Earthquakes Chapter 7

What Are Earthquakes? Section 1 Seismology = the study of earthquakes. Earthquakes are complex & present seismologist with many questions. Where Do Earthquakes Occur? Most earthquakes take place near the edges of tectonic plates. They move in different directions & at different speeds. Fault = a break in the Earth’s crust along which blocks of the crust slide relative to one another. Two plates can push towards each other or pull away from each other. They can also slip past each other like slow-moving trains traveling in opposite directions. Earthquakes occur along faults due to this sliding. The largest & most active earthquake zone lines along the plate boundaries surrounding the Pacific Ocean.

What Causes Earthquakes? Section 1 What Causes Earthquakes? As tectonic plates push, pull, or scrape against each other, stress builds up along faults near the plates’ edges. Deformation = the change in the shape of rock in response to stress. Rock deforms in: plastic manner, like a piece of molded clay elastic manner, like a rubber band. Elastic Rebound = the sudden return of elastically deformed rock to its undeformed shape. plastic deformation – does not lead to earthquakes Elastic deformation – does lead to earthquakes While rock can stretch farther than steel without breaking, it will break at some point. Think of elastically deformed rock as a stretched rubber band. You can stretch a rubber band only so far before it breaks. When the rubber band breaks, it releases energy, & the broken pieces return to their unscratched shape. Elastic Rebound occurs when more stress is applied to rock than the rock can withstand. During Elastic rebound, rock releases energy that causes an earthquake.

Are All Earthquakes the Same? Section 1 Are All Earthquakes the Same? Transform motion – occurs where two plates slip past each other creates strike-slip faults. Blocks of crust slide horizontally past each other along strike-slip faults. This motion produces moderate, shallow earthquakes. Convergent motion – occurs where two plates push together creates reverse faults. Blocks of crust that are pushed together slide vertically along reverse faults such that the foot wall moves down relative to the hanging wall. This motion produces strong, deep earthquakes.

Are All Earthquakes the Same? Cont. Section 1 Are All Earthquakes the Same? Cont. Divergent motion – occurs where two plates pull away from each other creates normal faults. Blocks of crust that are pulled away from each other slide vertically along normal faults such that the footwall moves up relative to the hanging wall. This motion produces weak, shallow earthquakes.

How Do Earthquakes Travel? Section 1 How Do Earthquakes Travel? Seismic waves = waves of energy that travel through the Earth. Body waves – seismic waves that travel through the Earth’s interior. Two types of body waves – P waves & S waves Surface waves – seismic waves that travel along the Earth’s surface Different types of seismic waves travel at different speeds & move the materials that they travel through differently. The energy that is released when rock springs back after being deformed.

How Do Earthquakes Travel? Cont. Section 1 How Do Earthquakes Travel? Cont. P is for Primary P waves = (pressure waves; primary waves) travel through solids, liquids, & gases, are the fastest seismic waves; always travel ahead of other seismic waves. P waves move rock back & forth between a squeezed position & a stretched position as they travel through it. If you squeeze an elastic material into a smaller volume or stretch it into a larger volume, the pressure inside the material changes. When you suddenly stop squeezing or stretching the material, it springs briefly back & forth before returning to its original shape.

How Do Earthquakes Travel? Cont. Section 1 How Do Earthquakes Travel? Cont. S is for Secondary S waves = (shear waves; secondary waves) cannot travel through parts of the Earth that are completely liquid & are slower then P waves. S waves rock back & forth as they travel through it. Shearing stretches parts of rock for other parts. Time-distance graph – compares the speeds of P waves & S waves Rock can also be deformed from side to side.

How Do Earthquakes Travel? Cont. Section 1 How Do Earthquakes Travel? Cont. Surface Waves Surface waves move the ground much like ocean waves move water particles (up & down in circles as the waves travel along the surface) Surface waves travel more slowly than body waves but are more destructive. Most damage during an earthquake comes from surface waves, which can literally shake the ground out from under a building. If you squeeze an elastic material into a smaller volume or stretch it into a larger volume, the pressure inside the material changes. When you suddenly stop squeezing or stretching the material, it springs briefly back & forth before returning to its original shape. Rock can also be deformed from side to side.

What Are Earthquakes?: REVIEW Section 1 What Are Earthquakes?: REVIEW Where do earthquakes occur? What directly causes earthquakes? Arrange the types of earthquakes caused by the three plate-motion types from weakest to strongest.

Earthquake Measurement Section 2 Earthquake Measurement Locating Earthquake Seismographs = instruments located at or near the surface of the Earth that record seismic waves. Seismogram = a tracing of earthquake motion created by a seismograph. Seismologist use seismograms to calculate when an earthquake started. They find an earthquake’s start time by comparing seismograms to the time-distance graph & noting the difference in arrival times of P & S waves. They also use it to find an earthquake’s epicenter.

Locating Earthquake: Cont. Section 2 Locating Earthquake: Cont. Epicenter = the point on the Earth’s surface directly above an earthquake’s starting point. Focus = the point inside the Earth’s where an earthquake begins. An earthquake’s epicenter is on the Earth’s surface directly above the earthquake’s focus. Seismologist often use the S-P-time method to find an Earthquake’s Epicenter.

Locating Earthquake: Cont. Section 2 Locating Earthquake: Cont. S-P-time method A circle is drawn around a seismograph station. The radius of the circle equals the distance from the seismograph to the epicenter. (This distance is taken from the time-distance graph) When a second circle is drawn around another seismograph station, it overlaps the first circle in two spots. One of these is the earthquake’s epicenter. When a third circle is drawn around a third seismograph station, all three circles intersect in only one spot. This spot is the earthquake’s epicenter.

Measuring Earthquake Strength Section 2 Measuring Earthquake Strength Richter scale – a commonly used tool for measuring earthquake strength. Each time the magnitude increases by 1 unit, the amount of energy release becomes 31.7 times larger.

Measuring Earthquake Strength: Cont. Section 2 Measuring Earthquake Strength: Cont. Modified Richer Scale Magnitude Estimated Effects 2.0 Can be detected only by seismograph 3.0 Can be felt at epicenter 4.0 Felt by most in area 5.0 Causes damage at epicenter 6.0 Causes widespread damage 7.0 Causes great, widespread damage

Earthquake Measurement: REVIEW Section 2 Earthquake Measurement: REVIEW What is the difference between a seismogram & a seismograph? How many seismograph stations are needed to use the S-P-time method? Why? If the amount of energy released by an earthquake with a magnitude of 7.0 on the Richter scale is x, what is the amount of energy released by an earthquake with a magnitude of 6.0 in terms of x?

Earthquakes & Society Section 3 Simply being award of earthquakes is not enough. It is important for people in earthquake-prone areas to be prepared. Earthquake Hazard Earthquake hazard measures how prone an area is to experiencing earthquakes in the future. An area’s earthquake-hazard level is determined by past & present seismic activity.

Earthquake Forecasting Section 3 Earthquake Forecasting Predicting when & where earthquakes will occur & how strong they will be is a very difficult task. Strength & Frequency The strength of earthquakes is related to how often they occur. Worldwide Earthquake Frequency Descriptor Magnitude Average occurring annually Great 8.0 & higher 1 Major 7.0-7.9 18 Strong 6.0-6.9 120 Moderate 5.0-5.9 800 Light 4.0-4.9 about 6,200 Minor 3.0-3.9 about 49,000 Very Minor 2.0-2.9 about 365,000 However, by closely monitoring active faults & other seismic activity, seismologist have discovered some patterns in earthquakes that allow them to make some broad predictions.

Earthquake Forecasting: Cont. Section 3 Earthquake Forecasting: Cont. The Gap Hypothesis Gap Hypothesis = states that sections of active faults that have had relatively few earthquakes are likely to be the sites of strong earthquakes in the future. Seismic Gap = the areas along a fault where relatively few earthquakes have occurred Seismologist have used this to predict the time & place of earthquakes.

Earthquakes & Buildings Section 3 Earthquakes & Buildings Mass damper – a weight placed in the roof of a building. Motion sensors detect building movement during an earthquake & send messages to a computer. The computer then signals controls in the roof to shift the mass damper to counteract the buildings movement. Cross-braces – are placed between floors. These braces counteract pressure that pushes & pulls at the side of a building during an earthquake

Earthquakes & Buildings: Cont. Section 3 Earthquakes & Buildings: Cont. Flexible pipe – help prevent water & gas lines from breaking. Engineers design the pipes with flexible joints so the pipes are better able to twist & bend without breaking during an earthquake. Active tendon system – works much the mass damper system in the roof. Sensors notify a computer that the building is moving. Then the computer activates devices to shift a large weight to counteract the movement.

Earthquakes & Buildings: Cont. Section 3 Earthquakes & Buildings: Cont. Base isolators – act as shock absorbers during an earthquake. They are made of layers of rubber & steel wrapped around a core. Base isolators absorb seismic waves, preventing them from traveling through the building.

Are you prepared for an Earthquake? Section 3 Are you prepared for an Earthquake? Plan ahead so you will know what to do & stick to your plan as closely as possible. Before the Shaking Starts Put heavier objects on lower shelves Have your home reinforced Make a plan with others (family, neighbors, or friends) to meet somewhere after the earthquake Store non-perishable food, water, a fire extinguisher, a flashlight with batteries, & a first aid kit in a place you can access after the earthquake.

Are you prepared for an Earthquake? Cont. Section 3 Are you prepared for an Earthquake? Cont. When the Shaking Starts Inside: Crouch or lie face down under a table or desk in the center of a room Outside: lie face down away from buildings, power lines, & trees, then cover your head with your hands. In car: stop your car & remain inside After the Shaking Stops Clam down, get your bearings, & remove yourself from immediate danger Recall your earthquake plan, & follow it through. Such as downed power lines, broken glass, & fire hazards. Be aware that there may be aftershocks

Earthquakes & Society: REVIEW Section 3 Earthquakes & Society: REVIEW How is an area’s earthquake hazard determined? Which earthquake forecast predicts a more precise location – a forecast based on the relationship between strength & frequency or a forecast based on the gap hypothesis? Describe two ways that buildings are reinforced against earthquakes.

Earthquake Discoveries Near & Far Section 4 Earthquake Discoveries Near & Far The study of earthquakes have led to many important discoveries about the Earth’s interior. Discoveries in Earth’s Interior Notice how light bends in water? Seismic waves bend in much the same way as they travel through rock. Moho = discovered in 1909, is a place within the Earth where the speed of seismic waves increases sharply. The Moho marks the boundary between the Earth’s crust & mantle.

Discoveries in Earth’s Interior: Cont. Section 4 Discoveries in Earth’s Interior: Cont. Shadow zone = discovered in 1906, an area on the Earth’s surface where no direct seismic waves from a particular earthquake can be detected. This suggested that the Earth has a liquid core. Solid inner core = discovered in 1936, before this discovery, seismologist thought that the Earth’s entire core was liquid.

Quakes & Shakes on Other Cosmic Bodies Section 4 Quakes & Shakes on Other Cosmic Bodies The Moon 1969 – scientist left a seismograph on the moon then purposefully crashed their landing vehicle back into the moon If the lander had crashed into the Earth, the equivalent seismograms would have lasted 20-30 seconds at most. On the moon it lasted for 90 minutes. Scientist think that the material in the moon’s interior has different properties than the moon’s surface material. Other planets seismologist have learned about.

Quakes & Shakes on Other Cosmic Bodies: Cont. Section 4 Quakes & Shakes on Other Cosmic Bodies: Cont. Mars 1976 – allowed seismologists to learn about seismic activity on Mars. Mars was so windy that the seismograph worked mainly as a wind gauge Seismographs recorded seismograms for months but only one possible “marsquake” happened. Several projects are currently underway to study Mar’s water supply, wind, soil, atmosphere & climate, & many other aspects of the planet. Other planets seismologist have learned about.

Quakes & Shakes on Other Cosmic Bodies: Cont. Section 4 Quakes & Shakes on Other Cosmic Bodies: Cont. The Sun Humans cannot directly access the sun scientist study it remotely using a satellite called SOHO. SOHO has shown that solar flares produce seismic waves Solar flares – are powerful magnetic disturbances in the sun. “sunquakes” – similar to earthquakes but are generally much stronger. Other planets seismologist have learned about.

Earthquake Discoveries Near & Far: REVIEW Section 4 Earthquake Discoveries Near & Far: REVIEW What observation of seismic-wave travel led to the discovery of Moho? Briefly describe one discovery seismologist have made about each of the following cosmic bodies: the moon, Mars, & the sun.