Introduction

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 participate 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 locating 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 slowly is moved along the drum, so that a spiral trace is written, showing a magnified version of any ground vibrations.

Earthquakes are located by recording the arrival time of the seismic waves 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 is required at each seismic station. This calls for a crystal-controlled clock to put time ticks on the seismic record as well as periodic comparison between the clock and National Bureau of Standards time transmitted over the radio stations WWV or WWVB. It is unlikely that most volunteer stations would be able to maintain the accuracy required to yield good earthquake locations.

There are two types of sound waves generated by earthquakes. P waves are compressional waves and they travel the fastest. S- waves are transverse waves and their speed is about 1.8 times slower than the P waves. Time 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. This time interval can be measured accurately if the drum rotation speed is known and knowledge of the exact time of day is not necessary. 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 P phase, either up or down, is used in estimating the orientation of the sudden slip which produced the earthquake waves. Although often difficult to read on drum recorders, any time it could be read with certainty, it would be a valuable piece of information.

Instrumental Options

To purchase seismic observatory grade equipment, it would cost about $3,590 per seismic station. Supplies, such as paper and ink, cost about $75 per year. A system in that price range would include a crystal clock, WWV receiver, calibrated amplifier, attenuator, and drum recorder with capillary ink pen. The price of a system could be reduced considerably at the cost of timing accuracy, amplitude calibration, and some labor. One could build a drum and set it up to rotate, 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 seismic amplifier can be built with inexpensive operational amplifiers. 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 (~$30).

Conclusions

The principal use 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. Although not available now, something in the price range of $700 or $800 could probably be produced commercially if there were sufficient demand.

Reference: Open-file Report 78-336, p. 64-69.