olelog

The greatest earthquakes occur in subduction zones, where one tectonic plate is sliding beneath another. Virtually all of the big earthquakes, the ones of magnitude eight or nine or above, happen at sea. The largest earthquake ever recorded is no exception to this rule.

The epicentre of the Great Chilean Earthquake of 22 May 1960 was about 160 km off the coast of Chile in the Peru-Chile Trench (39.5° S, 74.5° W) with a focal depth of 33 km. Two days later, on 24 May 1960, Cordón Caulle, a fissure vents system located in the Chilean Lake District, erupted, sending ash and steam as high as 6 km.

At the Peru-Chile trench the Nazca Plate is subducted beneath the South American Plate. In the area hit by the earthquake the dip of the subduction zone is about 30° and the subduction gives rise to an arc of still active volcanoes.

Buildings fell all along the Chilean coast from Conception to the southern end of Isla Chilor. The towns of Valdivia and Puerto Montt were devastated. (The earthquake is also known as the 1960 Valdivia earthquake / Gran terremoto de Valdivia).

The earthquake set off huge landslides and sent rocks and boulders tumbling down the mountain sides. The land around the city of Puerto Montt sank and coastal areas were flooded. Rivers had their courses changed and landslides created new lakes. Many of the landslides occurred in the Chilean Lake District from Lago Villarica to Lago Todos los Santos.

The earthquake set off huge tsunamis which radiated out from the epicentre, travelling at speed of up to 350 km/h, the Chilean coast was devastated by a 25 (or was it 12 ?) m high tsunami which arrived 10 to 15 minutes after the quake. Remains of houses were carried inland as much as 3 km. There was also severe damage in the Philippines, Hawaii and the japan.

Over 2000 people died and 3000 were injured. 2 million people became homeless. There were not extremely large numbers of victims, for such an earthquake, because the population was alerted on that something was going to happen by previous shakes and underground noise.

Map of some of the places mentioned and the most important volcanoes in the district. In 2005 we made our way from Puerto Montt to San Carlos de Barriloche in Argentina through the Chilean Lake District (bus, boat, bus, boat, bus) - as many tourists do. Under way I photographed the following volcanoes: Osorno (famous for its Fujiyama look), Puntiagudo ("Volcán Puntiagudo" (Spanish for "Sharp-pointed Volcano") is a stratovolcano with a prominent 2,493 m high sharp-pointed summit that results from glacial dissection and gets its name from this feature), and Tronador. See the 3 photos below.

• http://earthquake.usgs.gov/regional/world/events/1960_05_22.php
• http://en.wikipedia.org/wiki/Great_Chilean_Earthquake
• http://www.gochile.cl/html/ChileValdivia/Chile-Valdivia-Terremoto.asp
• http://www.geophys.washington.edu/tsunami/general/historic/chilean60.html
• http://www.usgs.gov/faq/list_faq_by_category/get_answer.asp?id=154





Notes:
The volcano Puyehue is often cited as the volcano that erupted on 24 May, but actually it was the nearby fissure volcano Cordón Caulle. Although Cordón Caulle is sometimes listed as part of Puyehue volcano, it is tectonically and magmatically distinct from Puyehue. No historical eruptions are known from Puyehue, and eruptions in 1921-22 and 1960 listed in some sources actually occurred at Cordón Caulle volcano located to the Northwest.

As far as I know the epicentre of the main quake was at 39.5° S, 74.5° W - some maps however show it inland (including the USGS map). Well of course there were more than one shock, but even then?

Can earthquakes trigger volcanic eruptions? The volcanic eruption 2 days after the 1960 Chilean earthquake has been taken as evidence, but that could still be a coincidence, and the question is still debated. That volcanoes, on the other hand, can cause earthquakes, is well known.

What I wanted to stress here is the role of subduction zones for important natural hazards like earthquakes, volcanoes, tsunamis and landslides.

I am back home from a week in Lisbon, where I was several times reminded of the Great Lisbon Earthquake.

On 1 November 1755 Lisbon was shaken by a violent earthquake. It occurred at 9:40 in the morning during High Mass - a mass to celebrate All Saints' Day, also known as All Hallows or Hallowmas, an important religious holiday in this strong Roman catholic country.

Geologists today estimate the Lisbon earthquake approached magnitude 9 on the Richter scale, with an epicentre in the Atlantic Ocean about 200 km west-south-west of Cape St. Vincent (See map). Estimates place the death toll between 60,000 to 100,000 people, making it one of the most destructive earthquakes in history.

More than 20 churches collapsed, and due to the candles lit for the celebration fire quickly broke out, and flames raged for five days. Gigantic fissures up to five metres wide appeared in the city centre. Survivors rushed to the open space of the docks for safety and watched as the water receded revealing the sea floor. Approximately forty minutes after the earthquake, an enormous tsunami engulfed the harbour and downtown, rushing up the Tagus river. It was followed by two more waves.

Many people at the time saw the disaster a God’s punishment - because the town was too rich, because of the inquisition (the Spanish Inquisition was established in 1478), because of idolatry or other sins? Apart from theological, philosophical (including work by Voltaire), and literary discussions, however, scientists got involved. It was the first earthquake studied scientifically for its effects over a large area, and it led to the birth of modern seismology.

I stayed at a hotel a few hundred metres from the Santa Justa Lift (also known as the Carmo Lift). The Santa Justa Lift was designed by an apprentice of Gustave Eiffel (the one with the Eiffel Tower). The iron lift is 45 metres tall and it brought me from the downtown streets to the uphill Carmo Square. From the roof of the lift construction (with a bar, where I had an espresso) there is a nice view over downtown Lisbon and the Carmo Convent. This mediaeval convent was ruined in the Earthquake, and the ruins of its Gothic church are the main trace of the great earthquake still visible in the city. The ruins were preserved to remind Lisboners of the destruction.

I went down to the docks and took the train to see one of the most impressive monuments in Lisbon, the Jeronimos Monastery. The vaulting in the church withstood the earthquake of 1755, which probably says something about the architecture. In the same district - called Belém, which is in fact Portuguese for Bethlehem, and pronounced more or less as “blem” - there is an old tower, the Torre de Belém. The tower was built in the same style as the Monastery between 1515 and 1519 in the middle of the river Tagus to defend Lisbon and the monastery. Today, however it stands on the water’s edge practically moored to the north bank, the river having altered course during (and after) the earthquake and tsunami of 1755.

From there the train moves on to the romantic fishing port - and holiday resort with yacht harbour - of Cascaias, about half an hour’s ride from Lisbon (30 kilometres west of Lisbon). A large portion of the village was destroyed during the earthquake in 1755. But today it is bustling, and I had a nice evening meal at the beach with cockles and local wine.



PS of 17 April 2008
The position of the epicentre is disputed (See comments). The source mechanism seems to require generation at a subduction zone, but where would that be?

The Norwegian Arctic archipelago of Svalbard was hit by a major earthquake in the early hours of Thursday morning 21 February 2008. The quake measured 6.2 on the Richter scale. This is the strongest quake in Norwegian history. The focus of the earthquake was localised on the sea floor, about 10 km down into the earth's crust. 15 aftershocks was registered shortly after.

The common concept assumes that earthquakes in Svalbard are due to its location in an active seismic zone along the Mid-Atlantic Ridge. The epicentre in Storfjorden lies close to a relatively large fault running in north-south direction . Through Svalbard's geological history this fault has repeatedly been the focus of seismic activity.

The reason was, however, according to Odleiv Olesen from NGU (Norwegian Geological Survey) the large displacement of sediments that took place in the last 1.1 million years near Svalbard. During the glaciations large masses of sediments were eroded from the bottom of Storfjorden on the East coast of Svalbard, and deposited elsewhere. This process changed the tension in the Earth crust and resulted in movements of the crust and earthquakes. This process is also known to cause minor earthquakes along the coast of mainland Norway.


There is an ongoing discussion about the role of change of gravity forces during the moon eclipse in the earth crust displacements. The total moon eclipse took place on the same night as the last large earthquake in Svalbard (21 February 2008).


See also my post on Svalbard Terranes.




• http://www.ssf.npolar.no/pages/news153.htm
• http://www.norwaypost.no/cgi-bin/norwaypost/imaker?id=131274
• http://npweb.npolar.no/english/articles/1203601816.69

Two strong earthquakes in the Kivu region on Sunday 3 February 2008 killed at least 40 people in Rwanda and D.R. Congo.

Kivu is a rift lake. The Kivu Graben is part of the great western East African Rift System. The graben, approximately 90 km wide and 200 km long, trends NNE-SSW and straddles both Rwanda and the Democratic Republic of Congo. Structurally, Kivu Graben is the southern extension of the Albertine Graben in Uganda. The western border fault (with tilted fault-block mountains rising on the west side) is also marked by destructive active volcanoes like Nyamulagira and Nyiragongo (approximate location shown as red dots on map).

The first quake which measured 6.1 on the Richter scale occurred 20 km N of Bukavu, D.R. Congo - marked with orange-coloured square on map. The second quake with a magnitude of 5 struck 40 km W of Butare, Rwanda, and 55 km E of Bukavu (See map). The quakes wsere followed by several aftershocks.

Hundreds of people were wounded. Many homes, as well as schools and churches, were damaged.

• http://www.terradaily.com/reports/Africa_quakes_kill_at_least_40_officials_hospitals_999.html
• http://www.foxnews.com/story/0,2933,327930,00.html
• http://news.bbc.co.uk/2/hi/africa/7225896.stm

Ole

Why should an earthquake swarm in Tanzania remind me of a place in the Rhine valley in Germany?



The highest magnitude of the earthquakes was 6.0 on the Richter scale registered on Tuesday 17 July 2007, which was felt throughout northern Tanzania and parts of neighbouring Kenya. The swarm is situated close to Mt. Oldonyo Lengai, a volcano in northern Tanzania, which erupted on Friday 20 July 2007, spewing small amounts of smoke and lava.

Although volcanic eruptions are often preceded and accompanied by earthquake swarms, most earthquake swarms are not associated with volcanic eruptions.

• http://news.yahoo.com/s/ap/20070720/ap_on_sc/kenya_tanzania_volcano
  

Oldonyo Lengai (or Ol Doinyo Lengai) is unique among active volcanoes in that it produces natrocarbonatite lava, a unique occurrence of volcanic carbonatite. Due to this unusual composition, the lava is erupted at relatively low temperatures (approximately 500-600°C). This temperature is so low that the molten lava appears black in sunlight, rather than having the red glow common to most lavas. It is also much more fluid than normal (silicate) lavas. The carbonate minerals of the lavas formed by Oldonyo Lengai are unstable at the Earth's surface and susceptible to rapid weathering, quickly turning from black to grey in color. The resulting volcanic landscape icolourferent from any other in the world.

Ol Doinyo Lengai is the only volcano known to have erupted carbonatite tephras and lavas in historical time.

Ol Doinyo Lengai is located in the eastern branch of the African Rift Valley and this brings me to another rift, however failed, the Rhine graben. here are the only carbonatites that I have personally ever seen - at the Kaiserstuhl. In the southern part of the Upper Rhine Graben the most prominent example of rift related magmatic activity is represented by the partly eroded Miocene Kaiserstuhl volcanic complex. Subvolcanic intrusions of carbonatites are now exposed in the center of the Kaiserstuhl. The activity took place somewhere between 16 and 14 million years ago.