Portuguese Seismic Networks
Graça M. Silveira1,2, Miguel Miranda1, Dina Vales3 and Luisa Senos3
1
Centro de Geofísica da Universidade de Lisboa, Edifício C8, Campo Grande, 1749-016 Lisbon, Portugal
2
Instituto Superior de Engenharia de Lisboa, Rua Conselheiro Emídio Navarro, 1950-062 Lisbon, Portugal
3
Instituto de Meteorologia, Rua C ao Aeroporto, 1700 Lisbon, Portugal
Introduction - Institute of Meteorology - SIVISA - ULISSEIS
Introduction
Portugal is an area of moderate to high seismic activity. In the past, the Portuguese territory has been affected by large earthquakes, like the 1755 1st of November earthquake which destroyed Lisbon and caused more that 30 000 casualties. More recently, during the last century, the Azores islands have also suffered significant human and economic losses.The very first installation of seismographic stations in Portugal occurred in the dawn of the former century. In 1902, two Milne pendulae have been deployed at the Azores islands, one in Ponta Delgada (São Miguel) and the other in Horta (Faial). Since then, and for almost eight decades, the development of seismology in Portugal always followed large earthquakes. In the aftermath of these earthquakes (mainly 1909, April 23rd; 1969, February 28th and 1980, January 1st) a few initiatives have been taken to expand the national seismic network. In the last two decades, several programmes have been started by different institutions to improve the seismic surveillance of the Portuguese territory.
The ex Instituto Nacional de Meteorologia e Geofísica, now Instituto de Meteorologia (IM), has been for a long time the main responsible for the portuguese seismic network. Nowadays, several groups are running a pool of seismic stations. The Instituto de Meteorologia continues to be the portuguese coordinator of national network, which is mainly devoted to the seismic surveillance of the portuguese economic zone. Other groups, connected to some Portuguese Universities, run seismic stations for research studies. Among them we can refer the Centro de Geofísica da Universidade de Lisboa (CGUL), the consortium SIVISA, between the IM and the University of Azores (UA), the Centro de Geofísica da Universidade de Évora (CGE), as well as the Instituto Superior Técnico from the Universidade Técnica de Lisboa (IST). We will present here the main characteristics of the networks runned by the IM and by the CGUL.
Institute of Meteorology Seismic Network
The Instituto de Meteorologia detains the seismic monitoring system in Portugal. The IM is the national institute responsible to provide timely notification of earthquake occurrences and the information needed by decision makers before and during times of crisis to the National Security Authorities in order to mitigate human and economic losses. Their research activity concerns mainly the identification of the active seismogenetic areas and the seismicity characterization in Portugal.
Figure 1. Geographical distribution of IM seismic stations in Portugal - November 2003.
The IM seismic network in Mainland Portugal and Madeira is composed by a total of 26 stations. The geographical location of these stations is presented in Figure 1. They can be distributed in four sub networks, as described below.
- The National Digital Network (NDN) has 14 stations (12 on Mainland and 2 on Madeira archipelago). These stations are equipped with sensors having a flat velocity response in the frequency band 0.2-40 Hz, with local data digitalisation, and a dynamic range of 120 dB. They can be accessed via a fully automatic dial-up process, over the public telecom network. All data is transmitted to a central computer network, at IM headquarters. Some enhancements on the transmitting process have been gathered with the introduction of ISDN lines.
- A second network, designated by Fóia Seismic Network (FSN), is a regional network covering the Algarve region. The FSN network groups seven short period stations, from which six are a single vertical component and the other one a three component station. Stations broadcast analogue signals in real time, trough a radio link, to a central acquisition system. The overall dynamic range is 72 dB. The central unit is dial-up accessed, from IM headquarters, via the public telecom network.
- The third sub-network group fours stations installed around Lisbon. This sub-network transmits in real time trough an analogue FM radio link and is the only one that sends data in real time to the IM monitoring service. It has a dynamic range of 72 dB.
- The fourth network has 5 paper recording stations. Four are in the mainland and one in Madeira. They are now more or less obsolete and need urgent replacement.
SIVISA Seismic Network
SIVISA stands for SIstema de VIgilância Sísmica dos Açores and it is a joint consortium between the IM and the Universidade dos Azores. SIVISA operates the Azores regional seismic network, composed by a total of 42 short period stations.
Figure 2. Geographical distribution of SIVISA/IM seismic stations on the Azores - November 2003.
The regional network is divided into three sub-networks, with data being concentrated at Horta (Faial), Angra do Heroísmo (Terceira) and Ponta Delgada (São Miguel).
- Horta Observatory concentrates data from 6 digital stations and 7 analogical-digital ones. All of them are telemetric ones, sending data by radio and they are installed trough the five islands of archipelago central group.
- Angra do Heroísmo Observatory concentrates data from 6 telemetric analogue stations locally digitised. All of them sending data by radio link. They also run one local analogue long period station. Five of these stations are installed in Terceira, one in Graciosa and the last one in Pico.
- Ponta Delgada concentrates data from 14 analogue stations, locally digitised. All the stations are installed in São Miguel around specific volcanic areas. Data is sent to the headquarters by radio link.
ULISSEIS Seismic Network
The main task of the ULISSEIS network, launched by the Centro de Geofísica da Universidade de Lisboa (CGUL) in 2001, is to serve the seismological community with high quality broad band seismic data for all kinds of scientific tasks. Portugal (mainland, Azores and Madeira) is presently covered with 4 permanent broad band stations belonging to the ULISSEIS network. Their sitting plan is shown in Figures 3a, 3b and 3c for Portugal mainland, Azores and Madeira, respectively. Table 4.1 resumes the actual status of the installed broad band stations, namely their code, location and main characteristics.| Station code | Coordinates | Characteristics | Operating date | Data |
|---|---|---|---|---|
| EVO | 38.532N 8.013W 25m |
STS1 BB, 20 bits |
02/96 to 12/99 |
Available |
| EVOP | 38.520N 8.120W 200m |
LDG sensors: LPZA 12s LPHA 12s 24 bits |
1996 to present |
Restricted |
| MDR | 32.710N 16.910W 1605m |
LDG sensors: LPZA 12s LPHA 12s 24 bits |
1996 to present |
Restricted |
| CDRO | 38.629N 28.699W 195m |
STS2 VBB, 24 bits |
07/03/01 to present |
Available |
Figure 3. Geographical distribution of ULISSEIS and other VBB stations in Portugal - November 2003;
left - mainland; right top - Azores; right bottom - Madeira
The Évora station (EVO) was the first broad band seismic station to be installed in Portugal as permanent. This station is equipped with a STS-1 seismometer (Figure 4a) and its 20-bit acquisition system was developed by EOST (École et Observatoire de Physique du Globe de Strasbourg). The EVO seismic station has been installed on a shelter belonging to the Évora University. A concrete pier for the STS1 seismic sensors deployment has been specifically constructed (Figure 4a) and for a better thermal insulation cork boxes are being used (Figure 4b). Data from EVO has been converted to SEED format and it is openly available to the scientific community by the usual request methods of GEOSCOPE. EVO closed in 1999, but will open again after being re-equipped with the Quanterra Q330 acquisition system.
Figure 4. Left - STS1 Évora seismic station sensors; right - STS1 sensors after insulation
Later, under the umbrella of the GITEC program, with the coordination of the LDG and cooperation of the CGUL, two Broad Band seismic stations have been installed in Évora and Madeira. Those stations where installed for a tsunami risk alert. This data are actually at the LDG headquarters, but belong to the CGUL. Consequently, to the ULISSEIS network. The necessary efforts to make them openly available at the CGUL are now underway. During 2001, the CGUL installed another VBB station in the Azores, namely in the Faial island, at Cedros (Figure 3). This station is part of the ULISSEIS network and was installed in the framework of the Memorandum of Understanding COSEA (Coordinated Seismic Experiment in the Azores). The permanent code CDRO was already assigned to the station by the NEIC/USGS. This station is equipped with a STS-2 seismometer and with a TITAN 3NT 24-bit acquisition system. Presently, the data are being pre-processed and sent to the IRIS archive data in SEED format. Since last April, data from CDRO are openly available.
Figure 5. Deployment of the CDRO (Faial) VBB seismic station; left - installing the STS 2 in the concrete pier; right - preparing the TITAN-3NT acquisition parameters.
The necessary efforts to install a data server in the CGUL, to concentrate all raw data acquired
locally, are now underway. A systematic backup policy will be implemented. All data
obtained in ULISSEIS network will be made available trough a WEB system via standard request procedures, in
particular, by anonymous ftp.
ULISSEIS intends to increase, re-distribute and upgrade their broad band stations. The deployment of a new
station in Trás-os-Montes region will enable a better coverage of Portugal mainland, in what concerns VBB seismic stations.