Unit IX Earthquakes Name: KEY California Geology The following assignment is based on information presented in Unit IX. Be sure to read the paragraphs (Note 1) provided before you attempt to answer the questions. Question 1 Read the following paragraph and answer question 1. Most people living in California have at one time or another experienced an earthquake. Without warning we wake up in the middle of the night because of the sometimes violent, sometimes slow rolling motion of the earth; other times we are driving on the freeway, walking in the mall, or even just watching television when an earthquake occurs. So what are earthquakes? What causes them? Why do earthquakes feel so different from one another? Earthquakes are the Earth's natural means of releasing stress. When the Earth's plates move against each other, stress is put on the lithosphere. When this stress is great enough, the lithosphere breaks or shifts. Imagine holding a pencil horizontally. If you were to apply a force to both ends of the pencil by pushing down on them, you would see the pencil bend. After enough force was applied, the pencil would break in the middle, releasing the stress you have put on it. The Earth's crust acts in the same way. As the plates move they put forces on themselves and each other. When the force is large enough, the crust is forced to break. When the break occurs, the stress is released as energy which moves through the Earth in the form of waves, which we feel and call an earthquake. Q-1 What happens when the earth s crust is broken? When the earth s crust is broken, stress is released as energy which creates an earthquake. Question 2 Read the following paragraph and answer question 2. There are two types of waves that are created when stress is released as energy in earthquakes: P, and S waves. The P wave, or primary wave, is the fastest of the three waves and the first detected by seismographs. They are able to move through both liquid and solid rock. P waves, like sound waves, are compressional waves, which means that they compress and expand matter as they move through it. S waves, or secondary waves, are the waves directly following the P waves. As they move, S waves shear, or cut the rock they travel through sideways at right angles to the direction of motion. S waves cannot travel through liquid because, while liquid can be compressed, it can't shear. S waves are the more dangerous type of waves because they are larger than P waves and produce vertical and horizontal motion in the ground surface. Both P and S waves are called body-waves because they move within the Earth's interior. Their speeds vary depending on the density and the elastic properties of the material they pass through, and they are amplified as they reach the surface. Q-2 What are the 2 seismic waves we discussed in lecture? For each wave: how is energy moved through rock by each wave, and what is the relative speed of each?
Wave type S wave P wave Way energy is moved by wave through rock They shear the rock they travel through They compress and expand the rock they travel through Relative speed Slower Faster Questions 3-6 Read the following paragraph and answer questions 3-6. Earthquakes vary in size. Those that do the most damage are extremely large, but some are so small they are almost undetectable. So, how are these measurements recorded? And how is their size determined? Geologists use seismographs to record the surface and body waves. Inside a seismograph designed to measure horizontal motion, a weight is freely suspended. As waves from earthquakes reach the seismograph the mass stays in relatively the same place, while the ground and the support move around it. This movement is recorded on magnetic tape by a pen attached to the mass. In a seismograph designed to measure vertical motion, the mass is connected to a spring, so as the ground and support move up and down, the pen on the mass measures the vertical motion. The metal tape which the motion is recorded on is marked with lines that correspond to one minute intervals. When motion is recorded a seismogram is created, which tells about the waves--how big they were and how long they lasted. P waves are recorded first, followed by S waves and then surface waves. While surface waves are the last to reach the seismograph, they last the longest time. Using the information from the seismogram, the epicenter and focus of the earthquake can be determined. The focus is the point on the fault at which the first movement or break occurred. The epicenter is the point on the surface directly above the focus. Once several seismograph stations have determined their distance from the epicenter, the actual epicenter can be located, using triangulation, on a map. Q-3 What is the instrument used to measure earthquakes called? seismograph Q-4 What is the epicenter? The epicenter is the point on the surface, directly above the focus. Q-5 What is the focus? The focus is the point on the fault at which movement first occurs during an earthquake. Q-6 Which seismic wave, the P or S wave, will reach the earthquake s focus first?
P wave Questions 7-10 Read the following paragraph and answer questions 7-10. Earthquakes can be measured in several ways. The first way is to describe the earthquake's intensity. Intensity is the measure, in terms of degrees, of damage to the surface and the effects on humans. Intensity records only observations of effects on the crust, not actual ground motion or wave amplitudes which can be recorded by instruments. While intensity helps to determine how large of an area was effected, it is not an accurate measure of the earthquake for many reasons. Two such reasons are: only the effect on an area showing the greatest intensity is reported, which can imply a greater or lesser intensity than what actually occurred, and the way in which seismic waves travel varies as they pass through different types of rocks, so some areas near by may feel nothing because they are built on faulted rock, while other areas quite a distance from the foci will feel the effects because they are built on compact homogenous rocks. The second type of measurement is the magnitude of the earthquake. Magnitude does not depend on population and effects to ground structures, but rather on wave amplitude and distance. Magnitude is determined using mathematical formulae and information from seismograms. One such magnitude scale is the Richter scale. This magnitude scale is logarithmic, meaning each step in magnitude is exponentially greater than the last. To determine the Richter magnitude, information collected by seismometers is used. Using a seismogram, the time difference between the recording of the P wave and the S wave is determined and matched to a corresponding distance value. The single maximum amplitude recorded on the seismogram is calculated and a line is drawn between the amplitude scale and the distance scale. The line crosses another scale, which corresponds to the magnitude. While this type of measurement is the most well known, the Richter scale is not as accurate a measurement as believed. Originally designed specifically for California, the Richter magnitude scale becomes an approximation in other states and countries. Also, the type of wave whose amplitude is to be measured is not specified, and it does not distinguish between deep and shallow foci. On the next page is a chart that shows how to measure Richter magnitude by an "eyeball" fit. First, the amplitude of the surface wave is measured on a seismogram produced by a Wood- Anderson seismometer (a specfic type of seismometer) and then it is compared with distance from the earthquake or the S-P time (which is the amount of time between the P-wave and S- wave arrival) to yield a magnitude.
Q-7 What is the difference between the intensity and the magnitude of an earthquake? Intensity is the measure of damage to the surface and the effects on humans. Q-8 Refer back to lecture. What scale, the Mercalli or Richter scale, measures the intensity of an earthquake? Mercalli Scale Q-9 Refer back to lecture. What scale, the Mercalli or Richter scale, measures the magnitude of an earthquake? Richter Scale
Q-10 What data are used to calculate a Richter Magnitude? The amplitude of the S wave on a seismogram, and the distance the seismograph was away from the epicenter of the earthquake are needed to calculate a Richter Magnitude. Be sure to read the paragraphs (Note 2) provided before you attempt to answer the questions. Questions 11-13 Read the following paragraph and answer questions 11-13. Damage in earthquakes is mainly from shaking. Although most earthquake damage is caused by shaking, other damaging effects of quakes can be just as devastating. For example, in the Great 1906 earthquake, the shaking damage in San Francisco was followed by fires that raged through the city almost uncontrolled, in part because water mains had broken in the quake. These and other destructive effects of quakes are discussed below. Fires Earthquakes in urban areas are often followed by destructive fires because (1) gas lines break, (2) electrical shorts ignite fires, (3) damaged water tanks and broken pipes limit water for firefighting, and (4) clogged roads and collapsed bridges prevent firefighter access. These factors can lead to fires spreading, causing extensive additional damage and burning entire neighborhoods. This photo shows fires in San Francisco s Marina District following the 1989 magnitude 6.9 Loma Prieta earthquake (photo below on left - courtesy of CBS 5). Damaged bridges, pipelines, powerlines, and roads Earthquakes often damage roads, hindering rescue and recovery efforts and causing accidents. Water and sewer pipeline breaks result in water loss and can cause sinkholes that undermine roads and buildings. Damage to natural gas and electrical distribution systems can cause fires, as well as major service outages. This car crashed when a section of the eastern span of the San Francisco-Oakland Bay Bridge collapsed in the 1989 magnitude 6.9 Loma Prieta earthquake (Earthquake Engineering Research Institute photo below at center). Dam failures Earthquake shaking can cause dams to fail, potentially causing catastrophic downstream flooding and reduced water supplies. In addition, many dams provide hydroelectric power, which could be critically needed following a quake. Cracks in the top of this dam were caused by the 1989 magnitude 6.9 Loma Prieta earthquake (USGS photo below on right). Hazardous material releases Earthquake damage can cause releases of hazardous materials from refineries and other chemical storage and distribution systems, research and industrial laboratories, manufacturing plants, and railroad tank cars. Oil was released and caught fire
when this storage facility was damaged by the 1999 magnitude 7.4 Izmit, Turkey, earthquake (photo below on left - by Kandilli Observatory and Earthquake Institute). Landslides Earthquakes can trigger landslides that damage roads, buildings, pipelines, and other infrastructure. Steeply sloping areas underlain by loose or soft rock are most susceptible to earthquake-induced landslides. This home was destroyed when the hillside beneath it gave way following the 1994 magnitude 6.7 Northridge earthquake (FEMA photo below at center). Liquefaction Earthquake shaking can cause soils to behave like a liquid and lose their ability to support structures. Liquefaction often causes buried gas and water lines to break. The highest hazard is in low-lying areas where there are loose, sandy soils or poorly compacted artificial fill. This photo shows liquefaction-related damage in the Marina District of San Francisco following the 1989 magnitude 6.9 Loma Prieta earthquake (USGS photo below on right). Surface rupture Fault movements can break the ground surface, damaging buildings and other structures. This fence near Point Reyes was offset 8 feet ( 2.5 m) when the San Andreas Fault moved in the Great (magnitude 7.8) 1906 earthquake (USGS photo below on left). Tsunamis Great earthquakes occurring anywhere in the Pacific Ocean may displace the ocean floor, generating tsunamis that could affect the California coast. Some coastal communities are designating Tsunami Hazard Zones and planning evacuation routes. Although the tsunami hazard in most of the Bay Area is low, coastal areas are still at risk. For example, this bait shop (Hazel s Fish Stand) in Half Moon Bay was ruined when it was hit by debris in the tsunami generated by the 1946 (magnitude 8) Alaska earthquake (photo below on right - copyright by MS & SB Collection). Q-11 What causes the most damage during an earthquake?
groundshaking Q-12 What are 8 other hazards related to earthquakes? fires damage to infrastructure dam failure hazardous material release landslides liquefaction surface rupture tsunamis Q-13 Is the earth opening up listed as a hazard? What is your opinion, do you think any of these hazard types could make this happen? The earth opening up is not listed as a hazard. Surface rupture can cause the ground to break up along the fault trace. Small gaps can be opened up, but not any that would be big enough to swallow something up. Liquefaction can cause the ground to liquefy and swallow up cars or homes, so that is probably the closest thing to the earth opening up that occurs. Question 14 Read the following paragraph and answer question 14. Many people think that all injuries in earthquakes are caused by collapsing buildings. Actually, most injuries in quakes are from objects that break or fall on people. For example, in the 1994 magnitude 6.7 Northridge earthquake, 55% of quake-related injuries were caused by falling objects, such as televisions, pictures and mirrors, and heavy light fixtures. Q-13 Are all quake injuries from collapsing buildings? Explain. Not all quake injuries are from collapsing buildings. More than half of all injuries are caused by falling objects. Questions 15-16 Read the following paragraph and answer questions 15-16. The best building code in the world does nothing for buildings built before the code was enacted. Although building codes used in California have some of the strictest seismic provisions in the world, many older buildings have not been retrofitted to meet updated codes. Retrofitting fixing problems in older buildings is the responsibility of a building s owner. In the early days of California, many homes were made of adobe bricks with wooden doorframes. After a powerful earthquake, doorframes were sometimes only parts of these houses still standing. From this came the myth that a doorway is the safest place to be during an earthquake. Today, few people in the Bay Area live in old, unreinforced adobe houses. In modern houses, doorways may be no stronger than any other part of the house and do little to protect you from falling debris. You are safer under a table, so DROP, COVER, AND HOLD ON. Q-15 We have good building codes, so we must have safe buildings. Explain.
This is not always true, as some buildings were built before building codes were implemented and have not been retrofitted. Q-16 Should you head for the doorway during an earthquake? Explain. You should not head for the doorway during an earthquake. In modern homes doorways are no stronger than the rest of the house. Question 17 Read the following paragraph and answer question 17. Many people wrongly believe that the U.S. Government will take care of all their financial needs if they suffer losses in an earthquake. The truth is that Federal disaster assistance is only available if the President formally declares a disaster. Even if you do get disaster assistance, it is usually a loan that you must repay, with interest, in addition to mortgages and other financial obligations you still owe, even on damaged property. If you don t qualify for loans, grants may be available to you. However, these are only designed to meet your most immediate needs, not to replace your losses. Q-17 Assess the statement: I don t need to worry about earthquakes, the government will save me. The government will only step in if the earthquake is declared to be a Federal disaster. Even so, grant money does not replace your losses, it is only designed to meet your immediate needs. Question 18 Refer to the lecture for Unit 9 (and Unit 1) to complete the final third of this activity. Q-18 Complete the chart below.