An earthquake is a
sudden, rapid shaking of the Earth
caused by the breaking and shifting of
rock beneath the Earth's surface. This
shaking can cause buildings and bridges
to collapse; disrupt gas, electric, and
phone service; and sometimes trigger
landslides, avalanches, flash floods,
fires, and huge, destructive ocean waves
(tsunamis) . Buildings with foundations
resting on unconsolidated landfill, old
waterways, or other unstable soil are
most at risk. Buildings or trailers and
manufactured homes not tied to a
reinforced foundation anchored to the
ground are also at risk since they can
be shaken off their mountings during an
earthquake. Earthquakes can occur at any
time of the year
- Duration of shaking. Duration depends on how the fault breaks during the
earthquake. The strongest shaking during the 1964 earthquake lasted 3 to 4 minutes.
During a magnitude 7 earthquake, the shaking may last 30 to 40 seconds. The longer
buildings shake, the greater the damage.
- Strength of shaking. Many damaging earthquakes occur within 15 miles of the Earth's
surface. In this case, shaking decreases rapidly with increasing distance from the fault
that produced the earthquake. In Alaska, these are most common in central and southeastern
Alaska. Deeper earthquakes are common beneath southern Alaska and the Aleutian Islands.
Because of their greater depth, the shaking directly above such shocks is reduced, and
the shaking decreases gradually with increasing distance from the epicenter of the
- Type of soil. Shaking is increased in soft, thick, wet soils. In certain soils the
ground surface may settle or slide. Damage is reduced in buildings located on bedrock.
- Type of building. Some existing buildings are not resistant enough to the side-to-side
and up-and-down shaking common during earthquakes.
by earthquakes are detected, recorded,
and measured by instruments called
seismographs. These devices may amplify
ground motions beneath the instruments
to over 1 million times, transcribing
the ground motion into a zig-zag or
wiggly trace called a seismogram. From
the data expressed in seismograms, the
time, epicenter, and focal depth of an
earthquake can be determined. Also,
estimates can be made of its relative
size and amount of energy it released.
The point on the
fault where rupture initiates is
referred to as the focus or hypocenter
of an earthquake. The hypocenter is
described by its depth in kilometers,
its map location in latitude and
longitude, its date and time of
occurrence, and its magnitude (a measure
of the amount of energy radiated as
seismic waves). The term epicenter,
which is more commonly used to refer to
an earthquake location, is the point on
the earth's surface directly above the
hypocenter. The description of an
epicenter is the same as for a
hypocenter except the depth is omitted.
The strength of an earthquake is
generally expressed in two ways:
magnitude and intensity. The magnitude
is a measure that depends on the seismic
energy radiated by the earthquake as
recorded on seismographs. An
earthquake's magnitude is expressed in
whole numbers and decimals (e.g., 6.8).
The intensity at a specific location is
a measure that depends on the effects of
the earthquake on people or buildings.
Intensity is expressed in Roman numerals
or whole numbers (e.g., VI or 6).
Although there is only one magnitude for
a specific earthquake, there may be many
values of intensity (damage) for that
earthquake at different sites.