Earthquakes occur around the globe, on every continent
and even in the ocean floor. In the maps to the right and below, you can
see that earthquakes cluster in particular locations. Most of these areas
of high earthquake activity are on plate boundaries. Other earthquakes
occur within plates (called intraplate earthquakes). All of Alabama’s
earthquakes are intraplate.
Below is SeismicMonitor
– a product of the Incorporated Research Institutions for Seismology (IRIS)
and the U.S. Geological Survey (USGS) showing real-time global earthquakes
of magnitude 4.0 and greater over the last five years. To learn more about
a particular quake on the map, click on the epicenter. To see USGS realtime
earthquakes in GoogleEarth, click
here .
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EARTH’S STRUCTURE
The outermost layer (crust) of the earth is divided into
huge plates, like a cracked eggshell. Heat generated in the core is
transferred into the mantle, a slow-moving plastic layer that drags the
overlying crustal plates (left). Stress builds up along plate boundaries
and within plates.
This can pull or push the crust forming different
structural features such as:
·Faults—breaks in the rock which allow movement on either side
·Mountains—areas of thickened crust subjected to compression
·Ocean ridges—areas of thinned crust where molten rock extrudes,
forming new crust
When stress exceeds the strength of the rocks in the
crust, the crust breaks along a fault and snaps into a new position. This
sudden movement releases energy—what we know as an earthquake.
Faults
The area of contact between two blocks of rock that
have moved relative to each other is called a fault (below). The direction of relative motion
of the blocks may be horizontal, vertical, or a combination of these
motions. The force that causes the stress within the rock that makes it
break or slip along a fault is a result of the movement of giant sections
(or plates) of the earth’s crust.
MEASURING EARTHQUAKES
Seismographs
Instruments
that measure magnitude of earthquakes are seismographs (or seismic stations). Seismic stations sense and
record vibrations from earthquakes as well as other sources such as mine
collapse, quarry blasts, explosions, construction, and even sonic booms
from aircraft or the space shuttle.
In order to more clearly sense earthquake waves,
seismic stations must be buried in hard rocks (right). This allows the
station to pick up seismic waves from around the world. To see examples of
seismograms (such as the one in the header of this page) from USGS seismic
stations, click
here.
Build Your Own Seismograph!
Interested
in learning more about seismographs? A great way to learn more is to
build your own. Click
here for links to building both simple and complex seismographs.
Magnitude
Magnitudeandintensityare terms used to describe the strength of an earthquake.
Magnitude relates to the energy
released or the amount of slip that occurred during an earthquake with
values ranging from 1 to 10. While many people may be familiar with the
term Richter Scale, there are actually many scales used to measure
magnitude that describe different aspects of the event. The table to the
right shows just a few of these magnitude types used today.
Magnitude
type
Magnitude
range
Application
Local (ML)
2 - 6
Maximum amplitude a
seismogram records on a Wood-Anderson torsion seismograph. Although these
seismographs are no longer widely in use, ML is calculated using modern
instruments with adjustments.
Duration (Md)
< 4
Duration of shaking.
Moment (Mw)
> 3.5
Moment of the
earthquake equal to rigidity of Earth, times average slip, times fault
area.
Energy (Me)
> 3.5
Amount of recorded
seismic energy radiated.
Body (Mb)
4 - 7
Amplitude of the p-wave (a type of seismic wave that travels
through the earth’s interior).
Surface wave (MLg)
5 - 8
For a distant quake.
Amplitude of the Lg wave (a type of seismic wave that travels through the
earth’s surface).
Intensity and ShakeMaps
Intensity is another method of
measuring earthquakes. It is a description of the intensity of shaking at a
location (not only at the epicenter). Intensity is based on reports and observations
from people in the area affected. Values range from I to XII on the
Modified Mercalli Intensity Scale (right), increasing from barely
detectable to catastrophic.
Intensity is shown on area maps such as USGS ShakeMaps (left) or historical
isoseismic maps. ShakeMaps (left) are USGS maps that show shaking
intensities of an area affected by an earthquake. ShakeMaps are produced by
combining user-submitted reports and other information. Scenario ShakeMaps
can also be produced when geologists are interested to see how an area of
interest might be affected by a particular magnitude event.
Click
here to explore USGS ShakeMaps of recent and previous earthquakes.
There are a number of good resources for earthquake
information for the United
States. The lead scientific agency in
the U.S.
for earthquake research and information gathering is the U.S. Geological
Survey (USGS). USGS conducts research not only in the U.S., but also in other
countries. For USGS earthquake information and data, click here
to go to their main earthquake webpage. Also explore the USGS interactive
hazards map below to learn more about seismic hazardsin our area. Note the seismic hazard in
the central U.S.
– this is the New Madrid Seismic Zone – a zone that has a history of large
magnitude earthquakes (click here to learnmore about New Madrid).
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Earthquake Preparedness
Learn more about how to protect yourself during an
earthquake and other disasters by clicking
here.
