HAZARDS

Natural hazards

Natural hazards are those potential events in the natural world that present a high risk to the population and infrastructure and which cannot be controlled. We all know the term ‘hazard’ from various contexts, e.g. health hazard, meaning anything that is detrimental to our physical well-being, but generally, hazard implies danger, the potential to harm, whereas risk is the quantifiable aspect, i.e. whether a hazard will harm and the amount of harm that can result.

The natural world knows many hazards: earthquakes, floods, extreme weather such as hurricanes or tornados, volcanic eruptions etc. The extreme rainfall experienced in England in July 2007, for instance, was a hazard which generated a high risk of flooding and subsequently a large populated area was inundated, resulting in the evacuation of the affected population, contamination of drinking water, destruction of buildings, bridges, roads, loss of power and disruption to all forms of transport. Economic losses are high and cover an area far greater than that under water. Compared to other natural hazards, the effect of this flooding may be considered as manageable in a country with enough resources to tackle the problems arising. Those affected may disagree, but, in contrast, the devastating Indian Ocean tsunami of 2004 resulted in high loss of life, widespread destruction and immeasurable suffering, causing problems that are still not solved and many questions that remain unanswered.

Although research does not necessarily solve problems caused by such disasters, by increasing understanding of natural phenomena, by producing models of potential events using known data, by exploring the Earth, on land and the seafloor for problem areas that may result in future hazards or to define hitherto unknown hazards, measures can be taken to increase awareness and to manage and reduce risk, and the more information collected, the further risk management and reduction will develop.

This website and the associated scientific project aim to provide new information on the hazards that are responsible for tsunamis, the processes that generate tsunamis and the areas that pose a potential threat and for this reason we will concentrate on these hazards and processes and the science behind the investigations. On this page we address the hazards that are responsible for the generation of tsunami, notably earthquake, landslides, volcanic eruption and impacts. Information on scientific methods for research and results obtained will be added to each separate section under the ‘popular science’ heading.

Earthquake

To understand why earthquakes are generated, it is necessary to first look at plate tectonics. The outer, rocky layer of the Earth is not a continuous solid mass, but is broken up into plates, which are slowly but continuously in motion. (http://worldatlas.com/aatlas/infopage/printpage/tectonic.htm). Some of these plates are moving away from each other (divergent plates), thus allowing magma from the Earth’s mantle to upwell and solidify to form new oceanic crust. This is happening, for instance, under the Atlantic Ocean at the Mid-Atlantic Ridge (http://www.platetectonics.com/oceanfloors/africa.asp). Other plates slide past each other at transform faults, for example, the San Andreas Fault in California, USA (http://quake.usgs.gov/kap/carrizo/). Plates also collide with each other (convergent plates). Colliding continental plates build mountains such as the Himalaya, formed when India moved north to collide with the Asian mainland. However, when an oceanic and a continental plate collide, the oceanic plate slides underneath the continental plate because it is denser. This process is termed subduction (http://www2.nature.nps.gov/GEOLOGY/usgsnps/pltec/converge.html).

Diagrams explaining these processes always show smooth boundaries for convenience, but this does not give a true representation of the actual processes. Boundaries are irregular and plates become locked, building up stresses along the faults, and when these stresses have reached a critical point, the rock fractures and sudden movement results, i.e. an earthquake is generated. If conditions are right, i.e. vertical plate movement, magnitude, depth of earthquake, location, a tsunami will result.

For more detailed information on plate tectonics, earthquake generation and tsunamis see: http://library.thinkquest.org/04oct/01724/home.html.

Landslides

Landslides entering the sea or large body of water and submarine landslides can be responsible for the generation of tsunamis. In this case, tsunamis may occur not only in the oceans, but also on lakes as happened in the 17^th century on Lake Lucerne, Switzerland. Landslides can result from earthquakes as in the case of the tsunami in Lituya Bay following a magnitude 8.3 earthquake. The resulting gravity wave was over 500 m high (http://www.drgeorgepc.com/Tsunami1958LituyaB.html). The second Storegga Slide in Norwegian waters, which may have been caused by the release of trapped gases, generated a tsunami that devastated the Scottish coast and the Shetland Islands some 8 000 years ago (http://environment.newscientist.com/channel/earth/tsunami/mg12717284.300). Other causes of landslides include erosion, freeze and thaw, heavy rainfall and continued deposition resulting in top-heavy accumulations.

Volcanic eruption

Volcanic eruptions have caused devastating tsunamis. Krakatao volcano in the Sunda Strait, Indonesia, erupted in 1883. The final stages of this event were a series of devastating explosions that tore the volcano apart. The resulting tsunami took the lives of 36 000 people and destroyed many coastal communities (http://www.geology.sdsu.edu/how_volcanoes_work/Krakatau.html). In the Southern Aegean, the volcano Santorini, part of the Cyclades group of islands, suffered a similar fate in ca. 1640 BC, generating tsunamis that impacted a large area of the Mediterranean and possibly led to the demise of the Minoan culture (http://www.decadevolcano.net/santorini/santorini.htm).

Impacts

Impacts of large extra-terrestrial objects such as asteroids or comets can be found in many places on the Earth, but such events are very rare. However, if a large body does collide with the Earth, the likelihood that it would land in the oceans is high, considering that 70% of the planet is covered with water. For a model of a tsunami generated by a potential impact event see: http://www.ucsc.edu/news_events/press_releases/text.asp?pid=355.

If you would like to learn more about tectonics, earthquakes, etc., the Open University, UK, provides open access to some of its educational material at Open Learn: http://openlearn.open.ac.uk/.

If you have questions on the material presented here, please contact: contact@shelfrisks.org

Man-made hazards

In contrast to natural hazards, some problems arise from hazards that are induced by human intervention. This will be discussed under “Environmental Risk”.