Larry J. Ruff
LEARNING FROM EXOTIC SOURCES
Exotic sources of seismic waves provide interesting educational opportunities. Most seismologists are concerned with earthquakes or controlled artificial sources of waves, thus our exposure to exotic sources depends on questions from others. Such a question usually begins as, "IÍm sure that this is a silly question, but did your seismograph record the ----," where "----" is some rather exotic source. Once people learn that any disturbance of the earth sends out waves and that our seismographs can record minuscule ground motions, their imagination generates seismograms that our sparse distribution of seismographs cannot record. Some examples of exotic sources are crashes of various types, sonic booms, industrial accidents, jackhammers, marching bands, various kinds of explosions, collapses of structures and natural features, thunder and other atmospheric phenomena, whales and other oceanic phenomena, etc. The recent underground nuclear tests by India and Pakistan have prompted many questions for seismologists. This interest reminds us that seismologists are obliged to be knowledgeable about exotic sources. In addition, any time that people come to us with questions, we have a window of educational opportunity.
Seismic source theory gives us the framework to calculate the radiation of body and surface waves from exotic sources in terms of the space-time distribution of equivalent forces. These detailed and complex calculations of waves are important and necessary for many technical reasons. However, if we are willing to ignore these details, then we can do some simple calculations that make the problem accessible to students. The simplest approach is to estimate the total energy available and assume that it all goes into seismic waves, then just use the standard empirical formulas that convert seismic energy to magnitude. In most cases, the exotic sources are tiny and the maximum equivalent magnitude is so small that there is little hope of any observations; hence this simple calculation is also scientifically useful. Basic seismology texts give the classic Gutenberg and Richter empirical connection between magnitude and energy. For our purposes here, let us work directly with the moment magnitude (Mw) and accept the standard energy relationship as a connection between Mw and the radiated wave energy. Thus, if we have some estimate for the seismic wave energy, E (units are Joules), then the predicted magnitude is: Mw = -3.1 + 2/3logE. This equivalent magnitude gives us the first clue whether the event will be detected, though we realize that the radiated waves could be quite different than those from an earthquake with the same Mw.
For nuclear explosions, there is great interest in the size of the explosion, denoted by yield and traditionally expressed in units of kilotons of TNT. We could convert kilotons TNT to Joules and then use the above formula to estimate the upper bound for Mw, but we can do better than this. Years of research provide an empirical connection directly between yield and magnitude. Only a fraction of the total energy release goes into seismic waves, since much of the explosive energy goes into "friction", i.e., the fracturing, heating, melting, and even vaporization of the rock in the near vicinity of the explosion. In general, to get an accurate estimate of the equivalent magnitude, we have to determine, or guess, the efficiency of conversion from total source energy into seismic wave energy. Otherwise, we get an upper bound on the equivalent magnitude by assuming 100% efficiency.
As one example of an exotic source question that I received one day, a turbine-generator at a local power plant went out of balance and caused considerable damage before it stopped. The vibrations triggered the accelerometer at the plant but were not felt a short distance away. I was asked if our station, at a distance of about 50 km, detected this event. I saw nothing above the background noise. Should I have seen something? We can use the energy approach to get a quick estimate by assuming that the entire rotational kinetic energy of the turbine-generator goes into seismic waves. With an energy value of about 5 X 10(8) J, the equivalent magnitude is 2.7--I would have seen an earthquake of this size, so only a small fraction of the kinetic energy was converted into seismic waves. I am not surprised that most of the rotational kinetic energy was deposited inside the power plant as "friction!"
Since most of the exotic sources that people ask about have very low total energy, there is no need to worry about the efficiency of energy conversion into waves. After all, if even nuclear explosions are only marginally recorded, what else in our environment could possibly be significant? Browsing through the seismological journals, you can find several articles that have documented recordings of exotic sources. There are several papers about sonic booms, typically from jet aircraft but others from spacecraft re-entry or meteorites, and some other cases remain unexplained. Volcanic explosions are exotic sources that seismologists have investigated, and there are several examples where other large explosions have prompted seismological investigations. Many of the questions about exotic sources involve falling objects and provide good problems for students who can calculate gravitational potential energy. This brings me to another exotic source question that I received one day: "... did the University of Michigan seismographs record the sinking of the Edmund Fitzgerald?" I had never thought about the seismic signal from a sinking ship, so I went to the library and quickly learned the basic facts, and lingering controversies, over the fate of the Edmund Fitzgerald. (For more details, see MichSeis newsletter, Spring 1996.) During the night of November 10, 1975, the Edmund Fitzgerald and its experienced crew of 29 people sank into the cold depths of Lake Superior while battling a storm with heavy snow, wind gusts of 90 mph, and wave heights of up to 30 feet. Large ships that sail the Great Lakes must be capable of surviving these storms, hence the importance of understanding exactly what happened to the Edmund Fitzgerald that night. There were three separate official investigations into this disaster, with three different conclusions about what caused the sinking. Here we focus just on the seismological aspect.
Given the images of the wreck, it is possible to concoct a hypothesis that maximizes the impact energy on the lake bottom. For that last journey, the Edmund Fitzgerald was loaded with 26,116 tons of taconite pellets, which are the iron ore concentrates from the Lake Superior banded iron formations. A typical density for taconite pellets is 5.2 gm/cm3. Wreckage of the Edmund Fitzgerald lies about 25 miles north of Whitefish Point, Michigan, under 530 feet of water. While the middle section is gone, the separate bow and stern sections are intact and rest on a circular mound that includes the iron ore. This picture allows a story where iron ore spilled out of the ship at shallow depths and then fell through the water. Thus, I focus on the falling cargo as the principal source of any seismic waves and ignore the effects of any explosions or implosions. It is easy to calculate the potential energy of the iron ore replacing the water at the bottom: 3.1 X 10(10) J. If all this potential energy is converted to seismic waves, then the equivalent magnitude is 3.8 to 3.9--earthquakes of this size are widely felt in eastern North America and their waves are well recorded by seismographs out to distances of several hundred kilometers.
The University of Michigan was running a "High Gain Long Period" instrument during November 1975 in the deep mine at White Pine, MI, which is 340 km from Whitefish Point, but I was not able to find any seismograms in either the national archive or in storage at Ann Arbor. I did look at the records at the Ann Arbor WWSSN station; the distance to Whitefish Point is about 540 km. There was no signal on either the long-period or short-period seismograms that I could attribute to the Edmund Fitzgerald. Given the noise level for that day, I should have seen something if the equivalent magnitude was 3 or larger. This negative result implies that just a small fraction of the potential energy was converted to seismic wave energy. To take this exercise one step further, we need to estimate the terminal velocity of the cargo falling through water. Note that if the cargo fell 530 feet through a vacuum, its velocity just before impact would be 56 m/s, or 200 km/hr. A fun calculation for students familiar with the basics of fluid flow would be to estimate the velocity just before impact for different shapes of the falling blob of taconite. This example illustrates how falling object exotic sources provide educational opportunities.
Are there other shipwrecks that might have been seismically observed? Certainly there are ships much larger than the Edmund Fitzgerald. However, I would guess that the break-up of a super oil tanker would not have the same effect, since its cargo just floats on top of the water! What about the sinking of the Titanic? Who knows how fast a shipÍs hull like the TitanicÍs might hit the bottom--what is its terminal velocity? Are there mechanisms of seismic wave generation that are more significant than bottom impact? Are there any seismograms of sinking ships? Send me your examples and thoughts about sinking ships and exotic sources (email@example.com), and I will have them posted on the EduQuakes web pages.
Current events--whether they be world news or Hollywood movies--do precipitate questions about exotic sources. Looking at some of the recent science news and movies, you might want to think about the seismic effects of giant impacts so that you will be prepared for "IÍm sure that this is a silly question, but ...."
SRL encourages guest columnists to contribute to "EduQuakes." Please contact Larry Ruff with your ideas. His e-mail address is firstname.lastname@example.org.
Posted: 14 September 1998