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EDUQUAKES November/December 1998 Larry J. Ruff HURRICANE SEASON People are fascinated by both earthquakes and the weather, hence there is a natural tendency to seek correlations between these two dynamic expressions of the Earth. On a recent trip to California, a well informed friend asked me about "earthquake weather." I had to confess that I was not very knowledgeable about this phenomenon, so my friend proceeded to tell me about it so that even I could forecast earthquakes. I tried to convince my friend that there is relatively little energy transferred from the fluid to the solid part of the Earth, but then a rather embarrassing question surfaced: How much energy do you need to trigger an earthquake? Needless to say, my friend still believes in earthquake weather. There is one phenomenon where seismologists readily acknowledge the influence of the fluid Earth upon our seismograms: noise. Yes, the wind that shakes the trees and buildings makes noise on our short-period instruments, and barometric pressure changes associated with fronts are a good source of long-period noise. But of all the noise sources, perhaps the most fascinating one is also the most important: microseisms. This noise appears on broad-band seismograms as a nearly harmonic modulated waveform with a period that can range from 4 to 12 seconds, though it is commonly referred to as "6-second microseisms." In the early days of seismology, this microseism noise was one of the foremost problems for seismologists. Empirical evidence suggested that it had something to do with ocean waves. When trying to explain the source of these microseism waves to novices, it is fun to say that it is the collective action of all the waves crashing on all the beaches of the continent--even if this description is not exactly true. One of the great theoretical breakthroughs in seismology was the work of Longuet-Higgins and others in the 1940's, which explained the generation of microseisms as an ocean gravity wave interference effect at the continent's edges. In the days of analog seismograms, it was important to use a seismograph that filtered out the 6-second microseisms. Today, we are in the era of broad-band digital seismographs that faithfully record ground velocity from periods of less than 1 second out to a hundred seconds or more. Thus, our modern seismograms proudly display this microseism noise. Unfortunately, some of the most crucial seismographic stations (e.g., Hawaii) for earthquake and global-scale structure studies have high levels of microseism noise--it can sometimes obscure the body waves from magnitude 7 earthquakes. As an experienced seismologist, I do my best to filter and ignore this microseism noise. As I recently discovered, though, other people find this microseism noise to be quite interesting and worthy of study. A few years ago, I was looking at some broad-band seismograms with Richard Ensign, a high school teacher who helped initiate the MichSeis program. When Richard asked about the source of the noise we were seeing, I quickly said that it was the standard microseism noise and moved on to look at something else. Richard was fascinated by the notion that oceanic processes can be recorded by our instruments in Michigan. Before long, Richard had embarked on a research project to chart the microseism noise levels and to compare it to weather at sea. Richard found web sites that give various weather and oceanographic information; the most useful for the microseism study is the global wave height forecasts from the U.S. Navy at http://www.fnoc.navy.mil/PUBLIC/WAM/all_glbl.html. The first research report was published in the Fall 1995 MichSeis newsletter (online at http://www.geo.lsa.umich.edu/MichSeis/), and it is a nice detective story of how Richard discovered the source of a large "microseism storm" in mid-September of 1995. Since then, we have followed a few tropical storms, and Richard found an interesting connection between microseism noise levels and the location of the eye of the hurricane (posted by the National Hurricane Center at http://www.nhc.noaa.gov/). The tracking of hurricanes and correlation to microseism noise level is a wonderful student project. The 1998 hurricane season has already produced three significant storms (Bonnie, Earl, and Georges); there are new opportunities and discoveries available for students. Where can students find microseism noise levels? I will post this information based on MichSeis seismographs; look for a link through the Seismological Society's web pages, or you can go directly to the MichSeis pages with the above-listed web address. I still do not believe in earthquake weather, but due to our modern broad-band seismographs and the curiosity of a high school teacher, I have learned a lot about microseism weather in recent years! This year has seen several workshops and new programs in educational seismology. The next issue of "EduQuakes" will describe some of these past events and future needs. SRL encourages guest columnists to contribute to "EduQuakes." Please contact Larry Ruff with your ideas. His e-mail address is eduquakes@seismosoc.org. Posted: 19 November 1998 |