2. What is an earthquake?
• Earthquakes are natural geologic
phenomena caused by the sudden
and rapid movement of a large
volume of rock. The violent shaking
and destruction caused by
earthquakes are the result of rupture
and slippage along fractures in Earth’s
crust called fault.
3. Terms to know:
• Fault- A break in a rock mass along which movement has
occurred.
• Focus- The zone within the Earth where rock displacement
produces an earthquake. (Also referred to as the hypocenter)
• Epicenter- The point at the surface directly above the focus.
• Seismic waves- A form of elastic energy that causes vibrations in
the material that transmits them.
• Elastic rebound- The sudden release of stored strain in rocks that
results in movement along a fault.
• Aftershock- A smaller earthquake that follows the main
earthquake.
• Foreshocks- small earthquakes that often precede a major
earthquake.
• Seismograph- an instrument that records and measures
earthquake waves.
4. The cause of earthquakes…
• Tectonic stresses acting over tens to
hundreds of years slowly deform the
crustal rocks on both sides of a fault.
When deformed by differential stress,
rocks bend and store elastic energy.
Eventually, the frictional resistance
holding the rocks in place is
overcome. Slippage allows the
deformed rock to “snap back” to its
original shape.
• Earthquakes are produced by the
rapid release of elastic energy stored
in rock that has been deformed by
differential stresses. Once the strength
of the rock is exceeded, it suddenly
ruptures causing the vibrations of an
earthquake.
5. Faults faults and more faults
• There are 3 basic types of faults: normal, reverse and
strike-slip.
• Normal faults occur where the crust is stretched and
elongated, therefore most earthquakes along normal
faults are associated with divergent plate boundaries.
• Reverse faults are most often found along convergent
plate boundaries. These faults are generated by
compressional forces associated with subduction zones
and continental collisions.
• Strike-slip faults are the result of stress that cause large
segments of Earth’s crust to slip horizontally past each
other. (Large strike-slip faults are called transform faults)
6. How are earthquakes
measured? a measure of the degree
• Intensity is
of earthquake shaking at a given
locale based on observed damage.
• Magnitude relies on data gleaned
from seismic records to estimate the
amount of energy released at an
earthquake’s source.
• In 1902 Giuseppe Mercalli developed
the Modified Mercalli Intensity Scale
to measure intensity.
• The Richter Scale is a scale of an
earthquake magnitude based on the
amplitude of the largest seismic
wave.
• Richter used a logarithmic scale to
express magnitude, in which a
tenfold increase in wave amplitude
corresponds to an increase of 1 on
The Richter Scale the magnitude scale.
7. Earthquake Facts
• The largest recorded earthquake in the world was a magnitude 9.5 in
Chile on May 22, 1960.
• In 1663 the European settlers experienced their first earthquake in
America.
• The largest recorded earthquake in the US was a magnitude 9.2 that hit
Prince William Sound, Alaska on March 28, 1964.
• The first “pendulum seismoscope” to measure the shaking of the ground
during an earthquake was developed in 1751, and it wasn’t until 1855 that
faults were recognized as the source of earthquakes.
• It is estimated that there are 500,000 detectable earthquakes in the world
each year. 100,000 of those can be felt, and 100 of them cause damage.
• From 1975-1995 there were only four states that did not have any
earthquakes. They were: Florida, Iowa, North Dakota, and Wisconsin.
• Most earthquakes occur at depths of less than 80 km (50 miles) from the
Earth’s surface.
