Solving the Mystery: The 21 October 1880 Portuguese Earthquake

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Solving the Mystery: The 21 October 1880
Portuguese Earthquake
by José R. Ribeiro, Ana I. Ribeiro, and
Ana P. Correia
Online Material: Relevant accounts for intensity estimates of
the 21 October 1880 Portuguese earthquake.
INTRODUCTION
Despite recent important contributions to the completeness
of earthquake catalogs, there are still significant gaps in our
knowledge about the preinstrumental seismicity of the Iberian
Peninsula. This is true even for the second half of the nineteenth century, in spite of the vast increase in the number of
scientific institutions and newspapers in these decades. In the
last extensive revision of the Iberian earthquake catalog
(Rodríguez de la Torre, 1990a,b), the number of known earthquakes occurring between 1851 and 1900 increased from 620
to 2066 as a result of the systematic search conducted by
Rodríguez de la Torre mainly using Spanish sources.
Relative to earlier earthquake catalogs, the new data enabled
(Martínez Solares and Mezcua, 2002; Mezcua et al., 2004) to
present a very much improved catalog of the earthquakes that occurred in the Iberian Peninsula and its vicinity, with a new assessment of epicenters and magnitudes of the major events, through
application of the Bakun and Wentworth (B&W) method
(Bakun and Wentworth, 1997, 1999; Bakun et al., 2003).
As Martínez Solares and Mezcua (2002) and Mezcua et al.
(2004) pointed out, the B&W procedure was only applied to
events for which there were enough intensity values. Although
registered in this catalog, one major Iberian earthquake that is
still waiting for epicenter and magnitude determinations is the
21 October 1880 (6:41 a.m.) earthquake.
The event was unknown until 1990, when it was discovered by Fernando Rodríguez de la Torre (1990a) in Spanish
and Portuguese nineteenth century newspapers. With this new
information revealed, the 1880 earthquake was included in the
catalog of Martínez Solares and Mezcua (2002), which was
subsequently transferred into the Spanish Catálogo Sísmico
Nacional (Instituto Geográfico Nacional [IGN]) and the
SHEEC (SHARE European Earthquake Catalogue, 1000–
1899) (Stucchi et al., 2013) with an entry based on Rodríguez
de la Torre’s compilation and comprising data from 27 localdoi: 10.1785/0220140178
ities. The search was later expanded (Rodríguez de la Torre,
1998) to eventually include macroseismic information from
44 Portuguese and Spanish cities, towns, and villages, although
not all are related to the 21 October 1880 event.
Based on the distribution of the intensities, Rodríguez de
la Torre (1990a) suggested an Atlantic location “far removed
from the coast” and a “high magnitude,” a hypothesis he later
reaffirmed (Rodríguez de la Torre, 1998). Martínez Solares and
Mezcua (2002) also located the epicenter in the Atlantic Ocean
at the approximate coordinates 41.0° N and 10.0° W (i.e., at
the latitude of Porto), a location that was also adopted by the
SHEEC. In both cases, however, no specific magnitude values
were reported, and no calculations were presented to support
these hypotheses.
We assume that the number and the spatial distribution of
intensity values were not enough to allow the assignment of a
reliable epicenter and magnitude to this earthquake through
the application of any of the several methods for macroseismic
locations now available. Thus, further advances in the knowledge of the macroseismic effects of that event are necessary to
determine these source parameters. Finding such additional
data and using it to determine the epicenter and the magnitude
of the earthquake was the main purpose of our present work.
METHODS AND RESULTS
To compile coeval accounts, a systematic search of the 1880
Portuguese and Spanish national and local press was carried
out at a wide range of libraries, archives, and online resources.
The search identified more than 100 new accounts of the
earthquake, mostly from local Portuguese newspapers. Scientific reports also provided useful data.
The geographic locations of the old and new observations
are shown in Figure 1. Macroseismic information is now available from 88 localities (more than tripling the current SHEEC
data file); 75 of these are Portuguese, and the vast majority
allow the assignment of an intensity value or interval (European Macroseismic Scale 1998 [EMS-98]). Ⓔ These are supported by a total of 189 independent accounts (available in the
electronic supplement to this article). Table 1 summarizes the
new set of intensities. In the case of localities already included
in the Martínez Solares and Mezcua (2002) dataset, there is a
broad agreement between the estimations of intensities of
Martínez Solares and Mezcua (2002) and this study.
The new data made it clear that the maximum intensities
(VI or even VII in the EMS-98 scale) were clustered within the
country in the vicinity of Serra da Estrela, the highest and
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▴ Figure 1. Macroseismic observations for the 21 October 1880 earthquake. The open circles indicate old observations, and the black
dots indicate new observations.
longest mountain range in mainland Portugal. Throughout
that region, houses and churches collapsed, people were injured, and there was even a death due to a fall (see Ⓔ the electronic supplement). In contrast, in the localities situated along
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the Atlantic coast of northern and central Portugal, the earthquake caused more fear than damage, with typical intensity values ranging from IV to V (EMS-98). In southern Portugal and
in the Spanish towns from which observations were reported,
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Table 1
Macroseismic Observations of the 21 October 1880 Earthquake
Locality
León
Pontevedra
Burgos
Vigo
Tui
Valença
Benavente (Zamora)
Caminha
Montalegre
Vila Praia de Âncora
Bragança
Ponte de Lima
Chaves
Viana do Castelo
Valladolid
Vidago
Braga
Zamora
Guimarães
Famalicão
Caldas de Vizela
Póvoa de Varzim
Felgueiras
Santo Tirso
Vila Real
Maia
Penafiel
Matosinhos
Torre de Moncorvo
Peso da Régua
Foz do Douro
Porto
Vila Nova de Gaia
Tabuaço
Lamego
Salamanca
Ovar
Oliveira de Azeméis
Pinhel
Vouzela
Avelãs da Ribeira
Viseu
Aveiro
Costa Nova
Latitude (° N)
42.600
42.435
42.342
42.230
42.046
42.033
42.003
41.876
41.826
41.813
41.806
41.767
41.741
41.694
41.654
41.640
41.552
41.500
41.442
41.412
41.383
41.383
41.366
41.342
41.296
41.233
41.207
41.185
41.176
41.167
41.152
41.150
41.136
41.117
41.096
40.969
40.860
40.840
40.776
40.723
40.663
40.658
40.641
40.614
Longitude (° W)
5.566
8.646
3.703
8.710
8.644
8.633
5.679
8.839
7.791
8.862
6.757
8.583
7.472
8.828
4.722
7.573
8.423
5.754
8.296
8.521
8.317
8.763
8.199
8.474
7.746
8.622
8.286
8.684
7.052
7.783
8.676
8.610
8.612
7.566
7.808
5.664
8.625
8.476
7.063
8.112
7.223
7.914
8.654
8.751
Intensity (EMS-98)*
III
IV
II–III
III–IV
III
IV–V
III–IV
IV
felt
III–IV
V
IV–V
IV–V
IV–V
III–IV
IV
V
III–IV
IV–V
V
V
IV
IV–V
IV–V
V
V
IV–V
V
felt
IV–V
V
V
V
IV
V–VI
III–IV
V
IV–V
IV–V
V
V
V–VI
V
IV
Sources†
RT1, RT2, MM, this work
RT1, RT2, MM, this work
This work
RT1, RT2, MM, this work
RT1, RT2, MM, this work
RT1, RT2, MM, this work
RT1, RT2, this work
This work
This work
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RT1, RT2, MM, this work
RT2, this work
This work
This work
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This work
This work
This work
This work
RT2, this work
This work
RT1, RT2, MM, this work
RT2, this work
This work
RT1, RT2, MM, this work
RT1, RT2, MM, this work
This work
RT1, RT2, MM, this work
This work
This work
RT1, RT2, MM, this work
This work
RT1, RT2, MM, this work
This work
RT1, RT2, MM, this work
RT1, RT2, MM, this work
This work
*. EMS-98, European Macroseismic Scale (1998).
†. RT1, Rodríguez de la Torre (1990a); RT2, Rodríguez de la Torre (1998); MM, Martínez Solares and Mezcua (2002).
(Continued next page.)
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Table 1 (continued)
Macroseismic Observations of the 21 October 1880 Earthquake
Locality
Águeda
Guarda
Gouveia
Ferreiros (Anadia)
Mira
Madrid
Manteigas
Sepins
Oliveira do Hospital
Valezim
Loriga
Covilhã
Coimbra
Maiorca
Serpins
Figueira da Foz
Cernache
Lavacolhos
Castelejo
Condeixa-a-Nova
Barroca (Fundão)
Alpedrinha
Ega
Castelo Novo (Fundão)
Castanheira de Pera
Figueiró dos Vinhos
Sarzedas
Marinha Grande
Leiria
Tomar
Serra (Tomar)
Abrantes
Vila Nova da Barquinha
Rio Maior
Alcoentre
Elvas
Agualva
Lisboa
Évora
Setúbal
Beja
Córdoba
Sevilla
Huelva
Latitude (° N)
40.574
40.537
40.494
40.448
40.428
40.410
40.401
40.387
40.360
40.359
40.326
40.281
40.210
40.166
40.156
40.149
40.141
40.132
40.124
40.113
40.109
40.099
40.097
40.078
40.000
39.902
39.851
39.751
39.744
39.604
39.596
39.463
39.459
39.337
39.210
38.881
38.773
38.710
38.571
38.525
38.016
37.881
37.387
37.261
Longitude (° W)
8.448
7.268
7.593
8.394
8.735
3.686
7.541
8.496
7.862
7.720
7.690
7.504
8.429
8.755
8.209
8.854
8.471
7.620
7.578
8.499
7.718
7.467
8.540
7.496
8.208
8.275
7.685
8.932
8.807
8.402
8.323
8.198
8.432
8.936
8.961
7.164
9.298
9.140
7.910
8.893
7.865
4.779
5.991
6.949
Intensity (EMS-98)*
V
V
V–VI
IV–V
V
III–IV
VI
Felt
Felt
VII
VII
V
V
IV–V
V
IV–V
IV–V
V–VI
IV
V–VI
VI
V
IV–V
VI–VII
IV–V
V
V–VI
IV
V
IV
V–VI
IV–V
V
IV–V
V
IV
IV
IV–V
III–IV
IV
Felt
III–IV
III
II–III
Sources†
This work
RT1, RT2, MM, this work
This work
RT1, RT2, MM, this work
This work
RT1, RT2, MM, this work
This work
This work
This work
This work
RT2, this work
RT2, this work
RT1, RT2, MM, this work
RT1, RT2, MM, this work
This work
RT1, RT2, MM, this work
This work
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This work
This work
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This work
This work
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This work
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This work
This work
This work
This work
This work
This work
RT2, this work
RT2, this work
This work
This work
This work
RT1, RT2, MM, this work
RT1, RT2, MM, this work
This work
This work
RT1, RT2, MM, this work
This work
RT1, MM, this work
*. EMS-98, European Macroseismic Scale (1998).
†. RT1, Rodríguez de la Torre (1990a); RT2, Rodríguez de la Torre (1998); MM, Martínez Solares and Mezcua (2002).
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the earthquake was weakly felt, with intensities rarely exceeding
III–IV (EMS-98). Hence, even without further data handling,
the emerging pattern (which is considerably distinct from the
one that could be extracted from the previous datasets) made
the previous assignments of an Atlantic Ocean epicentral
location very unlikely.
To obtain the earthquake location and magnitude using
the intensity data, the B&W method was applied. Thereafter,
the attenuation law (equation 1) obtained by Martínez Solares
and Mezcua (2002) and Mezcua et al. (2004) for the Iberian
Peninsula was used to compute the magnitude values M i for
the trial epicenters in accordance with the individual intensity
estimates of Table 1 (Ii), being di the distance between the
corresponding locality and the trial epicenter.
Mi I i − 1:96 2:93 × log d i
:
1:41
1
The trial epicenters were equally spaced (0.1°) and covered
an area from 41.5° N to 39.5° N and from 9° W to 7° W. To
draw the contour maps, we used the confidence levels tabulated
by Bakun and Wentworth (1999) for the number of available
intensity values for this earthquake (83).
The map resulting from this procedure is presented in Figure 2, and the corresponding coordinates of the intensity
center and value of the moment magnitude (with confidence
intervals) are given in Table 2. Our best estimate for the coordinates of the epicentral location, which coincides with the
minimum value for the residual root mean square, is 40.42° N
and 7.88° W. That places the earthquake source in the inner
region of central Portugal, which is almost 200 km away from
the previously hypothesized (Martínez Solares and Mezcua,
2002) epicenter in the Atlantic Ocean. The uncertainty associated with this assignment can be assessed (Mezcua et al.,
2004) by the radius of the circle of the equivalent area for the
different confidence levels: 9, 12, and 26 km for the 50%, 67%,
and 95% confidence levels, respectively. By this method, the
moment magnitude, obtained from the epicentral value of
M i , was M w 6.02 (95% CI 5:74–6:21).
DISCUSSION AND CONCLUSIONS
We obtained an initial and reliable estimate of the geographical
coordinates for the epicenter of the 21 October 1880 earthquake. In conjunction, we also calculated the magnitude of this
seismic event.
A question that inevitably arises from these results is why
the researchers who first found and studied the 21 October
1880 event were not able to obtain similar estimates with
the intensity data points that they gathered.
The B&W method was developed in 1997, so it was not
available to Rodríguez de la Torre for his first analysis (Rodríguez de la Torre, 1990a) nor did he apply it to his subsequent
dataset (Rodríguez de la Torre, 1998). However, Martínez Solares and Mezcua (2002) and Mezcua et al. (2004) were successful in the application of the B&W method to 35 earthquakes of
the Iberian Peninsula, 40% of them with fewer intensity points
than the 22 intensity values that were known for the 21 October
1880 earthquake (Martínez Solares and Mezcua, 2002).
The problem with the earlier epicenter estimates might
not be the number of observations but rather their geographic
distribution. It is worth noticing that previously there was not
a single intensity value from localities situated in the proximity
of the epicenter obtained in the present work. Instead, the intensity reports were mostly from the northern and central Portuguese coastal regions and from a few scattered localities in
western Spain.
Indeed, the application of the B&W method to that highly
biased dataset available prior to the addition of our new data
would not have led to reliable estimates of the epicentral location. We applied the B&W method to the earlier dataset and
obtained contour lines for the 95% confidence level (95% CI)
that extend from 10.4° W to 6.6° W in longitude and from
41.6° N to 39.6° N in latitude. Therefore, the best that the
previous investigators could have offered was a guess at the location of the epicenter, which was nevertheless very distant
from the earthquake epicenter calculated in the present work.
Despite this, the analysis of the epicenter could have been
significantly improved if the B&W method were applied to the
second set of observations compiled by Rodríguez de la Torre
(1998). In fact, this Spanish investigator presented new findings about the 1880 earthquake in February 1998, at the 4th
Portuguese–Spanish Assembly of Geodesy and Geophysics
(Rodríguez de la Torre, 1998). Together with a re-evaluation
of his previous intensity estimates, he released macroseismic
information for another 17 localities. Unfortunately, his new
dataset also included some accounts that (despite being true)
are probably misassigned due to confusion of the 21 October
1880 earthquake with the 20 October 1883 event, the latter
felt mainly in southern Spain. However, among the valid observations, there were also a handful of newspaper accounts
from localities within central Portugal, such as Castelo Novo,
Covilhã, and Loriga (see Table 1), which could have given a
clue as to the correct location for the earthquake epicenter.
In the present study, we collected data that allowed us to
fill some important geographic gaps, particularly within the
Portuguese territory. In these more remote locations, news
from the local correspondents emerged much later in the newspapers of large urban centers, a circumstance that perhaps was
not taken into account in the search by Rodríguez de la Torre
(1990a, 1998). In addition, we obtained a number of accounts
from local newspapers, most of which are not available in Lisbon’s Biblioteca Nacional de Portugal, where Rodríguez de la
Torre completed his inquiries (Rodríguez de la Torre, 1998).
Our own circumstances (living in Portugal) allowed easier access to such local sources, and there are now online resources
that simply did not exist in 1990 or even in 1998.
In short, we succeeded in our purpose of increasing the
initial set of macroseismic data: we advanced the dataset from
27 localities (44 independent accounts) in 1990 to the present
total of 88 localities (189 independent accounts). Not only were
we able to considerably expand the coverage of the territory, but
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▴
Figure 2. Contour map of the macroseismic magnitude M i values (dashed lines) and the confidence levels of rmsM i (thin lines; from
outside to inside: 95%, 67%, and 50%), obtained from the application of the Bakun and Wentworth (1997) method to the intensity data of the
21 October 1880 earthquake. The black dots indicate macroseismic observations, and the black star indicates the intensity center. Main
faults are also represented (solid lines).
Table 2
Estimates for Magnitude and Epicenter Location of the 21 October 1880 Earthquake
Statistical Parameters
Best value
95% confidence intervals
Moment Magnitude (M w )
6.02
5.74–6.21
Latitude (° N)
40.42
40.18–40.81
Estimates obtained from application of the Bakun and Wentworth (1997) method to the intensity data.
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Longitude (° W)
7.88
7.55–8.12
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▴
Figure 3. Epicentral location of all known earthquakes (M > 2) associated with the Seia–Lousã fault (SLF). Verin–Régua–Penacova
fault (VRPF) and Manteigas–Vilariça–Bragança fault (MVBF) are also represented. The inset map shows the site location on a larger
scale. Based on Serviço Meteorológico Nacional (SMN, 1972), Oliveira (1976), Carrilho et al. (2004), Martins and Mendes-Víctor (2005),
Correia and Ribeiro (2007), and Instituto Geográfico Nacional (IGN, 2014).
we were also able to increase the quality of the intensity estimates
such that most of them now are based on more than one account.
The estimated coordinates of the epicenter of the 21 October 1880 earthquake, together with the location of the places
where the maximum intensities were registered, suggest that
the Seia–Lousã fault (Fig. 3), a reverse fault of 90 km length,
could be the possible source of this event. This structure is
generally considered to have a low seismic activity, as only a
few (and relatively small) earthquakes have been assigned to
it (Fig. 3).
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The importance of the assignment of a magnitude and an
epicentral location to this recently discovered historical earthquake goes far beyond a hypothetical new vision of the seismicity
of a particular and obscure fault. In fact, the present magnitude
value of 6.0 places the 21 October 1880 earthquake as the sixth
greatest earthquake of the entire nineteenth century that took
place within or near the Iberian Peninsula (Martínez Solares and
Mezcua, 2002; Mezcua et al., 2004; Vilanova and Fonseca, 2007;
Stucchi et al., 2013). This is a notable achievement for a completely unknown earthquake only 25 years ago.
ACKNOWLEDGMENTS
Without the systematic search conducted by Fernando Rodríguez de la Torre, this earthquake probably would be still forgotten among pages of old newspapers. We are also grateful to
him, to José A. Peláez (Universidad de Jáen), to Viviana Castelli (Istituto Nazionale di Geofisica e Vulcanologia), and to
Yan B. Radziminovich (Institute of the Earth’s Crust, Irkutsk)
for providing scientific papers together with useful comments
and encouragement. We acknowledge William H. Bakun (U.S.
Geological Survey) for very useful information concerning his
method. We also wish to thank Cristina Domingues (Biblioteca
do Instituto Geofísico do Infante D. Luís), João Duarte Fonseca
(Universidade de Lisboa), José Martínez Solares (Instituto Geográfico Nacional, Spain), and Julio Mezcua (Instituto Geográfico
Nacional, Spain) for providing scientific papers. Finally, we are
indebted to Arquivo Histórico Municipal de Elvas (Rui Jesuíno),
Arquivo Municipal de Huelva (Luísa B. Oliver), Biblioteca de
Castilla y León, Valladolid (Amparo S. Rubio), Biblioteca Geral
da Universidade de Coimbra (Carlos Fiolhais, José Mateus, and
Helena Sousa), Biblioteca Municipal de Beja (Hermes Picamilho), Biblioteca Nacional de Portugal (Área de Reproduções),
Biblioteca Pública Municipal do Porto (Paula Bonifácio Marques), Faro de Vigo, and Museu Municipal de Penafiel (Manuel
Ribeiro) for helping in the newspaper search.
We are also truly grateful to the anonymous reviewer of our
manuscript, whose numerous comments and suggestions greatly
helped us to improve the article, and to Claire Niedzwiedz for
her work as our language editor, who made significant revisions
to the manuscript.
Carrilho, F., J. C. Nunes, J. Pena, and M. L. Senos (2004). Catálogo Sísmico
de Portugal Continental e Região Adjacente para o período 1970–2000,
Instituto de Meteorologia, Lisboa, Portugal (in Portuguese).
Correia, A. P. S., and J. R. Ribeiro (2007). Uma reavaliação dos tremores
de terra sentidos em Portugal em Janeiro de 1926 e em Janeiro
de 1927, Sismos e Imprensa 1, 1–2, https://www.researchgate.net/
publication/264375791 (last accessed January 2015) (in Portuguese).
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Madrid, http://www.ign.es/ign/layoutIn/sismoFormularioCatalogo.do
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Oliveira, C. S. (1976). Dados de base sobre risco sísmico em Portugal, Laboratório Nacional de Engenharia Civil, Proc. 36/11/4394, Lisboa,
Portugal (in Portuguese).
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Ibérica durante el siglo XIX, Ph.D. Thesis, Universitat de Barcelona,
Spain, 5 volumes, 2615 pp. (in Spanish).
Rodríguez de la Torre, F. (1990b). Iberian seismic catalogue, 1851–1900.
A revision, Proceedings and Activity Report 1988–1990 of the XXII
General Assembly of European Seismological Commission, Barcelona,
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Rodríguez de la Torre, F. (1998). A very large earthquake, until now unknown, in the Iberian Peninsula (21st October, 1880), Proc. of 4th
Portuguese–Spanish Assembly of Geodesy and Geophysics, Aguadulce,
Almeria, 9–13 February 1998.
Serviço Meteorológico Nacional (SMN) (1972). Anuário Sismológico de
Portugal number 25 (1971), Lisboa, Portugal (in Portuguese).
Stucchi, M., A. Rovida, A. A. Gomez Capera, P. Alexandre, T. Camelbeeck, M. B. Demircioglu, P. Gasperini, V. Kouskouna, R. M. W.
Musson, M. Radulian, et al. (2013). The SHARE European Earthquake Catalogue (SHEEC) 1000–1899, J. Seismol. 17, 523–544.
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José R. Ribeiro
Ana P. Correia
Escola Secundária c/3° ciclo de Henrique Medina
Av. Dr. Henrique Barros Lima
4740-230 Esposende, Portugal
[email protected]
REFERENCES
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Ana I. Ribeiro1
Instituto de Engenharia Biomédica (INEB)
Universidade do Porto
Rua do Campo Alegre, 823
4150-180 Porto, Portugal
Published Online 18 March 2015
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Also at Instituto de Saúde Pública da Universidade do Porto (ISPUP),
Rua das Taipas, 135, 4050-600 Porto, Portugal.
May/June 2015