Seismic activity from Hurricane Floyd recorded in Atlanta (Vertical Scale 250 mV per division)


Not Everything We Record Is an Earthquake

(An Exercise Using Hurricane Microseisms)

Prepared by Dorothy Victoria Smith, Georgia Institute of Technology, Atlanta, Georgia 30332, as part of course EAS 4900

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HELP: This is Sonny Day with Channel 3 weather with an important announcement. A tropical depression has developed in the North Atlantic basin. Last seen from satellite photos, it has been gaining strength with frightening speed. Unfortunately all of our satellite systems have crashed and we are no longer able to track this storm. We at Channel 3 have been informed from a reputable source that several local high schools have been provided with seismometers and that the readings from these machines can help us track this storm's path. This is your chance to make a difference. Can you help us? It is vital that we have adequate warning so that we evacuate our coastal residents in its path.

 

This scenario, although unlikely, is similar to conditions that existed during the 1940's and World War Two. No satellites were available and ships at sea could not radio their position, without revealing their location to German submarines. At that time seismic stations were located along the eastern coast to locate hurricanes and to allow broadcasting of the hurricanes location so that ships could avoid these dangerous storm areas. These stations measured the properties of storm microseisms, very much like those shown above, in a heroic effort to save lives. This exercise brings back an early chapter in seismology.

 

Try to see how well you can locate hurricanes.

 


Getting started.

 

Background:

The name hurricane refers to a tropical cyclone with sustained winds of 74 to greater than 155 miles per hour in the North Atlantic Ocean, Caribbean Sea, Gulf of Mexico, and the eastern North Pacific Ocean. The same tropical cyclone is known as a typhoon in the western Pacific and a cyclone in the Indian Ocean.

Wind is responsible for much of the structural damage caused by hurricanes. These winds can remain high even when the storm is well inland. Once the hurricane moves on land, the winds uproot trees and tear down power lines. Near the coast waves built up by the strong winds can damage beach houses. The low pressure of the storm causes a rise in sea level and near the coast flooding can cause great damage.

The large waves built up by the hurricane waves at sea generate the large microseisms we observe on seismograms during these storms. Microseisms are large continuous or pulsating waves with periods of 6 to 20 seconds. Scientists believed that microseisms are generated when the large ocean waves of hurricanes interfere or strike a coastline. From their area of generation they propagate to a seismic station as a surface wave.

Hurricane season typically lasts from mid-August through October, with occasional storms in November. The height of the season is in mid-September. The National Hurricane Center maintains a continuous watch on the development of hurricanes. Updates on the current position of a storm can be obtained from www.weather.com and www.nhc.noaa.gov.

 

Synopsis:

The microseism exercise requires only basic generation of seismograms and the use of Internet weather references to track a hurricane. Comparison of your station's distance (at a particular time) from the eye of the hurricane to the seismogram produced at that time can show that the maximum amplitude of the seismograph increases as the storm draws nearer or stronger. Likewise, a storm that moves away, grows weaker, or moves on land and no longer generates large ocean waves will show decreases in amplitudes of microseisms. A graph of distance versus maximum amplitude will show this and from this data, the approximate path of the hurricane may be extrapolated. Materials needed include Internet access, access to local news media, graphing tools, maps of the East Coast and data from your seismometer.

Choose the storm:

A typical hurricane season will provide 5 to 10 storms. Choose one that is currently active or still stored on your computer. The dates and locations of past hurricanes are available from www.hurricanehunters.com. This exercise is best applied to a developing storm. If the storm you choose takes an interesting path, this exercise will provide a week or two of day-to-day suspense.

 

Obtain appropriate seismograms:

You will need seismograph readings every twelve to twenty four hours for the approximate duration of the storm. (The hurricane hunter's web site gives location readings every six hours, so it will probably be more convenient to select times that correspond to published locations.)

Instructions for SCREAM:

1.While the program is running, select replay files under file on the toolbar. As you select the files, their dates will be shown in the right hand screen. Select the appropriate files for the storm.

2.When you have chosen the appropriate readings, select New Wave View under Windows on the toolbar.

3.You can now drag down the desired file to the new window. Once the seismogram is there, you can easily manipulate its amplitude and the time interval.

4.Change the scale until you have a sine wave with a 6-30 second period. As long as there is limited local noise, the microseism waves should be smooth. You can use the time and amplitude cursors to get exact readings. (To do this click on the box next to time and/or amplitude cursors in the upper right hand corner of the new wave window.)

5.You should make sure the time scale factors are the same for all your readings for a valid comparison. You can now print the screen directly from the icon on the toolbar.

 

Instructions for QUAKES:

1.Insert a formatted disk

2.Press Alt-F

3.Enter the start date and time of desired event in Greenwich Mean Time.

(09/30/1999 12:00) In this program, backspace will not work. If you make an error, write over the rest of the characters to return to the first space.

4. Enter the stop date and time for the desired event (09/30/1999 1:00). The necessary data will be written to your disk. Repeat for all desired events.

 

Use SWAP to view seismogram:

(for QUAKES only)

1. After opening SWAP, insert the disk with the storm data

2. Select open under File.

3. Look up 3 " floppy and select appropriate file.

4. Click on Open Detail Window. The large view of the seismogram will look relatively flat, but pick some background noise to enlarge You can select an area to zoom in on by left clicking to the left of the desired portion and dragging a box to the end of the part you want. This enlarged seismogram will now be shown in the bottom half of the screen. You can further zoom in and adjust by clicking on the top or bottom screen. You should eventually see the microseisms with 6-30 second periods.

5.Choose the same scale for all the seismogram so the amplitudes can be easily compared. (For example, a constant range for the y-axis, and about 4 minutes for the x-axis.)

6. To print this enlarged seismogram, press Alt-Print Screen to place the active window image onto the notepad. You can now paste this image into a Word document or into a graphics program. From these programs you can highlight the bottom screen, paste it to another document, and adjust its size as necessary for printing. (If you had printed directly from SWAP, only the whole image would have printed.)

7.Determine the amplitude. Do this for all other measurements. Make sure the scale is the same for all of them.

Determining Station Distance from the Hurricane:

The weather channel will give accurate and current latitude and longitude readings for any current storm. (They can be found at www.weather.com.) Local news media may also give this information for the times of your seismograms. You will need separate readings for the times corresponding to all of the enlarged seismograms. The locations (also in latitude and longitude) of past storms from the current season can be obtained at www.hurricanehunters.com (just double-click on the storm-it also has cool pictures!) You can convert differences in latitude and longitude to distances in miles or kilometers at www.nau.edu/~cvm/latlongdist.html, Simply plug in the latitude and longitude of the storm at the various times and the lat/long of your station which should be available from your station description. Choose whether you want the distance in miles or kilometers, and the rest is done for you.

Plotting Distance versus Amplitude:

Now that you have the maximum amplitudes from each twelve-hour interval and the distance from the storm at that time, you can graph distance versus amplitude. Observe the relationship between proximity to storm and wave amplitude. Also, observe how the amplitude varies with storm strength.

Tracking the Storm:

Use the station distance from the storm to plot its path along the coast. You can interpolate or extrapolate extra data points from your graph. And if somehow, miraculously, our satellite systems are restored; you can compare your tracks to the satellite tracks.

 

*Note to the teacher:

Students can learn the following:

Basics about waves and how to measure amplitudes

The decrease of signal with distance from the storm

Practice of graphing skills, including interpolation and extrapolation of data points

Use of this knowledge to make predictions and observations about the strength and path of the hurricane

 

Georgia Earth Science QCC standards met:

1.1-1.2 Uses science process skills in laboratory or field investigations, including observations, classification, communication, metric measurement, prediction, inference, collecting and analyzing data.

2 Uses traditional reference materials to explore background and historical information regarding a scientific concept. Uses current technologies to explore current research related to a science concept.

13.3 Collects data related to weather forecasting.

13.7 Uses a sequence of weather data to make a weather forecast.

15.7 Investigates factors that affect wave height, speed, magnitude, and rhythm.

18.3 Distinguishes between magnitude and intensity.

 

Georgia Physics QCC Standards met:

1 Uses science process skills in laboratory or field investigations, including observation, classification, communication, metric measurement, prediction, inference, collecting and analyzing data. Identifies problem, collects data, communicate data, and makes valid inferences and conclusions. Produces written reports of activities in accepted formats and uses precise language for presentations of procedure, tables of data, graphs, analytical methods, results, and analyses of error.

2 Collects time and distance data on objects in motion. Investigates problems that relate to time, distance, displacement, speed, velocity, and acceleration.

3.4 Makes and analyzes graphs showing direct, inverse, exponential relationships, and other variables.

7 Analyzes developments of the science and technology of mechanics that affect the quality of life, such as weather satellites, the space program, robots, etc.