# Lab Name Body contouring

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```					Name___________________________             Section________              Date__________
Geos251- Physical Geology                                                Lab #8

EARTHQUAKES
Objectives:
 Understand what causes an earthquake
 Explain with which geologic settings earthquakes are associated
 Explain the concentration of earthquakes in certain global regions
 Understand and define the following terms:
o Hypocenter/focus
o Epicenter
o P-waves
o S-waves
o Surface waves (Rayleigh and Love)
 Work with:
o Reading and interpreting topographic maps
o Constructing contour maps
o Constructing topographic profiles
 Recognize that different seismic waves travel differently through the Earth
 Triangulate an earthquake epicenter using a travel-time curve and seismograph readings
 Calculate the magnitude of an earthquake
 Estimate crustal shortening using a geologic cross-section

Due Date: Monday, 11/12 at 12; Tuesday, 11/13 at 12:30; Wednesday, 11/14 at 12
No late labs will be accepted.

Graded work: questions 1-18, all contour maps, cross-sections, and worksheets

Note: This is the most quantitative lab of the semester. Please read the handout carefully as it
contains the information that you need to complete this lab.
Introduction

If you watch the news or read the paper, then you are superficially familiar with earthquakes.
This lab is designed to introduce you to some basic principles of seismology and how geologists
use these principles to make calculations about earthquakes. You will also think about
earthquake occurrence relative to geologic settings.

Most earthquakes occur when stress from tectonic forces causes sudden movement of blocks in
the Earth. As rocks continue to experience stress, energy builds slowly. Eventually, so much
energy is stored that friction is overcome, allowing the two rock bodies to slip past each other
along a fault plane. This release of energy generates seismic waves, which are responsible for
earthquakes.

Questions:
1. Name the three types of stress that we talked about in the structural geology lab and illustrate
them with arrows.

2. Do you think that earthquakes are examples of brittle or elastic deformation in rocks? Briefly

3. Based on your answer to question 1, do earthquakes occur in the Earth’s: (circle one)
(a) crust?
(b) mantle?
(c) core?

EXERCISE 1: GLOBAL DISTRIBUTION OF EARTHQUAKES

4. The world map in the northeast corner of the room illustrates, among other things, the global
distribution of over 20,000 earthquakes that occurred between 1960 and 1990.
Study this map and answer the following questions:

(a) Each data point represents an earthquake. In general, are the earthquakes distributed
randomly, or does their occurrence follow a pattern? If applicable, describe the
pattern.
(b) Hypothesize the driving force behind these earthquakes.

(c) Other than earthquakes, name another geologic phenomenon that correlates with this
pattern of seismicity.

EXERCISE 2: BENIOFF ZONES

Among the world’s remaining frontiers are the deepest parts of the sea floor -- in the trenches
associated with subduction zones. These trenches mark areas in which oceanic plates are being
subducted or are beginning to descend into the mantle. As the oceanic plate goes further down
into the mantle, stresses and friction along the subduction zone produce earthquakes. This
surface of earthquakes is called the Benioff zone, and it defines the top of the plate that is being
subducted. Geologists can trace the downgoing plate into the mantle if they have data on the
depth and location of earthquakes in the Benioff zone. Figure A shows the relationship between
the Benioff zone (denoted by stars), trench, and the subducting plate.

.

5. In lecture, you have learned that subduction zones involve oceanic crust subducting beneath
continental crust. Why is it this way and not the other way around?
In this section of the lab, you will draw contour lines to produce a topographic profile and a
profile of a Benioff zone. Please read the directions carefully as you go.

Locate and read about the following worksheets:

Benioff Zone Topographic/Bathymetric Contour Map
Each data point on this map represents an elevation (in meters) above or below sea level.
-Positive numbers represent the elevations of points on land.
-Zero represents the continental shoreline (sea level.)
-Negative numbers represent the depth to the ocean floor.
-Elevation 4000 meters represents the location of a volcano produced by melting
within the subduction zone.

Benioff Zone Depth to Earthquake Contour Map
Again, there are data points representing elevations above or below sea level. Elevation zero
represents the continental shoreline. On this map, you also see a series of numbers in boxes.
Each boxed number represents the depth to an earthquake…in other words, the depth to the
Benioff zone. (For example, if you see “100” with a box around it, this means that an earthquake
originated 100 km below Earth’s surface.)

For both maps, notice the cross-section line running from the left of the map to the right of the
map. Walking along the cross-section line, you would be walking on land, reach the shoreline,
and then enter the ocean. Once in the ocean, you would drop down into a trench produced by the
subduction zone.

Topographic/Bathymetric Profile
This worksheet is where you will draw your profiles. The top profile will represent the
topography (what the Earth looks like on land) and the bathymetry (what the Earth looks like
below the ocean floor). The bottom profile will represent the Benioff zone.

Step 1: Contouring (USE PENCIL!!)
On the Benioff Zone Topographic/Bathymetric Contour Map:
Draw contour lines using a contour interval of 1000 m. Contouring the negative numbers will
produce a bathymetric map of the trench and the seafloor. Contouring the positive numbers will
produce a topographic map of the land and volcanic arc. The shoreline (zero elevation) and
several other contours are shown as examples.

On the Benioff Zone Depth to Earthquake Contour Map:
Draw contour lines using a contour interval of 100 km. Contour the depth to the earthquakes
using the numbers in boxes. This will illustrate the depth of the upper surface of the oceanic plate
subducting beneath the continental plate. The 100 km contour line is shown as an example.
Step 2: Profiling
Draw a topographic/bathymetric profile following the cross-section line A - A’ on the Benioff
Zone Topographic/Bathymetric Contour Map. Note that the vertical scale of this profile is
exaggerated, so slopes appear steeper than they actually are.

Then draw a profile that shows the Benioff zone and the down-going plate following the B - B’
profile line found on the Benioff Zone Depth to Earthquake Contour Map. In this section, the
vertical scale equals the horizontal scale, so there is no exaggeration.

6. Using a protractor, determine the average dip of the Benioff zone.

7. What is the approximate depth to the top of the downgoing plate directly beneath the chain of
volcano (in kilometers)?

8. Most volcanic arcs in the world are located where the Benioff zone (which represents the top
of the downgoing plate!) is at approximately the depth that you determined in question 6. Why
do you think this is so?

EXERCISE 3: DETERMINATION OF EARTHQUAKE EPICENTER

Earthquakes and Seismic Waves

Earthquakes are caused by sudden movements of large rock masses. When rocks suddenly slip
past each other along a fault, seismic waves are produced. The location within the Earth where
the seismic waves are produced is called the earthquake’s focus, or hypocenter. The location on
the surface of the Earth directly above the focus is called the epicenter.

Two major categories of seismic waves, body waves and surface waves, are produced at the foci
of earthquakes. Body waves travel directly through the earth in all directions from the focus.
Two types of body waves, P-waves (Primary waves) and S-waves (Secondary waves), are
produced during earthquakes.
This table compares some of the characteristics of P-waves, S-waves, and surface waves.

Name             Type      Motion            Can travel         Relative velocity      Arrival at
through:                                  seismic
stations
P-wave           Body      Compressional:    Solids, liquids,   Fastest seismic        First
(primary)                  Alternate         and gases          waves. ~300 m/s in
“push-pull”                between           (Earth’s           air; 300-1000 m/s
compressing       interior)          in soil; over 5000
and expanding                        m/s in solid rock at
media through                        the surface
which they
travel in the
same direction
that the waves
move
S-wave           Body      Transverse        Solids only        More slowly than P     Second
(secondary)                (shear): side-    (Earth’s           waves; faster than
“Side to side”             to-side and       interior)          surface waves
up/down
vibration of
rock
Surface waves    Surface                     Travels on         Slower than either     Last
(Love and                                    Earth’s surface    body wave, but
Rayleigh)                                    only, in all       greater amplitude
directions from    (wave height).
the epicenter      More destructive
than body waves.

Earthquake detection:
Seismographs are devices that record ground motion (seismic waves) at a site. They often
consist of a pen moving across a piece of paper, which is attached to a rotating drum. The pen is
electronically linked to a seismometer, which detects ground motion caused by body and surface
waves. The pen is moved back and forth by impulses from the seismometer, recording the
amount and direction of ground motion. The record of ground motion produced by a
seismograph is a seismogram. Modern seismometers record data digitally, making interpretation
of seismic data much easier than it used to be.
Your seismogram worksheet contains three seismograms from three locations on the map: Site X
on the volcanic island, Site Y in the river delta flats, and Site Z in the alluvial plains. The
seismograms all record ground motion (seismic waves) from the same earthquake, but the time,
location, and magnitude of the earthquake are unknown. Notice that the first arrivals of the
different types of seismic waves are marked on the seismograms. An important (and very
difficult) part of interpreting seismograms is determining the first arrival times of the different
seismic waves.
P-waves are fairly easy to identify because they are the first seismic waves to appear on a
seismogram. Notice that on the seismograms in the figure below, the P-waves quickly build up
to maximum amplitude (wave height), then slowly decrease in size until interrupted by the next
arrivals, the S-waves. The first arrival of S-waves can be identified on a seismogram as a sudden
increase in wave amplitude among the waning P-waves. Surface waves are the last (and longest
lasting) seismic waves to appear on a seismogram. The first surface-wave arrival can be
identified as a marked increase in seismic wave amplitude, much greater than P- or S-waves.

Determining the epicenter of an earthquake

Remember: each type of seismic wave travels at a different velocity through the Earth. The
relationships between velocities of P-, S-, and surface waves are fairly well known and can be
used to determine distance to the focus or epicenter of an earthquake. A seismograph acts as a
stopwatch, recording the time when P-, S-, and surface waves pass. By knowing this and the
velocities of each type of seismic wave, the distance to the focus of the earthquake can be
calculated.

Alternatively, a travel-time curve can be used to obtain an estimate of the distance to the
earthquake’s focus. This estimate is made by correlating the first arrivals of each wave type with
lines on a travel-time curve. The graph gives the distance to the focus, not the epicenter. (But if
the focus is shallow, the distance to the epicenter is roughly the same as the distance to the
focus.)

The following seismogram worksheet contains three seismograms from the different locations of
sites X, Y, and Z, all on the same time scale. (Time is not shown in detail because we are not
concerned with any time calculations in this exercise.)

Directions for determining the epicenter: (refer to the figure on the previous page)
-place each seismogram (for stations X, Y, and Z) over the large travel-time graph
-move the seismogram along the curves until the first P, S, and surface waves marked on the
seismogram intersect the corresponding lines on the travel time graph. Be as precise as possible.
-remember to keep the axes of both figures parallel during this process.
-read the distance to the epicenter off the horizontal scale of the large travel-time graph

9. How far is each seismic station from the earthquake’s epicenter? Use the seismograms and
travel-time curve to determine the distance. (Units in kilometers.)

Site X         ___________________________________

Site Y         ___________________________________

Site Z         ___________________________________

10. Use a compass to draw a circle around each of the above Section VI sites, representing the
distance from that site to the epicenter (see the above for an example). If the entire circle won’t
fit on the map, just draw what you can. The circles should all intersect or nearly intersect at one
point -- the earthquake’s epicenter! (Tips: Don’t forget to convert the distance units of
kilometers to centimeters using the map scale. Use the converted value to set the compass
11. Draw a star where all three circles intersect...the star represents the earthquake epicenter.

12. Which geologic structure is closest to the epicenter? Which type(s) of force(s) generate this
type of structure?

EXERCISE 4: DETERMINATION OF AN EARTHQUAKE’S MAGNITUDE

Seismograms also provide data about an earthquake’s magnitude. Magnitude is a measure of
the energy released by the earthquake at its focus and is related to the height of the waves
recorded on seismographs. Generally, strong earthquakes produce bigger seismic waves than
weak earthquakes. The vertical axis on a seismogram measures the amplitude (distance between
middle and top) of seismic waves. For the seismograms in this exercise, amplitude is measured
in micrometers ( 1,000,000 micrometers = 1 meter).
Magnitude can be measured in several ways that yield similar, but not identical, results.
Magnitude can be calculated using the amplitude of body waves or surface waves. Because the
seismograms in this exercise are long-period, surface wave magnitude is the easiest to calculate.

Formula:               Ms = log10(a) + 1.66[log10(s)] + 2.0

Where:        Ms = magnitude based on surface waves
a = amplitude of surface waves, measured in micrometers
s = distance to the epicenter, measured in km

(all logs base 10)
13. Using the surface-wave magnitude formula given in the exercise, calculate the magnitude of
the earthquake shown on the Site X, Site Y, and Site Z seismograms. Show your work.

Site X

Site Y

Site Z

14. Because of geologic variations along the paths of the seismic waves, the calculated
magnitudes are slightly different at each location. The average magnitude is a better estimate of
the magnitude of the earthquake. What is the average of your calculated magnitudes?
EXERCISE 4: CRUSTAL SHORTENING (review from structural geology lab)

Thrust faults result in the overall shortening of continental crust. This occurs due to compressive
forces that cause Earth’s crust to essentially crumple. Terranes can be displaced up to hundreds
of kilometers along a thrust fault. In fact, thrust faults are responsible for much of the uplift of
the Rocky Mountains in the western U.S. and Canada. Generally, the greater the amount of
shortening, the greater the number of thrust faults are needed to accommodate the total amount
of shortening. Note that the cross-section on the following page has many thrust faults,
indicating that the crust was shortened significantly.

We do not know how far the “marker bed” (shown in black) traveled until it came to rest on top
of the fold-belt rocks. However, you do have the means of making an estimate.
The following steps will allow you estimate the amount of crustal shortening that the thrust faults
have produced.

15. First, mark the faults using a marker or highlighter. Then label each hanging wall (HW) and
footwall (FW) for each fault.

16. Have the hanging walls moved up or down relative to the adjacent footwalls? Which type of
force (that you learned about last week) causes these thrust faults?

17. Determine the amount of crustal shortening in kilometers and as a percentage as follows:

a) Calculate and record the present length of the entire cross-section in kilometers.

b) Find the total length of the marker bed by measuring each individual section. Note that
folds in the right, upper portion of the cross-section need to be “unfolded” because the
layers were originally horizontal. Record your answer.

c) Calculate and record the amount of shortening in kilometers or in percent as follows:

kilometers: [original length - present length]
percent: [(100) x (original length - present length)/(original length)]

18. What surface process has been occurring continuously since the rock layers were deposited?

Based on this knowledge, do you think that the percentage of crustal shortening that you just
calculated is a minimum value (ie, this is the minimum amount that the crust has shortened), or a