Download Earthquakes!

Earthquakes!
An earthquake, or seismic event, is a
sudden movement of Earth’s crust that
releases energy.
Stress and Strain
• You might think of Earth’s mantle and crust
as solid and unbending. However, forces
applied to solid rock at high temperatures
and over a long time can bend, or deform,
rocks.
• Earthquakes occur when stress (force)
causes strain (bending) that exceed the
ability of the rocks to resist breakage.
• What you feel as an earthquake is the
sudden movement of the ground, which
releases stored energy.
Where do Earthquakes occur?
• Earthquakes do not occur randomly.
• Earthquakes occur at cracks in Earth’s crust
along which there is movement.
• These cracks are called faults.
• Faults are usually zones of weakness. The
places most likely to break are those that
have broken in the past.
• Earthquakes occur when the ground suddenly
shifts along geologic faults such as this small
fault near Kingman, Arizona.
• The displacement here is small, but major faults
can offset many miles.
• Perhaps the most
active fault in
the United
States is the San
Andreas Fault
system, which
extends from the
Gulf of California
north past Los
Angeles and
through the San
Francisco area.
• The focus of an earthquake is the place
underground where the rock breaks and
shifts.
• Energy waves (shaking) radiates from the
focus.
• Directly above
the focus is the
epicenter, where
the energy waves
first reach the
surface.
Earthquakes 101