|What are the hazards associated with tectonic events? Primary and Secondary Hazards
Natural hazards can be classified according to cause into tectonic, geomorphological (slopes), atmospheric and biological hazards. If a volcano erupts in Antarctica e.g. Mt. Erebus and no-one is affected it is merely a natural event (i.e. not hazardous). A natural hazard has to have human impact – either in terms of deaths or injuries or in terms of cost e.g. property damage.
Hazards vary in terms of such features as their
Frequency - how often they occur. Will affect perception and preparedness.
Magnitude - how much energy is released. Big is likely to be bad.
Areal extent - how big an area is affected e.g. Kilauea on Hawaii compared with the Asian tsunami of Boxing Day 2004.
Speed of onset – how quickly it starts. Earthquakes are very sudden and without recognisable warnings.
Concentration – as in where they occur. Tectonic hazards tend to be found in long narrow zones (i.e. the plate boundaries).
Temporal spacing – the time gap between events e.g Kilauea is very active regularly but Mount St. Helens erupts once every century or so. Basalt lava volcanoes tend to be more regularly active than volcanoes associated with Destructive Plate Boundaries.
We need to think about
the different sorts of hazards posed at each type of Plate Boundary
which hazards have the most impact e.g. death toll statistics
the differential impacts of hazard events between MEDCs and LEDCs
why some events are more hazardous than others.
Earthquakes are almost impossible to predict so protection by proper land use planning, public education schemes and by proper design and construction of structures is perhaps the best defence. Different countries have different levels of wealth and therefore access to technology and so their capacity to protect themselves varies. Differences can also occur amongst one society and often the poorest and weakest are the most vulnerable.
Volcanic eruptions are more predictable and evacuation is a possible defence mechanism.
Primary Hazards first – these are caused directly by the earthquake and the impacts can be both short and long term. The primary hazard is ground movement and shaking. Secondary hazards are more indirect and are other hazards triggered by the earth movements – they include soil liquefaction, landslides, avalanches and tsunamis (and fire and explosions).
Ground movement – surface seismic waves (Long waves) are the ones which shake the ground causing buildings and other structures to collapse. Underground pipes and power lines can be damaged by ground movement leading to fires and explosions (e.g. if gas escapes Kobe 1995). Near the epicentre (i.e. directly above the earthquake focus) all the waves arrive at once and so cause the most severe and complex ground motion. Different ground materials react in different ways to the shaking – so the amount of damage varies with rock type.
Mexico City 1985 effects of lake bed
Loma Prieta 1989 effects on Marina District
Leninakan 1988 effects of construction heights / building codes
Secondary hazards –
Soil liquefaction is when a solid material turns into a liquefied state due to an increase in pore water pressures as a result of ground shaking during an earthquake. It affects unconsolidated sediments at depths of less than 10m, which are saturated with water. These ground failures can destroy building foundations and cause buildings to sink or collapse. Structures such as bridges, dams and underground pipes will also be affected.
Landslides – sudden mass movements can result from causes other than earthquakes but stress from earthquakes can result in slope failure even on gentle slopes. These landslides, rock and snow avalanches can overrun people and structures, cause buildings to collapse, break underground pipes and disrupt rescue efforts by blocking roads. In many earthquakes, the landsliding has caused as much or more damage than the ground shaking. The greatest risk exists with high magnitude events (above 6.0 on the Richter scale) The 1964 Alaskan earthquake caused an estimated $1.26billion damage – 56% due to landsliding. Landsliding also caused at least 48 of the 130 deaths.
A road is blocked by a landslide
near Iwaki City in north eastern Japan
March 11, 2011
Tsunamis – the sea bed was shaken in the Banda Aceh (Indonesia) earthquake of December 2004 causing giant sea waves which killed approximately 200,000 people across the whole Indian Ocean region.
The giant tsunami following the March 2011 earthquake in Japan devastated coastal cities and had a massive impact on the economy and also endangered nuclear power stations causing the risk of radiation leaks. Buildings including oil refineries burst into flames.
Volcanic Hazards: Primary Hazards first…
Lava flows – are spectacular but they pose more threat to property than to lives. The most dangerous are fissure eruptions of basaltic lava which reach up to 50kph on steep slopes and which can spread tens of kms from their source. Andesitic and rhyolitic lavas flow as plastic rather than liquid and move more slowly, rarely reaching more than 8km from their source. Lava flows are most dangerous when large quantities are released quickly. These are rare events , but one example in 1977 killed 7 people. The Nyirangogo volcano, Congo, erupted through 5 fissures on the volcanoes flanks. This drained the lava lake which had collected in the summit crater in under 1 hour.
Lava flows destroy everything in their path and are likely to result in high but localised economic losses such as farmland, buildings and roads. Kilauea affecting Kalapana Gardens in Hawaii is a good case study. 78 sq km has been covered since 1983, destroying 180 houses but with no deaths.
Pyroclastic flows – are mixtures of hot rock fragments, lava particles and ash buoyed up by hot gases. They are sometimes called nuee ardentes. These are associated with Destructive Plate Boundary volcanoes and are deadly. The pyroclastic flow moves from the vent at very high speeds and can extend 40 kms from the source. It is denser than air and swo flows down slopes (like river valleys) close to the ground. Sometimes the blast can move laterally as in Mount St. Helens 1980. Lateral blasts have less climatic impact but cause severe damage in one direction and if unpredicted may exceed evacuation zones. Research: Mt. Pinatubo
Ash and tephra fall – (tephra is over 2mms) fine ash can go high into the atmosphere and can spread widely. More of a nuisance than a cause of death. Can affect world climate. Effects include – aeroplane engine failure, breathing problems, roof collapse (+rain). Agricultural production can be affected e.g. 30kg/hecare fell on crops in eastern Washington state after the Mount St. Helens eruption.
Volcanic gases – water vapour, carbon dioxide, sulphur dioxide, hydrogen sulphide etc. Carbon dioxide is dense than air and can collect in depressions or in buildings – one fatality for a fireman searching evacuated buildings on Heimaey without breathing apparatus. One big disaster was Lake Nyos, Cameroon August 1986. Research
Lahars - mudflows of volcanic material and are the second greatest threat after pyroclastic flows. Ash and debris can mix with water (rain or melted glacier ice) and can flow at very high speeds – e.g. 22 m/s over long distances, usually along river valleys. E.g. Pinatubo 1991. Research Nevada del Ruiz Colombia 1985.
Volcanic landslides – e.g. Mount st. Helens which was triggered by an earthquake.
Tsunamis – massive volcanic eruptions can trigger tsunamis as well as earthquakes can e.g after Krakatoa 1883. Research Papua New Guinea tsunami of 1999