SSM Network Renewal

A project of renewal of the Swiss Strong Motion Network was approved by the Swiss Federal Council in February 2009. The project is monitored and supervised by a steering committee headed by the Swiss Federal Office for the Environment (FOEN). The project, which final goal is the installation of some 100 new accelerometric stations in the next 8 years, is subdivided into two main phases.

During the first phase (2009-2012), 30 new state-of-the-art accelerometric stations will be installed throughout the country. New installation sites are planned, as well as replacement of existing strong motion dial-up stations, as displayed in Fig. 1. Orange squares in Fig. 1 are the presently operating dial-up stations, while blue squares represent the dam-related instruments. Green squares are the sites equipped with modern stations, continuously monitored at SED. Blue circles represent the priority sites for the first phase of the project. They include station replacements as well as sites for new free-field installations.

Fig. 1. Present outline of the SSMNet network and possible installation sites during the first phase of the renewal project.

Note that the outline of the new possible locations (blue circles in Fig. 6) provides on one hand better spatial coverage of the Swiss territory and, on the other hand, prioritizes the replacement of old strong motion instruments in the most hazardous and earthquake prone regions of the country. In particular, the epicentral areas of relevant past earthquakes will be instrumented in the first phase, namely Aigle (1584), Glarus (1971), Sarnen (1964), Sion-Sierre (1946), Yverdon (1929), Visp (1855), St. Gallen Rhine Valley (1796/96), Altdorf (1774), Brig (1755), Basel (1356), Churwalden/Vaz (1295/1991), etc. Further, the city areas of Zürich, Geneva, Basel, Bern, Lausanne, St. Gallen, Lucerne, Biel, Sion, Solothurn, Locarno, Chur, Sierre,are relevant sites for free-field installation.

The precise selection of sites will be a trade-off between the scientific objectives and the level of vibration disturbances. Indeed, modern stations are sensitive enough to record also small earthquakes but the signal-to-noise ratio may be too low due to traffic, industries, etc. in populated cities. Old stations located in transformer houses will be moved away from the transformers, stations close to train tracks and main roads will also be moved.

Fig. 2. New concept of station housing at SSMNet stations.

Within the framework of the renewal of the Swiss strong motion network, a completely new housing solution has been designed, as shown in Fig. 2. The housing system consists of a cylinder shaped concrete case covered by a metallic vault, for hosting the sensor and the electrical power devices. The datalogger and communication system are placed in a separate case (orange box in Fig. 2) and can be placed several meters away from the station, if necessary. Stations are anchored to the ground by means of metallic bars in order to avoid settlements, relative displacements and rotations (between concrete base and soil) in case of relevant ground shaking. This anchoring solution is implemented e.g. in the Italian strong motion network (RAN, see Gorini et al. 2010). The choice of sensors and dataloggers was guided by the need for retrieving realtime, continuous, very broadband 24 bit data. Data will be handled as those of the SDSNet stations. Each station will have detailed site assessment as proposed by the project Neries (Joint Research Activity Geotechnical Site Characterization JRA4).

References:

A. Gorini, M. Nicoletti, P. Marsan, R. Bianconi, R. De Nardis, L. Filippi, S. Marcucci, F. Palma, and E. Zambonelli. The italian strong motion network. Bulletin of Earthquake Engineering, in press, 2010.