Science Question

I have attached the assignment below

I need the assignment done within 6 hours

EAS-E141 Earthquakes and Volcanoes
Lab 5: Earthquake Location & Magnitude
Name:__________________________________
Determining Earthquake Location
*adapted from Brophy & Hamburger Earthquakes and Volcanoes: Lab Manual
and from Cronin & Tasa Earthquake Hazards
Goals:
•
•
Learn to use seismogram readings to determine seismic wave travel time
Learn to use seismic wave arrival times to locate earthquakes
Introduction
Seismograms are the main tool used by seismologists (or those who study earthquakes). They contain the
primary observational data that the scientists need to glean information on the earthquake sources, as well as
on the propagation of seismic waves along the path from source to receiver. For nearby earthquakes, the
main observables are:
• Arrival times of P and S waves
• Direction of wave movement
• Amplitudes of waves
• Seismic wave frequency
These observables can, in turn, be used to determine the main parameters of the earthquake source:
• Origin time of the earthquake
• Epicenter location (latitude and longitude)
• Focus (hypocenter) depth
• Earthquake magnitude
They also can be used to infer physical properties of the rock through which the seismic waves pass:
• Velocities of P and S waves
• Nature of the contacts between different rocks or faults
In this laboratory, we will examine seismograms recorded by seismic networks in the Southwest Pacific.
This remote part of the world, including the tiny island countries of Fiji, Tonga, and Vanuatu, is home to
some of the most active earthquake belts in the world. Then, in the second part of this lab, we will use some
“real-life” seismograms to determine earthquake parameters for several recent earthquakes.
Part 1: Working with Seismic Wave Travel Times
In general, P and S wave velocities within the earth vary strongly with depth, but only vary gradually with
lateral position Within the earth’s crust, P-wave velocity varies from about 3–7 km/sec (or 7,000–16,000
mph!!), and averages about 5.7 km/sec. S-wave velocities vary from about 2–4 km/sec and average about 3.3
km/sec. Here we set P-wave velocity to be 6 km/sec and S-wave velocity to be 3.4 km/sec. Use these
average seismic wave velocities to construct a travel -time chart for P-waves and S-waves.
Question 1: Using the blank graph paper on the following page, first make a graph of distance versus P and
S wave travel times, using the x-axis for distance in km, and the y-axis for travel time in seconds; next make
a graph of distance versus S-P travel times differences, using the x-axis for distance in km, and the y-axis for
travel time in seconds;
As the distance between an earthquake and seismograph increases, the P- and S-wave travel times and the
time delay between them, increases. Using the relations represented graphically on your travel-time chart
above, answer the following questions. Show your work where needed.
Question 2
If the “origin time” of an earthquake, known to be 9:00:10.0 from the world-wide seismic station reports, and
the P-wave arrives at 9:00:34.0, how far away was the seismic station from that earthquake’s focus
(hypocenter)?
Question 3
If a seismic station reports a P-wave arrival at 2:45:42.0 and an S-wave arrival at 2:46:01.0, what is the
distance to the focus? What is the origin time of the earthquake?
Question 4
If a seismic station at a distance of 300 km from an earthquake records a first arrival at 12:18:53.0, at what
time would you expect the S-wave arrivals to be recorded?
Question 5
You are hanging out in the seismograph station here at IU on the first floor of the geology building, chatting
on the phone with your friend from New Madrid, Missouri (distance = 380 km), when suddenly your friend
interrupts your conversation with a scream, “Jupiter’s thunder!! We are having an earthquake!!!” You look
down at the seismograph and notice that the pens are quiet. How long will you have to wait until the P-waves
would be expected to arrive at Bloomington? The S-waves?
P-waves: _______________________
S-waves: _____________________________
Part 2: Using Earthquake Arrival Times to Locate Earthquake Epicenters
We now know how to determine the distance of an earthquake from a seismic station, but this information
only gives us the distance, not the direction or location of the earthquake focus. For example. If we calculate
that an earthquake occurred 100 km from a seismic station, we know that the earthquake occurred anywhere
around a 1000 km radius from the seismic station. In the example below, the seismic station is shown with a
triangle, and the radius along which the earthquake occurred is drawn:
In order to determine the location of an earthquake, we need at least 3 seismic stations. This allows us to
triangulate the location where all three circles meet. In this last part of the lab, we will determine the location
of an earthquake epicenter. You are given a new S-P travel time plot for this which extrapolates the data to
longer time periods than what you plotted in your S-P travel time plots. Note that the y-axis is in minutes
rather than seconds. This new plot also includes the travel time of surface waves, but it is not necessary to
use for this exercise.
Surface wave travel time (do not use)
S-wave
P-wave
You are also given three recordings of the same earthquake event recorded at three different seismic stations.
One is Sitka, Alaska, another in Charlotte, North Carolina, and the third in Honolulu, Hawaii. Only the P and
S waves are shown on this seismogram, the surface waves are cut off. Also note that the time on the x-axis
is shown in hours:minutes.
Step 1: Using the seismogram readings, determine the times that the P-waves and S-waves first arrived at
each seismic station (try to be as accurate as possible; estimate to the nearest tenth of a minute) by filling in
the following table.
P arrival
S arrival
S–P (minutes)
Sitka, AK
Charlotte, NC
Honolulu, HI
Step 2: Using the new S–P travel time plot and the data you collected in Step 1, determine the distance from
the epicenter in km for each seismic station.
Sitka, AK: ________________
Charlotte, NC: ________________
Honolulu, HI:_______________
Step 3: On the map below, and using a drafting compass, draw a circle around each recording station. Make
the radius of each circle equal to the distance from the epicenter determined in Step 2. Hint: use the scale on
the map to set this radius on your drafting compass. The circles you draw should intersect at approximately
one point on the map. This point is the epicenter. If the three circles so not quite intersect at a single point,
find a point that is equidistant from the three edges of the circles, and use this as the epicenter.
Sitka
Charlotte
Honolulu
Question 6
Your three circles may not have intersected exactly at a single point. How could you improve your results?