The seismometer is THE most important component to consider when building your own seismograph station. You can build your own seismometer, purchase a new or used commercial seismometer, or receive the radio telemetry signals from an existing seismograph network by using a radio scanner and a telemetry demodulation board . You must decide if you want a seismometer which measures vertical or horizontal ground motion or both. Vertical seismometers are more difficult to build than horizontal seismometers. In fact, most of the amateur-built seismometers are horizontal because they are easier to construct and maintain. Some people build a Lehman Seismometer which is a horizontal seismometer. Others build a simple vertical seismometer described in: How to Build Earthquake, Weather, and Solar Flare Monitors by Gary Giusti, McGraw Hill (Tab), 1995. I prefer to use surplus commercial seismometers because I want to get a high-quality seismic signal. In the future, I may build a broadband vertical seismometer.
If you want to buy a ready-made seismic amplifier/filter, see Larry Cochrane's board. Or, build your own amplifier/filter.
I use a surplus IBM Pentium PC with 128 mb RAM, 80mb hard drive, and VGA video card/monitor as the data logger. This PC cost under $100 and works well. The data logging PC connect to the analysis PC via a serial cable.
I use the PSN-ADC-12 A/D board which has 12-bit A/D resolution, 1 KHz throughput, 8 single-ended input channels, and a fixed input range of +/- 5 volts. I also have a 8-channel GPS receiver connected to the A/D board which provides accurate timing for the seismograph station. Timing error with the GPS receiver is less than 1 millisecond. Timing accuracy is very important in operating a seismograph station so that seismic phase arrivals, P and S waves, can be compared to other stations for location of earthquakes.
Click here for a good description of seismic waves and how to locate earthquakes using P and S waves. Want to learn about locating earthquakes interactively? Click here for a really neat webpage!