Geology 10 – Environmental Geology
Activity 7. - 16 points (this is a 2 point change from the original
activity)
For this activity you will need to go to the Internet site listed below (URL). The
discussion and exercises are designed to introduce the tectonic setting of our area and to
provide an example of assessing the risks posed by frequent earthquakes. The last half of
the web site includes the questions and activities listed below.
http://msemac.redwoods.edu/~dbazard/Geography/Earthquakes/ind
ex.html
Activity Questions and Requirements:
1. Regional Recurrence Interval.
Calculate the recurrence intervals for large magnitude (6 or greater) and damaging
(intensity VI or greater) earthquakes that have impacted Humboldt County in the last 160
years. To do this, use the earthquakes listed in the following pdf file.
Download Humboldt Earthquakes PDF
You will need to download and print this file or keep it open it in a separate window.
a. Analyze the data and list the three longest and the three shortest intervals between
earthquakes.
b. Report the total number of earthquakes listed.
c. Determine and report the recurrence interval for this entire data set by using the
following calculation:
Total years divided by the quantity of total earthquakes minus one.
Formula: (total years) / {(total number of earthquakes) - 1}.
Do you see why you divide by earthquakes minus 1?
You could achieve this same result by adding up all of the intervals (7+11+2...) and
dividing this number by the number of intervals present.
d. Now recalculate the recurrence interval using only damaging earthquakes with an
intensity of VI or greater. Your total years will be the same, but the number of
earthquakes will decrease. Report this new recurrence interval.
e. Is it likely to experience a large, damaging earthquake if you live in Humboldt County
for a 10-year period? Explain.
2. Recurrence of Large Subduction Zone Earthquakes.
Calculate the recurrence interval for large subduction zone earthquakes occurring on the
Cascadia subduction zone.
To do this, we will use estimates based on geologic evidence. This evidence suggests that
7 large subduction zone earthquakes (mag 8+) have occurred in the last 3500 years. The
most recent one occurred on Jan 26, 1700 AD.
Calculate the recurrence interval for these large subduction zone earthquakes. It will help
to draw a time line to visualize how many intervals and the total time interval being
considered.
Note that newer geologic evidence indicates that these earthquakes occur more frequently
than the analysis provided here suggests. This is a work in progress.
3. Probability Determination:
Go to the following USGS site to determine the probability of a greater than magnitude
6.0 earthquake occurring within a 50km radius of Eureka in the next 30 years. Use the zip
code of 95501, time span of 30, magnitude of 6.0, No text report. The map generated will
include a legend with numeric probabilities. A 1.00 probability is a 100% chance; a 0.50
probability is a 50% chance. Report the probability of Eureka and the probability for
Ferndale. Link for Earthquake Probability Mapping,
4. Earthquake Risk Assessment
For the next exercise you will need to use the Humboldt GIS. You can open a window to
this by selecting the following link and answer the underlined questions:
Humboldt GIS - Hazards Mitigation
Once you have opened this window, go to the extreme left side of the map and click on
the magnifying glass with plus sign (+). After you click on this symbol, move the cursor
to Eureka on the map and click. Wait a moment and it should zoom in one level. Repeat
this for a total of four or five clicks. This may take a few moments - be patient. You can
zoom out by selecting the magnifying glass with a minus sign (-) and then clicking on the
map. Also, you can click on the "hand" symbol and then click and drag the map to move
the map around.
Next, go to the extreme right side of the map and make sure only the following layers
(under Map Themes) are "checked". You will have to uncheck a few of the layers lower
on the list. uncheck all "layer" boxes except the following:
 Places
 Background Layers
 Water Bodies
Then locate the two labels "Layers" and "Legend", above "Map Themes". After you
change a layer box, click on legend to see and explanation of the colors or patterns that
appear on the map.
For this exercise you will go through each of the following layers and answer questions
regarding the seismic risk associated with the area between Fortuna and Arcata.
 a. Check the Tsunami Evacuation Area box. List two locations that appear
susceptible to tsunami.
 b. Uncheck the tsunami box and then check: Area of Potential Liquefaction. List
two locations that appear susceptible to liquefaction.
 c. Uncheck the liquefaction box and then check: Seismic Safety. This layer is
related to slope stability. What general areas have potentially unstable slopes?
 d. Uncheck the safety box and check: Earthquake Faults. What areas are near
earthquake faults?
 e. Uncheck the faults box and then check: Earthquake Fault Hazard Zones. What
areas are near these hazard zones?
 f. Check all five boxes you evaluated above to get a collective map of the
earthquake hazards. Use this to describe specific areas that will experience the
greatest damage in an earthquake. You may need to zoom in a bit on one area to
complete this analysis. Your instructor may have you incorporate this answer into
question number 5, below.
 g. Describe at least two additional factors related to seismic risk and safety that
are not shown on this map.

5. Minimize Risk - Prepare for Earthquakes.
List some actions one can take to minimize the impact an earthquake will have on
your life (other than moving to a different area). You might consider looking at
the "Living On Shaky Ground", link, or this Red Cross site: Red Cross
Earthquake Safety Checklist.
6. Earthquake Risk Analysis.
Write an essay describing the risk of earthquake hazards in this west-central
portion of Humboldt County. Indicate which of the "Earthquake Sources"
discussed above is a likely source for a damaging earthquake. Use the results of
the recurrence and probability exercises to discuss the likelihood of a damaging
earthquake in this region. Discuss specific areas prone to earthquake damage and
discuss factors that may influence the risk, such as building types, preparation,
and responses of people. This essay should address each of these areas, be double
spaced, and be no more than two pages long.