Why are there earthquakes in Lisbon?
Lisbon is similar to San Francisco not only because of its position on the edge of an estuary and its Golden-Gate-like bridge. Another similarity is immediately obvious in the topography of the landscape—steep slopes and numerous hills separate distinctive neighborhoods and make the city feel larger than it is.
Both cities have streetcars. Whereas San Francisco’s streetcars stick to the flattest parts of the city, Lisbon’s streetcars worm their way along the more narrow, steep thoroughfares. Like the steep-climbing cable cars in San Francisco, the cute streetcars are a favorite of tourists.
Another common aid to those wishing to gain the views but not strain their legs is the ubiquitous Tuk-Tuk. It’s a wonder someone hasn’t figured out how well these would work in San Francisco. Here is a couple who has just gotten a ride to one of the many viewpoints scattered throughout the city (the intrepid San Franciscans walked of course!)
Another similarity is only obvious after walking around and reading information about buildings newly rebuilt after being destroyed in the 1755 earthquake. Although Lisbon is one of the oldest cities in Europe, you wouldn’t know it from walking around. Here is one piece of evidence still visible—church that was destroyed but never rebuilt. It now serves as an open-air archeology museum—to me one of the most interesting sights in the city
A castle on one of the highest hills (Castelo de Sao Jorge) was built over various layers of culture. On the back side it’s still possible to see the ancient walls from the time of the Moors (8th to 12th century in Lisbon), with newer edifices built on top. The main center of activity, and where tourists tend to concentrate) is a valley heading northwest from the Placa do Comercio (Plaza of Commerce). Here is a view overlooking the valley, which is where a river formerly flowed into the sea. The Castelo do Sao Jorge is just off the left side of the photo.
The arc below, on the north side of the Placa do Comercio, was designed to celebrate the rebuilding that occurred after the 1755 earthquake. Since it was built on an old river bed, on the edge of the bay where sediments are very weak, one wonders it they did not set themselves up for another catastrophe during the next major earthquake. You may remember that the Marina district in San Francisco, which suffered so much damage in the 1989 earthquake, was the site where land was filled in to create a site to celebrate the rebuilding from the great 1906 San Francisco earthquake.
How big was the earthquake? According to the USGS (search on earthquake.usgs.gov, which includes eyewitness I accounts and other interesting information) it was a magnitude 8.7. This is a very large earthquake! Why did it occur here? The history of Portugal is much like that of the U.S. east coast, which collided with the west coasts of Europe and Africa during the Paleozoic (around 400–250 million years ago). Beginning in the late Triassic (about 200 million years ago), the U.S. east coast and the Eurasian and African continents began to separate. Since then, the Atlantic Ocean has gotten larger, as the continents continued to move farther away from each other.
So both edges of the Atlantic are what referred to as passive continental margins—they are far from the active plate boundary that lies in the middle of the ocean. But the plate boundary between Europe and Africa is a collision zone, and as the Mediterranean Sea has gotten smaller, and Europe and Africa have gotten closer, the plate boundary has oozed westward and has now included southern Portugal in its sphere. So the earthquakes will continue….
When in Lisbon, be sure to visit the Geological Museum. It’s not in the tour guides but easy to find on Rua Da Academia Das Ciencias (Street of the Academy of Sciences) in the Barrio Alto district.
The museum is “old school”, with samples laid out in neat rows. One could spend many hours pursuing their incredible collections, including awesome rock and mineral samples and fossils extending from Paleozoic trilobites to Pleistocene large vertebrate, including the fierce crocodile shown below. They also have a stellar archeological exhibit.
Final Footnote. Jay took this photo of the blogger working on photos while drinking a local Portuguese beer (stout) on the terrace of a quaint restaurant on the streetcar line in the Alfama district of rambling, narrow alleyways. Life is good!