GEOSCI 106 Lab 4_ Earthquakes- MARCH
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University of Wisconsin, Madison *
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Course
106
Subject
Geology
Date
Dec 6, 2023
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8
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GEOSCI/ENVIR ST 106: Environmental Geology
Lab 4: Earthquake detection and seismic hazards
Assignment Overview:
Plate tectonics can have major impacts on human life in the form of earthquakes.
Geologists collect a variety of data about historical earthquakes to understand future risks. In this lab, you
will use data from maps and seismic stations, along with simple computations, to explore these hazards.
You will be acting in the role of a seismologist, using data from seismic sensors to determine the location
and time of an earthquake that has already taken place. You will then use this information to determine
which nearby sites can be effectively warned in time, and how much advance warning you can give them
before damaging surface waves hit them.
Instructions:
1.
Download the “Pacific NW seismicity.kmz” dataset associated with this lab in Canvas.
2.
If you do not have Google Earth Pro, download and install it. It is freely available here:
https://www.google.com/earth/versions/
. Note that you will need Google Earth Pro, not the
version accessible in a web browser. The web browser version does not have all the
functionalities that you will need to complete this lab.
3.
Fill out each red highlighted field (_________) according to each question’s instructions.
Submission:
To submit the assignment on Canvas, use the following steps:
1.
In Google Docs, generate a PDF: File → Download as → PDF Document
2.
In Google Docs, use Share → Get Shareable Link, and copy the link address
3.
In Canvas, upload your PDF to the assignment.
4.
In Canvas, paste the link address to your Google Doc in the assignment comments.
1
Background
The energy released by an earthquake is transmitted through the Earth as vibrations in several different
ways. Body waves – much like sound waves through air – travel through the Earth itself, and are
relatively fast. These include pressure (or primary) P-waves and shear (or secondary) S-waves. Surface
waves – much like ripples moving across a pond – travel primarily along the surface of the Earth, and are
somewhat slower. Surface waves are especially important because they are the waves that are almost
entirely responsible for damage to structures. The propagation and speed of these waves is shown below
in Figure 1.
Figure 1:
Cartoon of energy movement away from an earthquake and toward a seismic observation
station, including approximate propagation velocities for P-waves, S-waves, and surface waves.
An example of a record that might be recorded by a seismometer is shown in Figure 2, along with the
different wave arrivals highlighted on the record.
Figure 2:
Example of an seismograph record (seismogram) with arrows showing the arrivals of the P-
and S-waves. Notice that the surface waves arrive after both the P- and S-waves. This implies that the P-
and S-waves provide a brief warning before the damaging surface waves arrive.
Seismologists and, increasingly, computers, can detect and differentiate these different types of vibrations
to determine the arrival time of each wave, i.e., the time at which they arrive at the seismometer. As
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described in the textbook, the difference in the arrival times of seismic waves of different speeds (e.g., a
P-wave and an S-wave) tells us how far away the earthquake was. Just like anything moving at a set
speed, its arrival time at a location is determined by when it started, how fast it was moving, and how far
it had to go. Said more mathematically:
Arrival time = start time + distance / velocity
For an earthquake, we are able to measure when each wave arrives (arrival time) and we can roughly
know its velocity, based on prior knowledge of how different types of waves travel through different
types of rock. We don’t know in advance how far away the earthquake was from the seismometer that’s
measuring the waves (distance), or the time the earthquake started. In this lab, you will use information
from multiple seismometers to determine where and when an earthquake happened.
Based on the equation above and some algebra (see the appendix at the end of this lab), we can use the
following two equations to estimate an earthquake’s distance from a seismometer and the earthquake’s
start time.
Distance = (S arrival time - P arrival time) / (1 / S velocity - 1 / P velocity)
Start time = S arrival time - distance / S velocity
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