1957 Aleutian Tsunami Photograph Credit: Henry Helbush. Source: National Geophysical Data Center