Module 2_ EQs
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Feb 20, 2024
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Module 2: EQs
Define the three characteristics of a wave
Amplitude- displacement
Wavelength- distance between successive waves
period/frequency-P is time between waves (=1/frequency) ; F is number of waves per time
(Hertz=1 sec)
What are the 3 types of seismic waves? Which is fastest? Which is most destructive?
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Primary wave, Secondary wave, Surface wave. The fastest wave is the Primary wave.
The most destructive wave is a Surface wave. It causes the most damage.
What is a seismograph?
-
A seismograph is the record of ground motion produced by the earthquake. .
What do these waves tell us about layers of Earth?
-
The waves tell us what the different densities and velocities are in the different layers of
the earth due to the earthquake's speed.
How do you locate the source of an EQ? How many stations do you need (at least)?
-
P waves travel about 1.7 times faster than S waves. Farther from hypocenter, greater lag
time of S wave behind P wave (S-P). (S-P) time indicates how far away the earthquake
was from the station using the three seismic stations. Must determine distance of EQ
from at least three seismic stations, P wave arrives, then S wave arrives, and lastly
Surface wave arrives last, this intersection of the circles gives the location.
What is the relationship between earthquake numbers and magnitude?
-
The relationship between earthquake numbers and magnitudes is found using logarithms
with a low magnitude causing a high frequency, and a high magnitude causing a low
frequency.
What is the difference between mainshock and aftershock?
-
The mainshock is the biggest event to occur and the aftershocks are smaller events that
continuously happen close together after the mainshock which then trails off.
What do you need to determine magnitude on Richter Scale?
-
The magnitude on the Richter scale is a logarithmic scale that combines the S-P time
movements and the amplitude to find the magnitude scale in the middle.
What is Richter scale good for? Bad for? Why bad?
-
Richter scale is good for magnitude of shallow, small moderate nearby earthquakes. It
does not work well for distant or large earthquakes, this is because it cant accurately
calculate the magnitude due to the farther distance of the earthquake.
What is the difference between magnitude and intensity?
-
The difference between magnitude and intensity is the magnitude of an EQ is a measure
of how big it is-how much energy is released compared to the Intensity (Modified
Mercalli) of an EQ which is a measure of how much shaking occurs at any place and how
much damage it does.
What kind of shaking does massive damage to buildings? Why?
-
Horizontal shaking does massive damage to buildings, because of the side to side motion
as well as acceleration this furthers damage because of how buildings are designed to
only handle vertical forces (weight of building and contents).
What level in the Modified Mercalli Intensity does damage possibly start to create many loss of
lives on a poor country (i.e. Haiti)?
-
Weak buildings suffer more damage from horizontal accelerations of more than 0.1g
(=0.98 m/s^2)
What ground type is most destructive in terms of wave energy?
-
The ground type that is most destructive in terms of wave energy is the water-saturated
sand and mud (softer ground)
What is liquefaction? What happens to large structures during liquefaction?
-
Liquefaction is water saturated soils that become a slurry (liquid-like) when shaken.
Buildings that can tilt, sink and fall.
How does the natural period of swaying change for different heights of buildings?
-
Natural Periods of swaying are about 0.1 second per story. 1- story houses shake at about
0.1 second per cycle (short period=fast=high frequency). 30-story buildings sway at
about 3 seconds per cycle (long period=slow=low frequency).
How tall of a building would you prefer to be in during an earthquake?
-
I would prefer to be in a modern 1-2 story wood frame house with shear walls, bracing ,
tying walls, and foundations and roof together.
What lacking structure in building properties causes building floors and elevated highways to
collapse?
-
Lack of shear walls can cause collapses in buildings like parking garages which have
many floors, and bridges/elevated highways.
What can be installed on the ground to assist buildings in an EQ?
-
You can install base isolations to assist buildings in an EQ which separate the ground
movements from the building absorbing the earthquake's energy.
Where do the largest EQs occur?
-
In subduction zones you have the biggest earthquakes as well as the most. This is usually
in the Ring of Fire. This is due to the fact that plate tectonics are coming together creating
more pressure. The world's largest earthquakes “Megathrust” on the plate boundary
which are shallow.
Where are the most EQs in the US (i.e. seismic risk)? What plate boundaries are here (there are
two)?
-
The most EQs in the US happen in California, the plate boundaries here are the San
Andreas Fault boundary, and the coastal convergent boundary.
What causes the EQ wave amplification in basins?
-
The cause of EQ wave amplification in basins is due to the sedimentary ground that is
present in valleys interacting with the waves.
What was the ground acceleration in the Japanese earthquake?
-
The ground acceleration in the Japanese earthquake was 2.99 x gravity, 0-60 mph in 1s.
What does the HTEW video on Japan coast illustrate?
-
The HTEW video on Japan illustrates the subduction zone showing us how the coast was
pushed out of water and snapped. The town dropped several feet, the coast went from 9 ft
to 6ft. The sea defense was overcome by the land dropping and the waves. There was
mass devastation as everything in the waves path was destroyed.
Was the Japanese earthquake felt in Tucson? Did it effect Earth’s spin? How?
-
Yes, the Japanese earthquake was felt in Tucson 6 times. It went around the earth as we
are a third of the world away so we felt it. It did affect the earth's spin by changing the tilt
by 4-10 inches increasing the earth's spin by 1.8 microseconds.
Where do continental collision earthquakes occur?
-
Continental collision earthquakes occur in convergent zones.
Why was the magnitude 7.8 Nepal EQ in 2015 so destructive?
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The 7.8 Nepal EQ in 2015 was so destructive because there are constant earthquakes
going on along the coast because India is moving 5 cm per year. This is a problem
because building codes in this area aren't up to date and have very little rebar and codes.
There was also a major avalanche at the base camp of Mt. Everest killing 17 people.
Another factor was the fact that the earthquake hit in Paleo Lake which had lots of
sentiments causing amplification of the shaking.
Where is the largest strike-slip boundary on Earth?
-
The largest strike-slip boundary on Earth is The San Andreas Fault.
Where was the earthquake shaking amplified in southern California?
-
The earthquake shaking was amplified in southern California in the valleys with
unconsolidated sediment.
What is causing the basin and range faulting in SW US?
-
A spreading center caused by a subduction zone is stretching creating a Basin and Range
which is repetitive even creating the Catalina Mountain range.
When and where was the largest EQ in AZ?
-
The largest EQ in AZ was the 1887 Pitaycachi Sonora earthquake with a M7.6, there was
great destruction near the epicenter with 42 casualties due to weak structures since there
wasn't the right technology at the time to prepare.