Hazard Case Studies

Hazards - Volcanic eruptions, earthquakes and different hazards linked within them + case studies.

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  • Created by: Kay
  • Created on: 09-05-11 09:53

What is a hazard?

A hazard is anything that puts you or others in danger.

In this set of revision cards you will look at many different hazards as a result of natural disasters. You may be able to apply these to your AS level geography, geology and many other levels of these subjects.

You will also look at different case studies that you can use in your exams for essay questions.

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Pyroclastic Flows

These are boiling hot clouds of gas, tephra (ash) and other volcanic debris. They can reach temperatures of 1,000°C.

In general, they travel down the flanks of a volcano at tremendous speeds or spread laterally under gravity. Their speed depends on the density of the cloud - some can reach 700km/h

They are a result of an andesitic eruption (explosive).


  • Collapse of an eruption column.
  • Blast. (eg Mount St Helens)
  • Frothing at the mouth.
  • Collapse of lava dome.

What they do:

Pyroclastic flows will destory almost anything in their way. Due to their boiling temperatures no one can withstand them. In recorded history, over 100,000 people are estimated to have died as a result of a pyroclastic flow.


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Ash Fall

Volcanic ash is made up of small tephra created during a volcanic eruption. Each individual piece is generally <2mm.

It is classed as a hazard as it is significantly denser than snow. Just two milimetres of ash is enough to close down an airport. When it falls during the day, it turns the sky a pale yellow-ish colour. If you are close enough to the erupting volcano, it can completely blacken out the sky and decrease temperatures.

When ash is mixed with rain water, glacial melt or other forms of liquid they form lahars - see next slide - and is dense enough to collapse most roofs.

Ash can also cause floride poisoning in livestock that eat ash-covered grass if there is enough floride present.

Inhaling volcanic ash can damage the respiratory system and eyes. If too much is inhaled and it mixes with moisture, it can create a cement-like liquid in the lungs.

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These hazards are also known as mudflows - even though there is often very little mud involved.

The consistency of a lahar is similar to that of wet concrete. When it stops moving, it sets in a similar way and is very difficult to remove.

Lahars can be huge - the Osceola lahar produced 5,600 years ago by Mount Rainier in Washington created a wall of mud 140 metres deep in the White River canyon and covered an area of over 330 square kilometres.


  • Snow & glacier melt (See Nevado Del Ruiz slide)
  • Flood caused by a crater lake break out
  • Rainfall or typhoons (See Mount Pinatubo slide)

More people have died as a result of lahars than any other volcanic hazard.

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Volcano - Case studies.

Nevado Del Ruiz - 1985

When the volcano Nevado Del Ruiz erupted in '85, the pyroclastic flows melted the glaciers. Enormous lahars flew down the flanks of the volcano at ~60km/h.

The lahars reached the small town of Amero, killing more than 20,000 of its 29,000 inhabitants.

Casualties in other towns brought the death toll up to 23,000 people as a result of lahars from the volcano.

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Volcanoes - Case Studies

Mount Pinatubo - 1991

Pinatubo is an active stratovolcano on the island of Luzon in the Philippines. On July 16, 1990 a large magnitude 7.7 earthquake hit Luzon, signalling the awakening of Mount Pinatubo. 

On March 15, 1991 earthquake swarms started on the island. The first eruption was on April 2nd of that year. The volcano spewed out ash which dusted the surrounding area. An evacuation was issued to the immediate area. 

Later that year on June 12, a larger eruption sent pyroclastic flows for around 4km around the volcano. Eruptions continued up until about 4 days later. On June 15, Typhoon Yunya hit the island causing massive lahars to spread down the flanks of the volcano. 

The ash cloud covered an area of 125,000km2 causing almost total darkness across the island. Tephra fall covered most of the South China Sea and was recorded as far away as Cambodia and Malaysia.

As evacuations moved most of the people in danger away from the volcano, only just over 400 people were killed as a result of the immediate eruption. It is considered a success in volcanic prediction.

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Volcanoes - Case Studies

Mount St Helens - 1980

The stratovolcano, located in Washington State erupted on May 18, 1980 early in the morning. It was rated at a VEI 5 event. During the build up to the eruption, the volcano gave plenty of warning.

It had remained dormant for over 120 years, with limited activity in the 1950s. In March 1980, a series of small earthquakes indicated the movement of magma below the volcano. During the same month, 174 earthquakes measuring M2.6 or greater were recorded in just two days.

The shocks increased in magnitude and frequency through to April and May. Gas emissions were recorded in late March as blue flames appearing from a new crater. A large bulge also grew on the north flank of the volcano.

Around 8:30am on May 18, a shallow M5.1 earthquake occurred right under the bulge on the north flank. The initial blast released the magma below to a much lower pressure, causing a pyroclastic flow, which killed the volcanoes first victims. The 13km surrounding the volcano was almost completely destroyed.

Ash covered most of Washington State and reached as far away as Yellowstone National Park, Minnesota, Oklahoma and took less than a day to travel around the globe.

Lahars, created by glacier and snow melt took out 27 bridges, washed away Camp Baker and followed river channels for over 40km.

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Shaking and Ground Rupture

This is a primary effect of an earthquake. It results in damage to buildings and infrastructure and can result in massive loss of life.

The severity of the damage depends on the magnitude of the earthquake, the depth at which it occurs, geology of an area and the distance you are from the epicentre.

Ground ruptures are also known as fissures. It is the visible displacement of the earth's surface. It is a major risk for large engineering structures such as dams, bridges and nuclear power stations. Careful mapping is required to track existing faults to determine high-risk areas.

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Mass Movements & Liquifaction

These are common hazards linked with earthquakes. Soil liquifaction occurs when - due to shaking - water saturated ground mass looses its strength and transfers from a solid to a liquid.

It can cause buildings, bridges and other structures to sink or tilt. In Japan, 2011 after the M9.0 'quake, levees sank into the soil in some places up to 3 metres. This allowed the tsunami (see later slide) to wash further in land.


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These are long wavelength, long period sea waves caused by the movement of oceanic crust or large volumes of water. In open ocean, the size of the wave can be enormous - sometimes surpassing 100km.  The lag-time between waves can be anywhere between five minutes to an hour.

Some tsunamis travel 600-800 kilometers per hour depending on water depth.

Waves produced by an earthquake can hit nearby coastal areas in a matter of minutes. Tsunamis can also travel thousands of kilometers across open ocean and destroy far shores hours after the earthquake that generated them has faded to nothing.

Usually, earthquakes caused by destructive plate margins (under magnitude 7.5 on the Richter scale) do not cause tsunamis, although some instances of this have been recorded. Most destructive tsunamis are caused by earthquakes of magnitude 7.5 or more.

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Earthquakes - Case Study

The 2011 Tohoku Earthquake, Japan

The Great East Japan Earthquake occurred on March 11, 2011 was generated under the ocean at the subduction zone (Pacific under the Honshu) off the eastern coast. It was classed as a M9.0 on the Richter Scale.

So far, the government have estimated there will be around 30,000 deaths as a result of the initial shaking and the tsunami that followed. The earthquake itself was a shallow one at only 32km below the crust.

Land subsidence was recorded in over 15 towns across the eastern half of the country. Numerous power plants have been damaged, the most covered by the media  is Fukushima which has now caused the evacuation of a 30km radius around it.

Dam's ruptured, washing away homes. 1.5 million houses have registered that they have lost all water supplies. 4.4 million homes are without electricity at all.

Your best bet is to read up on this yourself. There is so much information online that it simply won't fit on this note.

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Earthquakes - Case Studies

Haiti - 2010

The Haiti M7.0 'quake struct Port Au Prince on January 12, 2010 at 4:53pm. The country sits on a conservative plate boundary - like California. Here, the North American plate slides past the Caribbean in the opposite direction.

As a result of the earthquake, 230,000 people died whilst 300,000 were injured. Many people migrated to the Dominican Republic in order to get treatment in the packed hospitals. A year later, diseases such as cholera are running rampage through the refugee camps where over 1 million people now reside.

20% of structures collapsed in Port Au Prince whilst the remaining 80% suffered serious damage due to lack of reinforcements. There was significant damage to the port as liquifaction occurred. This stopped aid from being shipped in. The airport was also highly damaged.

The estimated cost to rebuild the country is anywhere between $8-14billion. Over 3 million people are now relying on aid. The tourism of the country has been strongly affected.

1.1 million people were displaced and a third of civil servants were killed. On top of this, all government buildings were destroyed, making it very difficult to bring about order. The prison opened it's doors, creating a gang-based law system in the refugee camps.

Dead bodies have been very difficult to deal with.

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Earthquakes - Case Studies

San Francisco - 1906

On April 18, 1906 a M7.9 earthquake hit the busy city of San Francisco in Northern California. The epicentre was located about 2 miles outside of the city. At the time, only ~500 death were reported as most from Chinatown were ignored. Now, the lowest estimate is at 3000. Roughly 300,000 people were left homeless out of a population of 410,000.

The earthquake occurred because of a rupture on the San Andreas fault - a conservative fault that runs almost the length of the state. Fire ran rife throughout the city of San Francisco. Due to collapsed buildings, the fire service was unable to travel to the main disaster zones. It is estimated that 90% of the damage was as a result of the fires that broke out. They burned for 4 days and nights.

Throughout California's history, there have been over 60 recorded disasterous earthquakes  rating over a M4.4 listed on Wikipedia with over 30 in the past 100 years.

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need more information on the responses and effects 

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