1.14: Lab 14 - Seismology

This lab contains potentially inaccessible interactive resources. Please work with your instructor and local campus resources to identify accommodations for these resources.

Learning Objectives

• Identify the three types of stresses that cause normal, reverse, and strike-slip faults.
• Analyze the spatial distribution of faults in southern California.
• Describe the differences between p-waves and s-waves.
• Explain how an earthquake’s epicenter is located.
• Differentiate between earthquake magnitude and earthquake intensity.
• Analyze liquefaction hazard probability estimates and earthquake probability data.
• Explain how earthquake vulnerability varies for different communities.

Introduction

Seismology is the study of earthquakes and the energy produced by earthquakes. Each earthquake represents movement along a fault. This movement could be caused by pressure put on the rock from tectonic activity, volcanic activity, or human activity such as fracking. A fault is a fracture in a rock; this is where energy is released by an earthquake. You could think of the fault as having potential energy due to the pressure that is placed on it. Once the pressure builds up and it can no longer withstand it, kinetic energy is released when the rocks move along the fault.

Figure 14.1 illustrates the fault line between two rock blocks. The earthquake occurs and energy is released at the hypocenter, also called the earthquake’s focus. The epicenter is the location on the surface of the Earth directly above the hypocenter. Tip: the prefix hypo- means under (e.g., a hypodermic needle is one that injects underneath the skin); the prefix epi- means above or on (e.g., epidermal means the outer layer of the skin).

Figure 14.1: Simplified Fault Diagram. Figure by Waverly Ray is licensed under CC BY-NC-SA 4.0

Think About It…Earthquake Frequency

Millions of earthquakes occur across the world each year. The U.S. Geologic Survey estimates that in any given year only 16 major earthquakes will occur globally (major earthquakes are defined as a 7.0 on the moment magnitude scale). Why do you think there are millions of earthquakes but so few are powerful ones that measure at least a 7.0?

Part A. Fault Types

There are three main types of stress: compression (pushing together), tension (pulling apart), and shear (sliding past). Each of these types of stress results in a different fault type (Figure 14.2) with particular features as follows:

➢ A fault scarp occurs when rocks are offset in compressional and tensional faults. The offset could be small (a few centimeters) or larger (dozens of meters). Larger fault scarps create cliffs.

➢ A hanging wall is a block of rock that is on top of the fault.

➢ A footwall is a block of rock that is underneath the fault.

Notice that for reverse faults, compression causes the hanging wall rocks to rise up relative to the footwall rocks. For normal faults, tensional stress causes the hanging wall rocks to drop down relative to the footwall rocks. And, for strike-slip faults, shear stress does not create a fault scarp because the two rock blocks slide laterally across the fault.

Figure 14.2: Three Basic Fault Types. Figure by Steven Earle is licensed under CC BY 4.0

Check It Out! The Three Basic Fault Types

View the three basic fault types in motion in this IRIS Earthquake Science video. (Video length is 51 seconds).

It is easy to confuse the scale and vocabulary of tectonic plate boundary types and fault types. This is because the three types of stress (compression, tension, and shear) apply to both plate tectonics and earthquakes. But, it’s important to remember that plate tectonic boundaries represent very large blocks of rock whereas faults are relatively small-scale. Also consider that along any plate tectonic boundary, the movement caused by the main stress will likely place different stresses around the boundary.

For example, along a mid-oceanic ridge, tensional stress is pulling apart the oceanic tectonic plate. This causes shear stress to occur along the oceanic rocks being torn apart. As you learned in the previous lab, the San Andreas Fault Complex represents the transform plate tectonic boundary between the Pacific plate and the North American plate, so the main stress is shear. But, there are areas with compressional stress and tensional stress that are created when these two massive blocks of rock slide past each other. Therefore, California has all three fault types.

Pin It! Fault Vocabulary

Remember to use the correct vocabulary when talking about faults:

➢ Normal faults are caused by tensional stress.

➢ Reverse faults are caused by compressional stress.

➢ Strike-slip faults are caused by shear stress.

For example, you would never call a fault convergent—there are convergent tectonic plate boundaries caused by compressional stress, but we don’t use the term convergent for faults. Compressional stress creates reverse faults.

Exercise \(\PageIndex{1}\)

1. Complete Table 14.1 below.

Table 14.1: Fault Types. Figures by Ralph L. Dawes and Cheryl D. Dawes are licensed under CC BY 3.0

Diagram	Stress Type that Created the Fault	Fault Type Name
Figure 14.3: Fault Example 1
Figure 14.4: Fault Example 2
Figure 14.5: Fault Example 3

Figure 14.6 shows where the three fault types are located in Southern California. The dashed lines represent normal faults caused by tensional stress. The dotted lines represent reverse faults caused by compressional stress. The black lines represent strike-slip faults caused by shear stress. The San Andreas Fault Complex includes the numerous faults that mark the transform boundary between the Pacific and North American tectonic plates. Notice that the San Andreas Fault Complex is not a straight line, which makes sense because two tectonic plates are sliding past each other so a smooth surface wouldn’t be expected. There is a lot of accumulating pressure and different rock types so the transform boundary is jagged.

Figure 14.6: Map of Fault Types in Southern California. Figure by Waverly Ray is licensed under CC BY-NC-SA 4.0

Exercise \(\PageIndex{2}\)

2. With a highlighter or colored pencil, trace the main fault of the San Andreas Fault Complex. Hint: find the Salton Sea in the southeast area of the map. Tip: if needed, use an atlas or the internet to find where the Salton Sea is located. The faults extending on the northeast side of the Salton Sea are part of the San Andreas Fault Complex. Highlight those faults as they extend all the way to the northwest part of the map.
3. Use an atlas or the internet to find the locations of Bakersfield, Los Angeles, San Diego, and El Centro. Label these cities on Figure 14.6.
4. In one sentence, describe the location of the San Andreas Fault Complex. Hint: use the scale bar to estimate how far inland the fault is located.

5. Why are there three types of faults found in Southern California when the San Andreas Fault Complex represents a transform plate boundary? Your response should be two to three sentences in length and include an explanation of the location of the compressional faults seen along the coast and inland (around Los Angeles County). Tip: refer to this diagram from the Southern California Earthquake Center at USC.

Part B. Seismic Energy

Earthquake energy, known as seismic energy, travels through the Earth in the form of seismic waves. Seismic waves are an expression of the energy released after an earthquake in the form of body waves and surface waves (Figure 14.7). When seismic energy is released, the first waves to propagate out are body waves that pass through the body of the planet. Body waves include primary waves (P waves) and secondary waves (S waves). Primary waves are the fastest seismic waves, averaging 6 kilometers per second in the outer crust. This is equivalent to a speed of about 13,422 miles per hour. They move through rocks via compression, very much like sound waves move through the air. Particles of rock move forward and back during the passage of the P waves (Figure 14.8). Primary waves can travel through both fluids and solids. Secondary waves travel slower—averaging 3.5 kilometers per second in the outer crust—and follow primary waves, propagating as shear waves. Particles of rock move from side to side during the passage of S waves (Figure 14.9). Because of this, secondary waves cannot travel through fluids, plasma, or gases. Surface waves only travel at the Earth’s surface; they cause the most destruction to human-built structures.

Figure 14.7: Three Types of Seismic Waves. Figures adapted by Waverly Ray from the USGS are licensed under CC BY-NC-SA 4.0

Figure 14.8: P Waves. Figure by Brian Brondel is licensed under CC BY-SA 2.5

Figure 14.9: S Waves. Figure by Brian Brondel is licensed under CC BY-SA 2.5

The equipment used to detect and record earthquakes are called seismographs (also known as seismometers). There are thousands of seismographs across the world. Figure 14.10 shows a basic horizontal-motion seismograph. A pen is suspended by a weight that touches a rotating drum. As the ground shakes, the drum moves and the pen records the movements on a record called a seismogram.

Figure 14.10: A Seismograph. Figure by Dollynarak is licensed under Free Art License 1.3

Figure 14.11 is the seismogram from a magnitude 4 earthquake with time in seconds along the x-axis and wave amplitude in millimeters per second along the y-axis. Notice how on the left, the line is flat until the P wave arrives at 0.7 seconds on the seismogram. The S wave arrives 8 seconds after the P wave. The difference in time between the P and S wave arrivals is used to determine how far the seismograph is relative to the earthquake’s location (this is known as the S-P time). The amplitude of the largest seismic wave is used to estimate the earthquake’s magnitude.

Figure 14.11: An Example Seismogram. Figure by Steven Earle is licensed under CC BY 4.0

Check It Out! A Seismograph Recording P and S Waves

View a seismograph recording P and S waves in this IRIS Earthquake Science video. (Video length is 11 seconds).

P and S waves travel at different speeds through the different layers of the Earth. On average, P waves travel between 6.0 and 13.0 kilometers per second (as mentioned above, in the outer crust P waves travel at about 6 kilometers per second—the interior layers of the Earth are more dense so the P waves do not travel as fast). On average, S waves travel between 3.5 and 7.5 kilometers per second.

If we assume a P wave speed of 7.5 kilometers per second and an S wave speed of 4.3 kilometers per second, we can use observations from thousands of earthquakes to understand how far a seismograph is from the epicenter of an earthquake. Think of it this way: the moment the fault ruptures, seismic energy starts moving away from the hypocenter. P waves travel fastest and reach a seismograph first and are recorded on the seismogram. Some time later, the slower S waves arrive and are recorded on the seismogram. By knowing the average speeds of the P and S waves, we can work out the distance the seismograph is to the epicenter by calculating the S-P time. When the P wave speed is 7.5 kilometers per second and the S wave speed is 4.3 kilometers per second, every second of S-P time equates to a distance of 10 kilometers. Therefore, if S-P time recorded on a seismogram is 60 seconds then the seismograph is located 600 kilometers from the earthquake’s epicenter (60 times 10 kilometers equals 600 kilometers).

Exercise \(\PageIndex{3}\)

6. What are the main differences between P waves and S waves?

7. What is the difference between a seismograph and a seismogram?

8. Refer to Figure 14.11. What is the S-P time?

9. How far is the seismograph that recorded the seismogram shown in Figure 14.11 from the earthquake’s epicenter? Show your calculations.

10. Suppose you felt an earthquake’s P waves arrive to your location, causing shaking. Thirty seconds after the P waves first arrive, you feel the S waves arrive, which causes severe shaking. Assuming a S-P time of 30 seconds, a P wave speed of 7.5 kilometers per second, and an S wave speed of 4.3 kilometers per second, how far are you from the earthquake’s epicenter? Provide an answer in both kilometers and miles. Hint: to convert kilometers to miles, multiply kilometers by 0.621. Be sure to show your work.

The S-P time lets us know the distance of a seismograph from an earthquake but doesn’t tell you where the epicenter is in relation to the seismograph. Is it to the north, south, east, or west? For example, if the seismograph is 500 kilometers from the epicenter, you don’t know in which direction the epicenter is located. It could be 500 kilometers to the east; it could be 500 kilometers to the west; etc. A triangulation method is used to locate the earthquake’s epicenter. With data from at least three seismographs, you can pinpoint the earthquake’s epicenter (Figure 14.12). For each of the seismographs (shown by green dots in Figure 14.12), a black circle is drawn representing the distance the seismograph is to the epicenter. Where the three circles intersect is the earthquake’s epicenter.

Figure 14.12: Epicenter Triangulation with Distances from Three Seismographs. Figure by USGS is in the public domain

Before we look at a specific example, let’s make sure we understand the x-axis and the y-axis for the seismograms provided on the “Wilber 3” data service from the Incorporated Research Institutions for Seismology (IRIS) website. Figure 14.13 is a seismogram recorded on July 22, 2020, in Corvallis, Oregon. It recorded a 7.8 magnitude earthquake that occurred near the Alaskan Peninsula (to be exact, the latitude of the epicenter was 55.0298° North, the longitude was 158.5217° West). The P wave arrival is shown by the vertical red line on the left and the S wave arrival is shown by the vertical blue line on the right. At the far left of the x-axis is the date, 2020-07-22 (representing July 22, 2020). The x-axis shows the time in UTC. UTC is Coordinated Universal Time, which allows scientists to know the precise time of an event no matter what the local time is. The first time notch on the left represents 06:18:00 UTC (06 is the hours, 18 is the minutes, and 00 is the seconds). So the first time notch is 6:18 a.m. UTC. The next time notch is 2 minutes later. The y-axis shows the raw number recorded from the seismograph itself, which represents the movement caused by the seismic waves.

Figure 14.13: An Example Seismogram from the “Wilber 3” Data Service. Figures adapted from the Wilber 3 Data Service of the Incorporated Research Institutions for Seismology (IRIS) are used with permission

To triangulate an earthquake’s epicenter, first you will need to answer questions 11 through 13. Note that the latitudes and longitudes provided do not indicate north, south, east, or west. Positive latitudes are in the northern hemisphere (N) and negative longitudes are in the Western hemisphere (W).

Exercise \(\PageIndex{4}\)

11. The first seismogram (Figure 14.14) is located at Station 1. Station 1 is in the Disney Wilderness Preserve in Florida. It’s latitude is 28.11° (this is the same as 28.11° N) and it’s longitude is -81.43° (this is the same as 81.43° W). The P wave arrival is marked by the red vertical line on the left and arrived at 15:33:15 UTC. The S wave arrival is marked by the blue vertical line on the right and it arrived at 15:36:38 UTC. Find the S-P time and the kilometers distance Station 1 is from the epicenter.
    1. S-P time:
    2. Distance in kilometers:

Figure 14.14: Station 1 Seismogram. Figure by Bruce Jones Design and FreeUSandWorldMaps.com is licensed under CC0 1.0

12. The second seismogram (Figure 14.15) is located at Station 2. Station 2 is at Las Juntas de Abangares in Costa Rica. Its latitude is 10.29° and its longitude is -84.95°. The P wave arrival, marked by the red vertical line on the left, arrived at 15:31:55 UTC. The S wave arrival, marked by the blue vertical line on the right and it arrived at 15:34:11 UTC. Find the S-P time and the distance in kilometers from the epicenter to Station 2.
    1. S-P time:
    2. Distance in kilometers:

Figure 14.15: Station 2 Seismogram. Figures by Jeremy Patrich is licensed under CC BY-NC-SA 4.0

13. The third seismogram (Figure 14.16) is located at Station 3. Station 3 is at Sierra la Laguna Baja, California Sur, in Mexico. Its latitude is 23.69° and its longitude is -109.94°. The P wave arrival,marked by the red vertical line on the left and arrived at 15:32:37 UTC. The S wave arrival, marked by the blue vertical line on the right and it arrived at 15:35:27 UTC. Find the S-P time and the distance in kilometers from the epicenter to Station 3.
    1. S-P time:
    2. Distance in kilometers:

Figure 14.16: Station 3 Seismogram. Figures adapted from the Wilber 3 Data Service of the Incorporated Research Institutions for Seismology (IRIS) are used with permission

Next, you’ll use the Earthquake Triangulation tool from the Incorporated Research Institutions for Seismology (IRIS) website.

Step 1

Go to the Earthquake Triangulation website.

Step 2.

Click on “+Station”.

Step 3

At the bottom of the website, enter the Station 1 data; you will need the latitude and longitude (provided in the question 11 text) and distance from the epicenter in kilometers that you found for question 11b. Note: the website automatically creates data in the latitude, longitude, and distance kilometers fields, but you need to add your own data to those fields instead.

Step 4

On the upper-right of the window, click on “+Station”.

Step 5

At the bottom of the website, enter the Station 2 data; you will need the latitude and longitude (provided in the question 12 text) and distance from the epicenter in kilometers that you found for question 12b.

Step 6

On the upper-right of the window, click on “+Station”.

Step 7

At the bottom of the website, enter the Station 3 data; you will need the latitude and longitude (provided in the question 13 text) and distance from the epicenter in kilometers that you found for question 13b.

Step 8

Use the + sign to zoom in on the location where the radius circles around each station come close to intersecting

Exercise \(\PageIndex{5}\)

14. What are the names of the towns closest to the earthquake’s epicenter?

15. Use Your Critical Thinking Skills: Why do you think that the three lines don’t perfectly intersect? (This would be needed to accurately triangulate the earthquake’s epicenter).

The earthquake that you analyzed for questions 11 through 15 occurred on June 23, 2020, in Oaxaca, Mexico. The hypocenter was at a depth of 26.3 kilometers (16 miles). Figure 14.17 shows the tectonic setting for this earthquake. The IRIS Teachable Moment page (2020, p. 4) explains that:

As part of the circum-Pacific “Ring of Fire”, Mexico is one of the most seismologically and volcanically active regions on Earth. Most of Mexico is on the North American Plate. Offshore of southern Mexico, the oceanic Cocos Plate subducts beneath the North American Plate at the Middle America Trench. In the area of this earthquake, the Cocos Plate subducts toward the northeast at a rate of approximately 6.5 centimeters per year.

Figure 14.17: Tectonic Setting Map for the June 23, 2020 Earthquake in Oaxaca, Mexico. Figure by Incorporated Research Institutions for Seismology (IRIS) is used with permission

The June 23, 2020, earthquake was a 7.4 magnitude earthquake and more than two million people felt moderate to strong shaking. But how strong is a 7.4 magnitude earthquake? And what does moderate to strong shaking mean? The next section of the lab explains the difference between an earthquake’s magnitude and an earthquake’s intensity so you will be able to answer these questions!

Part C. Earthquake Magnitude and Intensity

Most of us have heard of the Richter scale for determining the magnitude of an earthquake. This scale uses two pieces of data from seismograms: the time difference between the first arrival of the P wave and the first arrival of the S wave (recall that this is the S-P time), and the S wave maximum amplitude. Figure 14.18 shows a sample seismogram on the top and the three scales below it. A line drawn from the S-P time plotted on the left scale to the S wave maximum amplitude on the right scale marks a magnitude of 5.0 on the middle Richter scale. The Richter scale is logarithmic so each time the magnitude increases by 1.0 there is a ten-fold increase in the measured amplitude. In terms of the energy released by an earthquake, each number increase on the Richter scale represents approximately 32 times more energy released. The Richter scale is also called M_L, where M stands for magnitude and _L stands for local. Typically, seismologists no longer use the Richter scale except for local earthquakes (within about 150 kilometers) that are smaller than 4.0 on the scale. The Richter scale was developed in the 1930s and there are now more sophisticated ways to determine an earthquake’s magnitude.

Figure 14.18: The Richter Scale. Figure by USGS is in the public domain

For earthquakes larger than 4.0, the Moment Magnitude scale (M_W) is used. Like the Richter scale, it is logarithmic. And, the Moment Magnitude scale was designed so it is roughly equal to the Richter scale (for example, a 6.5 earthquake on the Moment Magnitude scale is roughly equal in magnitude to a 6.5 earthquake on the Richter scale). The Moment Magnitude scale is calculated based on the distance the fault moved (it’s slip), the strength of the rocks along the fault (the rock’s rigidity), and the area of the fault that slipped.

To give you an idea of how much more energy is released by earthquakes of different magnitudes you will use an online calculator from the United States Geological Survey.

Step 1

Go to the “How Much Bigger…?” Calculator website.

Step 2

In the box beneath “How much bigger is a magnitude…” type 6.0.

Step 3

In the box beneath “than a magnitude…” type 5.0.

Step 4

Click on the Calculate button.

Step 5

Scroll down to read the information provided and fill in the blanks for the following question.

Exercise \(\PageIndex{6}\)

16. A magnitude 6.0 earthquake is _________ times bigger than a magnitude 5.0 earthquake, but it is ________ times stronger (energy release).

Step 6

Scroll up and click the Reset button. Follow steps 1 through 5 using the magnitudes indicated to answer the following questions.

Exercise \(\PageIndex{7}\)

17. A magnitude 7.0 earthquake is _________ times bigger than a magnitude 5.0 earthquake, but it is ________ times stronger (energy release).
18. A magnitude 8.0 earthquake is _________ times bigger than a magnitude 5.0 earthquake, but it is ________ times stronger (energy release).
19. A magnitude 9.0 earthquake is _________ times bigger than a magnitude 5.0 earthquake, but it is ________ times stronger (energy release).

Keep this website open as you will need to use it later in this lab.

Table 14.2 provides the number of earthquakes that occurred at different magnitude ranges above 5.0 M_w from 2000 to 2019. By comparison:

➢ approximately 1,000,000 earthquakes with a magnitude of 2.0-2.9 occur each year,

➢ approximately 100,000 earthquakes with magnitudes of 3.0-3.9 occur each year, and

➢ approximately 10,000 earthquakes with magnitudes of 4.0-4.9 occur each year.

The number of earthquakes with magnitudes of 7.0 and greater has remained relatively consistent since recordkeeping began. Although the average number of large earthquakes per year is fairly constant, they can occur in clusters. However, that does not imply that earthquakes that are distant in location, but close in time, are related. There has definitely been an increase in the number of earthquakes that have been detected and located due to a more than 10-fold increase in the number of seismic stations worldwide over the past century.

Table 14.2: Worldwide Earthquakes, 2000-2019. Data from USGS

Year	5-5.9 Mw Count	6-6.9 Mw Count	7-7.9 Mw Count	8.0+ Mw Count
2000	1,344	146	14	1
2001	1,224	121	15	1
2002	1,201	127	13	0
2003	1,203	140	14	1
2004	1,515	141	14	2
2005	1,693	140	10	1
2006	1,712	142	9	2
2007	2,074	178	14	4
2008	1,768	168	12	0
2009	1,896	144	16	1
2010	2,209	150	23	1
2011	2,276	185	19	1
2012	1,401	108	12	2
2013	1,453	123	17	2
2014	1,574	143	11	1
2015	1,419	127	18	1
2016	1,550	130	16	0
2017	1,455	104	6	1
2018	1,674	117	16	1
2019	1,492	135	9	1

Exercise \(\PageIndex{8}\)

20. Refer to Table 14.2.

1. In what year did the highest number of 8.0+ magnitude earthquakes occur?

1. Is this the same year that the highest number of 5-5.9, 6-6.9, and 7-7.9 earthquakes occurred?

2. On average, how many 8.0+ magnitude earthquakes occurred between 2000 and 2019? Tip: Add the number of earthquakes in the 8.0+ M_w column and divide that number by twenty. Show your work.

3. On average, how many 7-7.9 magnitude earthquakes occurred between 2000 and 2019? Tip: Add the number of earthquakes in the 7-7.9 M_w column and divide that number by twenty. Show your work.

4. Use Your Critical Thinking Skills: Suppose a friend asks you, “Why are we having so many more earthquakes this year?” Explain your response in one to two sentences.

While magnitude scales measure the strength of the earthquake—the amount of energy it releases—intensity scales measure the impact of the earthquake in a specific area. Seismologists in the United States typically use the Modified Mercalli Intensity scale to measure an earthquake’s intensity. On this scale, the impact of the earthquake is represented by Roman numerals as shown in Table 14.3. An earthquake’s intensity is based on two main factors:

➢ The local geology: Bedrock shakes the least while soft mud shakes the most.

➢ The hypocenter depth: Deeper hypocenters cause less shaking than hypocenters closer to the surface.

The strength of shaking experienced and the length of shaking also influence an earthquake’s intensity.

Table 14.3: Modified Mercalli Intensity Scale. Adapted from USGS

Intensity	Shaking	Description/Damage
I	Not felt	Not felt except by very few under especially favorable conditions.
II	Weak	Felt only by a few people at rest, especially on upper floors of buildings.
II	Weak	Felt quite noticeably by people indoors, especially on upper floors of buildings. Many people do not recognize it as an earthquake. Standing motor cars may rock slightly. Vibrations similar to the passing of a truck. Duration estimated.
IV	Light	Felt indoors by many, outdoors by few during the day. At night, some awakened. Dishes, windows, doors disturbed; walls make cracking sound. Sensation like heavy truck striking building. Standing motor cars rocked noticeably.
V	Moderate	Felt by nearly everyone; many awakened. Some dishes, windows broken. Unstable objects overturned. Pendulum clocks may stop.
VI	Strong	Felt by all, many frightened. Some heavy furniture moved; a few instances of fallen plaster. Damage slight.
VII	Very Strong	Damage negligible in buildings of good design and construction; slight to moderate in well-built ordinary structures; damage considerable in poorly built or badly designed structures; some chimneys broken.
VIII	Severe	Damage slight in specially-designed structures; considerable damage in ordinary substantial buildings with partial collapse. Damage great in poorly built structures. Fall of chimneys, factory stacks, columns, monuments, walls. Heavy furniture overturned.
IX	Violent	Damage considerable in specially-designed structures; well-designed frame structures thrown out of plumb. Damage great in substantial buildings, with partial collapse. Buildings shifted off foundations. Liquefaction.
X	Extreme	Some well-built wooden structures destroyed; most masonry and frame structures destroyed with foundations. Rails bent.

Exercise \(\PageIndex{9}\)

21. Why would seismologists prefer to use the Moment Magnitude scale instead of the Richter scale? Hint: compare the variables used for the two scales.

22. What is the difference between earthquake magnitude and intensity?

23. About 26,000 people felt very strong shaking during the June 23, 2020, earthquake in Oaxaca. What type of damage occurs with this level of earthquake intensity? Hint: refer to Table 14.3.

24. What is the highest earthquake intensity that you have experienced? Indicate the Roman numeral of the Modified Mercalli Intensity scale and in two to three sentences describe what you experienced. If you have not experienced an earthquake, ask a classmate or friend about their experience and respond with that information.

Figure 14.19 shows the Modified Mercalli Intensity scale mapped for two different earthquakes. The map on the left shows the intensity of the 1994 Northridge 6.7 M_w earthquake (hypocenter at a 18.20 kilometer depth) and the map on the right shows the intensity of the 2001 Nisqually 6.8 M_w earthquake (hypocenter at a 57 kilometer depth).

25. Use the earthquake magnitude comparison calculator (which you used in questions 16 through 19 above) to determine how much more energy is released in a 6.8 magnitude earthquake compared to a 6.7 magnitude earthquake.

26. Which earthquake had the greatest intensity? Explain your response in at least one sentence.

27. Why does the lower magnitude earthquake have a higher intensity? Explain your response in two to three sentences.

Figure 14.19: The Modified Mercalli Intensity Scale Mapped near the Epicenters of Two Different Earthquakes. Figure by USGS is in the public domain

Liquefaction

Liquefaction occurs when sediments at or near the surface behave like a liquid due to the shaking caused by an earthquake. These loosely compacted and water-saturated sand and silt sediments move and flow during strong shaking. Liquefaction occurs in a variety of natural locations such as marshlands, alluvial (river-deposited sediments) floodplains, beaches, and areas with high water tables. Places with artificial fill are also susceptible to liquefaction. Artificial fill are sediments used for construction purposes to provide support for human-built structures and to build up areas that were once submerged.

Check It Out! Liquefaction

IRIS Earthquake Science has a neat video showing seismic waves and liquefaction called “Amplification and Liquefaction Animation”. (Video length is 1:24). You can see the damage caused by liquefaction from a September 28, 2018 earthquake in Indonesia on a BBC News video called “Indonesia quake turns ground into liquid” (caution: the video shows upsetting scenes of destruction).

Figure 14.20 is a map of the liquefaction hazard probability estimates for northwestern Alameda County in the Bay Area. The map includes the communities of Alameda, Berkeley, Emergyville, Oakland, and Piedmont. Based on seismic models, the map predicts the percentage of area that would liquefy if a 7.1 magnitude earthquake occurred on the Hayward fault.

Figure 14.20: Liquefaction Hazard Map for Northwestern Alameda County. Figure by USGS is in the public domain

Figure 14.21 is a map excerpt that shows the types of sediments found in northwestern Alameda County. It provides information for part of the area mapped in Figure 14.20. Each sediment type is labeled on the sediment map as follows:

➢ Qhf, Qpf, Qf, and Qof are alluvial fan sediments,

➢ afem is artificial fill, and

➢ Qds are beach and dune sands.

Figure 14.21: Map of Sediments found in Northwestern Alameda County. Figure adapted from the USGS is in the public domain

Exercise \(\PageIndex{10}\)

28. Compare Figures 14.20 and 14.21. Which type of sediment has the largest area that is predicted to liquefy if a 7.1 magnitude earthquake on the Hayward fault occurs?

29. Search google maps for Alameda, California, (or search for the coordinates of 37.7906, -122.2801). Click on satellite view and zoom in on the area of the online map that matches the maps of Figure 14.20 and Figure 14.21. List five types of infrastructure and buildings that are found in areas that will likely liquefy during a 7.1 magnitude earthquake on the Hayward fault. Tip: use google’s streetview to better understand what infrastructure and buildings are present.

30. If a 6.1 magnitude earthquake occurred in northwestern Alameda County, do you think the same areas would liquefy as shown in Figure 14.20? Why or why not? Explain your response in two to three sentences.

Let’s find out if your college is located in an area that has the potential to liquefy. We’ll use the Regulatory Maps Geo Application created by the California Department of Conservation. This geographic information system (GIS) identifies areas that are prone to three earthquake hazards: liquefaction; earthquake-induced landslides; and amplified ground shaking. Factors that cause an area to be mapped as prone to liquefaction include having groundwater within 40 feet of the Earth’s surface, the presence of sands and silts deposited in the last 15,000 years, and a prediction that strong levels of shaking caused by earthquakes will occur during the next 50 years.

In California, there are environmental and earthquake hazard disclosure requirements for sellers of residential real estate. Additionally, for new construction or significant remodels, site-specific geologic studies are required so builders can avoid known hazards or incorporate mitigation features. Mitigation means to lessen the impact of something. So, mitigation features for a building might be to install reinforced concrete that could withstand higher levels of seismic energy without failing.

It is important to note that the map may not show all faults that have the potential for surface fault rupture, either within the Earthquake Fault Zones or outside their boundaries. Additionally, the map may not show all areas that have the potential for liquefaction, landsliding, strong earthquake ground shaking or other earthquake and geologic hazards. Also, a single earthquake capable of causing liquefaction or triggering landslide failure will not uniformly affect the entire area zoned.^[256] Therefore, the information presented in the GIS is limited.

Check It Out! Seismic Hazards Program Information

Read more about the laws that require the Regulatory Maps Geo Application and how the GIS was created at the California Department of Conservation Seismic Hazards Program website.

Step 1

Go to the Earthquake Zones of Required Investigation website

Step 2

Read the information about the Regulatory Maps Geo Application.

Step 3

Click the checkbox to agree to the terms and conditions.

Step 4

Click OK to access the map.

Step 5

In the upper-right portion of the map, click on the legend icon. (This icon has three shapes next to three horizontal lines).

Exercise \(\PageIndex{11}\)

31. What color represents a fault zone?

32. What color represents a liquefaction zone?

33. What color represents a landslide zone?

Step 6

Click the “x” on the upper-right to close the map legend.

Step 7

In the search box that says Esri World Geocoder, type in the address of your college.

Step 8

Click the magnifying glass.

Step 9

Click on the map where your college is located.

Step 10

Click on the arrow to open the pop-up window.

Exercise \(\PageIndex{12}\)

34. Is your college located in a liquefaction zone?

35. Has your college area been evaluated for a liquefaction hazard?

36. Use Your Critical Thinking Skills: Some colleges are located in areas that have not been evaluated for these earthquake hazards. Does this mean that the colleges are not at risk of earthquake hazards? Why or why not? Your response should be one to two sentences in length.

Step 11

Click the “x” on the upper-left corner to close the pop-up window.

Step 12

Zoom out of the map until you find a large area that is mapped in a liquefaction zone.

Exercise \(\PageIndex{13}\)

37. Where is the largest liquefaction zone closest to your college?

Part D. Earthquake Probability and Risk

Before we take a look at earthquake risks, we need to make sure everyone understands probability and the difference between a hazard and a risk.

➢ If you flip a coin (let's say a quarter), there is a 1 in 2 chance of "heads".

• 1 divided by 2 = 50%
• So a coin flip has a 50% probability.

➢ If you roll a six-sided die (one dice), there is a 1 in 6 chance of you rolling a 1.

• 1 divided by 6 = ~17%
• So rolling a 1 on one die has about a 17% probability.

A hazard has the potential to cause harm while risk is the probability of a hazard event causing harm. So, earthquakes are an example of a hazard and the risk of an earthquake is the probability that a particular earthquake will cause loss of life, injuries, or property damage.

In 2015, the United States Geological Survey in cooperation with the Southern California Earthquake Center, the California Geological Survey, and the California Earthquake Authority released a report on the third Uniform California Earthquake Rupture Forecast (a computer model known as UCERF3). The goal for this model is to determine where and when faults might slip. Keep in mind that the UCERF3 model provides a forecast, not a prediction. Seismologists are unable to predict earthquakes—they are unable to determine the time, date, location, and magnitude of future earthquakes. Seismologists can only calculate the probability that an earthquake will occur in an area within a certain number of years. The USGS explains the four major components of the UCERF3 model as follows:

➢ geodesy (precise data on the slow relative movement of the Earth’s tectonic plates),

➢ geology (mapped locations of faults and documented offsets on them),

➢ seismology (occurrence patterns of past earthquakes), and

➢ paleoseismology (data from trenches across faults documenting the dates and offsets of past earthquakes on them).^[257]

UCERF3 mathematically models each of these components but it’s important to remember that the datasets are not 100% complete. For example, not all faults in the state are mapped or have documented offsets (slips) from previous earthquakes.

Figure 14.22 provides a map that was created by the UCERF3 model. The map shows the likelihood that a 6.7 magnitude earthquake will occur in the next thirty years (starting from 2014). The 6.7 magnitude was selected because this is the same magnitude as the 1994 Northridge earthquake and a thirty-year timeframe is used because that is the typical time for a residential mortgage.

Figure 14.22: Map of Likelihood of Experiencing a 6.7 or Larger Earthquake in the Next 30 Years. Faults are shown as rectangles outlined in black. For the purposes of the source report, the white line in the middle of the map represents the boundary between Northern and Southern California. The San Francisco and Los Angeles Regions are shown as rectangles outlined in white. Figure by WGCEP 2014 team (USGS, CGS, and SCEC) is in the public domain

Exercise \(\PageIndex{14}\)

38. Refer to Figure 14.22. Where is the highest probability (likelihood) that a 6.7 or larger earthquake will occur in the next thirty years? Hint: find the location with the pink/purple shading.

Table 14.4 provides the probabilities of earthquakes of particular magnitudes occurring in the next thirty years (starting from 2014). Refer to the caption of Figure 14.22 to see the boundaries of the locations listed in Tables 14.4a through 14.4b.

Table 14.4a: Earthquake Probabilities in the Northern California Region. Data from USGS

Magnitude	30-Year Likelihood of One or More Events
≥ 6.0	100%
≥ 6.7	95%
≥ 7.0	76%
≥ 7.5	28%
≥ 8.0	5%

Table 14.4b: Earthquake Probabilities in the Southern California Region. Data from USGS

Magnitude	30-Year Likelihood of One or More Events
≥ 6.0	100%
≥ 6.7	93%
≥ 7.0	75%
≥ 7.5	36%
≥ 8.0	7%

Table 14.4c: Earthquake Probabilities in the San Francisco California Region. Data from USGS

Magnitude	30-Year Likelihood of One or More Events
≥ 6.0	96%
≥ 6.7	60%
≥ 7.0	46%
≥ 7.5	31%
≥ 8.0	7%

Table 14.4d: Earthquake Probabilities in the Los Angeles Region. Data from USGS

Magnitude	30-Year Likelihood of One or More Events
≥ 6.0	96%
≥ 6.7	60%
≥ 7.0	46%
≥ 7.5	31%
≥ 8.0	7%

Exercise \(\PageIndex{15}\)

Refer to Tables 14.4a through 14.4d to answer the following six questions.

39. Which regions have the highest likelihood of a 6.0 magnitude or greater earthquake?

40. Which region has the highest likelihood of a 6.7 magnitude or greater earthquake?

41. Which region has the highest likelihood of a 7.0 magnitude or greater earthquake?

42. Which region has the highest likelihood of a 7.5 magnitude or greater earthquake?

43. Which region has the highest likelihood of a 8.0 magnitude or greater earthquake?

44. What are three conclusions that you could make based on the data from Table 14.4?

As mentioned above, slip rate data is utilized in UCERF3. Figure 14.23 shows the average slip rates in millimeters per year on major faults. In addition to forecasting probabilities in Northern California, Southern California, the San Francisco Region, and the Los Angeles Region, UCERF3 also assigned individual 30-year probabilities for a 6.7 magnitude or larger earthquake occurring on major faults as follows:

➢ Southern San Andreas: 59%

➢ Hayward-Rodgers Creek: 31%

➢ San Jacinto: 31%

➢ Northern San Andreas: 21%

➢ Elsinore: 11%

➢ Calaveras: 7%

➢ Garlock: 6%

Figure 14.23: Map of Average Slip Rates on Major Faults. Figure by WGCEP 2014 team (USGS, CGS, and SCEC) is in the public domain

The California Earthquake Authority (CEA) website provides information on earthquake risks by county. In the wake of the loss of life, injuries, and catastrophic damage caused by the 1994 Northridge earthquake, the California legislature established the California Earthquake Authority. The CEA is a publicly managed and privately financed organization that provides residential earthquake insurance.

Exercise \(\PageIndex{16}\)

47. Go to the California Earthquake Risk Map & Faults by County website. and select your county from the drop-down menu next to the text that says “What are the earthquake risks near”. What are three facts that you learned about your county and its major faults from this website? Be sure to indicate what types of faults are found in your county (e.g., normal, reverse, and/or strike-slip).

48. Risk perception is the subjective way that people interpret the severity of a potential harm or loss. Risk perception is a complex process based on numerous factors including, but not limited to, perceived self-efficacy (whether or not an individual feels they are able to control or influence a situation), previous experience, knowledge of the underlying causes of the risk, and peer group perspectives. Based on the probability statistics presented in Table 14.4, how do you perceive the risk of a future earthquake in your area? Write three to four sentences that incorporate the information that you have learned in this lab.

49. How prepared are you and your loved ones for an earthquake? Read the CEA’s Seven Steps to Earthquake Safety webpage, or search the internet for an earthquake preparedness guide. In two to three sentences, discuss how prepared you feel for an earthquake.

50. Which of the earthquake preparedness tips could you do right now without having to spend money? List at least three steps that you could take to become more prepared for an earthquake without spending money.

Earlier in the lab, you learned that the local geology, hypocenter depth, strength of shaking, and length of shaking influence an earthquake’s intensity. The damage from an earthquake is influenced by other variables such as building age and type. Although local building codes to mitigate (lessen the impact of) the damage caused by earthquakes have been enacted since 1925, it wasn’t until the early 1970s that the State of California passed widespread legislation requiring compliance with building and infrastructure standards and in 1977 the federal government adopted the Earthquake Hazards Reduction Act. Therefore, older buildings may not have been constructed with seismic safety in mind. In terms of building type, the building design and the materials used for construction play a major role in the type of damage that will be incurred from ground shaking due to an earthquake. Table 14.5 provides a brief overview of building types and seismic risks.

Table 14.5: Buildings and Seismic Risks

Building Type	Seismic Risk
Concrete	Without steel-reinforced columns, concrete buildings crack and collapse. Also, cheaper concrete (that uses more water) is weaker than concrete made with more cement
Brick	When unreinforced, likely to crack and collapse
Houses with concrete foundations	Without bracing and anchoring, houses may slide off their foundations
Living spaces above garages (or apartment buildings with parking spaces on the first floor)	Without retrofitting, garage and parking space doors and walls are not braced for ground shaking
Post and pier house (these are built without foundations an have crawl spaces underneath)	Without bracing and new footings, houses are at risk of shifting and collapsing
Buildings on slopes	Without retrofitting, these buildings may not withstand shaking

So far, you’ve learned about multiple factors that influence the damage done by an earthquake. These variables can be further understood by thinking in terms of a community’s vulnerability to earthquake hazards. Vulnerability refers to the geographic, demographic, and economic conditions that increases a community’s susceptibility to the impact of a hazard event. We can also think about how vulnerability influences a community’s ability to prepare for a hazard event.

In 1998, researchers Robert Bolin and Lois Stanford published The Northridge Earthquake: Vulnerability and Disaster.^[261] Several quotes from their book shed light on the relationship between vulnerability and the threat of hazards and their impacts:

➢ While earthquakes and other natural hazards may cause grave losses for a diversity of people, some of those people are much more likely to avoid or be able to cope with loss and damage. For those who cannot, the accumulated deficits of poverty, discrimination, and other structural constraints can only serve to amplify the effects of the hazard on their lives, deepening the daily struggles they face (Cannon 1994; Maskrey 1993). (p. 5)

➢ The most vulnerable are typically those with the fewest choices, those whose lives are constrained, for example, by discrimination, political powerlessness, physical disability, lack of education and employment, illness, the absence of legal rights, and other historically grounded practices of domination and marginalization (Blaikie et al. 1994). (p. 9-10)

➢ The estimated risks posed by such hazards are by no means equally distributed across the state or in Southern California specifically. They vary according to spatial factors, geological conditions, and land use patterns that locate some people and settlements nearer hazardous areas than others. The asymmetries of risk are reinforced by social inequalities that pertain in employment, income, wealth, legal protections, and the availability of secure housing. As we have suggested, it is precisely access to these social resources that can influence people’s vulnerability to disaster (Blaikie et al. 1994; Dreze and Sen 1990). Employment, income, legal status, and ethnicity all bear directly on where people live, the kinds of structures they live in, and whether they have the resources to cope with untoward events in their lives. (p. 15)

➢ California’s earthquake risk was produced by Anglo-European settlement and subsequent large-scale urbanization. Rapid population growth in its two primary urban centers, San Francisco and Los Angeles, has situated more than half the state’s thirty million residents in areas of high seismic activity (Bolt 1993). (p. 64-65)

Exercise \(\PageIndex{17}\)

51. List the vulnerability-related demographic and economic factors that are mentioned in the quotes from Bolin and Stanford (1998).

52. Table 14.6 below provides selected data for four census tracts located within 25 miles of the San Andreas Fault Complex. What are three conclusions that you can make regarding this data set? For example, one conclusion is that Census Tract 2 has the highest percentage of children under 10 years of age.

Table 14.6: Community Data. Data from California Office of Environmental Health Hazard Assessment

Variable	Census Tract 1	Census Tract 2	Census Tract 3	Census Tract 4
Population	3,035	5,339	6,989	4,143
% of population children under 10 years old	22%	25%	18%	9%
% of population over 65 years old	6%	7%	8%	10%
% of adults that have less than a high school education	48%	36%	31%	9%
% of population living below twice the federal poverty level	80%	86%	54%	19%
% of population making less than 80% of their counties' median family income and paying greater than 50% of their income towards housing costs	34%	43%	9%	18%
% of households with limited English proficiency	32%	13%	16%	4%

53. Based on the data provided in Table 14.6, do you think each census tract would feel the same impacts from a severe earthquake? Explain your response in three to four sentences.

54. Let’s suppose that you are working on state plans to improve the earthquake preparedness of vulnerable communities in California. What is one recommendation that you would make for each census tract shown in Table 14.6?
    1. Census Tract 1:
    2. Census Tract 2:
    3. Census Tract 3:
    4. Census Tract 4:

Part E. Wrap-Up

Consult with your geography lab instructor to find out which of the following wrap-up questions you should complete. Attach additional pages to answer the questions as needed.

Exercise \(\PageIndex{18}\)

55. What is the most important idea that you learned in this lab? In two to three sentences, explain the concept and why it is important to know.

56. What was the most challenging part of this lab? In two to three sentences, explain why it was challenging. If nothing challenged you in the lab, write about what you think challenged your classmates.

57. What is one question that you have about what you learned in this lab? Write your question along with one to two sentences explaining why you think your question is important to ask.

58. Review the learning objectives on page 1 of this lab. How would you rate your understanding or ability for each learning objective? Write one sentence that addresses each learning objective.

59. Sketch a concept map that includes the key ideas from this lab. Include at least five of the terms shown in bold-faced type.

60. Create an advertisement to educate your peers on the important information that you learned in this lab. Include at least three key terms in your advertisement. The advertisement should be about half a page in size (about 4 inches by 6 inches).

61. One way to think of physical geography is that it is the study of the relationships among variables that impact the Earth's surface. Select two variables discussed in this lab and describe how they are related.

62. How does what you learned in this lab relate to your everyday life? In two to three sentences, explain a concept that you learned in this lab and how it relates to your day-to-day actions.

63. How does what you learned in this lab relate to current events?
    1. Write the title, source, and date of a news item that relates to this lab.
    2. In two to three sentences, discuss how the news item relates to what you have learned in this lab.
    3. In one to two sentences, discuss whether or not the news item accurately represents the science that you learned. Tip: consider whether or not the news item includes the complexity of the topic.

64. Search O*NET OnLine to find an occupation that is relevant to the topics presented in today's lab. Your lab instructor may provide you with possible keywords to type in the Occupation Quick Search field on the O*NET website.
    1. What is the name of occupation that you found?
    2. Write two to three sentences that summarize the most important information that you learned about this occupation.
    3. What is one question that you would want to ask a person with this occupation?

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^[256]Text by the California Department of Conservation is in the public domain

^[257] Text by USGS is in the public domain

^[261] Bolin, R., & Stanford, L. (1998). The Northridge Earthquake: Vulnerability and Disaster. London: Routledge.