All about Earthquakes
In this section of the course we will look at the nature and origin of
earthquakes.
Areas that we will progress to look at include:


Seismology
Seismic Waves
Earthquakes
Earthquakes occur when energy stored in elastically strained rocks is
suddenly released. This release of energy causes intense ground shaking in
the area near the source of the earthquake and sends waves of elastic
energy, called seismic waves, throughout the Earth. Earthquakes can be
generated by bomb blasts, volcanic eruptions, and sudden slippage along
faults. Earthquakes are definitely a geologic hazard for those living in
earthquake prone areas, but the seismic waves generated by earthquakes are
invaluable for studying the interior of the Earth.
Origin of Earthquakes
Most natural earthquakes are caused by sudden slippage along a fault zone.
The elastic rebound theory suggests that if slippage along a fault is hindered
such that elastic strain energy builds up in the deforming rocks on either side
of the fault, when the slippage does occur, the energy released causes an
earthquake. This theory was discovered by making measurements at a
number of points across a fault. Prior to an earthquake it was noted that the
rocks adjacent to the fault were bending. These bends disappeared after an
earthquake suggesting that the energy stored in bending the rocks was
suddenly released during the earthquake.
(origin_earthquakes.gif)
Seismology, The Study of Earthquakes
When an earthquake occurs, the elastic energy is released and sends out
vibrations that travel throughout the Earth. These vibrations are called seismic
waves. The study of how seismic waves behave in the Earth is called
seismology.
Seismographs - Seismic waves travel through the Earth as vibrations. A
seismometer is an instrument used to record these vibrations and the
resulting graph that shows the vibrations is called a seismograph. The
seismometer must be able to move with the vibrations, yet part of it must
remain nearly stationary.
This is accomplished by isolating the recording device (like a pen) from the
rest of the Earth using the principal of inertia. For example, if the pen is
attached to a large mass suspended by a spring, the spring and the large
mass move less than the paper which is attached to the Earth, and on which
the record of the vibrations is made.
Seismic Waves - The source of an earthquake is called the focus, which is an
exact location within the Earth were seismic waves are generated by sudden
release of stored elastic energy. The epicenter is the point on the surface of
the Earth directly above the focus. Sometimes the media get these two terms
confused. Seismic waves emanating from the focus can travel in several
ways, and thus there are several different kinds of seismic waves.
(seismograph.gif)
Types of Seismic Waves
Body Waves - emanate from the focus and travel in all directions through the
body of the Earth.
(seismicwaves.gif)
Surface Waves - Surface waves differ from body waves in that they do not
travel through the Earth, but instead travel along paths nearly parallel to the
surface of the Earth. Surface waves behave like S-waves in that they cause
up and down and side to side movement as they pass, but they travel slower
than S-waves and do not travel through the body of the Earth.
The record of an earthquake, a seismograph, as recorded by a seismometer,
will be a plot of vibrations versus time. On the seismograph, time is marked at
regular intervals, so that we can determine the time of arrival of the first Pwave and the time of arrival of the first S-wave. Because P-waves have a
higher velocity than S-waves, the P-waves arrive at the seismographic station
before the S-waves.
All about Glaciers
In this section of the course we will look at the nature and origin of Glaciers.
Areas that we will progress to look at include:
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

Glacier Speeds
Glacial Ice
Glacier Size
Glaciers
Glaciers constitute much of the Earth that makes up the cryosphere, the part
of the Earth that remains below the freezing point of water. Most glacial ice
today is found in the polar regions, above the Arctic and Antarctic Circles.
While glaciers are of relatively minor importance today, evidence exists that
the Earth's climate has undergone fluctuations in the past, and that the
amount of the Earth's surface covered by glaciers has been much larger in the
past than in the present. In fact, much of the topography in the northern part
of North America, as well as in the high mountain regions of the west, owe
their form to erosional and depositional processes of glaciers. The latest
glaciation ended only 10,000 years ago.
Definition of a glacier
A glacier is a permanent (on a human time scale, because nothing on the
Earth is really permanent) body of ice, consisting largely of recrystallized
snow, that shows evidence of downslope or outward movement due to the
pull of gravity.
Types of Glaciers
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
Mountain Glaciers - Relatively small glaciers which occur at higher
elevations in mountainous regions. Smallest of these occupy hollows or
bowl-shaped depressions on sides of mountains (cirque glaciers).
Ice Sheets: (Continental glaciers): are the largest types of glaciers on
Earth. They cover large areas of the land surface, including mountain
areas. Modern ice sheets cover Greenland and Antarctica.

Ice Shelves: Ice shelves are sheets of ice floating on water and
attached to land. They usually occupy coastal embayments, may
extend hundreds of km from land and reach thicknesses of 1000 m.
Glaciers can also be classified by their internal temperature.
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Temperate glaciers - Ice in a temperate glacier is at a temperature near
its melting point.
Polar glaciers - Ice in a polar glacier always maintains a temperature
well below its melting point
Glacier Speeds
Temperature
Very cold
Pretty cold
Cold
Chilly
Mountain
Glaciers
1.5mph
1.7mph
1.8mph
1.9mph
Ice Sheets
Ice Shelves
2.0mph
2.1mph
2.5mph
3.2mph
2.3mph
2.4mph
2.5mph
2.7mph
Which glacier type moves fastest in very cold conditions?
Answer = ice shelves
The Mountain Glacier moves faster than an Ice Sheet in chilly conditions, true
or false?
Answer = False
The Formation of Glacial Ice
Glaciers can only form at latitudes or elevations above the snowline, which is
the elevation above which snow can form and remain present year round. The
snowline, at present, lies at sea level in polar latitudes and rises up to 6000 m
in tropical areas. Glaciers form in these areas if the snow becomes
compacted, forcing out the air between the snowflakes. As compaction
occurs, the weight of the overlying snow causes the snow to recrystallize and
increase its grain-size, until it increases its density and becomes a solid block
of ice.
Changes in Glacier Size
A glacier can change its size by Accumulation, which occurs by addition of
snowfall, compaction and recrystallization, and Ablation, the loss of mass
resulting from melting, usually at lower altitude, where temperatures may rise
above freezing point in summer. Thus, depending on the balance between
accumulation and ablation during a full season, the glacier can grow or shrink
Additional Glacier Resources
Link to additional Glacier resources
http://www.google.com/search?sourceid=navclient&ie=UTF-8&oe=UTF8&q=glaciers
Flash Animation of an Earthquake
Evolution of expected seismicity rate at Landers 1975-2010
When an earthquake occurs on a fault, the seismicity in the area surrounding
the fault failure is modified. In some places it is promoted and in others it is
inhibited. This effect tends to disappear with time but is also further modified
when subsequent earthquakes occur.
Since 1975, in the Mojave desert (Southern California), four consecutive
earthquakes increased the expected seismicity rate at the location of the
future Landers rupture. Consequently, three hours and 26 minutes after
Landers, the Big Bear fault failed in the area of highest predicted seismicity.
And again seven years later, the Hector Mine earthquake occurred in an area
where the predicted seismicity rate was increased. The seismicity rate
change, as derived from the stress change pattern through Dieterich's
state/rate friction law (JGR, 1994), appears to be an appropriate descriptor of
the seismicity affecting an area.
This animation was presented by Ross Stein at the American Geophysical
Union lecture 'Frontiers of Geophysics' in December 2001. Animations by
Ross Stein and Keith Richards-Dinger.
(insert seismicity_animation.swf)
This animation downloaded from
http://quake.wr.usgs.gov/research/deformation/modeling/animations/