Geology 101 – Environmental Geology - Spring 2007
Lab #3 – Earthquakes and Faults
The first part of today’s lab is to complete the GPS location exercise that is ‘left-over’
from Lab #2. If you have already answered questions 1-3, we will head outside for a
short while to use the GPS units. Otherwise complete those questions using the maps
provided, and join the second group for the outside GPS session.
Part A: Use the enlarged campus area map with UTM grid for this part of the lab.
1. What is the elevation of the Colgate main quad, as determined from the map?
2. What are the UTM co-ordinates (WGS 84) of the main quad sidewalk immediately
south of Lathrop Hall?
3. What is the bearing of a line from the Colgate Chapel to Hascall Hall?
Using the GPS Unit:
We will go outside for a few minutes for a quick lesson on GPS (Global Positioning
System) use.
4. What is the GPS-determined UTM location of the point determined in #2 above?
The elevation?
Briefly discuss the significance of any differences between #1, #2 and #4.
Part B: Exercise using travel times to locate earthquake epicenter.
As we discussed in class, seismographs record the first arrival of earthquake waves from
distant events. The velocity of P-waves exceeds that of S-waves so we can use the
difference in arrival time to determine the distance between the earthquake and the
seismograph station. Knowing that distance for three stations, we can locate the
earthquake source.
Using the seismograms on the next page, calculate the time lag between the P-wave and
S-wave arrival times for each of the stations. Record the data below:
St. Louis: _______________
Bloomington:____________________
Minneapolis: _____________
Bowling Green: __________________
To determine the distance from the earthquake to each station based on the time lag
between P and S arrivals, we need to know the average velocities of P and S waves and
the effect this has on travel time. Average P wave velocity in the crust is 6.1 km/sec;
average S wave velocity is 4.1 km/sec. This means that for each second of time since the
earthquake, the P waves have traveled 2 km farther than the S waves. Another way of
thinking about this is to calculate the time lag at some distance from the epicenter, say
100 km.
P-waves travel 100 km in _______ seconds
S-waves travel 100 km in _______ seconds
Thus the time lag at a distance of 100 km from the epicenter (T100) is ________ seconds.
For longer distances we can use a simple proportion equation to determine the distances
based on the time lag:
X
Tx
=
100km
T100
Where X is the unknown distance in kilometers; Tx is the time lag for the distance X;
T100 is the time lag at 100 km.. Using this relationship, calculate the distances from each
of the stations to the earthquake epicenter:
St. Louis(SLM): _______________
Bloomington (BLO):___________________
Minneapolis (MNM): _____________
Bowling Green (BGO):_________________
Use the map of the continental US on the following page, and a drawing compass or ruler
to draw arcs of appropriate scaled length, and locate the earthquake epicenter.
Part C: Earthquake research assignment using the USGS website
Use the United States Geological Survey site – http://earthquake.usgs.gov/regional/neic/ to investigate recent worldwide earthquake events. Select a recent event (within the last
2 years) and write a short (250 words) description of the event which includes the
following information:
a. Location (include a map showing the epicenter and nearby geography)
b. Magnitude and Intensity
c. Damaging effects
d. Type of fault
e. Plate tectonic setting
f. Influence of local geology on the distribution of damaging effects