Fault Extent Student Exercise
Answer Sheet
1. Which direction do most of the faults go (north, south, east, west, etc…)? Do they
all go the same direction? Why do you think this might be?
They all run roughly northwest – southeast. This orientation is parallel to the plate
boundary in the area because the faults basically define the plate boundary.
2. Where are most of the earthquake epicenters located? Why? Are there
exceptions? Why?
Most earthquake epicenters are located near the faults because earthquakes are shaking
caused by slip along faults. Earthquakes can't happen without faults, so if an earthquake
appears to be far from mapped faults, it is most likely because the fault was left off the
map or we never knew that there was an active fault there before.
3. Find the earthquake that occurred on April 18, 1906. What was its magnitude? Is
this bigger or smaller than most of the earthquakes in this list? Mark its epicenter
with a star, and write '1906' next to it. Which fault did this earthquake rupture?
At Magnitude 7.9, this earthquake is one of the largest earthquakes ever recorded in
California. It occurred along the San Andreas fault, but its epicenter is right near the
junction of the San Andreas and San Gregorio faults.
4. The 1906 earthquake started at the epicenter you marked with a star, but
scientists observed fault offset (like the offset fences in the slide show) along a huge
section of the California coast from Cape Mendocino in the north to as far south as
San Juan Bautista. What is the distance, in kilometers, from the earthquake
epicenter to Cape Mendocino? What is the distance form the epicenter to San Juan
Bautista? What is the total length that this fault ruptured? Draw a gray line along
the entire rupture length and label it 1906 Earthquake rupture.
Map reading skills are required to answer this question.
Cape Mendocino to Epicenter: 300 kilometers.
Epicenter to San Juan Bautista: 130 kilometers.
Total rupture length: 430 km.
5. Find the earthquake that occurred on May 3, 2002. On which fault did this
earthquake occur? What was its magnitude? This earthquake was barely felt by
anybody except students at the University of California at Berkeley. The earthquake
occurred directly below the campus football stadium, which is cut by a major fault
(the earthquake was in the middle of the night, so nobody was playing football at the
time). The earthquake rupture was, coincidentally, about the same size as the
football field (about 100 meters). Imagine that the earthquake caused a patch of
DLESE TEACHING BOXES | October 2005
fault the size of a football field to slip. What is 100 meters in kilometers? On your
map, how many inches across is a 100-meter long line? (This is sort of a trick
question).
100 meters = 0.1 kilometers.
A 100 m long line would be very tiny on the map (the map of California prints out on a
standard sheet of letter sized paper at a scale of about 1: 5,000,000, so 100 meters would
be 0.02 millimeters, or about the width of a human hair).
6. Write one sentence comparing the size of the 1906 and 2002 earthquake rupture
lengths.
The 1906 earthquake ruptured a much longer portion of the fault than the time 2002
earthquake (about 4300 times longer!).
7. Based on the information about these two earthquakes, write a sentence
explaining how earthquake magnitude is related to the length of the fault patch that
ruptures in an earthquake.
Longer earthquake ruptures release more total energy (because they slip over a much
larger area), so they therefore have higher magnitudes.
8. Can you tell anything about the magnitude of an earthquake from the location of
its epicenter alone? Why or why not?
No. The epicenter is just the point on the earth's surface above where the earthquake
starts rupturing. It tells you nothing about how big the earthquake was at the end or how
much area the fault ruptured.
DLESE TEACHING BOXES | October 2005