Observatories and Research Facilities for EUropean Seismology
Volume 3, no 1 June 2001 Orfeus Newsletter


The French National Network of Seismic Survey (RéNaSS)

Michel Granet
EOST-IPGS, 5 rue René Descartes, F-67084 Strasbourg Cedex

Introduction - Description of RéNaSS
The regional short-period seismic survey networks
The national broad-band network - The national headquarters - Scientific outputs
Assessment and future objectives - Acknowledgements -

Introduction

The French metropolitan territory is an area of moderate seismic activity compared to the Euro-Mediterranean domain. However, based on a historical catalogue and one century of instrumental records the possibility that a major earthquake occurs on the French motherland should be considered. During the previous century, the earthquake which occurred at Lambesc (1909, I0=IX) is the most significant. The recent earthquakes in the Pyrenees (18/02/96, M=5.2) and in the Alps (15/07/96, M=4.9) remind us that large and destructive events could happen in France. Real-time monitoring of the French seismic activity is the main task of the National Network of Seismic Survey ( RéNaSS, INSU-CNRS).
The National Institute of the Science of the Universe (INSU-CNRS) supports since the middle of the seventies a national program to survey the seismicity of France. Nowadays, RéNaSS gathers all the regional seismic networks and isolated seismological stations installed and maintained by the Observatories of the Science of the Universe (OSU), the Institute of Physics of the Earth of Paris and the seismological laboratories of different universities. The two objectives of RéNaSS are:
  • 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

Born from the necessity to organise and improve the instrumental seismic coverage of France at the beginning of the eighties, RéNaSS is a federation of regional seismic networks and broad-band seismological stations whose main components are linked at a central site located in Strasbourg. The current organisation is as follows:
  • 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.
Nowadays, RéNaSS consists of:
  • 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

We distinguish between real-time networks (which directly contribute to the rapid determination service) and the networks for which the seismological data are collected afterwards by either the regional or national centers.

'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

The national broad-band network is currently composed of fourteen stations equipped with Streckeisen STS2 sensors and two equipped with Streckeisen STS1 sensors (Figure 1):
  • 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).
The stations of Arbois, Calern, Chute-du-Sampolo and St-Étienne-de-Tinée are under the responsibility of the university of Nice Sophia-Antipolis (TGRS-RéNaSS network). The stations of Argentières-La-Bessée, Digne, Grand-Maison and Sixt are under the responsibility of the Observatory of Grenoble (ROSALP-RéNaSS network). The remaining sites are directly operated by the national headquarters at Strasbourg.
The stations equipped with STS2 sensors are endowed with an acquisition system called TITAN-ATLAS developed by the manufacturer AGECODAGIS. They have been designed by the manufacturer on the basis of a 'request book' issued jointly by the seismologists responsible for different seismic survey networks. The data are continuously recorded on a ring buffer of 2 Gb (4 Gb in the near future). It is possible to record up to 6 channels with a sigma-delta A/D converter having a 24 bits dynamic range.
The connection with central or regional hubs is made through switched phone lines.


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

The national headquarters

The national headquarters at Strasbourg are responsible for the daily exploitation of the 69 stations linked to Strasbourg. Approximately 2500 local earthquakes per year are processed. The parametric data are distributed on a daily and a weekly basis both as classical paper bulletins and electronic reports through the Internet. The flow chart of the general operations at the national center is shown in Figure 2. At Strasbourg, raw data (seismograms) and parameter data (phase pickings, earthquake location and magnitude) are immediately accessible on the RéNaSS website.

klick for originaly large figure
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 applications of the short-period regional seismic networks concern mainly studies on seismic tomography and seismotectonics including some local studies on the instrumental seismicity and the seismic hazard. With seismic tomography, very new results were obtained for deep structure investigations in the Pyrenees, Alps, Rhine Graben and the Armorican Massif. Additional studies on seismic anisotropy at a regional scale or local scale provide new results on deformation at depth related with recent or old orogenies. A main contribution to seismotectonics based on data obtained by these regional networks is the new seismotectonic zoning map. Papers based on focal mechanisms are regularly published. Publications on the two large earthquakes which occurred recently in France address the seismogenesis processes acting in the crust.

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.
On a long-term, the calculation of new structural models and the constitution of a data base of focal mechanisms and centroid moment tensors of moderate to large earthquakes - similar of what is currently existing at a worldwide scale - would largely contribute to a better knowledge of regional tectonics and seismic hazard in France. A list of publications to date is available.

Assessment and future objectives

RéNaSS is an operating network for the real-time seismic surveillance of France as well as for its scientific activity. Its federal structure allows a strong regional presence. In addition, its 'university' label has a great impact towards the general public and the media. Its main forces are:
  • 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.
Concerning the dispatching of the earthquake information, both the regional and national headquarters will act on:
  • The data base access from a large set of services.
  • The improvement of the distribution tools: GIS and voice server.

Acknowledgements

RéNaSS will not exist without a strong, positive and unfailing support by INSU (CNRS) since the idea was born at the beginning of the eighties. The Ministry of Education, Research and Technology (MENRT) contributes significantly to the development of this Observatory task. Seismic surveillance would not be possible without the following Observatories of the Science of Universe: Clermont-Ferrand, Grenoble, Paris, Toulouse, Strasbourg. Different universities contribute to maintain this activity through a strong staff support: Marseille St-Jérôme (UMSJ), Nice Sophia-Antipolis (UNSA) and La Rochelle (ULR). Many scientists, engineers, technicians, students have been and are currently involved in RéNaSS. Without their contribution, nothing would have been possible.

Table 1. List and coordinates of broad-band stations.
Code Latitude Longitude Elevation Name Installation date
ARBF 43.4920 N 5.332 E 185 Arbois (06) 10/1998
ATE 43.0850 N 0.700 W 480 Arette (09) 12/1998
CALF 43.7523 N 6.922 E 1242 Calern (06) 10/1994
CFF 45.7630 N 3.111 E 400 Clermont Ferrand (63) 05/1995
CHIF 46.1330 N 0.408 W 62 Chizé 10/2000
DOU 50.0050 N 4.595 E 224 Dourbes (Belgium) 02/1997
MLS 42.9560 N 1.095 E 450 Moulis 09/1995 (closed 10/1998)
OGAG 44.7880 N 6.541 E 1300 L'Argentière (05) 06/1996
OGDI 44.1100 N 6.217 E 770 Digne (05) 09/1997
OGGM 45.2040 N 6.116 E 1575 Grand-Maison (38) 01/1996
OGSI 46.0666 N 6.754 E 750 Sixt (74) 07/1997
RENF 48.1190 N 1.635 W 40 Rennes (35) 03/1996
SAOF 43.9864 N 7.555 E 574 Saorge (06) 05/1995
SMPL 42.0940 N 9.285 E 405 Sampolo (2B) 09/1996
STET 44.2500 N 6.929 E 1200 St-Étienne-de-Tinée (06) 03/1997
ECH 48.2159 N 7.158 E 580 Echery Station of Geoscope
SSB 45.2792 N 4.542 E 700 Saint-Sauveur-de-Badole Station of Geoscope


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