Portuguese Seismic Networks
Graça M. Silveira1,2,
Dina Vales3 and
Centro de Geofísica da Universidade de Lisboa, Edifício C8, Campo Grande, 1749-016 Lisbon, Portugal
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.
Instituto Superior de Engenharia de Lisboa, Rua Conselheiro Emídio Navarro, 1950-062 Lisbon, Portugal
Instituto de Meteorologia, Rua C ao Aeroporto, 1700 Lisbon, Portugal
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
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.
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.
All data, digital and analogue, is treated, compiled, archived and publish by the IM
headquarters. IM is an International Seismological Centre (ISC) member.
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.
- 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.
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).
Digital and analogue stations are similar to those installed
in the mainland. All data from the three observatories is transmitted to the
central headquarters, located at Ponta Delgada Observatory. All the
pre-processing, compilation and archive is organised there.
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.
- 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.
BB, 20 bits
VBB, 24 bits
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.