Transcript Lesson 1: What are earthquakes and where do they occur

Lessons 1-4
Lesson 1:
What are earthquakes
and where do they
occur
Where do Earthquakes Occur?
• Earthquakes can occur near the Earth’s surface or
far below the surface.
• Most earthquakes occur at plate boundaries, but
some happen at faults located in the middle of
tectonic plates.
http://www.dosecc.org/html/volcanic_monitoring.ht
ml
PLATE BOUNDARIES
Earthquake Locations Around the World
What Causes Earthquakes?
• Earthquakes are caused by movement along faults.
• When stress is placed on rocks it deforms, or changes.
• This is called Elastic Deformation (remember faults and
folds, tension and compression)
• Rock is stretched and bent until it can no longer take
the stress.
• When enough stress builds up in the rock it, it slips
and energy is released.
• The rock then returns to its original shape. This is
called Elastic Rebound. Think of a rubber band, you
can only stretch it so far until it breaks and return to
its original shape.
• This energy is felt as an earthquake.
Elastic
ELASTIC REBOUND
Earthquakes in Alabama??
• Do we live near a plate boundary?
• Do we live near a fault?
The New Madrid Fault
• If there is no plate boundary in the middle of the United
States, why did these earthquakes take place?
• Geologists are beginning to understand the answer. The New
Madrid Fault Zone is part of an ancient plate boundary. In
this area, the North American Plate tried to form a divergent
plate boundary about 500 million years ago. The splitting
stopped before new plates could form. The faults in the New
Madrid Zone are remnants of this old event. Earthquakes
occur because the North American Plate is still "settling
down". The faults in the New Madrid Zone do not reach the
Earth’s surface. They are buried beneath thousands of feet of
rock and sediment deposited by the Mississippi River.
Geologists have located them by looking at the patterns of
earthquakes in the zone.
Several of the largest
earthquakes ever
recorded in the United
States occurred in the
Midwest, far from any
plate boundary. These
earthquakes took place
in an area called the
New Madrid Fault
Zone, named after the
town of New Madrid,
Missouri.
• Over a three-month period in the winter of
1811 to 1812, the New Madrid Fault Zone
was struck by three huge earthquakes
estimated to be greater than magnitude 8.0
• The New Madrid Fault is what causes
Alabama to experience earthquakes.
• The map shows the
earthquakes recorded in
Alabama since 1886.
The last
earthquake to
occur in Alabama
was on July 27,
2007. It
measured a 2.6
on the Richter
scale.
This was not a
very strong
earthquake, but it
was recorded by
a seismograph,
an instrument
used to measure
earthquakes.