9.1 Hazardous environments resulting from crustal (tectonic) movement

Global distribution of tectonic hazards

Tectonic hazards include seismic activity (earthquakes), volcanoes and tsunamis. Most of the world’s earthquakes occur in clearly defined linear patterns (Figure 9.1). These linear chains generally follow plate boundaries.

Earthquakes

Broad belts of earthquakes are associated with subduction zones (where a dense ocean plate plunges beneath a less dense continental plate) whereas narrower belts of earthquakes are associated with constructive plate margins, where new material is formed, and plates are moving apart. Collision boundaries, such as in the Himalayas, are also associated with broad belts of earthquakes, whereas conservative plate boundaries, such as California’s San Andreas fault line, give a relatively narrow belt of earthquakes (this can still be over 100km wide). In addition, there appear to be occurrences of earthquakes related to isolated plumes of tectonic activity, known as hotspots.

Revised

Typical mistake

Although most earthquakes are associated with plate boundaries and tectonic activity, many earthquakes occur at great distances from plate boundaries and are not readily explained by tectonic activity.

African plate Eurasian plate

Arabian plate

Java Trench

Indo-Australian plate

Aleuthian Trench

North American plate

Philippine plate Cocos plate

San Andreas fault

‘Ring of Fire’

Hawaiian ‘Hot Spot’

East Pacific Rise

Pacific plate Nazca plate

MidAtlantic ridge

Caribbean plate

South American plate

Antarctic plate Antarctic plate

Volcanoes

Earthquake zones Subduction zone

Motion of plate Spreading ridge offset by transform faults

Collision zone

Figure 9.1 Distribution of plates, plate boundaries, volcanoes and earthquakes

Volcanoes

Most volcanoes are found at plate boundaries although there are some exceptions, such as the volcanoes of Hawaii, which occur over hot spots (isolated plumes of rising magma). About three-quarters of the Earth’s 550 historically active volcanoes lie along the Pacific Ring of Fire. At subduction zones volcanoes produce more viscous lava, and tend to erupt explosively and produce much ash. By contrast, volcanoes that are found at midocean ridges or hot spots tend to produce relatively fluid basaltic lava, as in the case of Iceland and Hawaii.

Tsunamis

Up to 90% of the world’s tsunamis occur in the Pacific Ocean. This is because they are associated with subduction zones, most of which are found in the Pacific. Tsunamis are generally caused by earthquakes (usually in subduction zones) but can be caused by volcanoes (e.g. Krakatoa, 1883) and landslides (e.g Alaska, 1964).

Earthquakes and resultant hazards

Table 9.1 Earthquake hazards and impacts

Hazards

Primary hazards: l Ground shaking l Surface faulting

Secondary hazards: l Ground failure and soil liquefaction l Landslides and rockfalls l Debris flow and mud flow l Tsunamis

Impacts

Loss of life Loss of livelihood Total or partial destruction of building structure Interruption of water supplies Breakage of sewage disposal systems Loss of public utilities such as electricity or gas Floods from collapsed dams Release of hazardous material Fires Spread of chronic illness

Factors affecting earthquake damage

The extent of earthquake damage is influenced by: l the strength and depth of earthquake and number of aftershocks l population density l the type of buildings l the time of day l the distance from the centre (epicentre) of the earthquake l the type of rocks and sediments l secondary hazards l economic development

Most earthquakes occur with little, if any, advance warning. Most problems are associated with damage to buildings, structures and transport.

Dealing with earthquakes

The main ways of dealing with earthquakes involve better forecasting techniques, warning systems and emergency procedures, and improvements to building design and location.

Revised

There are a number of ways of predicting and monitoring earthquakes. These include measurement of: l small-scale uplift, subsidence or ground tilt l changes in rock stress l microearthquake activity (clusters of small quakes) l anomalies in the Earth’s magnetic field l changes in radon gas concentration l changes in electrical resistivity of rocks

Tsunami warning systems

At present it is impossible to predict precisely where and when a tsunami will happen. In most cases it is only possible to raise the alarm once a tsunami has started (early warning system).

1 What are the main hazards associated with earthquake activity? 2 In what ways is it possible to predict and monitor earthquakes? 3 Outline the causes of tsunamis. 4 To what extent is it possible to manage the impacts of tsunamis?

Answers on pp.218–219

Tested

Volcanic hazards

Volcanic hazards can be divided into six main categories l lava flows l ballistics and tephra clouds l pyroclastic flows and ash fallout l gases and acid rain l lahars (mud flows) l glacier bursts (jokulhlaups)

Table 9.2 Hazards associated with volcanic activity

Direct hazards (primary hazards) Indirect hazards (secondary hazards)

Pyroclastic flows Volcanic bombs (projectiles) Lava flows Ash fallout Volcanic gases Nuees ardentes Earthquakes Atmospheric ash fall out Landslides Tsunamis Acid rainfall Lahars (mudflows)

Socio-economic impacts

Destruction of settlements Loss of life Loss of farmland and forests Destruction of infrastructure – roads, airstrips and port facilities Disruption of communications

Predicting volcanoes

Volcanoes are easier to predict than earthquakes because there are certain signs. The main ways of predicting volcanoes include: l seismometers to record swarms of tiny earthquakes that occur as the magma rises l chemical sensors to measure increased sulfur levels l lasers to detect the physical swelling of the volcano l ultrasound to monitor low-frequency waves in the magma, resulting from the surge of gas and molten rock l direct observation

Revised

A pyroclastic flow is a fast-moving cloud of extremely hot gas, ash and rock fragments, which can reach temperatures of about 1000°C and travel at speeds of up to 700km/h.

A nuee ardente is a mass of hot gas, superheated steam and volcanic dust that travels down the side of a volcano as a ‘glowing avalanche’ following a volcanic eruption.

Expert tip

It is a good idea to have contrasting case studies of volcanoes – one MEDC and one LEDC – to bring out differences in impact, management, land-use etc.

Now test yourself

5 What are the main hazards associated with volcanoes? 6 In what ways is it possible to predict volcanoes?

Answers on p.219

Tested