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EARTHQUAKES
Earthquakes result from a release of energy accompanying failure
of rock material at depth. Several important pieces of information
about an earthquake are gathered by seismologists. These include
the location of the following:
Epicenter: The area on the surface
of the Earth immediately above the place where the energy
was released.
Focal Point: the place where the rupture occured.
Depth to the focal point (or focus) and a measure of
the "strength" of the earthquake.
One intensity scale developed for assessing
the perceptions of individuals who experienced an earthquake
is the Modified Mercalli Scale (MM). The MM scale ranges
from I to XII. Surveys are sent to the resdents of an area and
they are asked to judge the "intensity" by comparing their rememberance
to a list of 12 experiences. Lines of equal earthquake damage
(isosesimals) are drawn on a map of the area.
- Using resources on the Internet, identify
the MM which is "felt by almost everyone, some people awakened.
Small objects moved. Trees and poles may shake."
- Using resources on the Internet, locate
a isoseismal map of the Oct. 17, 1989 Loma Prieta (World
Series) earthquake and give its address (URL). Note
that there are very high MM values located in San Francisco
even though the epicenter of the fault was several hundred
miles south. WHY?
MM measures a complex set of interactions between the strength
of the surface material, the type of architecture, building
codes, and population density (the more people that experienced
the earthquake, the better the survey will capture an "average"
perception).
A certain amount of energy is released by an earthquake.
In the 1930's, Dr. Charles F. Richter developed an earthquake
magnitude scale to express the differences between the small
and medium-sized earthquakes he observed in southern California,
and great earthquakes he was recording around the world.
A tremendous range of energies are released by earthquakes,
so Richter decided that a logarithmic scale would
be appropriate. He used log10 so that there is
an order of magnitude difference between Richter magnitudes.
A magnitude 6 event, for example, has ground motion of 10
times greater than a magnitude 5 and 100 times less than
a magnitude 8 event.
- Is a negative magnitude defined?
[Recall that the pH of a solution is defined as a
logarithm (actually as the negative of a logarithm) of the
acivity of the hydrogen ion. Can pH be negative?]
- Using Internet resources, find the conditions
that Richter used to define his measure of earthquake magnitude.
Are there concerns about applying this measure to areas
that do not meet these conditions?
Energy travels away from the focus (the hypocenter) in all
possible directions and is propagated by different methods.
It is convenient to separate earthquake waves into body
waves (the familiar P and S waves) and surface waves.
- Using Internet resources, describe a Raleigh
wave. Is this a body wave or a surface wave?
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