Abstract

Objective

The goal of this project is to use archived, online seismometer data from the Berkeley Digital Seismic Network to create your own seismograms in order to measure how fast seismic waves from distant earthquakes travel through the Earth's crust.

Introduction

A seismograph is an instrument that detects and records ground motion. As an analogy, think of trying to draw a straight line on a piece of paper while someone is holding your elbow and jiggling it back and forth. It would be hard to get the line straight, wouldn't it!

You can think of a seismograph as a machine that is kind of like your arm, holding the pen. The 'elbow' end of the machine detects the vibrations, causing the pen to move back and forth. Meanwhile, a roll of paper is moving at a constant speed under the pen. When there are no vibrations, the pen draws a straight line on the paper. When the ground shakes, it causes the pen to move back and forth, so instead of straight lines, you get up and down squiggles. The greater the vibrations, the larger the squiggles.

In the digital age, seismographs have been replaced by seismometers, which measure and record ground motion digitally. The data from seismometers can be collected automatically and analyzed with computers.

When an earthquake occurs, how do scientists know how powerful it is? A network of seismometers constantly monitors movements of the Earth's crust. When an earthquake occurs, waves of motion travel out from the epicenter, through the crust, and are detected by the seismometers. By analyzing the differences in the timing of the waves between multiple staions, scientists can pinpoint the source of the waves: the epicenter of the original quake.

In this project you will use online seismometer data from the Berkeley Digital Seismograph Network (BDSN) to measure how fast seismic waves from distant earthquakes travel through the earth's crust.

Terms, Concepts, and Questions to Start Background Research

To do this project, you should do research that enables you to understand the following terms and concepts:

• earthquake,
• tectonic plate,
• seismograph,
• seismogram,
• seismometer,
• seismology,
• Coordinated Universal Time.

More advanced students will also want to study the physics of waves, and should do research to understand the following terms and concepts:

• frequency,
• period,
• wavelength, and
• wave velocity.

Questions

• Do seismic waves generally travel at the same speed or is there variation in wave speed?
• Do seismic waves from more powerful earthquakes travel faster than waves from weaker quakes?
• Do seismic waves travel faster through oceanic crust or continental crust (or about the same through both)?

Bibliography

• To find earthquake information (date, time, location, magnitude), see this webpage:
  USGS, 2006. "Historic Worldwide Earthquakes," United States Geological Survey, Department of the Interior [accessed December 21, 2006] http://earthquake.usgs.gov/regional/world/historical.php.
• To create a seismogram, see:
  UC Regents, 2005. "Make Your Own Seismogram!" Northern California Earthquake Data Center, University of California, Berkeley and USGS [accessed December 21, 2006] http://www.ncedc.org/bdsn/make_seismogram.html.
• To measure the great circle distance between two points on the globe, see:
  Byers, J.A., 1997. "Surface Distance Between Two Points of Latitude and Longitude," United States Department of Agriculture, Agricultural Research Service [accessed December 21, 2006] http://www.chemical-ecology.net/java/lat-long.htm .
• For getting started on your background research, here are two good references on seismology:
  
  • Wood, M.M. and W. Pennington, 2006. "Seismic Education Site: UPSeis," Department of Geological and Engineering Sciences, Michigan Technical University [accessed December 21, 2006] http://www.geo.mtu.edu/UPSeis/index.html.
  • Wikipedia contributors, 2006. "Seismology," Wikipedia, The Free Encyclopedia [accessed December 21, 2006] http://en.wikipedia.org/w/index.php?title=Seismology&oldid=96601497.

Materials and Equipment

To do this experiment you will need the following materials and equipment:

• computer with Internet access and a printer.

Shop for Supplies at Science Buddies Online Store

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Experimental Procedure

Select Historic Earthquakes to Analyze

1. Do your background research so that you are knowledgeable about the terms, concepts, and questions, above.
2. Make a table of historic earthquakes to analyze. You can find a list at Historic Worldwide Earthquakes (USGS, 2006).
3. For each earthquake, you'll need at least the following information:
   • magnitude,
   • time (UTC),
   • epicenter location (latitude, longitude, depth).
4. We'll walk through one example to show you where to find the data you need.
   1. The screenshot below shows a portion of the list at the Historic Worldwide Earthquakes webpage. To get more information about a particular earthquake, click on the link. We're going to be looking at data from a magnitude 6.3 temblor with epicenter on Kyushu Island, Japan that occurred on June 11, 2006, at 20:01:29 UTC.

   2. Clicking on the link for this quake brings us to a page with the information shown in the screenshot below.

   3. Copy the information you need for each earthquake (highlighted in yellow rectangles, above).
   4. Notice that there are additional tabs (Summary, Maps, Scientific & Technical, highlighted in green rectangle, above) with additional information about each quake. You'll probably be interested in checking out the Summary and Maps tabs for each quake you study.
5. For a simple analysis of seismic wave speed, choose 10–20 earthquakes from (roughly) the same part of the world. For other ideas on criteria for selecting earthquakes to analyze, see the Variations section. Note that, on the right-hand side of the Historic Worldwide Earthquakes page, you can sort the quake data by Date, Magnitude, Magnitude 6.0 and Greater, Country & Date, Country & Magnitude (see screenshot, below).

Make a Seismogram

1. The webpage for making a seismogram is pretty straightforward. Again, we'll walk through an example to show you how to use it.
   1. Select a station. Click on one of the radio buttons to select from the 28 available stations. For more information on the stations, see Table 1, below. For our example seismogram, we used data from station BKS, the Byerly Seismographic Vault, near the UC Berkeley campus.

   2. Select the data channel. There are two types of data channels: 'long period' and 'broadband.' The period of a seismic wave is the time that elapses between successive wave crests. Long period waves, then, are low-frequency waves. The long period channels are good for viewing seismic activity from distant earthquakes, so you should use these channels for this project. You can choose to look at vertical motion, or horizontal motion (either north-south or east-west).
      The broadband channels collect information more frequently, and so have information about higher-frequency waves. Broadband channels are good for viewing seismic activity from local earthquakes. See the Variations section for project ideas that can make use of the broadband data channels.
   3. Set the desired time period. The seismogram plot can be a maximum of 24 hours for long period data channels (1 hour for broadband data channels). It's a good idea to view seismic data both before and after the time of the earthquake. An easy way to do this is to set the time period to one entire day. Enter the date and time in the format "yyyy/mm/dd,hh:mm:ss". Separate date and time with a comma, but no spaces. In the screenshot below, we have set the plot to cover the entire day of June 11, 2006.

   4. Set the plotting parameters. Often, the default parameters will work just fine. For higher-magnitude quakes, however, you may need to decrease the Amplitude Scaling parameter by a factor of 10 or 20 (or more) so that the active traces do not overwhelm the entire plot. You can also increase the spacing between the traces (third parameter), but this will increase the vertical size of the plots, making them harder to view and print.

   5. Make the seismogram! Click on the "Create Plot" button near the bottom of the page. That's it!
2. Here are some tips on "reading" the seismogram plot. As an example, here is a seismogram for June 11, 2006, the day of the magnitude 6.3 quake on Kyushu. (Plotting parameters were: compression factor = 1, amplitude scaling = .004, and pixels between adjacent output traces = 5.)
   1. The plot contains a series of traces. The y-axis shows the intensity of ground motion, and the x-axis is time.
   2. Most of the traces in a random 24-hour plot will be flat; squiggles in the traces indicate seismic activity.
   3. Each trace represents 15 minutes (900 s) of data. The first trace of each hour is colored red. It is followed by three black traces—one for each of the remaining quarter-hours.
   4. Look for the first sign of activity following the UTC time given for the earthquake. In this case, the time of the original earthquake was 20:01:29 UTC (= 20:00:00 hours plus 89 seconds). The first squiggle appears in the red trace at 20:00:00 hours + 810 s.
   5. Thus, the elapsed time was 810 - 89 = 721 seconds (just over 12 minutes).
   6. If you are not sure that a squiggle is related to the earthquake you are studying, try creating a plot for the same time period using one (or more) additional data station(s). This way you can confirm or reject your hunch.
3. Here are some tips on printing the seismogram. Since the plots are large, you probably won't have much luck printing directly from the browser window. Instead, save the plot as a file and print it with another program.
   1. Right click on the plot,
   2. Choose Save Image As... to save image to file
   3. Then import into another program (e.g. Word) for printing.
4. Here are some tips if you have problems creating a seismogram.
   1. Data may not be available for all stations at all times, which means that sometimes you may end up with an error message instead of the seismogram you were looking for. If you see an error message like: "Cannot create seismogram -- apparently there is no data.", try using another station. If that doesn't work, try selecting a different earthquake to study.
   2. If you have a hard time seeing the individual traces in a plot, try decreasing the amplitude scaling by a factor 10. (You can also increase the number of pixels between adjacent output traces, but this will increase the vertical size of the plot.)
   3. If these tips don't help solve your problem, try the help links on the Make Your Own Seismogram! page.

Calculate the Average Velocity of the Seismic Waves

1. velocity = distance / time
2. Calculate distance from epicenter to seismometer station using the latitude and longitude of each point. Here's how:
   1. You already have the latitude and longitude of the epicenter of the quake (step 4 of Select Historic Earthquakes to Analyze, above).
   2. You can find the latitude and longitude of each of the data stations in the table, below. (Note that you can click on the data station code to see a webpage with detailed information about each station.)

      Table 1. Make Your Own Seismogram! Station Locations
      station code	latitude (decimal degrees)	longitude (decimal degrees)	elevation (m)	place name
      BDM	+37.9540	-121.8655	+220	Black Diamond Mines Park, Antioch, CA, USA
      BKS	+37.8762	-122.2356	+244	Byerly Seismographic Vault, Berkeley, CA, USA
      BRIB	+37.9189	-122.1518	+220	Briones Reserve, Orinda, CA, USA
      BRK	+37.8735	-122.2610	+49	Haviland Hall, Berkeley, CA, USA
      CMB	+38.0346	-120.3865	+697	Columbia College, Columbia, CA, USA
      CVS	+38.3453	-122.4584	+295	Carmenet Vineyards, Sonoma, CA, USA
      ELFS	+40.6183	-120.7279	+1554	Eagle Lake Biol. Field Stn., Susanville, CA, USA
      FARB	+37.6978	-123.0011	-18	Farallon Islands, CA, USA
      GASB	+39.6547	-122.7160	+1355	Alder Springs, CA, USA
      HATC	+40.8173	-121.4705	+1009	Hat Creek Radio Astronomy Obs., Cassel, CA, USA
      HELL	+36.6801	-119.0228	+1140	Rademacher Property, Miramonte, CA, USA
      HOPS	+38.9935	-123.0723	+299	Hopland Field Station, Hopland, CA, USA
      HUMO	+42.6071	-122.9567	+555	Hull Mountain, OR, USA
      JCC	+40.8175	-124.0296	+27	Jacoby Creek, Bayside, CA, USA
      JRSC	+37.4037	-122.2387	+70	Jasper Ridge Biol. Preserve, Stanford, CA, USA
      KCC	+37.3236	-119.3187	+888	Kaiser Creek, CA, USA
      MHC	+37.3416	-121.6426	+1250	Lick Observatory, Mt. Hamilton, CA, USA
      MNRC	+38.8787	-122.4428	+710	McLaughlin Mine, CA, USA
      MOD	+41.9025	-120.3029	+1554	Modoc Plateau, CA, USA
      ORV	+39.5545	-121.5004	+335	Oroville Dam, Oroville, CA, USA
      PACP	+37.0080	-121.2870	+844	Pacheco Peak, CA, USA
      PKD	+35.9452	-120.5416	+583	Bear Valley Ranch, Parkfield, CA, USA
      RAMR	+35.6360	-120.8698	+417	Ramage Ranch, Paso Robles, CA, USA
      SAO	+36.7640	-121.4472	+317	San Andreas Geophysical Obs., Hollister, CA, USA
      SUTB	+39.2291	-121.7861	+252	Sutter Buttes, CA, USA
      WDC	+40.5799	-122.5411	+268	Whiskeytown Dam, Whiskeytown, CA, USA
      WENL	+37.6221	-121.7570	+139	Wente Vineyards, Livermore, CA, USA
      YBH	+41.7320	-122.7104	+1060	Yreka Blue Horn Mine, Yreka, CA, USA

   3. Use the online calculator, Surface Distance Between Two Points of Latitude and Longitude (Byers, 1997), to find the distance between the epicenter of the earthquake and the data station.
   4. The calculator requires you to enter the latitude and longitude in degrees, minutes seconds, instead of decimal degrees. However, it has a handy converter you can use. Click on the button that says, "Decimal Degrees <=> Deg Min Sec". A popup window will open where you can convert from decimal degrees to degrees, minutes and seconds.
   5. Enter the latitude of the epicenter in decimal degrees, and click on the "Converts to" button (see screenshot below).

   6. Now go back to the distance calculator and enter the latitude of the epicenter in degrees, minutes and seconds.
   7. Repeat for the longitude of the epicenter, and the latitude and longitude of the data station (see screenshot, following the next step). (Note that a positive value of latitude is North of the equator, and a negative value is South of the equator. For longitude, a positive value is East of the prime meridian and a negative value is West of the prime meridian.)
   8. Once you have entered the latitude and longitude of both points, click on the "Distance between" point to calculate the distance (see screenshot, below).

   9. The distance between the epicenter of the Kyushu earthquake and the BSK data station is 9030 km.
3. Divide the distance by the elapsed time you calculated from the seismogram. For our example, the distance is 9030 km, and the time is 721 s. The calculated velocity is 12.5 km/s.
4. Repeat all of above the steps for 10–20 earthquakes from (roughly) the same area of the world.
5. What is the average velocity of the seismic waves from the earthquakes you studied? (More advanced students should also calculate the standard deviation.)

Variations

• The seismometers measure shaking in three dimensions: vertically (up and down), north-south, and east-west. Do seismic waves in each of these dimensions travel at the same or different speeds?
• Speed vs. magnitude. Do seismic waves from more powerful earthquakes travel faster than those from less powerful earthquakes? Measure speed from ten earthquakes for each of several differnent magnitudes and compare.
• Interested in making your own seismograph? Try the Science Buddies project Is There a Whole Lot of Shaking Going On?
• You can also use the Make Your Own Seismogram! webpage to create hour-length seismograms using broadband channels, which are good for viewing local earthquakes. Is the seismic wave speed over shorter distances the same as that over larger distances? You can use the USGS website to locate historical earthquakes in northern California. In addition, for a list of recent earthquakes in the region, see http://www.ncedc.org/cgi-bin/finger-quake.
• For a more advanced project that uses seismometer data, try the Science Buddies project Locating the Epicenter of an Earthquake.
• Advanced. Design an experiment to see if there is a relationship between seismic wave speed and the geological features between the earthquake epicenter and the seismic recording station. For example, do seismic waves travel differently through oceanic crust vs. continental crust?

• For more science project ideas in this area of science, see Geology Project Ideas.

Credits

Andrew Olson, Ph.D., Science Buddies

Sources

This project was inspired by the "Make Your Own Seismogram!" webpage:

• UC Regents, 2005. "Make Your Own Seismogram!" Northern California Earthquake Data Center, University of California, Berkeley and USGS [accessed December 21, 2006] http://www.ncedc.org/bdsn/make_seismogram.html.

Last edit date: 2007-04-24 21:40:00