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.
amp
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. | ![]() |
phase
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. | ![]() |