Name: _______________________
Date: ________________________
Laboratory Exercise 15
Natural Hazards
Introduction
"The costs of natural disasters, lives lost, homes destroyed, economies disrupted, have skyrocketed
in this century," reported USGS Director Chip Groat in 1999. A natural disaster means that a substantial
number of deaths or economic losses occurred as a result of a natural hazard. We can’t prevent
earthquakes, hurricanes, tornadoes, or volcanoes; no place on Earth is safe from natural hazards. Natural
hazards can occur anywhere and often without warning, and understanding the spatial and temporal
distributions of natural hazards may save your life.
Objective
Upon completion of this lab, you will be able to:
• Understand when and where natural hazards can occur
Spatial Distribution
Some hazards have very specific spatial distributions. For example, we know the locations of all
volcanoes across the globe, and have identified all major faults in the Earth’s crust. Subsequently,
geologists have a good understanding of where volcanic eruptions and earthquakes can occur in the United
States.
However, most atmospheric hazards can impact widely different areas. Hurricanes are limited to
coastal areas, but hurricane remnants can travel hundreds of miles inland. Blizzards are more common in
high latitudes, but snow occurs in all 50 states, and it may only take a little bit to wreak havoc. Floods can
occur in all parts of the country, but major floods tend to happen near major rivers and the adjacent lowlying areas. Lightning and tornadoes are restricted to thunderstorms, but thunderstorms can occur
anywhere in the country. The key to understanding the spatial distribution of hazards lies in identifying
and assessing the causation of the hazard.
Temporal Distribution
Some hazards are seasonal, while others seem random. The Atlantic hurricane season runs from
June 1 through November 30, but blizzards tend to happen in the winter. Thunderstorms, and the
associated lightning and tornadoes, are more frequent during certain times of the year (spring and early
summer), but it is not uncommon for strong thunderstorms to occur during any month of the year. By
comparison, volcanoes and earthquakes rarely occur; due to poorly understood subterranean forces,
seasonality does not apply to these hazards. Instead, a statistical return interval is used to understand the
frequency of these events.
A return interval is the statistical calculation for reoccurrence of the hazard. The return interval on
a particular fault of smaller earthquakes may be more frequent than for a larger earthquake on the same
fault. Return intervals are commonly applied to blizzards, hurricanes, thunderstorms, floods, and even
damaging winds (Figure 15.1). Return intervals trace the frequency and intensity of the hazardous events,
helping determine potential trends and possibly building a predictive measure to the periodicity of these
events.
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Figure 15.1. Percentage of likelihood of damaging winds in Florida.
Lightning
At any given minute, there are 6,000 bolts of lightning striking somewhere on the Earth.
Lightning is a discharge of static electricity between the ground (usually positively charged) and a storm
cloud base (usually negatively charged). Lightning cannot occur in the absence of a thunderstorm, and
thunder always follows lightning, although thunder may not be heard if the storm is far away or winds
direct the sound in the opposite direction.
In the United States, an average of 100 people are killed every year due to lightning strikes.
Lightning is easily avoided; if you can hear thunder, then you can be struck by lightning. When a
thunderstorm is in the vicinity, it is unsafe to be outside, especially around tall objects. The safest shelter is
inside an enclosed building. If you cannot reach shelter, remain as low to the ground as you can while
touching as little of the ground as you can (i.e. stay on your toes and squat down).
Since light travels faster than sound, we always see lightning before hearing the thunder. While
we see the lightning nearly instantaneously, the sound travels at a speed of 330 meters per second (1100
feet per second; roughly one-fifth of a mile each second). The National Weather Service advocates the
30/30 rule for lightning safety: If thunder is heard within 30 seconds of the strike, seek shelter
immediately, then wait 30 minutes after the last lightning strike before leaving shelter.
1. If you hear thunder 10 seconds after seeing lightning, how far away is the bolt? _______________
meters _______________ miles
Figure 15.2. (source: noaa.gov)
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2. Looking at Figure 15.2, why is her hair standing on end? Is she in danger?
3. Is there a spatial or temporal distribution to lightning?
Tornadoes
Over 90% of all tornadoes on Earth occur in the United States (Figure 15.3), hosting an average of
1000 tornadoes each year. Small in spatial scale, the damage caused by these violently rotating columns of
air can be incredible. While tornadoes are most common in the central part of the country during the
spring, they can occur any time of the year at almost any location in the country. Tornadoes are triggered
from large, severe storms common along strong frontal boundaries.
A tornado outbreak occurs when atmospheric conditions are right to spawn several tornadoes. The
tornado outbreak of May 4-10, 2003, triggered 384 tornadoes over 19 states, and caused 42 deaths. The
Super Outbreak of April 3-4, 1974 produced 147 tornadoes in 13 states in 16 hours; 307 people lost their
lives during the Super Outbreak. Forecasts and warnings have improved substantially in the last 30 years,
undoubtedly saving hundreds of lives.
Figure 15.3. Tornado risk areas in the United States.
1. Why would the frequency of tornadoes increase during the spring?
2. What controls the spatial distribution of the tornado risk areas in the United States?
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Hurricanes
Hurricanes are intense low-pressure systems. Generating winds greater than 74 miles per hour,
hurricanes develop over warm tropical waters and move poleward as directed by upper-level steering
winds. An average of two hurricanes annually make landfall along the East Coast (Figure 15.4).
The most expensive natural disaster in US history, Hurricane Andrew caused over $30 billion in
damage and killed 61 people as it made landfall in southern Florida in August, 1992. The deadliest natural
disaster in US history was a result of the Galveston, TX hurricane in September, 1900. Estimates of 6,000
to 10,000 people died from the 20-foot storm surge that inundated the city of Galveston. The homes of
more than 20,000 people were destroyed in the one-day event.
Figure 15.4. Hurricane risk areas in the United States.
1. Why is hurricane activity in the United States constrained to coastal areas along the Gulf of Mexico and
the Atlantic Ocean, and not the Pacific Ocean?
Floods
Flooding is a major natural hazard, causing more loss of life than hurricanes, tornadoes, and
lightning combined. It can occur in any stream system and can be deadly for people living in floodplains.
The 1993 flood of the Mississippi River was the most devastating flood in United States history, causing
$20 billion in damage and record-breaking discharges (Figure 15.5). Five Midwest states were impacted by
this flood, which inundated low-lying areas and damaged or destroyed everything in its path. Any area
within a floodplain is susceptible to flooding.
A flash flood is different from a stream system flood. A flash flood is a sudden, short-lived torrent
of water that exceeds the capacity of a stream channel or drainage system. Flash floods are extremely
dangerous in urban areas and desert regions where infiltration rates are low. In 1995, the Arizona State
Legislature passed the “Stupid Motorist Law” (Article 28-910) to provide consequences for motorists that
ignored barricades and drove through temporarily flooded washes, usually necessitating their subsequent
rescue.
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Figure 15.5. Flooding risk areas in the United States per major flood events from 1993-1997.
1. What type of flooding usually occurs in Arizona?
2. What contributes to the frequency of flood events in the Midwest?
Blizzards
Each year, from October to April, there are over 100 snowfall events in the United States. Some
cause minor traffic inconveniences, while other events plunder entire regions. The March, 1993 Storm of
the Century was such an episode, overwhelming the entire eastern seaboard for several days with frigid
temperatures, heavy snow, and strong winds. The Storm of the Century ultimately caused over $5 billion
in damage and more than 270 deaths.
The exact name and type of winter storm depends on the geographic location (Figure 15.6). The
mountainous west can have heavy snows that last for days, producing up to ten feet of snow at one time,
and several hundred inches over one season. Lake-effect snow occurs in locations immediately downwind
of the Great Lakes, at times lasting for several days and producing several feet of snow during that time.
The Great Plains are home to powerful blizzards that produce up to 20 inches of snow coupled with winds
exceeding 35 mph and temperatures well below freezing. Nor’easters like the “Storm of the Century”
populate the East Coast from North Carolina to Maine.
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Figure 15.6. Snow risk areas in the United States.
1. What is the spatial control on hazardous snow events in the United States?
Volcanoes
In 1980, the composite volcano Mount St. Helens in the Cascade Range in southwest Washington
violently erupted, spewing ash as far as Idaho, snapping trees like twigs, destroying buildings in a 230-mile
radius, and killing over 50 people. Seventeen separate pyroclastic flows, intense avalanches of hot gases
and ash fragments, occurred during the Mount St. Helens eruption. Continental volcanoes tend to form
along plate subduction zones (Figure 15.7), which provide a ready source of highly explosive magma.
Volcanoes usually issue several, sometimes benign, warnings of impending eruption. These
warnings include earthquake swarms and earth deformations, which signal a potentially dangerous buildup
of pressure below the crater. Recent advances in the detection of these signals have saved countless lives.
Figure 15.7. Volcanic risk areas in the United States.
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1. Why aren’t there volcanic hazards mapped in the eastern United States?
Earthquakes
Approximately 95% of all earthquakes are caused by a buildup of pressure along tectonic plates
boundaries (Figure 15.8). An earthquake is a sudden release of that energy along a fault line. The focus, or
location of the earthquake, is usually tens to hundreds of miles below the surface, and the earthquake’s
epicenter is the location on the surface directly above the release. Shallow earthquakes are the most
dangerous, causing the most destruction at the surface. The USGS locates around 18,000 to 20,000
earthquakes annually across the globe, about 50 per day.
The April 18, 1906 earthquake in San Francisco killed more than 3,000 people and caused
approximately $524 million in damages. While the shaking lasted only a minute, it was of such an
intensity that the streetcar tracks bent into wavelike forms and sewers and water mains broke. Only $20
million in damage is attributed to the earthquake; the remaining damage occurred from the resulting fires
that raged throughout the city, uncontrolled. The only water supply to the city was destroyed during the
earthquake, and streets were blocked from collapsed structures, preventing fire control.
While we cannot predict earthquakes, we can significantly mitigate their effects by identifying
earthquake potential, building safer structures, and providing education on earthquake safety. San
Francisco learned from the 1906 earthquake, and now their water supply is aligned along a gridded
network. This design proved effective in the 1989 Loma Prieta earthquake, undoubtedly saving many lives
and structures in San Francisco.
Figure 15.8. Earthquake risk areas in the United States.
1. What controls the spatial distribution of earthquakes in the western United States?
Landslides
Lethal mixtures of water, rocks, and mud occur in every state, causing approximately $2 billion in
damage and an average of 25 deaths annually. Any area of weak or fractured rock is susceptible, but places
where erosion or vegetation removal weakens slopes, heavy rain or snowmelt saturates slopes, or triggers
like earthquakes and volcanoes destabilize slopes are places where landslides can occur (Figure 15.9).
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Figure 15.9. Landslide risk areas in the United States.
1. What topographic feature controls the spatial distribution of landslides in the United States?
GIS
Geographic information systems (GIS) are digital management databases that allow the user to
graphically overlay the spatial distribution of different features. The user can then easily identify
significant relationships between the features. In Figure 15.3, the GIS overlay could provide the user with
the quickest bus route to the closest shopping center.
Figure 15.3. GIS allows overlays of spatial features.
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1. Carefully overlay the different hazard maps from the previous sections onto your map of the United
States. Use a different color for each hazard, and create a legend for your hazards. Place a dot on your
hometown, or where you want to live after graduation.
Using your completed map, answer the following questions regarding natural hazards in the United States.
2. Which location in the United States seems safest from natural hazards? Why?
3. Which location in the United States seems at greatest risk from natural hazards? Why?
4. What other natural hazards occur in the United States that aren’t depicted on your hazard map?
5. What types of hazards is your hometown susceptible to?
6. Re-read the introduction section. Write a paragraph answering the question: Given the premise that the
rates of natural hazard events aren’t increasing, why have natural disasters skyrocketed? Consider the
following when writing your paragraph: natural hazard temporal and spatial distributions, impacts of
climate change, population levels and settlement patterns, and advances in technology.
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