1. Introduction
This report is going to outline some of the key details surrounding high magnitude earthquake that struck the South West coast line of Portugal in 1755. Details about the earthquake, the tsunami that followed and the fire that they both helped to cause. The report will also touch on the importance of earthquake proofing buildings and the consequences suffered by Lisbon due to the lack of technology they had to offer.
The earthquake occurred on the Eurasian, African plate boundary and subsequently the damage suffered by Lisbon the capital of Portugal and a major trading port at the time. Lisbon is located on the south west coast of Portugal.
Figure 1 An isoseismal map showing the Mercalli results over Spain and Portugal. (Silva, Gómez-Diego, Elez, & Giner-Robles, 2017)
On the 1st November 1755 the city of Lisbon was hit with a devastating tsunami triggered by an earthquake measuring a magnitude of 8.5 on the Richter Scale (Zitellini, Mendes, Cordoba, & Danobeitia, 2001) and a X-XI in Lisbon on the Mercalli Intensity Scale (Baptista, Heitor, Miranda, Miranda, & Mendes Victor, 1998) this classifies as few buildings surviving and severe landslides, making this earthquake Western Europe’s largest and most destructive in over 1700 years in the roman republic times (Zitellini, Mendes, Cordoba, & Danobeitia, 2001). Following both the earthquake and the tsunami the city was ravaged by fire. Lisbon did not suffer alone other cities including Faro and C.S.Vicente along the coast also felt the effects of both the earth quake and the tsunami and it is said that the earthquake was felt as far away as Finland (Zitellini, Mendes, Cordoba, & Danobeitia, 2001) more than 3500 km away.
2. Tectonic setting
The tectonic setting for this earthquake is the African – Eurasian plate boundary, a convergent ocean – ocean plate margin, shown here in [fig 2]. Further West along the plate boundary it begins to become a transform boundary, eventually divergent. The plate margin undergoes compressional forces due to the convergence from the North West against the South East, although slow (Baptisa, Miranda, Miranda, & Mendes Victor, 1998), over long periods of time the friction between the plates is capable of building up and storing elastic energy. According to (Baptisa, Miranda, Miranda, & Mendes Victor, 1998) there is no reason to believe there is a Benioff zone here. The Gorringe Bank is made up of sub marine mountains and are formed by the compressive forces either side. The Gorringe Bank acts as a margin to the Horseshoe Abyssal Plain (Zitellini, et al., 2009). These compressional structures are prime locations for faulting and therefore earthquake epicentres.
The likely Fault zone area according to (Zitellini, et al., 2009) the fault responsible for the earthquake was a reverse thrust fault with an area of 27,000 km2 and a length of around 370 km. This fault would have been a result of the release of elastic energy displacing the water column above leading to the tsunami.
Figure 2 A plate tectonic map of the Eurasian African margin. (PORTUGALPRESS, 2017)
3. Earthquake Epicentre
Working out an earthquakes epicentre is an important part of understanding more about how it occurred. This is not an easy process when the earthquake occurred hundreds of years ago; however using isoseismal maps the earthquake was estimated to be a magnitude 8.5 on the Richter scale. A supposed epicentre of 37’00’N 10’00’W according to (Baptista, Heitor, Miranda, Miranda, & Mendes Victor, 1998) was derived from this method. This may not be a precise location for the epicentre of the 1755 Earthquake. Seismologists have found it difficult to pin point the epicentre. Due to other points matching a lot of the criteria for an epicentre of an earthquake.
Figure 3 A map showing three proposed locations for the Lisbon earthquake. Richardson et al. 2010
Evidence points to three locations one just North of Gorringe Bank (Zitellini, Mendes, Cordoba, & Danobeitia, 2001), a point just South of Gorringe Bank and the third point indicates closer to the shore line [Fig 3]. Model B just above the Gorringe bank which is a series of submarine mountains formed by compressional forces and model C is on the oceanic – continental plate boundary the oceanic crust is getting subducted here both areas are likely locations for the Epicentre. Model A is a large thrust structure thought to stretch for over 50Km and shows displacement that other areas don’t (Zitellini, Chierici, Sartori, & Torelli, 1999) this displacement is characteristic of an earthquakes epicentre. In this area the African and Eurasian plates are converging (Zitellini, Mendes, Cordoba, & Danobeitia, 2001).
4. Tsunami
Tsunamis are waves that carry far more energy than the average wave, they are formed by sudden movements on the ocean floor. This could be an earthquake causing a large displacement in the column of water above the seafloor, landslide or even a meteorite impact. These waves are often very destructive as when they reach land and start to grow in height. In the case of the 1755 Lisbon Earthquake it was submarine and caused a tsunami to form. The sheer reach of this tsunami is portrayed in [fig 4] waves reaching as far as 6500 km or more.
Figure 4 A model showing the time in hours for the waves to travel and the extent of how an earthquake this large can effect areas in excess of 6500 km away. (Institute of Computational Mathematics and Mathematical Geophysics, n.d.)
The tsunami of 1755 didn’t just effect Lisbon just like the earthquake was felt far away the tsunami was witnessed in Morocco, as far as Cornwall and even the West Indies (Baptista, Miranda, Chierici, & Zitellini, 2003). Reports from the time suggest flooding of Lisbon town with waves as high as 6 metres, a run up height of greater than 15 metres and making it as far as 250 metre from the shore line (Baptista, Heitor, Miranda, Miranda, & Mendes Victor, 1998). In the case of the disaster in Lisbon, there was a large outbreak of fire across the city, which is not uncommon in large cities after a tsunami or earthquake. Modern cities have gas lines and electricity, which can break and start a fire. However in 1755 the fires would have had to be caused by other methods. The tsunami hit on the 1st of November 1755 which happened to be all saints day where there where candles lit all over the city. Following the ground movement of the earthquake and the of the tsunamis shear pushing power candles inevitably fell and caught fire subsequently spread fire through the city.
5. Impacts and Prevention
When natural disaster strikes there is often a toll on human life and the economic climate of a city or even country. This event is thought to have covered 800 000 km2 and to have killed up to 100 000 (Chester, 2001) in total. The level of damage caused by an earthquake can be put down to different factors including ground quality, geological structures, building quality or location for example coastal towns are at far more risk from tsunamis. The amount of damage caused by an earthquake is used to make an eye witness account of the damage done to buildings, roads and natural structures like rivers. This damage is recorded on the Mercalli scale, Lisbon was rated between a IX and X which means the ground has sustained serious damage i.e. cracks and many buildings have been destroyed. Lisbon is a coastal town and hence was devastated by the tsunami generated by the 1755 earthquake, something that Lisbon lacked was suitable sea defences especially ones strong enough to deal with a tsunami. A sea wall used to defend Japan today can be seen in [fig 5].
Figure 5 Japan’s 4 storey tsunami sea wall (Independant, 2016)
In 1755 the buildings where not reinforced or built with earthquakes or tsunamis in mind. A result of this was that buildings did not hold their structure and therefore sustained serious damage. Today we build buildings with anti-earthquake technology included. For example we put large buildings on rollers and pendulums in the centre of them to counter the movement. Buildings are also built to be flexible however back in the 1700s the technology simply wasn’t there. The lack of these safety measure lead to the destruction of many buildings in Lisbon and surrounding towns and cities. The fire would have potentially been caused by fallen candles, oil lamps or stoves left on it the panic.
6. Conclusion
This report set out to inform and go in to light detail about the 1755 Lisbon earthquake disaster. Areas covered in this report include the tectonic setting of the South West coast of Portugal near Lisbon, the report highlighted the likely epicentres and explained why. Following this there is information on the devastating tsunami triggered by the earthquake, explaining the distance waves can travel and the obdurate impact they have on surrounding areas. To bring the report to a close there is some information on the failings of the architecture of Lisbon. There are points made about the lack of structural reinforcement for events such as this earthquake showing the importance of technology we have today. Also highlighted the absence of sea defences that is like to have played a great part in the downfall of Lisbon. Overall the earthquake Lisbon suffered on the 1st November 1755 was a high magnitude earthquake one that still remains one of the highest experienced in the area. With this in mind, the devastation it brought to Lisbon and the surrounding area is justified and, this only amplified by the lack of anti-earthquake technology in buildings and sea defences in the city of Lisbon.
7. References
- Baptisa, M. A., Miranda, P. M., Miranda, J. M., & Mendes Victor, L. (1998). CONSTRAINS ON THE SOURCE OF THE 1755 LISBON TSUNAMI INFERRED FROM NUMERICAL MODELLING OF HISTORICAL DATA ON THE SOURCE OF THE 1755 LISBON TSUNAMI. J. Geodynamic Vol. 25, 159-174.
- Baptista, M. A., Heitor, S., Miranda, J. M., Miranda, P., & Mendes Victor, L. (1998). The 1755 Lisbon tsunami: parameter. J. Geodynamics Vol. 25, 143-157.
- Baptista, M. A., Miranda, J. M., Chierici, F., & Zitellini, N. (2003). New study of the 1755 earthquake source based on multi-channel seismic survey data and tsunami modeling. Natural Hazards and Earth System Sciences, 333-340.
- Chester, D. K. (2001). The 1755 Lisbon earthquake. Progress in Physical Geography 25,3, 363–383.
- Independant. (2016, March 5). Japan’s sea wall: Storm brews over plans to construct giant £5bn barrier against tsunamis. Retrieved from Independant: https://www.independent.co.uk/news/world/asia/japans-sea-wall-storm-brews-over-plans-to-construct-giant-5bn-barrier-against-tsunamis-a6914781.html
- Institute of Computational Mathematics and Mathematical Geophysics. (n.d.). Analysis of the Tsunami Travel Time maps for damaging tsunamis in the World Ocean. Retrieved from Tsunami Labratory: http://tsun.sscc.ru/ttt_rep.htm
- Johnston, A. C. (1996). Seismic moment assessment of earthquakes in stable continental regions-111. New Madrid 1811-1812, Charleston 1886 and Lisbon 1755. Geophys.J. Int., 314-344.
- PORTUGALPRESS. (2017, March 30). Lisbon’s next devastating quake now has a date! Retrieved from Portugal Resident: http://portugalresident.com/lisbon’s-next-devastating-quake-now-has-a-date
- Silva, P. G., Gómez-Diego, P. V., Elez, J., & Giner-Robles, J. L. (2017). EARTHQUAKE ENVIRONMENTAL EFFECTS OF THE AD 1755 LISBON EARTHQUAKE- TSUNAMI IN SPAIN. IX Reunión del Cuaternario Ibérico, 53-57.
- Zitellini, N., Chierici, F., Sartori, R., & Torelli, L. (1999). The tectonic source of the 1755 Lisbon earthquake and tsunami. Annali Di Geofisica, 49-55.
- Zitellini, N., Gràcia, E., Matias, L., Terrinha, P., Abreu, M., Abreu, M., . . . Diez, S. (2009). The quest for the Africa–Eurasia plate boundary west of the Strait of Gibraltar. Earth and Planetary Science Letters, 13-50.
- Zitellini, N., Mendes, L. A., Cordoba, D., & Danobeitia, J. (2001). Source of 1755 Lisbon Earthquake and Tsunami Investigated. Eos, Transactions, American Geophysical Union, 285-296.
08.03.2019