To experience the plate tectonics--the jostling of giant plates that carry
continents and oceans--try this experiment:
Hold your hand in front of you and watch you finger nails grow. That is
roughly the average speed of a tectonic plate. Even at this slow rate,
over time movement can produce dramatic and deadly consequences.
Plate tectonics tells us that the Earth's rigid outer shell, or
lithosphere, is broken into a mosaic of oceanic and continental plates
which can slide over the plastic aesthenosphere, which is the uppermost
layer of the mantle. The plates are in constant motion. Where they
interact, along their margins, important geological processes take place,
such as the formation of mountain belts, earthquakes, and volcanoes.
The lithosphere covers the whole Earth. Therefore, ocean plates are also
involved, more particularly in the process of sea-floor spreading. This
involves the midocean ridges which are a system of narrow submarine
cracks that can be traced down the center of the major oceans. The ocean
floor is being continuously pulled apart along these midocean ridges. Hot
volcanic material rises from the Earth's mantle to fill the gap and
continuously forms new oceanic crust. The midocean ridges themselves are
broken by offsets know as transform faults.
Map of the Earth's Plate Boundaries
Plate tectonics revealed that there are four types of seismic zones. The
first follows the line of midocean ridges. Activity is low, and it occurs
at very shallow depths. The lithosphere is very thin and weak at
these boundaries, so the strain cannot build up enough to cause large
earthquakes. Associated with this type of seismicity is the volcanic
activity along the axis of the ridges (for example, Iceland, Azores,
Tristan da Cunha).
The second type of earthquake associated with plate tectonics is the
shallow-focus event unaccompanied by volcanic activity. The San Andreas
fault is a good example of this. In faults like this, two mature plates
are scraping by one another. The friction between the plates can be
so great that very large strains can build up before they are periodically
relieved by large earthquakes. Although, activity does not always occur
along the entire length of the fault during any one earthquake. For
instance, the 1906 San Francisco event was caused by breakage only along
the northern end of the San Andreas fault.
The third type of earthquake is related to the collision of oceanic and
continental plates. One plate is thrust or subducted under the other plate
so that a deep ocean trench is produced. This type of earthquake can be
shallow, intermediate, or deep, according to its location on the downgoing
lithospheric slab. Such inclined planes of earthquakes are know as Benioff
The fourth type of seismic zone occurs along the boundaries of
continental plates. Typical of this is the broad swath of seismicity
from Burma to the Mediterranean, crossing the Himalayas, Iran, Turkey, to
Gilbraltar. Within this zone, shallow earthquakes are associated with high
mountain ranges where intense compression is taking place.
Intermediate-and deep-focus earthquakes also occur and are known in
the Himalayas and in the Caucasus. The interiors of continental plates are
very complex, much more so than island arcs. For instance, we do not yet
know the full relationship of the Alps or the East African rift system to
the broad picture of plate tectonics.
Recycling of Crustal Material
If the Earth was not to be blown up like a balloon by the continual influx
of new volcanic material at the ocean ridges, then old crust must be
destroyed at the same rate where plates collide. The required
balanced occurs when plates collide, and one plate is forced under the
other to be consumed deep in the mantle, a process kown as plate
It is now known that there are seven major crustal plates, subdivided into
a number of smaller plates. They are about 80 kilometers thick, all in
constant motion relative to one another, at rates varying from 10 to130
millimeters per year.
Their pattern is neither symmetrical nor simple. As more and more are
learned about the major plates; many complicated and intricate maneuvers
are taking place.
We know that most large-scale geological action--such as the formation of
mountains, rift valleys, volcanoes, earthquakes, faulting--is due to
different types of interaction at plate boundaries.
Plate Tectonic Interactions
Relationship between Plate Tectonics and Earthquakes
The Earth is formed of several layers that have very different
physical and chemical properties. The outer layer, which averages about 70
kilometers in thickness, consists of about a dozen large, irregularly
shaped plates that slide over, under and past each other on top of the
partly molten inner layer. Most earthquakes occur at the boundaries where
the plates meet. In fact, the locations of earthquakes and the kinds of
ruptures they produce help scientists define the plate boundaries. There
are three types of plate boundaries: spreading zones, transform faults,
and subduction zones
Cross section of the Earth's Plate Tectonic Structure
At spreading zones, molten rock rises, pushing two plates apart and adding
new material at their edges. Most spreading zones are found in
oceans; for example, the North American and Eurasian plates are
spreading apart along the mid-Atlantic ridge.
Spreading zones usually have earthquakes at shallow depths (within 30
kilometers of the surface).
Transform faults are found where plates slide past one another. An example
of a transform-fault plate boundary is the San Andreas fault, along the
coast of California and northwestern Mexico. Earthquakes at transform
faults tend to occur at shallow depths and form fairly straight linear
Subduction zones are found where one plate overrides, or subducts another,
pushing it downward into the mantle where it melts. An example of a
subduction-zone plate boundary is found along the northwest coast of the
United States, western Canada, and southern Alaska and the Aleutian
Islands. Subduction zones are characterized by deep-ocean trenches,
shallow to deep earthquakes, and mountain ranges containing active
volcanoes. Earthquakes can also occur within plates, although
plate-boundary earthquakes are much more common. Less than 10 percent of
all earthquakes occur within plate interiors.
As plates continue to move and plate boundaries change over geologic time,
weakened boundary regions become part of the interiors of the plates.
These zones of weakness within the continents can cause earthquakes in
response to stresses that originate at the edges of the plate or in the
What is an earthquake?
An earthquake is the vibration, sometimes violent, of the Earth's surface
that follows a release of energy in the Earth's crust. This energy
can be generated by a sudden dislocation of segments of the crust, by a
volcanic eruption, or even by manmade explosions.
But the most common kind of quakes, the most destructive and the kind
people generally have in mind when we think of earthquakes are the ones
that are caused by the sudden dislocation of large rock masses along the
faults within the earth's crust. These are known as tectonic earthquakes.
A fault is a fracture within some particular rocky mass within the earth's
Fault sizes can vary greatly, as some faults can be miles long.
Earthquakes are caused by active faults, which are, faults along which the
two sides of the fracture move with respect to each other.
In short, an earthquake is caused by the sudden movement of the two sides
of a fault with respect to another.
There are three different groups of faults, depending on the way they move
(refer to diagram):
- Normal faults
These occur in response to pulling or tension: the overlying block moves
down the dip of the fault plane.
- Thrust (reverse) faults
These occur in response to squeezing or compression: the overlying
block moves up the dip of the fault plane.
- Strike-slip (lateral) faults
These occur in response to either type of stress: the blocks move
horizontally past one another.
The slow and continuous movement of two sides of an active fault relative
to one another can noticed over time; this movement is called fault slip.
The rate of this movement may be as little as a few inches or so per year.
The movement of these two sides of the fault cannot be an entirely smooth,
easy type of movement. We can infer the existence of conditions or forces
deep with the fault which resist this relative motion of the two sides of
the fault. This is because the motion along the fault is accompanied
by the gradual buildup of elastic strain energy within the rock
along the fault. The rock stores this strain like a giant spring being
Eventually, the strain along the fault becomes too much for the rock to
bear. The fault then ruptures, or suddenly moves a comparatively large
distance in a short amount of time. The rocky masses which form the
two sides of the fault then snaps into a new position. This snapping back
into position, upon the release of strain, is the elastic rebound.
The rupture of the fault also results in the sudden release of the strain
energy that had been built up over the years. The most important form that
this suddenly released energy takes is that of seismic waves.
What are Seismic Waves?
Seismic energy travels through the crust in the form of waves. There
are two basic kinds of seismic waves: body waves and surface waves. Body
waves travel outward in all directions, including downward, from the
quake's focus -- that is, the particular spot where the fault first began
to rupture. Surface waves, by contrast, are confined to the upper
few hundred miles of the crust. They travel parallel to the surface, like
ripples on the surface of a pond. They are also slower than body waves.
Following an earthquake, the body waves strike first. The fastest kind are
the primary waves, or P-waves. People often report a sound like a train
just before they feel a quake, which is the P-wave moving as an
acoustic wave in the air. P-waves can travel through solids, liquids
and gases. When these waves travel throught the air, it is called
sound waves. In most rocks, p-waves will travel about 1.7 and 1.8 times
faster than the secondary, or S-waves.
A person in a building perceives the arrival of S-waves as a sudden
powerful jolt, as if a giant has pounded his fist down on the roof.
Finally, the surface waves strike. In very strong earthquakes, the
up-and-down and back-and-forth motions caused by surface waves can make
the ground appear to roll like the surface of the ocean, and can
literally topple buildings over.
This wave can only travel through solids, and do no travel through the
The picture below shows how an S wave travels by vibrating up and down.
The black box shows how an area of rock deforms as the wave passes. The
hammer represents the initial release of energy at fault rupture
Intensity of an Earthquake
TheRichter Magnitude Scale
Seismic waves are the vibrations from earthquakes that travel through the
Earth; they are recorded on instruments called seismographs. Seismographs
record a zig-zag trace that shows the changing amplitude of ground
oscillations beneath the instrument. Sensitive seismographs, which greatly
magnify these ground motions, can detect strong earthquakes from sources
anywhere in the world. The time, location, and magnitude of an
earthquake can be determined from the data recorded by seismograph
stations. The richter scale was developed as a mathematical device
to compare the size of eathquakes.
The magnitude of an earthquake is determined from the logarithm of the
amplitude of waves recorded by seismographs. On the Richter Scale,
magnitude is expressed in whole numbers and decimal fractions. For
example, a magnitude of 5.0 might be computed for a moderate earthquake,
and a strong earthquake might be rated as magnitude 6.0. Each whole
number increase in magnitude represents a tenfold increase in
measured amplitude; as an estimate of energy, each whole number step in
the magnitude scale corresponds to the release of about 31 times more
energy than the amount associated with the preceding whole number
The Richter Scale is not used to express damage. An earthquake in a
densely populated area which results in many deaths and considerable
damage may have the same magnitude as a shock in a remote area that does
nothing more than frighten the wildlife. Large-magnitude earthquakes that
occur beneath the oceans may not even be felt by humans. Earthquakes
with magnitude of about 2.0 or less are usually called
microearthquakes; they are not commonly felt by people and are generally
recorded only on local seismographs. Events with magnitudes of about 4.5
or greater--there are several thousand such shocks annually--are strong
enough to be recorded by sensitive seismographs all over the world.
Great earthquakes, such as the 1964 Good Friday earthquake in
Alaska, have magnitudes of 8.0 or higher. On the average, one
earthquake of such size occurs somewhere in the world each year.
TheModified Mercalli Intesity Scale
The effect of an earthquake on the Earth's surface is called the
intensity. The intensity scale consists of a series of certain key
responses such as people awakening, movement of furniture, damage to
chimneys, and finally--total destruction. The current intensity scale
being used in the U.S. is the Modified Mercalli (MM) Intensity Scale.
The scale is composed of 12 increasing levels of intensity that range from
imperceptible shaking to catastrpic destruction. This scale does not
have a mathmatical basis, instread it is an arbitrary ranking system based
on observed effects.
The following is an abbreviated description of the 12 levels of
Modified Mercalli intensity (MMI). I.
Not felt except by a very few under especially favorable conditions.
Felt only by a few persons at rest, especially on upper floors of
Delicately suspended objects may swing.
Felt quite noticeably by persons indoors, especially on upper floors of
Many people do not recognize it as an earthquake.
Standing motor cars may rock slightly.
Vibration similar to the passing of a truck. Duration estimated.
Felt indoors by many, outdoors by few during the day.
At night, some awakened.Dishes, windows, doors disturbed; wallsmake
Sensation like heavy truck striking building. Standing motor cars rocked
Felt by nearly everyone; many awakened.
Some dishes, windows broken. Unstable
objects overturned. Pendulum clocks may stop.
Felt by all, many frightened.
Some heavy furniture moved; a few instances of fallen plaster. Damage
Damage negligible in buildings of good design and construction;
slight to moderate in well-built ordinary structures; considerable
damage in poorly built
or badly designed structures; some chimneys broken.
Damage slight in specially designed structures; considerable damage in
substantial buildings with partial collapse. Damage great in
poorly built structures.
Fall of chimneys, factory stacks, columns, monuments, walls. Heavy
Damage considerable in specially designed structures; well-designed
thrown out of plumb. Damage great in substantial buildings, with partial
Buildings shifted off foundations.
Some well-built wooden structures destroyed; most masonry and frame
destroyed with foundations. Rails bent.
Few, if any (masonry) structures remain standing. Bridges destroyed.
Rails bent greatly.
Damage total. Lines of sight and level are distorted. Objects thrown
into the air.
How do earthquakes affect
The dynamic response of the building to earthquake ground motion is the
most important cause of earthquake-induced damage to buildings. Ground and
soil failure beneath buildings is also a major cause of damage.
The sudden movement along the plane of faults within the earth's crust
releases enormous amount energy. This energy then travels through the
earth by seismic waves. The waves will travel great distances before
losing it's energy.
After the generation of the waves, it will reach the earth's surface and
set it in motion, which is refered to as, earthquake ground motion.
When the earthquake ground motion occurs beneath the building and is
strong enough, it will tranfer the waves and set the building into motion
as well. It starts with the building's foundation and then to the rest of
the building creating damages along the way.
Ground Motion And Building Frequencies
The most important characteristics of earthquake ground motions for
buildings are the duration, amplitude (of displacement, velocity and
acceleration) and frequency of the ground motion. Frequency is the number
of complete cycles of vibration made by the wave per second. We can
consider a complete vibration to be be the same as the distance between
one crest of the wave and the next, in other words one full
wavelength. Frequency is often measured in units called Hertz. If
two full waves pass in one second, the frequency is 2 hertz (abbreviated
as 2 Hz).
Response of the building to ground motion is as complicated as the ground
motion itself, yet typically quite different. It also begins to vibrate in
a complex manner, and because it is now a vibratory system, it also
possesses a frequency content. However, the building's vibrations tend to
center around one particular frequency, which is known as its natural or
fundamental frequency. So the shorter a building is, the higher its
natural frequency. The taller the building is, the lower its natural
Building Frequency and Period
Another way to understand this is to think of the building's response in
terms of it's natural period. The building period is simply the inverse of
the frequency: Whereas: the frequency is the number of times per second
that the building will vibrate back and forth, the period is time it takes
for the building to make one complete vibration.
The relationship between
T = 1 / f
This means that a short building with a high natural frequency also has
short natural period. A very tall building with a low frequency has a long
period. For example, it takes the Empire State Building a comparatively
long time to sway back and forth during a strong gust of wind.
How do buildings respond to
The absolute movement of the ground and building during an earthquake is
all that large at all, even during a major earthquake.
That means that the building does not undergo displacements that are large
compared to the building size itself. So it is not the distance that the
building moves that causes damage, instead, it is more of the sudden force
that causes the building to shift quickly that causes the building to
suffer damage. Think of someone pulling a rug from beneath you. If
they pull it quickly (i.e., accelerate it a great deal), then they dont
have to pull it very far to throw you off balance. On the other hand, if
they pull the rug slowly and gradually increase the speed of the rug, they
can move (displace) it and you a great distance without that same
In other words, the damafe that a building suffers is not because of its
displacement, but upon acceleration. Whereas displacement is the actual
distance the ground and the building may move during an earthquake,
acceleration is a measure of how quickly they change speed as they move.
During an earthquake, the speed at which both the ground and building are
moving will reach some maximum. The more quickly they reach this maximum,
the greater their acceleration.
Acceleration has this important influence on damage, because, as an object
in movement, the building obeys Newton' famous Second Law of
Dynamics. The simplest form of the equation which expresses the Second Law
of Motion is:
F = m a
This states the Force acting on the building is equal to the Mass
of the building times the Acceleration. So, as the acceleration of
the ground, and in turn, of the building, increase, so does the force
which affects the building, since the mass of the building doesn't change.
Of course, the greater the force affecting a building, the more damage it
will suffer; decreasing Fis an important goal of earthquake resistant
design. When designing a new building, for example, it is desirable to
make it as light as possible, which means, of course, that M,and in turn
,Fwill be lessened. There are also various techniques now for reducing A.
It is important to know that Fis actually what's known as an
inertial force, that is, the force created by the building's tendency to
remain at rest, and in its original position, even though the ground
beneath it is moving. This is in accordance with another important
physical law known as D'Alembert's Principle, which states that a mass
acted upon by an acceleration tends to oppose that acceleration in an
opposite direction and proportionally to the magnitude of the acceleration
(see picture above). This inertial force F imposes strains upon the
building's structural elements. These structural elements primarily
include the building's beams, columns, load-bearing walls, floors, as well
as the connecting elements that tie these various structural elements
together. If these strains are large enough, the building's structural
elements suffer damage of various kinds.
To help you understand the concept and process of inertial generated
strains, consider this example. Imagine a simple block of stone that is
perfectly rigid. During an earthquake, if this block is simply sitting on
the ground without any attachment to it, the block will move freely in a
direction opposite to the ground motion, with a force proportional to the
mass and acceleration of the block. If the same block, however, is solidly
founded in the ground and no longer able to move freely, it must in some
way absorb the inertial force internally. In the picture to the right,
this internal uptake of force is shown to result in cracking near the base
of the block .
The taller a building, the longer its natural period tends to be. But the
height of a building is also related to another important structural
characteristic: the building's flexibility. Taller buildings tend to be
more flexible than short buildings. (consider a thin metal rod. If it
is very short, it is difficulty to bend it in your hand. If the rod
is somewhat longer, and of the same diameter, it becomes much easier to
bend. Buildings behave similarly.) So we conclude that a short
building is a stiff, while a taller building is flexible.
Stiffness greatly affects the building's uptake of earthquake generated
force. Reconsider the example above, of the rigid stone block deeply
founded in the soil. The rigid block of stone is very stiff; as a result
responds in a simple, dramatic manner. Real buildings, of course, are more
inherently flexible, being composed of many different parts.
Furthermore, not only is the block stiff, it is brittle; and because of
this, it cracks during the earthquake. This leads us to the next important
structural characteristic affecting a building's earthquake response and
Ductility is the ability to undergo distortion or deformation--bending,
for example-- without resulting in complete breakage or failure. The
ductility or flexability of structure is one of the most important factors
affecting its earthquake performance. One of the primary tasks of an
engineer designing a building to be earthquake resistant is to ensure that
the building will possess enough ductility to withstand the size and types
of earthquakes it is likely to experience during its lifetime. Damping
All vibrating objects, including buildings, will eventually stop vibrating
as time goes on. More precisely, the amplitude of vibration decays with
time. Without damping, a vibrating object would never stop vibrating, once
it had been set in motion. In a building during an earthquake,
damping--the decay of the amplitude of building's vibrations--is due to
internal friction and the absorption of energy by the building's
structural and nonstructural elements. All buildings have some intrinsic
damping. The more damping a building possesses, the sooner it will stop
vibrating--which of course is better. Today, some of the more advanced
techniques of earthquake resistant design and construction employ
added damping devices like shock absorbers to increase artifically the
intrinsic damping of a building and so improve its earthquake performance.
What is liquefaction?
Liquefaction is a type of ground failure. This happens when loose, moist
soil or sand is shaken so hard that individual grains separate, turning
the earth into a soft, fluid slurry that can swallow entire buildings. And
ground motions in regions of soft sediment are drastically amplified
relative to surrounding areas, so that much greater earthquake
damage results, such as that in the Marina District of San Francisco
following the 1906 and Loma Prieta (1989) earthquakes. During the
Loma Prieta earthquake, liquefaction of the soils and debris used to fill
in a lagoon caused major subsidence, fracturing, and horizontal sliding of
the ground surface in the Marina district in San Francisco
How to Construct
Eqrthquake Resistant Buildings
>Fig.1 The conventional approach to earthquake resistant design of
buildings depends on providing the building with strength, stiffness and
inelastic deformation capacity which are strong enough to withstand a
given level of earthquake-generated force. This is generally accomplished
through the selection of an appropriate structural configuration and the
careful detailing of structural members, such as beams and columns, and
the connections between them.
But in contrast, we can say that the basic approach to more advanced
techniques for earthquake resistance is not to strengthen the building,
but to reduce the earthquake-generated forces that is put on the
building. The most important advanced techniques of earthquake
resistant design and construction are:
- Base Isolation Energy
- Dissipation Devices Active Control Systems
This is the most widely used method against earthquake damage. A
base isolated structure is supported by a series of bearing pads which are
placed between the building and the building's foundation. (See
Figure 1). A variety of different types of base isolation bearing pads
have now been developed, including ones called lead-rubber bearings.
Fig.2 A lead-rubber bearing is made from layers of rubbers
sandwiched together with layers of steel. In the middle of the bearing is
a solid lead "plug." On top and bottom, the bearing is fitted with steel
plates which are used to attach the bearing to the building and
foundation. (See Figure 2) The bearing is very stiff and strong in the
vertical direction, but flexible in the horizontal direction.
Fig. 3 shows how this isolation system works. As a result of an
earthquake, the ground beneath each building begins to move. In Figure 3,
it is shown moving to the left. Each building is undergoing displacement
towards the right, which is due to inertia.In addition to displacing to
the right, the un-isolated building is also changing shape into more of a
parallelogram from a rectangular. This is the process if deforming. And of
course, the primary cause of earthquake damage is deformation which the
building goes through as a result of inertial force acting on it.
The base-isolated building, though is still displacing, retained it's
original retangular shape. Only the lead-rubber bearing supporting
the building are deformed. The base-isolated building escaped the
deformation and damage--which shows that the inertial forces acting on the
base-isolated building have been reduced. Experiments and observations of
base-isolated buildings in earthquakes have been shown to reduce building
accelerations to as little as 1/4 of the acceleration of fixed-base
buildings Since the rubber isolation bearings are highly elastic,
they don't suffer any damage. The lead plug in the middle of the example
bearing experiences the same deformation as the rubber. However, it also
generates heat as it does so. In other words, the lead plug reduces, or
dissipates, the energy of motion--i.e., kinetic energy--by converting that
energy into heat. And by reducing the energy entering the building, it
helps to slow and eventually stop the building's vibrations sooner--in
other words, it damps the building's vibrations.
Fig.4 A second type of base-isolation is called Spherical Sliding
Isolation Systems. With this, the building is supported by bearing
pads that have a curved surface and low friction. During an earthquake,
the building is free to slide on the bearings. Since the bearings have a
curved surface, the building slides both horizontally and vertically (See
Figure 4). The force needed to move the building upwards limits the
horizontal or lateral forces which would otherwise cause building
deformations. Also, by adjusting the radius of the bearing's curved
surface, this property can be used to design bearings that also lengthen
the building's period of vibration.
Preparing for an Earthquake
By planning and practicing what to do in case of an earthquake, your
family can learn to react correctly and automatically when the shaking
begins. During an earthquake, most deaths and injuries are caused by
collapsing building materials and heavy falling objects, such as
bookcases, cabinets, and heating units. Learn the safe spots in each room
of your home.
Participating in an earthquake drill will help children understand what to
do in case you are not with them during an earthquake.
During your earthquake drill:
Get under a sturdy table or desk and hold on to it.
If you're not near a table or desk, cover your face and head with
your arms; and
stand or crouch in a strongly supported doorway OR . . .
brace yourself in an inside corner of the house or building.
Stay clear of windows or glass that could shatter or objects
that could fall on you.
Remember: If inside, stay inside. Many people are injured at entrances
of buildings by falling debris.
Evacuation Plans If an earthquake occurs, you may need to evacuate a
damaged area afterward. By planning and practicing for evacuation,
you will be better prepared to respond appropriately and efficiently to
signs of danger or to directions by civil authorities.
Take a few minutes with your family to discuss a home evacuation plan.
Sketch a floor plan of your home; walk through each room and discuss
Plan a second way to exit from each room or area, if possible.
If you need special equipment, such as a rope ladder, mark where
it is located.
Mark where your emergency food, water, first aid kits, and fire
extinguishers are located.
Mark where the utility switches or valves are located so that they can
be turned off, if possible.
Indicate the location of your family's emergency outdoor meeting place.
Write Down Important Information
Make a list of important information and put it in a secure location.
Include on your list:
important telephone numbers, such as police, fire, paramedics, and
the names, addresses, and telephone numbers of your insurance agents,
including policy types and numbers
the telephone numbers of the electric, gas, and water companies
the names and telephone numbers of neighbors
the name and telephone number of your landlord or property manager
important medical information, such as allergies, regular medications,
the vehicle identification number, year, model, and license
number of your automobile, boat, RV, etc.
your bank's or credit union's telephone number, account types, and
radio and television broadcast stations to tune to for emergency broadcast
information Gather and Store Important Documents in a Fire-Proof Safe
Ownership certificates (automobiles, boats, etc.)
Social Security cards
Household inventory, including:
list of contents
photographs of contents of every room
photographs of items of high value, such as jewelry, paintings,
Emergency Supplies Stock up now on emergency supplies that can be used
after an earthquake. These supplies should include a first aid kit,
survival kits for the home, automobile, and workplace, and emergency water
and food. Store enough supplies to last at least 3 days.