A national observation network was developed with the aim to monitor natural and induced seismicity and infrasonic events. The present network consists of 14 broad-band (BB, Figure 1) and short-period (SP) stations monitoring natural seismicity, 17 borehole SP installations for the monitoring of induced seismicity (see Figure 2), 23 accelerometers and five infrasound arrays. BB data are open and key stations are included in the European real-time network VEBSN.

Figure 1: KNMI seismic broad-band network the Netherlands.

Figure 2: KNMI monitoring network in the northern part of the Netherlands. Triangles show borehole installations, squares accelerometer stations. Gas fields are shown in green.

The infrasound arrays consist of in-house developed microbarometers capable of measuring low-frequency air pressure fluctuations in the range of 0.002 to 20 Hz. Typical infrasound amplitudes are in the range of hundredths to tens of pascals. Arrays are used to reduce the negative effect of wind noise on the measurements. Furthermore, the slowness of coherent infrasonic waves can be derived, which is a measure of the direction-of-arrival of the signal. The location and configurations of the KNMI infrasound arrays can be seen in Figure 3.

Figure 3: KNMI infrasound array network. Apertures are in the range of 75 to 1500 meters while the number of microbarometers varies between six and 16. The KNMI-LOFAR array is a collaboration with the radio-astronomical LOFAR initiave. Note that the scale of this array is in kilometers instead of meters for the other arrays.

The institute has also an archive of historical analog data for the period 1908-1994. These data are easily accessible and are regularly requested. A subset of scanned seismograms are available through the SISMOS project.

References:

• Dost, B. and H.W. Haak, Comparing waveforms by digitization and simulation of waveforms for four Parkfield earthquakes observed in station DBN, the Netherlands Bulletin of the Seismological Society of America, 2006, 96, 4, S50-S55, doi:10.1785/0120050813.
• Bakun, W.H., B. Aagaard, B. Dost, W.L. Ellsworth, J.L. Hardebeck, R.A. Harris, C. Ji, M.J.S. Johnston, J. Langbein, J.J. Lienkaemper, A.J. Michael, J.R. Murray, R.M. Nadeau, P.A. Reasenberg and M.S. Reichle, Implications for prediction and hazard assessment from the 2004 Parkfield earthquake Nature, 2005, 437, 969-974, doi:10.1038/nature04067.
• Evers, L.G., The inaudible symphony: on the detection and source identification of atmospheric infrasound, PhD Thesis, 2008, TU Delft, ISBN 978-90-71382-55-0, 170 pp.