Understanding Earthquake Hazards and Minimizing Risks

Exercise 6 Earthquake Hazards James S. Reichard Georgia Southern University Student Name: Kevin Hoffman In this lab you will: examine various earthquake hazards and some of the techniques that can help minimize the loss of life and property damage. Background Reading and Needed Supplies Prior to doing this exercise you should read Chapter 5, Earthquakes and Related Hazards in the textbook. With respect to supplies, you will need a calculator, ruler, and colored pencils. Part I – Earthquakes and Plate Tectonics Each year, the vibrational (seismic) waves from earthquakes cause buildings to fail somewhere in the world, resulting in the death of large numbers of people. In addition to buildings, earthquakes cause considerable damage to human infrastructure, such as utility lines and transportation systems. Seismic waves also can trigger highly destructive tsunamis and landslides. Although humans cannot prevent earthquakes, we can use science and engineering to help minimize the hazards. In this exercise we will explore some of the earthquake hazards and ways we can reduce the loss of life and property damage. 1) The map in Figure 3.1 shows the location of earthquake epicenters that occurred over a 35year period. Note how the epicenters clearly define the edge of Earth's tectonic plates. a) Using a red colored pencil, place a set of arrows at the plate boundaries listed below so as to indicate whether the boundary is dominated by compressional, tensional, or shearing forces (i.e., indicate whether the boundaries are convergent, divergent, or transform. mid-ocean ridge in the Atlantic Aleutian Islands of Alaska western coast of South America Japan western coast of North America Islands of Indonesia b) What is the name of the geologic process that takes place along the plate boundary running through the Atlantic basin? The geologic process that takes place along the plate boundary running through the Atlantic basin is called seafloor spreading.

c) What geologic process occurs along the convergent boundaries you identified on the map? At convergent boundaries, the geologic process that occurs is called subduction. This is when one tectonic plate is forced beneath another plate, creating a subduction zone. The subducting plate sinks into the mantle, causing intense heat and pressure. This process can result in the formation of volcanic arcs, mountain ranges, and earthquakes. d) What is the name of the earthquake hazard that can develop along convergent boundaries which is capable of bringing death and destruction to distant shorelines? The name of the earthquake hazard that can develop along convergent boundaries and bring death and destruction to distant shorelines is a subduction zone earthquake. The name of the earthquake hazard that can develop along convergent boundaries and bring death and destruction to distant shorelines is a subduction zone earthquake. e) Explain why large-magnitude earthquakes relatively rare along divergent boundaries are, but quite common along convergent boundaries. Large-magnitude earthquakes are relatively rare along divergent boundaries due to the nature of the tectonic plates pulling apart. However, they are quite common along convergent boundaries where plates collide and create intense pressure and friction.

Figure 3.1

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Part II – Seismic Waves and Buildings 2) Figure 3.2 illustrates how earth materials vibrate differently in response to the passing of different types of seismic waves. Based on this illustration, explain why surface waves generally cause the most damage to human structures. Surface waves generally cause the most damage to human structures due to their unique characteristics. Unlike other types of seismic waves, surface waves travel along the Earth's surface and have a horizontal motion that can amplify the shaking effect. This amplification leads to increased ground motion, which in turn applies greater stress and strain on buildings and infrastructure, resulting in more severe damage. Additionally, surface waves have longer wavelengths compared to other seismic waves, allowing them to travel greater distances and affect a larger area, further increasing the potential for destruction. Figure 3.2 3) Figure 3.3 is a simplified sketch showing the structural skeleton of a typical multi-story building. A supporting structure usually consists of steel or steel-reinforced concrete. Based on what you learned in the textbook about seismic waves and buildings, indicate on the sketch the three types of engineering features that could be added to help prevent this building from collapsing in the event of an earthquake.

24 Figure 3.3 Ex 3 – Earthquakes 4) Seismic engineering controls in multi-story buildings are especially important should the building experience the phenomenon known as resonance , which causes it to shake more violently during an earthquake. Describe how resonance works and explain why it may occur in some multi-story buildings within a city and not others. Resonance occurs when the natural frequency of a building matches the frequency of ground motion during an earthquake, resulting in amplified shaking. In multi-story buildings, resonance may occur if the building's height and structural characteristics align with the seismic waves' wavelength. Factors such as the building's material, design, and foundation can influence whether resonance occurs. Therefore, some multi-story buildings within a city may experience resonance while others do not, depending on these factors. Seismic engineering controls are crucial in mitigating the effects of resonance and ensuring the safety of occupants during earthquakes. 5) As seismic waves travel outward from the focus through solid bedrock they naturally lose energy and become weaker. However, if the waves encounter unconsolidated sediment, such as in a sedimentary basin, they will begin to slow down and lose energy at a faster rate. As illustrated in Figure 3.4, this more rapid transfer of energy can lead to ground amplification where the ground itself shakes more violently, thereby increasing the threat to buildings and infrastructure. Ex 3 – Earthquakes

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The ground started to shake violently at the bedrock shown on the graph because of the hard surface and then as the vibrations started to spread outward it started to lighten up and become more subtle. Figure 3.4 a) The relief map on the following page shows the amount of ground amplification that can be expected to occur in the Los Angeles area. Using a black colored pencil, outline those areas where ground amplification is expected to occur (i.e., non-purplish areas where amplification is > 1). b) Add labels to your map indicating what areas are underlain by bedrock and those underlain by sediment. c) Provide an explanation as to why some parts of the sedimentary deposit are expected to experience as much as a five-fold increase in ground shaking (yellow color), whereas in other areas the increase may be by a factor of three or less (reddish colors). There is a possibility of a five-fold increase in ground shaking in certain areas of the sedimentary deposit. Geological formations in those regions play a role in the heightened seismic activity, as well as their composition and structure. In the map, yellow coloration represents these areas where ground shaking is more intense. However, in some other areas with reddish colors, the amount of shaking may be comparatively lower, between three and five times. These variations can be attributed to differences in geological characteristics and

26 underlying tectonic forces, highlighting the complexity of seismic activity within the sedimentary deposit. d) Notice that the yellow area in the northwest portion of the map has a distinct shape. Describe what this feature represents and explain why the ground amplification is expected to be so high there. This feature represents a geological formation characterized by a significant increase in ground amplification. Ground amplification refers to the phenomenon where seismic waves are amplified as they pass through certain types of soil or rock layers. In this particular location, the ground amplification is expected to be exceptionally high due to the presence of soft, loose sediments and a shallow water table. These conditions result in a low shear wave velocity and high-water content, which in turn leads to increased amplification of seismic waves. It is important to note that the high ground amplification in this area poses a potential risk for increased ground shaking during earthquakes, which can have detrimental effects on structures and infrastructure. Therefore, understanding and accounting for the ground amplification in this location is crucial for accurate seismic hazard assessment and effective engineering design. Ex 3 – Earthquakes

Ex 3 – Earthquakes

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28 6) Explain why the areas in metro Los Angeles with an increased risk of ground amplification also happen to be places where the population density is the highest. In other words, why did most of the development occur where the risk of earthquake damage is the greatest? The areas in metro Los Angeles with an increased risk of ground amplification coincide with high population density due to a combination of factors. Firstly, these areas are often located in desirable locations, such as near the coast or in close proximity to job centers, making them attractive for development. Additionally, historical factors, such as the availability of infrastructure and established communities, have contributed to the concentration of population in these areas. Furthermore, the demand for housing in a densely populated region like Los Angeles has led to the development of high-rise buildings and multi-story structures, which are more susceptible to ground amplification effects during earthquakes. Overall, the combination of desirable location, existing infrastructure, and housing demand has resulted in the concentration of population in areas with an increased risk of ground amplification. 7) Imagine that you are an executive with a large insurance company. Explain how a map of potential ground amplification might affect your company's rates for earthquake insurance. A map of potential ground amplification would significantly impact our company's rates for earthquake insurance. By identifying areas with higher ground amplification, we can assess the increased risk of damage and adjust our rates accordingly. This allows us to provide more accurate coverage and ensure that our customers are adequately protected. 8) Suppose you were to move to a city where the risk of a major earthquake is relatively high, but ground amplification maps are not available. Describe the types of geologic information that you could use instead which might help you avoid moving into a high-risk area. Ex 3 – Earthquakes

29 In the absence of ground amplification maps, one could rely on various types of geologic information to assess the risk of a high-risk area prone to earthquakes. Firstly, studying regional geology can provide insights into the tectonic setting and the presence of active faults. Identifying fault lines and their proximity to potential settlement areas is crucial in avoiding high-risk zones. Additionally, analyzing the seismic history of the region can provide valuable information on the frequency and magnitude of past earthquakes. Historical records, geological surveys, and paleo-seismic studies can help determine the level of seismic activity and potential future risks. Furthermore, understanding the soil and rock types prevalent in the area is important as certain geological formations can amplify ground shaking during an earthquake. By considering factors such as liquefaction potential, slope stability, and the presence of soft soils, one can make informed decisions about suitable locations to avoid high-risk areas. Overall, while ground amplification maps may not be available, a comprehensive analysis of regional geology, seismic history, and soil characteristics can help individuals make informed choices when relocating to a city with relatively high earthquake risk. Part III – Intraplate Earthquakes Large earthquakes are most common along convergent and transform plate boundaries, but they can also take place along old, buried faults in the middle of tectonic plates. These so- called intraplate earthquakes are of great concern because they occur rather infrequently, which means people are generally less prepared. We can get an idea of the risk of intraplate quakes in the U.S. by examining the USGS seismic hazard map shown in Figure 3.5. This map shows the strength of the horizontal ground motion (relative to gravity) that has a 10% chance of occurring over a 50-year period. For example, we can see that in the Los Angeles area, there is a 10% probability in next 50 years of an earthquake with a lateral ground motion of more than 0.32 times the force of gravity. Although the seismic hazard map in Figure 3.5 provides information as to the probability of different levels of ground shaking in a given area, it says nothing about the level of earthquake Ex 3 – Earthquakes

30 preparedness. Along the transform plate boundary in California, for example, major earthquakes are relatively common. Consequently, state and local building codes in this region generally require seismic engineering controls that are designed to help structures withstand lateral ground shaking. With the exception of California, most of the high-risk areas in the United States have not experienced a major earthquake in modern times, thus are far less prepared in terms of seismic engineering and emergency management planning. Ex 3 – Earthquakes

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27 Figure 3.5

32 9) Notice in Figure 3.5 how the area along the Mississippi River valley has a seismic-risk level similar to what is found along the plate boundary in California. In the winter of 1810-11, a series of magnitude 8 quakes occurred in the Mississippi valley region, but caused little structural damage because the area was largely undeveloped at the time. a) Using a U.S. map or atlas, list two major U.S. cities in the Mississippi Valley seismic zone that lie within the 8 to 32% gravity shake zones (yellow-orange-red). Using a U.S. map or atlas, we can identify two major U.S. cities situated in the Mississippi Valley seismic zone that fall within the 8 to 32% gravity shake zones, indicated by shades of yellow, orange, and red. These cities experience a higher risk of seismic activity compared to other areas. By pinpointing these cities on the map, we can better understand the potential impact of earthquakes in this region and take appropriate measures to ensure safety and preparedness for residents and visitors alike. b) Explain why the Mississippi Valley region, despite experiencing large magnitude earthquakes in the early 1800s, has so few buildings with the types of seismic engineering designs that are so common in California and Japan. The Mississippi Valley region, despite experiencing large magnitude earthquakes in the early 1800s, still lacks buildings with the types of seismic engineering designs that are commonplace in California and Japan. This discrepancy can be attributed to a combination of factors unique to the region. Firstly, the historical context plays a significant role, as seismic engineering knowledge and techniques were not as advanced during the time of those earthquakes. Additionally, geographical differences contribute to this disparity, as regions prone to frequent earthquakes tend to prioritize stronger building codes and regulations. Lastly, cultural attitudes and awareness about earthquake preparedness may have also influenced the development of seismic engineering in these areas. Ex 3 – Earthquakes

10) The area around Charleston, South Carolina, is another active intraplate seismic zone in the eastern United States. In 1886, a large-magnitude quake occurred near Charleston, causing considerable damage. a) Using an atlas and Figure 3.5, list the major U.S. cities that lie within the 4 to 24% gravity shake zones (green-yellow-orange) surrounding Charleston. The major U.S. cities that fall within the gravity shake zones range from 4 to 24% surrounding Charleston. These zones, represented by the colors green, yellow, and orange, indicate the levels of seismic activity in these areas. Within this range, cities such as Atlanta, Charlotte, Raleigh, and Jacksonville are located, highlighting the potential impact of earthquakes in a significant part of the Southeastern United States. It is crucial for residents and visitors to be aware of these zones and take necessary precautions to ensure their safety in case of any seismic events. b) How well prepared do you think these cities are for a future earthquake? Explain your answer. Do you think cities like Atlanta, Charlotte, Raleigh, and Jacksonville are well prepared for earthquakes? The preparation of such a community for natural disasters needs to take into account a variety of factors. First of all, these cities do not lie in seismically active areas, which may indicate that their infrastructure and emergency response systems are less robust than those in earthquake-prone areas. Furthermore, residents and local authorities may be less prepared if they have not experienced earthquakes previously. In order to be able to effectively respond to an earthquake if it occurs, these cities should invest in comprehensive earthquake preparedness plans and training programs.

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34 11) Notice in Figure 3.6 how western Washington state lies in a high-risk zone. Here the earthquake risk is related to a convergent plate boundary and associated subduction zone. Figure 3.6 illustrates how earthquakes are generated along subduction zones. Figure 3.6 (Source: USGS Fact Sheet 150-00) a) Explain why the largest earthquakes on record, around magnitude 9, are found associated with subduction zones. In other words, what it is about the subduction process itself that generates such large earthquakes? The largest earthquakes, around magnitude 9, are found associated with subduction zones due to the subduction process itself. Subduction occurs when one tectonic plate is forced beneath another, creating immense pressure and stress. This build-up of stress over time results in the release of energy in the form of earthquakes. The subduction process involves the interaction of two massive plates, causing the potential for larger areas of slip and greater displacement along the fault line. Additionally, the subduction zones are characterized by the presence of a subduction thrust fault, which allows for the accumulation of strain energy. When this energy is released, it can result in a significantly larger earthquake compared to other types of fault systems. Overall, the subduction process generates such large earthquakes due to the combination of intense Ex 3 – Earthquakes

pressure, extensive plate interaction, and the accumulation of strain energy along the subduction thrust fault. b) Although the Puget Sound and Seattle area in Washington state lies in a high-risk zone, relatively few buildings have incorporated seismic engineering controls similar to those required in California. Explain how this reduced level of preparedness is related to the subduction zone process itself. The reduced level of seismic engineering controls in the Puget Sound and Seattle area is related to the subduction zone process itself. Unlike California, where the San Andreas Fault produces frequent and noticeable earthquakes, the subduction zone in Washington state experiences less frequent and less intense seismic activity. As a result, the perceived risk of earthquakes is lower, leading to fewer buildings incorporating seismic engineering controls. 12) Communities along the Washington, Oregon, and northern California coast all face the prospect of a major subduction zone earthquake that generates a series of large tsunami waves. Describe why it is difficult for emergency management officials to develop effective evacuation procedures for coastal communities should a subduction zone earthquake occur in this area. Hint: examine how tsunamis travel in Figure 3.6. Developing effective evacuation procedures for coastal communities in the event of a subduction zone earthquake is challenging for emergency management officials due to several factors. First, the unpredictable nature of earthquakes makes it difficult to accurately predict when and where they will occur. Additionally, subduction zone earthquakes can generate powerful tsunami waves, which adds another layer of complexity to evacuation planning. Furthermore, coastal communities often have limited infrastructure and resources, making it challenging to efficiently evacuate large populations in a short amount of time. Finally, coordinating and communicating evacuation procedures with multiple agencies and community members can be a logistical challenge. Overall, these factors contribute to the difficulty

36 emergency management officials face in developing effective evacuation procedures for coastal communities in the event of a subduction zone earthquake. Ex 3 – Earthquakes

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