Introduction

• A real-time monitoring of the French seismicity including the distribution as a 'routine service' or as a 'rapid determination service' of earthquake parameters. This activity is labelled as a 'public service' mission by the Home Office and implies a 24h duty.
• To record, collect, archive and distribute the seismic data (seismograms of local and teleseismic events and earthquakes parameters) for scientific purposes.

Description of RéNaSS

• The national headquarters of RéNaSS is in charge of the rapid determination of earthquake parameters on a 24h schedule, the collection, the archiving and the distribution of seismograms, arrival times and source parameters.
• The regional headquarters have the technical and scientific responsibility of the regional seismic network. They are also the main partners of local and regional administrative councils.

• A short-period university network consisting of 103 seismological stations distributed over 7 regional networks and some independent stations. These regional dense seismic networks are located in areas where the historical and instrumental seismic activity are the most important, thus allowing both a low triggering threshold and a very precise location of earthquakes.
• A broad-band network equipped with a high dynamic acquisition system (Figure 1).
• A central site which is in charge of the real-time monitoring of the seismic activity and of the routine processing of the short-period and broad-band networks.

The regional short-period seismic survey networks

'Real-time' short-period regional seismic networks
The real-time short-period networks are the Rhine Graben network (9 1comp stations), the Nice hinterland area network (5 1comp and 2 3comp stations), the Provence network (8 1comp stations) and the region of Massif Central network (4 1comp and 3 3comp stations). The transmission of seismological data is achieved using radio-telemetry from the remote station in the field to a central station equipped with a continuous data recording system. A PC Linux-based data acquisition system installed by the central site of RéNaSS normally complements a regional recording system. Such a PC-based recording system enables one to look at the seismic noise in real-time through the Internet simultaneously at Strasbourg and at the regional headquarters. Upon a detection, the seismograms and the automatic phase pickings are automatically transmitted to Strasbourg using the Internet again.

'Off-line' networks
There are three regional networks off-line or in near real-time: the Pyrenean network composed of 20 isolated stations (15 3comp and 5 1comp), the region of Alps network (so-called 'Sismalp') composed of 44 stations (9 3comp and 34 1comp) and the region of Charentes network composed of 5 stations (1 3comp).
The data are transmitted to the regional center or to the central site at Strasbourg using either switched phone links or the METEOSAT satellite (5 Pyrenean stations). The connections between the central site and these regional centers is described later on. Three additional autonomous stations are operated: Mencas, Dompierre and Brest. These independent stations are connected with the central site at Strasbourg through a PC-PC link on a switched phone network.

The national broad-band network

• STS 1: Saint-Sauveur-de-Badole (SSB) and Echery (ECH) belonging to the Geoscope global network.
• STS 2: Arbois (ARBF), Argentières-La-Bessée (OGAG), Calern (CALN), Chizé (CHIF), Chute-du-Sampolo (SMPL), Clermont-Ferrand (CFF), Digne (OGDI), Dourbes (DOU, Belgium), Grand-Maison (OGGM), Arette (ATE), Rennes (RENF), Saorge (SAOF), St-Étienne-de-Tinée (STET) and Sixt (OGSI).

Figure 1. Location of broad-band stations on the French metropolitan territory.

The national headquarters

Figure 2. Synoptic scheme of the data processing at the national headquarters.

The seismic duties of RéNaSS consist of a routine survey of the national and worldwide seismicity from seismic signals (seismograms) collected through the regional seismic networks and isolated stations using different transmission links (radio-telemetry, switched or dedicated phone lines, Internet, satellite). These duties also include the distribution of obtained earthquake parameters and the archiving and distribution of the raw data, the phase picking and the hypocenter parameters. Seismograms are continuously analysed and phases picked automatically. When several phase pickings become available, an initial location of the event is obtained using an algorithm depending on the 'probable' nature of the earthquake (local, regional, teleseismic). The estimated location and magnitude are compared with globaly obtained event detections and, if appropriate, an alarm is declared. In case of such an alarm, the seismologist on duty comes to the data center and collects all the available information from other seismic networks or agencies - the national headquarters at Strasbourg are in permanent connection through the Internet with other national or international seismological institutions - . An improved location is obtained and within 60 minutes after the earthquake occurred its parameters are made publicaly available. In other cases, the events are routinely processed each morning. Data are then stored in the data base where they are made available through a website for either scientific purposes or general public information and a monthly bulletin is published.

The national headquarters of RéNaSS is also responsible for the fast determination of large events, and has to inform the civil authorities when an earthquake has been felt. This kind of activity of fast determination requires scientists on duty, 24 hours per day and seven days per week.

Scientific outputs

The scientific output of broad-band data concerns mainly source studies and new structural models of Europe. At the central headquarters at Strasbourg we are presently developing automatic procedures to retrieve moment tensors of large earthquakes within near-real time and well in advance of any field actions:

• The seismic moment of all earthquakes having a magnitude M>4 and located in France or its immediate surroundings.
• The seismic moment and the focal mechanism for all earthquakes located in the same area with a magnitude M>4.5.

Assessment and future objectives

• A continuous link with the general public and a systematic educational information by a seismologist in case of destructive earthquakes.
• Continuous upgrade of the information in case of a seismic crisis.
• On site intervention in case of a major earthquake.
• A positive scientific output of the data.

RéNaSS is constantly improving its professionalism, which implies to work on a better homogeneity of all used instrumental equipment and a greater strictness in collecting and archiving the seismic data on a long-term basis, which, at the end, is the main and primary mission of seismological observatories. Among the future objectives, a better instrumental coverage of the territory and a more user-friendly and complete access to all possible 'earthquake' information are on the top of the list.

Concerning the instrumental covering RéNaSS will be instrumentally and technically improved by:

• The addition of a four stations network located in the region of Normandy.
• The progressive and systematic replacement of 1 component by 3 components seismometers; in addition, some networks will be equipped with long period sensors.
• Upgrading and securing the computer-based fast determination.
• The use of satellite for data transmission: the fast development of satellite technology with reduced costs compared to the rapidly increasing technical, administrative and financial difficulties to maintain radio-telemetry transmissions which motivate a transition to new technologies.

• The data base access from a large set of services.
• The improvement of the distribution tools: GIS and voice server.

Acknowledgements