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Ch 1: intro to natural hazards
Most tsunamis are triggered at SUBDUCTION ZONES — long, narrow strips of Earth’s lithosphere
(outermost layer containing the crust and upper mantle) where one tectonic plate moves beneath another.
2 tsunami warning signs:
1) Strong ground-shaking close to the epicenter
2) A rapid withdrawal of the sea to very low levels
Deﬁning and minimizing disasters
Natural hazards are caused by both internal (from inside Earth) and external processes (from the sun).
PROCESSES are the ways in which events a!ect the surface of the Earth.
HAZARD = any natural process that threatens human life or property
RISK = the probability of a hazardous event times the expected damage
DISASTER/CATASTROPHE = events that cause serious injury, loss of life, and property damage in a brief
amount of time over a speciﬁc area (catastrophe tends to be more severe and takes longer to recover from)
MITIGATION = e!orts to prepare for disasters to minimize their e!ects
What we can do to minimize impact of disasters:
•Improve existing warning systems and install new ones
•Improve communication infrastructure and chain-of-command protocol so emergency o"cials in coastal
communities are warned ASAP
•Public education program ➞ where to go, where to take, how to get into
•Evacuation routes should be publicized + marked by signs
Role of time in understanding hazards
Hazards are recurrent events. Thus, by combining our knowledge of a history of past events with a
surveying of geologic features created by the past disasters, our hazard forecasts and warnings will
become more accurate.
The geologic cycle
The 4 associated sequences of the Earth processes that produce the minerals, fuels, land, water, and
atmosphere needed for survival through physical, chemical, and biological forces. They are all include a
renewal process, are driven by energy, and are cyclic. They do not all relate to the movement of the Earth’s
The tectonic cycle
The creation, movement, and destruction of tectonic plates. Oceanic plates are denser than continental
plates and thus will always move underneath continental plates.
DIVERGENT BOUNDARIES: two plates pulling away from each other. If its in the ocean, then it is known as
SEAFLOOR SPREADING, where magma comes up to create new crust.
CONVERGENT BOUNDARIES: two plates collide into each other, usually resulting in a subduction zone. If an
oceanic plate goes under a continental plate, then the oceanic plate heats up as it moves under and
releases gases that melt the lower crustal rocks. Magma moves up slowly until it reaches the surface and
erupts, building volcanoes. Colliding plates with similar density usually produces mountains. Ocean plates
— older one subducts younger one
September 2010 Elaine Jessica Giang
SUBDUCTION adds material to continents (sediments/rock/crustal fragments on the subducting plate are
added to the continent.
TRANSFORM (shear/transverse) BOUNDARY: two plates slide horizontally past each other.
HOTSPOTS: an almost stationary source of heat within the mantle (mantle is just below crust) that
produces volcanoes at speciﬁc, ﬁxed points of Earth’s surface as plates move over it.
The rock cycle
The recycling of 3 major groups of rocks; it is driven by energy from the core and sun.
IGNEOUS ROCK: crystallization of molten rock.
SEDIMENTARY ROCK: sediment is produced as rocks at/near the surface break down chemically and
physically (WEATHERING). LITHIFICATION then compacts and cements these sediments into sedimentary
METAMORPHIC ROCK: the morphing of rock by chemical ﬂuids, pressure, or heat (naturally occurring
agents). If sedimentary rock is buried deep enough, it will melt and thus the potential for the cycle to start
all over again is renewed.
The hydrologic cycle
The cycling of water form the oceans to the atmosphere, continents and islands, and back to the oceans. It
is driven by solar energy and by evaporation, precipitation, surface runo!, and subsurface ﬂow.
RESIDENCE TIME: avg. amount of time that a drop of water spends in any one compartment
The transfers of cycling of elements through the atmosphere, lithosphere, hydrosphere, and biosphere; it
is intimately related to the other cycles — tectonic provides water, gases, heat, and energy for transferring
dissolved solids in gases, aerosols, and solutions. The other cycles transfer and store chemical elements
by a series of compartments/reservoirs (water, soil, and rock).
Fundamental concepts for understanding natural hazards
1. Hazards can be predicted through scientiﬁc analysis; most can be predicted from a history of similar
events, patterns in their occurrences, and by warning events that precede them
2. Risk analysis is an important part of understanding the e!ects of a hazard; you can use risk analysis to
assess the probability that a hazard will occur and what its severity will be. This helps people decide
whether or not living in a hazard prone area is worth the risk.
3. Natural hazards are linked together and to the physical environment; one hazard will often produce
another. I.e. Loose rocks are more prone to landslides than granite.
4. Damage from natural disasters is increasing due to population growth, property development in
dangerous areas, and poor land-use practices. Higher magnitude events occur less often and lower
magnitude events occur a lot [inverse relationship].
5. Damage from disasters can be minimized by using science, good land-use planning and regulation,
engineering, and being prepared and proactive
September 2010 Elaine Jessica Giang
Hazards are natural
Attempts to control hazards (like ﬂood control) may amplify its hazards. The best way to mitigate hazards
is to identify their location, and to avoid putting people and property there.
Prediction and warning
To forecast and event and issue a warning, we must ﬁrst:
1. Locate the hazard (by creating maps of geological features, past history of hazards, etc)
2. Estimate the probability that it will occur (using history of events + current geography)
3. Identify any precursor events that may signal the hazard
4. Try to predict when the hazard will occur
5. Warn the people — even if it doesn’t occur, communities will be left o! better prepared and informed
Minimizing damage from hazards
Proactive planning is just as important as our actions right after a hazard:
1) Emergency Work — normal activities have ceased or changed. Focus on search and rescue, clearing of
rubble, and repairing roads and utilities.
2) Restoration — normal activities return, refugees return, rubble is mostly cleared
3) Reconstruction I — bridges, buildings, and other structures are rebuilt with hazards in mind. Normal
activities are at pre-disaster levels of greater
4) Reconstruction II — activities improved and developed, major construction projects are completed.
Proactive: avoiding and adjusting to hazards
Good land use planning ➞ not building near hazard-prone areas, requiring stricter building regulations to
reduce potential damage from landslides, earthquakes, etc. Also being prepared: knowing evacuation
routes, having survival kits ready, educating yourself on disasters.
Return period: RP
RP is the average amount of years that pass between hazards. Determined by dividing the time span of
data by the number of hazards that has occurred.
I.e.. 70 years/2 (for number of ﬂoods that occurred during those years) = 35 year ﬂood
Some hazards concentrate energy for a long time and release it all very fast (earthquakes, volcanoes,
hurricanes, storms, landslides). Others form in a very short time and dilute their energy over a long period
of time (tsunamis, ﬂoods).
Composition of the Earth, ocean, and atmosphere
The Earth’s core is mostly iron and nickel; the crust is oxygen and silicon. The ocean’s made mostly of
oxygen and hydrogen, while the atmosphere is composed of nitrogen and oxygen.
VISCOSITY: a measure of how much ﬂuids resist changing their shape (the greater the viscosity the more it
resists change & thus needs more force to change it). Air has low viscosity.
COMPRESSIBILITY: how much an object can be squeezes d or expanded so that it ﬁlls more or less space.
Results in a change in density (mass/volume) because of the volume change.
September 2010 Elaine Jessica Giang