Project IDA (International Deployment of Accelerometers) is a global array of accelerometers that collects low-frequency seismic data (Agnew, et. al., 1986). This network has been in place since 1975, and has had as many as 18 stations in operation between 1978 to 1987. The original purpose of the network was to collect data for earth structure and earthquake mechanisms, and it has been used to detect slow and silent earthquakes (Beroza and Jordan, 1990). The data from 1978 to 1987 were available, although only 1984 to 1987 were considered carefully because of the sparse global coverage in the earlier years. The stations now have an almost uniform distribution around the globe and respond to frequencies from about one cycle per minute to below one cycle per day. An example of a month of the data is shown in Figure later in this paper. Figure shows the locations of the accelerometers.
The amplitude and phase response of each station is plotted in Figures and . Note that the amplitudes and the phases of the stations are similar enough not to require removing the response before imaging, except for the two stations with reversed polarities. These were corrected in the processing. The station showing the high response in Figure did not exist in the data processed here.
Figure 3 The amplitude responses of the IDA stations. The unusual amplitude response is from station 14, MCM, which did not exist in our dataset.
Figure 4 The phases of the IDA stations. The two unusual phase responses are from stations 10 and 20, GUA and SPA, i.e. Guam and the South Pole, indicating the polarities are reversed there.