The Geology P.A.G.E.: Geology Through Literature

The next up on my Geology Through Literature thread is Candide by Voltaire published in 1759. You can get my complete thoughts on the book/story over at my website - Dinojim.com, but for here I will just go into the geological or basic scientific aspects that are brought up in the story.

I had heard about the geological content of Candide many years ago after I had read the story. So, I had it on my list to eventually go back and find the information in order to present it here. Within the story of Candide, the titular character comes upon the shores of Lisbon, just as an earthquake begins. This is a historical event that took place on November 1st, 1755 (just four years prior to publication).

Chapter 5

"Scarcely had they set foot in the city (Lisbon), still weeping over the death of their benefactor, than they felt the earth quake beneath their feet. In the port a boiling sea rose up and smashed the ships lying at anchor. Whirlwinds of flame and ash covered the streets and public squares: houses disintegrated, roofs were upended upon foundations, and foundations crumbled.

Thirty thousand inhabitants of both sexes and all ages were crushed beneath the ruins. The sailor said with a whistle and an oath:

'There'll be some rich pickings here.'

'What can be the sufficient reason for this phenomenon?' wondered Pangloss.'

The end of the world is come!' Candide shouted."

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...'This earthquake is nothing new,' replied Pangloss. 'The city of Lima felt the same tremors in America last year. Same causes, same effects. There must be a vein of sulphur running underground from Lima to Lisbon.'

'Nothing is more probable,' said Candide, 'but for God's sake get me some oil and wine.'

'What do you mean, "probable"?' the philosopher retorted. 'I maintain that the thing is proven.'

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...'For all this is the best there us, If the volcanic activity is in Lisbon, it means it could not have been anywhere else. For it is impossible for things not to be where they are. For all is well.'"

Chapter 6

"After the earthquake which had destroyed three quarters of Lisbon, the wise men of the country had not been able to come up with any more effective means of preventing total ruin than to give people a splendid auto-da-fé. It was decided by the University of Coimbra that the spectacle of a few people being ceremonially burnt over a low flame is the infallible secret of preventing earthquakes.

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...A week later...the earth quaked once more.

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For my reading I had the Everyman's Library version of Candide and at the beginning of most Everyman's Library books is an Introduction. This introduction, by Roger Pearson talks about the actual earthquake which is portrayed in Candide and was experienced by Voltaire.

"...Voltaire's faith in God had been severely shaken by the Lisbon Earthquake on 1 November (All Saints' Day) 1755, which killed 40,000 or more people; and his poem on the subject, published in 1756, is a devastating cri de coeur against Pope and Leibniz, not to mention the Almighty. Subtitled 'An Examination of the Axiom: All is well', the poem begins by asking, first, how such carnage can be in accordance with the eternal laws of a good and free God and, second, how it can be a punishment from God. Why Lisbon, Why not London or Paris ('Lisbon lies in ruins, while in Paris they dance.') Did the volcanic activity that caused the earthquake really have to be part of the Creation?"

The Great Lisbon Earthquake is well known as one of the widest felt earthquakes ever on record. It was felt from its epicenter off the shores of Portugal, up through Great Britain and well into Africa (see image below).

Shakemap of the Great Lisbon Earthquake. (Gutscher et al., 2006)

Best estimates are that between 10,000 and 15,000 people died within the city of Lisbon. Many died later due to injuries, fires, and tsunamis and outside the city limits, hence giving the higher numbers often cited elsewhere. As to the damage, many of the finer buildings in the city were mostly ruined and smaller houses and shops were completely destroyed. Observers in ships were said to see the city swaying corn before the buildings collapsed. These estimates and observations are based on eyewitness accounts in The Lisbon Earthquake by T. D. Kendrick.

However, contrary to what was believed (at least by Candide at the time), this was not a result of volcanic activity. As you can see on the map below, there aren't even any even any volcanoes on the Iberian Peninsula.

Active, dormant, and extinct volcanoes of the world.

Even with this blown up view of Europe showing any earthquake hazards in Europe, there isn't even a hazard within 1,000 km of Lisbon. And even that one has an uncertain eruption date.

Earthquake Hazard Map of Europe

My guess is that Candide is aware of the volcanic activity and their corresponding earthquakes in Italy and attributed this earthquake to those causes. However this does not have the hallmarks of a volcanic eruption. As the seismograms below can illustrate, volcanic eruptions have a drawn out shaking due to the magma moving through the Earth called harmonic tremors. This provides a steady shaking over a long period of time. Earthquakes due to fault action have a sharp start and peter out fairly quickly (over the course of 1-3 minutes).

Volcanic harmonic tremors

Fault based seismic tremors

The earthquake had to be a result of fault movement then and not volcanic activity, so let's looks at possible plate tectonic activity. Looking at a plate tectonic map of the Iberian Peninsula we have the following:

Fault slip rates along plate tectonic boundaries (http://diapiro.ictja.csic.es/gt/ivone/research_AFEU.html)

There is a plate boundary running right near Lisbon (which is located right about where the "5" is in 0.05 north of the plate boundary). So where would the earthquake epicenter have been? Looking at the map below, a recent study has pinpointed the likely epicenter to being within the Marques de Pombal (Zitellini et al., 2001).

Bathymetric map of the southwestern Iberia with location of seismic stations (Zitellini et al., 2001).

It has also been determined that the cause of the earthquake was likely a shallow, eastward dipping thrust fault (Gutscher et al., 2006). This type of fault zone, along with it's location can cause the amount of damage caused in the fault as well as the following tsunamis that are associated with this fault. It is interesting to note that even though they compare this Lisbon earthquake to a Lima, Peru earthquake as both being caused by volcanic activity, both are actually caused by eastward dipping thrust faults. Lima, Peru was hit and destroyed by an earthquake and tsunami combo in 1746 (history.ac.uk). These events are so similar, that it is no wonder that Voltaire used this example in his writing. Although, he did have the date off, it wasn't "last year" from the Lisbon earthquake, but close enough (9 years prior).

It is estimated that the Lisbon Earthquake had the magnitude of 8.5-9.0 on the moment magnitude scale (the Richter Scale). The Richter Scale measures the amount of energy released from an earthquake. This number is comparable to other earthquakes because it does not care about the amount of damage that occurs. However, this can only really be measured with modern day equipment. Any earthquakes that occurred before the advent of earthquake reading equipment needs to be estimated differently to gain an accurate estimate.

What we can measure without the direct scientific readings of the energy released from the fault is the amount of damage that had been recorded by contemporary media. This information is then translated into the Modified Mercalli Intensity Scale. It is on this scale that we are able to place estimates on the size of the earthquake. Based on this scale, the Lisbon earthquake would likely be at least a IX in the area of Lisbon (as shown on the first map above):

Violent

Damage considerable in specially designed structures; well-designed frame structures thrown out of plumb. Damage great in substantial buildings, with partial collapse. Buildings shifted off foundations.

And due to the construction at the time (wooden and brick houses, high percentage of people living in poverty), this type of damage would wipe out an entire city.

As a final note, the text states that some shaking started about a week later (presumable a week after the original earthquake). These later earthquakes are what are known as aftershocks. Aftershocks are smaller earthquakes than the original, however they can cause just as much, if not more, damage due to the already susceptible state that the city is in from the original earthquake. Aftershocks are fairly common after large earthquakes as the fault settles out from a large movement (USGS). This was the case for the Lisbon earthquake as well (drgeorgepc.com).

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References

https://www.researchgate.net/profile/M-A_Gutscher/publication/223469777_The_Gibraltar_Arc_seismogenic_zone_Part_2_Constraints_on_a_shallow_east_dipping_fault_plane_source_for_the_1755_Lisbon_earthquake_provided_by_tsunami_modeling_and_seismic_intensity/links/09e41507675e5267e8000000.pdf

https://archive.org/details/lisbonearthquake010555mbp

http://earth.rice.edu/mtpe/geo/geosphere/hot/volcanoes/volcanoes_b2.html