Classic Seismograms

For most of the last century, seismograms were recorded on sheet of paper, either with ink or photographically. We call such records "analog" records to distinguish them from digital recordings. These records are read just like a book - from top-to-bottom and left-to-right.

The classic paper seismogram is read like a book, from left-to-right and top to bottom. A continuous record is constructed by drawing the line as a sheet of paper fastened to a rotating drum that steadily moves horizontally on a threaded attachment. When the ground vibrates the pen moves up or down creating th seismic record of the vibrations. Seismograph station and component, date and start time are recorded on the upper left of this paper. You can see many interesting historical seismograms at http://www.iris.edu/seismo/.

One problem with these mechanical systems was the limited range of ground motion that could be recorded - vibrations smaller than a line thickness and those beyond the physical range of the ink pen were lost. To circumvent these limitations we often operated high and low-gain instruments side-by-side, but that was neither as efficient nor effective as the modern digital electronic instruments. However, modern "digital" or computerized instruments are relatively new, only about 15-20 years old, and most of our data regarding large earthquakes are actually recorded on paper (or film). Additionally, we still use paper recording systems for display purposes so we can see what is going on without a computer.

Digital Seismograms

Today, most seismic data are recorded digitally, which facilitates quick interpretations of the signals using computers. Digital seismograms are "sampled" at an even time interval that depends on the type of seismic instrument and the interest of the people who deploy the seismometer. The same principle is used to provide "digital" sound on compact disks. The motion of the ground is continuous, but we can pick only certain positions and reconstruct the motion (within certain limits).

 

A digital seismogram is a record of the ground movement stored as an array of numbers that indicate the time and the movement of the ground for a range of times and are easily analyzed using computers. The principle is the same as that used for digital audio signals that are stored in MP3 files or Music CD's.

Also, since with live in a three-dimensional space, to record the complete ground motion, we must record the motion in three directions. Usually, we usually choose:

  • Up-down
  • North-south
  • East-west

With three records of ground motion in three directions, a single seismic station that records about 20 samples per second must store or transmit about

3 x 1.7 x 106 samples/day ~ 5 x 106 samples/day

which equals about 5 megabytes per day per seismic station. These numbers are very crude, and this does not account for clever data compression schemes that are widely used to reduce the communications load substantially.