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Natural Hazards 2

„Nature to be commanded, must be obeyed“ (Francis Bacon, 1561-1626)

W. Eberhard Falck eberhard.falck@uvsq.fr

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Natural Processes

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Driving forces behind natural processes

•  There are three fundamental phenomena that drive all natural processes:

– radioactive decay – gravity –  the Earth‘s rotational inertia

•  They cause/drive mass and energy flows throughout the natural environment

•  Neither of these processes can be stopped or controlled by us humans

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Radioactive decay in the environment

•  It ‚powers‘ the sun

•  Solar radiation causes the movements in the atmosphere (= wind) due to differential heating of different surfaces (e.g. water, land)

•  Solar radiation causes evaporation and (indirectly) evapotranspiration from plants

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Radioactive decay and geology

•  It keeps the Earth‘s interior hot

•  Without the decay heat, the Earth would have cooled down in about 30,000 years (Lord Kelvin)

•  The heat flow from the earths drives phenomena such as

– vulcanism - earthquakes

– plate tectonics - orographic changes

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Gravity

•  Drives the surface water flow from the mountains to the sea

•  Drives the groundwater flow (pressure differences due to different elevation)

•  Causes eroded material to move downwards

•  ‚Mountains‘ are a higher degree of order - according to the 2nd law of thermodynamics this order will dissipate

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Rotational Inertia

•  Causes, together with gravity, the tidal waves in the oceans

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Overview: Natural Hazards

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Hazard classification by root cause

•  Endogenic –  having their root in processeses in the Earth‘s interior

•  Exogenic –  having their root in processes above ground

•  Extraterrestrial –  e.g. meteorite impacts

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Normal processes vs. catastrophic events

•  Many exogenic, e.g. weather-related, processes occur regularly and do not pose any hazards

•  Changes in intensity, in timing or co-occurence of several phenomena can lead to catastrophic events

– heavy rainfall after a long dry seasons can lead to flooding or mud-slides

– heavy rainfall in several different regions of the same catchment area can lead to flooding

– prolonged wet weather can lead to mud-slides

•  Normal events can trigger chains of events that finally lead to catastrophic events

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Endogenic hazards

• earthquakes

• volcanic phenomena

•  tsunamis

• eustasis related phenomena

• natural radioactivity

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Exogenic hazards

• particularly relevant in high-energy environments – mountain areas – coastal areas – river valleys – because of

•  high relief energy, i.e. steep slopes, altitude differences •  lack of protection from wind, waves etc. •  tidal forces

• climatic changes/variations – natural or human induced – change the dynamic equilibria in nature – humans have arranged themselves with a particular state of

nature, which is changing later

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Weather-related hazards

•  storms

•  heavy rainfall

•  hail

•  snow

•  severe frost

•  severe heat

•  droughts

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Water-related hazards

•  inundations

•  (flash)floods

•  avalanches

•  tidal phenomena

•  rogue ocean waves

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Geology-related hazards

•  torrents

•  mud flows

•  rock falls

•  landslides

•  cave-ins

•  permafrost-related

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Extraterrestrial hazards

•  magnetic storms

•  meteorite impacts

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Endogenic hazards

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Earthquakes

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Causes •  The Earth‘s crust is made up of numerous plates that slowly

move due to convections in the mantle

•  Slow movements occur along other fault zones in the crust

•  The frictional energy built up is released spasmodically - an earthquake

•  Small earthquakes can also occur due to volcanic activity

•  Sometime earthquakes have an anthropogenic origin: e.g. collapsing underground mines

•  Large earthquakes occur about once a year

•  Small earthquakes occur about once an hour

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Plate tectonics

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Tectonic plates

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Tectonic plates: animation

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Global distribution of earthquakes

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Phenomenology •  Earthquakes generate different types of waves

•  The strength and characteristics of these waves can be recorded by seismographs

•  From these measurements their location, the focus and the epicentre, can be determined

•  The strength of an earthquake can be measured by magnitude and intensity

•  The Richter scale measures the energy of the seismic waves (open logarithmic scale)

•  The Mercalli scale measures the intensity or effect on the surface of the earth (descriptive scale)

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Mercalli vs.Richter Scale

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Seismographs and seismograms •  The seismograph utilises the inertia of a mass (e.g. a heavy

metal ball) relative to the moving ground

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Earthquake hazards Direct hazards •  Total or partial collapse of structures •  Falling debris and dust from rubble •  Transportation casualties due to collapse of bridges etc. •  Floods from collapsed dams or river banks •  Landslides and soil liquefaction •  Tsunamies

Indirect Hazards •  Fires •  Release of hazardous material •  Electrocution •  Exacerbation of pre-existing hazardous situations

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Earthquake impacts •  Total or partial

destruction of structures •  Blockage or interruption

of transport systems •  Interruption of water, gas

and electricity supplies •  Breakage of sewage

systems •  Interruption of land-use

due to landslides or inundation

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Can they be predicted ?

•  Science cannot yet predict earthquakes as to time or location of their occurrence

•  We can only predict probabilities for regions and time spans

•  For instance, the United States Geological Survey (USGS) calculates a probability of 67% for a major earthquake to occur in the San Francisco area within the next 30 years

•  Research to understand possible warning signals is ongoing

•  ‚Urban myths‘: – Earthquake weather/season

– Animals or certain people can sense an oncoming earthquake

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Mitigation measures

•  Measures and strategies to mitigate the effects and impacts

•  Earthquake resistant buildings/ infrastructure

– Lightweight construction – Cross-bracing – Decoupling – High-strength door-frames – Brick buildings are

unsuitable •  Emergency preparedness •  Behavioural advice, e.g.

„drop-cover-hold on“ (USA)

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Emergency preparedness

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Earthquakes - socio-economic impact After the event •  Desaster relief costs •  Lost economic opportunities •  Loss of land-use due to landslides etc. •  Cost of rebuilding houses and infrastructure •  Disruption of societies •  Health impacts due to traumatisation and poorer healthcare

Precautionary measures •  Emergency preparedness costs •  Higher cost of safer building practices / retrofitting •  People / companies avoid earthquake zones - lost

employment / economic opportunities

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Volcanoes

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What is a volcano ? •  An opening, or rupture, in the Earth's crust that allows hot

magma, ash and gases to escape from below the surface •  Volcanoes occur

– along plate boundaries – above mantle plumes (hot spots) within plates – as non-hotspot intraplate volcanism where a thinning of the Earth‘s

crust occurs

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Typical occurence of volcanoes

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The classical volcano: Stratovolcano

•  Examples are Vesuvius, Aetna, Stromboli, Fuji

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Volcano hazards

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Arctic Vulcanos

•  Sub-glacial vulcanos can pose the added risk of causing glacier surges, e.g. on Iceland

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Volcanic eruption impacts •  Earthquakes •  Lava/mud flows destroy houses /

infrastructure •  Pyroclastic flows (up to 700 km/h, 1000°C)

cannot be escaped •  Volcanic bombs •  Ash rain suffocates animal and plant life •  Large-scale eruptions eject ash into the

strathosphere, where it can circulate for years and change the global climate (e.g. Krakatau, 1883)

•  Tsunamis •  Poisonous and/or suffocating exhalations

(H2S, SO2, CO2) •  Aerosols causing health problems •  Acid precipitation •  Glacier surges and melt-water torrents

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Forecasting volcanic activities

•  Most volcanoes (on land) and zones with volcanic activity are well known

•  These zones are monitored

•  Monitoring for seismic activity

•  Precision geodesy to detect surface distortions

•  Monitoring of effluent gas composition

•  However, predicting the precise period for an eruption is difficult with the risk of false alarms or too late evacuation

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Volcanic risk mitigation

•  Mapping of hazard/danger zones

•  Land-use restriction in danger zones

•  Emergency preparedness plans

•  Monitoring seismic/volcanic activities

•  Evacuation when eruptions are iminent

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Mapping hazard zones

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Socio-economic impacts •  Cost of mitigation measures •  Cost of emergency relief operations •  Cost of re-building houses and infrastructure •  Cost of reforestation •  Cost of slope stabilisation measures •  Loss of farmland •  Exacerbated health problems due to poorer healh care and

exposure to health hazards (e.g. dust) •  Disruption of economic activities (e.g. the Philipine GDP fell by

3% in the years after the Mt. Pinatubo eruption) •  Disruption of social life •  But also benefits, such as geothermal energy on Iceland

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Tsunamis

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The Japan tsunami on 11 March 2011

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The Japan tsunami on 11 March 2011

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What is a tsunami ?

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Mechanisms of tsunami generation

•  Displacement of rock masses cause displacement of water

•  Root causes can be –  earthquakes, –  seabed slides –  volcanic eruptions –  cliff collapse –  iceberg calving

Before the earthquake

Earthquake

Tsunami spreads out

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The travel of the tsunami of 26/12/2004

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Tsunami early warning systems •  Following the tsunami that hit various regions around the Indian

Ocean on 26/12/2004, the efforts to set up early warning systems were considerably intensified.

•  Early warning systems integrate the world-wide network of seismic stations with oceanic observation stations

•  Like earthquakes, tsunamis cannot be predicted, but there are often several hours before a tsunami hits a coast and its spreading can be predicted

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Tsunami emergency preparedness

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Tsunami risk maps

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Socio-economic impacts •  Cost of mitigation measures •  Cost of emergency relief operations •  Cost of re-building houses and

infrastructure •  Cost of re-building coastal

infrastructure •  Loss of fishing grounds and

infrastructure, e.g. fishfarms •  Exacerbated health problems

due to poorer healh care •  Disruption of economic activities •  Disruption of social life

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Isostatic processes

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Definition

•  Can have endogenic and/or exogenic causes

•  A collection of processes that result in changes of the mean sea level

•  The mean sea level depends on the volume of the oceanic water

•  The mean sea level depends on the topography of the ocean floors

•  The topography of the ocean floors has changed significantly over geological timescales

•  Isostatic rebound following the ice-age is a major mechanism e.g. in Northern Europe

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Mean sea level

•  Local mean sea level (LMSL) is defined as the height of the sea with respect to a land benchmark, averaged over a period of time (such as a month or a year) long enough that fluctuations caused by waves and tides are smoothed out

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Isostatic rebound

•  The continental crust floats on top of the mantle •  Changes in load on the continents results in variations in

‚draft‘ and dipping •  The appearance and retreat of continental ice sheets are such

a change in load

before glaciation during glaciation after glaciation

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Large-scale uplift of continents

•  In some areas accompanied by subsidence of neighbouring areas

•  For instance, The Netherlands are sinking

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Hazards and impacts •  Slow, long-term movements in the order of 0.1 to 10 mm/year

•  Rising areas: – Receding coastline – Drying-up of harbours

•  Subsidence areas: – Increasing probability of flooding – Increase of coastal erosion – Need to improve coastal defence measures – Considerable socio-economic impact e.g. in the

Netherlands – Salination of near-coast groundwaters

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Next lesson

•  Natural radioactivity •  Exogenic hazards