Station COI: Dusting Off an Old Seismic Station

E
○
Station COI: Dusting Off an Old Seismic
Station
by Susana Custódio, Josep Batlló, Décio Martins,
Fábio Antunes, João Narciso, Sara Carvalho, Vânia
Lima, Fernando C. Lopes, Paulo Ribeiro, Reinoud
Sleeman, E. Ivo Alves, and Celeste R. Gomes
Online Material:Analysis of the background seismic noise of
station COI, and yearly inventory of the historical seismograms
and bulletins of station COI.
INTRODUCTION
The seismic station COI, located in Coimbra, Portugal, is the
oldest seismic station in Portugal and among the oldest in the
world. Experimental seismic recording began in 1891 with an
Angot seismograph (V. G. S. S. Santos, unpublished data, 1995;
Custódio et al., 2010). Continuous recording of ground
motion began in 1903 with a Milne seismograph, and is still
on-going at the same site, now with a broadband Streckeisen
STS-2 seismometer. Coimbra’s observatory owns an over
100-year long, well-preserved collection of seismograms, seismographs and bulletins. Station COI is taken into account
in early seismological works, for example, in the 1901 compilation of stations equipped with seismographs recommended by
the British Association for the Advancement of Science
(BAAS, 1901), in the earthquake catalog of the International
Association of Seismology for the year of 1904 (Rosenthal,
1907), in the San Francisco earthquake report (Reid, 1910)
or in the classical book “Seismicity of the Earth” (Gutenberg
and Richter, 1954). However, throughout time station COI
lost its early importance and was progressively forgotten.
Station COI is missed even by recent compilations of historical seismic data (e.g., Ferrari, 2000; Schweitzer and Lee,
2003).
Recently we were able to start undusting the seismological
heritage of Coimbra’s station. In this paper, we present a brief
history of the station. We also describe its seismographs, seismograms and bulletins, including their present state of preservation and availability. Station COI is particularly relevant for
studies of earthquakes in the pre-digital era given that: (1) it is
doi: 10.1785/0220120014
one of the westernmost stations in Europe (Fig. 1); (2) it has
operated for more than one century at the same site; and
(3) the instruments are located inside an underground shelter
with double walls, on well-consolidated ground, in a site favorable for seismic observations.
HISTORY
The University of Coimbra (UC) was founded in 1290, at the
time of the establishment of the first universities in Europe. In
1755 the largest historical earthquake in Europe—the Lisbon
earthquake—occurred offshore Portugal with a magnitude
estimated between 8.5 and 8.8 (Abe, 1979; Johnston, 1996).
Both the earthquake and the resulting tsunami devastated several locations in Portugal, including the capital city of Lisbon
(Coimbra was almost unaffected). In Europe, the great Lisbon
earthquake marked a turning point in the thinking about the
Earth (Kendrick, 1955; Fonseca, 2005). A few years later, the
Portuguese education system was reformed. In 1772 the faculties
of Mathematics and Natural Philosophy were created at the
UC, giving a new impetus to modern science in Portugal.
The study of the Physics of the Earth was greatly developed
in Europe over the following (nineteenth) century. In order
to keep up with the new advances, the UC requested funds from
the Portuguese government for the establishment of a meteorological and magnetic observatory (Carvalho, 1872; Leonardo
et al., 2011). The observatory, named Observatório Meteorológico
e Magnético da Universidade de Coimbra, was founded in 1864.
In 1891 the observatory initiated seismographic observations
with an Angot instrument (V. G. S. S. Santos, unpublished data,
1995; Custódio et al., 2010). These were the earliest seismographic observations in Portugal. Continuous seismic recording
started a decade later, in 1903, with a Milne seismograph
(Viegas, 1910). In 1925 the observatory was renamed Instituto
Geofísico da Universidade de Coimbra—IGUC (Geophysical
Institute of the University of Coimbra) (V. G. S. S. Santos,
unpublished data, 1995; Custódio et al., 2010).
In 1909, an M 6 earthquake occurred in Ribatejo, lower
Tagus valley (mainland Portugal), causing considerable damage
to the village of Benavente and strong shaking in Lisbon (Stich
et al., 2005; Fonseca and Vilanova, 2010; Teves-Costa and Batlló, 2011). The 1909 event reminded the country of the danger
posed by earthquakes. Coimbra’s station was then re-equipped
with Wiechert seismographs, and another two seismic stations
were installed in Lisbon (LIS) and Porto (PTO). The first
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863
−30°
−15°
0°
15°
30°
45°
60°
60°
50°
50°
COI
40°
40°
30°
30°
−30°
−15°
0°
15°
30°
45°
▴ Figure 1. Map of the location of seismic stations (triangles) in
Europe in 1909–1910 based on the compilation of Batlló et al.
(2008), after Schweitzer and Lee (2003) and Merlin and Somville
(1910). The star marks station COI.
seismic bulletin produced at station COI was published in 1910
and contains information about the year of 1909 (Viegas, 1910).
The seismic station COI was operated independently by
the University of Coimbra from the time of its foundation
until 1946. In 1946, under the dictatorial government of
Salazar, the staff of the three Geophysical Institutes (Lisbon,
Porto and Coimbra) were integrated into the Serviço Meteorológico Nacional—SMN (National Meteorological Service).
This decision was not well accepted in Coimbra and had negative consequences in the operation of the institute
(Carvalho, 1946; V. G. S. S. Santos, unpublished data, 1995;
Leonardo et al., 2011). During the 1950s and 1960s efforts
were undertaken to modernize the seismic station: In 1954
the vertical Wiechert was modified to electromagnetic recording and a new vertical short-period Grenet seismograph was
acquired in 1961. At the same time, station COI applied to
join the World Wide Standardized Seismographic Network
(WWSSN). However, the only station in mainland Portugal
selected to integrate the WWSSN was PTO.
In 1969 a large M 7.9 earthquake occurred offshore
Portugal (Fukao, 1973). This earthquake was strongly felt in
Portugal and prompted an upgrade of the national seismic network. The new network was composed of nine stations in
mainland Portugal, including COI, LIS and PTO. The seismic
data collected by the network were centralized in Lisbon and
analyzed by SMN. Coimbra’s station was then completely
re-built and equipped with three short-period and three longperiod Geotech seismometers.
In April 25, 1974, the Carnation Revolution put an end to
the dictatorial government. The months that followed the revolution were marked by political and civil turmoil. During
this period several manuscripts and records of the institute,
including seismograms labelled “with no earthquakes”, were
destroyed in a bonfire (seismograms in Coimbra’s archive are
divided into “with earthquakes” and “with no earthquakes”
stacks) [J. S. Bandeira, personal comm., 2012]. To the
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Volume 83, Number 5
best of our knowledge, this was the only occasion in the
history of the institute in which records were destroyed.
At present, station COI is again operated independently
by the University of Coimbra. Ground motion is currently
recorded by a broadband seismometer. Hundreds of high
school students visit the station yearly in order to learn about
seismology: Students are given a tour of the historical instruments, shown posters and animations about earthquakes and
seismology, and have the opportunity to jump next to the
Geotech sensors in order to see their footsteps on the seismograms on the computer screen.
The Geophysical Institute of the University of Coimbra
remains at the same site where it was initially established. At
the time of the observatory’s founding, it was situated in a
quiet rural area on top of a hill, in a location that was well
outside the town of Coimbra. Throughout the twentieth century urban development engulfed the observatory, compromising most significantly the recording of high-frequency groundmotion. In order to assess the evolution of seismic noise at
station COI, we plan to digitize and analyze a sample of background noise for every year in the archive. Station COI is
located on well-consolidated triassic sandstones.
Analysis of present-day broadband records of station COI
reveal that the site has a low noise level at long periods, with the
minimum noise approaching the low noise model around
0.1 Hz (Fig. 2). At higher frequencies the data becomes noisier,
but remains below the high noise model. The amplitude of the
seismic noise around 2 Hz shows a clear daily variation, probably due to urban noise. More details on the computation of
the power spectral density (PSD) of background noise, as well
as a near-real-time monitor of the PSD, can be obtained from
the website of the Observatories and Research Facilities for
European Seismology—ORFEUS (http://www.orfeus‑eu
.org/Data‑info/dataquality.html). Ⓔ The electronic supplement S1 shows the probability density function (PDF) of the
PSD of ground acceleration from April through December 2011.
SEISMOGRAPHS, SEISMOGRAMS AND
BULLETINS
The collection of seismometers preserved at station COI
comprises a Milne, an astatic Wiechert, a vertical Wiechert, a
Grenet, three Geotech S13, three Geotech SL210, and a
Streckeisen STS-2 instruments (Table 1). Only the Geotechs
and the STS-2 are currently in operation. The response of each
instrument is shown in Figure 3. The historical collection of
seismographs, seismograms and bulletins of station COI is
mostly well preserved. The earliest seismograms found at the
station date back to 1903.
The first seismograph of the observatory—an Angot—was
acquired in 1891. The Angot seismograph and seismograms
cannot be found at the observatory today, but fortunately
these are the only seismograph and seismogram collection
missing from the observatory. Everything that we know about
the Angot is based on indirect evidence. V. G. S. S. Santos,
September/October 2012
PSD (10 * log (m 2/s 4/Hz))
(a)
−80
−100
−120
−140
−160
−180
−200
100
PSD (10 * log (m 2/s 4/Hz))
(b)
2.0 Hz
0.5 Hz
0.05 Hz
0.01 Hz
150
200
250
Julian day
300
350
−80
−100
0.5 Hz
2.0 Hz
−120
−140
−160
−180
−200
320
0.05 Hz
0.01 Hz
322
324
326 328
Julian day
330
332
334
PSD (10 * log (m 2/s 4/Hz))
(c)
−100
−120
HNM
−140
−160
LNM
−180
0.01
0.1
1
Frequency (Hz)
10
▴ Figure 2. Power spectral density (PSD) of background noise
recorded at station COI from April though December 2011. All plots
come from data recorded by the broadband STS-2 sensor, channel
BHZ (vertical). Similar plots for other channels and components
can be obtained from the ORFEUS website. (a) PSD of the background noise as function of time for selected frequencies of
0.01 Hz, 0.05 Hz, 0.5 Hz and 2.0 Hz. (b) Same as (a) for 15 days
of observations. The background noise at 2 Hz displays a clear diurnal modulation, which is probably due to urban noise. (c) Minimum
background noise level at 0.05 Hz. The minimum noise level at station COI, channel HHZ, overlaps the low noise model around 0.1 Hz.
unpublished data (1995) gives a detailed list of documents
attesting the ordering, payment, installation and operation of
this seismograph. The seismometer, Angot B, N° 5388, was built
in the Breguet workshop, in Paris. Other Angot seismographs
existed in France (Lyon, Grenoble, Clermont-Ferrand/Puy-de-
Dôme, Paris and maybe Nice) and one instrument also existed in
Algeria (Algiers) (J. Fréchet, personal comm., 2009). To the best
of our knowledge, none of these seismographs can be found
today. Price (1887) describes the Angot seismograph as composed of three inertial masses suspended by two pendula and
one vertical spring. The two pendula were orthogonal and
recorded ground motion along the directions EW and NS.
The apparatus was enclosed by a wooden coverture and attached
to a vertical column. The pendula were connected to a drum of
slow rotation speed and a clock was integrated in the system. The
seismograms were written on paper by ink. Curiously, the existence of the Angot seismograph at station COI is denied, or
ignored, by the two directors who pioneered seismology in
Coimbra. In 1909, Viegas wrote a letter to Laurence Martin,
U.S. Geological Survey, Madison (Wisconsin, U.S.A.) in which
he stated: “Our seismograph was established in the year 1904; we
have no seismological observations before that year” (V. G. S. S.
Santos, unpublished data, 1995). In this sentence Viegas refers
to the Milne as the first seismograph in Coimbra. A few years
later, Carvalho (1916) wrote: “In 1903 the seismological services
were inaugurated. A Milne horizontal pendulum, built by Munro,
was installed (…) in April of that year”. The two directors probably considered the Milne seismograms as the first reliable seismic records of station COI.
The Milne seismograph was acquired for IGUC in 1901
and started to operate regularly in 1903. The pendulum was oriented as to record EW ground displacement. The recording
system was photographic, the record speed was 1 mm=min,
and the amplification was approximately a factor of 10. The
pendulum motion was undamped and had a natural period
of approximately ∼20 sec. Viegas (1910) notes that the Milne
seismograph was good at recording distant earthquakes but was
not so good at recording nearby earthquakes. The Milne seismograph is the oldest seismic instrument in Coimbra today.
It is well preserved, with a few parts missing (e.g., lamp, mirror,
boom, parts of the wooden coverture). Its seismogram collection
is very complete and well preserved. Of the 692 rolls of seismograms recorded between 1903 and 1916, ∼50 days are not recorded due to malfunctions of the seismometer and only three
rolls are missing (corresponding to 3 weeks, or 21 days). The
Milne seismograph started to operate irregularly in 1918, with
interruptions that sometimes lasted several months. Probably
the Milne seismograph was given less importance after the
acquisition of the astatic Wiechert, which started to operate regularly in 1915. The last Milne seismogram found at the institute
contains data from July 1926. Ⓔ The electronic supplement S2
contains a list of the years for which we do and do not have
seismograms of each instrument.
Wiechert seismographs were acquired for the institute on
the wake of the 1909 M 6 Benavente earthquake. The astatic
Wiechert arrived at the institute in 1911 but only started to
operate regularly in 1915. The inverted pendulum had an
inertial mass of 1,000 kg and natural period of 13–15 sec. It
recorded two orthogonal components of ground motion: EW
and NS. The motion of the inverted pendulum was damped
through air pistons. Seismograms were written on smoked
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Table 1
Seismometers and seismograms of station COI.
Type1
Mech
Mech, LP
Mech, LP
Mech, LP
Mech/EM, LP
EM, SP
EM, SP
EM, LP
FB, BB
Seismograph
Angot
Milne
Astatic Wiechert
Vertical Wiechert
Wizine
Grenet
Geotech S13
Geotech SL210
STS-2
Component2
EW, NS, Z
EW
EW, NS
Z
Z
Z
EW, NS, Z
EW, NS, Z
EW, NS, Z
Recording System3
Ink
Photo.
Smoked
Smoked
Photo.
Photo.
Heat/Ink/Dig.
Heat/Ink/Dig.
Dig.
Operating Period
1891–1899(?)
1903–1926
1915–1969
1926–1953
1954–1979
1961–1985
1972–today
1974–today
2007–today
Seismograms4
missing
1903–1926
1915–1969
1933–1953
1954–1979
1962–1985
1972–today
1974–today
2007–today
1. Type of seismograph: mechanical (Mech); electromagnetic (EM); force balance (FB); long period (LP); short period (SP);
broadband (BB).
2. Component(s) of ground-motion recorded by the seismometers.
3. Recording system: photographic (Photo.); smoked paper (Smoked); hot stylus writing over sensitive paper (Heat); digital (Dig.)
4. Time period for which seismograms exist at the station. Not all years exist and/or are complete. Ⓔ The electronic
supplement S2 contains a more detailed list of seismograms available.
(a)
Displacement Response
6
10
5
10
4
Amplification
10
3
10
2
10
1
10
0
10
−1
10
−2
10
−3
10
−2
10
−1
0
10
10
Frequency (Hz)
1
10
2
10
Velocity Response
(b)
3
10
Amplification
2
10
1
10
0
10
−1
10
−2
10
−3
10
−2
10
−1
0
10
10
Frequency (Hz)
1
10
2
10
▴ Figure 3. (a) Displacement and (b) velocity response spectra
of the seismometers operated at station COI: 1. Milne; 2. Astatic
Wiechert; 3. Vertical Wiechert; 4. Grenet; 5. Short-period Geotech;
6. Long-period Geotech; 7. Broadband STS-2. Note the evolution
in amplification, damping, and frequency band of the recorded
ground motion. Details on the computation of the response curves
can be found on the logbook of the station.
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paper, with an amplification on the order of 100–200, at a speed
of 14–15 mm=min. Coimbra’s archive contains seismograms of
the astatic Wiechert for the years 1915–1969. This collection of
seismograms is heterogeneous: Some years are very complete
and well preserved, some seismograms are in poor conditions,
and a few whole years are missing (Ⓔ electronic supplement S1).
The seismograph itself is well preserved.
The vertical Wiechert seismograph was acquired in 1926.
Alike the astatic Wiechert, it recorded seismograms on smoked
paper. The amplification of the instrument was on the order of
100. The seismograph had an inertial mass of 80 kg and natural
period of ∼5 sec. The pendulum motion was damped by air
pistons. In 1953, Custódio de Morais, director of the institute
at the time, visited the observatory of Malaga, Spain, where he
saw a vertical Wiechert adapted to electromagnetic transduction
and photographic recording (V. G. S. S. Santos, unpublished
data, 1995; Batlló, 2004). Coimbra’s vertical Wiechert seismograph was then adapted likewise. The new parts imported from
Spain were signed by Eng. Guillamón, Observatorio Sismológico de Málaga. This modification added magnets, a coil and a
galvanometer to the system, as well as a photographic recorder
(Fig. 4). The seismograph thus became a coupled mechanical
and electromagnetic instrument. Old station logbooks evidence
an effort to calculate the amplification and working characteristics of the coupled sensor. In these old logbooks the modified
seismograph is referred to asWizine, which results from the concatenation of Wiechert with Galitzin. The electromagnetic
adaptation rendered the instrument much more sensitive: the
amplification of the mechanical instrument was approximately
100, whereas that of the modified instrument was probably
around 1500. Curiously, the vertical Wiechert is listed as a seismograph in operation in the bulletins of Coimbra, but no phase
arrivals are reported based on its seismograms. We have not yet
been able to identify the reason for this and for its many missing
smoked paper seismograms. We can point to a couple reasons
September/October 2012
▴ Figure 4. Photographs of the modified vertical Wiechert, renamed Wizine after the adaptation to electromagnetic transduction and photographic recording.
that may have contributed to the little importance given to the
seismograms of this instrument: The natural period of the mechanical Wiechert was not very stable and its 80-kg mass was too
light to overcome friction. The friction problem was eliminated
with the modification to electromagnetic recording. However,
the coupling between the mechanical sensor and the electromagnetic device was difficult to calculate. Also, the coils of the Wizine
did not move in a uniform magnetic field, so the gain G of the
motor was not stable. All these reasons rendered the seismograph
difficult to calibrate. The amplitudes of the seismic waves were thus
meaningful only in a relative sense. Nevertheless, it should have
been possible to extract arrival times from the seismograms.
A short-period vertical seismometer, which was provided
by Prof. Grenet, Univ. Algiers (Algeria), started to operate
regularly in Coimbra in 1961 (V. G. S. S. Santos, unpublished
data, 1995). It was the first electromagnetic seismograph by
initial design in Coimbra. It was also the first short-period
instrument at the observatory. The Grenet seismograph had a
natural period of 1.4 sec and was connected to a photographic
recorder with a speed of 30 mm=min. The collection of
Grenet seismograms is quite complete and the seismograms are
well preserved (Ⓔ electronic supplement S2).
Coimbra’s seismic station was completely rebuilt following
the M 7.9 earthquake that occurred offshore southwest Iberia
in 1969 (Fukao, 1973). Both Wiechert seismographs were
stopped at this time, while the Grenet was kept in operation
in a different room of the observatory. The astatic Wiechert
never operated again. The Wizine was set back into service and
operated for almost another decade until 1979. The new station was placed fully underground and had thick double walls
to ensure proper thermal insulation. WWSSN-like instruments were then installed at the station: three long-period (LP)
seismometers Geotech SL210, three short-period (SP) seismometers Geotech S13, analog instrumentation for amplification and filtering, helicorders, a quartz-crystal clock and a
radio for accurate timing. The SP and LP sensors started to
operate in 1972 and 1974, respectively. The seismograms were
written by a hot stylus passing over sensitive paper. All three
components (EW, NS and Z) of the LP seismometers were
recorded on one page, leading to entangled traces for highamplitude signals. In order to avoid this problem, the horizontal components were disconnected and only the vertical
component was actually recorded. The recording system was
modified to ink in 1983. In 1991 the Grenet recording drum
was adapted to record the EW LP component of ground motion. Data became digitized and saved directly to a local disk in
2003. In 2006 the helicorders were deactivated. The Geotech
seismogram collection is quite complete and well preserved.
The seismometer most recently installed at station COI is
a broadband Streckeisen STS-2 sensor, which started to operate
in 2007. The sensor is connected to a Quanterra Q330 digitizer. The data are currently relayed in real-time to Instituto de
Meteorologia, Portugal, to the Data Center (DC) of ORFEUS,
to the Data Management Center of the Incorporated Research
Institutions for Seismology (IRIS DMC), and to other interested institutions. The data can be freely accessed via ORFEUS
DC (http://www.orfeus‑eu.org/Data‑info/data.html) or IRIS
DMC (http://www.iris.edu/dms/dmc/), using station code
COI (Coimbra) and network code SS (Single Station). The
station code COI is internationally registered at the International Seismological Centre (ISC, http://www.isc.ac.uk/) and
the network code SS is assigned by the International Federation of Digital Seismograph Networks (FDSN, http://www
.fdsn.org/) for single stations.
Seismic bulletins of station COI were published during
about two decades (Ⓔ electronic supplement S1). The first
bulletin can be found on-line at https://bdigital.sib.uc.pt/
republica/UCSIB‑A‑23‑43/globalitems.html. Typed bulletins
that are in their final format but which were never published
exist for over another decade of observations. Preliminary
bulletins, with phase identification, arrival times, and time
corrections, exist for many other years. Coimbra’s archive also
contains bulletins received from other stations around the
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world, some of which are very old (end of the nineteenth century, beginning of the twentieth century). Amongst the bulletins stored in Coimbra, we highlight bulletins from stations in
former Portuguese colonies, as Mozambique and Angola. We
also found bulletins of Spanish seismic stations for the years of
the Spanish civil war that cannot be found in Spain today.
There are likely other bulletins of interest in Coimbra’s
archive—a full inventory of these bulletins is ongoing.
PRESENT AND FUTURE
In the last years we pulled together resources from different
projects in order to start to document and preserve the seismological heritage of station COI. Historical seismograms
are now roughly divided by years and a detailed seismogramby-seismogram inventory is ongoing. Some seismograms of
station COI have already been digitized within the European
Seismological Comission project EUROSEISMOS, led by
Istituto Nazionale di Geofisica e Vulcanologia (INGV, Italy),
and a few other individual initiatives. The website of EUROSEISMOS (http://storing.ingv.it/es_web/) contains a list of
the seismograms scanned within the project.
Seismic bulletins of station COI, both published and unpublished, as well as other relevant documents, are currently being
digitized with funding from a Portuguese project that concerns
the history of science at the University of Coimbra. All these
documents will be freely available on-line as they become digitized. The Museum of Science of the University of Coimbra
is currently documenting the seismic instruments of station COI.
Station COI recently became a reference station of the
world in SeismoArchives (http://www.iris.edu/seismo/).
The SeismoArchives are being constructed under the auspices
of the International Committee for Preservation of WWSSN
and Historical Seismograms (ICPWHS) of the International
Association of Seismology and Physics of the Earth’s (IASPEI),
in collaboration with IRIS DMC and the United States
Geological Survey (USGS). Selected seismograms of station
COI will be digitized within the scope of SeismoArchives.
We have compiled a detailed logbook of station COI, with
relevant information concerning the seismographs (including
transfer functions and poles and zeros), seismograms and bulletins.
The logbook and the day-by-day inventory of seismograms, both
of which are frequently updated, can be accessed at http://
geofisico.dyndns.org/media/10143/coistation.pdf and http://
geofisico.dyndns.org/media/10419/inventory.xls, respectively.
All the digitized material of station COI is freely available
for research purposes. Some seismograms have already been
used for the study of historical earthquakes (e.g., Batlló et al.,
2010). We do not have the capability of running a database or
server with the historical material ourselves, but we will make
all existent material available to interested data centers (e.g.,
SISMOS, IRIS-DMC). We welcome suggestions about the
next seismograms to digitize and/or comments on how to best
preserve the heritage of station COI.
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ACKNOWLEDGMENTS
Our first and foremost acknowledgment goes to all directors and
operators who throughout more than a century went out of
their way to diligently care for seismic recording in Coimbra.
We also thank J. Fréchet (École et Observatoire des Sciences
de la Terre—Institut de Physique du Globe de Strasbourg,
EOST-IPG) for informations about the Angot seismographs.
This work was supported by project EUROSEISMOS and
SeismoArchives. The Portuguese Science and Technology
Foundation (Fundação para a Ciência e Tecnologia) funded this
work through grants HC/0119/2009 and PEst-OE/CTE/
UI0611/2012. The lead author (S.C.) acknowledges a Marie
Curie International Reintegration Grant awarded within the
7th European Community Framework Programme (PIRG03GA-2008-230922).
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Susana Custódio
Fábio Antunes1
João Narciso2
Sara Carvalho
Vânia Lima
Fernando C. Lopes1
Paulo Ribeiro3
E. Ivo Alves1,3
Celeste R. Gomes1
Centro de Geofísica
Universidade de Coimbra
Portugal
[email protected]
Josep Batlló
Instituto Dom Luiz, Universidade de Lisboa
Portugal
Décio Martins
Centro de Física Computacional
Departamento de Física
Universidade de Coimbra
Portugal
Reinoud Sleeman
Observatories and Research Facilities for European Seismology
Netherlands
1
Also at Departamento de Ciências da Terra, Universidade de Coimbra,
Portugal.
2
Now at ICIST, Instituto Superior Técnico, Portugal.
3
Also at Instituto Geofísico, Universidade de Coimbra, Portugal.
Seismological Research Letters
Volume 83, Number 5
September/October 2012
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