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# How Do We Measure Earthquakes

By Helen Clark,2014-08-09 05:23
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How Do We Measure Earthquakes

How Do We Know Where an Earthquake Originated?

Teacher's Guide & Student Worksheet

Time: Approximately 1 hour

5E Phase: Explain This lesson is designed for the Explain phase of a 5E Model and might follow http://www.iris.edu/hq/resource/seismac_activity_1 as an engage and

explore activity. Other Engage/Explore options include the use of http://www.iris.edu/hq/programs/education_and_outreach/aotm/9

Objectives: By the end of this activity, the student will be able to: 1. Identify P and S waves on seismograms,

2. Determine the distance of an epicenter from a seismic station using travel time curves,

3. Locate the epicenter of an earthquake by triangulation, and 4. Calculate the time of origin of an earthquake based on seismic data

Materials Needed for each group of 4 students:

; 4 three-component seismograms for the same earthquake (see Page 6 for instructions)

; 1 copy of the Travel time curve (Appendix A or B)

; 1 globe (inflatable globes are available online for \$3-\$7 each), or map

(http://www.iris.edu/hq/resource/online_triangulation) for locating epicenters online ; String and overhead pen, if using globe, or safety compass (available online for \$2), if

using map

; Map with tectonic plates illustrated (http://pubs.usgs.gov/imap/2800/)

; Map with global and/or regional seismicity illustrated

(http://earthquake.usgs.gov/regional/world/seismicity/index.php)

Grades 6-8: Scientific Inquiry 1 B/1, 2

Mathematical Inquiry 2 C/2

Technology and Science 3 A/2

Processes that shape the Earth 4 C/ 1, 2

Grades 9-12: Scientific Inquiry 1 B/2

Technology and Science 3 A/3

Processes that shape the earth 4 C/5

Symbolic Relationships 9 B/3

Common themes: Models 11 B/3

1 Jeffrey Barker, Binghamton University & Michael Hubenthal, IRIS Consortium Questions or comments: jbarker@binghamton.edu

Prior to the Activity:

1. Provide students with instruction covering the basic differences between P and S waves.

A suggested activity for this instruction includes:

http://www.iris.edu/hq/files/programs/education_and_outreach/aotm/6/Activity-HumanWaves.pdf

2. Provide students on the basics of seismographs.

http://www2.bc.edu/%7Ekafka/BCESP_WebCurriculum/1_BuildYourOwn_Seismograph/Build_Your_Own_Seismograph.html

Lesson Description

1. Instruct students how to identify P & S waves on a seismogram

2. The student should be familiar with the travel time curve, and what it represents.

They should be able to identify the P and S wave travel time curves (See

Appendices A & B)

3. Note that different time and distance scales are used on seismograms, travel time

curves, globes and/or maps.

Student Procedures

1. Identify P and S waves in their seismograms

2. Measure the time between arrival of the P and S wave

3. Use this time to look-up the distance the epicenter is away from the station using the travel-time-curve

4. Measure out a length of string equal to the distance the earthquake epicenter is from the station. If using a globe, use the distance scale on the equator, or the distance scale on a map

5. Locate the station on the globe using the latitude and longitude of the station 6. Hold one end of the string on the stations location

7. Use the string to sweep out an arc on the globe representing the possible locations of the earthquake epicenter

8. Work together with other students that had the same earthquake (they should sweep out arcs from their stations as well) to narrow down the possible locations for your earthquake

2 Jeffrey Barker, Binghamton University & Michael Hubenthal, IRIS Consortium

Teacher Background:

A tremendous number of earthquakes occur every year around the world. The vast majority of these earthquakes are too small to be felt by humans. The larger ones, having a magnitude greater than 5, can effect human property and life if they originate near a populated area. However, populated areas do not make up a very large portion of the earth's surface. For this reason, most of the earthquakes that occur each year are not felt either because they are too small, or they are in an area where no one is around to make a direct observation of their movement.

To understand Plate Tectonic processes on the Earth, and to better understand where future earthquakes are likely to occur, it is important to know how to locate earthquakes as they occur. The primary tool seismologists use to locate an earthquake's point of origin is a network of seismometers. Seismometers are designed to be sensitive enough to feel even the smallest motion of the waves coming from distant locations on Earth. By combining measurements of wave arrival times at several seismograph stations, the earthquake’s source can be located.

There are several hundred seismic stations around the world; each one continuously monitoring the Earth's surface for shaking caused by an earthquake. Organizations such as the Incorporated Research Institutions for Seismology (IRIS) gather and archive data from these stations in a central database. These data can then be accessed by anyone interested in learning more about earthquakes.

Some other things to be aware of are that:

; Modern seismic stations produce a 3-

component seismogram simultaneously. One

seismogram shows vertical motion, one

shows east to west motion, and another

shows north to south motion (Figure 1)

; Because body waves come up from below

the surface, P wave motion is usually greatest

on a vertical component seismogram, while

the S wave usually registers a stronger signal

on the horizontal component seismograms

(Figure 2).

Figure 1 Modern seismic stations produce a 3-; Students will need help in identifying P and component seismogram.

S waves on seismograms. The P wave is the

first signal to appear on the seismogram, and is best identified on the vertical

component. The S wave arrives later, and should be the first signal to arrive with its

amplitude on the horizontal components larger than that on the vertical component.

The S wave may be a broader pulse than the P wave (longer period, or lower

frequency). There may be several arrivals of P waves before the S wave arrives;

students will interpret only the FIRST P wave and FIRST S wave.

3 Jeffrey Barker, Binghamton University & Michael Hubenthal, IRIS Consortium

P Wave S Wave UP UP

Motion

NORTH NORTH

EAST EAST

Motion

P Wave S Wave

from below from below

FIGURE 2 Since the motion of a P wave is in the direction the wave travels, for a wave coming up from below the seismograph station, the motion of the ground will be mostly vertical (up and down), with a smaller amount of motion on the horizontal components (North and East). By contrast, since the S wave motion is perpendicular to the direction of wave travel, an S wave coming up under the station will produce larger motion in a horizontal direction which will be some combination of the North and East component directions, and smaller motion on the vertical component. This concept can be clearly seen in Figure 1 and can also be shown using the animation available from

http://www.iris.edu/hq/programs/education_and_outreach/aotm/9

; Travel time curves (Appendix A & B) indicate the arrival times of various waves

with increasing distance from the earthquake source. Time is plotted in minutes and 0distance is plotted in degrees (the opposite side of the Earth is at a distance of 180).

The P wave curve and the S wave curve should be indicated, and these are the ones the students will use. There are many other P and S wave curves, representing reflections and refractions from layers within the Earth. These will not be used, but will help students see what the later-arriving P waves and S waves represent.

; Measuring the S wave and P wave arrival times from only one station gives the distance from that station to the epicenter. However, the earthquake could be located anywhere on a circle centered on that station with a radius equal to the calculated distance. An estimate of the time of origin is also calculated.

; Measuring arrival times at two stations should give you two possible locations of the epicenter, where the two circles intersect.

4 Jeffrey Barker, Binghamton University & Michael Hubenthal, IRIS Consortium

; Arrival times measured at three stations, minimum, are needed to triangulate

epicenter location to a single point. Using more than three stations supplies

redundant information, which is helpful if any errors in measurement or interpretation

occur with the data from any one of the stations.

; Each student should work on seismograms from one station. Only by cooperating

with other students who have worked on the same earthquake, but at different stations,

can the earthquake be located. By teaming up, sharing data, and discussing

interpretations and procedures, students can find their own mistakes in calculations.

; Due to the inherent distortion in most maps, caused by making a 3 dimensional object

into 2 dimensions, the scale changes as you move north or south of the equator. In

order to triangulate for earthquake location, it is important that both distance and

angle from each station to the earthquake epicenter be accurate. This is possible only

for carefully chosen map projections centered near the earthquake location. Of

course, this gives away the answer; the earthquake will be near the center of the map.

; By using a globe, the problems of map projection is eliminated. The shortest path

between seismograph station to earthquake epicenter is often over the North or South

pole, and this is easily measured (and seen) on a globe. To measure distance on a

globe, students should measure a length of string along the equator (a great circle).

Placing one end of the string at the seismograph station location, the circle swept out

by rotating the string around this gives all possible locations for the earthquake source.

; It can be a bit unwieldy having three or four students simultaneously measuring

distances on a single globe, but this enables them to find and visualize the intersection

point needed for triangulation. It will be necessary to have several globes or to

schedule their use so that groups who have interpreted seismograms for the same

earthquake can access the globe together.

5 Jeffrey Barker, Binghamton University & Michael Hubenthal, IRIS Consortium

Instructions for Retrieving Seismic Data:

1. Visit the Rapid Earthquake

Viewer (REV) at

http://rev.seis.sc.edu/. Select

“Earthquake View”

2. In the Earthquake View,

select a recent event of

interest by clicking on a

circle. Hint: Larger

Earthquakes generally

produce clearer arrivals so

choose large events when

possible.

Note: If multiple events overlap, a drop down menu will appear to allow you to select the event that interests you.

6 Jeffrey Barker, Binghamton University & Michael Hubenthal, IRIS Consortium

3. From the Record Section window, you will be able to select which stations you want to get data from. Do this by noting the station symbol (colored triangles at the top of the record section) and clicking on corresponding station code in the table below the seismograms. If there are gaps in the records or you don’t

zone. Choose stations with a distance less than about 100 degrees.

4. This will open a three-

component view of the

seismograms for the

earthquake. From this

window you can do the

following;

a. Make note of the date,

time and location of

the earthquake (the

solution), or print this

screen.

b. Toggle the calculated

p/s wave arrivals on

and off. (This is

picks.)

5. Save or print the seismograms by clicking on the

“Save seismogram button.”

6. User your browser’s back button to return to the Record Section view to select additional stations (See step #3) so that each group has 3 to 4 stations’ recordings of

the same event.

7 Jeffrey Barker, Binghamton University & Michael Hubenthal, IRIS Consortium

7. Once you have enough stations for that event, use your browser’s back arrow to

the class.

Possible Student Pitfalls:

1. Students may fail to realize that the three components of the seismograms indicate ground motion simultaneously in three directions. They sometimes think that one component follows the others in time.

2. Students may have trouble identifying P and S waves. Rather than interpreting the first P wave and the first S wave, they may be drawn to the largest waves.

3. Students may not measure the difference in time between the S wave arrival and the P wave arrival. Instead, they may simply measure the arrival time of one or the other wave. Students may also have difficulty with measurements on the time scale of the seismograms.

4. Students may not realize that the travel time curve likely has a different time scale than the seismograms. After measuring the time difference appropriately, students may not keep their measurements on the travel time curve aligned with the time axis. This is necessary in order to measure time difference at a single distance. Finally, students may have difficulty reading this distance from the travel time curves.

5. Students may have difficulty measuring angular distance on a globe. They may try to measure distance along lines of latitude which are not great circles. They may confuse the latitude and longitude of their station with the angular distance determined from travel time curves.

6. Students may have difficulty locating their seismograph station on a globe. They may confuse latitude and longitude, or may neglect whether these are N, S, E or W. This is a useful learning exercise. The seismograph stations have letter names which indicate the nearest town or region, but these may not be labeled on the globe.

7. Even with no apparent errors in procedure, students’ triangulations may not meet at a

point. The most likely cause is that the earthquake occurred at a depth significantly different than assumed for the travel time curve. With the expected level of measurement error in time and distance in the various stages of this exercise, it is not unusual to have a gap of a few degrees in triangulating. Larger misfits probably indicate misunderstandings in interpretation or incorrect procedure. If more than three students interpret seismograms from a single earthquake, some sets of three may find consistent results, while others have discrepancies. This indicates an error by one of the students in the group with discrepancies.

8 Jeffrey Barker, Binghamton University & Michael Hubenthal, IRIS Consortium

Student Worksheet Name:

Date:

1. Identify P and S waves on your seismogram

What is the P arrival time? ____________________

What is the S arrival time? ____________________

2. Determine the amount of time between arrival of the P and S wave (S-P time)

________________________

3. Determine the distance the epicenter is away from your station using the travel time

curve. __________________ Degrees

a. Do this by creating a marker (on a

piece of paper) on the y-axis of the

graph that is equal to the S-P time you

found in #2 above.

b. Keeping the paper edge parallel with the y-

axis, slide the paper to the right and upward.

Find the spot in the graph where the marker

you created fits between the S and P curves on

the travel time curves.

c. Read down from the edge of the maker to the

x-axis to determine the distance from the

station to the epicenter. Read down to the x-axis here to get distance to the epicenter

4. Measure a length of string proportional to the distance from earthquake’s epicenter to

the station (determined in 3c above) on the globe. Hint: the equator is the best place for

measuring this length.

9 Jeffrey Barker, Binghamton University & Michael Hubenthal, IRIS Consortium Questions or comments: jbarker@binghamton.edu

5. Find the station on the globe using the latitude and longitude of the station

6. Hold one end of the string on the station location. Stretch out the string and use the other end to sweep out an arc on the globe. This arc represents all the possible locations of the earthquake epicenter.

7. Describe what you learned from analyzing your data alone.

8. Work together with other students that had the same earthquake to narrow down your search by having them sweep out an arc for their station.

9. What was the latitude and longitude of the location where all three (or four) strings met? Lat: ________________________

Long: _______________________

a) What is the nearest geographic location to this point?

10. Describe, step-by-step, how using more data narrowed down the possible epicenter locations.

11. Locate the epicenter on the map showing Earth's tectonic plates on the last page of this handout. Based on the map, is your solution logical? Would you expect an earthquake in this area to be fairly common or rare? Explain your answer fully.

10 Jeffrey Barker, Binghamton University & Michael Hubenthal, IRIS Consortium