Chapter 11- Earthquakes Continuation of plate tectonics- convection in the asthenosphere is still the driving force of moving lithospheric plates. Where do earthquakes occur? At plate boundaries where pressure builds up and eventually breaks rock – Usually not in the middle of plates Faults occur where boundaries meet and rocks “pass their elastic limit” – Earthquakes often occur near these fault lines – Three types of faults Types of faults 1. Normal faults – Caused by TENSIONAL forces – Forces PULL OR STRETCH rocks – Rocks above fault line move downward 2. Reverse- - caused by COMPRESSIONAL forces - rocks are PUSHED toward each other - rocks above fault line move upward 3. Strike-slip fault - Caused by SHEAR forces - Rocks move PAST EACH OTHER with little up or down motion. Oral Pop Quiz (24points) Be able to NAME, SHOW and DESCRIBE how the rocks move at the fault line Be able to tell which type of forces (compressional, tensional or shear) cause each type of fault. Locating earthquakes using seismic waves An earthquake creates seismic waves that travel away from the epicenter of an earthquake. – Epicenter is the place on the Earth’s surface directly above where the earthquake occurred. Primary waves (P-waves) move away from the earthquake’s epicenter at a certain rate Secondary waves (S-waves) move away from the epicenter at a different, slower rate. Surface waves (no cool abbreviation) move at the slowest rate. Point where plate movement occurs and energy is originated is called the focus (can be miles below the surface). Point on Earth’s surface above the focus is called the epicenter Seismic waves All types of seismic waves (s,p and surface) are detected by seismographs and recorded on seismograms. To be shown during class discussion: * Recent earthquakes * Seismograph video clips: Three types of seismic waves: P waves travel fastest – Move particles back and forth in the same direction – Cause little destruction S-waves travel slower, cause more damage – Move particles back and forth at a ninety degree angle to wave motion Surface waves travel slowest, cause the most damage – Move particles side-to-side and in a swaying motion Calculating distance from the location of an earthquake The difference in arrival time between p-waves and s-waves can be timed to determine how far away from the seismograph station the earthquake occurred. With at least three stations reporting, we can pinpoint the earthquake’s location using TRIANGULATION (see next slide or page 312 for example). Plate Boundaries Divergent: very little earthquake activity Volcanic activity can vary Convergent: earthquake activity is strong and often very deep. Also, great amount of volcanic activity at subduction zones. Transform: Earthquakes are frequent and often shallow. Little to no volcanic activity. Other Info: Seismic waves are recorded on paper called seismograms. The energy from earthquakes sometimes causes tsunamis- giant seismic ocean waves. The Richter Scale is used to measure the intensity of earthquakes – Largest recorded 9.3 2004 Indian Ocean – Each number jumps x32 in intensity. “Hot spots” are places on Earth that possess volcanic and earthquake activity but are NOT near plate boundaries. Lab Report How do seismologists locate the epicenter of earthquakes? Similarly, how did we find the epicenter of an earthquake in class? Extra Credit Earthquake Report By Permission Only Location, date and time Pictures Damage (cost), deaths, injuries Magnitude Difference in P/S waves in Punxsy Search USGS Top Ten for list of earthquakes. Also a list on p. 318. Other research Plates involved Chapter 11 Test Questions Explain how Primary and Secondary waves are used to pinpoint the location of an earthquake’s epicenter Calculate the difference in earthquake magnitude using the Richter Scale Homework: Page 330 Questions 1-9. – Write out answers Answer these questions. – THIS IS YOUR CHANCE!!!
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