# HKDSE Geography/M1/Impacts of Tectonic Hazards
## Earthquakes

Along active faults, earthquakes cause fault rupture and ground deformation. Cracks and deformation result.
When seismic waves are released from the focus onto the ground's surface, the ground will shake. Areas around the epicentre will be seriously affected:

### Damage to Structures and Facilities

* Collapse of structures: Earthquakes will cause buildings, bridges and other structures to collapse. Roads may split open. Loss of life will result from collapsing buildings and falling fragments.
* Fires: Underground gas pipes and power lines may break. This causes gas leaking and electrical short circuits, and hence fires, causing further casualties because of burning and the inhalation of smoke.
* Disruption of transport: Road networks, railways and highways will be destroyed. If the important lifelines are damaged, economic loss will result, rescue will be blocked and normal life will take longer to resume.
* Disruption of communications: Telephone and Internet cables may be damaged by the earthquake. In addition to increasing the cost of repair, this will also hinder communication inside and outside the damaged area, hindering rescue work.

### Damage to the Natural Environment

* During an earthquake, loose slope materials and debris may slide down slopes. This may cause landslides and avalanches, leading to more loss of life and property, especially at steep and unstable slopes.
* When dams, artificial dykes and other structures collapse because of the shaking, the water from the reservoir will flood downstream. This further leads to loss of life and property.
* Submarine earthquakes may trigger tsunamis, leading to more damage (see below).
* An earthquake may result in tremors called aftershocks which bring more damage to the area for weeks or months.
* Earthquakes may increase ground water pressure. If the soil is poorly compacted, it may mix with the ground water to cause soil liquefaction. The strength of the soil supporting buildings' foundations will be weakened, so buildings may sink or collapse. This may cause casualties and economic losses.
* If no swift action is taken to bury dead bodies and ensure clean water supplies after an earthquake, diseases and epidemics (e.g. malaria) will spread among victim, leading to further casualties.

## Volcanic Eruptions
### Effects of Lava Flow

Molten lava is scalding hot. It has the ability to burn everything in its path. It:
* Sets hills, buildings, structures, crops and communications links on fire.
* Causes extensive economic losses and loss of life.
* Animals are killed, habitats are destroyed and ecological restoration may take decades.
Less viscous lava can move at speeds of up to 50 km per hour on steep slopes, and spread tens of kilometres away from the volcano.

### Effects of Pyroclastic Flow and Ash Fall

Strong eruptions eject volcanic ash and cinder, broken rock fragments and volcanic bombs (lava particles) from the volcano. These pyroclastic materials mix with hot gases and travel downhill at soaring speeds because of the force of gravity. This is called pyroclastic flow and has the following impacts:
* High temperature and large rocks cause loss of human and animal life.
In addition, ash is ejected into the atmosphere:
* Volcanic ash may blow into the atmosphere to form eruption clouds. The surrounding areas will be covered in volcanic ash as the clouds set down. This disturbs people's lives.
** If farmlands are covered by ash, crop failure occurs, leading to food shortage or even famines.
* Volcanic ash and dust in the atmosphere acts as condensation nuclei. This accelerates the condensation of water vapour into water droplets in the atmosphere. This favours the formation of heaby rainstorms and thunderstorms.
* Volcanic ash in the stratosphere reflects insolation back into space. This leads to a regional or even global temperature drop. It takes a very long time for ash to settle, so the climate may be changed for a very long time.
* Volcanic ash may mix with rainwater and flow quickly through river valleys and low-lying areas, leading to mudflows that cause great loss of life and property.
Ash in the air also causes problems:
* Volcanic ash in the air causes breathing difficulties. People may suffocate as a result.
* Volcanic ash in the air blocks out sunlight, lowering visibility and hinder traffic.

### Effects of Gas Ejection

Water vapour and gases like sulphur dioxide, carbon dioxide, hydrogen sulphide and carbon monoxide are ejected from volcanoes, with the following effects:
* Carbon dioxide drives oxygen away. Humans and animals suffocate.
* Carbon dioxide intensifies the greenhouse effect and accelerates global warming.
* Hydrogen sulphide is a toxic gas. People may be poisoned by it.
* Acid gases like sulphur dioxide attack the human respiratory system.
* Sulphur dioxide, hydrogen sulphide and carbon dioxide mix with rainwater to form acid rain, which damages natural habitats and human settlements.

### Effects of the Force of Ejection

* Landslides occur after a particularly violent eruption if the volcano is steep and unstable.
* The release of accumulated pressure and gases underground causes the crust to shake violently, leading to an earthquake.
* The shock generated by a volcanic eruption may displace water and create big sea waves known as tsunamis.

### Other

If no swift action is taken to bury dead bodies and ensure clean water supplies after a volcanic eruption, diseases and epidemics will spread among victims, causing further casualties.
* Lava, mud and slope materials don't just flow anywhere
* If you are given a map with an asymmetric hill - i.e. the contour lines on either side are of different closeness - then be careful. Lava flows, landslides, etc., are more likely to occur on the steep side.

## Tsunamis
### Sweep Effect

The sweep effect of the waves may destroy buildings and drown people. People, buildings and objects may be carried inland by the swash and carried offshore by the backwash. These are hydrostatic effects. Structures may collapse because of their damaged foundations, or because of the force of incoming water. These are hydrodynamic effects. Load carried by the tsunami waves will also damage buildings and facilities. These are shock effects.

### Flooding

* A temporary flood is caused by the tsunami. It may inundate buildings and infrastructure until the tsunami waves subside.
* The coastline of the affected area may be changed permanently by submergence.

### Other

* Seaside tourism is hit hard.
* Trade activities come to a halt.
* Not everything kills
* Sometimes, you may be asked what kind of hazards threaten people's lives. Not all of these impacts do! For example, in CE07Q13, the candidate was asked about the possible threats to human life during a volcanic eruption. 'Acid rain' was one of the choices. Yes, it's a result of volcanic eruptions, but it doesn't kill! Therefore, acid rain is excluded from the correct answer.

## Interpreting Pictures of Destruction

Very often, photo interpretation questions of the aftermath of tectonic hazards occur. Refer to this picture of an earthquake + tsunami in Chile:
In this image, only collapse of buildings is shown. Therefore, poor building structure/lack of quake-proof designs is a valid reason accounting for this destruction. However, the lack of evacuation routes is NOT as no loss of human life was shown. It was possible that the area was evacuated, leaving only the buildings prone to tsunami destruction. (Example: CE07Q42)