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A hazard becomes a disaster through exposure and vulnerability, reduced by capacity; rates per million and return periods measure the pieces.
Paper packet. Every task here also exists on screen, where it is checked automatically; answers written on paper are not assessed by Nydus. When you are back at a device, enter your answers there.
By the end of this lesson you will be able to separate hazard, exposure, vulnerability and capacity in a stated event, and compare disasters with rates.
You can compare places with rates per thousand or per million, and you know that the same average can hide very different places. This lesson applies those ideas to earthquakes, floods and storms, and asks why the same natural event can be a small emergency in one place and a catastrophe in another.
| Term | What it means |
|---|---|
| Hazard | A natural event, such as an earthquake, flood or storm, that could cause harm. |
| Exposure | The people, buildings and property in the path of a hazard. |
| Vulnerability | How easily people and property are harmed: weak buildings, poverty, no warning. |
| Capacity | A community's ability to prepare for, respond to and recover from a hazard. |
| Disaster | Serious harm to people and property beyond what a community can cope with alone. |
| Magnitude | A measure of an earthquake's size; each whole step is ten times the ground-shaking amplitude. |
| Return period | The average time between events of a given size, such as the one-in-a-hundred-year flood. |
Disaster risk grows with three things and shrinks with one:
$$\text{risk} \approx \dfrac{\text{hazard} \times \text{exposure} \times \text{vulnerability}}{\text{capacity}}.$$
The formula is a way of thinking, not an equation to compute: it says a disaster needs all of hazard, exposure and vulnerability, and that capacity can shrink it.
Another way: picture
Picture the same storm passing over two coastal towns. In one, homes stand on stilts behind a seawall, sirens sound hours before landfall and buses carry people inland. In the other, homes sit at sea level, there is no warning and few people own cars. The storm is identical; the disasters are not.
Another way: steps
An earthquake in an empty desert harms no one. It is a hazard, a natural event that could cause harm, but not a disaster. A disaster happens only when a hazard meets people and property that cannot withstand it.
Keeping the two words apart is the key to explaining disasters. The size of the hazard matters, but so do the number of people in its path, how easily they are harmed and how well they can respond.
Exposure is who and what is in the hazard's path. More people living on floodplains, along fault lines or on hurricane coasts means more exposure. Coastal counties of the United States have grown quickly for decades, so the same hurricane now threatens far more people and property than it would have in 1950.
Exposure is partly a choice and partly not. People live near rivers and coasts for jobs, trade and beauty, and land in safer places is often more expensive.
Vulnerability is how easily people are harmed once exposed. Weak buildings, poverty, poor health, old age, isolation and lack of warning all make harm more likely. Two families on the same street can be very differently vulnerable if one owns a car and the other cannot leave.
Vulnerability is shaped by society, not by nature. That makes it the part of disaster risk that people can most directly reduce, through building codes, planning and support for the most vulnerable.
Capacity is a community's ability to prepare, respond and recover: warning systems, trained responders, evacuation plans, hospitals, insurance and savings. It pulls risk down, while hazard, exposure and vulnerability push it up.
The National Weather Service's tornado and hurricane warnings, and the drills held in schools across the country, are forms of capacity. So is flood insurance, which does not stop a flood but lets families rebuild afterward.
An earthquake's size is given by its magnitude. Each whole step on the scale multiplies the ground-shaking amplitude recorded on a seismograph by ten, and the energy released by about thirty-two.
So a magnitude $8$ earthquake shakes the ground ten times as hard as a magnitude $7$, and releases about thirty-two times the energy. The scale is logarithmic: small differences in the number mean large differences in the event.
A disaster that kills $2000$ people in a country of $10$ million has a death rate of $200$ per million. One that kills $90$ in a country of $18$ million has $5$ per million, forty times lower.
Rates per million let countries of any size be compared. Comparing events of similar size with rates reveals the differences in exposure, vulnerability and capacity that the raw counts hide.
In January 2010 a magnitude $7.0$ earthquake struck near Port-au-Prince, Haiti. Estimates of the dead range from about $100000$ to more than $300000$, in a country of about ten million. In February 2010 a magnitude $8.8$ earthquake struck Chile, releasing hundreds of times more energy, and killed about $500$ people in a country of about seventeen million.
Chile had strict building codes, enforced for decades, and experience with large earthquakes. Haiti had many poorly built concrete homes on crowded hillsides. The smaller hazard produced the far larger disaster.
A one-in-a-hundred-year flood is a flood so large that it has a one percent chance of happening in any given year. It does not mean the flood comes once a century, neatly spaced. Two can come in successive years.
Over a thirty-year mortgage, a home in that flood zone faces about a one-in-four chance of at least one such flood, which is why the Federal Emergency Management Agency requires flood insurance for many homes in these zones.
Checking an answer. A one-in-T flood's yearly chance is always small when T is large. Each step of magnitude is a factor of ten in amplitude, never an addition of ten.
Separating the hazard from exposure, vulnerability and capacity is allowed because each can change independently. A stronger building code changes vulnerability without changing the earthquake; a new suburb on a floodplain changes exposure without changing the river.
Dividing deaths by population is allowed because the rate describes how hard a country was hit relative to its size, which is what a comparison of disasters needs.
Hurricanes form over warm ocean water and bring three hazards: wind, heavy rain and storm surge, a rise in sea level pushed ashore by the storm. Storm surge is often the deadliest.
The National Hurricane Center rates hurricanes by wind speed on the Saffir-Simpson scale, from category $1$ to $5$. But a slow-moving category $1$ storm can bring more flooding rain than a fast category $4$, so the category alone does not predict the disaster.
Across the world, poorer countries lose far more lives to hazards of similar size, while richer countries lose more money, because they have more property to damage. Poverty raises vulnerability and lowers capacity at the same time.
The same pattern appears inside countries. Within American cities, poorer neighborhoods often sit on lower, flood-prone land, have older housing and fewer resources to evacuate or rebuild.
A disaster does not start when the hazard strikes. Decisions made years before, about where to build, how strongly and for whom, set up the harm. And it does not end when the water recedes: recovery can take years and leave some neighborhoods permanently changed.
Geographers study the whole process, before and after, because that is where the differences between places are made.
The U.S. Geological Survey records every significant earthquake's magnitude and location. The National Weather Service tracks storms and floods, and FEMA maps flood zones and records disaster declarations.
Death tolls are often uncertain, especially in large disasters in poorer countries, and can be revised for years. A careful comparison gives the source and year of each figure and treats uncertain tolls with caution.
The most common slip is to read the size of a disaster from the size of the hazard. Another is treating a one-in-a-hundred-year flood as one that comes on a schedule.
A third is adding ten for each step of magnitude instead of multiplying by ten. A fourth is comparing raw death counts between countries of very different sizes instead of rates per million.
Hurricane Katrina struck the Gulf Coast in August 2005 and killed more than $1800$ people, most of them in Louisiana and Mississippi. In New Orleans the greatest harm came not from the wind but from the failure of levees and floodwalls, which let water cover about four fifths of the city.
Every part of risk was at work. The hazard was a large hurricane and its storm surge. Exposure was high because much of the city lies below sea level. Vulnerability was concentrated among older residents and households without cars, many of whom could not leave before the storm. Capacity failed when the levees gave way and rescue and shelter were overwhelmed.
After Katrina, the Army Corps of Engineers rebuilt the city's flood defenses, and evacuation plans now include buses and trains for residents without cars. The storm showed that a hazard becomes a disaster through decisions made long before it arrives, and that reducing vulnerability can save as many lives as any wall.
California sits astride the boundary between the Pacific and North American plates, and large earthquakes are certain. After the 1933 Long Beach earthquake collapsed many school buildings, fortunately after classes had ended for the day, the state passed the Field Act, requiring public schools to be built to strict earthquake standards.
Building codes have been strengthened after each major earthquake since. When the magnitude $6.7$ Northridge earthquake struck the Los Angeles area in 1994, it caused billions of dollars of damage but killed about sixty people in a region of millions, a low toll for so strong a quake in so crowded a place.
California has continued to require the retrofitting of older buildings most likely to collapse, such as some apartment buildings built over open parking. These rules reduce vulnerability without changing the hazard or the exposure, and they are why geographers describe building codes as among the most effective disaster measures a society can take.
It is natural to think the size of a disaster follows the size of the hazard: the stronger the earthquake, the more people die. But harm depends on exposure, vulnerability and capacity as well. In 2010 a magnitude $7.0$ earthquake in Haiti killed tens of thousands, while a magnitude $8.8$ earthquake in Chile, hundreds of times more energetic, killed a few hundred.
So explain a disaster by separating its parts. Name the hazard, then ask who was in its path, what made them easy to harm, and what helped them respond.
A hurricane with $120$ mph winds strikes a coast. Classify the hurricane.
$\text{hazard}$
The natural event.
Fifty thousand people live in the surge zone. Classify this.
$\text{exposure}$
People in the path.
Many of them have no car to evacuate. Classify this.
$\text{vulnerability}$
Harder to escape harm.
Buses are sent to carry people inland. Classify this.
$\text{capacity}$
It reduces the harm.
A home is in the one-in-$50$-year flood zone. Find the chance in one year.
$\dfrac{1}{50} = 2\%$
Each year.
Find the chance of no such flood in a year.
$100 - 2 = 98\%$
Everything else.
Find the floods expected over thirty years.
$\dfrac{30}{50} = 0.6$
On average.
Say whether a flood last year makes one this year less likely.
$\text{no: still } 2\%$
Each year stands alone.
Name the part of risk that flood insurance changes.
$\text{capacity to recover}$
It does not stop the flood.
Country A, with $12$ million people, loses $3600$ lives. Find its deaths per million.
$\dfrac{3600}{12} = 300$
A's rate.
Country B, with $30$ million, loses $150$. Find its rate.
$\dfrac{150}{30} = 5$
B's rate.
Compare the two rates.
$\dfrac{300}{5} = 60$
Sixty times higher in A.
A's quake was magnitude $7$ and B's $8$. Compare their amplitudes.
$10^{8 - 7} = 10$
B's was ten times larger.
State the conclusion.
$\text{the smaller hazard, the larger disaster}$
Hazard size did not decide the harm.
Name what to investigate next.
$\text{exposure, vulnerability, capacity}$
The explanation lies there.
Write the rate as a fraction.
$\dfrac{1200}{8}$
Deaths over millions.
Evaluate the rate.
$150$
Deaths per million.
Say what the rate lets you do.
A report on a disaster notes that a hundred thousand people live on the river's floodplain. Which part of disaster risk does this describe?
Complete the worked solution: in similar earthquakes, country A, with $5$ million people, lost $350$ lives, and country B, with $12$ million people, lost $96$. Find each country's deaths per million and how many times higher A's rate was.
Find country A's deaths per million.
$\dfrac{\text{deaths}}{\text{millions}} =$ s
A's rate.
Find country B's deaths per million.
$\dfrac{\text{deaths}}{\text{millions}} =$ t
B's rate.
Divide A's rate by B's.
$\dfrac{\text{A}}{\text{B}} =$ k
Times higher.
Name what could explain the gap.
$\text{exposure, vulnerability and capacity}$
The hazards were similar.
Match each term to its meaning.
| a natural event that could cause harm | people and property in the path of the event | how easily people and property are harmed | serious harm beyond what the community can cope with | |
|---|---|---|---|---|
| hazard | ||||
| exposure | ||||
| vulnerability | ||||
| disaster |
A flood map marks a neighborhood as reached by the one-in-$100$-year flood. Fill in the chance of that flood in any one year, the number of such floods expected over a thirty-year mortgage, and the chance of no such flood in a given year.
| value | |
|---|---|
| chance in one year (%) | |
| floods expected in 30 years | |
| chance of none in a year (%) |
A country of $15$ million people is struck by an earthquake. Write the deaths per million people as a function of the number of deaths $d$.
Answer:
One earthquake has a magnitude of $5.5$ and another a magnitude of $8.5$. How many times larger is the ground-shaking amplitude of the second, as recorded on a seismograph at the same distance?
Answer: times
Suppose a flood in a town in Vermont caused $250$ million dollars of damage, of which $50$ million was covered by flood insurance. What percent of the damage was uninsured?
Answer: %
Lesson test: one question per skill, one attempt each, no hints. Your answers are checked when you submit.
A country of $15$ million people is struck by an earthquake. Write the deaths per million people as a function of the number of deaths $d$.
Answer:
You can explain a disaster. Explain how a smaller earthquake can kill far more people than a larger one.
26. Your turn: a country of $8$ million people loses $1200$ lives in an earthquake. What is its death rate per million?, step 3
$\text{compare with other countries}$
Whatever their size.