The seismic spectral amplitude measurement (SSAM) system is a new, inexpensive tool for monitoring the spectra of seismic signals in near real time on a low-cost PC. The heart of the system is a digital signal processing board that is capable of continuously computing fast Fourier transforms (FFT) for up to 64 channels of data digitized at a rate of 100 samples per second. Parameters such as the frequency range of each spectral band and the time interval over which the spectral amplitude within each band is averaged are easily modified through a startup control file for the data acquisition program. In the current system, spectral amplitudes are computed approximately every 5 sec and then averaged within each of 16 user-defined frequency bands over a 1-min interval. Data for each time interval are output through a parallel port for use in real-time display and written into binary files on disk for archiving and later analysis. Spectrograms generated from these data proved to be an effective tool for assessing the nature of long-period (LP) event swarms that accompanied the 1989-1990 eruption sequence at Redoubt volcano, Alaska, and for distinguishing these signals from seismic noise. In particular, one of the eruptions was successfully forecast principally on the basis of identifying the precursory LP swarm on SSAM records.