Seismic Recording in an Indigenous
Earthquake Prediction Program
by John Lahr
U. S. Geological Survey
There are many reasons why it would be useful to have greater volunteer involvement in seismic recording. It would lead to a greater public awareness of seismological problems and reduce the mystery surrounding earthquakes, the Richter magnitude scale, and earthquake prediction. There may be some instances where a group of trained volunteers could assist seismologists in a temporary earthquake recording program. Following a large earthquake, speed is important in setting up a temporary network, and a local volunteer group could help with tasks such as site permitting, geophone and antenna emplacement, felt report and damage surveys, and reporting surface fault breaks.
The cost of seismic instruments is the limiting factor in obtaining useful scientific data from volunteer seismic recording stations, although for institutions, such as schools, this may be less of a problem. In this paper the type of measurement one needs to make in order to record earthquakes, as well as the cost of various equipment options, will be discussed.
What does a seismic instrument measure?
When an earthquake occurs, sound waves, called seismic waves, are generated which travel outward in all directions in the earth. For small earthquakes, the principal frequencies of these waves are from 5 to 20 cycles per second (Hz). A seismic sensor, called a seismometer or geophone, usually consists of a coil of wire suspended by a spring in the field of a magnet. The slightest vibration of the ground causes the coil to vibrate with respect to the magnet, thus producing a small fluctuating voltage at the coil leads. This voltage is amplified and used to drive a pen motor. The displacement of the pen turns out to be proportional to the velocity of the coil. Most seismic recorders write a record on a sheet of paper taped to a drum. The drum rotates as the pen motor is moved slowly along the drum, so that a spiral trace is written, showing a magnified version of any ground vibrations.
Earthquakes are located using the arrival time of the seismic waves recorded at four or more stations. For good locations, the relative arrival times must be accurate to at least 0.1 sec. Therefore, accurate and precise timing and uniform drum rotation is required at each seismic station. This calls for a crystal controlled clock to put a time tick on the seismic record each minute as well as periodic comparison between the clock and National Bureau of Standards time transmitted over the radio stations WWV or WWVB.
There are two types of sound waves generated by earthquakes. P (primary) waves are compressional waves and they travel the fastest. S (secondary) waves are transverse waves and their speed is about 1.8 times slower than the P waves. The time interval between the P wave and the S wave, called the S minus P time, is directly proportional to the distance from the station to the earthquake. The distance in kilometers from the seismic station to the earthquake is approximately 8 times the S - P interval in seconds. This time interval can be measured if the drum rotation speed is constant and is known. Although the beginning of the S wave is difficult to pick with sufficient accuracy to be useful in precise earthquake locations, the S - P interval would give the amateur seismologist some idea of the earthquake location, and in conjunction with S - P readings at two or more other stations, an estimate of the earthquake location can be made.
Other parameters of interest are the amplitude of the seismic signal and the duration of disturbance from the P phase to the end of the earthquake coda. The coda is the series of waves recorded on the seismogram following the P and S waves. Both the amplitude and the duration of the seismic signal can be used to estimate an earthquake's magnitude. The direction of first motion of the ground at the onset of the P phase, either up or down, is used in estimating the orientation of the fault plane and the direction of fault movement which produced the earthquake waves.
An observatory-grade seismograph system costs about $3,500. Such a system would include a sensor, crystal clock, WWV receiver, calibrated amplifier with switchable gain, and drum recorder with capillary ink pen. Supplies, such as paper and ink, cost about $75 per year.
The cost of a system could be reduced considerably by home or school fabrication of parts at the possible sacrifice of timing accuracy and amplitude calibration. The minimum specifications for a seismograph system would be a magnification of 50,000 to 100,000 at 10 Hz and a paper speed of 30 to 60 mm per minute. One could build a drum and rotate it, using an AC synchronous motor. A mount to hold the pen motor which translates along the drum on a lead screw must also be built The cost of scrap and/or surplus parts for the mechanical assembly might be from $20 to $40. The seismic amplifier can be built with inexpensive operational amplifiers for about $15. The items which one would probably want to purchase would be the geophone (~ $25), the pen motor (from $100 to $160) and the capillary pen (~$3O). Therefore the total cost would be roughly $200 to $300.
Derivation of Useful Data from Volunteer Recording
In order to obtain useful seismic data, not only must equipment be purchased, but it must be routinely operated and maintained over a substantial period of time. This requires a long-term commitment of about one half hour per day which only the exceptional individual is likely to make. The measurement of data from many non-standardized systems also poses a problem. For these reasons, the expected result of individual recording efforts is educational rather than scientific.
Institutions, such as schools and museums, however, in many cases have the resources and staff to purchase and operate seismic equipment that would yield data of scientific interest as well as serve educational functions. Data from stations placed so as to augment current government or university run networks and capable of maintaining 0.1 sec timing accuracy would definitely be of value. In addition, even if timing accuracy has not been maintained, amplitude, duration and first-motion data could be derived from the records.
Dissemination of Information on Earthquakes and Seismic Equipment
General information on earthquakes is available now at probably every public library. The latest editions of most earth science textbooks for high school now include the fundamentals of new global tectonics, including the global distribution of earthquakes and continental drift. However, more detailed information is not readily available. Of 46 libraries on the San Francisco peninsula, from San Jose to San Francisco, C. F. Richter's book, Elementary Seismology, one of the best texts in Seismology, is available at only five. The Earthquake Information Bulletin, published bimonthly by the U.S. Geological Survey, is carried by only two of these libraries. It is a publication directed at the public and includes short articles of general interest on geophysical topics as well as a summary of current earthquakes around the world. Emphasis will need to be placed on making books and periodicals such as these much more readily available if the public is ever to take a more serious interest in seismological problems.
There have been a number of articles in Scientific American on building amateur seismic equipment, and these are listed in the appendix. these is ideally suited, however, to recording local earthquakes. the indigenous earthquake prediction program, it would be helpful instrument designed that would be inexpensive and relatively easy
The principal value of an indigenous earthquake recording program would be to further the interest and education of the public in seismic problems. In addition, a volunteer group could be quite helpful in some recording programs. It is not expected, however, that a significant amount of earthquake data directly useful to the earthquake prediction program would be obtained.
The cost of seismic instrumentation runs from about $3,500 for a first class setup to about $200 or $300 for a home built version. Institutions, such as schools and museums, should be encouraged to set up and maintain earthquake recording equipment. Someone from the scientific community could act as liaison between the institutions and the seismologists in order to validate that timing and calibration quality was maintained and to make copies of seismograms available to those interested. In addition, the records should be available to the public so that a person interested in local seismicity would have access to locally recorded data.
Richter, C. F., Elementary Seismology, Freeman and Co., San Francisco, 768 pp., 1958.
The following are from Scientific American, Amateur Scientist. SeriesDate, Volume, No, Pages, and title are shown for each.
April, 1952, 186, 4, pp 94--98, Larkin Seismograph
June, 1953, 188, 6, pp 114-118, About an ingenious electronic seismograph.
July, 1957, 197, 1, pp 152-162, Concerning simple and Ingenious devices to record the waves made by earthquakes
May, 1961, 204, 5, pp 182-186, A mechanical seismograph that detects the vibrations of water in a well.
March, 1972, 226, 3, pp 114-119, A simple motor with compensating devices Is the key to a homemade chart recorder.
November, 1973, 229, 5, pp 124-129, A sensitive mercury tiltmeter that serves as a seismometer.
September, 1974, 231, 3, pp 192-198, A venerable clock is made highly accurate by equipping it with quartz-crystal works.
September, 1975, 233, 3, pp182-188, Electronic stratagems are the key to making a sensitive seismometer.
Ward, P.L. (editor), Proceedings of Conference IV, The use of volunteers in the Earthquake Hazards Reduction Program, convened under auspices of National Earthquake Hazards Reduction Program, February 2-3, 1978, Menlo Park, California, Open-File Report - U.S. Geological Survey, of 78-336, P. 548.