Sunday, March 4, 2012

Philosophy, literature, religion, and geology (Part II)

Are you then sure, the power which would create
The universe and fix the laws of fate,
Could not have found for man a proper place,
But earthquakes must destroy the human race?
Voltaire (1755)

Yesterday, in Part I of this post, I talked about Voltaire's mention of the great Lisbon earthquake of 1755 in his novella Candide.

Lisbon isn't a place we normally associate with earthquakes.  Portugal is a small country nestled up against Spain, on the western coast of the Iberian Peninsula, and Lisbon, its capital, is located at the mouth of the Tagus River (Tejo in Portugeuse) where it flows into the Atlantic.

Note the nicely curved, southwest-facing coastline funneling into the estuary of the Tagus River.  This feature will be significant when we discuss the tsunami which struck Lisbon shortly after the earthquake.

Seismicity in this part of the world is due to the tectonic plate boundary between the African Plate and the Eurasian Plate (which runs through the Straits of Gibralter out to the Mid-Atlantic Ridge).

This boundary is called the Azores-Gibralter Fault Zone (AGFZ) and movement along this boundary is complex.  East of the Azores, the AGFZ has divergent movement where the plates pull apart along the Tercieira Ridge.  In the middle segments, there's transform movement along the Gloria Fault where the plates grind sideways past each other.  And, finally, near the Gulf of Cadiz and Straits of Gibralter, the plates have a convergent motion where they're pushing together.

So, around 9:40 am (local time), on Saturday, November 1, 1755 (All Saints Day), the convergent portion of this plate boundary slipped, possible 10 - 20 meters, by thrusting along a shallowly-dipping eastward-directed fault plane.  This fault is just offshore from the Cape St. Vincent (Cabo de São Vicente), the extreme southwestern tip of Portugal  (reference).  The movement along this fault generated an estimated 8.5 - 9.0 magnitude earthquake (no seismic instruments back then, the magnitude is estimated from the size of the fault and damage estimates).

The first strong shock wave (the P wave arrival) damaged many of the buildings, but it was the arrival of the second shock (surface waves) which caused the extensive damage and collapsed structures throughout the city.  Among those structures were many of the stone churches filled with worshippers celebrating mass.  This phase lasted a reported 3.5 minutes (a long time when you're experiencing such a terrifying event).  Fissures up to 15 feet wide opened up in the city.  After the main shocks, people worked their way down to the riverside quays of the Tagus River (Rio Tejo in Portuguese) where they gathered for comfort.

Lisbon, across the Tagus River

People presumably watched in confusion as the water in the Tagus estuary pulled out to see exposing a "sea floor littered by lost cargo and old shipwrecks."  Today we all should know what that presages but people in the mid-1700s in Portugal had no idea.  Some 40 minutes after the quake, the waters came rushing back up the river.  So fast, according to contemporary accounts, people on horseback, galloping at full speed, barely escaped.  At least three waves of tsunami came up the river, the largest being an estimated 20 meters (65 feet) in height.  All of the ships in Lisbon's harbor were destroyed and tens of thousands of people drowned.

Just as happened with the 1906 San Francisco earthquake, Lisbon was then beset by several days of uncontrolled fire which burned through the city killing and injuring even more people.  As one researcher writes:

Fanned by steady northeast winds, the great fire burned out of control through the ruins of the city for more than 3 days. It swept everything in its path and destroyed houses, churches and palaces. Lisbon's magnificent museums, and its magnificent libraries - housing priceless documents and papers dealing with the great history of Portugal's great past - burned to the ground. Archives and other precious documents were completely destroyed. Works of art, tapestries, books, manuscripts, including the invaluable records of the India Company were destroyed. Also burned was the king's palace and its 70,000-volume library. Over two hundred fine, priceless paintings , including paintings by Titan, Reubens, and Coreggio, were burned in the palace of the Marques de Lourcal.

The earthquake caused damage in many more places than Lisbon.  Other cities in Portugal, Spain, and Morocco were heavily damaged as well.  The earthquake was felt as far away as Finland.  The tsunami raced across the Atlantic causing damage in England and Ireland and large waves struck several islands in the Caribbean.  Seismologists have estimated that this was a 1,000 year earthquake.

Tomorrow, in Part III of this series, I'll talk about how this earthquake affected Europe.  I'll leave you with a basically unanswerable question: If God exists, why would He allow an earthquake to drop a cathedral on devout worshippers celebrating mass? 


  1. The usual question about theodicy - if God exists then why would God allow x-- depends, of course, entirely on what sort of God you are trying to understand. If you are arguing the nature of God from the Bible you have a lot of contradictory evidence. If you stick with the Gospels, you get an entirely different view, or possibly views, since even the synoptics disagree on some matters. The easiest outs are : "God is so much bigger/wiser/wholly other from us, we can hardly expect to know why God would allow X to happen;" or the other fave "God was smiting the sinners. Just because people were in church doesn't mean they were not sinners." You also can have the Greek-philosophical sort of impassible God, who wouldn't care as long as the plate tectonics worked out. And the good-cop, bad-cop God and CounterGod/Demiurge of a lot of the Gnostics. But you really can't have the biddable, reasonable God who might be bought off by one's good deeds, which is a terrible blow to nice middle-class people everywhere, like me. God should try harder to behave in a more godly manner.

    The best answer of course, is that it's a mystery, and you might as well go on trying to be a decent person even if God doesn't know any better.

    1. Or one could take the atheist position that there is no God so the question is essentially meaningless.

  2. I still don't buy the AGFZ being capable of a M9 thrust earthquake. My money is on the relict subducted Alboran plate that is still producing large earthquakes up to 600km deep below southern Spain.

    1. Possibly. We'll just have to wait for the next quake to see (although it is a bit ghoulish to want large earthquakes to occur so we can learn more about the Earth!).