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Introduction:
This activity is designed to provide a better understanding of
earthquake activity, the locations of faults, and earthquake hazards in the
Satellite Image and Base Map: Figure 1 (or original color poster of bay area) is a Landsat Satellite image that depicts a 50,000 square kilometer area from Lake Berryessa in the upper right of the poster (north) to Monterey on the lower left (south) and from about 30 km out into the Pacific Ocean on the west to Stockton on the east. The image depicts the Bay area as it looks from about 900 km above the earth.
The satellite image was taken using the infrared portion of the electromagnetic spectrum that is not visible to the human eye. The poster uses false-color imaging to enhance the landforms and surface features. The colors, though brilliant, are not as objects normally appear to the human eye. The color key follows:
Deep red –
ground covered by heavy vegetation (
Light red –
cultivated crops (rectangular areas most abundant in the
Shades of
green – grass-covered areas (east of the Bay) and marshy areas (northwest shore
of
Turquoise Blue – urbanized areas (Note the obvious street grids. Red flecks represent parks and greenways)
Royal Blue – bodies
of water (
The bright yellow circles
represent the epicenters of the more than 12,000 earthquakes of magnitude 2 or
larger that occurred in the
The magnitude of the earthquakes is depicted by the diameter of the circles—from the smallest (one millimeter on the poster), representing earthquakes of magnitude 2-3, to the largest, an eight-millimeter (on the poster) circle in the Santa Cruz Mountains, representing the 7.1 magnitude Loma Prieta earthquake of October 17, 1989.
The area north and east of the Loma Prieta epicenter circle is heavily dotted with more than 150 smaller circles representing some of the aftershocks of that earthquake.
In areas where earthquakes occur most frequently, a solid yellow cluster or line clearly traces the major Bay Area faults. Note how closely the trace of the computer-generated epicenters compares with the faults shown in Figure 2.
William Bakun, who heads the USGS Bay Area Future Earthquakes Project, said the poster is more than "just another pretty map."
"While this is one of the more visually appealing renditions of the Bay Area," Dr. Bakun said, "it also is an excellent way to comprehend the amount of earthquake activity in the area and where that activity is concentrated. Even seismologists who have studies the area for years look at this map and are surprised. They see features and relationships that they had not noticed before."
A base map showing faults and selected geographical features is shown in Figure 2 for comparison with the satellite image. Note that the map and the satellite image (Figure 1 or poster) are not at the same scale and do not cover exactly the same area. In fact, although the map is oriented in the traditional manner with "north" toward the top of the map (note North latitude and West longitude tic marks along the edge of the map), the satellite image is "rotated" to be more parallel to the coastline (see north arrow in lower left hand corner of the image or poster).
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Figure 1. False
color satellite image of the |
Figure 2.
Faults in the
Activities for the Classroom:
1. Using a road map or an
atlas, find and mark the following geographic localities on he reference
map:
2. Find the largest earthquake circle on the map. Can you identify this earthquake?
3. If there were 12,000 earthquakes from January of 1972 to December of 1989, how many earthquakes were there per year and per month, on average?
4. Trace the surface expression of four faults using the clues given by the epicenter circles. Identify the faults using the reference map.
5. Dr. Bakun noted that seismologists were surprised about some of the features that they could see on the poster. Write your own paragraph about interesting features that you see on the poster.
6. Blow up a balloon and predict when it will pop. What variables are you using to make your prediction? Does your prediction mechanism work for multiple balloons? Try this activity with a group of students. What prediction methods can you develop? How accurate is your prediction method with different students blowing up the balloon?
7. Make a hypothesis about the
lack of earthquakes on the
8. As reported in "The
Next Big Earthquake," "the peninsula segment of the San Andreas fault
between
9. Translate the Spanish place names to English.
10. Scientists know that
buildings and other man-made structures built on fill areas or areas of
uncompacted sediments are more likely to be damaged in an earthquake because
these materials can intensify shaking.
Areas that are steep sloped are more likely to experience landslides and
mud slides. With this information and
information gathered from the earthquake epicenters on the poster, where would
you look for a house if you were moving to the
11. What supplies would you want to have in your home if you were in an earthquake prone area? What changes would you make in your home if you lived in an earthquake prone area? Would you change or move furniture or belongings?
12. If the Pacific Plate moves up
the peninsula at an average rate of 5 cm per year toward the northwest, how
long will it take for
Discussion of Earthquake Hazards and Forecasting in
the
Many
people look at this poster and the trace of the San Andreas Fault just south of
Scientists know that the Pacific Plate is sliding to the northwest relative to the North American Plate at an average rate of about 5 centimeters per year. The faults in this area are the surface expression of the edges of these two tectonic plates grinding past each other. In fact, the movement along these faults is neither smooth nor constant. The motion of the plates builds up strain along these faults until the stress becomes too much and the built up pressure is released through an earthquake.
By
studying these, and other faults scientists recognize the patterns of
earthquakes that relieve the stress in this constantly moving system. The history of earthquakes in the
Scientists continue to study earthquake patterns. As our understanding of earthquake systems increases, so does our ability to predict the impact on the human environment.
In
the early to mid 80s, scientists began forecasting the Loma Prieta
earthquake. Scientists had judged the
In
July of 1990, after the Loma Prieta Earthquake, the National Earthquake
Prediction Evaluation Council convened a panel of experts to re-evaluate
predictions for the
The article goes on to say, "More importantly, when the probabilities of earthquakes on all of these segments are combined mathematically, there is a 67 percent chance for at least one earthquake of magnitude 7 or larger in the San Francisco Bay Area between 1990 and 2020. Such an earthquake could strike at any time."
While we cannot predict the exact date and time of an earthquake, nor can we control them, we know enough to mitigate much of the damage that can be caused by an earthquake. Our schools play a critical role in educating people about the potential hazards to human life and property and about appropriate safety precautions in earthquake territory. With this information, our students begin to learn about designing and developing buildings and communities in balance with their natural environment.
Additional
and more recent information on
Progress Toward a Safer Future Since the 1989 Loma Prieta Earthquake, Fact Sheet 151-99, http://geopubs.wr.usgs.gov/fact-sheet/fs151-99/
Major Quake Likely to Strike Between 2000 and 2030, Fact Sheet 152-99, http://geopubs.wr.usgs.gov/fact-sheet/fs152-99/
When Will the Next Great Quake Strike Northern California?, http://quake.wr.usgs.gov/QUAKES/Fact Sheets/When/