Seismic network

For a long time, the station of Uccle, set up in 1899, was the only seismic station in Belgium. From 1958 onward, a real seismic network was gradually established. A summary of the most important developments is given in History.
The present-day seismic network in Belgium consists of two different parts, the seismometer network and the accelerometer network. Seismometers register continuously the ground vibrations, with a sensitivity that is tuned to detect even the smalest trembling that surpasses the noise level. The accelerometers on the contrary are only activated by stronger ground motions.

The Seismometric Network

The main objective of the seismometric network is the collection of reliable information about the seismic activity in Belgium and surroundings. Besides this, the network also measures the seismic waves generated by earthquakes all over the world. The arrival times of these waves are routinely measured and transferred to international seismic centres.

The actual permanent network consists of 28 measuring stations of which 3 in Luxemburg. These last belong to the Musée National d'Histoire Naturelle. The Royal Observatory of Belgium (ROB) guarantees the maintenance and data analysis of all these stations. To perform good quality measurements, most of the seismic stations are positioned where hard rock surfaces and where the disturbing influence caused by human activity is as low as possible.

Most of the seismic stations are equipped with differend types of short period seismometers (Mark-Product en Lennartz), or with broadband seismometers (Güralp CMG-40, CMG-3E, CMG-3T). The acquisition systems were developed at the ROB and are based on PC's. Several stations are provided with ISDN or ADSL connections, which allows quasi real time data consultation. The registration is performed continuously, but only the registered earthquakes and other particular events are preserved.

Besides the permanent network, the ROB also disposes of mobile stations that can be quickly put up, for exaple to register aftershocks in case of an important earthquake.

The Accelerometric Network

For earthquakes with magnitudes >3.5 the ground movements in the vicinity of the epiceter are mostly too strong to be registered by a seismometer. Ground accelerations higher than one twentieth of the gravity (> 0,05 g) saturate the signal of a seismometer, which makes the measurements useless for further analysis. This was for instance the case for The Liege earthquake in 1983, and The Roermond earthquake in 1992. For these earthquakes there are no registrations of the ground movements in the epicentral area, where most of the damage occured (Melchior, 1985; ...).

Accelerometers have lower sensitivities than seismometers and are designed to measure strong ground accelerations, up to 1 g, even close to the epicenter during important earthquakes. In the seismically active zones in and around Belgium ground accelerations can reach some tenths of the gravity. The study of these strong ground movements plays an important role in the analysis of focal mechanisms and of seismic risk. Registrations of such movements in more active zones show that these are more complex than expected. This shows the importance of collecting as many data as possible. For this reason from 1999 onwards, a network of accelerometers was gradually installed in Belgium. The first earthquake that was recorded by this network was the Alsdorf earthquake in 2002.

At the moment the accelerometer network of the ROB consists of 19 stations. The accelerometers are concentrated in the most active zones. Their precise location is chosen in function of the type of sub-soil and in some occasions also in funcion of the nearness of important infrastructures. Most of the stations are equipped with an accelerometer of type Episensor ETNA, with which the vertical and two horizontal components of the ground acceleration are recorded. Registration starts only when the ground acceleration exceeds a predetermined treshold. The transfer of the data to the ROB is realised with the help of a modem, via analogous telephone lines.