Hazard Basics

An event meeting people

An earthquake in an empty stretch of desert is not a hazard. It is geology. The same earthquake beneath a city is a catastrophe, and nothing about the rock has changed. A natural hazard is a natural event that threatens people or property, which means it takes two things to make one: an event, and somebody in its way. Hold on to that and the whole topic falls into place, because every risk factor the specification lists describes either the event or the people. Magnitude, frequency, duration and predictability are properties of the event. Where people live, what they live in, what warning they can act on and what they can afford to do are properties of the people. Risk is what happens where the two meet. It also explains the thing most candidates find counter-intuitive, which is that a smaller event can kill far more people than a larger one. In 2010 two earthquakes struck seven weeks apart. Haiti, on 12 January, measured 7.0, with a shallow focus close to Port-au-Prince: the death toll ran into the hundreds of thousands, estimates differ widely, and around 1.5 million people were left without homes. Chile, on 27 February, measured 8.8, which released on the order of five hundred times more energy, and killed in the region of 500 people. The geology did not decide that. Chile had been building to a seismic code since its magnitude 9.5 earthquake of 1960, the largest ever recorded, while much of Port-au-Prince was unreinforced masonry with no enforced code. The difference was the people half of the equation.

Six terms, and the first two are not the same thing

Candidates lose marks on the first pair constantly, and the last three are the ones to pin to a real event rather than define in the air.

From event to disaster

Six stages turn a natural event into a disaster. Put them in order.

• A natural event occurs, of a size set by physical processes alone
• It happens where people live, which is what makes it a hazard rather than geology
• How many people are in its path depends on population density and where settlement has spread
• How badly they are affected depends on buildings, income, health and whether a warning could be acted on
• The response depends on what emergency services, money and planning were in place beforehand
• The losses that follow are the product of all of those together, not of the event alone

Why did the smaller earthquake kill more?

In 2010 a magnitude 7.0 earthquake in Haiti caused vastly greater loss of life than a magnitude 8.8 in Chile seven weeks later. Which explains it best?

• Because losses depend on the people as much as on the event. The Chilean building code, enforced since the enormous earthquake of 1960, meant structures were designed to survive severe shaking, while much of the Haitian capital was unreinforced masonry built without an enforced code, and the focus lay shallow and close to a dense city. The same shaking met two very different populations.
• Because the Haitian earthquake was shallower, and depth is what determines loss of life.
• Because the magnitude scale does not measure the shaking people actually feel.
• Because the emergency response in Haiti was slower.

Two halves of one risk

Split the risk factors into the event and the people, and the specification list stops being something to memorise. The third column is the part that earns the higher marks, because it is where the two halves interact.

Match each fact to the half of the risk it belongs to

Tie every factor to a place

The content map gives this node one instruction and it is worth following exactly: link the risk factors to a real scenario rather than defining them in the abstract. A definition of vulnerability earns a mark; the same definition attached to a named place and a date earns the argument. So build the habit of pairing. Vulnerability is not "how easily people are harmed", it is unreinforced masonry in a dense capital against an enforced seismic code, in two countries struck seven weeks apart in 2010. Predictability is not "how much warning there is", it is days of hurricane tracking followed by an evacuation order that thousands of households had no means to obey. Two traps to avoid. Hazard and disaster are different words: a hazard is a threat and a disaster is a threat realised, so a well-prepared place can live with high hazard and few disasters, and using the two interchangeably costs marks on the very first question of the paper. And magnitude is not destiny, which is the point the 2010 pair exists to prove, so an answer that ranks events by size alone has missed the topic entirely. Finally, the register. Nobody dies in an earthquake through carelessness. Vulnerability is a circumstance, usually one that somebody was born into, and an answer explaining it as circumstance rather than as failure is both fairer and more accurate.

Complete the hazard risk record

A natural hazard is a natural event that threatens people or property, so an event in an empty place is not one; a hazard that has actually caused serious loss is called a _____. Hazards are grouped as geophysical, atmospheric and _____. Magnitude scales are usually logarithmic, and on the moment magnitude scale each whole number stands for roughly _____ times more energy. That is why the pairing of 2010 is so useful: the magnitude 8.8 event in Chile killed in the region of 500 people while the magnitude _____ event in Haiti, seven weeks earlier, killed a number counted in the hundreds of thousands. Warning is no guarantee either, since New Orleans was told to evacuate the day before Hurricane Katrina arrived in _____ and roughly 1,800 people still died.

Spot the three sound hazard statements

Select the THREE statements that could stand as they are in an answer.

• An event only becomes a hazard where there are people or property to threaten, so the same earthquake is geology in an empty desert and a catastrophe beneath a city.
• A smaller event can cause far greater loss than a larger one, since building quality, density and income decide how the same shaking is experienced.
• A forecast helps only those able to act on it, so a well-predicted hazard can still cause heavy loss among people with no means of leaving.
• The largest magnitude events are always the most damaging, since they release the most energy.
• Hazard and disaster mean the same thing and can be used interchangeably.
• Losses are higher in poorer countries largely because people there do not prepare properly.

A 6-mark risk answer, marked

Explain why two hazard events of similar magnitude can cause very different levels of loss. [6 marks] The event sets the shaking and the people decide what it does. The clearest demonstration is the pair of earthquakes in 2010. On 12 January a magnitude 7.0 struck Haiti with a shallow focus close to Port-au-Prince, and the death toll ran into the hundreds of thousands, with estimates differing widely, while around 1.5 million people were left without homes. On 27 February a magnitude 8.8 struck Chile, releasing on the order of five hundred times more energy, and killed in the region of 500 people. The larger event was by far the less deadly. The main reason is building quality and its enforcement. Chile had been building to a seismic code since its magnitude 9.5 earthquake of 1960, the largest ever recorded, so structures were designed to move with severe shaking rather than collapse. Much of Port-au-Prince was unreinforced masonry put up without an enforced code, and buildings of that kind fail suddenly and completely, which is why so many deaths occurred in the first minutes. Density compounded it, since the shallow focus lay close to a crowded capital, so a great many people were exposed to the strongest shaking at once. Capability matters as well. Wealth decides whether homes can be strengthened, whether emergency services can be equipped in advance and how quickly help arrives, and a country with less of it is starting the response from further back. None of that is a failure by the people affected; it describes the circumstances they were living in when the ground moved. What earned the marks: the event and the people separated explicitly; two named events with dates, magnitudes and outcomes; the energy difference quantified so the comparison bites; building enforcement explained as a mechanism rather than named; and the register kept to circumstance rather than blame.

Find the two that would cost marks

Five sentences from answers about hazard risk. Two would lose credit for the argument they make rather than for being factually wrong. Tap those TWO.

• The larger the magnitude of an event, the greater the loss of life it will cause.
• The same event is geology in an empty area and a hazard where there are people and buildings in its path.
• Losses are worse in poorer countries because people there do not take the warnings seriously enough.
• One country had enforced a seismic code for decades and the other had not, which is why similar shaking produced very different outcomes.
• An evacuation order only reduces deaths among people who have the means to leave when it is given.

Build the definition sentence

Assemble the sentence that defines a hazard properly.

Four judgements about risk

Each situation asks you to work out what is actually driving the risk.

• A magnitude 7.5 earthquake occurs beneath an uninhabited mountain range and nothing is damaged.
• A city has spread onto its floodplain over twenty years. River flows and rainfall are unchanged.
• A storm is tracked for four days and an evacuation is ordered, and many people in the poorest districts do not leave.
• Two regions face the same hazard. One is struck every few years, the other once in a lifetime.

Explain hazard risk

Explain the factors that affect the risk a natural hazard poses to people. [9 marks]

• Open with the definition, since a hazard needs an event and people in its path
• Separate the factors that belong to the event from those that belong to the people
• Tie at least two factors to a named place with a date, as the specification expects rather than defining them in the abstract
• Explain why a smaller event can cause greater loss, using magnitude and vulnerability together
• Finish on the point where the two halves meet, such as a warning that only helps those able to act on it, and write about circumstances rather than failings