January, 2001
This month we go to the BBC for a story on climate research. This study has a seismological twist - the researchers at the University of Bremen in Germany used data from a seismograph operating in Hamburg in order to measure changes in the ocean!
How did they do this? Well, although seismographs are primarily used to monitor earthquakes, other activities such as such as volcanoes, explosions, and even falling trees and passing cars can generate seismic waves. Disturbances in the oceans and atmosphere can also produce vibrations in the Earth. In southern California, the seismic network operated by Caltech and the USGS has recorded the sonic booms generated by the Space Shuttle!
In this month's feature, scientists used "microseisms" measured over a 40-year period by a seismograph in Hamburg, Germany, to make their analysis. Microseisms are vibrations of the Earth, typically with periods between 6 and 12 seconds, primarily caused by wave surf and standing waves in the ocean. Although seismologists generally think of microseisms as noise (as opposed to an earthquake - which is a signal!), seismic networks were once considered as a tool to monitor the movement of storm fronts (satellites have since provided a better alternative). The scientists in Germany used seismological records in order to see whether the amplitude and duration of the microseisms had changed over time. They explain an increase in the duration of the ground motions at Hamburg in terms of global warming.
To illustrate what microseisms look like, we've created several images in the table below. To make it easy, we've compared a summer day (June 1, 2000) with a winter day (December 1, 2000) from two stations in the Berkeley Digital Seismic Network. BKS is located in Berkeley, which is near the coast of California. CMB is located in Colombia, which is a town in the Sierra Nevada, and is over 150 km away from the coast.
| Date | Berkeley | Columbia |
| 06/01/2000 |
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| 12/01/2000 |
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| 12/20/2000 |
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Each figure shows 24 hours of ground motion recorded at the site, using a helicorder-style display. Each line on the plot shows 15 minutes of motion from left to right (the vertical marks are drawn every 90 seconds), and can be read like a book: once you reach the end of a line on the right-hand side of the plot, go down one line and start at the far left. Every 4th line (the beginning of each hour) is drawn in red. All seismograms are plotted to the same scale.
The background oscillations are microseisms. You can see that the background noise has higher amplitudes in the 12/01 seismogram than in the 06/01 seismogram. This is a seasonal variation, due to winter storms in northern California. And that the microseisms have higher amplitudes at Berkeley than Columbia. For comparison, we've added a record with several earthquakes. The 12/20/2000 record shows several very long period oscillations caused by earthquakes in Chile as well as a more high-frequency waveform from an earthquake in Burney, California.
Related Web Sites
Hurricane Season: This article appeared in the "EDUQUAKES" column of Seismological Research Letters. It discusses microseisms and using a seismometer in a school environment to monitor hurricanes. More details about this can be found at the MichSeis web site Hurricane season and microseisms.
A Microseism Tutorial: A discussion of microseisms, particularly in the context of their impact on a project to measure gravity waves. Somewhat technical in its description.
How can I make my own seismometer?: A collection of resources for building a seismometer system from the Berkeley Seismological Laboratory.
Related Classroom Activities
Make your own seismogram: This Web site allows one to generate a seismogram for specified dates and times and was used to make the figures above.
Science News Write-Up: Write a report on this article for your class! Here is a suggested outline to help you organize the report.
News Archive
Earth Science News Archive: We are building an archive of these news features. Feel free to browse earlier articles and resources!