The Alaska Volcano Observatory (AVO), a cooperative program of the U.S. Geological Survey, the Geophysical Institute of the University of Alaska, Fairbanks, and the Alaska Division of Geological and Geophysical Surveys, began a program of seismic monitoring at potentially active volcanoes in the Cook Inlet region in 1988. Seismic monitoring of this area was previously accomplished by two independent seismic networks operated by the U.S. Geological Survey (Northern Cook Inlet) and the Geophysical Institute (Southern Cook Inlet). In 1989 the AVO seismic program consisted of three small-aperture networks of six, five, and six stations on Mt. Spurr, Redoubt Volcano, and Augustine Volcano respectively. Thirty-five other stations were operated in the Cook Inlet region as part of the AVO program. During 1990 six additional stations were added to the Redoubt network in response to eruptive activity, and three stations were installed at Iliamna Volcano. The principal objectives of the AVO program have been the seismic surveillance of the Cook Inlet volcanoes and the investigation of seismic processes associated with active volcanism.

The locations of seismograph stations operated by AVO between 1988 and 1990 are shown in Figure 1. Geographic coordinates, elevations, and the installation date of each of the stations are contained in Appendix A. Signals from each of the stations are telemetered to Fairbanks, where recording and analysis take place. Data is recorded digitally in event-detection mode on a PC/AT system and continuously and in event-detection mode on a Masscomp system (Alaska Volcano Observatory Staff, 1990). The two event-detection systems employ different triggering parameters, with the PC/AT system having greater sensitivity for events in close proximity to the volcanoes. Data is also recorded on Helicorders, a Real-time Seismic Amplitude Measurement (RSAM) system (Murray and Endo, 1989; Endo and Murray, 1991), and a Seismic Spectral Amplitude Measurement (SSAM) system (Stephens and others, in press). A description of the various components of the computer systems, their networking, and associated software is given by March and Power (1990).

Following a 23 year period of quiescence, Redoubt Volcano erupted between December 14,1989 and April 21,1990. The eruption was accompanied by thousands of earthquakes (Alaska Volcano Observatory Staff, 1990). Throughout the eruption sequence, data from the PC/AT system provided the primary means of determining earthquake hypocenters. This report catalogs the earthquake hypocenters and magnitudes calculated from data collected between October 12, 1989 and December 31, 1990 on the PC/AT acquisition system, provides station locations, statistics, and calibrations, and outlines which stations were recorded and used in triggering the PC/AT system.