What is a digital seismograph?

Just as a microphone and tape recorder are used to receive and save the acoustic waves produced by an orchestra, a seismograph is used for seismic waves. Of course, the human ear can hear the acoustic waves of music; and it can directly sense the passage of seismic waves if a large earthquake is nearby. However, to obtain an accurate permanent record of the ground motion, and to look at earthquakes occuring around the world, a more sensitive instrument is needed. A complete seismographic station is composed of: (1) a seismometer that continuously translates ground motion into an electrical output; (2) some method to permanently record this output, e.g., on a piece of paper for analog recording, or in a computer file for digital recording; and (3) time, the seismograph must keep accurate time. In addition, modern digital seismographs add a fourth component: display, "user-friendliness", and rapid data transfer. MichSeis and OhioSeis seismographs emphasize "user-friendliness".

The Macintosh family of computers provides an advanced and consistent user interface with a "multi-tasking" system that allows the digital seismograph to keep working in the "background" as other tasks are performed. The software needed to run the seismographic station is SeismoGraf, and the companion program is SeismoView, which allows anyone with a Macintosh to display and analyze the digital seismograms. The SeismoGraf and SeismoView programs are currently supplied free of charge to OhioSeis participants (contact Dr. Larry Ruff ruff@umich.edu at the Seismological Observatory, Department of Geological Sciences, The University of Michigan.)

To look closer at a seismograph, the seismometer can be built to look at different periods of waves. "Short period seismometers" respond best to wave periods of less than 1 second; these seismometers are very small, relatively inexpensive, and record local earthquakes quite well. "Broad-band seismometers" respond to periods from 1 second up to, say 20 seconds for the minimal requirements, or up to 1,000 seconds or more for the most expensive instruments. A recent advance is the development of a small-sized inexpensive "broad band" instrument: the EAI S-102 vertical seismometer. Also, the SeismoGraf software utilizes inexpensive hand-held GPS receivers as the absolute time source.