Earthquakes Lesson 9 Scientists believe that convection currents inside the Earth cause tectonic plates, (crustal plates) to separate, collide, or slip past each other. Each type of plate movement creates faults in the Earth's crust. Most earthquakes occur at plate boundaries and are the result of plate movement. Scientists have been able to study Earth's plates by studying earthquake patterns. Earthquakes are often the primary evidence that a fault exists beneath the surface of the Earth when there is no physical evidence above ground. San Francisco, California home destroyed in the Marina District due to soil liquefaction during the 1989 Loma Prieta earthquake. The saturated, loosely packed coarsegrained soils of the Marina District changed from a solid state to a liquid state during the earthquake. The invention of seismographs allowed scientists to record detailed information about earthquakes. Since the invention of seismographs, scientists have recorded millions of earthquakes. Using data from seismographs scientists pinpointed the focus of earthquakes, point within the Earth where earthquakes originate. They also located the epicenter of an earthquake, which is the point on the surface of the Earth directly above the focus. Scientists next plotted each earthquake on a world map and found they formed a pattern around tectonic plates, which are large blocks of the Earth s crust. Scientists invented sonar during World War II to locate enemy submarines that were cruising underwater off the coastlines of the United States. After the war, scientists began to map the Atlantic Ocean floor using sonar. Scientists found to their amazement, a great mountain chain ran down the center of the Atlantic Ocean. They named the chain of mountains the Mid-Atlantic Ridge. The Mid-Atlantic Ridge runs from north of Iceland down the middle of the Atlantic Ocean almost to Antarctica. Scientists found that the Mid-Atlantic Ridge is one section of the mid-oceanic ridge that circles our globe like seams on a baseball. Each of these mid-ocean ridges is an earthquake zone. Less than five percent of recorded earthquakes are located in these zones and the earthquakes are usually of low magnitude. USGS
Subduction zones are areas where one tectonic plate slips beneath another. Oceanic plates, which are heavier, subduct beneath continental plates because the minerals in basalt are heavier than minerals in granite. Around the Pacific Ocean basin, there is a series of subduction zones. Scientists coined the term Ring of Fire for the region because 80% of all recorded earthquakes and 75% of the world's active volcanoes are located there. A graben is a valley that forms between parallel faults. Pressure building inside the Earth forces land to break apart along cracks in the Earth's surface called faults. The type of fault depends on whether the land is pulling apart or pushing together. A normal fault occurs when one section of the land drops down as the crust thins due to forces pulling it apart. A reverse fault occurs when two landmasses are compressed. This type of fault movement thickens and shortens the crust. Normal faults and reverse faults usually move blocks of land upward in steeply inclined planes. The Great Rift Valley of East Africa developed when a series of grabens, valleys, 6000 km long formed. The valleys develop when land drops downward between two normal parallel faults. Two plates moving in a horizontal direction past each other develop a transform fault. The San Andreas Fault in California is more than 1200 km long. The Pacific Plate is moving northward at approximately 5.3 cm per year relative to the North American Plate. Streets, sidewalks, buildings and bridges all cross the fault in California. The plates slipping past each other in some areas cause only very small earthquakes detected by seismographs. The plates lock together at other places along the fault allowing energy to build up before a major earthquake. During the 1906 San Francisco earthquake, a road that crossed the head of Tomales Bay was offset approximately seven meters. Lesson summary Convection currents inside the Earth cause tectonic plates movement. The invention of seismographs allowed scientists to record detailed information about earthquakes. The focus of an earthquake is the point within the Earth that is the origin of an earthquake. Most of the world's earthquakes occur at boundaries between crustal plates. The mid-oceanic ridge runs through the middle of the world s oceans like seams on a baseball.
Name Date Earthquakes Quiz 9 Fill in the blanks using words from the Word Bank 1. The of an earthquake is the point within the Earth that is the origin of the earthquake. 2. The Mid-Atlantic Ridge runs from north of down the middle of the Atlantic Ocean almost to Antarctica. 3. The is a region where 80% of all recorded earthquakes and 75% of the world's active volcanoes are located. 4. Most of the world's occur at boundaries between crustal plates. 5. Scientists invented during World War II to locate enemy submarines that were cruising underwater off the coastlines of the United States. 6. The San Andreas Fault in California is a more than 1200 km long. 7. The invention of allowed scientists to record detailed information about earthquakes. 8. Most earthquakes occur at plate due to crustal plate movement. 9. The mid-oceanic ridge runs through the middle of the world s like seams on a baseball. 10. currents inside the Earth cause tectonic plates to separate, collide, or slip past each other. Word Bank sonar Iceland Ring of Fire focus seismographs transform fault boundaries convection oceans earthquakes
Wave Watching Activity 9 Introduction This is a two-part activity. In the first part, you will explore how P-waves, which are compression and expansion waves, move through a slinky. The wave box demonstrates how energy moves through solid materials as transverse or S-waves. Materials Metal slinky Yarn or tape Shoe box 2 metal washers 6 paper clips Rubber band Creating waves with a slinky Step 1 Studying P-waves 1. Place a piece of string on the slinky somewhere near the middle. 2. Place the slinky on a table or the floor. 3. Two people hold the ends of the slinky stretching it apart approximately two meters. 4. One person compresses their end of the slinky by holding between ten and twenty coils between their fingers. 5. The person holding onto the compressed coils releases all but the last coil. 6. Observe what happens to the string on the slinky after releasing the coils. 7. Repeat the activity several times watching as the waves travel back and forth through the slinky. 8. P-waves are compression waves and the waves traveling through the slinky are similar to the compression waves traveling through rocks after an earthquake.
Step 2 Studying S-waves 1. The second part of this activity creates a wave box to observe motion produced by S-waves, transverse waves. 2. Punch a hole at each end of a shoebox near the center. 3. Cut a rubber band in half, thread it through the hole in the box, and tie it to a paper clip. 4. Stretch the rubber band, run it through the hole in the other end of the box, and tie it around another paper clip. 5. Make sure the rubber band is tight enough so that when you pluck it, it will vibrate. 6. Place the six paper clips evenly along the rubber band so they are all facing the same way. 7. Pluck the rubber band and observe the motion of the paper clips. Wave box