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Natural Hazards Summary 2021/2022
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Natural Hazards summaryIntroduction to Natural Hazards
Natural process: the physical, chemical and biological ways by which events, such as volcanic
eruptions and erthquakes affect Earth’s surface, internal (tectonic plats) or external (energy from the
sun)
Natural hazard: a natural process and event that is a potential threat to human life and property
Disaster: a hazardous event that occurs over a limited time span within a defined area
Criteria for a natural disaster are:
1. 10 or more people are killed 3. A state of emergency is declared
2. 100 or more people are affected 4. International assistance is requested
Catastrophe: a massive disaster that requires significant expenditure of money and a long time (ofter
years) for recovery to take place
The effects of natural hazards change with time because of changes in patterns of human land use,
as well as climate change → Urban growth, marginal lands, change the physical properties of earth
materials → Warming oceans causing an increase in storm intensity (more energy), rising sea levels,
drought.
To understand the natural processes we call hazard, some background knowledge (history) of the
geologic cycle processes that produce and modify earth materials, such as rocks, minerals and water
is necessary.
Geologic cycle: a group of interrelated sequences of Earth processes known as the hydrologic, rock,
tectonic and biogeochemical cycles
Tectonic cycle: involves the creation, movement and destruction of tectonic plates, production and
distribution of rock and mineral resources as well as volcanoes and earthquakes
Rock cycle: the largest of the geologic subcycles and it is linked to all the other subcycles. Tectonic
cycle for heat and energy, biogeochemical cycle for materials, hydrologic cycle for water →
weathering, erosion, transportation, deposition and lithification of sediment
Three types of rocks: igneous rock, sedimentary rock and metamorphic rock
Hydrologic cycle: the movement of water from the oceans tot he atmosphere and back again, water
is stored in compartments as oceans, atmosphere, rivers, streams, groundwater, lakes, ice caps and
glaciers
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,Biogeochemical cycle: the transfer or cycling of a chemical element or elements through the
atmosphere (the layer of gases surrouding Earth), lithosphere (Earth’s rocky outer layer),
hydrosphere (oceans, lakes, rivers and groundwater) and biosphere (the part of Earth where life
exists) → related to the tectonic, rock and hydrologic cycles → tectonic water, heat and energy to
form and change the earth materials transferred in biogeochemical cycles, rock and hydrologic for
transfer and store chemical elements in water, soil and rock
5 Fundamental concepts for understanding natural processes as hazards
1. Science helps us predict hazards
Natural hazards, such as earthquakes, are natural processes that can be identified and studied using
the scientific method. Most hazardous events and processes can be monitored, mapped and their
furture activity predicted, on the basis of frequency of past events, patterns in their occurrence and
types of precursor events.
Uniformitarianisme (James Hutton): concept that the present is key tot he past; that is, we can read
the geologic record by studying present processes, processes we observe today also operated in the
past
Environmental unity: one action causes others in a chain of actions and events, for example
removing native vegetation from a steep slope may lead to a landslide, to obstruction of a stream…
Prediction: with respect to a hazardous event, such as an earthquake, the advance determination of
the date, time, and size of the event
Forecast: with respect to a natural hazard, an announcement that states that a particular event, such
as a flood or storm, is likely to occur during a particular time interval, often with some statement of
the degree of its probability
The effects of a hazardous event can be reduced if we can forecast or predict it and if we can issue
a warning, this involves the following elements:
- Identifying the location where a hazardours event is likely to occur
- Determining the probability that an event of a given magnitude will occur
- Observing any precursor events
- Forecasting or predicting the event
- Warning the public
2. Knowing hazard risks can help people make decisions
Hazardous processes are amenable to risk analysis, which estimates the probability that an event will
occur and the consequences resulting from that event. Estimating the risk for hazard scenarios is a
proactive step in minimizing impacts of hazards.
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,Risk: from a natural hazards viewpoint, risk may be considered as the product of the probability of an
event times the consequences
Wicked problem: resistance to a solution
3. Linkages exist bewteen natural hazards
Hazardous processes are linked in many ways, for example tsunamis and hurricanes often cause
flooding and coastal erosion.
4. Human scan turn disastrous events into catastrophes
As a result of increasing human population and poor land-use practices, events that uased to cause
disasters are now often causing catastrophes.
The impact of a hazardous event is in part a function of the amount of energy released, that is it, its
magnitude and the interval between occurrences, that is its frequency
Magnitudefrequency concept: the idea that the intensity and extent of an event are inversely
proportional to how often it occurs
5. Consequences of hazards can be minimized
Minimizing the consequences and effects of natural hazards requires an integrated approach that
includes scientific understanding, land-use planning and regulation, engineering and proactive
disaster preparedness.
Land-use planning: the preparation of an overall master plan for future development of an area; the
plan may recommend zoning restrictions and infrastructure both practical and appropriate for the
community and its natural environment; based on mapping and classification of existing human
activities and environmental conditions, including natural hazards.
Natural service function: a benefit that arises from an event caused by nature that is also a hazard to
people or the environment
Chapter 4: Tsunamis
Introduction to Tsunamis
Tsunamis: (the Japanese word for ‘large harbor waves’) are produced by the sudden vertical
displacement of ocean water → triggered by earthquake (underwater landslide), collapse of part of a
volcano, a submarine volcanic explosion and an impact in the ocean of an extraterrestrial object
(asteriod or comet)
Four-stage process from earthquake to tsunami
1. Earthquake rupture in seafloor pushes water upwards staring the tsunami.
2. Tsunami moves rapidly in deep ocean reaching speeds of up to 900 km/hr (wave height <1m).
3. As the tsunami nears land, it slows to about 45 km/hr but is squeezed upwards, increasing in
height.
4. Tsunami heads inland destroying all in its path (trough of wave may arrive first, exposing seafloor.
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, Runup: refers tot he furthest horizontal and vertical distance that the largest wave of a tsunami
moves inland, varies considerably with the shape of the seafloor immediately offshore and with the
type of topography and vegetation landward of the beach
Distant tsunami: a series of gravity waves in the ocean originating from a source typically thousands
of kilometers away from the shoreline that is inundated. Travels out across the ocean. These waves
are produced by the displacement of the entire water column of the ocean by for example an
underwater earthquake
Local tsunami: a series of gravity waves in the ocean originating from a source typically thousands of
kilometers away from the shoreline that is inundated. Heads towards nearby land. These waves are
produced by the displacement of the entire water column of the ocean by for example an
underwater earthquake
Landslides have both generated and contributed to huge tsunamis. These landslide scan take place
underwater (submarine landslides) or they can be large rock avalanches that fall from moutains into
the sea.
Geographic Regions at Risk from Tsunamis
The heightened risk from tsunamis comes from the geographic location of a coast in relation to
potential tsunami sources. Coasts in near proximity to a major subduction: considered to be capable
of periodically generating a major earhquake of M9 or greater, zone are at greatest risk.
Why is the tsunami bigger in some places?
Offshore: bathymetry and shelf slope
Onshore: topography and vegetation
(low relief, more destruction)
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