Lab 1 Key

Question 1

Choose a map projection and explain what it depicts accurately, what it depicts inaccurately,
and what it may be useful for.

Answer 1

Mercator, doesn’t preserve area, used for navigation.

Question 2

Which volcanic eruption led to the year without a summer in 1816? (1 pt)
a. Mt. St. Helens
b. Mt. Pinatubo
c. Mt. Kilauea
d. Mt. Tambora

Answer 2

Mt. Tambora

Question 3

Why did the eruption from question 2 cause the year without a summer?

Answer 3

The aerosols from the eruption blocked the sun from reaching Earth, thus lowering its temperature by
~.5-1 degree C. (+1 for mention of aerosols +1 for relating to global climate change).

Question 4

Which type of seismic wave will reach each hospital first? (1 point)
(a) P waves (b) S waves (c) Surface waves

Answer 4

P Waves

Question 5

Which earthquake poses the greatest potential hazard to the city of LA: the next “Big One”
earthquake modeled after the M 7.9 1857 Fort Tejon earthquake, or a smaller M 7.1
earthquake on the Puente Hills fault, located within Los Angeles?

Answer 5

Puente Hills

Question 6

In order to further reduce risk to LA, list three other types of municipal targets besides
hospitals that you, as mayor, may be interested in retrofitting.

Answer 6

schools fire stations
power stations water mains
gas mains sewage
bridges city hall/courtrooms, etc.
tunnels skyscrapers
Food Warehouses office buildings

Question 7

What type of plate boundary does the San Andreas Fault lie on?

Answer 7

Right-lateral Strike-slip

Question 8

What type of stress (compressional, tensional, or shear) is causing earthquakes along the
San Andreas Fault?

Answer 8

Shear

Question 9

Given your answer above, how is it possible to have thrust faults like the Puente Hills fault
in Southern California? Examine the diagram below to aid in your answer

Answer 9

Thrust faults are a result of compression –
because there is a bend in the SA Fault, the
strike slip shear stresses are no longer sliding
past each other, but instead are moving into
each other, or towards each other, in a
compressional manner.

Question 10

Which of the following layer(s) of the Earth is a liquid?

Answer 10

Outer core

Question 11

In which layer of the Earth do most earthquakes occur?

Answer 11

Crust

Question 12

Give two pieces of evidence for plate tectonics.

Answer 12

Continent borders match up
Fossil zones match across the Atlantic
Paleoclimate record shows that places have moved from tropical climates to
places that are no longer tropical (or vice-versa)
Rock types match across the Atlantic
Ages of ocean rocks
Magnetic stripes on seafloor
Volcanic/earthquake/geothermal activity at plate boundaries
Hot spot tracks
Mid-ocean ridge topography
Wadati-Benioff zones at subduction zones

Question 13

A Magnitude 8 earthquake is expected to occur in California during our lifetimes! It has been
17
suggested that if it were possible to trigger small earthquakes, perhaps the strain over the San
Andreas Fault could be relieved intentionally by producing small earthquakes, and thereby
avoiding the big one. Use the figure below to find the total energy released by a Magnitude 8
earthquake and compare this value with the amount of energy released by a Magnitude 4
earthquake. How many small earthquakes (Mw = 4) would have to occur in order to release
the same amount of energy as one Magnitude 8 (Mw = 8) earthquake?

Answer 13

56,000,000,000/56,000 = 1000000 … so one million times more energy
You would need 1 million M4 earthquakes to release the same amount of energy as a M8
earthquake.

Question 14

Given your answer above, how would you respond to a suggestion that we should
intentionally trigger small human-induced earthquakes to avoid large, more intense natural
earthquake shaking? Is this a sound mitigation strategy? Why or why not?

Answer 14

It’s just not feasible to trigger a million small earthquakes.

Question 15

Earthquake focal mechanisms (“beach balls”) are produced automatically following large
earthquakes. What important piece of information can these focal mechanisms provide? Can
you determine the fault orientation from the diagrams and why?

Answer 15

Sort of…you can narrow down the fault orientation to two locations, but you need to be given
the specific orientation

Question 16

What type of surface wave is this shake table capable of simulating?

Answer 16

Love waves - only horizontal motion, no vertical ‘dip’ slip motion.

Question 17

Given your answer above, what is the orientation of stresses you expect to be strongest on
your model? You may answer this question by drawing a depiction below.

Answer 17

We expect the orientation of the stresses to be along ‘strike’ or horizontal in 2
directions. This will cause shearing between layers of the building.

Question 18

What is resonance? How is resonance different from regular shaking?

Answer 18

Resonance is the tendency of systems to vibrate (or oscillate) at large amplitude at
certain frequencies. When the frequency of the force or motion coincides with the
inherent frequency of the object, structure or building, resonance occurs and the
amplitude of oscillation increases.

Question 19

What are the four variables that must be accounted for when calculating Peak Ground
Acceleration?

Answer 19

a) Magnitude of Earthquake (M), full points for Magnitude
b) Distance between A and epicenter (r), full points for Distance
c) Depth of the sedimentary basin (d), full points for Depth of basin
d) Velocity of the S-waves traveling through the earth at 30 meters depth (Vs), full points for
Velocity of S-waves

Question 20

Two earthquakes discussed in lecture in detail, 2010 Chile and 2010 Haiti, had a
magnitude difference of 1.8, occurred in the same year, but yielded vastly different
fatality numbers. A comparison of these two earthquakes showed that the earthquake
with the lower magnitude resulted in a significantly greater loss of life and number of
people injured. List the magnitudes of these two earthquakes and then cite the
approximate loss of life for your two chosen examples.

Answer 20

2010 Haiti – Port au Prince (7.0) with >200000 dead
2010 Chile - Maule (8.8) with >700 dead.

Question 21

What was the main factor that contributed to the difference in life lost for these two
earthquakes.

Answer 21

building designs in Haiti – brittle, unreinforced buildings which tend to fail easier during
an earthquake

Question 22

What is liquefaction? Why might Back Bay be particularly dangerous if an earthquake
occurred in Boston?

Answer 22

Liquefaction is when sand or loose soil behaves like a liquid when shaken or
vibrated…because the grains are moving with so much energy in the system, the sand starts
to flow like a liquid.
Back bay would be dangerous because it’s made of filled in sand – it used to be the Charles
River! So, it could liquify in a large earthquake.

Question 23

It has been said by some that San Francisco was built by the grace of the 1906
earthquake. Given that this earthquake was particularly devastating for San Francisco,
what does this expression mean?

Answer 23

The Coulomb failure stress due to the 1906 earthquake is negative in the bay area. This
means that the 1906 earthquake released the stress in the area, and the area has been in a
“stress shadow” since 1906. Very few earthquakes have occurred in the bay area since 1906
because there was not that much accumulated stress, and so people had time to build up San
Francisco without any major destructive earthquakes

Question 24

What is the average displacement that you would expect from an earthquake with
Mw=8.3? (Please show all your work.)

Answer 24

log10(AD) = (0.69Mw) – 4.8
log10(AD) = (0.69*8.3) – 4.8
log10(AD) = 0.927
AD = 100.927
AD = 8.45 m

Question 25

The velocity of a tsunami wave can be calculated based on two parameters: the gravitational
constant and the depth of the ocean basin. Using the equation for tsunami wave velocity,
calculate the velocity of a tsunami wave traveling across the Pacific basin (average depth = 4350
meters).

Answer 25

Velocity = sqrt(gD) = sqrt(9.8 m/s^2 x 4350 m) = 206.47 m/s

Question 26

Assuming the velocity remained relatively unchanged, how long would it take a tsunami to travel
from Maule, Chile to the Hawaiian Islands some 10,842 km away. Use the velocity you
calculated above and report your answer in hours.

Answer 26

Time = Distance/velocity =(10842 km x 10^3 meters/1km) / 206.47 m/s = 5.2511 x 10^4 s =
14.5-14.6 hours

Question 27

On what type of plate boundary did the 2024 Sea of Japan/Noto earthquake occur?

Answer 27

Subduction Zone

Question 28

Japan has built an extensive network of well-developed sea barriers to protect against oncoming
tsunami. Despite these barriers, the Tohoku earthquake and its resulting tsunami caused extensive
devastation and flooding. Name two factors that )contributed to the failure of the sea walls
following Tohoku?

Answer 28

1. big tsunami weakened them
2. earthquake lowered coast by up to 1 m
3. water got behind the sea walls, eroded behind them, and then they toppled over
4. accept damage due to earthquake shaking even though it wasn’t that intense because the
   subduction zone was so far away.
5. They weren’t high enough

Question 29

Which of the following was the focal mechanism for the mainshock (Circle the correct answer)?
North is up.

Answer 29

Reverse fault

Question 30

Why are early warning systems for tsunamis easier to develop than early warning systems for
earthquakes?

Answer 30

Tsunamis take much longer to get to places that they affect. There’s time for the warning to have an
effect.

Question 31

Is it likely that an earthquake on the Puente Hills fault (from Lab 2) would cause a large
tsunami?

Answer 31

No, because earthquake on the PH fault would occur on land.

Question 32

What information available within minutes after the earthquake would tell you if a tsunami
could have been generated?

Answer 32

Location of Earthquake

Question 33

If you are sitting on a beach, what is a major warning sign that might indicate a tsunami if you do
not feel any shaking?

Answer 33

Receding Water

Question 34

Why is it important to consider both the wind speed and wind direction when assessing
tephra hazard zones? Give one reason each for wind speed and wind direction.

Answer 34

wind direction because it defines which direction the tephra will blow

Question 35

What aspects of Mount Rainier’s magma composition results in the explosive nature of most
stratovolcanoes? Give two aspects and explain why.

Answer 35

Silica content, High volatile content

Question 36

Which of the three hotspots is located in a region that is associated with volcanic melt
production in two distinct ways?

Answer 36

Iceland

Question 37

Draw phase diagrams for and explain the two melt generation processes that occur at
the hotspot you listed in (a). Make sure to include the solidus and geotherm in your
diagram and clearly explain how each will be affected during each melt generation
process.

Answer 37

Increase the temperature [1 point]
i. The geotherm approaches the solidus
ii. When the two intersects, melting occurs in a parcel of mantle that rises
Lower the pressure [1 point]
i. Pressure is decreased as a result of plate spreading and decompression
melting occurs as mantle buoyantly rises adiabatically (due to the decrease in
pressure)
ii. Occurs at divergent plate boundaries