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Risk assessment for Benton County, Oregon, Part II: Risk of a catastrophic earthquake

Benton County, Oregon, lies close to the Cascadia Subduction Zone. What is the risk of a catastrophic earthquake for this area?

Thomas JusterDepartment of Geology, University of South Florida© 2010 University of South Florida Libraries. All rights reserved,

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Core Quantitative Literacy TopicsCalculating risk

Supporting Quantitative Literacy TopicsProbability

Core Geoscience SubjectEarthquakes, Risk Equation

Pacific Northwest Seismic Network image

The module you are viewing is a PowerPoint slide presentation.

•Navigate from slide to slide using the up/down arrow keys, or the scroll wheel on the mouse if one is available

•Use the mouse to select hyperlinks (underlined, in blue type)

•When done, use the escape key to exit the presentation.

You can and probably should have a spreadsheet open in a separate window, so you can try out things that are explained in the presentation.

PowerPoint applications use lots of memory, so you may want to exit other programs while running this presentation, especially if it starts to act slowly or sluggishly. If you don’t immediately see the slideshow when switching back and forth between windows, use the up/down arrow keys (or scroll wheel on mouse) to ‘wake it up’.

Close this window to proceed with the slide show.

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Getting started

After completing this module you should be able to:

•Estimate probability using the frequency approach•Switch between the annual probability and recurrence interval•Use Excel to perform a simple risk assessment of Benton County, Oregon

Benton County

You should also remember where Oregon is!

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Re-statement of the problem

Benton County lies near the Cascadia Subduction Zone, which produces frequent earthquakes, and is capable of producing giant earthquakes. Benton County officials make decisions that are affected by this risk—for example, they create zoning laws which restrict activities in certain locations.

Your task: calculate the annual risk produced by a giant earthquake (magnitude M ≈ 9.0) for Benton County in terms of both (a) fatalities; and (b) monetary losses, in dollars.

This is the second of two modules that tackle this task. You must have completed the first one before you can start this one.

Click on the icon to open the embedded spreadsheet link, and immediately save with a unique name. This is where you will enter your answers.

All photos from the Oregon State archives website

Hazard-Risk-II

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Your assessment tool: the Risk Equation

The tool you will use to compute the risk of a major earthquake along the Cascadia Subduction Zone is the Risk Equation, which states:

where

Risk is a quantitative measure of the expected affects of the hazard, on an average basis;

P(hazard) (sometimes just called Hazard) is the probability that the hazard will occur, in any given time period;

Exposure is the number of people or structures that would be affected if the hazard were to occur;

Vulnerability is the probability that a person will be injured or killed, or that a structure will be damaged, if it were to be exposed to the hazard.

Risk can be measured in terms of fatalities (fatality risk) or money (for monetary losses). In this activity you will do both.

ityVulnerabilExposurehazardPRisk )(

In detail, big earthquakes can be produced in several different ways in this area. We will focus on the ‘Big One’ that would occur if there were major slip along the subduction zone, as occurred last in the year 1700.

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Step 1: What is the probability of a catastrophic earthquake?

‘Catastrophic’ is a term used to describe earthquakes with a magnitude around 9.0. As you will learn in the section on earthquakes, the recurrence interval for these powerful earthquakes worldwide is about 10 years. It’s a good thing they are so rare, because when they do occur they can wreak terrible destruction over a large area, and often spawn tsunami waves that can threaten coastal areas as far as 10,000 miles away!

We can estimate the recurrence interval for catastrophic earthquakes using the frequency approach to probability. Nelson and others (Endnote 1) studied far-flung tsunami deposits to infer the timing of mega-quakes on the Cascadia Subduction Zone and identified 9 events:

1700AD

1110AD

660AD

490BC

890BC

1390 BC

1790BC

2390BC

2890BC

Task #1: Enter these data into your spreadsheet and, in a separate column, calculate the number of years before 2010 for each event. Remember, “AD” means “after the death of Christ” and “BC” means “Before Christ”.

Your spreadsheet should look like this. Notice I’ve color-coded the cells to indicate the familiar content (yellow for numbers, orange for formulas), though your spreadsheet doesn’t have to do this.

Also notice the formula for cell C2. You’ll need a different formula for the BC dates (why?)

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Step 1: Probability of a catastrophic earthquake, con’t.

Recall that the frequency approach to probability states that the probability is estimated as:

For mega-quakes, years is the time period of observation (there either was or wasn’t a mega-quake during any given year).

Q1. Using the frequency approach to probability, what is the probability of a catastrophic earthquake on the Cascadia Subduction Zone in any given year? Use your spreadsheet as a calculator. (HINT: your answer should lie between 0.001 and 0.002.)

nobservatioofperiodstimeofNumber

soccurrenceofNumberP

Q2. Using your spreadsheet to do calculations, convert the probability you calculated in Q1 to a recurrence interval. What are the advantages of expressing the likelihood in terms of recurrence interval instead of probability? Go direct to End-of-module questions.

Often it’s easier to describe the likelihood that an event occurs with a recurrence interval (symbol RI), which is calculated as:

Remember, the units of the recurrence interval are the same as the windows of time in which probability is defined; i.e., for a probability “in any given year” the recurrence interval will be in years.

PRI

1

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Step 2: What is the Exposure?

The exposure is a number that represents the total number of people or value of property exposed to a hazard. Since you will be calculating both the mortality risk (people) and the monetary risk (property), you will need to know both.

Q3. Use the Internet to find the current population of Benton County, Oregon, as estimated by the US Census Bureau. Save the value in your spreadsheet. This is the mortality exposure.

The only data on property value for Benton County are from 1994 (Endnote 2), when the total property in the county was valued at 3.9 billion dollars ($3,900,000,000). Obviously the value is much greater today, but by how much? How do we estimate the value today?

One strategy is to multiply the value in 1994 by two factors:

a. A factor to account for inflation, since $1.00 in 1994 is worth about $1.45 in 2010. The inflation factor is thus 1.45.

b. A factor to account for growth, since Benton County’s population (and presumably the value of its property) has expanded since 1994. Using census statistics, we can estimate that Benton’s population has increased by about 11% since 1994. The growth factor is thus 1.11.

Q4. Estimate the total value of all property in Benton County in 2010 by multiplying the value in 1994 by both factors identified above. Do the calculation in your spreadsheet! This is the property value exposure. Go direct to End-of-module questions.

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Step 2: Exposure, con’t.

There is also monetary exposure due to infrastructure—bridges, roads, etc.—which are owned by the government and would have to be replaced or repaired if they were damaged. In 1994, the value of this infrastructure was $2,247,000,000.

Q5. Adjust the infrastructure value to 2010 values the same way you adjusted the property values. Use your spreadsheet for the calculation!

Q6. Now tally the total value of property and infrastructure in Benton County in 2010. Again, use your spreadsheet for the calculation.

Go direct to End-of-module questions.

Photo the Oregon State archives website Photo the www.oregon.gov

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Step 3: What is the vulnerability?

Actual risk assessment calculations compute vulnerability with equations that take into account the type of structure (what is the structure made of?), substrate (is the structure sitting on hard rock or soft mud?), and location (is the structure sitting at the base of a hill that could produce a landslide? How far is the structure from the likely epicenter?).

For this activity you’ll use a much simplified vulnerability factor that takes into account all of these things and lumps them into a single factor.

For mortality, the vulnerability factor represents the fraction of the population that would die if the hazard were to occur.

For monetary losses, the vulnerability factor represents the fraction of the total value that would be lost due to damage from the hazard.

The mortality hazard depends on the time of day when the earthquake strikes:

Time 2 a.m. 2 p.m. 5 p.m.

Mortality Factor 0.00001 0.00007 0.00003

The monetary factor is approximately 0.18. This is just the cost to rebuild or repair structures; it does not include the loss of business that would inevitably occur after a large earthquake.

Q7. Why do you think the mortality depends on the time of day?

Go direct to End-of-module questions.

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Step 3: Putting it all together: calculating the mortality risk

At this point you have all the information you need to calculate the risk for Benton County, Oregon, posed by a catastrophic earthquake on the Cascadia Subduction Zone. You will calculate risk using the Risk Equation (Slide 4). We’ll start with the mortality risk.

Task #2: Create a table in your spreadsheet that calculates the mortality risk at three different times of day. Your table should look like the this one:

I’ve used the spreadsheet color scheme to remind you that the yellow cells should contain numbers and the orange cells formulas. Your table doesn’t have to use these colors.

The probability you estimated on Slide 6 goes in column Q.

The exposure you looked up on Slide 7 goes in column R.

The three different vulnerability factors for mortality go here.

Risk is calculated with the Risk Equation

Q8. What are the units of risk for this calculation?

Go direct to End-of-module questions

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Step 3: Putting it all together: calculating the monetary risk

The monetary risk is calculated the same way, except the values for exposure and vulnerability are different.

Task #3: Add another row to the table you just created to calculate the monetary risk of a catastrophic earthquake on the Cascadia Subduction Zone.

The probability you estimated on Slide 6 goes in column Q.

The monetary exposure you estimated on Slide 7 goes here.

The monetary vulnerability factor goes here

Mortality risk

Monetary risk, calculated with the Risk Equation

Q9. What are the units of risk for this calculation? Format your answer so it has the proper units.

Q10. Why doesn’t the monetary risk depend on the time of day?

Go direct to End-of-module questions

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Step 3: Putting the risk in context

As you’ve seen, it’s relatively easy to calculate the risk in terms of both fatalities and monetary losses due to a catastrophic earthquake. As with any calculation, when you’re done your first question should be: “what does it mean?”

One way of understanding the significance of these numbers is to compare them to other numbers that you understand.

For example, the table below shows the risk of mortality from other causes of death in Benton County, Oregon:

For comparing the monetary risk, the city budget of Corvallis—the largest city in Benton County, with 60% of its population—was $7,553,710 in 2010.

Cause Risk (deaths/year)

Traffic accident 18.2

Accidental injury 24.0

Tobacco-related 140

Q11. The risk of death from these hazards is not equal for everyone in Benton County. For example, who is most at risk for dying from tobacco-related deaths?

Go direct to End-of-module questions.

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End-of-module assignment

1. Answer questions Q1-Q11 on slides 6, 7, 8, 9, 10, 11, and 12.

2. Create the risk spreadsheet shown on Slides 11 and 12.

3. Calculate the intervals between catastrophic earthquakes in the Cascadia Subdction Zone from the data on Slide 5. Then use Excel’s MIN(), AVERAGE(), and MAX() functions to determine the smallest, average, and largest interval on this list.

4. Does the average value look similar to anything else you’ve calculated? What is it nearly equal to?

5. For the sake of comparison, these are the recurrence intervals for great earthquakes along some other important fault systems:

Create a table that calculates the probability in any given year for great earthquakes on each of these faults, then enter the results onto your answer sheet.

6. Why do you think the mortality risk you calculated on Slide 10 is so low?

7. Comment on the size of the risk for both mortality and monetary value based on the comparison values on Slide 12. Is the mortality large compared to the risk of other hazards? Is the monetary risk large compared to the government budget of Benton County?

RI (years)Hayward Fault, near San Francsico 6.9 150San Andreas Fault, northern segment 8.0 300Wasatch fault, near Salt Lake City 7.5 350

Largest Quake

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End-of-module assignment, con’t.

7. Which of the two risks from a catastrophic earthquake—mortality or monetary value—do you think is a greater concern for Benton County? Explain.

8. The second tab of the embedded spreadsheet (labeled “Hazard by zone”) contains a large table summarizing the local hazard potential, building construction, and predicted damages from a catastrophic earthquake according to zone. The zones are census tracts from the 2000 census, and are identified on the map below the table.

The Local Hazard Potential columns describes how severe the local hazard would be in the event of a catastrophic earthquake. Remember, the local hazard depends on the type of substrate, slope, etc. ‘Liquefaction’ refers to the process by which shaking turns moist sediment into quicksand.

The Construction Methods columns describe the percentage of structures on each tract that are made of wood, steel, etc. ‘Mason.’ refers to masonry (e.g., brick), which may be either reinforced with steel (‘Reinf.’) or unreinforced (‘Unreinf.’)

The Damage from a M=9.0 Earthquake columns describe just that: the degree of damage that the structures on each tract would suffer in the event of a catastrophic earthquake. The column labeled Heavy+Total(%) shows the percentage of structures in each tract that would suffer either ‘heavy’ or ‘total’ damage. The tracts with the greatest percentage of heavy or total damage are indicated with shading—pink, orange, and yellow in order of decreasing susceptibility. For example, 27.1% of Tract 41003000600 would be extensively damaged by a catastrophic Cascadia Subduction Zone earthquake.

Your task: study the table and figure, and try to understand why certain areas have higher property risk than other areas. Is the local geology the most important factor determining risk? Or is it the construction methods? Then write a few sentences explaining and justifying your conclusion.

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1. Great earthquakes of variable magnitude at the Cascadia subduction zone, by Alan R. Nelson, Harvey M. Kelsey, and Robet C. Witter, published in the journal Quaternary Research, volume 65, issue 3, pp. 354-365, 2006. Go back to Slide 5.

2. Most of the data for this risk calculation come from Earthquake Hazard and Risk Assessment and Water-induced Landslide Hazard in Benton County, Oregon, by Wang, Graham, and Madin, 2001. Go back to Slide 7.

Endnotes